Westminster" Series 
 
 GI ASS MANUFACTURE 
 
GLASS 
 MANUFACTURE 
 
 BV 
 
 WALTER ROSENHAIN B.A. B.C.E. 
 
 SUPERINTENDENT OF THE DEPARTMENT OF METALLURGY AND 
 
 METALLURGICAL CHEMISTRY AT THE NATIONAL 
 
 PHYSICAL LABORATORY 
 
 NEW YORK 
 
 D. VAN NOSTRAND COMPANY 
 23 MURRAY AND 27 WARREN STREETS 
 
 1908 
 
GENERAL 
 
 BRADBURY, AGNEW, & CO. LD., PRINTERS, 
 LONDON AND TONBRIDGE. 
 
PREFACE 
 
 THE present volume on Glass Manufacture has been 
 written chiefly for the. benefit of those who are users of 
 glass, and therefore makes no claim to be an adequate guide 
 or help to those engaged in glass manufacture itself. For 
 this reason the account of manufacturing processes has 
 been kept as non-technical as possible ; no detailed drawings 
 of plant or appliances have been given, and only a few 
 illustrative diagrams have been introduced for the purpose 
 of avoiding lengthy verbal descriptions. In describing each 
 process the object in- view has been to give an insight into 
 the rationale of each step, so far as it is known or under- 
 stood, and thus to indicate the possibilities and limitations 
 of the process and of its resulting products rather than to 
 provide a detailed guide to the technique of the various 
 operations. The practical aim of the book has further been 
 safeguarded by the fact that the processes described in these 
 pages are, with the exception of those described as obsolete, 
 to the author's definite knowledge, in commercial use at the 
 present time. For this reason many apparently* ingenious 
 and beautiful processes described in earlier books on glass 
 have not been mentioned here, since the author could find 
 no trace of their employment beyond the records of the 
 various patents involved. On the other hand the reader 
 
 207120 
 
vi PREFACE. 
 
 must be warned to bear in mind that the peculiar conditions 
 of the glass manufacturing industry have led to the practice 
 on the part of manufacturers of keeping their processes as 
 secret as possible, so that the task of the author who would 
 give an accurate account of the best modern processes used 
 in any given department of the industry is beset with great 
 difficulties. The author has endeavoured to steer the best 
 course open to him under these circumstances, and he 
 would appeal to the paucity of glass literature in the 
 English language as evidence of the difficulty to which he 
 refers. 
 
 In addition to these difficulties, which arise largely from 
 considerations of a commercial nature, the writer of a book 
 on glass is further confronted with technical difficulties of 
 no inconsiderable order. As already indicated, the aim of 
 the present author has been to describe processes from the 
 point of view of principles and methods rather than as mere 
 rule-of-thumb descriptions of manufacturing manipulations, 
 but in doing this he is met at every turn by the fact that 
 from the scientific side the greater part of the field of glass 
 manufacture is a " terra incognita." In making this 
 statement the labours of many eminent scientific workers 
 are by no means forgotten, but the entire field is so large 
 and beset with such great experimental difficulties that even 
 the labours of a list of investigators that includes the 
 names of Fraunhofer and Faraday, Stokes, Hopkinson, 
 Abbe and Schott, have resulted in little more than an 
 accumulation of empirical data which, while they have been 
 productive of great direct practical results, have left the 
 science of glass still in a very elementary condition. To 
 take two examples in illustration of this fact we may mention 
 
PEEFACE. vii 
 
 the question of the connection between chemical composition 
 and any of the physical properties of glass, such as refrac- 
 tion and dispersion of light, and on the more mechanical 
 side the question why all processes, such as rolling or 
 moulding, which involve the contact of hot glass with metal 
 result in a roughening of the glass surface. The former 
 question has been studied by several of the investigators 
 named above, Schott and Abbe having particularly devoted 
 an enormous amount of labour and money to the study of 
 the question with results which have proved disappointing 
 from the scientific point of view. By prolonged experi- 
 menting and the employment of a costly system of trial and 
 error an important series of novel and useful glasses has 
 been produced by these workers, but no law by whose aid 
 the optical properties of a glass of given chemical composi- 
 tion could be predicted has yet been discovered, and as a 
 summary of the known facts only the vaguest general 
 principles are available for the guidance of those who wish 
 to produce glasses of definite properties. The same applies 
 in a similar degree to most of the other properties of glass, 
 with the exception, perhaps, of density and thermal expan- 
 sion ; attempts to generalise from the known data of a 
 limited number of glasses generally meet with unqualified 
 failure. The conclusion which one is forced to admit is 
 that the fundamental principles underlying the nature and 
 constitution of glasses have yet to be discovered. A study 
 of the other question mentioned above as an example of the 
 limitations of our knowledge leads to the same conclusion ; 
 an almost endless succession of inventors have busied 
 themselves with devices for overcoming the roughening 
 action of rollers and moulds upon glass, but without any 
 
viii PEEFACE. 
 
 real success. A long list of other examples of the same 
 kind could be given, our knowledge of the physical and 
 chemical principles underlying many of the phenomena 
 met with in glass manufacture being deplorably deficient. 
 It will thus be seen that to write a truly scientific account 
 of glass manufacture is at the present time impossible, and 
 the reader is asked to bear this in mind if he should find 
 the chemical or physical explanations given in this book 
 less frequent or less adequate than could be desired. 
 
 Having dwelt somewhat emphatically on the limitations 
 of our present scientific knowledge as applied to glass 
 manufacture, it is perhaps scarcely necessary at the present 
 time to emphasise the fact that this state of affairs should 
 act as the strongest incentive to further investigation of the 
 whole subject. The difficulty, however, lies in the fact that 
 such investigation can scarcely be carried on by voluntary 
 workers in ordinary laboratories, but must be undertaken 
 with the active help of glass manufacturers at their works. 
 Glass is essentially a substance that cannot be satisfactorily 
 handled in small quantities, particularly so far as all the 
 phenomena connected with its production and manipulation 
 while hot are concerned; the influences of containing 
 vessels, of furnace gases and of rapid cooling are all 
 enormously exaggerated if ounces instead of hundredweights 
 or tons of glass are used for experimental purposes, and 
 these influences and others of the same nature vitally affect 
 all the results of small-scale laboratory operations. The 
 progress of our scientific knowledge of glass and the 
 consequent development of the glass industry from its 
 present state where rule-of- thumb and "practical experience" 
 still hold excessive sway lies in the hands of those 
 
PEEFACE. ix 
 
 concerned in the industry itself. It must be admitted that to 
 undertake such work involves the expenditure of much time 
 and money on the part of a manufacturer, while the field is 
 so large and the problems so complicated that any adequate 
 return cannot be promised for the immediate future ; on 
 the other hand the very size of the field and the difficulty 
 of the problems offers the promise of the greatest ultimate 
 reward ; a really important scientific discovery in connection 
 with glass would be certain to bring in its train industrial 
 developments whose limits it is impossible to foresee. The 
 industrial success of the glass-works of Schott in Jena is 
 often quoted as a brilliant example of commercial success 
 resulting from purely scientific investigations in this actual 
 field ; an example of still greater magnitude is furnished by 
 the success of the aniline dye works of Germany which are 
 built up on purely scientific achievements. The glass 
 industry as a whole, supplying some of the absolute 
 necessaries of modern life, should be capable of offering the 
 greatest rewards to success, and the example of other 
 industries has shown that ultimate success is bound to reward 
 properly-conducted and perseverant scientific research. 
 Nowhere is this more urgently needed than in the whole 
 field of glass manufacture. 
 
 The author is indebted to Mr. W. C. Hancock for valuable 
 assistance in the reading of proofs and various suggestions 
 in connection with the contents of this book. 
 
TABLE OF CONTENTS 
 
 PAGE 
 
 PREFACE v 
 
 CHAPTEE I. 
 
 THE PHYSICAL AND CHEMICAL PROPERTIES OF GLASS. 
 
 Definition of the term "Glass" Amorphous structure the common 
 feature of all vitreous bodies Glass a congealed fluid Glasses 
 not definite chemical compounds but complex solutions Eange 
 of chemical composition available for glass-making Considera- 
 tions governing chemical composition Influence of composition 
 on physical properties Chemical stability of glass Permanence 
 of glass surfaces Action of water, acids, and alkalies on glass 
 Action of light on glass . . . . . . . p. 1 
 
 CHAPTER II. 
 
 THE PHYSICAL PROPERTIES OF GLASS. 
 
 Mechanical properties : tensile strength, crushing strength, elasticity, 
 ductility, and hardness Thermal properties of glass : thermal 
 endurance, coefficient of expansion, thermal conductivity Ther- 
 mometer glass Electrical properties of glass Transparency and 
 colour of glass . . . . . . . . . p. 18 
 
xii TABLE OF CONTENTS. 
 
 CHAPTEE III. 
 
 THE RAW MATERIALS OF GLASS MANUFACTURE. 
 
 General considerations Chemical purity, moisture, and physical 
 condition, constancy of quality Sources of silica, sand and 
 sandstone Felspar Sources of alkali: Soda ash (carbonate of 
 soda), salt cake (sulphate of soda), pearl ash (carbonate of 
 potash) Alkali nitrates Natural minerals containing alkalies 
 Sources of other bases : Lime, chalk, limestone, slaked lime 
 Gypsum (sulphate of lime) Barium compounds Magnesia and 
 zinc Lead oxide, red lead Aluminium, manganese, arsenic 
 Carbon Coke, charcoal, anthracite coal . . . . p. 35 
 
 CHAPTER IV. 
 
 CRUCIBLES AND FURNACES FOR THE FUSION OF GLASS. 
 
 Fire-clay and silica-brick Manufacture of glass-melting pots 
 Drying and first heating of pots Blocks for tank and other 
 furnaces Uses of silica brick Furnaces Coal-fired and gas- 
 fired furnaces Gas producers Regenerative furnaces, principles 
 and construction of Siemens' furnaces Recuperative furnaces 
 General arrangements of modern tank furnaces Relative 
 advantages of tank and pot furnaces . . . . p. 54 
 
 CHAPTER V. 
 
 THE PROCESS OF FUSION. 
 
 Mixing of raw materials by hand and by machinery The charging 
 operation Chemical reactions during melting of carbonate 
 mixtures, and of sulphate mixtures Influence of carbon on 
 the reactions The fining process . . . . . p. 73 
 
 CHAPTER VI. 
 
 PROCESSES USED IN THE WORKING OF GLASS. 
 
 Ladling, gathering, and casting Limitations of ladling Ladling 
 used for rolled glass, gathering for blown glass Rolling of glass 
 Blowing processes and operations Use of moulds Pressing 
 Moulding />. 84 
 
TABLE OF CONTEXTS. xiii 
 
 CHAPTEE VII. 
 
 BOTTLE GLASS. 
 
 Raw materials Furnaces Predominance of tank furnaces Process 
 of blowing bottles by hand Gathering, marvering, blowing 
 Use of fire-clay and metal moulds Formation of neck 
 Improved appliances, moulds and tools Manufacture of bottles 
 by machinery The "Boucher" bottle-blowing machine 
 Annealing of bottles Large bottles, carboys Aids to the blower 
 Sievert's process Large shallow vessels, bath-tubs . p. 95 
 
 CHAPTEE VIII. 
 
 BLOWN AND PRESSED GLASS. 
 
 Eaw materials Bohemian glass and flint glass Gathering and 
 blowing Chair work Hand work Production of tumblers by 
 hand Application of coloured glass to blown articles Use of 
 moulds as aids to blowing Eoughening effect of moulds Fire- 
 polishing by reheating Use of compressed air Pressed glass 
 Moulds and presses Capacity and limitations of pressing 
 process .......... p. 108 
 
 '* 
 CHAPTEE IX. 
 
 ROLLED OR PLATE GLASS. 
 
 Eolled plate glass Furnaces Eaw materials Process of ladling 
 The rolling table Annealing Cutting and sorting Patterns on 
 rolled plate " Figured" rolled plate Machine used for double- 
 rolling Polished plate Eaw materials Casting from melting 
 pots Special casting pots The rolling table Importance of 
 flatness Annealing kilns Grinding and polishing processes 
 Machines used for grinding and polishing Method of holding 
 the glass Abrasives and polishing materials Theory of the 
 polishing process Limiting sizes of polished plate Homogeneity 
 of polished plate Uses of plate glass Bent polished plate 
 Mirrors Bevelling, process and machines Wired plate glass, 
 rolled and polished Difficulties and limitations Advantages of 
 wired glass p. 122 
 
xiv TABLE OF CONTENTS. 
 
 CHAPTEE X. 
 
 SHEET AND CROWN GLASS. 
 
 Comparison of sheet with polished plate Raw materials for sheet 
 Furnaces : various forms of tank furnaces Blowing process 
 Gathering, forming the gathering on blocks, forming the shoulder 
 of the cylinder, blowing the cylinder, opening the end of the 
 cylinder, detaching cylinder from pipe Cutting off the "cap" 
 Splitting the cylinder Flattening and annealing Cutting and 
 sorting sheet-glass Defects of sheet-glass Variations of the 
 process Attempts to produce " sheet" glass by rolling Sie vert's 
 process Direct drawing processes The America.n process for 
 drawing cylinders Fourcault's processes Difficulties and limita- 
 tions Crown glass The blowing process Limitations . p. 149 
 
 CHAPTER XI. 
 
 COLOURED GLASSES. 
 
 Definition of coloured glass Physical causes of colour Colouring 
 substances : copper, silver, gold, carbon, tin, arsenic, sulphur, 
 chromium, uranium, fluorine, manganese, iron, nickel, cobalt 
 Range and depths of tints available Intensely coloured glasses 
 The process of "flashing" Character of "flashed" glass- 
 Colours produced on glass by painting: use of coloured "glazes" 
 as paints Ancient stained glass and modern glass Technical 
 uses of coloured glass, photography, railway and marine 
 '. . . 178 
 
 CHAPTER XII. 
 
 OPTICAL GLASS. 
 
 Nature and properties of optical glass Homogeneity Formation 
 and removal of striae in solutions and in glass Transparency 
 and colour Absorption of light in "decolourised" glasses 
 Refraction and dispersion Definitions Refractive index, dis- 
 persion, medium dispersion, the quantity v Specification of 
 optical properties in terms of certain spectrum lines Table of 
 
TABLE OF CONTENTS. xv 
 
 typical optical glasses and their optical constants Crown and 
 flint glasses Eelation between refraction and dispersion in the 
 older and newer glasses Work of Abbe and Schott Applications 
 of the new glasses Non-proportionality of dispersion in different 
 types of glass Resulting imperfections of achromatism -The 
 relative partial dispersions of glasses Pairs of glasses giving 
 perfect achromatism not yet fully available Constants of 
 Schott's telescope crown and flint - Narrow range of optical 
 glasses, consequent limitations in lens design Causes of these 
 narrow limits Possible directions of extension Chemical 
 stability of optical glasses Double refraction in optical glass 
 arising from imperfect annealing p. 205 
 
 CHAPTER XIII. 
 
 OPTICAL GLASS. 
 
 The manufacture of optical glass Raw materials Mixing Furnaces 
 and crucibles Kilns for heating pots Transfer of pots from kiln 
 to melting furnace Introduction of cullet and raw materials 
 The fining process, difficulties and limitations The stirring 
 process The final cooling of the glass Rough sorting of the 
 glass fragments Moulding and final annealing of the moulded 
 glass Grinding and polishing of plates and discs for examina- 
 tion ; smallness of yield obtained Difficulty of obtaining large 
 blocks of perfect glass p. 223 
 
 CHAPTER XIV. 
 
 MISCELLANEOUS PRODUCTS. 
 
 Glass tubing Gathering and drawing of ordinary tubes Special 
 varieties of tube Combustion tubes Tubes of vitreous silica 
 Varieties of vitreous silica Transparent, glass -like silica ware 
 Great cost of production Translucent "milky" silica ware 
 produced electrically Great thermal endurance of vitreous silica 
 Sensitiveness to chemical action of all basic substances at high 
 temperatures Glass rod and fibre Glass wool Quartz fibres 
 Glass beads Artificial gems Use of very dense flint glass 
 coloured to imitate precious stones Means of distinguishing 
 
xvi TABLE OF CONTENTS. 
 
 imitations Precious stones produced by artificial means Chilled 
 glass Great strength and fragility of chilled glass Bupert's 
 drops Manufacture of "tempered" glass by Siemens De La 
 Bastie's process Massive glass, used for house construction and 
 paving blocks Water-glass (silicate of soda or potash), manu- 
 facture in tank furnaces Glass for lighthouse lenses and search- 
 light mirrors Production by casting glass in iron moulds Sizes 
 and types of lenses and prisms produced . . . .p. 238 
 
 APPENDIX Bibliography of Glass Manufacture . . . p. 253 
 
GLASS MANUFACTURE 
 
 CHAPTEE I. 
 
 THE PHYSICAL AND CHEMICAL PROPERTIES OF GLASS. 
 
 ALTHOUGH the term " glass " denotes a group of bodies 
 which possess in common a number of well-defined and 
 characteristic properties, it is difficult to frame a satisfactory 
 definition of the term itself. Thus while the property of 
 transparency is at once suggested by the word " glass," 
 there are a number of true glasses which are not transparent, 
 and some of which are not even translucent. Hardness and 
 brittleness also are properties more or less characteristic of 
 glasses, yet very wide differences are to be found in this 
 respect also, and bodies, both harder and more fragile than 
 glass, are to be found among minerals and metals. Perhaps 
 the only really universal property of glasses is that of 
 possessing an amorphous structure, so that vitreous bodies 
 as a whole may be regarded as typical of "structureless" 
 solids. All bodies, whether liquid or solid, must possess an 
 ultimate structure, be it atomic, molecular or electronic in 
 character, but the structure here referred to is not that of 
 
 G.M. B 
 
2 GLASS MANUFACTURE. 
 
 individual molecules but rather the manner of grouping or 
 aggregation of molecules. 
 
 In the great majority of mineral or inorganic bodies the 
 molecules in the solid phase are arranged in a definite 
 grouping and the body is said to have a crystalline structure ; 
 evidences of this structure are generally visible to the 
 unaided eye or can be revealed by the microscope. Vitreous 
 bodies on the other hand are characterised by the entire 
 absence of such a structure, and the mechanical, optical 
 and chemical behaviour of such bodies is consistent only 
 with the assumption that their molecules possess the same 
 arrangement, or rather lack of arrangement, that is found 
 in liquids. 
 
 The intimate resemblance between vitreous bodies and 
 true liquids is further emphasised when it is realised 
 that true liquids can in many instances pass into the 
 vitreous state without undergoing any critical change or 
 exhibiting any discontinuity of behaviour, such as is 
 exhibited during the freezing of a crystalline body. In the 
 latter class of substances the passage from the liquid to the 
 crystalline state takes place at one definite temperature, and 
 the change is accompanied by a considerable evolution of 
 heat, so that the cooling of the mass is temporarily arrested. 
 In the case of glasses, on the other hand, the passage from 
 the liquid to the apparently solid condition is gradual and 
 perfectly continuous, no evolution of heat or retardation of 
 cooling being observed even by the aid of the most delicate 
 instruments. We are thus justified in speaking of glasses 
 as " congealed liquids," the process of congealing in this 
 case involving no change of structure, no re-arrangement of 
 the molecules, but simply implies a gradual stiffening of 
 
PHYSICAL AND CHEMICAL PROPEKTIES OF GLASS. 3 
 
 the liquid until the viscosity becomes so great that the body 
 behaves like a solid. It is, however, just this power of 
 becoming exceedingly stiff or viscous when cooled down to 
 ordinary temperatures that renders the existence of vitreous 
 bodies possible. All glasses are capable of undergoing the 
 change to the crystalline state when kept for a sufficient 
 time at a suitable temperature. The process which then 
 takes place is known as " devitrification," and sometimes 
 gives rise to serious manufacturing difficulties. 
 
 Molten glass may be regarded as a mutual solution of a 
 number of chemical substances usually silicates and 
 borates. When cooled in the 'ordinary way these bodies 
 remain mutually dissolved, and ordinary glass is thus 
 simply a congealed solution. The dissolved substances 
 have, however, natural freezing-points of their own, and if 
 the molten mass be kept for any length of time at a 
 temperature a little below one of these freezing-points, that 
 particular substance will begin to solidify separately in the 
 form of crystals. The facility with which this will occur 
 depends upon the properties of the ingredients and upon 
 the proportions in which they are present in the glass. In 
 some cases this devitrification sets in so readily that it can 
 scarcely be prevented at all, while in other cases the glass 
 must be maintained at the proper temperature for hours 
 before crystallisation can be induced to set in. In either 
 of these cases, provided that the glass is cooled sufficiently 
 rapidly to prevent crystallisation, the sequence of events 
 during the subsequent cooling of the mass is this : as the 
 temperature falls further and further below 7 the natural 
 freezing-point of one or other of the dissolved bodies, the 
 tendency of that body to crystallise out at first rapidly 
 
 B 2 
 
4 GLASS MANUFACTURE. 
 
 increases ; as the temperature falls, however, the resistance 
 which the liquid presents to the motion of the molecules 
 increases at a still greater rate, so that two opposing forces 
 are at work, one of them an increasing tendency towards 
 crystallisation, the other a still more rapidly increasing 
 resistance to any change. There is thus for every glass 
 a certain critical range of temperature during which the 
 greatest tendency exists for the crystallising forces to over- 
 come the internal resistance ; through this range the glass 
 must be cooled at a relatively rapid rate if devitrification is 
 to be avoided ; at lower temperatures the crystallising forces 
 require increasingly longer periods of time to produce 
 any sensible effect, until, as the ordinary temperature is 
 approached, the forces of internal resistance entirely prevent 
 all tendency to crystallisation. 
 
 The phenomena just described in reality constitute the 
 natural limit to the range of bodies which can be obtained 
 in the vitreous state : as we approach this limit the glass 
 requires more and more rapid cooling through the critical 
 range of temperature, and is thus more and more liable to 
 devitrify during the manufacturing processes, until finally 
 the limit is set when no industrially feasible rapidity of 
 cooling suffices to retain the mass in the vitreous state. 
 
 While the range of bodies that can be obtained in the 
 vitreous state is very large, only a comparatively small 
 number of substances are ordinarily incorporated in indus- 
 trial glasses. With the exception of certain special glasses 
 used for scientific purposes, such as the construction of optical 
 lenses, thermometers and vessels intended to resist unusual 
 treatment, all industrial glasses are of the nature of mixed 
 silicates of a few bases, viz., the alkalies, sodium and 
 
PHYSICAL AND CHEMICAL PROPERTIES OF GLASS. 5 
 
 potassium, the alkaline earths, calcium, magnesium, stron- 
 tium, and barium, the oxides of iron and aluminium 
 (generally present in minor quantities), and lead oxide. 
 The manner in which these various elements enter into 
 combination and solution with one another has been much 
 investigated, and the more general conclusions have been 
 anticipated in what has been said above. It is abundantly 
 evident that glasses are not definite chemical compounds, 
 but rather solutions, in varying proportions, of a series of 
 definite compounds in one another. In many cases the 
 actual constitution of industrial glasses is so complex as, 
 for the present at all events, to baffle adequate chemical 
 expression. 
 
 One of the factors that limit the range of possible com- 
 positions of glasses has already been indicated, and two 
 others must now be discussed. For industrial purposes, 
 the cost and rarity of the ingredients becomes a vital bar at 
 a certain stage ; thus the use of such elements as lithium, 
 thallium, etc., is prohibitively costly. In another direction 
 the glass-maker is very effectively restrained by the 
 limitations of his furnaces as regards temperature. The 
 presence of excessive proportions of silica, lime, alumina, 
 etc., tends to raise the temperature required for the free 
 fusion of the glass, and when this temperature seriously 
 exceeds 1600 C., the manufacture of the glass in ordinary 
 furnaces becomes impossible. Thus pure silica can be 
 converted into a glass possessing very valuable properties, 
 but the requisite temperature cannot be attained in regenera- 
 tive gas-fired furnaces such as are ordinarily used by glass 
 manufacturers.. The production of this glass has accordingly 
 been carried on upon a small scale only by means of 
 
6 GLASS MANUFACTURE. 
 
 laboratory furnaces heated by oxy-acetylene flames, while 
 latterly a less perfect variety of silica glass-ware has been 
 produced on a large scale by the aid of electric furnaces. 
 Such methods are, however, obviously limited to very 
 special products commanding special prices. 
 
 A further limitation in the choice of chemical components 
 is placed upon the manufacturer by the actual chemical 
 behaviour of the glass both during manufacture and in use. 
 As regards chemical behaviour during manufacture, it must 
 be borne in mind that, although glasses are of the nature 
 of solutions rather than of compounds, yet these solutions 
 tend towards a state of saturation ; thus a glass rich in 
 silica and deficient in bases will readily dissolve any basic 
 materials with which it may come in contact, while, on the 
 other hand, a glass rich in bases and poor in acid con- 
 stituents such as silica, boric acid or alumina, will readily 
 absorb acid bodies from its surroundings. During the 
 process of melting, glass is universally contained in fire- 
 clay vessels. These are chosen, as regards their own 
 chemical composition, so as to offer to the molten glass a 
 few of those materials in which the glass itself is deficient ; 
 yet a limit arises in this respect also, since glasses very 
 rich in bases, such as the very dense lead and barium glass 
 made for optical purposes, rapidly attack any fire-clay with 
 which they may come in contact. The finished glass 
 also betrays its chemical composition by its chemical 
 behaviour towards the atmospheric agents, such as moisture 
 and carbonic acid, with which it comes in contact; glasses 
 containing an excessive proportion of alkali, for example, 
 are found to be seriously hygroscopic and to undergo rapid 
 decomposition, especially in a damp atmosphere. 
 
PHYSICAL AND CHEMICAL PROPERTIES OF GLASS. 7 
 
 Within the limits set by these considerations, the glass 
 manufacturer chooses the chemical composition of his glass 
 according to the purpose for which it is intended ; for most 
 industrial products the cheapest and most accessible raw 
 materials that will yield a glass of the requisite appearance 
 are employed, while for special purposes the dependence 
 of physical properties upon chemical composition is utilised, 
 as far as possible, in order to attain a glass specially 
 suited to the particular requirements in question. Thus 
 the flint and barium glasses used for table and ornamental 
 ware derive from the dense and strongly refracting oxides 
 of lead and barium their properties of brilliancy and weight. 
 The fusibility and softness imparted to the glass by the 
 presence of these bases further adapts it to its purpose by 
 facilitating the complicated manipulations to which the 
 glass must be subjected in the manufacturing processes. 
 
 Taking our next example at almost the opposite extreme, 
 the hardest "combustion tubing, 1 ' which is intended to 
 resist a red heat without appreciable softening, is manufac- 
 tured by reducing the basic contents of the glass to the 
 lowest possible degree, especially minimising the alkali 
 content, and using the most refractory bases available, such 
 as lime, magnesia, and alumina in the highest possible 
 proportions. Such glass is, of course, difficult to melt, and 
 special furnaces are required for its production, but on the 
 other hand this material meets requirements which ordinary 
 soda-lime or flint glass tubing could never approach. 
 Another instance of these refractory glasses is to be found 
 in the Jena special thermometer glasses and in the French 
 (Tonnelot) " Verre dur " ; the best of these glasses show 
 little or no plasticity at temperatures approaching 500 C., 
 
8 GLASS MANUFACTUEE. 
 
 and have thus rendered possible a considerable exten- 
 sion of the range of the mercury thermometer. Further 
 modification of chemical composition has resulted in the 
 production of glasses which are far less subject to those 
 gradual changes which occur in ordinary glass when used 
 for the manufacture of thermometers changes which 
 vitiated the accuracy of most early thermometers. A still 
 more extensive adaptation of chemical composition to the 
 attainment of desired physical properties has been reached 
 primarily as a result of the labours of Schott and Abbe, in 
 the case of optical glasses. The work of these men, and 
 the developments which have followed from it, both at the 
 works founded by them at Jena and elsewhere, have so 
 profoundly modified our knowledge of the range of possi- 
 bilities embraced by the class of vitreous bodies, that it is 
 not at all easy at the present time to realise the former 
 narrow and restricted meaning of the term "glass." The 
 subject of the dependence of the optical properties of glass 
 upon chemical composition will be referred to in detail in 
 Chapter XII. on " Optical Glass," but the outline of the 
 influence of composition on properties here given could not 
 be closed without some reference to this pioneer work of 
 the German investigators. 
 
 The chemical behaviour of glass surfaces, to which we 
 have already referred, is of the utmost importance to all 
 users of glass. The relatively neutral chemical behaviour 
 of glass is, in fact, one of its most useful properties, and, 
 next to its transparency, most frequently the governing 
 factor in its employment for various purposes. Thus the 
 entire use of glass for- table-ware depends primarily upon 
 the fact that it does not appreciably affect the composition 
 
PHYSICAL AND CHEMICAL PEOPERTIES OF GLASS. 9 
 
 and flavour of edible solids or liquids with which it is 
 brought into contact a property which is only very 
 partially shared even by the noble metals. Again, the 
 use of glass windows in places exposed to the weather 
 would not be feasible if window-glass were appreciably 
 attacked by the action of water or of the gases of the 
 atmosphere. For these general purposes, it is true, most 
 ordinary glasses are adequately resistant, but this degree 
 of perfection in this respect is only the outcome of the 
 centuries of experience which the practical glass-maker 
 has behind him in the manufacture and behaviour of 
 such glass. When, however, a higher degree of chemical 
 resistance is required for special purposes, as for instance 
 when glass is called upon to resist exposure to hot, damp 
 climates, or is intended to contain corrosive liquids, the 
 rules which are an adequate guide to the glass-maker in 
 meeting ordinary requirements are no longer sufficient, 
 particularly when the glass is expected to meet other 
 stringent requirements as well. It has, in fact, frequently 
 happened that a glass-maker, in striving to improve the 
 colour or quality of his glass, as regards freedom from 
 defects, brilliancy of surface, etc., has spoilt the chemical 
 durability of his products. The reason lies in the fact, 
 long known in general terms, that an increased alkali con- 
 tent reduces the chemical resistance of glass, while at 
 the same time such an increase of alkali is the readiest 
 means whereby the glass-maker can improve his glass in 
 other respects by making it more fusible and easier to 
 work in every way. 
 
 This subject of the chemical stability of glass surfaces 
 attracted much attention during the later part of last 
 
10 GLASS MANUFACTUBE. 
 
 century, and careful investigations on the subject were 
 carried out, particularly at the German Keichsanstalt 
 (Imperial Physical Laboratory) at Charlottenburg. Here 
 also the labours of Schott and Abbe proved helpful, until 
 at the present time such glass as that used by the Jena 
 firm in the production of laboratory ware, and certain 
 other special glasses of that kind, are fitted to meet the 
 most stringent requirements. 
 
 Leaving aside the inferior glasses, containing, generally, 
 more than 15 per cent, of alkali, the behaviour of glass 
 surfaces to the principal chemical agents may be summed 
 up in the following statements. Pure water attacks all 
 glass to a greater or lesser extent ; in the best glasses the 
 prolonged action of cold water merely extracts a minute 
 trace of alkalies, but in less perfect kinds the extraction of 
 alkali is considerable on prolonged exposure even in the 
 cold, and becomes rapidly more serious if the temperature 
 is raised. Superheated water, i.e., water under steam 
 pressure, becomes an active corroding agent, and the best 
 glasses can only resist its action for a limited time. For 
 the gauge-glass tubes of steam boilers working at the high 
 pressures, which are customary at the present time, specially 
 durable glasses are required and can be obtained, although 
 many of the gauge-tubes ordinarily sold are quite unfit for 
 the purpose, both from the present point of view and from 
 that of strength and " thermal endurance." 
 
 In certain classes of glass, the action of water, especially 
 when hot, is not entirely confined to the surface, some 
 water penetrating into the mass of the glass to an appreci- 
 able depth. The exact mechanism of this action is not 
 known, but the writer inclines to the view that it arises 
 
PHYSICAL AND CHEMICAL PEOPEETIES OF GLASS. 11 
 
 from a partial hydration of some of the silica or silicates 
 present in the glass. If such glasses be dried in the 
 ordinary way and subsequently heated, the surface wfll be 
 riddled with minute cracks, some glass may even flake off, 
 and the whole surface will be dulled. As such penetrating 
 action sometimes takes place in the poorer kinds of glass 
 by the action of atmospheric moisture when the glass is 
 merely stored in a damp place, it is often mistaken for 
 " devitrification." This latter action, however, is not known 
 to occur at the ordinary temperature, although glass when 
 heated in a flame frequently shows the phenomenon ; it 
 is, however, entirely distinct from the surface " corrosion " 
 just described. Water containing alkaline substances in 
 solution acts upon all glasses in a relatively rapid manner ; 
 it acts by first abstracting silica from the glass, the alkali 
 and lime being dissolved or mechanically removed at a 
 later stage. Water containing acid bodies in solution 
 i.e., dilute acid on the other hand acts upon most varieties 
 of glass decidedly less energetically than even pure water, 
 and much less vigorously than alkaline solutions ; this 
 peculiar behaviour probably depends upon the tendency of 
 acids to prevent the hydration of silica, this substance 
 being thereby enabled to act as a barrier to the solvent 
 action of the water upon the alkaline constituents of the 
 glass. The better varieties of glass are alo practically 
 impervious to the action of strong acids, although certain 
 of these, such as phosphoric and hydrofluoric, exert a 
 rapid action on all kinds of glass. Only certain special 
 glasses, containing an excessive proportion of basic con- 
 stituents and of such substances as boric or phosphoric 
 acid, are capable of being completely decomposed by the 
 
12 GLASS MANUFACTUEE. 
 
 action of strong acids, such as hydrochloric or nitric, the 
 bases entering into combination with the acids, while the 
 silicic and other acids are liberated. 
 
 In connection with the action of acids upon glass, mention 
 should be made of certain special actions that are of prac- 
 tical importance. The dissolving action of hydrofluoric 
 acid upon glass is, of course, well known. It is used in 
 practice both in the liquid and gaseous form, and also in 
 that of compounds from which it is readily liberated (such 
 as ammonium or sodium fluoride), for the purpose of 
 " etching " glass, and also in decomposing glass for pur- 
 poses of chemical analysis. Next in importance ranks the 
 action of carbonic acid gas upon glass, especially in the 
 presence of moisture. The action in question is probably 
 indirect in character ; the moisture of the air, condensing 
 upon the surface of the glass, first exerts its dissolving action, 
 and thus draws from the glass a certain quantity of alkali, 
 which almost certainly at first goes into solution as alkali 
 hydrate (potassium or sodium hydroxide) ; this alkaline 
 solution, however, rapidly absorbs carbonic acid from the 
 air, anci.the carbonate of the alkali is formed. If the glass 
 dries, this carbonate forms a coating of minute crystals on 
 the surface of the glass, giving it a dull, dimmed appear- 
 ance ; this, however, only occurs ordinarily with soda 
 glasses, since the carbonate of potassium is too hygroscopic 
 to remain in the dry solid state in any ordinary atmos- 
 phere. Potash glasses are, as such, no more stable chemi- 
 cally than soda glasses, but they are for the reason just 
 given less liable to exhibit a dim surface. If the dimming 
 process, in the case of a soda glass, has not gone too far, 
 the brightness of the surface of the glass may be practically 
 
PHYSICAL AND CHEMICAL PROPEKTIES OF GLASS. 13 
 
 restored by washing it with water, in which the minute 
 crystals of carbonate of soda readily dissolve, while separated 
 silica is removed mechanically. An attempt made to clean 
 the same dimmed surface by dry wiping would only result 
 in finally ruining the surface, since the small sharp crystals 
 of carbonate of soda would be rubbed about ovrr the 
 surface, scratching it in all directions. 
 
 The dimming process in the case of the less resistant 
 glasses is not only confined to the formation of alkaline 
 carbonates ; the films of alkaline solution which are formed 
 on the surface of glass form a ready breeding-ground for 
 certain forms of bacteria and fungi, whose growth occurs 
 partly at the expense of the glass itself ; the precise nature 
 of these actions has not been fully studied, but there can 
 be little doubt that silicate minerals and glass is to be 
 reckoned among these are subject to bacterial decom- 
 position, a well-known example in another direction being 
 the " maturing " of clays by storage in the dark, the 
 change in the clay being accompanied by an evolution of 
 ammonia gas. In the case of glass it has been shown that 
 specks of organic dust falling upon a surface give rise to 
 local decomposition. In this connection it is interesting 
 to note the effect of the presence of a small proportion 
 of boric acid in some glasses. The presence of this 
 ingredient in small proportions is known to render the 
 glass more resistant to atmospheric agencies, and more 
 especially to render it less sensitive to the effects of 
 organic dust particles lying upon the surface. It 
 has been suggested probably rightly that the boric 
 acid, entering into solution in the film of surface 
 moisture, exerts its well-known antiseptic properties, 
 
14 GLASS MANUFACTURE. 
 
 thus protecting the glass from bacterial and fungoid 
 activity. 
 
 The durability of glass under the action of atmospheric 
 agents is a matter of such importance that numerous 
 efforts have been made to establish a satisfactory test 
 whereby this property of a given glass may be ascertained 
 without actually awaiting the results of experience obtained 
 by actual use under unfavourable conditions. One of the 
 earliest of the tests proposed consisted in exposing surfaces 
 of the glass to the vapour of hydrochloric acid. For this 
 purpose some strong hydrochloric acid is placed in a glass 
 or porcelain basin, and strips of the glass to be tested are 
 placed across the top of the basin, the whole being covered 
 with a bell-jar. After several days the glass is examined, 
 and as a rule the le&& stable glasses show a dull, dimmed 
 surface as compared with the more stable ones. A more 
 satisfactory form of test depends upon the fact that 
 aqueous ether solutions react readily with the less stable 
 kinds of glass ; if a suitable dye, such as iod-eosin, be dis- 
 solved in the water-ether solution, then the effect upon the 
 less stable glasses when immersed in the solution is the 
 formation of a strongly adherent pink film. The density 
 or depth of colour of this film may be regarded as measur- 
 ing the stability of the glass ; the best kinds of glass 
 remain practically free from coloured film even on pro- 
 longed exposure. A test of a somewhat different kind is 
 one devised in its original form by Dr. Zschimmer, of the 
 Jena glass works ; this depends upon the fact that the dis- 
 integrating action of moist air can be very much accelerated 
 if both the moisture and the temperature of the air sur- 
 rounding the glass be considerably increased. For this 
 
PHYSICAL AND CHEMICAL PBOPEKT1ES OF GLASS. 15 
 
 purpose the samples of glass are exposed to a current of 
 air saturated with moisture at a temperature of about 
 80 C. in a specially arranged incubator for one or more 
 days, means being provided for securing a constant stream 
 of moist air during the whole time. On examining the 
 glass surfaces after this exposure any wiping or other 
 cleaning of the surfaces being avoided various qualities of 
 glass are found to show widely varying appearances. The 
 best and most stable glasses remain entirely unaffected ; 
 less stable kinds show small specks, which merge into a 
 generally dulled . surface in unstable kinds. There is 
 no doubt that this test gives a sharp classification of 
 glasses, but it yet remains to be proved that this classi- 
 fication agrees with their true relative durability in 
 practice ; the writer is inclined to doubt whether this 
 is really the case, since certain glasses that have proved 
 very satisfactory in this respect in practical use all over 
 the world were classed among the less stable kinds by 
 this test. 
 
 Before leaving the subject of the chemical behaviour of 
 glass, a reference should be made to the changes which glass 
 undergoes when acted upon by light and other radiations. 
 Under the influence of prolonged exposure to strong light, 
 particularly to sunlight, and still more so to ultra-violet 
 light, or the light of the sun at high altitudes, practically 
 all kinds of glass undergo changes which generally take the 
 form of changes of colour. Glasses containing manganese 
 especially are apt to assume a purple or brown tinge under 
 such circumstances, although the powerful action of radium 
 radiations is capable of producing similar discoloration in 
 glasses free from manganese. Apart from these latter 
 
16 GLASS MANUFACTUBE. 
 
 effects, of which very little is known as yet, there can be no 
 doubt that the action of light brings about chemical changes 
 within the glass, but it is by no means easy to ascertain 
 the true nature of these changes, although they most pro- 
 bably consist in a transfer of oxygen from one to another 
 of the oxides present in the glass. Although it has not 
 been definitely proved, it seems very unlikely that the glass 
 either loses or gains in any constituent during these 
 changes. Good examples of the changes undergone by 
 glass under the action of sunlight are frequently found in 
 skylights, where the oldest panes sometimes show a decided 
 purple tint which they did not possess when first put in 
 place. The glass spheres of the instruments used for 
 obtaining records of the duration of sunshine at meteoro- 
 logical stations also show signs of the changes due to light 
 the glass of these spheres when new has a light greenish 
 tint, but after prolonged use the colour changes to a 
 decided yellow. The coloured glass in stained-glass 
 windows also shows signs of having undergone changes of 
 tint in consequence of prolonged exposure to light ; glass 
 removed from ancient windows usually shows a deeper tint 
 in those portions which have been protected from the 
 direct action of light by the leading in which the glass was 
 set, and it is at least an open question whether the beauty 
 of ancient glass may not be, in part, due to the mellowing 
 effect of light upon some of the tints of the design. This 
 photo-sensitiveness of glass is also of some importance in 
 connection with the manufacture of photographic plates. 
 It has been found that if the glass plate of a strongly- 
 developed negative be cleaned, a decided trace of the former 
 image is retained by the glass, and this image is apt to re- 
 
PHYSICAL AND CHEMICAL PEOPERTIES OF GLASS. 17 
 
 appear as a " ghost " if the same glass be again coated with 
 sensitive emulsion and again exposed and developed. The 
 best makers of plates recognise this fact and do not re-coat 
 glass that has once been used for the production of a 
 negative. 
 
 G.M. 
 
CHAPTEE II. 
 
 THE PHYSICAL PROPERTIES OF GLASS. 
 
 The Mechanical Properties of Glass are of considerable 
 importance in many directions. Although glass is rarely 
 used in such a manner that it is directly called upon to 
 sustain serious mechanical stresses, the ordinary uses of 
 glass in the glazing of large windows and skylights depend 
 upon the strength of the material to a very considerable 
 extent. Thus in the handling of plate-glass in the largest 
 sheets, the mechanical strength of the plates must be relied 
 upon to a considerable extent, and it is this factor which 
 really limits the size of plate that can be safely handled 
 and installed. The same limitation applies to sheet-glass 
 also, for, although its lighter weight renders it less liable 
 to break under its own weight, its thinner section renders 
 it much more liable to accidental fracture. In special 
 cases, also, the mechanical strength of glass must be relied 
 upon to a considerable - extent. Gauge tubes of high- 
 pressure boilers, port-hole glasses in ships, the glass prisms 
 inserted in pavement lights, and the glass bricks which 
 have found some use in France, as well as champagne 
 bottles and mineral water bottles and syphons, are all 
 examples of uses in which glass is exposed to direct 
 stresses. It is, therefore, a little surprising that while the 
 
THE PHYSICAL PKOPEKTIES OF GLASS. 19 
 
 mechanical properties of metals, timbers, and all manner 
 of other materials have been studied in the fullest possible 
 manner, those of glass have received very little attention, 
 at all events so far as published data go. One reason for 
 this state of affairs is probably to be found in the fact that 
 it is by no means easy to determine the strength of so 
 brittle and hard a body as glass. As a consequence even 
 the scanty data available can only be regarded as first 
 approximations. The following data are only intended to 
 give an idea of the general order of strength to be looked 
 for in glass : 
 
 Tensile strength : 
 
 From 1 to 4 tons per sq. in. (Trautwine). 
 
 J to 1J (Henrivaux). 
 
 ., 2 to 5j ,, (WinkelmannandSchott). 
 
 5 to 6 (Kowalski). 
 
 Crushing strength : 
 
 From 9 to 16 tons per sq. in. (Trautwine). 
 
 3 to 8 ,, (WinkelmannandSchott). 
 20 to 27 (Kowalski). 
 
 Of the above figures the experiments of Winkelmann 
 and Schott are probably by far the most reliable, but these 
 refer to a series of special Jena glasses, selected with a 
 view to determining the influence of chemical composition 
 on mechanical properties, and, unfortunately, this series 
 does not include glasses at all closely resembling those 
 ordinarily used for practical purposes. The attempt to 
 connect tensile and crushing strength with chemical 
 composition was also only very partially successful; but 
 
 c 2 
 
20 GLASS MANUFACTUEE. 
 
 the results serve to show that the chemical composition 
 has a profound influence on the mechanical strength of 
 glass, so that hy systematic research it would probably be 
 possible to produce glasses of considerably greater mechanical 
 strength than those at present known. It must be noted 
 in this connection that the mechanical properties of glass 
 depend to a very considerable extent upon the rate of 
 cooling which the specimen in question has undergone. It 
 is well known that by rapid cooling, or quenching, the 
 hardness of glass can be considerably increased ; such 
 treatment also increases the strength both as against 
 tension and compression, and numerous processes have 
 been put forward for the purpose of utilising these effects 
 in practice. Unfortunately the " hardened " glass thus 
 obtained is extremely sensitive to minute scratches, and 
 flies to pieces as soon as the surface is broken, and 
 the great internal stress which always exists in such glass 
 is thereby relieved. All these peculiarities are, of course, 
 dependent as to their degree upon the rapidity with which 
 the glass has been cooled, and the aim of inventors in this 
 field has been to devise a rapid cooling process which 
 should strike the happy mean between the increased 
 strength and the induced brittleness resulting from 
 quenching. Thus processes for "tempering" glass by 
 cooling it in a blast of steam or in a bath of hot oil or 
 grease have been brought forward ; but, although some 
 such glass is manufactured, no very extensive practical 
 application has resulted. 
 
 Elasticity and Ductility of Glass. In a series of glasses 
 investigated by Winkelmann and Schott, the modulus of 
 elasticity (Young's Modulus) varied from 3,500 to 5,100 
 
THE PHYSICAL PROPERTIES OF GLASS. 21 
 
 tons per sq. in., the value being largely dependent upon 
 the chemical composition of the glass. Measurable ductility 
 has not been observed in glass under ordinary conditions 
 except in the case of champagne bottles under test by 
 internal hydraulic pressure ; in these tests it was found 
 that a permanent increase of volume of a few tenths of a 
 cubic centimetre could be obtained by the application of 
 an internal pressure just short of that required to burst 
 the bottle pressure of the order of 18 to 30 atmospheres 
 being involved. This small permanent set has been 
 ascribed to incipient fissuring of the glass, and this 
 explanation is probably correct. On the other hand, it is 
 in the writer's opinion very probable that glass is capable 
 of decided flow under the prolonged action of relatively 
 small forces ; the behaviour of large discs of worked optical 
 glass suggests some such action, but the view as yet lacks 
 full experimental confirmation. 
 
 The Hardness of glass is a property of some importance 
 in most of the applications of glass. The durability of 
 glass objects which are exposed to handling or to periodical 
 cleaning must largely depend upon the power of the glass 
 to resist scratching ; this applies to such objects as plate- 
 glass windows and mirrors, spectacle and other lenses, and 
 in a minor degree to table-ware. On the other hand, the 
 exact definition and means of measuring hardness are not 
 yet satisfactorily settled. Experimenters have found it 
 very difficult to measure the direct resistance to scratching, 
 since it is found, for example, that two glasses of very 
 different hardness are yet capable of decidedly scratching 
 each other under suitable conditions. Resort has therefore 
 been had to other methods of measuring hardness ; the 
 
22 GLASS MANUFACTUBE. 
 
 method which, from the experimental point of view, is, 
 perhaps, the most satisfactory, depends upon principles 
 laid down by Hertz and elaborated experimentally by 
 Auerbach. This depends upon measuring the size of the 
 circular area of contact produced when a spherical lens is 
 pressed against a flat plate of the same glass with a known 
 pressure. Auerbach himself found some difficulty in 
 deciding the exact connection between the "indentation 
 modulus " thus determined and the actual hardness of the 
 glass. This method is, therefore, of theoretical interest 
 rather than of use in testing glasses for hardness. A test 
 of a more practical kind consists in exposing specimens of 
 the glasses to be tested to abrasion against a revolving disc 
 of cast-iron fed with emery or other abrasive, and to 
 measure the loss of weight which results from a given 
 amount of abrading action under a known contact pressure. 
 If a number of specimens of different glasses are exposed 
 to this test at one time, a very good comparison of their 
 power of resisting abrasion can be obtained. It is not 
 quite certain that this test measures the actual "hardness" 
 of the glass, but it affords some information as to its power 
 of resisting abrasion, and for many purposes this power is 
 the important factor. 
 
 Hardness being, as indicated above, a somewhat indefinite 
 term, it is not possible to give any precise statement as to 
 the influence of chemical composition upon the hardness 
 of glass. In general terms it may be said that glasses rich 
 in silica and lime will be found to be hard, while glasses 
 rich in alkali, lead or barium, are likely to be soft. It 
 must, however, be borne in mind that rapid cooling, or 
 even the lack of careful annealing, will produce a very 
 
THE PHYSICAL PROPERTIES OF GLASS. 23 
 
 great increase of hardness in even the softest glasses. The 
 actual behaviour of a given specimen of glass will, there- 
 fore, depend at least as much upon the nature of the 
 processes which it has undergone as upon its chemical 
 composition. 
 
 The Thermal Properties of Glass, although not of such 
 general importance as the mechanical properties, are yet 
 of considerable interest in a large number of the practical 
 uses to which glass is constantly applied. Perhaps the 
 most important of these properties is that known as 
 thermal endurance, which measures the amount of sudden 
 heating or cooling to which glass may be exposed without 
 risk of fracture; the chimneys employed in connection 
 with incandescent gas burners, boiler gauge glasses, 
 laboratory vessels, and even table and domestic utensils 
 are all exposed at times to sudden changes of temperature, 
 and in many cases the value of the glass in question 
 depends principally upon its power of undergoing such 
 treatment without breakage. The property of " thermal 
 endurance " itself depends upon a considerable number of 
 more or less independent factors, and their influence will 
 be readily understood if we follow the manner in which 
 sudden change of temperature produces stress and, some- 
 times, fracture in glass objects. If we suppose a hot liquid 
 to be poured into a cold vessel, the first effect upon the 
 material of the vessel will be to raise the temperature of 
 the inner surface. Under the influence of this rise of 
 temperature the material of this inner layer expands, or 
 endeavours to expand, being restrained by the resistance 
 of the central and outer layers of material which are 
 still cold ; the result of this contest is, that while the inner 
 
24 GLASS MANUFACTURE. 
 
 layer is thrown into a state of compression, the outer and 
 central layers are thrown into a state of tension. Accord- 
 ingly, if the tension so produced is sufficiently great, the 
 outer layers fracture under tension and the whole vessel 
 is shattered by the propagation of the crack thus initiated. 
 From this description of the process it will be seen that a 
 high coefficient of expansion and alow modulus of elasticity 
 will both favour fracture, while high tensile strength will 
 tend to prevent it. The thermal conductivity of the glass 
 will also affect the result, because the intensity of the 
 tensile stress set up in the colder layers of glass will 
 depend upon the temperature gradient which exists in the 
 glass ; thus if glass were a good conductor of heat it would 
 never be possible to set up a sufficient difference of 
 temperature between adjacent layers to produce fracture ; 
 for the same reason, vessels of very thin glass are less apt 
 to break under temperature changes than those having 
 thick walls, since the greatest difference of temperature that 
 can be set up between the inner and outer layers of a thin- 
 walled vessel can never be very considerable. It also 
 follows from these considerations, that if a cold glass vessel 
 be simultaneously heated or cooled from both sides, it can be 
 safely exposed to a much more sudden change of temperature 
 than it could withstand if heated from one side alone ; on 
 the other hand, when very thick masses of glass have to 
 be heated, this must be done very gradually, as a con- 
 siderable time will necessarily elapse before an increment 
 of temperature applied to the outside will penetrate to the 
 centre of the mass. It should also be noted here, that in 
 addition to the thermal conductivity of the glass, its heat 
 capacity or specific heat also enters into this question, since 
 
THE PHYSICAL PROPERTIES OF GLASS. 25 
 
 heat will obviously penetrate more slowly through a glass 
 whose own rise of temperature absorbs a greater quantity 
 of heat. It will thus be seen that " thermal endurance " 
 is a somewhat complicated property, depending upon the 
 factors named above, viz. : coefficient of expansion, thermal 
 conductivity, specific heat, Young's modulus of elasticity, 
 and tensile strength. 
 
 The coefficient of thermal expansion varies considerably 
 in different glasses, and we can here only state the limiting 
 values between which these coefficients usually lie ; these 
 are 37 x 10 ~ 7 as the loweri and 122 x 10 ~ 7 as the upper 
 limit. These figures express the cubical expansion of the 
 glass per degree Centigrade, the corresponding figures for 
 steel and brass respectively being about 360 x 10 ~ 7 and 
 648 x 10 ~ 7 respectively. It should be noted that vitreous 
 bodies of extremely low expansibility are obtainable by the 
 suitable choice of ingredients, but in some cases these 
 " glasses " are white opaque bodies, and in all cases they 
 present great difficulty in manufacture, owing to the fact 
 that alkalies and lime must be avoided in their composition. 
 
 Quite apart from the question of thermal endurance, the 
 expansive properties of glass are of some importance. 
 Thus when several kinds of glass have to be united, as, 
 for example, in the process of producing "flashed" coloured 
 glass, it is essential that their coefficients of expansion 
 should be as nearly as possible the same ; otherwise con- 
 siderable stresses will be set up when the glasses, which 
 have been joined at a red heat, are allowed to cool. On 
 the other hand, this mutual stressing of two glasses owing 
 to differences in their thermal expansion has been utilised 
 for the production of tubes and other glass objects possess- 
 
26 GLASS MANUFACTUEE. 
 
 ing special strength. If a tube be drawn out of glass 
 consisting of two layers, one considerably more expansible 
 than the other, and the cooling process be rightly conducted, 
 it is possible to produce a tube in which both the inner and 
 outer layers of glass are under a considerable compressive 
 stress. Not only is glass, as we have seen above, enormously 
 stronger as against compression than it is against tension, 
 but glass under compressive stress behaves as though it 
 were a much tougher material, being less liable to injury 
 by scratches or blows. Moreover, if a tube in this condition 
 be heated and then exposed to sudden cooling, the first 
 effect of the application of cold will be a contraction of the 
 surface layers, resulting in a relief of the initial condition 
 of compression. These tubes are, therefore, remarkably 
 indifferent to sudden cooling, although they are naturally 
 more sensitive to sudden heating. In this respect they 
 differ entirely from ordinary glass, which is considerably 
 more sensitive to sudden cooling than to sudden heating, 
 particularly when the heat or cold is applied to all the 
 surfaces of the object at the same time. The special tubes 
 made of two layers of glass above referred to are manu- 
 factured by the Jena Glass Works for special purposes, 
 among which boiler gauge glasses are the most important. 
 It should be also mentioned here that the remarkable 
 thermal endurance of vitrified silica, which can be raised 
 to a red heat and then immersed in cold water without risk 
 of breakage, is chiefly due to its very low coefficient of 
 expansion. 
 
 In another direction the expansive properties of glass are 
 of importance wherever glass is rigidly attached to metal. 
 At the present time this is done in several industrial 
 
THE PHYSICAL PEOPEETIES OF GLASS. 27 
 
 products, such as incandescent electric lamps and " wired " 
 plate glass. In certain varieties of incandescent lamps, 
 metallic wires are sealed into the glass bulbs, and the only 
 metal available for this purpose, at all events until recently, 
 has been platinum, whose coefficient of expansion is low 
 as compared with most metals, and whose freedom from 
 oxidation when heated to the necessary temperature makes 
 it easy to produce a clean joint between glass and metal. 
 More recently the use of certain varieties of nickel steel has 
 been patented for this purpose, since it is possible to obtain 
 nickel steel alloys of almost any desired coefficient of 
 expansion from that of the alloy known as " invar," having 
 a negligibly small expansion compared with that of ordinary 
 steel. By choosing a suitable member of this series a metal 
 could be obtained whose coefficient of expansion corresponds 
 exactly with that of the glass to which it is to be united. 
 The oxidation of the nickel steel when heated to the 
 temperature necessary for effecting its union with the glass 
 presented serious difficulties to the production of a tight 
 joint, and several devices for avoiding this oxidation have 
 been patented. In the incandescent electric lamp, although 
 the joint between glass and metal is required to be perfectly 
 air-tight, the two bodies are only attached to one another 
 over a very short length. In wired plate glass, however, 
 an entire layer of wire netting is interposed between two 
 layers of glass, the wire being inserted during the process 
 of rolling. Here a certain amount of oxidation of the wire 
 is not of any serious importance, as it only appears to give 
 rise to a few bubbles, whose presence does not interfere 
 with the strength and usefulness of the glass ; but any 
 considerable difference of coefficient of expansion will 
 
28 GLASS MANUFACTUEE. 
 
 produce the most serious results on account of the great 
 lengths of glass and metal that are attached to each other. 
 This factor has been neglected hy some manufacturers, with 
 the result that much of the wired glass of commerce is 
 liable to crack spontaneously some time after it has left the 
 manufacturer's hands, while there is also much loss by 
 breakage during the process of manufacture. 
 
 Thermal expansion is a vital factor in yet another of the 
 uses of glass. Our ordinary instrument for measuring 
 temperature the mercury thermometer is very consider- 
 ably affected by the expansive behaviour of glass. When 
 a mercury thermometer is warmed the mercury column 
 rises in the stem because the mercury expands upon 
 warming to a greater extent than the glass vessel, bulb 
 and stem, in which it is contained. The subject of the 
 graduations and corrections of the mercury glass ther- 
 mometer is a very large one and somewhat outside the 
 scope of the present volume ; but attention should be 
 drawn in this place to the peculiarities of the behaviour of 
 glass that have been discovered in this connection. One 
 of these is that when first blown the bulb of a thermometer 
 takes a very considerable time to acquire its final volume, 
 the result being, that if a freshly made thermometer is 
 graduated, after some time the zero of the instrument will 
 be found considerably changed, generally in a direction 
 which indicates that the volume of the bulb has slightly 
 increased. By a special annealing or " ageing " process 
 this change can be completed in a comparatively short time 
 before the instrument is graduated. There is, however, a 
 further peculiarity \vhich is prominent in some thermometers, 
 although very greatly reduced in the best modern glasses. 
 
THE PHYSICAL PKOPERTIES OF GLASS. 29 
 
 This becomes apparent in a decided change of zero when- 
 ever the thermometer has been exposed for any length of 
 time to a high temperature, the zero gradually returning 
 more or less to its original position in the course of time. 
 With thermometers made of glasses liable to these aberra- 
 tions, the reading for a given temperature depended largely 
 upon the immediate past history of the instrument ; but, 
 thanks to the Jena Works, thermometer glasses are now 
 available which are almost entirely free from this defect. 
 In this connection the curious fact has been observed that 
 glass containing both the alkalies (potash and soda) shows 
 these thermal effects much more markedly than a glass 
 containing one of the alkalies only. 
 
 The thermal conductivity of glass, except in so far as it 
 affects the thermal endurance, is not a matter of any great 
 direct practical importance, although the fact that glass is 
 alwajs a comparatively poor conductor of heat is utilised 
 in many of its applications, as, for example, the construction 
 of conservatories and hot-houses, although even in that 
 case the opacity of glass to thermal radiations of long wave 
 lengths is of more importance than its low thermal con- 
 ductivity. Similar statements apply, in a still more marked 
 degree, to the subject of the specific heat of glass. 
 
 The electrical properties of glass are of much greater 
 practical importance, glass being frequently used in 
 electrical appliances as an insulating medium. The 
 insulating properties of glass, as well as the property 
 known as the specific inductive capacity, vary greatly 
 according to the chemical composition of the material. 
 Generally speaking, the harder glasses, i.e., those richest 
 in silica and lime, are the best insulators, while soft glasses, 
 
30 GLASS MANUFACTURE. 
 
 rich in lead or alkali, are much poorer in this respect. In 
 practice, particularly when the glass insulator is exposed 
 to even a moderately damp atmosphere, the nature of the 
 glass affects the resulting insulation or absence of insula- 
 tion, in another way. Almost all varieties of glass have the 
 property of condensing upon their surfaces a decided film 
 or layer of moisture from the atmosphere, and, as we have 
 seen above, glasses differ very considerably in the degree 
 to which they display this hygroscopic tendency. The 
 softer glasses are much more hygroscopic than the hard 
 ones, and the resulting film of surface moisture serves to 
 lessen or even to break down the insulating power of the 
 glass, the electricity leaking away along the film of moisture. 
 In the case of appliances for static electricity, where very 
 high voltages have to be dealt with, an endeavour is some- 
 times made to avoid this leakage by varnishing the surface 
 of the glass with shellac or other similar substance, and 
 this proves a satisfactory remedy up to a certain point. 
 Quite recently a variety of glass has been brought forward 
 which is peculiar in having a comparatively low electrical 
 resistance, so that for certain purposes it can be used as 
 an electric conductor. Although interesting in itself, this 
 glass is not very likely to prove useful even for the limited 
 number of applications that could be found for an electri- 
 cally conducting glass, since it is very rich in alkali, and 
 is, therefore, likely to be unstable chemically, even under 
 the action of atmospheric agencies alone. 
 
 The most valuable and in many ways the most in- 
 teresting of the properties of glass its transparency has 
 not been dealt with as yet, and all mention of this subject 
 has been postponed to the end of the present chapter, 
 
THE PHYSICAL PEOPEETIES OF GLASS. 31 
 
 because the whole subject of the optical properties of glass 
 will be dealt with more fully in the chapter on optical glass 
 (Chap. XII.), so that a very brief reference only need be 
 made to the matter here. 
 
 There can be no doubt that, in most of its practical appli- 
 cations, transparency is the fundamental and essential 
 property which leads to the employment of glass in the 
 place of either stronger or cheaper materials. By trans- 
 parency, in this sense, we wish to include mere translucence 
 also, since very frequently it is as necessary to avoid un- 
 disturbed visibility as it is to secure the admission of light. 
 It is indeed hard to find any use to which glass is extensively 
 put into which the function of transmitting light does not 
 very largely enter. Almost the only such example of use is 
 the modern application of opal glass to the covering of walls, 
 and the use not as yet widely extended of pressed glass 
 blocks as bricks and paving stones ; in these cases it is 
 the hardness and smoothness of surface that gives to the 
 vitreous body its superiority over other materials, but apart 
 from these special cases, the fact remains that well over 
 95 per cent, of the glass used in the world is employed for 
 purposes where transmission of light is essential to the 
 attainment of the desired result, either from the point of 
 view of utility or from that of beauty. It is interesting to 
 note that the power of transmitting light is not shared by 
 many solid bodies. Some colloidal organic bodies, such as 
 gelatine and celluloid, possess the property to a degree com- 
 parable with glass, while certain mineral crystals, such as 
 quartz and fluor-spar, may even surpass the finest glass in 
 this respect; while some of the other optical properties of 
 glass are greatly exceeded by such natural substances as 
 
32 GLASS MANUFACTUEE. 
 
 the diamond and the ruby. But the very brevity of this 
 list is in itself striking, because it must be borne in mind 
 that transparency by no means constitutes the only 
 common characteristic of vitreous bodies. 
 
 Although the transparency of glass is so valuable and 
 indeed so essential a property of that substance, it must be 
 remembered that no kind of glass is perfectly transparent. 
 Quite apart from the fact that of the light that falls upon a 
 glass surface, however perfectly polished, a considerable 
 proportion is turned back by reflection at the surface of 
 entry and again by reflection at the surface of exit from 
 the glass, a certain proportion of light is absorbed during 
 its passage through the glass itself, and the transmitted 
 beam is correspondingly weakened. In the purest and 
 best glasses this absorption is so small that in any moderate 
 thickness very delicate instruments are required to show 
 that there has been any loss of light at all ; but even the 
 best glass, when examined through a thickness of 20 in. 
 or more, always shows the effects of the absorption of light 
 quite unmistakably. In fact, not only does all glass 
 absorb light, but it does this to a different degree accord- 
 ing to the colour of the light, so that in passing through 
 the glass a beam of white light becomes weakened in one 
 of its constituent colours more than in the others, with the 
 result that the emergent light is slightly coloured. Thus 
 the purest and whitest of glasses, when examined in very 
 thick pieces, always show a decided blue or green tint, 
 although this tint is quite invisible on looking through a 
 few inches of the glass. The ordinary glass of commerce, 
 however, is far removed from even this approach to perfect 
 transparency. The best plate glass shows a slight greenish- 
 
THE PHYSICAL PEOPERTIES OF GLASS. 33 
 
 blue tint, which is just perceptible to the trained eye 
 when a single sheet of moderate thickness is laid down 
 upon a piece of white paper. When a sheet of this glass 
 is viewed edgewise, in such a way that the light reaching 
 the eye has traversed a considerable thickness, the greenish- 
 blue tint of the glass becomes more apparent. By 
 holding strips of various kinds of glass, cut to an equal 
 length, close together and comparing the colour exhibited 
 by their ends, a means of comparing the colours of 
 apparently " white " glasses is readily obtained. It will be 
 found that different specimens of glass differ most markedly 
 in this respect. Sheet glass is, as a rule, decidedly deeper 
 in colour than polished plate, but rolled plate is as a rule 
 much greener the. colour of this glass can, in fact, in most 
 cases be seen quite plainly in looking through or at the 
 sheets in the ordinary way. 
 
 The question of how far the colour of glass affects the 
 value of the light which it transmits depends for its answer 
 upon the purpose to which the lighted space is to be put. 
 Where delicate comparisons of colour are to be made, or 
 other delicate work involving the use of the colour sense is 
 to be carried on, it is essential that all colouration of the 
 entering daylight should be avoided, and the use of the 
 most colourless glass obtainable will be desirable. Again, 
 in photographic studios it is important to secure a glass 
 which shall absorb as small a proportion of the chemically 
 active rays contained in daylight as possible, and special 
 glasses for this purpose are available. Although for the 
 present the price of these special glasses may prove pro- 
 hibitive for the glazing of studio lights, their use is found 
 highly advantageous where artificial light is to be used to 
 
 G.M. D 
 
34 GLASS MANUFACTUEE. 
 
 the best advantage. On the other hand, for every-day 
 purposes, the slight tinge of colour introduced into the 
 light by the colour of ordinary sheet and plate glass, or 
 even of greenish rolled plate glass, has no deleterious effect 
 whatever, the majority of persons being entirely unconscious 
 of its presence. The transmission of light by glass, its 
 absorption, refraction, dispersion, etc., are, however, best 
 grouped together as the " optical " properties of glass, and 
 under that heading they will receive a fuller treatment in 
 connection with the subject of the manufacture of glass for 
 optical purposes. 
 
CHAPTEE III. 
 
 THE RAW MATERIALS OF GLASS MANUFACTURE. 
 
 THE choice of raw materials for all branches of glass 
 manufacture is a matter of vital importance. As a rule all 
 ''fixed" bodies that are once introduced into the glass- 
 melting pot or furnace appear in the finished glass, while 
 volatile or combustible bodies are more or less completely 
 eliminated during the process of fusion. Thus while the 
 chemical manufacturer can purify his products by filtration, 
 crystallisation or some other process of separation, the 
 glass-maker must eliminate all undesirable ingredients 
 before they are permitted to enter the furnace, and the 
 stringency of this condition is increased by the fact that 
 the transparency of glass makes the detection of defects of 
 colour or quality exceedingly easy. For the production of 
 the best varieties of glass, therefore, an exacting standard 
 of purity is applied to the substances used as raw materials. 
 As the quality of the product decreases, so also do the 
 demands upon the purity of raw materials, until finally 
 for the manufacture of common green bottles, even such 
 very heterogeneous substances as basaltic rock and the 
 miscellaneous residues of broken, defective and half-melted 
 glass forming the refuse of other glassworks may be 
 utilised more or less satisfactorily. 
 
 D 2 
 
36 GLASS MANUFACTURE. 
 
 For the best kinds of glass the most desirable quality in 
 raw materials is thus as near an approach to purity as 
 possible under commercial conditions, and next to that, as 
 great a constancy of composition as possible. For instance, 
 the quantity of moisture contained in a ton of sand 
 appreciably affects the resulting composition of the glass, 
 and if the sand cannot be obtained perfectly dry, it should at 
 least contain a constant proportion of moisture, otherwise it 
 becomes necessary to determine, by chemical tests, the per- 
 centage of moisture in the sand that is used from day to day, 
 and to adjust the quantityused in accordance with the results 
 of these tests, a proceeding which, of course, materially com- 
 plicates the whole process. In other cases, variable com- 
 position is not so readily allowed for, and uncontrollable 
 variations in the composition of the glass result at times 
 the quality falls off unaccountably, or the glass refuses to 
 melt freely at the usual temperature. The systematic 
 employment of chemical analysis in the supervision of both 
 the raw materials and of various products will frequently 
 enable the manufacturer to trace the causes of such un- 
 desirable occurrences; but however necessary such control 
 undoubtedly is, it cannot entirely compensate for the use of 
 raw materials liable to too great a variation in composition 
 or physical character. For not only the chemical com- 
 position, but also the physical condition and properties of 
 the material are of importance in glass manufacture. Thus 
 it is essential that materials to be used for glass-melting 
 should be obtainable in a reasonably fine state of division, 
 and in this connection it must be remembered that both 
 exceedingly hard bodies and soft plastic substances can 
 only be ground with very great difficulty. Further, where 
 
THE RAW MATERIALS OF GLASS MANUFACTURE. 37 
 
 a substance occurs naturally as a powder, this powder 
 should be of uniform and not too fine a grain, more 
 especially if it belongs to the class of refractory rather 
 than of fluxing ingredients. In that case the presence of 
 coarser grains will result in their presence in the undis- 
 solved state in the finished glass, unless excessive heat and 
 duration of " founding " be employed to permit of their 
 dissolution. This applies chiefly to siliceous and calcareous 
 ingredients, but hardened nodules of salt-cake may behave 
 in a similar manner. 
 
 A further consideration in the choice of raw materials is 
 facility of storage. Thus limestone in the shape of large 
 lumps of stone which are only ground to powder as 
 required, is readily stored, and undergoes no deleterious 
 change even if exposed to the weather ; on the other hand, 
 sulphate of soda (salt-cake), if stored even in moderately 
 dry places, rapidly agglomerates into hard masses, at the 
 same time absorbing a certain percentage of moisture. 
 Such properties are not always to be avoided, salt-cake for 
 example being at the present time an indispensable in- 
 gredient in many kinds of glass-making, but the value of a 
 substance is in some cases materially lessened by such 
 causes. 
 
 The raw materials ordinarily employed in glass-making 
 may be grouped into the following classes : 
 
 (1) Sources of silica. 
 
 (2) Sources of alkalies. 
 
 (3) Sources of bases other than alkalies. 
 
 (1) Sources of Silica. The principal source of silica is 
 sand. This substance occurs in nature in geological 
 
38 GLASS MANUFACTURE. 
 
 deposits, often of very considerable area and depth. 
 These deposits of sand have always been formed by the 
 disintegration of a siliceous rock, and the fragments so 
 formed have been sifted and transported by the agency of 
 water, being finally deposited by a river either in the sea 
 (marine deposits) or in lakes (lacustrine deposits), while the 
 action of the water, either during transport or after deposi- 
 tion, has frequently worn the individual particles into the 
 shape of rounded grains. 
 
 In consequence of this origin, the chemical composition 
 of sand varies very greatly with the nature of the rock 
 whose denudation gave rise to the deposit. Where rocks 
 very rich in silica, or even consisting of nearly pure silica, 
 have been thus denuded, the resulting sand is often very 
 pure, deposits containing up to 99'9 per cent, silica being 
 known. More frequently, however, the sand contains 
 fragments of more or less decomposed felspar, which 
 introduce alumina, iron and alkalies into its composition. 
 Finally, " sands" of all ranges of composition from the 
 pure varieties just referred to down to the clay marls, very 
 rich in iron and alumina, are known. 
 
 For the best varieties of glass, viz., optical glass, flint 
 glass and the whitest sheet-glass, as well as for the best 
 Bohemian glass, a very pure variety of sand is required, 
 preferably containing less than 0*05 per cent, of iron, 
 and not more than 0'05 per cent, of other impurities such 
 as alumina, lime or alkali. As a matter of fact, sands 
 containing so little iron rarely contain any other impurity 
 except alumina in measurable quantities. The best-known 
 deposit of such sand in Europe is that at Fontainebleau 
 near Paris, but equally good sand is found at Lippe in 
 
THE KAW MATEEIALS OF GLASS MANUFACTURE. 39 
 
 Germany, whence sand is delivered commercially with 
 a guaranteed silica content of 99'98 per cent. Sand of 
 excellent quality, although not quite so good as the above, 
 is obtained at Hohenbocka in Germany (Saxony) and at a 
 few other places in Europe. In England no deposit of sand 
 of such purity is at present being exploited. 
 
 Next in order of value to these exceedingly pure sands, 
 come the glass-making sands of Belgium, notably of Epinal. 
 These usually contain from 0'2 to 0*3 per cent, of iron and 
 rather more alumina, but they are used very largely for the 
 manufacture of sheet and plate-glass. When the standard 
 of quality is further relaxed, a large number of sand 
 deposits become available, and the manufacturers of each 
 district avail themselves of more or less local supplies ; thus 
 in England the sands of Leighton in Bedfordshire and of 
 Lynn on the East Coast, are largely used. Finally, for the 
 manufacture of the cheapest class of bottles, sands contain- 
 ing up to 2 per cent, of iron and a considerable proportion 
 of other substances are employed. 
 
 Silica, in various states of purity, occurs in nature in a 
 number of other forms than that of sand. By far the 
 commonest of these is that of more or less compact sedi- 
 mentary rock, known as " sandstone." As far as chemical 
 composition is concerned, some of these stones are admirably 
 suited for making the best kinds of glass, although as a rule 
 a stone is not so homogeneous as the material of a good 
 sand-bed. The stone has the further disadvantage that it 
 requires to be crushed to powder before it can be used for 
 glass-making, and the crushed product is generally a 
 mixture of grains of all sizes ranging from a fine dust to the 
 largest size of grain passed by the sieves attached to the 
 
40 GLASS MANUFACTURE. 
 
 crushing machine. The presence of the very fine particles 
 is a distinct objection from the glass-maker's point of view, 
 so that it would probably be necessary to wash the sand so 
 as to remove this dust a process that in itself adds to the 
 cost of the crushed stone and at the same time leads to the 
 loss of a serious percentage of the material. Objections of 
 the same kind apply, but with still greater force, to the use 
 of powdered quartz or flint as sources of silica for the 
 glass-maker ; further, these materials are exceedingly hard 
 and therefore difficult to crush, so that the price of the 
 materials is prohibitive for glass-making purposes. The 
 use of ground quartz and flint is therefore confined to the 
 ceramic industries in which these substances serve as sources 
 of silica for both bodies and glazes ; in former times, how- 
 ever, ground flint was extensively used in the manufacture 
 of the best kinds of glass, as the still surviving name of 
 " flint glass " testifies. 
 
 Minerals of the felspar class, consisting essentially of 
 silicates of alumina and one or more of the alkalies, are 
 extensively used in glass-making and should be mentioned 
 here, since their high silica-content (up to 70 per cent.) 
 constitutes an effective source of silica. As a source of this 
 substance, however, most felspars would be far too expensive, 
 and their use is due to their content of alumina and alkali. 
 
 (2) Sources of Alkali. Originally the alkaline constituents 
 of glass were derived from the ashes of plants and of seaweed 
 or " kelp " ; in both cases the alkali was obtained in the 
 form of carbonate and was ordinarily used in a very impure 
 form : at the present time, however, the original source of 
 alkali for industrial purposes is found in the natural deposits 
 and other sources of the chlorides of sodium and potassium. 
 
THE RAW MATEEIALS OF GLASS MANUFACTUKE. 41 
 
 At the present time it is not yet industrially possible to 
 introduce the alkalies into glass mixtures in the natural 
 form of chlorides. The principal difficulty in doing this arises 
 from the fact that the chlorides are volatile at the tempera- 
 ture of glass-melting furnaces and are only acted upon by 
 hot silica in the presence of water vapour. Introduced into 
 an ordinary glass furnace, therefore, these salts would be 
 driven off as vapour before they could combine with the 
 other ingredients in the desired form of double silicates. 
 
 Alkalies are, therefore, introduced into the glass mixture 
 in less volatile and more readily attackable forms. Of 
 these the carbonate is historically the earlier, while the 
 sulphate is at the present time industrially by far the more 
 important. The Carbonate of Soda, or soda ash, which is 
 used in the production of some special glasses, and is an 
 ingredient of English flint glasses, is produced by either of 
 two well-known chemical processes. One of these is the 
 " black ash," or " Le Blanc " process, in which the chloride 
 is first converted into sulphate by the direct action of sul- 
 phuric acid, and the sulphate thus formed is converted into 
 the carbonate by calcination with a mixture of calcium 
 carbonate and coal. The sodium carbonate thus formed is 
 separated by solution and subsequent evaporation. A purer 
 form of sodium carbonate can be obtained with great 
 regularity by the " ammonia soda " process, in which a 
 solution of sodium chloride is acted upon by ammonia and 
 carbonic acid under pressure. Soda ash produced by this 
 process is now supplied regularly for glass-making purposes 
 in a state of great purity and constancy of composition. 
 It is upon these qualities that the great advantages of this 
 substance depend, since its relatively high cost precludes 
 
42 GLASS MANUFACTUEE. 
 
 its use except for special kinds of glass, and for these 
 purposes the qualities named are of great value. 
 
 For most purposes of glass-making, such as the produc- 
 tion of sheet and plate-glass of all kinds, the alkali is 
 introduced in the form of salt-cake i.e., sulphate of soda. 
 This product is obtained as the result of the first step of 
 the Le Blanc process of alkali manufacture i.e., hy the 
 action of sulphuric acid on sodium chloride ; salt-cake is 
 thus a relatively crude product, and its use is due to the 
 fact that it is by far the cheapest source of alkali available 
 for glass-making. There are, however, certain disadvantages 
 connected with its use, The chief of these is the fact that silica 
 cannot decompose salt-cake without the aid of a reducing 
 agent ; such a reducing agent is partly supplied by the 
 flame-gases in the atmosphere of the furnace, but in 
 addition to these a certain proportion of carbon, in the 
 form of coke, charcoal or anthracite coal must be added to 
 all glass mixtures containing salt-cake. The use of a 
 slightly incorrect quantity of carbon for this purpose leads 
 to disastrous results, while even under the best conditions 
 it is not easy to remove all traces of sulphur compounds 
 from glass made in this way. A further risk of trouble 
 arises in connection with salt-cake from the fact that it is 
 never entirely free from more or less deleterious impurities, 
 According to the exact manner in which it has been pre- 
 pared, the substance always contains a small excess either 
 of undecomposed sodium chloride or of free sulphuric acid, 
 or the latter may be present in the form of sulphate of 
 lime. A good salt-cake, however, should contain at least 
 97 per cent, of anhydrous sodium sulphate, and not more 
 than 1*0 per cent, of either sodium chloride or sulphuric 
 
THE KAW MATEEIALS OF GLASS MANUFACTUKE. 43 
 
 acid. While pure sodium sulphate is readily soluble in 
 water, ordinary salt-cake always leaves an insoluble residue, 
 consisting frequently of minute particles of clay or other 
 material derived from the lining of the furnace in which it 
 was prepared, or from the tools with which it was handled ; 
 and these impurities are liable to become deleterious to the 
 glass if present in any quantity. The insoluble residue 
 should not exceed 0'5 per cent, in amount, and in the best 
 salt-cake is generally under 0*2 per cent. 
 
 Salt-cake possesses certain other properties that make it 
 somewhat troublesome to deal with as a glass-making 
 material. Thus, on prolonged exposure, particularly to 
 moist air, the powdered salt-cake absorbs moisture from 
 the atmosphere and undergoes partial conversion into the 
 crystalline form of " Glauber's Salt," a process which 
 results in the formation of exceedingly hard masses. 
 Ground salt-cake, therefore, cannot be stored for any 
 length of time without incurring the necessity of re- 
 grinding, and this accretive action even comes into play 
 when mixtures of glass-making materials, containing salt- 
 cake as one ingredient, are stored. In practice, therefore, 
 salt-cake can only be ground as it is wanted, and its physical 
 properties make it difficult to grind it at all fine, while the 
 dust arising from this process is peculiarly irritating, 
 although not seriously injurious to health. 
 
 Potash is utilised in glass-making almost entirely in the 
 form of carbonate, generally called " pearl-ash." Origin- 
 ally derived from the ashes of wood and other land plants, 
 this substance is now manufactured by processes similar to 
 those described in the case of soda, the raw material being 
 potassium chloride derived from natural deposits such as 
 
44 GLASS MANUFACTUEE. 
 
 those at Stassfurth. The pearl-ash thus commercially 
 obtainable is a fairly pure substance, but its use is com- 
 plicated by the fact that it is strongly hygroscopic and 
 rapidly absorbs water from the atmosphere. Where it is 
 desired to produce potash glasses of constant composition, 
 frequent analytical determinations of the moisture contents 
 of the pearl-ash are necessary, and the composition of the 
 glass mixture requires adjustment in accordance with the 
 results of these determinations. 
 
 The alkalies are also introduced into glass in the form of 
 nitrates (potassium nitrate, or saltpetre, and sodium nitrate, 
 or nitre) ; but although these substances act as sources of 
 alkali in the glass, they are employed essentially for the 
 sake of their oxygen contents. Such oxidising agents are 
 not, of course, added to glass mixtures containing sulphates 
 and carbon, but are employed to purify the mixtures con- 
 taining alkali carbonates, and more especially to oxidise the 
 flint glasses. Since these substances are only introduced into 
 glass in small quantities their extreme purity is not of such 
 great importance to the glass-maker, and the ordinary 
 " refined " qualities of both nitrates are found amply pure 
 enough to answer the highest requirements. 
 
 A certain number of natural minerals which contain an 
 appreciable quantity of alkali are sometimes utilised as raw 
 materials for glass manufacture. The most important of 
 these are the minerals of the felspar class already referred 
 to. These, however, contain a considerable proportion of 
 alumina, while all but the purest varieties also contain 
 more or less considerable quantities of iron. Some glass- 
 makers regard alumina as an undesirable constituent, while 
 others take the opposite view, and upon this view their use 
 
THE RAW MATEEIALS OF GLASS MANUFACTURE. 45 
 
 of felspathic minerals will depend. For the cheaper 
 varieties of glass, however, such as bottle glass, felspathic 
 minerals and rocks, such as granite and basalt, are freely 
 used as raw materials. Another mineral in which both 
 alkali and alumina are found is cryolite. This mineral 
 is a double fluoride of soda and alumina, whose properties 
 are particularly valuable in the production of opal and 
 opalescent glasses. As a mere source of alkali, however, 
 cryolite is much too expensive. 
 
 (3) Sources of Bases other than Alkalies. The most impor- 
 tant of these are lime and lead oxide, the former being re- 
 quired for the production of all varieties of plate and sheet- 
 glass, as well as for bottles and a large proportion of pressed 
 and blown glass, while lead is an essential ingredient of all 
 flint glass. The only other base having any considerable 
 commercial importance in connection with glass-making 
 is barium oxide, while oxide of zinc, magnesia, and a few 
 other substances are used in the manufacture of special 
 glasses for scientific, optical or technical purposes, where 
 glass of special properties is required. The metallic oxides 
 \vhich are used for the production of coloured glass are, of 
 course, also basic bodies. These will be treated in connection 
 with coloured glasses, with the exception of manganese 
 dioxide, which is used in large quantities in the manufacture 
 of many ordinary " white " glasses. 
 
 Calcium Oxide (lime) is generally introduced into glass 
 mixtures in the form of either the carbonate or the hydrated 
 oxide (slaked lime). The carbonate may be derived either 
 from natural sources, or it may be of chemical origin, while 
 the hydrate is always obtained by the calcination of the 
 carbonate, followed by " slaking" the lime thus produced. 
 
46 GLASS MANUFACTUBE. 
 
 Natural calcium carbonate occurs in great quantities in the 
 form of chalk and limestone rocks. Both varieties are used 
 for glass-making. Chalk is a soft friable material which 
 is apt to clog during the grinding operations, particularly 
 as the natural product is generally somewhat moist. As 
 regards the greater part of its mass, chalk is often found 
 in a state of great purity, but it is frequently contaminated 
 by the presence of scattered masses of flint. Chemically 
 this impurity is not very objectionable to the glass-maker, 
 since it merely introduces a small proportion of silica whose 
 presence need scarcely be allowed for in laying down the 
 mixture. On the other hand, if any fragments of flint 
 remain in the mixture when put into the furnace, they 
 prove very refractory, and are apt to be found as opaque 
 enclosures in the finished glass. Natural limestone can 
 also be obtained in great purity in many parts of the 
 world. It is generally a hard and rather brittle rock that 
 can be readily ground to powder of the requisite degree of 
 fineness. Flint concretions are not so frequently found in 
 this material, but, on the other hand, it is often contaminated 
 with magnesia and iron. The former ingredient, when 
 present in small quantities, tends to make the glass hard 
 and viscous, so that limestone of the lowest possible 
 magnesia content should be used, especially for the harder 
 kinds of glass, such as plate and sheet-glass, etc. The iron 
 contents of the limestone used must also be low where a 
 white glass is required ; but since a smaller quantity of 
 limestone is used for a given weight of glass produced than 
 the quantity of sand used for the same purpose, the 
 presence of a somewhat higher percentage of iron is 
 permissible in the limestone as compared with the sand ; 
 
THE RAW MATERIALS OF GLASS MANUFACTURE. 47 
 
 for the better varieties of glass, however, the iron should 
 not exceed 0'3 per cent, of the limestone. 
 
 Slaked lime is sometimes used as the source of lime for 
 special glasses where the process of manufacture renders 
 it desirable to avoid the evolution of carbonic acid gas 
 which takes place when the carbonate is heated and 
 attacked by silica. When slaked lime is used only the 
 water vapour of the hydrate is driven off, and this occurs 
 at a much lower temperature. For the production of 
 slaked lime, an adequately pure form of limestone, prefer- 
 ably in the form of large lumps, is burnt in a kiln until 
 the carbonic acid is entirely driven off; after cooling, the 
 lime so formed is slaked by hand. The product so obtained 
 is, however, apt to vary both as regards contents of 
 moisture and carbonic acid, which latter is readily absorbed 
 from the atmosphere ; the use of this material, therefore, 
 requires frequent analytical determinations of the lime 
 contents and corresponding adjustments of the mixture if 
 constant results are required. 
 
 It is possible to introduce lime into glass mixtures in the 
 form of gypsum or calcium sulphate, but the decomposition 
 of this compound, like that of sodium sulphate, requires 
 the intervention of a reducing agent such as carbon, and 
 the difficulties arising from this source in connection with 
 the use of salt-cake are still further increased in the case 
 of the calcium compound. Since limestones of considerable 
 purity are more or less plentiful in many districts, the 
 commercial value of calcium sulphate for glass-making is 
 probably slight. 
 
 The Compounds of Barium may best be dealt with at this 
 stage, since they are chemically so closely allied to the 
 
48 GLASS MANUFACTURE. 
 
 compounds of lime just described. Barium occurs in 
 nature in considerable quantities in the minerals known 
 as barytes (heavy spar) and witherite respectively. The 
 former is essentially sulphate of barium, while the 
 latter is a carbonate of barium. The use of the sulphate 
 meets with the same objection here as in the case of 
 calcium sulphate discussed above, except that the barium 
 compound is much more easily reduced and decomposed 
 than the lime compound. The natural mineral witherite 
 is used to a considerable extent in the production of barium 
 glasses, and these have been found capable of replacing 
 lead glasses for certain purposes. On the other hand, for 
 the best kinds of barium glasses, viz., those required for 
 optical purposes, the element is introduced in the form of 
 artificially prepared salts. Of these the most important is 
 the carbonate, commercially described as " precipitated 
 carbonate of barium " ; this precipitated compound, how- 
 ever, does not ordinarily correspond to the chemically pure 
 substance, but contains more or less considerable quantities 
 of sulphur compounds. The question whether these* 
 impurities are or are not objectionable can only be deter- 
 mined for each particular case, since much depends upon 
 the special character of the glass to be produced. Both the 
 nitrate and the hydrate of barium are commercially avail- 
 able, but they are very costly ingredients for use in the 
 production of even the most expensive kinds of glass ; these 
 substances are, however, obtainable in a state of considerable 
 purity, although the hydrate has the inconvenient property 
 of rapidly absorbing carbonic acid from the atmosphere, 
 thus becoming converted into the carbonate. 
 
 Magnesia is another glass-forming base that is closely 
 
THE RAW MATERIALS OF GLASS MANUFACTURE. 49 
 
 related, chemically, to calcium and barium. This element 
 is usually introduced into glass mixtures in the form of 
 either the carbonate or the oxide. The carbonate occurs in 
 nature in a more or less pure state in the form of magnesite, 
 and by calcination, the oxide is obtained. The natural 
 mineral and its product are, of course, by far the cheapest 
 sources of magnesia, but as the element is only used in 
 comparatively small quantities, the artificial precipitated 
 carbonate or calcined magnesia are frequently preferred. 
 Magnesia is only introduced intentionally in notable 
 quantities in special glasses where the properties it confers 
 are of special value ; in ordinary lime glasses this element, 
 as has already been mentioned, is to be regarded as an 
 undesirable impurity. 
 
 Zinc oxide lies, chemically, between the bases already 
 discussed on the one hand, and lead oxide on the other. 
 This element is only introduced into special optical glasses, 
 a special " zinc crown" having found some application. 
 Chemically prepared zinc oxide is almost the only form in 
 which the element is used, but the very volatile character 
 of this substance must be borne in mind when it is 
 introduced into glass mixtures. 
 
 Lead is one of the most widely-used ingredients of glass ; 
 the glasses containing this substance in notable quantity 
 are all characterised to a greater or less degree by similar 
 properties, such as considerable density and high refractive 
 power, and are classed together under the name " flint 
 glasses." Lead is now almost universally introduced into 
 glass mixtures in the form of red lead, although the other 
 oxides of lead might be employed almost equally well. Eed 
 lead is a mixture of two oxides of lead (PbO and Pb 2 3 ) in 
 
 G.M. E 
 
50 GLASS MANUFACTURE. 
 
 approximately such proportions as to correspond to the 
 formula Pb 3 04. It is prepared by the roasting of metallic 
 lead in suitable furnaces, where the molten lead is exposed 
 to currents of hot air. The product is obtainable in con- 
 siderable purity, very small proportions of silica, derived 
 from the furnace bed, and of iron derived from the tools 
 with which the lead is handled, being the principal foreign 
 substances found in good red lead. Silver would be an 
 objectionable impurity, but owing to the modern perfect 
 methods of de-silvering lead, that element is rarely found 
 in lead products. Analytical control of red lead as used in 
 the glass mixtures, and consequent adjustments of the 
 mixture, are, however, necessary where exact constancy in 
 the glass produced is desired. The reason for this necessity 
 lies in the fact that the oxygen content, and therefore the 
 lead-oxide (PbO) content, varies decidedly from batch to 
 batch, while the material as actually delivered and used 
 frequently contains notable proportions of moisture. 
 
 A word should perhaps be said here as to methods of 
 handling red lead on account of the injurious effects which 
 the inhalation of lead dust produces upon the workmen 
 exposed to it. For glass-making purposes it is not feasible 
 to adopt the method adopted by potters of first " fritting " 
 the lead and thus rendering it comparatively insoluble and 
 innocuous ; even if this were done, the difficulty would only 
 be moved one step further back, and would have to be over- 
 come by those who undertook the preparation of the frit. 
 The proper solution of the problem, in the writer's opinion, 
 is to be found in properly preventing the formation of lead 
 dust, or at all events in protecting the workmen from the 
 risk of inhaling it. Where only small quantities of lead 
 
THE RAW MATERIALS OF GLASS MANUFACTURE. 51 
 
 glass are made, and therefore only small quantities of lead 
 are handled and mixed at a time, it is no doubt sufficient to 
 provide the workmen engaged on this task with some 
 efficient form of respirator to be worn during the whole of 
 the time that they are engaged on such work, and to take 
 the further precautions necessary by way of cleanliness 
 and the provision of proper mess-rooms to avoid any risk 
 of lead dust either directly or indirectly contaminating 
 their food. Where, however, large quantities of flint-glass 
 are made every day, it is possible and proper to make 
 more perfect arrangements for the mechanical handling and 
 mixing of the lead with the other ingredients by the provision 
 of suitable mixing and transporting machinery, so arranged 
 as to be dust-tight. It is only fair to state, however, that 
 partly under their own initiative, partly under pressure 
 from the authorities, glass makers in this country are 
 complying with these requirements in an adequate manner. 
 Aluminium. There are several varieties of glass into 
 which alumina enters in notable quantities, the principal 
 examples being certain optical and many opal glasses, while 
 most ordinary glasses contain this substance in greater or 
 less degree. In the latter, the alumina is derived by the 
 inevitable processes of solution, from the fire-clay vessels or 
 walls within which the molten glass is contained, while in 
 some cases the element is intentionally introduced in small 
 proportions (about 2 per cent, to 3 per cent, of A1 2 3 ) by the 
 use of felspar as an ingredient of the mixture. Where 
 larger proportions of alumina are required, the substance is 
 introduced in the form of the hydrate, which is obtainable 
 commercially in a state of almost chemical purity, but of 
 course at a correspondingly high cost. In opal glasses 
 
 E 2 
 
52 GLASS MANUFACTUEE. 
 
 alumina is derived partly or wholly from felspars, or in 
 some cases from the use of the mineral cryolite. This is a 
 double fluoride of aluminium and sodium which is found in 
 great natural masses, chiefly in Greenland. Owing to the 
 high price of this mineral, however, artificial substitutes of 
 nearly identical composition and properties have been 
 introduced and are used successfully in the glass and 
 enamelling industries. 
 
 Manganese. Although the oxides of this element really 
 belong to the class of colouring compounds, they are so 
 widely used in the manufacture of ordinary " white " glasses 
 that it is desirable to deal with them here. The element 
 manganese is most usually introduced into glass mixtures 
 in the form of the per-oxide (MnC^), although the lower 
 oxide (MnsOi) can also be used. The material ordinarily 
 used is the natural manganese ore, mined chiefly in Eussia ; 
 the purest forms of this ore consist almost entirely of the 
 per-oxide, but " brown " ores, containing more or less of 
 the lower oxide, are also used with success. These ores 
 always contain' small amounts of iron and silica, but 
 provided the iron is not present in any considerable quantity, 
 the value of the ore is measured by the percentage of 
 manganese which it contains. The colouring and " decolour- 
 ising " action of manganese will be discussed in a later 
 chapter. Certain other substances, which have been 
 suggested as either substitutes for, or improvements upon, 
 manganese for this purpose need only be mentioned here, 
 viz., nickel, selenium and gold. 
 
 Arsenic is another substance frequently introduced into 
 " white " glass mixtures. This element is universally 
 introduced in the form of the white arsenic of commerce 
 
THE EAW MATEEIALS OF GLASS MANUFACTUEE. 53 
 
 (i.e., arsenious acid, As 2 3 ) which is obtained in a pure form 
 by a process of sublimation. Owing to the very poisonous 
 nature of this material, special precautions must be taken 
 in its use for glass-making purposes to avoid all risk of 
 poisoning. 
 
 Carbon. As has already been indicated, an admixture 
 of carbon in some suitable form is essential in the case of 
 certain glass mixtures. The carbon for this purpose may 
 be used in the form of either charcoal, coke, or anthracite 
 coal. Of these, charcoal is undoubtedly the purest form of 
 carbon, but it is excessively expensive in this country. 
 Coke varies very much in quality according to the coal 
 from which it has been produced, but it always contains 
 notable proportions of ash rich in iron, and also some 
 sulphur. Anthracite coal can be obtained in a very pure 
 form, containing considerably less ash than that found in 
 most kinds of coke, and this is therefore probably the most 
 convenient form of carbon for this purpose. 
 
CHAPTEK IV. 
 
 CRUCIBLES AND FURNACES FOR THE FUSION OF GLASS. 
 
 FOR the successful production of substances which are 
 formed by a process of fusion, the use of refractory 
 materials of a proper kind is of great importance. In the 
 production of glass the double difficulty has to be overcome 
 of finding substances capable of being formed into furnaces 
 and crucibles which shall not only resist the softening and 
 melting action of the furnace heat for long periods of time, 
 but shall also resist the dissolving action of the molten 
 glass itself. The refractory materials employed in connec- 
 tion with glass-making thus fall into two distinct groups, 
 members of one group being those which meet both of the 
 above requirements and can therefore be used in positions 
 exposed to direct contact with molten glass, while members 
 of the second group are materials which resist the action 
 of the heat and flame gases but cannot resist the dissolving 
 effect of the glass itself; these, of course, can only be 
 placed where molten glass is not liable to touch them. We 
 shall deal with the former group first. 
 
 Those portions of glass-melting plant which come into 
 contact with molten glass are almost universally made of 
 some form of fire-clay. To discuss in detail the composi- 
 tion and properties of the varieties of fire-clay best suited 
 
 
CRUCIBLES AND FURNACES FOR FUSION OF GLASS. 55 
 
 to this purpose would exceed the entire limits of this book, 
 so that only a few leading principles can be stated. Taking 
 first the clays intended for the production of crucibles or 
 " pots," we find that for the purposes of the production of 
 such objects the prepared clay must possess a certain 
 degree of plasticity while damp and a considerable degree 
 of strength when dried. The dried and burnt material 
 must be so refractory as to resist the high temperatures 
 used in glass-melting without undergoing fusion or even 
 serious softening. Clays of various composition and 
 physical nature also differ very widely in their power of 
 resisting the chemical attack of molten glass ; all clays are 
 more or less dissolved under these circumstances, but not 
 only the rate, but also the manner, of dissolution is of 
 importance, so that frequently a clay which dissolves 
 rapidly but uniformly is preferred to one which dissolves 
 more slowly but in such an irregular manner as to throw 
 off particles of undissolved material which contaminate the 
 glass in the form of opaque enclosures or " stones.' 7 It is 
 also to be noted that the best results in this direction can 
 only be obtained by careful adaptation of the clay employed 
 to the particular kind of glass which is to be melted in the 
 crucibles in question. In England this question has not 
 received the amount of attention it deserves, but in 
 Germany and America the available fire-clays of the 
 country have been systematically studied and exploited. 
 As a result the glass-maker has at his disposal a large 
 selection of materials of accurately known physical and 
 chemical properties. By carefully correlating these with 
 the performance of his " pots " in the furnaces, the manu- 
 facturer is able to select the most suitable material, and is, 
 
56 
 
 GLASS MANUFACTUBE. 
 
 moreover, in a position to know in what direction to look 
 for improvement or for replacement if the supply of a 
 satisfactory brand should cease. 
 
 We may now follow briefly the process of manufacture of 
 a fire-clay pot or crucible. The size and shape of the 
 crucible will depend upon the particular purpose for which 
 it is intended. Crucibles varying in capacity from 4 cwt. 
 to 2J tons of glass are used for various kinds of glass, but 
 the more usual sizes lie between 30 in. and 50 in. in 
 diameter. For many kinds of glass the shape of the pot is 
 
 FIG. 1. Open " pot " or crucible 
 for glass melting. 
 
 FIG. 2. Covered pot for glass 
 melting, as used for flint 
 glass and optical glass. 
 
 simply that of an open basin, circular or oval in plan and 
 larger in diameter at the brim than at the base (Fig. 1), 
 but for the production of flint glass, and of other glasses 
 which are to be protected from contact with the flame and 
 gases of the furnace, so-called "covered" pots are used. 
 In these the basin here of a more nearly cylindrical 
 shape is covered over by a dome, and access is allowed 
 only by a relatively small hooded opening (Fig. 2). 
 Covered pots are built up on wooden moulds, which are 
 made collapsible, and are removed before the drying of the 
 pot is begun. 
 
 The material for pot-making is first prepared with great 
 
CRUCIBLES AND FURNACES FOR FUSION OF GLASS. 57 
 
 care. The proper variety of clay having been selected, it 
 is ground to a fine powder in suitable mills and carefully 
 sieved ; with this fine clay powder is mixed, in accurately 
 determined proportions, a quantity of crushed burnt fire- 
 clay. In some works this burnt material is obtained by 
 simply grinding up fragments of old used pots, but the 
 better practice is to burn specially-selected fire-clay sepa- 
 rately for this purpose. The quantity of such burnt 
 material added to the mixture depends upon the chemical 
 nature and especially on the plasticity of the virgin clay 
 employed ; with so-called " fat " or very plastic clays up to 
 50 per cent, of burnt material is added, but with the leaner 
 clays, such as those of the Stourbridge district in England, 
 very much smaller proportions are used. The object of 
 this addition of burnt material is to facilitate the safe dry- 
 ing of the finished pots and to diminish by dilution the 
 total amount of contraction which takes place both when 
 plastic clay is allowed to dry, and further when the dry 
 mass is subsequently burnt; the burnt material or 
 "charnotte," having already undergone these shrinking 
 processes, acts both as a neutral diluent and also as a 
 skeleton strengthening the whole mass and reducing the 
 tendency to form cracks. 
 
 The virgin clay and chamotte having been intimately 
 mixed, the whole mass is " wet up " by the addition of a 
 proper proportion of water and prolonged and vigorous 
 kneading, usually in a suitable pug mill. The mass leaves 
 this mill as a fairly stiff, plastic dough, but the full tough- 
 ness and plasticity of such clay mixtures can only be 
 developed by prolonged storage of the damp mass. In the 
 next stage of the process, the plastic clay is passed to the 
 
58 GLASS MANUFACTUBE. 
 
 " pot maker " in the form of thick rolls, and with these he 
 gradually builds up the pots or crucibles from day to day, 
 allowing the lowest parts to dry sufficiently to enable them 
 to bear the weight of the upper parts without giving way. 
 The building of large pots in this way occupies several 
 weeks, and during this time the premature drying of any 
 part of the pot must be carefully avoided. After the com- 
 pletion of the pot, drying is allowed to take place, slowly 
 at first, but more vigorously after a time when the risk of 
 cracking is smaller ; when it is taken into use, the pot is 
 usually many months old and is thoroughly air-dry. The 
 clay, however, is still hydrated, i.e., contains chemically 
 combined water, and this is only expelled during the early 
 stages of the burning process. This process is carried out 
 in smaller furnaces or kilns placed near the melting 
 furnaces. In these the pot or pots are exposed to a very 
 gradually increasing temperature until a bright red heat is 
 finally attained. This is a delicate process in which great 
 care is required to secure gradual and uniform heating, 
 especially during the earlier stages, otherwise the pots are 
 apt to crack and become useless. Finally, when a bright 
 red heat has been maintained for at least a day, the pots 
 are ready to be placed in the furnace, and this is ordinarily 
 done while both pots and furnace are at a red heat, the 
 pots never being allowed to cool down again once they have 
 been burnt. 
 
 Fire-clay is also used in the manufacture of bricks and 
 blocks of various sizes required for the construction of glass- 
 melting furnaces. Here fire-clay is only used in positions 
 where contact with molten glass is expected, as in the walls 
 of the basin or tank proper in " tank " furnaces, or at a 
 
CEUCTBLES AND FUENAOES FOB, FUSION OF GLASS. 59 
 
 level below that of the pot or crucible in pot furnaces ; in 
 the latter position leakage of glass from broken pots or 
 overflow being liable to result in an accumulation of 
 molten glass on the floors or walls of the furnace and 
 passages. The fire-bricks used in these latter positions are 
 usually of a much poorer quality of fire-clay than that used 
 for the manufacture of pots, and this is justified in so far as 
 certain of the requirements that apply to crucibles do not 
 apply here but on the other hand the use of more 
 refractory bricks would result in a longer life for the 
 furnace. Such bricks, it should be noted, are not laid in 
 mortar when used for furnace construction, but are set in 
 a thin paste of fire-clay in water, and these joints are kept 
 as thin as possible. The part of the furnace known as the 
 " siege" (French " siege"), i.e., the floor of the furnace 
 upon which the pots are placed, is usually built of very 
 large blocks of fire-clay, made of coarse materials calculated 
 to give great strength. At or near the points where the 
 flame enters the furnace, these blocks rapidly wear away, 
 partly by melting but chiefly by a process of abrasion, for 
 it seems that a rapidly moving flame has an abrading 
 action of a very marked kind. 
 
 The actual tanks or basins which contain the molten 
 glass in tank furnaces are also built of large blocks of fire- 
 clay, but these are made of the best procurable materials, 
 and should receive at least as much care in every respect 
 as crucibles ; it is true that their shape and size gives them 
 greater strength, but on the other hand these blocks are 
 expected to resist the contact of molten glass for very much 
 longer periods of time than the average crucible. To 
 understand the requirements for tank-blocks it is necessary 
 
60 GLASS MANUFACTURE. 
 
 to anticipate the next section to the extent of stating that 
 in tank furnaces the glass is contained, during melting, 
 refining and working, in a basin built up of large blocks. 
 These blocks are not cemented together in any way, but 
 are built up " dry " and are supported on the outside by a 
 system of iron bars and rods. The molten glass penetrates 
 between the blocks to a certain extent, but as the outside of 
 all such blocks is intentionally kept as cold as possible the 
 glass rapidly stiffens as it penetrates further into these 
 interstices, and this stiffened glass effectually binds the 
 blocks together and prevents all leakage. It will thus be 
 seen that the blocks are exposed to the full heat of the 
 furnace and to the corroding action of the glass on the 
 inner side, but are kept cold on the outer side. As this 
 state of affairs tends to produce cracks, these blocks are 
 necessarily made of rather coarse material. On the other 
 hand, the material of a block never gets so hot as the wall 
 of a crucible, which is heated from both sides, so that 
 extreme refractoriness is not so essential. 
 
 It is impossible, within the limits of this chapter, to go 
 into the details of the choice of materials for tank-blocks ; 
 it is a subject upon which no finally satisfactory conclusion 
 has yet been reached, and what has been said above will 
 suffice to show the nature of the considerations upon which 
 such choice must be based. 
 
 We now turn to the second class of refractory materials 
 used in the construction of glass-melting furnaces, viz., 
 those which are so placed as not to come into contact with 
 molten glass. Here mechanical strength and refractoriness 
 are almost the only considerations, but in the roof-vaults or 
 " crowns " of tank furnaces and also of furnaces in which 
 
CEUOIBLES AND FUENACES FOE FUSION OF GLASS. 61 
 
 glass is melted in open pots, there is the further considera- 
 tion that the material of the bricks used shall not contain 
 notable quantities of any colouring oxide, since small 
 flakes, etc., are apt to drop down into the molten glass, and 
 would thus be liable to cause serious discolouration. Such 
 a material as chrome-ore brick is therefore excluded. As a 
 matter of fact, some form of " silica brick " is in universal 
 use. Bricks of this material, otherwise known as " Dinas 
 bricks" from the place of their first origin, in Wales, con- 
 sist of about 98 per cent, of silica (Si02). Pure silica 
 cannot be baked or burnt into coherent bricks entirely by 
 itself, since it possesses neither plasticity when wet nor any 
 binding power when burnt, but an admixture of about 
 2 per cent, of lime and a little alumina makes it possible 
 first to mould the bricks when wet and then to burn them 
 so as to form fairly strong, coherent blocks. These are of 
 amply adequate refractoriness for the highest temperatures 
 that can be attained in industrial gas-fired furnaces, and 
 their mechanical strength is sufficient to make it possible 
 to build vaults of considerable span, but on the other hand 
 this material requires very gradual heating and constant 
 watching while the temperature is rising or falling to any 
 considerable extent ; the reason for this difficulty lies in the 
 fact that silica bricks swell very markedly during heating, 
 so that unless a vault built of this material is given room 
 to spread somewhat, it will rise seriously and may even 
 break up completely. This risk is avoided by gradually 
 slackening the tie-bolts that hold the vault together, and 
 correspondingly " taking up the slack " as the vault cools 
 when the furnace is let out. Sudden local heat also has a 
 Disastrous effect on this material, producing serious flaking. 
 
62 GLASS MANUFACTURE. 
 
 For positions where intense heat is to be borne, and 
 at the same time mechanical strength is required, silica 
 brick is a most valuable material, but owing to its chemical 
 composition it is rapidly attacked by molten glass or by any 
 material containing a notable proportion of basic con- 
 stituents, so that the silica bricks can only be employed 
 out of contact with glass. 
 
 We now turn to consider, very briefly, the general design 
 and arrangement of some typical glass-melting furnaces. 
 The oldest and simplest form of furnace is, in effect, simply 
 a box built of fire-brick, in the centre of which stands the 
 crucible, while a fire of wood or coal is placed upon either 
 side. To attain any great degree of heat by such means, 
 however, the size of the box or chamber and especially of 
 the grates in which the fires are maintained must be 
 properly proportioned both to the dimensions of the crucible 
 and to each other. The grates are generally wide and 
 deep, while draught is provided by means of a tall conical 
 chimney which stands over the entire chamber and com- 
 municates with it by a number of small openings. In a 
 more refined furnace, the chamber itself is double, and the 
 flame, after playing around the crucible in the inside of the 
 chamber, is made to pass through the space between the 
 outer and inner chamber before passing to the chimney or 
 cone. We need not give any greater attention to these 
 primitive furnaces, since they are practically obsolete at the 
 present time. In modern furnaces the process of com- 
 bustion is carried on in two distinct stages ; the first stage 
 takes place in a subsidiary appliance known as a " gas pro- 
 ducer," where part of the heat which the fuel is capable of 
 generating is utilised for the production of a combustible 
 
CRUCIBLES AND FURNACES FOR FUSION OF GLASS. 63 
 
 gas ; this gas passes into the furnace proper, either direct, 
 while it is still hot from the producer, or after being con- 
 veyed some distance, when it is again heated up by the 
 waste heat of the furnace. In either case the gas is hot 
 when it enters the furnace proper, and there it meets a 
 current of air, also heated by the aid of the waste heat of 
 the furnace. Hot gas and hot air burn rapidly and com- 
 pletely, and if properly proportioned yield exceedingly high 
 temperatures. Seeing that in this process a part of the 
 heat of combustion yielded by the fuel is generated in a 
 subsidiary . appliance and is thus lost to the furnace, it 
 appears at first sight somewhat surprising that this system 
 of firing is very considerably more efficient than the old 
 " direct " system where the whole of the fuel is burnt in 
 the furnace itself. But the advantage arises from the fact 
 that in the newer system the fuel is handled in the gaseous 
 form. This has the advantage, first and most important, 
 that the heat escaping from the furnace in the hot pro- 
 ducts of combustion (chimney gases) can be transferred to 
 the incoming unburnt gas and air and can thus be returned 
 to the furnace. The manner in which this is accomplished 
 will be considered below, but it may be noted here that in 
 some furnaces the escaping products of combustion are so 
 thoroughly cooled that they are unable to produce an 
 effective draught in the chimney of the furnace. Another 
 advantage of the use of gaseous fuel is the fact that com- 
 plete combustion can be obtained without the use of so 
 great excess of air, such as is required when solid fuels are 
 to be burnt completely. For this reason much higher 
 temperatures can be readily obtained with gaseous fuel, 
 while the pre-heating of both gas and air also facilitates the 
 
64 GLASS MANUFACTURE. 
 
 attainment of high temperatures ; further, the great facility 
 with which the flow of either gas or air can be regulated by 
 means of suitable valves, makes it possible to secure much 
 greater regularity in the working of the furnaces. Finally, 
 in modern gas-producers, the amount of sensible heat 
 generated and therefore lost to the furnace, is kept very 
 low, the greater part of the heat set free by the partial 
 combustion of coal in the producer being absorbed by the 
 decomposition of a corresponding quantity of steam into 
 hydrogen and carbonic oxide gas. The gas as it leaves one 
 of these producers is not very hot, and the percentage of 
 heat lost in this way is therefore much smaller than in the 
 older forms of gas-producer. 
 
 It is again impossible, within the limits of this chapter, 
 to enter into the details of construction and working of 
 gas-producers. We must content ourselves with saying that 
 most modern producers are of the form of a tower in which 
 a thick bed of fuel is partially burnt and partly gasified 
 under the action of a blast of air mixed with steam. The 
 chemical actions that take place are complicated, but the 
 final result is the production of a gas containing from 2 to 
 8 or 10 per cent, of carbonic acid, 10 to 20 per cent, of 
 hydrogen, 8 to 25 per cent, of carbonic oxide (CO), 1 to 3 
 per cent, methane (CH 4 ), and 45 to 60 per cent, of nitro- 
 gen, with varying quantities of moisture, tarry matter, and 
 ammonia. In good producer gas, the combustible con- 
 stituents (hydrogen, carbonic oxide and methane) should 
 total from 30 to 48 per cent, of the whole by volume, but 
 the exact composition to be expected depends very much on 
 the type of producer and the class of fuel used. Some pro- 
 ducers are capable of dealing with exceedingly low-grade 
 
CKUCIBLES AND FUENACES FOE FUSION OF GLASS. 65 
 
 fuels, and the gas which they yield can still be utilised for 
 obtaining the highest temperatures a proceeding that 
 would have been impossible if it had been attempted to 
 burn these fuels directly in the furnace. 
 
 The gas on leaving the producer passes along fire-brick 
 
 REGENERATOR 
 
 REGENERATOR 
 
 FIG. 3. Diagram of the arrangements of a regenerative furnace. 
 
 flues or passages to the furnace proper ; the path which it 
 is now caused to take varies somewhat according to the 
 arrangement of the furnace in question. Modern gas-fired 
 furnaces usually belong to one of two distinct types accord- 
 ing to the manner in which the heat of the escaping pro- 
 ducts of combustion is utilised for heating the incoming 
 gas and air ; these two types are known as the " regenerative " 
 G.M. F 
 
66 GLASS MANUFACTURE. 
 
 and the "recuperative" respectively. In regenerative 
 furnaces the hot products of combustion, after leaving the 
 furnace chamber proper, and before reaching the chimney, 
 pass through chambers which are loosely stacked with fire- 
 bricks; these chambers absorb the heat of the escaping 
 gases, and thus rapidly become hot. As soon as a sufficiently 
 high temperature is attained in these chambers or " re- 
 generators," the path of the gas-currents is altered ; the 
 escaping products of combustion are made to pass through, 
 and thus to heat a second set of regenerating chambers, 
 while the incoming gas and air are drawn through the 
 heated regenerator chambers before entering the furnace 
 chamber proper. The incoming gas and air are thus heated, 
 absorbing in turn the heat stored in the brickwork of the 
 regenerators. It is evident that two sets of such regenera- 
 tors are sufficient, the one set undergoing the heating 
 process at the hands of the escaping products of combustion, 
 while the other set is giving up its heat to the incoming gas 
 and air; when this process has gone far enough, it is only 
 necessary to interchange the two sets of chambers, by the 
 operation of suitable valves, and this series of alternations 
 may be continued indefinitely. The arrangement is shown 
 diagrammatically in Fig. 3. 
 
 In recuperative furnaces the same principle is utilised in 
 a somewhat different manner ; the outgoing products of 
 combustion pass through tubular channels formed in fire- 
 clay blocks, while the ingoing gas and air pass around the 
 outside of these same blocks; the heat of the outgoing 
 gases is thus transferred to the incoming gases by the 
 process of conduction through the fire-clay walls of the 
 recuperator tubes. The relative merits of the two systems 
 
CEUCIBLES AND FUENACES FOE FUSION OF GLASS. 67 
 
 have been hotly contested ; the regenerative system has the 
 advantage of avoiding all reliance on the heat conductivity 
 of fire-clay, while it also avoids the somewhat complicated 
 special tubular blocks required for the other system ; on 
 the other hand, the recuperative system avoids the necessity 
 for all " reversing " valves and their regular periodical 
 working, while it also occupies somewhat less space. 
 Temperatures sufficiently high for all glass-melting pur- 
 poses can be attained by both means. 
 
 In both systems of furnace, heated gas and heated air 
 are admitted to the furnace by separate fire-brick flues or 
 passages, air and gas being allowed to mix just before they 
 enter the furnace chamber proper. The economy and 
 efficiency of the furnace depend to a very great extent upon 
 the manner in which this mixing is accomplished. Eapid 
 and complete mixing of air and gas results in an intensely 
 hot, but short and local flame, while slower mixing tends to 
 lengthen the flame and spread the heat through the entire 
 furnace chamber ; on the other hand if the mixing of gas 
 and air is too slow, combustion may not have been com- 
 pleted in the short time occupied by the gases in passing 
 through the furnace, and combustion may either continue 
 in the outflow flues and regenerators, or it may be prevented 
 by the narrowness of these passages, and unburnt gases may 
 pass to the chimney. When the openings or "ports " are 
 properly proportioned, and the draught of the chimney is 
 properly regulated, combustion should be just complete as 
 the gases leave the furnace chamber, and under these cir- 
 cumstances small tongues of keen flame will escape from 
 every opening in the furnace ; large smoky flames issuing 
 from a gas-fired furnace indicate incomplete combustion. 
 
 F 2 
 
68 
 
 GLASS MANUFACTUEE. 
 
 As has already been indicated, glass is melted either in 
 pots or crucibles of various shapes and sizes, or in open 
 tank furnaces. The general arrangement of a pot furnace 
 working with closed or " covered " crucibles is shown in 
 Fig. 4. In this particular furnace, the " ports " or aper- 
 tures by which the gas and air enter the furnace chamber, 
 are placed in the floor of the chamber, but these apertures 
 
 FIG. 4. Sectional diagram of a regenerative pot furnace working with , 
 
 covered pots. 
 
 are often placed in the side or end walls, or even in a central 
 column, the object being in all cases to heat all the pots as 
 uniformly as possible and to avoid any intense local heating, 
 which would merely endanger the particular crucible 
 exposed to it, without greatly aiding the real work of the 
 furnace. In pot furnaces, however, in which the more 
 refractory kinds of glass are to be melted, it is generally 
 considered desirable that the flame should be made to play 
 
CRUCIBLES AND FUENACES FOR FUSION OF GLASS. 69 
 
 about the pots in such a way as to heat the lower parts of 
 the pots most strongly. In connection with the question of 
 the uniformity of heat distribution in a gas-fired furnace it 
 must further be borne in mind that in the case of regenera- 
 tive furnaces the direction of the flame is reversed every 
 time the valves are thrown over, and in practice this is done 
 about once every half-hour ; this proceeding, of course, 
 tends very much to equalise the temperature of the two 
 sides of the furnace. In recuperative furnaces, on the other 
 hand, the direction of the flame is not changed, and for 
 
 FIG. 5. Diagram of a furnace with " horse-shoe " flame. 
 
 that reason a flame returning upon itself, usually called a 
 horse-shoe flame, is often employed; this is obtained by 
 placing the entry and exit ports side by side at one end of 
 the furnace ; the impetus of the flame gases and their rapid 
 expansion during combustion carry the flame out across 
 the furnace, while the chimney draught ultimately sucks it 
 back to the exit ports, the shape of the flame being shown 
 in Fig. 5. 
 
 In general arrangement, a tank furnace for glass melting 
 resembles an open-hearth steel furnace. The tank or basin, 
 as already indicated, is built up of a number of large fire-clay 
 
70 
 
 GLASS MANUFACTUKE. 
 
 blocks, forming a bath varying in depth from 20 in. to 
 42 in. according to the design of the furnace and the kind 
 of glass to be melted in it. The ports for entry of gas and 
 air and for exit of the products of combustion are in most 
 modern furnaces placed in the side walls of the furnace 
 just above the level of the glass, the whole being covered by 
 a vault built of silica brick. Figs. 6 and 7 show the general 
 arrangement of a simple form of tank-furnace such as that 
 used in the manufacture of rolled plate glass. The furnace 
 
 F IG< 6. Longitudinal sectional diagram of tank furnace. 
 
 indicated in the diagram is intended for regenerative work- 
 ing with alternating directions of flame ; in recuperative 
 furnaces the horse-shoe flame is always used in tanks, 
 while this arrangement of ports is sometimes adopted for 
 regenerative tanks also, particularly in the manufacture of 
 bottles. For the production of sheet glass, tank furnaces 
 are generally subdivided into two compartments and are 
 also provided with various constrictions intended to arrest 
 impurities and to allow only clear glass to pass, but as 
 regards the arrangement of flues and ports there is a very 
 general similarity between various furnaces of this type. 
 
CRUCIBLES AND FUENACES FOE FUSION OF GLASS. 71 
 
 It is beyond the scope of this book to discuss the relative 
 merits of tank and pot melting furnaces ; wherever the 
 former can be made to produce glass of adequate quality for 
 the purpose desired, the great economy of the tank furnace 
 inevitably carries all before it, so that bottle glass, for 
 example, is now made exclusively in tanks, and the same 
 
 FIG. 7. Transverse sectional diagram of tank furnace, showing 
 regenerators and gas and air passages. 
 
 applies also to rolled plate of the ordinary kind, and to the 
 great majority of sheet glass. On the other hand, where 
 special qualities of glass are required in relatively small 
 quantities, or where the requirements as to quality are very 
 extreme, the pot furnace remains indispensable. Optical 
 glass and coloured glasses are examples of this kind, 
 although some tinted glasses are used in sufficient quan- 
 tity to justify the use of small tank furnaces for their 
 
72 GLASS MANUFACTURE. 
 
 production. The causes of the greater economy of the tank 
 furnace are numerous, and complicated by the detailed 
 requirements of each particular manufacture, but the most 
 important factors in the question may be summed up thus : 
 
 (1) The tank furnace utilises the heat of the flame more 
 efficiently, as the glass is exposed to the heat in a basin 
 whose surface covers the entire area of the furnace, while 
 in a pot furnace there is much vacant, unused space. 
 
 (2) The tank furnace permits of continuous working, the 
 raw materials being introduced at one end while the glass 
 is being withdrawn and worked at the other end. There 
 are thus no idle periods, and each part of the furnace 
 remains at or near the same temperature during the whole 
 time that a furnace is alight. For a given size of plant, 
 therefore, a tank furnace yields a much larger output, with 
 a relatively smaller fuel consumption. 
 
 (3) The tank furnace obviates the need for pots or crucibles, 
 which are not only costly and troublesome to produce, but 
 are liable to premature failure and require periodical re- 
 newal, which involves a serious loss of time for the furnace. 
 
 (4) Finally, the molten glass in a tank furnace can be 
 always maintained at or near one constant level and is, 
 therefore, always convenient for withdrawal by means of 
 the gatherer's pipe or the ladle. 
 
 In pot furnaces, on the other hand, the composition of 
 the glass can be more accurately regulated, and the molten 
 glass itself can be more effectively protected from contami- 
 nation either by matter dropping into it or by the action of 
 the furnace gases, while in pots it is also possible to effec- 
 tually melt together materials which, in the open basin of 
 a tank, could not be kept together long enough to combine. 
 
CHAPTEK V. 
 
 THE PROCESS OF FUSION. 
 
 IT has already been indicated that, for glass-making pur- 
 poses, the raw materials are required in a state of reasonably 
 tine division. The exact degree of fineness required depends 
 very much upon the nature of the ingredient in question, 
 the general rule being that the more refractory and 
 chemically resistant materials require to be most finely 
 ground, while substances which melt and react readily, 
 such as soda ash and salt-cake, do not require very fine 
 grinding. 
 
 Assuming that the materials are available in a suitable 
 state of fineness, the first step in the process of glass melt- 
 ing consists in securing their admixture in the proper 
 proportions. This may be done by hand entirely, by hand 
 aided by some machinery, or entirely automatically. The 
 process of hand mixing is only available for relatively small 
 quantities of material and requires very careful supervision 
 if inadequate mixing is to be avoided. In most cases the 
 actual weighing out is done by hand, while the mixing is 
 done by machinery. In this process the separate ingre- 
 dients are weighed out from barrows or skips and are 
 tipped into a large hopper whence each batch, as soon as it 
 is completed, passes into the mixing chamber of the mixing 
 
74 GLASS MANUFACTUEE. 
 
 machine. This may consist of nothing more than a cylin- 
 drical chamber in which steel arms revolve and stir up the 
 contents, but more modern appliances take the form of 
 rotating barrels or cylinders, set up on an inclined axis and 
 provided with suitable shelves and baffles ; in these the 
 materials are very thoroughly shaken over and mixed. 
 Where hand mixing is adopted, the various ingredients of 
 each batch are thrown into a large bin and are there turned 
 over several times with shovels, the entire material being 
 ultimately sieved through a wire sieve of suitable mesh. 
 In all cases the resulting mixture should be perfectly 
 uniform in colour and texture, and analyses of different 
 samples should show only small variations. With the 
 mixture thus prepared the " cullet " or broken glass which is 
 to be re-melted is now incorporated ; ideally this should 
 also be uniformly distributed, but this is rarely attempted 
 in practice on the large scale. 
 
 The next step in the process is the introduction of the 
 mixture into the furnace. In the case of tank furnaces 
 this is a simple matter, since in these the temperature is 
 kept as nearly constant as possible, and raw materials may, 
 therefore, be introduced at almost any time, the amount 
 introduced being so regulated as to keep the level of the 
 molten glass or "metal" as nearly constant as possible. 
 The actual introduction is managed by means of a large 
 opening or door at what is known as the " melting end " of 
 the furnace. Normally this opening is covered by a large 
 firebrick block suspended by a chain running over pulleys 
 and counterbalanced by a counterpoise weight. When 
 charging is to begin, this block is raised and the opening is 
 uncovered. The raw materials are then introduced either 
 
THE PEOCESS OF FUSION. 75 
 
 by hand, by the aid of long-handled shovels, or they are 
 first filled into a long scoop moved by mechanical means 
 forward into the furnace, where it is given a half-turn, 
 which empties the contents out, and is then rapidly 
 withdrawn. 
 
 This charging process may be repeated every half-hour, 
 or larger quantities may be introduced once every four 
 hours, according to the practice that may be adopted at 
 any particular furnace. 
 
 In the case of pot furnaces the charging process is not so 
 simple. Here the first charge of raw materials has to be 
 introduced into a pot which has been almost entirely 
 emptied during the working-out process, and the tempera- 
 ture of the furnace has also fallen very considerably during 
 this time. Before new material is introduced, the heat of 
 the furnace must first be adequately restored. If this is 
 not done, the fusion of the glass takes an abnormal course 
 and very imperfect results arise. Further, the quantity of 
 material introduced at one time must be carefully adjusted 
 to the capacity of the pot. During the earlier stages of 
 fusion most glass mixtures form large masses of foam, and 
 if the crucible has been too heavily charged this foam over- 
 flows, with the result that valuable material is lost and the 
 floor and passages of the furnace are clogged with glass. 
 A certain amount of overflow, as well as leakage from 
 defective crucibles, is, however, unavoidable, and for this 
 purpose every pot furnace is provided with a chamber so 
 placed that the glass will flow into it and so be prevented 
 from finding its way into the regenerators or other parts 
 where its presence would hinder the working of the 
 furnace. These receptacles or ''pockets " must, however, be 
 
76 GLASS MANUFACTUEE. 
 
 periodically cleared of their contents from outside, and this 
 constitutes one of the most irksome operations connected 
 with glass manufacture. Owing to the occurrence of 
 foaming and to the fact that the raw materials occupy 
 much more space than the glass formed from them, it is 
 necessary to fill the pot with fresh batches of raw materials 
 several times, the quantity which can be introduced 
 decreasing each time. The number of times that this 
 must be done depends upon the particular circumstances, 
 but from four to eight " fillings " are commonly used for 
 various kinds of glass and size of pot. The precise stage 
 at which a fresh batch of raw materials should be intro- 
 duced is another matter requiring careful attention. For 
 some purposes it is necessary to wait until the previous 
 batch is completely melted, while in other cases raw 
 material may be added whilst some of the previous batch 
 is still floating on the surface of the glass in the pot. 
 
 We have/ now to consider the chemical reactions which 
 take place in the mixture of raw materials that are intro- 
 duced into the hot furnace. The exact course of these 
 reactions is not known in very great detail, as this could 
 only be ascertained by an elaborate research on the nature 
 of the intermediate products that result under various 
 circumstances. A research of this kind would throw much 
 light on the whole of the melting processes but is in itself 
 so difficult that it has not yet been carried out at all fully. 
 We can therefore only give an account of the chemical 
 changes from our knowledge of the end-results and of a 
 few intermediate products that are known. To take the 
 simplest case, we may consider a mixture consisting of 
 sand, carbonate of lime and carbonate of soda mixed in 
 
THE PEOOESS OF FUSION. 77 
 
 suitable proportions. In such a case we know that the mere 
 action of heat alone will produce two changes the carbon- 
 ate of soda will melt and the carbonate of lime will lose its 
 carbonic acid and be " burnt " or converted into caustic 
 lime. The first stage of the fusion process thus probably 
 results in a mass consisting of sand grains and grains of 
 carbonate of lime undergoing decomposition, all cemented 
 together by molten carbonate of soda. This mass will be 
 full of bubbles, some derived from the air enclosed between 
 the grains of the original mixture and thus trapped by the 
 melting mass, and others formed by the carbonic acid 
 which is being driven off in the form of gas by the decom- 
 position of the carbonate of lime. At the temperature of 
 the furnace, however, silica has the properties of a strong 
 acid, and not only attacks the carbonate of lime much in 
 the same manner as, for instance, hydrochloric acid would 
 do in the cold, but the silica also attacks the carbonate of 
 soda, which heat alone can scarcely decompose. The exact 
 order in which these reactions take place will depend upon 
 the temperature of the furnace and the degree of mixing 
 attained in the preparation of the raw materials. Although 
 in the long run the final result will probably be the same 
 as regards purely chemical constitution, much of the 
 technical success of the process must depend upon the 
 exact sequence of the changes involved, as this must govern 
 the number and size of the bubbles that are formed in the 
 glass and the fluidity of the mass from which these bubbles 
 have to free themselves. In the present state of our 
 knowledge, however, we can only say that the final result 
 is the complete expulsion of all carbonic acid from the 
 compounds present (although it may remain entangled in 
 
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Till'! IMUH'I'KS OK KIIHION. 79 
 
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80 GLASS MANUFACTURE. 
 
 carbon required in practice is very considerably less than 
 that given by this theory ; it follows therefore that either 
 this very large amount of reducing action must be ascribed 
 to the furnace gases, or that the actual reactions are not 
 strictly of the kind we have described. Both explanations 
 are probably partly correct, and in practice the amount of 
 carbon to be used in a given mixture and furnace can only 
 be found by actual trial, in which the manufacturer is, of 
 course, guided by the results obtained with other furnaces 
 of a similar type. The end-product of the reactions is 
 again a mixture of silicates, but a certain amount of unde- 
 composed sulphate is always found in such glasses, while 
 gaseous oxides of sulphur escape from these furnaces in 
 considerable quantity. Under exceptional circumstances 
 the glass may even contain sulphides of soda or of lime, 
 and sometimes even suspended carbon, but these are 
 abnormal constituents and result in the serious discolouration 
 of the glass. 
 
 It is obvious that to a mixture containing carbon as a 
 reducing agent such oxidising materials as nitrates cannot 
 be added, but small quantities of arsenic and of manganese 
 dioxide are added because their other properties are 
 sufficiently valuable to outweigh their disadvantages as 
 oxidising agents. 
 
 Having now briefly considered the process of fusion 
 proper, we pass to the second stage in the melting of 
 glass. In a properly conducted glass-furnace, when the 
 last trace of undecomposed raw materials has disappeared, 
 we find the glass as a transparent mass throughout which 
 gas bubbles are thickly disseminated. For the majority of 
 purposes it is necessary to free the glass as perfectly as 
 
THE PEOCESS OF FUSION. 81 
 
 possible from these bubbles before it is worked into its final 
 form. This freeing or "fining" process is carried out by 
 further and more intense heating of the molten glass, which 
 is thereby rendered more fluid and allows the bubbles to 
 disengage themselves by rising to the surface. This occurs 
 much more readily when the bubbles are large ; very minute 
 bubbles, in fact, show no inclination to rise through the 
 fluid mass. The glass-maker accordingly compounds his 
 mixtures of raw materials in such a way as to yield large 
 bubbles, or, failing that, he adds to the molten mass some 
 substance that evolves a great many large bubbles, and 
 these in their upward course through the glass sweep the 
 small ones away with them. The added substance may be 
 an inorganic volatile body, such as arsenic, or more 
 frequently some vegetable substance containing much 
 moisture is introduced into the glass. The most usual 
 method is to place a potato in the crook of a forked iron rod 
 and then to dip the rod with the attached potato into the 
 molten glass ; the heat at once begins to drive off the 
 moisture and to decompose the potato, so that there is a 
 violent ebullition of the whole mass. This "boiling up" 
 process assists the fining considerably and also serves to 
 mix the whole contents of the pot very thoroughly, but it 
 has some attendant disadvantages, such as the introduction 
 of oxide of iron into the glass from the rod which is used in 
 the operation, while the contaminated material adhering to 
 the walls of the pot itself is dragged off and mixed with the 
 rest of the glass by the violent stirring action that takes 
 place. It is, of course, further obvious that this process can 
 only be usefully applied to glass melted in pots, since the 
 bulk of the molten glass in a tank furnace could not be 
 G.M. G 
 
82 GLASS MANUFACTURE. 
 
 reached at all in this manner. Mixtures that are to be 
 melted in tanks must therefore be capable of freeing them- 
 selves of their enclosed bubbles without such outside aid. 
 In a tank, in fact, the whole melting process proceeds on 
 somewhat different lines, since the temperature of the 
 furnace is never intentionally varied, while on the other 
 hand the melting glass travels down the furnace into regions 
 whose temperature can be regulated to favour the various 
 stages of the process that take place in each part of the 
 furnace. On the whole, however, it is an undoubted fact 
 that while the running of a pot furnace can be varied, 
 within wide limits, to suit the requirements of whatever 
 mixture it is desired to melt, in the case of tank furnaces 
 the mixture must be closely adjusted to the requirements 
 of the furnace, whose general " run " cannot be very readily 
 altered. 
 
 The completion of the " fining " process is generally 
 determined by taking samples of the glass out of the pot 
 or tank and examining them for enclosed bubbles. Such 
 samples may be obtained in a variety of ways, the most 
 usual method being to dip a flat iron rod just below the 
 surface of the glass and to lift it out vertically upwards, 
 thus retaining on the flat surface of the rod some of the 
 glass that lay there at the moment when the rod was 
 immersed. These test samples or "proofs " are examined 
 very carefully, and if no trace of bubbles can be observed 
 the glass is generally regarded as " fine," but it is by no 
 means certain that the absence of bubbles from such a 
 small sample will prove that the whole mass is free ; that, 
 however, is a point where the melter's experience enables 
 him to judge how far he may rely upon the indications 
 
THE PEOCESS OF FUSION. 83 
 
 given by the "proofs." When the glass is "fine" it 
 frequently happens that the surface of the molten mass is 
 contaminated by specks of foreign matter floating on the 
 glass ; for the purpose of removing these, the surface of all 
 glass is skimmed before work is begun upon it. This is 
 done by removing the surface skin of glass by means of 
 suitably shaped iron rods, upon which small masses of 
 molten glass are first "gathered." Finally, it only 
 remains to reduce the temperature of the glass from that 
 of the melting and fining process to the much lower 
 temperature at which the various methods of working the 
 glass are carried out. In pot furnaces this is accomplished 
 by lowering the temperature of the entire furnace, while 
 in tank furnaces the fine glass flows into the working 
 chamber of the tank which is always kept at the working 
 temperature. 
 
 G 2 
 
CHAPTEK VI. 
 
 PKOCESSES USED IN THE WOKKING OF GLASS. 
 
 IN the previous chapter we have followed in outline the 
 process of fusion and fining of glass, leaving the molten 
 material ready for working up into the final shape. Up to 
 that point the process is very similar in all kinds of glass, 
 although the furnaces, pots and utensils employed vary 
 considerably, as do also the temperatures to which the 
 materials are heated at various stages. ' The working 
 processes, however, differ entirely from one class of 
 product to another, as obviously the process employed for 
 the production of a sheet of plate-glass can have little in 
 common with that used in the manufacture of a wine-glass. 
 On the other hand, the modes of working hot glass are not 
 so numerous as the products that are produced, so that we 
 find very similar appliances and manipulation recurring in 
 various branches of the industry. For that reason we 
 propose to deal here with the principal methods of manipu- 
 lating glass, leaving the details of each method as applied 
 to special purposes to be discussed in connection with the 
 special product in question. 
 
 The first stage in the working of all glass is the removal 
 of a suitable quantity of molten glass from the furnace. 
 Practically only three methods are available, viz., ladling, 
 
PEOCESSES USED IN THE WORKING OF GLASS. 85 
 
 pouring and gathering. If we think of a familiar substance 
 of physical properties somewhat resembling those of glass, 
 we may take thick treacle and suppose it confined in a jar 
 or bottle ; there are three obvious ways of extracting it 
 from the bottle : we may ladle it out with a spoon, or we 
 may pour it out by tilting the whole bottle, or we may dip 
 a spoon or fork into the thick liquid, slowly draw it out 
 and turn it round as we do so, thus bringing out on the 
 spoon or fork a round adherent mass or " gathering " of 
 treacle. In the case of molten glass, the process of ladling 
 is by far the simplest, but it has certain very decided 
 limitations and disadvantages. These arise from the fact 
 that a ladle cannot be introduced into molten glass without 
 contaminating the whole mass of glass, at any rate with 
 numerous air bubbles. - The metal of the ladle carries with 
 it a considerable amount of closely adherent air which is 
 partially detached while in contact with the hot glass, so 
 that both the contents of the ladle and the glass remaining 
 in the furnace are contaminated. These bubbles might 
 perhaps be avoided if hot ladles were used, but in that case 
 the glass would adhere to the surface of the metal, and 
 each ladle would require laborious cleaning after each time 
 that it was used. In practice, therefore, ladling is only 
 used for the production of those classes of glass where the 
 presence of a certain number of air-bells is not injurious, 
 and the ladles are kept cold by immersion in water after 
 each time of use. The use of the cold ladle has, however, 
 the further disadvantage that a certain quantity of the 
 glass withdrawn in the ladle is very considerably chilled by 
 contact with the cold metal, and is thus too stiff to undergo 
 the further processes satisfactorily this chilled glass has, 
 
86 GLASS MANUFACTURE. 
 
 therefore, to be rejected from each ladleful ; this not only 
 involves loss of glass, but also necessitates the separation 
 of this spoilt glass from the rest. 
 
 The general process of rolling requires little treatment 
 here. Two essentially different processes are used ; in one 
 the glass is thrown on a flat table and rolled out by a 
 moving roller passing along the table ; in the other the 
 glass passes between two moving rollers, and the sheet so 
 formed is received on a moving table or slab. The former 
 mode of rolling is used for the production of the ordinary 
 rolled plate glass ; if the surface of both table and roller 
 is smooth, the glass also has a comparatively smooth 
 surface, but the surface is far from being level or free from 
 irregularities. It has been found that it is quite impossible 
 to prevent these irregularities, which appear to arise from 
 the buckling of the glass against the iron surfaces with 
 which it comes into contact ; when rolled, the glass is too 
 stiff to recover its true, smooth surface under the influence 
 of surface tension, so that it retains all the marks of roller 
 and table nor can the roller be made perfectly smooth, 
 since in that case it appears to slip over the glass and does 
 not roll it out properly. All efforts, therefore, to produce 
 a glass having a true and smooth surface by direct rolling 
 . have failed, and are likely to fail, so long as tables and 
 rollers are made of materials similar to those now in use. 
 The process of rolling on a stationary table is, however, 
 used for the manufacture of plate-glass ; but here the slab 
 as rolled has still the rough, uneven surface similar to that 
 of ordinary " rolled plate," and this is removed and replaced 
 by a true polished surface by the mechanical processes of 
 grinding and polishing. The second mode of rolling, i.e., 
 
PEOCESSES USED IN THE WOEKING OF GLASS. 87 
 
 with two or more " stationary " rollers and a moving table, 
 is used for the production of rolled plate having special 
 surface features or patterns; the variety of rolled glass 
 known as " figured rolled plate," having a deeply imprinted 
 pattern, is produced in this way. This method requires 
 much more complicated mechanical appliances, some of 
 which are still protected by patent rights. 
 
 Ladling being thus limited to the production of inferior 
 kinds of glass, the better varieties are dependent upon 
 either gathering or pouring. The former process is limited 
 as regards the quantity of glass that can be dealt with in 
 one piece, although surprisingly large quantities can be 
 gathered upon a single pipe; the great masses of glass, 
 however, that are required for the production of modern 
 polished plate could not be handled in this way, and the 
 method of pouring is accordingly adopted. For this purpose 
 either the pots in which the glass has been originally 
 melted, or others specially designed for this purpose, and 
 into which the molten glass has been transferred, are 
 removed bodily from the furnace by the aid of powerful 
 mechanical appliances ; they are then carried by overhead 
 cranes to the place where the glass is to be rolled into the 
 form of a plate, and there the pot is tilted and the molten 
 glass is allowed to run out and to form a pool on the rolling 
 table, the passage of the great roller ultimately rolling the 
 pool out into a sheet much as dough is rolled out with a 
 rolling-pin. This process is obviously only possible with 
 pots or crucibles of a suitable size, and is, moreover, very 
 destructive to these pots, since they are exposed to such 
 great variations of temperature. In the case of tank 
 furnaces, numerous devices have been patented for allowing 
 
88 GLASS MANUFACTtJKE. 
 
 the glass to flow out over a sill or weir of suitable size, 
 ready to be rolled or drawn into the form of sheets or slabs; 
 but none of these devices have, so far as the writer is aware, 
 found their way into practice ; the reason for this probably 
 lies in the fact that it is not easy to find a material which 
 will present a smooth face to the outflowing glass, such 
 materials as fire-clay leading to contamination from 
 detached fragments, while chilled metal leads to local chilling 
 of the glass. Unless, therefore, the various processes of 
 drawing glass into sheets direct from the furnace undergo 
 very material improvement, the laborious process of 
 gathering is likely to retain its importance even in the 
 production of such large objects as sheets of window 
 glass. 
 
 In its essence the process of gathering consists in intro- 
 ducing into the glass a heated iron rod or tube to which a 
 small quantity of glass is allowed to adhere ; rod and glass 
 are removed from the furnace together, and the small 
 adherent ball of glass is allowed to cool so far as to become 
 stiff enough to carry its own weight. The rod with its 
 adherent ball is then again dipped into the glass, where a 
 fresh layer of glass attaches itself to the ball already on 
 the rod. The whole is again withdrawn, allowed to cool 
 down, and then dipped into the molten glass again to 
 gather a fresh quantity. This cycle of operations is repeated 
 until the desired quantity of glass is attached to the rod or 
 tube. These operations, particularly when weights of thirty 
 or forty pounds of glass have to be gathered, require the 
 exercise of a great deal of skill and care ; the introduction 
 of the gathering into the molten glass is each time liable 
 to produce air bells which would spoil the whole mass of 
 
PROCESSES USED IN THE WORKING OF GLASS. 89 
 
 glass or would contaminate the contents of the crucible, 
 while subsequently the mass of hot glass adhering to the 
 rod or pipe tends to run down and even to drop off entirely 
 if not properly checked by suitable rotation of the pipe. 
 Further, the manual labour and exposure to heat involved 
 for the operator all tend to increase the cost of such work. 
 Mechanical aids have been invented, and some of these are 
 in actual use, but they are chiefly confined to mechanism 
 for relieving the operator of the great weight of the gathering 
 in its later stages. 
 
 Just as ladling is nearly always preliminary to rolling, 
 so gathering is usually the preliminary to some blowing 
 process, although the blowing is often combined with and 
 sometimes replaced by the mechanical pressing of the glass. 
 Where the glass is to be blown, the gathering is always 
 made on a glass-maker's pipe. This is an iron tube from 
 4 to 6 ft. long, provided at one end with a wooden casing 
 to serve as a handle, and with a suitably arranged mouth- 
 piece for blowing. The shape of the lower or " butt " end 
 of the pipe depends upon the character and size of the 
 objects to be blown ; for small articles the pipe must be 
 narrow and light, but for heavy sheet-glass the butt of the 
 pipe is extended into a conical mass whose base is from 
 2 to 3 in. in diameter. The bore of the pipe at both ends 
 also depends upon the class of work for which it is intended. 
 The first stage of all blowing processes consists in the 
 formation of a hollow sphere by blowing into the pipe, the 
 pressure of the breath being as a rule sufficient to cause 
 the gradual distension of the hot mass of glass. From this 
 rudimentary hollow sphere the various shapes of blown 
 articles are then evolved by a series of manipulations which 
 
90 GLASS MANUFACTUKE. 
 
 vary very widely in different branches of manufacture. 
 They generally consist, however, in gradually changing the 
 shape of the mass of glass by the pressure either of hand 
 tools or of specially prepared moulds or blocks against 
 which the glass is held or turned, either with or without 
 simultaneous blowing into the pipe. The extent to which 
 the aid of such moulds and blocks is invoked varies 
 continuously from the production of the hand -made vase 
 or glass to the moulded bottle ; in the former, practically 
 only hand tools, whose shape bears no direct resemblance 
 to that of the finished article, are employed, while in the 
 latter the elongated hollow 7 mass of glass is placed inside a 
 mould, and internal air pressure is used to press the glass 
 into contact with the mould from which the shape of the 
 finished bottle is thus directly derived. 
 
 The art of the blower further takes the fullest advantages 
 of the peculiar physical properties of glass while in the 
 heated viscous condition, the material being made to flow 
 under the action of gravity and centrifugal forces, as well 
 as under the pressure of the breath, the glass being held 
 aloft, twirled or swung about to ensure the production of 
 the various shapes required. For the great majority of such 
 purposes the unaided manipulations of the operator are 
 sufficient, but various mechanical aids are used to facilitate 
 the more laborious stages of the work, while for the simpler 
 forms that are required in very great numbers, such as 
 bottles, the whole of the operations are now carried out by 
 automatic machines. Of the more usual mechanical aids 
 at the disposal of the glass-blower, we have already 
 mentioned hand-tools, blocks, and moulds of various kinds. 
 Next in importance to these is the use of compressed air 
 
OF 
 
 PEOCESSES USED IN THE WOKKING OF GLASS. 91 
 
 for blowing large or heavy articles ; the pressure available 
 by the human breath is very limited, and the volume of 
 air that can be thus delivered is not very large, while the 
 constant use of the lungs for such a purpose is trying for 
 the workman. In many works, therefore, air under pres- 
 sure is supplied to the benches or stages where the blowing 
 is done, and the blowers' pipes can be coupled to this air- 
 supply by means of flexible connections when required. 
 The principal difficulty lies in the correct regulation of the 
 air-pressure for each special purpose ; but this difficulty 
 has been overcome by the use of delicate valves under the 
 control of each blower, who can thus regulate the pressure 
 to his own exact requirements. Such a system, of course, 
 requires some little practice on the part of the men using 
 it, but when they have become accustomed to the working 
 of the plant the results achieved are decidedly better and 
 more regular than those obtained by mouth blowing. 
 Besides the use of compressed air supplied in the way just 
 indicated, several other devices are in use to aid the blower 
 in producing the requisite pressure in the interior of the 
 hollow bodies he is producing. The simplest of all these 
 consists in utilising the expansive force of the air enclosed 
 in the hollow body when that body is exposed to heat. 
 Thus, for instance, in blowing a cylinder of sheet-glass, if 
 the blower holds his thumb over the aperture of his pipe, 
 and brings the closed end of the cylinder near the hot 
 "blowing hole," the heat which softens that end of the 
 glass will also act upon the enclosed air, and will very 
 rapidly produce such an expansive effect as to burst open 
 the softened end of the cylinder, and this means of opening 
 the closed ends of the cylinder is frequently employed in 
 
92 GLASS MANUFACTURE. 
 
 practice. It is, of course, obvious that any other expansive 
 fluid might be employed in a similar manner, and in some 
 blowing processes it has long been the practice to introduce 
 a small quantity of water into the interior of the hollow 
 body, when the rapid expansion of the steam produced 
 thereby is utilised for the purpose of generating the 
 requisite internal pressure. This use of the expansive 
 force of steam generated by the heat of the hot glass body 
 has received great development at the hands of Sievert in 
 Germany, whose process is described in Chapter VII. 
 
 Whatever mechanical aids are employed to facilitate the 
 various stages of the process, all glass blowing involves a 
 series of operations requiring considerable skill, while the 
 whole manner of dealing with the glass is essentially 
 extravagant of material, except perhaps in the production 
 of bottles or flasks having narrow mouths. The reason for 
 this latter statement lies in the fact that by blowing it is 
 only possible to produce closed or nearly closed hollow 
 bodies or vessels ; thus a blown wine-glass or tumbler is 
 formed with a hood or dome closing in the open top of 
 the glass, and this hood or dome has subsequently to be 
 removed by subsidiary processes, such as cutting off by the 
 aid of strong local heat or by grinding, and the cut edge 
 has to be provided with a smooth finish. In the case of 
 comparatively small articles like glasses the loss involved 
 from this cause is not so very great, but were large flat 
 bowls or dishes to be produced by blowing, the loss in the 
 dome or covering would be very serious. This difficulty is 
 entirely avoided by the process of pressing glass. We have 
 already indicated the manner in which moulds are used for 
 the production of the desired shape in the case of bottles, 
 
PKOCESSES USED IN THE WOBKING OF GLASS. 93 
 
 etc., but in these cases, where the final object is to be a 
 hollow vessel, the glass is readily forced into contact with 
 the mould by means of internal air or steam pressure ; 
 in the process to which we are now referring, however, the 
 hot glass is forced into contact with the external mould by 
 means of an internal plunger which is forced downward 
 with considerable force. By this means, flat or shallow 
 bodies can be produced without the preliminary formation 
 of a completely closed vessel, while it is obvious that by 
 the use of suitable moulds, complicated and elaborate 
 shapes can be produced. It is true, of course, that 
 pressed articles do not show the same smooth and brilliant 
 surface which is characteristic of the fire-polish of blown 
 articles, while the facility with which elaborate surface 
 ornamentation can be applied by this process has not 
 tended to artistic refinement in design, but the great 
 majority of cheap and useful glass articles of domestic use 
 have been made available by the development of the 
 pressing industry. 
 
 In the ordinary course, pressed glass is produced direct 
 from the molten material, which is introduced into the 
 presses either by gathering or by means of ladles, but for 
 some special purposes glass is brought into its final shape 
 by mechanical pressure after having first been allowed to 
 solidify and having then been specially re-heated to undergo 
 the pressing or moulding process. This is principally done 
 in the case of the best kinds of optical glass, where the 
 molten glass is first allowed to cool in the actual crucible 
 and is then broken up into lumps of a suitable size, from 
 which the more defective portions can be rejected, the 
 more perfect portions only being heated up again in special 
 
94 GLASS MANUEACTUKE. 
 
 kilns and then forced to take the desired shape by being 
 pressed sometimes with hand tools only and sometimes 
 by the aid of powerful presses into moulds of the required 
 shape. Small lenses, however, for which the requirements 
 of quality are not so high are sometimes pressed direct 
 from small gatherings taken from the molten glass in the 
 crucible. 
 
CHAPTEE VII. 
 
 BOTTLE GLASS. 
 
 ALTHOUGH bottles are in some respects the cheapest and 
 crudest products that are manufactured of glass, their uses 
 are so innumerable and their numbers so enormous that 
 their production constitutes a most important branch of 
 the industry. 
 
 In the choice of raw materials for the production of 
 ordinary bottles cheapness is necessarily the first considera- 
 tion. Natural minerals, bye-products of other industries, 
 and the crudest chemicals are utilised so long as it is 
 possible by compounding these ingredients in suitable pro- 
 portions to obtain a glass whose composition meets the 
 somewhat crude requirements which bottles are expected 
 to meet. The most essential of these requirements are 
 that the bottles shall be strong enough to resist the internal 
 pressure which may come upon them when used for the 
 storage of fermented or effervescent liquors as well as the 
 shock of ordinary use, while the glass itself must possess 
 sufficient chemical resistance to remain unattacked by the 
 more or less corrosive liquids which it is called upon to 
 contain. Further, from the point of view of the bottle 
 manufacturer it is desirable that the glass shall be readily 
 fusible, easily worked, and easily annealed. In other 
 
96 GLASS MANUFACTURE. 
 
 branches of glass manufacture increased fusibility is often 
 attained by increasing the alkali contents of the glass, but 
 in bottle making this is inadmissible, both on account of 
 the prohibitive cost of alkali and because an increased 
 alkali content renders the glass more liable to chemical 
 attack. On the other hand, in many varieties of bottle the 
 colour of the glass is nearly, or quite, immaterial so that 
 the introduction of relatively large proportions of iron oxide 
 is permissible. This substance acts as a flux and assists in 
 the production of a fusible, workable glass containing little 
 alkali. Such alkali as bottle glass does contain is fre- 
 quently derived from felspathic minerals, which generally 
 also contain considerable proportions of iron. The use of 
 these minerals also introduces notable proportions of 
 alumina into the glass. In certain classes of bottles, 
 notably those used for special wines, certain shades of 
 colour are required the well-known " Hock bottle " colour 
 being an example. The presence of iron in the glass tends 
 to the production of a green or greenish yellow colour 
 deepening to a black opacity if the quantity of iron be high. 
 The lighter shades of this green tint may be " neutralised " 
 by the introduction of manganese into the glass, the 
 resulting colours ranging from light amber to purple ; 
 nickel oxide is also sometimes used as a colouring material 
 in these glasses. 
 
 In the production of ordinary bottles the continuous 
 tank furnace has now entirely superseded the old pot 
 furnaces, the character of the product being in this case par- 
 ticularly suited to this process of production. The modern 
 bottle-glass tank is generally an oblong basin having one 
 semi-circular end. The flame is often of the " horse-shoe J> 
 
BOTTLE GLASS. 97 
 
 type, the gases both entering and leaving the furnace at 
 the flat or charging end of the furnace. The raw materials 
 are thrown into the furnace at the square end of the tank, 
 and the glass flows uninterruptedly down the furnace to 
 the colder semi-circular end where the working holes are 
 situated. At these points fire-clay rings are kept floating 
 on the glass, and from within these the gatherer takes his 
 gathering, the rings serving to retain the grosser impurities 
 carried down by the glass. The producing power of such 
 a furnace, even when the bottles are blown by hand, is 
 very considerable ; a furnace having ten working holes 
 and containing normally about 85 tons of molten glass 
 will yield some four million bottles per annum, and furnaces 
 of considerably larger capacity are in use. 
 
 The methods of bottle making are at the present time 
 passing through what is probably a stage of transition. 
 Up to the middle of last century the processes in use were 
 little better than those of the middle ages ; the first step of 
 a more modern development of the industry took the direc- 
 tion of improved tools and implements for carrying out the 
 old operations. More recently a whole series of inventions 
 have been put forward with the aim of producing bottles by 
 entirely different and wholly mechanical processes with the 
 object of eliminating the uncertain element of skilled labour 
 entirely. While it must be admitted that some of the 
 earlier of these inventions proved to be brilliantly ingenious 
 failures, there is little doubt that here, as in other manu- 
 facturing processes, the machine-made article will ultimately 
 supersede the hand-made product. Even now, mechanical 
 processes are largely in use both in America and Europe, 
 and at some recent exhibitions machine-made bottles have 
 
 G.M. H 
 
98 GLASS MANUFACTUEE. 
 
 been shown which in every point of quality were superior 
 to the best hand-made goods. 
 
 The first stage in the production of bottles by hand, and 
 also for most of the machine processes, is that of gathering 
 the requisite quantity of glass. The bottle-blower's pipe is 
 between 5 and 6 ft. long, and is provided with a slightly 
 enlarged end or " nose " upon which the glass is gathered. 
 Three gatherings are generally sufficient for the production 
 of ordinary bottles, but for extra large bottles, and especi- 
 ally for carboys, heavier gatherings are necessary, and for 
 these the gatherer must go to the furnace four, five, or even 
 six times. When the requisite quantity of glass has been 
 gathered on the pipe the gathering is worked and rounded 
 by rolling it either on a flat metal plate or "marver," or 
 in a hollowed block made of wood or more rarely of metal ; 
 by this process the glass is formed into a well-rounded, 
 symmetrical pear-shaped body. The blower now distends 
 the mass gradually by the pressure of his breath, at the 
 same time swinging the pipe, the effect of these movements 
 being to draw the bulk of the glass downwards, leaving a 
 thinner and colder portion having the rudimentary shape 
 of the neck of the bottle next to the pipe. In the oldest 
 form of the process the next stage in the production of the 
 bottle is accomplished by the aid of a cylindrical mould of 
 fire-clay, whose diameter is that of the external size of the 
 finished bottle. The pear-shaped bulb of glass is for this 
 purpose re-heated at the melting furnace, and is then placed 
 inside the fire-clay mould. By vigorous blowing, and a 
 rapid rotation of the pipe and glass, the bulb is forced to 
 assume the cylindrical shape of the mould, the glass form- 
 ing the neck of the bottle being at this stage of the process 
 
BOTTLE GLASS. 99 
 
 too cold and stiff to be further deformed. The next step is 
 the formation of the concavity found in the base of wine 
 and beer bottles ; this is produced by pushing up the hot 
 plastic glass that forms the bottom of the bottle as it leaves 
 the clay mould. This is done by a second workman using 
 an iron rod known as the " pontil," upon which a small mass 
 of glass has previously been gathered. This mass of glass 
 remains attached to the bottom of the bottle, which is thus 
 for the moment fastened both to the "pontil " and to the 
 blower's pipe. The blower, however, immediately detaches 
 the bottle from the pipe at the point where the neck of the 
 bottle is intended to end, effecting this by locally chilling 
 the glass a process known by the descriptive term of 
 " wetting off." The unfinished bottle is now attached to 
 and handled by means of the " pontil." The neck is 
 softened by re-heating it over the furnace, and is then 
 moulded into the desired shape by the aid of specially- 
 shaped tongs. Finally a thread of glass is wound round 
 the end of the neck to produce the thickening usually 
 found at that point. The finished bottle, still attached to 
 the "pontil," is now carried to the annealing kiln, where 
 it is placed in position and detached from the " pontil " by 
 a sharp blow, which severs the glass that had been gathered 
 on the " pontil " from the bottom of the bottle. 
 
 The process, in the form described above, has been obso- 
 lete for many years, improvements, consisting of appliances 
 for facilitating the various operations, having been gradually 
 introduced. The most important of these is the substitu- 
 tion of metal moulds for the fire-clay moulds of earlier 
 times. These metallic moulds are made to open and close 
 at will by the action of a pedal, and are designed to give the 
 
 E 2 
 
100 GLASS MANUFACTUKE. 
 
 entire bottle its final shape, except for the indentation of the 
 bottom, although this is sometimes produced by a convex 
 piece placed on the bottom of the mould. In the forma- 
 tion of the neck thickening, also, important mechanical aids 
 have become almost universal. These last consist of tongs 
 provided with rollers and arranged to rotate about an axis 
 that terminates in a tapered spike which enters the neck of 
 the bottle ; by pressing the tongs together so as to bring 
 the rollers against the outside of the neck and rotating the 
 whole, the rollers are made to form the neck thickening in 
 an accurate and rapid manner. 
 
 Important and valuable as these improvements of the 
 ancient process of bottle-blowing undoubtedly are, they do 
 not touch the main disadvantages of the process dis- 
 advantages that seriously affect the economy of the process 
 and the well-being of the workers employed upon it. It is 
 consequently not surprising that a great number of inventors 
 have laboured at the problem of the purely mechanical 
 production of bottles. A large number of patents have 
 accordingly been taken out in connection with bottle-making 
 machinery. The first of these to attain any favour was 
 that devised by Ashley, but although great claims were 
 made for it, its use has not extended. At the present time, 
 however, there are a number of bottle-works actually at 
 work producing bottles by mechanical means ; one of the 
 most successful of these machines is that devised by 
 Boucher, of Cognac. The products of this machine, 
 exhibited in Paris at the exhibition of 1900, were equal, 
 and possibly superior, to the best hand-made bottles. The 
 Boucher machine, although by no means entirely auto- 
 matic, requires no highly-skilled labour beyond that of a 
 
BOTTLE GLASS. 101 
 
 workman whose duty it is to operate the various levers of 
 the machine at the right instant and in the proper order. 
 
 The details of the machine, as set forth in the patents 
 and other published descriptions, are somewhat complicated, 
 and vary somewhat in the different models ; the general 
 principle and mode of operation is, however, the same in all 
 varieties of the machine, and we shall therefore give a brief 
 account of it here. 
 
 In the Boucher process, the glass is first gathered from 
 the furnace, but as no blowing-pipes are required, the 
 gathering is done on a light iron rod, thus saving the 
 gatherer much of the labour of carrying the heavy pipes. 
 The requisite quantity of the glass so gathered is then dropped 
 into the first or "measuring" mould of the machine, the 
 " thread " being cut by hand by the operator. From the 
 measuring mould, the glass is next caused to pass into the 
 "neck" mould; the glass flows into this mould, and is 
 further pressed into it by the aid of compressed air, applied 
 above the free surface of the glass. At this stage the still 
 liquid glass has the external shape of the neck of the bottle, 
 but the mass of glass is solid, i.e., no cavity has yet been 
 produced in it. The formation of the cavity is next begun 
 by the action of a plunger which is driven into the " solid " 
 mass of glass filling the neck mould, this plunger thus 
 punching out the passage through the neck of the bottle. 
 As soon as the plunger is withdrawn, compressed air is 
 admitted into the cavity so formed, and the mass of glass is 
 at the same time inverted, and that part occupying the 
 position of what is to be the shoulder of the bottle is allowed 
 to descend while being blown out by the compressed air. 
 This process of distension is limited, and the desired shape 
 
102 GLASS MANUFACTURE. 
 
 is imparted to the mass by bringing towards it a third 
 mould, by contact with which the glass is considerably 
 stiffened a row of jets of compressed air, impinging on the 
 outside of the glass forming the shoulder of the bottle, being 
 further used to stiffen the glass, once the requisite extension 
 has been attained. The mass has now a shape very similar 
 to that known as a " parason " in hand bottle-blowing, and 
 is by this time decidedly stiff. It is now introduced into 
 the finishing mould and is blown into perfect contact with 
 the mould by powerful air-pressure, thus attaining the 
 proper shape of barrel and base ; the indentation of the 
 base is, however, sometimes produced on a separate machine 
 or press. During all these operations the neck of the 
 bottle, which was the first part to be formed, has remained 
 firmly held in the neck mould, and all the movements that 
 have been described are performed by means of levers 
 actuating movements of this mould as a whole, which, of 
 course, carry the glass with them. The last movement of 
 the levers, which releases the bottle from the finishing 
 mould, also opens the neck mould, and thus leaves the 
 bottle finished and entirely free. 
 
 It will be seen that the process adopted in this machine 
 follows as closely as possible the various stages of hand 
 blowing, but that the mechanical movements of the machine 
 replace the laborious and difficult technique of the blower. 
 One such machine is capable of producing as many as 120 
 bottles, each weighing If Ibs., per hour, but this is accom- 
 plished only by having some of the moulds in duplicate and 
 so arranged as to come into use alternately. The machine 
 itself is attended by one " moulder," who operates the 
 levers, and by a youth' who carries the finished bottles to 
 
BOTTLE GLASS. 103 
 
 the annealing kiln, while, of course, the services of a 
 gatherer are also required. The appearance of a bottle 
 works equipped with these machines is in striking contrast 
 to that of a hand-blowing works, where the stages around 
 the working-holes are crowded with men doing arduous 
 work under very severe conditions of temperature and 
 atmosphere. Finally, it must be pointed out that the use 
 of the Boucher machine is by no means confined to the 
 production of the cheapest kinds of bottles, but that it has 
 shown itself especially well suited to the production of 
 champagne and other bottles that are required to withstand 
 a high internal pressure, the machine-made bottles showing 
 excellent results under pressure tests. The machine is also 
 used for the production of moulded glass-ware of white 
 glass, since it can be adapted to the production of any kind 
 of glass vessel that can be produced by blowing into a 
 mould. 
 
 The. annealing of bottles was formerly carried out in large 
 chambers or kilns of very simple construction, in which 
 the bottles were stacked as made, the kiln being previously 
 heated to the requisite temperature : when full, the kiln 
 was closed up in a rough temporary manner and allowed to 
 cool naturally, thus annealing the bottles stacked within it. 
 In this branch of glass-making also, however, the continuous 
 annealing kiln has superseded the older kinds, and con- 
 tinuous kilns are now almost universal in bottle-making. 
 In these kilns, which consist of long tunnels, kept hot at one 
 end and having a gradually decreasing temperature as the 
 other end is approached, the bottles are stacked on trucks 
 which are slowly drawn through the kiln from the hot to 
 the cold end. At the cold end the trucks are unloaded and 
 
104 GLASS MANtJFACTtMS. 
 
 are then returned, by an outside route, to the charging end, 
 but of course the bottles cannot be stacked on the truck 
 until it has actually entered the hot end of the tunnel and 
 acquired the temperature there prevailing. In a slightly 
 different form of kiln, the bottles are carried down the kiln 
 on a species of conveyer belt formed of iron plates, but the 
 principle of all these appliances is similar even when used 
 for very different kinds of glass. 
 
 In the account of bottle manufacture given above we have 
 referred almost exclusively to the mode of production of the 
 ordinary bottles used for the storage of such liquids as wine, 
 beer, spirits, etc., and we will now deal with some other 
 branches of manufacture closely allied to these. 
 
 An important branch of glass manufacture is the pro- 
 duction of vessels of large dimensions. Those most closely 
 allied to ordinary bottles are the vessels known as carboys, 
 used for the storage and transportation in bulk of chemical 
 liquids, and especially of acids. Formerly these were blown 
 by hand in a manner closely resembling that used for 
 ordinary bottles, but the weight of the mass of glass to be 
 handled by gatherer and blower is very great, while the 
 lung-power of a blower is not sufficient to produce the great 
 expansion required. Formerly the only aid available to 
 the blower was the device of injecting into the hot, hollow 
 glass body, at an early stage of the process, a quantity of 
 water or alcohol ; this liquid was immediately vapourised 
 by the heat of the glass, and if the blower closed the mouth- 
 piece end of his pipe by placing his thumb over it, the expan- 
 sive force of the vapour so generated served to blow out the 
 glass to the desired extent. More recently mechanical 
 aids to the production of these large vessels Lave become 
 
BOTTLE GLASS. 105 
 
 available, first in the shape of mechanical arrangements for 
 relieving the workmen of the full weight of the glass and 
 pipe by providing suitable arms upon which the whole can 
 be supported without interfering with the blower's freedom 
 of manipulating the pipe and glass in the desired way ; 
 further, a supply of compressed air, which can be readily 
 connected with the pipe at any desired moment, facilitates 
 the blowing process. 
 
 A process of producing hollow glass vessels of very large 
 size by purely mechanical means has, however, been intro- 
 duced during recent years by P. Sievert, of Dresden. By 
 the methods of this inventor, glass vessels of quite unprece- 
 dented size such as bath-tubs freely accommodating full- 
 grown men can be produced. For this purpose the glass 
 is spread out on the surface of a large cast-iron plate, pro- 
 vided with numerous small holes through which steam or 
 compressed air may be blown when desired. The slab of 
 viscous glass, when properly spread over this plate, is 
 clamped down against it all around the outside edge by 
 means of a suitably-shaped iron collar, which holds the 
 glass in air-tight contact against the plate beneath. The 
 whole iron plate, with the slab of glass clamped to it, is 
 now turned over, so that the glass hangs down under the 
 plate. The glass immediately begins to sag under its own 
 weight, and is assisted in this tendency by a suitable blowing 
 of steam or air into the space between the plate and the 
 glass. In blowing bath-tubs in this way the glass is 
 allowed to distend downwards until the desired depth is 
 attained, when further distension is arrested by bringing a 
 flat supporting plate under the glass, which is pressed 
 against this flat plate by the pressure of the air, thus 
 
106 GLASS MANUFACTUEE. 
 
 forming the flat bottom of the tub. In this process the 
 outline of the object is determined by the shape of the 
 clamping bars or plate that fix the edges of the hot glass 
 against the iron plate described above, and by this means 
 almost any desired shape can be given to objects of simple 
 form. 
 
 It is obvious that this process can also be employed for 
 blowing a hollow body into contact with a mould of any 
 desired form and forcing the hot glass to take the exact 
 shape of the mould ; for smaller bodies, however, the 
 blowing in of separately generated steam is not required, 
 the heat of the molten glass itself being used to generate the 
 necessary steam. For this purpose the requisite quantity 
 of glass is dropped on the surface of a wet slab of asbestos. 
 On this surface the glass remains floating upon a layer of 
 steam, which is constantly renewed by the intense heating 
 action of the hot glass on the water contained in the asbestos 
 below. The moulds used in this process are provided with 
 a sharp edge or lip, and as soon as the glass has spread 
 into a slab of sufficient size, the inverted mould is brought 
 down upon the glass and pressed against it. The sharp lip 
 or edge of the mould forces the glass into close contact with 
 the asbestos under it all around the edge of the mould, 
 thereby enclosing the space existing between the rest of the 
 glass and the wet asbestos. The heat of the glass continues 
 to generate steam at a rapid rate, but now the steam can 
 no longer escape from under the glass around the edges, 
 and therefore blows the glass upwards into the mould, 
 ultimately forcing the glass into intimate contact with the 
 surface of the mould ; when this is accomplished, the 
 pressure of the steam rises rapidly, and ultimately lifts the 
 
BOTTLE GLASS. 107 
 
 entire mould and glass sufficiently to allow the excess steam 
 to escape and this is the sign that the blowing is complete. 
 The whole process takes only a very few seconds, and is 
 very successful when applied to suitable glass and used with 
 moulds of proper shape. It is, of course, obvious that 
 ordinary narrow-mouthed bottles could not be produced in 
 this way, but wide-mouthed bottles and jars are made in 
 this manner, although the chief utility of the process lies 
 in the production of comparatively shallow articles, which 
 are not of a shape that lends itself to pressing. 
 
CHAPTEK VIII. 
 
 BLOWN AND PRESSED GLASS. 
 
 IN many ways very similar to the processes employed in 
 the production of hottles are those used in the manufacture 
 of all hollow glass vessels that are produced by blowing, 
 either with or without the aid of moulds. Apart from the 
 actual shapes of the articles themselves, however, the 
 principal difference between bottles and the better classes of 
 hollow glass-ware lies in the composition and quality of the 
 glass itself. In this respect all grades of manufacture are 
 to be met with, from the light-coloured greenish or bluish 
 glass used for medicine bottles to the most perfectly 
 colourless and brilliant " crystal " or flint glass. This 
 gradation in the perfection of the glass represents a corre- 
 sponding gradation in the care bestowed upon the choice of 
 raw materials and the various manipulations of melting 
 the glass. As we have seen, for the commonest kinds of 
 bottles, where colour and quality are immaterial, all kinds 
 of fusible materials can be utilised, loamy or ferruginous 
 sands and refuse glass of all kinds being employed. Where 
 somewhat higher requirements have to be met, rather 
 purer sands have to be used as sources of silica, while lime 
 and alkali must be introduced in purer forms, the alkali 
 in the shape of the cheapest qualities of salt-cake and the 
 
BLOWN AND PEESSED GLASS. 109 
 
 lime in that of lime- stones reasonably free from iron and 
 magnesia. Finally, for the best qualities of glass the 
 purest sand obtainable is used, being often specially washed 
 to remove all loamy matter, while the alkali is introduced 
 in the form of carbonate, a chemical product which in its 
 better qualities is practically free from injurious impuri- 
 ties. In these high-class products two very distinct kinds 
 of glass are met with. One class, of which the Bohemian 
 " crystal " is the highest example, is chemically of the 
 nature of an alkali-lime silicate, the alkali in the case of the 
 Bohemian glass being j^otash ; the other variety of glass 
 contains no lime, its place being taken by lead, typical of 
 this class being English flint glass. In some varieties of 
 glass, lead is also replaced, partially or entirely, by barium, 
 but this material is chiefly used for the manufacture of 
 pressed glass. 
 
 The higher grades of quality in glass, which thus require 
 increased refinement in the raw materials, also demand 
 increased refinement in the furnaces and appliances 
 employed in their melting. The tank-furnace, which holds 
 the field in bottle manufacture, is scarcely met with in the 
 production of the better qualities of hollow glass-ware ; 
 medicine bottles and other articles of moderate quality 
 might be produced in tanks, but the quantity of glass 
 required for such purposes is seldom large enough to 
 justify such large plant. For the best qualities of colour- 
 less glass-ware, however, the tank-furnace could not be 
 used on account of the fact that both as regards colour and 
 freedom from defects, the product of a tank-furnace is 
 never equal to the best product of pot-furnaces. For 
 flint-glass, indeed, covered pots or crucibles must be used in 
 
110 GLASS MANUFACTUKE. 
 
 order to adequately protect the molten glass from the reduc- 
 ing action of the furnace gases and from contamination by 
 dust. The materials of which the pots are constructed are 
 also chosen with a view to avoiding all risk of introducing 
 colouring or otherwise injurious impurities from that 
 source. 
 
 In all processes for the production of hollow glass-ware, 
 the glass or " metal " is taken from the pot by the process 
 of gathering which has already been described; where 
 blown articles are to be produced, as distinct from pressed 
 goods, the initial stage is always the formation of a small 
 hollow globe or bulb at the end of the glass-blower's pipe. 
 The subsequent manipulations depend upon the nature of 
 the article to be produced. The article may either be 
 made entirely by hand work, or rather "chair" work, as 
 it is usually called, or the manipulations may be facilitated 
 and the product cheapened while its character is, of course, 
 also modified by the aid of moulds, which are used to 
 bring the object to its proper shape and to impress upon it 
 certain decorative mouldings or markings. As we have 
 already seen, ordinary bottles are now always blown with 
 the aid of moulds, and the same applies to medicine 
 bottles, lamp chimneys, and the bulbs for electric light ; 
 in connection with lamp-chimneys it should be noted that 
 they are blown in moulds in the form of cylindrical bottles 
 with a flat bottom and a domed top, the ends being 
 subsequently cut off. 
 
 Many of the cheaper varieties of tumblers and glasses 
 are also blown in moulds, but they can be, and sometimes 
 are, produced by hand, and as their manufacture is typical 
 of that of all hand-blown hollow ware, we shall now 
 
BLOWN AND PRESSED GLASS. Ill 
 
 describe it in some detail as an example of this class of 
 work. 
 
 The implements used by the glass-blower and his assis- 
 tants for this work are few and simple. The largest item 
 is the glass-blower's bench or chair, which is simply a 
 rough wooden bench provided with two projecting side- 
 rails or arms. When finishing a piece of work the blower 
 sits on this bench, and the pipe lies across the two rails in 
 front of him in such a position that by rolling it backwards 
 and forwards along the rails he can readily keep the pipe in 
 gentle rotation. In addition to the ordinary blower's pipe 
 and a " pontil " or rod for attaching small quantities of 
 glass whereby the piece in hand can be held, the only other 
 tools used by the blower are a number of shears and 
 pincers of various shapes which serve for cutting off, press- 
 ing in, and distending the glass as required, a flat board 
 and a stone or metal plat or " marver " being also used for 
 the purpose of moulding the glass. 
 
 As already indicated, the first step in the production of 
 such an object as a tumbler consists in gathering a suitable 
 quantity of glass on the pipe and blowing it into a small 
 bulb. This bulb is blown out to the proper size and is then 
 elongated by gently swinging the pipe. The next step is 
 the flattening of the lower end of the bulb by gently press- 
 ing it on the " marver " or flat plate provided for such 
 purposes ; in this way the flat bottom of the glass is formed, 
 and the bulb now has the shape of the finished glass, but 
 remains attached to the pipe by a shoulder and neck. The 
 earliest practice was to separate the tumbler from the pipe 
 at such a point as to leave the tumbler of the correct length, 
 the remaining operation consisting in holding the glass, 
 
112 GLASS MANUFACTURE. 
 
 first fixed to a pontil for the purpose, into the furnace so as 
 to heat the broken edge ; this edge was thereby rounded off, 
 and the brim of the glass could be widened or otherwise 
 shaped by rotating the glass or pressing it in or out by the 
 aid of pieces of wood. In modern practice, however, this is 
 not usual, the glass being separated from the pipe well 
 above the shoulder and annealed in this shape. Subse- 
 quently the glass is finished in a trimming room or work- 
 
 FIG. 8. Sectional diagram of the evolution of a tumbler. 
 
 shop by being cut off at the desired point and having the 
 rough edge rounded off by the aid of a blowpipe flame. 
 The cutting-off operation is carried out in a great variety 
 of ways, the most usual being by the action of heat applied 
 locally and suddenly, either by the aid of specially-shaped 
 flat blowpipe flames or by an electrically-heated wire. 
 Machines for carrying out this operation, as well as the 
 subsequent rounding of the edge automatically, are in use, 
 but the latter process is sometimes replaced by slightly 
 grinding and polishing the edges. 
 
BLOWN AND PEESSED GLASS. 113 
 
 The evolution of an ordinary tumbler, as just described, 
 and- as illustrated diagrammatically in Fig. 8, is typical of 
 the whole process of hollow-glass blowing, but of course the 
 number of operations, as well as the care and skill involved 
 in each step, increases rapidly as the form of the vessel 
 becomes more complex ; in the highest class of work a very 
 considerable element of artistic taste and judgment on the 
 part of the operative also becomes essential, for, although 
 the form of the object as well as the choice of colour and 
 ornamentation are chosen by the designer, the blower has 
 to translate the drawing of the designer into glass, and 
 although his skill enables him to attain a considerable 
 degree of fidelity in his rendering, many details remain at 
 his own option, and the proper management of these is no 
 small factor in the success of the whole work. 
 
 In this connection mention should perhaps be made of 
 the application of colour and other decorations to this kind 
 of glass. A very considerable range of effects of this kind 
 is now available to the glass-worker. In the first place the 
 body of the glass used for the production of the articles in 
 question may be coloured by the addition of suitable 
 colouring materials to the molten glass or raw materials, as 
 explained in Chapter XI., but this procedure has very obvious 
 limitations ; where the article is built up of glass from 
 several gatherings as, for example, is the case in an 
 ordinary wine-glass, where the bowl, leg and foot are each 
 made of separate gatherings it is possible to use glass of 
 different colours for these different parts, and this is 
 commonly done in the production of wine glasses having 
 ruby or green bowls and white legs and feet. A further 
 modification in the application of colour is obtainable by 
 G.M. T 
 
114 GLASS MANUFACTURE. 
 
 taking up two or more gatherings on the same pipe and 
 superposing a large gathering of white glass on a smaller 
 one of coloured glass; this is analogous to the process of 
 "flashing" sheet glass, described in Chapter X. and this 
 process lends itself to a variety of manipulations resulting 
 in the distribution of the coloured layer of glass in almost 
 any desired manner over the object in hand. The principal 
 objection to this process, however, lies in the fact that pots 
 of molten glass of all the colours desired must be kept 
 available to the blower at the same time, and this is not 
 easily arranged for in any reasonably economical manner. 
 For this reason, and also because the manipulations are 
 simpler, coloured glass intended for application to blown 
 glass-ware is generally used in the form of short rods 
 previously prepared ; these rods are suitably heated, and the 
 coloured glass can then be applied to the article in hand at 
 any desired place and in as small or large a quantity as 
 required. If the two glasses thus brought into contact are 
 properly related to one another as regards chemical composi- 
 tion and physical properties, they blend very readily and 
 perfectly, and the result is quite as good as could be obtained 
 by using the coloured glass in the molten condition. Other 
 decorations, such as gilding or other metallic lustres and 
 also various kinds of iridescence, are produced upon the 
 finished glass. Metallic lustres are obtained by placing 
 upon the surface of the glass, and slightly fusing into it a 
 layer of particles of the actual metal. In some cases this is 
 done by rolling the glass vessel, while still hot, in a mass 
 of metallic foil of the kind desired, when a sufficient quantity 
 readily adheres ; in other cases the metal is applied in the 
 form of a flux or glaze containing a large proportion of an 
 
BLOWN AND PKESSED GLASS. 115 
 
 easily-reduced compound of the metal, and this is afterwards 
 reduced to the metallic state by the action of heat, some- 
 times aided by that of smoke or other reducing gases. An 
 iridescent surface is produced upon certain varieties of 
 glass by the corrosive action of acid vapours ; in fact, in 
 localities where the atmosphere is tainted with sulphur 
 fumes it is quite usual to see an iridescent lustre on the 
 surface of ordinary window glass. There are, of course, 
 numerous other means of decorating blown and other glass, 
 such as cutting, engraving, etching, silvering, etc., but it would 
 lie beyond the scope of the present volume to deal with these, 
 since they are outside the field of actual glass manufacture. 
 In the production of hollow glass-ware by hand, the glass- 
 blower avails himself to the full of the property so charac- 
 teristic of glass of assuming a pasty or viscous condition 
 when suitably heated ; by raising or lowering the temperature 
 of his material, the blower can at will render it stiffer or 
 more fluid ; by blowing he can distend it, draw it out by 
 the aid of gravity or centrifugal action, or he can mould it 
 with the aid of rods and tongs of suitable shape, while at 
 times he allows it to fall or festoon under its own weight 
 while held aloft. With all these manipulations at his 
 disposal, the skilful operative is able to work the glass to 
 his will and to fashion objects of great variety and beauty, 
 but it should be noted that objects produced by hand in this 
 way will bear the mark of the processes employed in their 
 production in the fact that they do not possess the extreme 
 regularity of size and shape which are associated with 
 machine-made articles ; there is a certain natural variability 
 in the exact shape of curves and festoons that is foreign to 
 tbe products of mechanical processes. For some purposes 
 
 i 2 
 
116 GLASS MANUFACTURE. 
 
 this variability is a disadvantage, while to some minds it 
 appears as a defect, and methods have been devised for 
 facilitating the production of strictly uniform glass-ware by 
 the use of moulds as an aid to the work of the glass-blower. 
 While undoubtedly reducing the value and beauty of the 
 ware from the purely artistic standpoint, these aids to hand- 
 work have rendered possible an immense expansion of the 
 entire industry, since, with the use of moulds, presentable 
 glass-ware can be produced by hands far less skilled than 
 those required for pure hand-work. 
 
 In the description given above of bottle- blowing by hand 
 we have already seen an example of the use of moulds in 
 aiding the blower to form his object to the desired size and 
 shape. Much more complicated and decorative objects can, 
 however, be produced by the use of moulds. Such objects as 
 globes and shades for gas, oil and electric lamps, when of a 
 light substance and suitable shape, are usually produced by 
 blowing bulbs of glass into moulds, where they acquire the 
 general shape as well as the detailed decorated surface con- 
 figuration which they afterwards present. Here again the 
 body remains a closed vessel, and is only opened and 
 trimmed to the final shape at the end of the operation 
 when all the blowing and moulding have been done. 
 Articles blown in this way very frequently show "mould 
 marks," since the contact of the hot glass with the relatively 
 cold surface of the mould results in a certain crinkling or 
 roughening of the glass, much as in the process of rolling. 
 This effect can be minimised by dressing the interior sur- 
 faces of the moulds with suitable greasy dressings, whose 
 chief property should be that they do not stick to the hot 
 glass and leave little or no residue when gradually burnt 
 
BLOWN AND PKESSEB GLASS. 117 
 
 away in the mould ; the proper care of the moulds and 
 their maintenance is in fact the first essential to successful 
 manufacture in this as well as in the pressed-glass industry. 
 Even under the most favourable conditions, however, the 
 surface of glass blown into moulds is not so good as that of 
 hand-blown articles which have never come into contact 
 with cold materials, and therefore retain undiminished the 
 natural " fire polish " which glass possesses when allowed 
 to cool freely from the molten state. An effort at pro- 
 ducing a similar brilliance of surface on moulded and 
 pressed articles is often made by exposing them, after they 
 have attained their final form, to the heat of a furnace to 
 such an extent as to soften the surfaces and allow the glass 
 to re-solidify under the undisturbed influence of surface- 
 tension much as it would do in solidifying freely in the first 
 place. Unfortunately this process cannot be carried out 
 without more or less softening the entire article, so that 
 skilful manipulation is required to prevent serious deforma- 
 tion of the object, while a certain amount of rounding off 
 in all sharp corners and angles cannot be avoided. 
 
 The air-pressure required to bring the whole of the 
 surfaces of a large and possibly complicated piece of glass 
 into contact with the surfaces of the mould is sometimes 
 very considerable, and the lung-power of the blower is often 
 insufficient for the purpose ; in many works, therefore, 
 compressed air is supplied for the purpose, arrangements 
 being employed whereby the operative can quickly connect 
 the mouthpiece of his pipe with the air-main, while he can 
 accurately control the pressure by means of a suitable 
 valve. The Sievert process of moulding by the aid of steam 
 pressure has already been described. 
 
118 GLASS MANUFACTURE. 
 
 Although the evolution of the industry scarcely followed 
 this path, it is not a large step to pass from a process in 
 which air pressure is used to drive viscous glass into 
 contact with a mould to a process in which the pressure of 
 the air is replaced by the pressure of a suitably-shaped 
 solid plunger, and this is essentially the widely-used process 
 of glass pressing. In the first instance this mode of manu- 
 facture is obviously applicable to solid or flat and shallow 
 articles which could not be conveniently evolved from the 
 spherical bulb which stands as embryo of all blown glass ; 
 at first sight it would seem in fact as though the process 
 must be limited to articles of such a shape that a plunger 
 can readily enter and leave the concave portions. By the 
 ingenious device, however, of pressing two halves of a 
 closed or nearly closed vessel simultaneously in two adjacent 
 moulds and then pressing the two halves together while 
 still hot enough to unite, it has been made possible to pro- 
 duce by the press alone such objects as water- jugs, for 
 example, into which a plunger could not possibly be intro- 
 duced when finished. The process of pressing being a 
 purely mechanical one and requiring no very elaborate 
 plant and little skilled labour, has placed upon the market 
 a host of cheap and extremely useful articles, thus serving 
 to widen very considerably the useful applications of glass. 
 On the other hand, the process has been and is still used to 
 some extent for the production of articles intended to 
 imitate the products of other processes such as hand-blown 
 and cut glass, with the result that a great deal of glass has 
 been produced which cannot possibly be classed as beautiful 
 and much of which can lay as little claim to utility. 
 
 The essential feature of the process of glass press- 
 
BLOWN AND PEESSED GLASS. 119 
 
 ing consists, as already indicated, in forcing a layer of 
 glass into contact with a mould by the pressure of a 
 mechanically actuated plunger. For this purpose a, suit- 
 able mould and plunger as well as a press for holding the 
 former and actuating the latter are required. The moulds 
 are generally made of a special quality of close-grained 
 cast-iron, and they are kept trimmed and dressed in much 
 the same manner as the moulds used for blowing (except 
 that the latter are sometimes made of wood). For the 
 purpose of facilitating the removal of the finished article, 
 the moulds are generally made in several pieces which fit 
 into one another and can be separated by means of hinges. 
 A very important point about these moulds is that the 
 various pieces should fit accurately into one another, since 
 otherwise a minute." fin " of glass will be forced into every 
 interstice, and the traces of these fins will always remain 
 visible on the finished article ; the very perfect fit required 
 to entirely prevent the formation of such fins is, of course, 
 scarcely attainable in practice except in the case of new 
 moulds, so that the traces of fins are generally to be found 
 on all pressed articles, anil serve as a ready means of 
 identifying these products when an attempt is made to 
 imitate better classes of glass-ware by their means. The 
 presses used in this process are generally of the hand-lever 
 type ; power presses could no doubt be used, but it is con- 
 tended that the hand-press has a very great advantage in 
 allowing the operator to judge by touch when sufficient 
 pressure has been exerted, and this is an important con- 
 sideration, since an excessive pressure would either force 
 the glass out of the mould altogether or would be liable to 
 burst or injure the mould seriously. The actual presses 
 
120 GLASS MANUFACTUEE. 
 
 consist of vertical guides and levers for controlling the 
 movement of the plunger and a table for holding the 
 moulds, and in some cases a system of cranks and levers 
 for opening and closing the moulds. The process of 
 pressing is exceedingly simple. The proper quantity of 
 glass is gathered from the pot on a solid rod and dropped 
 into the mould. The thread of glass which remains 
 between the glass in the mould and that remaining on the 
 iron is cut off with a pair of shears, and then the plunger is 
 lowered into the mould and allowed to remain there until 
 the glass has stiffened sufficiently to retain its shape, when 
 the plunger is withdrawn. In this proceeding it will be 
 seen that the glass is forced into intimate contact with the 
 relatively cold surfaces of mould and plunger, and while 
 undergoing this treatment the glass must remain sufficiently 
 plastic to readily adapt itself to the configuration of the 
 mould. It is therefore not surprising to find that the 
 pressing process can only be used successfully with glass of 
 a kind specially adapted for it. Certain varieties of flint 
 glass and some barium glasses are used for this purpose, 
 but the greater quantity of pressed glass, particularly as 
 produced on the Continent, is made of a lime-alkali silicate 
 containing considerable quantities of both soda and potash 
 and relatively little lime ; while sufficiently resistant for 
 most purposes, this glass is particularly soft and adaptable 
 while in the viscous condition. 
 
 The deleterious effect produced upon glass surfaces when 
 brought into contact with relatively cold metal has already 
 been referred to above, and it only remains to add that this 
 is the principal difficulty with which the glass-pressing 
 process has to contend. It is overcome to some extent by 
 
BLOWN AND PEESSED GLASS. 121 
 
 the aid of the reheating process described above ; but this 
 is only a partial remedy, and in the majority of pressed 
 glass products the surface is " covered " as far as possible 
 by the application of relief decorations such as grooves, 
 spirals, and ribbings. An attempt is sometimes made to 
 imitate the appearance of cut glass, but the rounding of the 
 angles during the reheating process destroys the sharpness 
 of the effect and allows of the ready detection of the 
 imitation, while the cheapness of the decoration when 
 applied in the mould has frequently led manufacturers to 
 grossly over-decorate, and, therefore, destroy all claim to 
 beauty in their wares. 
 
CHAPTER IX. 
 
 ROLLED OR PLATE-GLASS. 
 
 IN the present chapter we propose to deal with all those 
 processes of glass manufacture in which the first stage 
 consists in converting the glass into a slab or plate by some 
 process of rolling. We have already considered the general 
 character of the rolling process, and have seen that, 
 although hot, viscous glass lends itself readily to being 
 rolled into sheets or slabs, these cannot be turned out with 
 a smooth, flat surface. In practice the surface of rolled glass 
 is always more or less dimmed by contact with the minute 
 irregularities of table or roller, and larger irregularities 
 of the surface arise from the buckling that occurs at a 
 great many places in the sheet. These limitations govern 
 the varieties of glass that can be produced by processes 
 that involve rolling, and have led to the somewhat curious 
 result that both the cheapest and roughest, as well as the 
 best and most expensive kinds of flat glass, are produced 
 by rolling processes. Ordinary rough "rolled plate," such 
 as that used in the skylights of workshops and of railway 
 stations, is the extreme on the one hand, while polished 
 plate-glass represents the other end of the scale. The 
 apparent paradox is, however, solved when it is noted that 
 in the production of polished plate-glass the character of 
 

 ROLLED OE PLATE-GLASS. 123 
 
 the surface of the glass as it leaves the rollers is of very 
 minor importance, since it is entirely obliterated by the 
 subsequent processes of grinding, smoothing, and polishing. 
 Intermediate between the rough " rolled " and the 
 "polished" plate-glass we have a variety of glasses in 
 which the appearance of the rolled surface is hidden or 
 disguised to a greater or lesser extent by the application of 
 a pattern that is impressed upon the glass during the 
 rolling process ; thus we have rolled plate having a ribbed 
 or lozenge-patterned surface, or the well-known variety of 
 ''figured rolled" plate, sometimes known as " Muranese," 
 whose elaborate and deeply-imprinted patterns give a very 
 brilliant effect. 
 
 Eolled plate-glass being practically the roughest and 
 cheapest form of glazing, is principally employed where 
 appearance is not considered, and its chief requirement is, 
 therefore, cheapness, although both the colour and quality 
 of the glass are of importance as affecting the quantity and 
 character of the light which it admits to the building where 
 the glass is used. On the ground of cheapness it will be 
 obvious from what we have said above (Chapter IV.), that 
 such glass can only be produced economically in large tank 
 furnaces, and these are universally used for this purpose. 
 The requirements as regards freedom from enclosed foreign 
 bodies of small size and of enclosed air-bells are not very 
 high in such glass, and, therefore, tanks of very simple 
 form are generally used. No refinements for regulating 
 the temperature of various parts of the furnace in order to 
 ensure perfect fining of the glass are required, and the 
 furnace generally consists simply of an oblong chamber or 
 tank, at one end of which the raw materials are fed in, 
 
124 GLASS MANUFACTUKE. 
 
 while the glass is withdrawn by means of ladles from one 
 or two suitable apertures at the other end. For economical 
 working, however, the furnace must be capable of working 
 at a high temperature, because a cheap glass mixture is 
 necessarily somewhat infusible, at all events where colour 
 is considered. This will be obvious if we remember that 
 the fusibility of a glass depends upon its alkali contents, 
 and alkali is the most expensive constituent of such 
 
 The actual raw materials used in the production of 
 rolled plate-glass are sand, limestone and salt-cake, with 
 the requisite addition of carbon and of fluxing and purifying 
 materials. The selection of these materials is made with 
 a view to the greatest purity and constancy of composition 
 which is available within the strictly- set limits of price 
 which the low value of the finished product entails. These 
 materials are handled in very large quantities, outputs of 
 from 60 to 150 tons of finished glass per week from a 
 single furnace being by no means uncommon ; mechanical 
 means of handling the raw materials and of charging 
 them into the furnace are therefore adopted wherever 
 possible. 
 
 * The glass is withdrawn from the furnace by means of 
 large iron ladles. These ladles are used of varying sizes 
 in such a way as to contain the proper amount of glass to 
 roll to the various sizes of sheets required. The sizes used 
 are sometimes very large, and ladles holding as much as 
 180 to 200 Ibs. of glass are used. These ladles, when filled 
 with glass, are not carried by hand, but are suspended 
 from slings attached to trolleys that run on an overhead 
 rail. The ladler, whose body is protected by a felt apron 
 
EOLLED OE PLATE-GLASS. 125 
 
 and his face by a mask having view-holes glazed with 
 green glass, takes the empty ladle from a water-trough, in 
 which it has been cooled, carries it to the slightly inclined 
 gangway that leads up to the opening in the front of the 
 furnace, and there introduces the ladle into the molten 
 glass, giving it a half-turn so as to fill it with a " solid " 
 mass of glass. By giving the ladle two or three rapid 
 upward jerks, the operator then detaches the glass in the 
 ladle as far as possible from the sheets and threads of 
 glass which would otherwise follow its withdrawal ; then 
 the part of the handle of the ladle near the bowl is placed 
 in the hook attached to the overhead trolley, and by bearing 
 his weight on the other end of the handle, the workman 
 draws the whole ladle up from the molten bath in the 
 furnace and out through the working aperture. This 
 operation only takes a few seconds to perform, but during 
 this time the ladler is exposed to great heat, as a more or 
 less intense flame generally issues from the working aper- 
 ture, whence it is drawn upward under the hood of the 
 furnace. From the furnace opening, the ladler, generally 
 aided by a boy, runs the full ladle to the rolling table and 
 there empties the ladle upon the table just in front of the 
 roller. In doing this, two distinctly different methods are 
 employed. In one, only the perfectly fluid portion of the 
 glass is poured out of the ladle by gradually tilting it, the 
 chilled glass next to the walls of the ladle being retained 
 there and ultimately returned to the furnace while still hot. 
 In the other method, the chilling of the glass is minimised 
 as far as possible, and the entire contents of the ladle are 
 emptied upon the rolling table by the ladler, who turns the 
 entire ladle over with a rapid jerk which is so arranged 
 
126 
 
 GLASS MANUFACTUEE. 
 
 as to throw the coldest part of the glass well away from the 
 rest. When the sheet is subsequently rolled this chilled 
 portion is readily recognised by its darker colour, and since 
 it lies entirely at one end of the sheet it is detached before 
 the sheet goes any further. Neither method appears to 
 present any preponderating advantage. 
 
 The rolling table used in the manufacture of rolled plate 
 is essentially a cast-iron slab of sufficient size to accommo- 
 
 GUIDES 
 
 VlM 
 
 Q O 
 
 SECTIONAL ELEVATION. 
 
 GUIDES 
 
 TABLE 
 MOLTEN GLASS 
 
 DIRECTION OF 
 ROLLING 
 
 PLAN. 
 
 FIG. 9. Boiling table for rolled plate-glass. 
 
 date the largest sheet which is to be rolled ; over this slab 
 moves a massive iron roller which may be actuated either 
 by hand or by mechanical power the latter, however, 
 being now almost universal. The thickness of the sheet to 
 be rolled is regulated by means of slips of iron placed at 
 the sides of the table in such a way as to prevent the 
 roller from descending any further towards the surface of 
 the table : so long as the layer of glass is thicker than 
 these slips, the entire weight of the roller comes upon the 
 
EOLLED OE PLATE-GLASS. 127 
 
 soft glass and presses it down, but as soon as the required 
 thickness is attained, the weight of the roller is taken by 
 the iron slips and the glass is not further reduced in thick- 
 ness. The width of the sheet is regulated by means of a 
 pair of iron guides, formed to fit the forward face of the 
 roller and the surface of the table, in the manner indicated 
 in Fig. 9. The roller, as it moves forward, pushes these 
 guides before it, and the glass is confined between them. 
 When the roller has passed over the glass, the sheet is left 
 on the iron table in a red-hot, soft condition, and it must 
 be allowed to cool and harden to a certain extent before it 
 can be safely moved. In this interval, the chilled portion 
 if any is partially severed by an incision made in the 
 sheet by means of a long iron implement somewhat like 
 a large knife, and then the sheet is loosened from the bed 
 of the table by passing under it, with a smooth rapid stroke, 
 a flat-bladed iron tool. The sheet is next removed to 
 the annealing kiln or " lear," being first drawn on to a stone 
 slab and thence pushed into the mouth of the kiln. At 
 this stage the chilled portion of the sheet is completely 
 severed by a blow which causes the glass to break along the 
 incision previously made. 
 
 The rolled-plate annealing kiln is essentially a long, low 
 tunnel, kept hot at one end, where the freshly-rolled sheets 
 are introduced, and cold at the other end, the temperature 
 decreasing uniformly down the length of the tunnel. The 
 sheets pass down this tunnel at a slow rate, and are thus 
 gradually cooled and annealed sufficiently to undergo the 
 necessary operations of cutting, etc. Although thus simple 
 in principle, the proper design and working of these "lears " 
 is by no means simple or easy, since success depends upon the 
 
128 GLASS MANUFACTUEE. 
 
 correct adjustment of temperatures throughout the length 
 of the tunnel and a proper rate of movement of the sheets, 
 while the manner of handling and supporting the sheets 
 is vital to their remaining flat and unbroken. The actual 
 movement of the sheets is effected by a system of moving 
 grids which run longitudinally down the tunnel. The 
 sheets ordinarily lie flat upon the stone slabs that form the 
 floor of the tunnel, and the grids are lowered into 
 recesses cut to receive them. At regular intervals the iron 
 grid bars are raised just sufficiently to lift the sheets from 
 the bed of the kiln, and are then moved longitudinally a 
 short distance, carrying the sheets forward with them and 
 immediately afterwards again depositing them on the stone 
 bed. The grids return to their former position while 
 lowered into their recesses below the level of the kiln bed. 
 
 When they emerge from the annealing kiln or " lear " 
 the sheets of rolled plate-glass are carried to the cutting 
 and sorting room. Here the sheets are trimmed and cut 
 to size. The edges of the sheets as they leave the rolling 
 table are somewhat irregular, and sometimes a little 
 " beaded," while the ends are always very irregular. Ends 
 and edges are therefore cut square or " trimmed " by the 
 aid of the cutting diamond. For this purpose the sheet is 
 laid upon a flat table, the smoothest side of the sheet being 
 placed upwards, and long cuts are taken with a diamond 
 good diamonds of adequate size and skilful operators being 
 necessary to ensure good cutting on such thick glass over 
 long lengths. Strips of glass six or eight feet long and 
 half an inch wide are frequently detached in the course of 
 this operation, and the final separation is aided by slight 
 tapping of the underside of the glass just below the cut 
 
EOLLED OK PLATE-GLASS. 129 
 
 and if necessary by breaking the strip off by the aid of 
 suitable tongs. 
 
 No very elaborate " sorting " of rolled plate glass is 
 required, except perhaps that the shade of colour in the 
 glass may vary slightly from time to time, and it is 
 generally preferable to keep to one shade of glass in filling 
 any particular order. Apart from this, the rolled plate 
 cutter has merely to cut out gross defects which would 
 interfere too seriously with the usefulness of the glass. 
 As we have already indicated, air-bells and minute 
 enclosures of opaque matter are not objectionable in 
 this kind of glass, but large pieces of opaque material 
 must generally be cut out and rejected, not only because 
 they are too unsightly to pass even for rough glazing 
 purposes, but also because they entail a considerable 
 risk of spontaneous cracking of the glass in fact, visible 
 cracks are nearly always seen around large "stones," as 
 these inclusions are called. These may arise from various 
 causes, such as incomplete melting of the raw materials, 
 or the contamination of the raw materials with infusible 
 impurities, but the most fruitful source of trouble in this 
 direction lies in the crumbling of the furnace lining, which 
 introduces small lumps of partially melted fire-clay into the 
 glass. In a rolled plate tank furnace which is properly con- 
 structed and worked, the percentage of sheets which have to 
 be cut up on account of such enclosures should be very 
 small, at all events until the furnace is old, when the linings 
 naturally show an increasing tendency to disintegrate. 
 
 Keturning now to the rolling process, it is readily seen 
 that a very slight modification will result in the production 
 of rolled plate-glass having a pattern impressed upon one 
 
 G.M. K 
 
130 GLASS MANUEACTUKE. 
 
 surface ; this modification consists in engraving upon the 
 cast-iron plate of the rolling table in intaglio any pattern 
 that is to appear upon the glass in relief. As a matter of 
 fact only very simple patterns are produced in this way, 
 such as close parallel longitudinal ribbing and a lozenge- 
 pattern, the reason probably being that the cost of cutting 
 an elaborate pattern over the large area of the bed-plate of 
 
 
 FIG. 10. Sectional diagram of machine for rolling "figured rolled" 
 
 plate-glass. 
 
 one of these tables would be very considerable. Further, 
 as these tables and their bed-plates are so very heavy, they 
 are not readily interchanged or left standing idle, so that 
 only patterns required in very great quantity could be 
 profitably produced in this way. These disadvantages are, 
 however, largely overcome by the double-rolling machine. 
 In this machine, into whose rather elaborate details we 
 cannot enter here, the glass is rolled out into a sheet of 
 the desired size and thickness by being passed between 
 two rollers revolving about stationary axes, the finished 
 sheet emerging over another roller, and passing on 
 
EOLLED OE PLATE-GLASS. 131 
 
 to a stone slab that moves forward at the same rate 
 as the sheet is fed down upon it. In this machine a 
 pattern can be readily imprinted upon the soft sheet as 
 it passes over the last roller by means of a fourth roller, 
 upon which the pattern is engraved ; this is pressed down 
 upon the sheet, and leaves upon it a clear, sharp and deep 
 impress of its pattern. The general arrangement of the 
 rollers in this machine is shown in the diagram of Fig. 10, 
 which represents the sectional elevation of the appliance. 
 After leaving the rolling machine, the course of the 
 " figured rolled plate " produced in this manner is exactly 
 similar to that of ordinary rolled plate, except that as a 
 somewhat softer kind of glass is generally used for "figured," 
 the temperature of the annealing kilns requires somewhat 
 different adjustment. The cutting of the glass also requires 
 rather more care, and it should be noted that such glass 
 can only be cut with a diamond on the smooth side ; the 
 side upon which the pattern has been impressed in relief 
 cannot be materially affected by a diamond. This is one 
 reason why it is not feasible to produce such glass with a 
 pattern on both sides. 
 
 Figured rolled glass, being essentially of an ornamental 
 or decorative nature, is generally produced in either 
 brilliantly white glass or in special tints and colours, and 
 the mixtures used for attaining these are, of course, the 
 trade property of the various manufacturers ; the whiteness 
 of the glass, however, is only obtainable by the use of very 
 pure and, therefore, expensive materials. As regards the 
 coloured plate-glasses, a general account of the principles 
 underlying the production of coloured glass will be found 
 in Chapter XI. 
 
 K 2 
 
132 GLASS MANUFACTUEE. 
 
 The manufacture of polished plate-glass really stands 
 somewhat by itself, almost the only feature which it has 
 in common with the branches of manufacture just described 
 being the initial rolling process. 
 
 The raw materials for the production of plate-glass are 
 chosen with the greatest possible care to ensure purity and 
 regularity ; owing to the very considerable thickness of 
 glass which is sometimes employed in plate, and also to 
 the linear dimensions of the sheets which allow of numerous 
 internal reflections, the colour of the glass would become 
 unpleasantly obtrusive if the shade were at all pronounced. 
 The actual raw materials used vary somewhat from one 
 works to another ; but, as a rule, -they consist of sand, 
 limestone, and salt-cake, with some soda-ash and the usual 
 additions of fluxing and purifying material such as arsenic, 
 manganese, etc. The glass is generally melted in pots, 
 and extreme care is required to ensure perfect melting and 
 fining, since very minute defects are readily visible in this 
 glass when finished, and, of course, detract most seriously 
 from its value. 
 
 The method of transferring the glass from the melting- 
 pot to the rolling table differs somewhat in different works. 
 In many cases the melting-pots themselves are taken bodily 
 from the furnace and emptied upon the bed-plate of the 
 rolling machine, while in other cases the glass is first 
 transferred to smaller " casting" pots, where it has to be 
 heated again until it has freed itself from the bubbles 
 enclosed during the transference, and then these smaller 
 pots are used for pouring the glass upon the rolling slab. 
 The advantage of the latter more complicated method lies, 
 no doubt, in the fact that the large melting-pots, which 
 
EOLLEI) OE PLATE-GLASS. 133 
 
 have to bear the brunt of the heat and chemical action 
 during the early stages of melting, are not exposed to the 
 great additional strain of being taken from the hot furnace 
 and exposed for some time to the cold outside air. Apart 
 from the mechanical risks of fracture, this treatment 
 exposes the pots to grave risks of breakage from unequal 
 expansion and contraction on account of the great differences 
 of temperature involved. Where smaller special casting- 
 pots are used, these are not exposed to such prolonged 
 heat in the furnace, and are never exposed to the chemical 
 action of the raw materials, so that these subsidiary pots 
 may perhaps be made of a material better adapted to with- 
 stand sudden changes of temperature than the high-class 
 fire-clay which must be used in the construction of melting 
 pots. On the other hand, the transference of the glass 
 from the melting to the casting-pots involves a laborious 
 operation of ladling and the refining of the glass, with its 
 attendant expenditure of time and fuel. Finally, the 
 production of plate-glass in tank furnaces could only be 
 attempted by the aid of such casting-pots in which the 
 glass would have to undergo a second fining after being 
 ladled from the tank, and this would materially lessen the 
 economy of the tank for this purpose, while it is by no 
 means an easy matter to produce in tank furnaces qualities 
 of glass equal as regards colour and purity to the best 
 products of the pot-furnace. 
 
 The withdrawal of the pots containing the molten glass 
 from the furnace is now universally carried out by powerful 
 machinery. The pots are provided on their outer surface 
 with projections by which they can be held in suitably- 
 shaped tongs or cradles. A part of the furnace wall, which 
 
134 GLASS MANUFACTUEE. 
 
 is constructed each time in a temporary manner, is broken 
 down; the pot is raised from the bed or "siege" of the 
 furnace by the aid of levers, and is then bodily lifted out 
 by means of a powerful fork. The pot is then lifted and 
 carried by means of cranes until it is in position above the 
 rolling table ; there the pot is tilted and the glass poured 
 out in a steady stream upon the table, care being taken to 
 avoid the inclusion of air-bells in the mass during the 
 process of pouring. When empty, the pot is returned to 
 the furnace as rapidly as possible, the glass being mean- 
 while rolled out into a slab by the machine. Except for 
 the greater size and weight of both table and roller, the 
 plate-glass rolling table is similar to that already described 
 in connection with rolled plate. Of course, since the glass 
 is poured direct from the pot, there is no chilled glass to 
 be removed. Further, owing to the large size of sheets 
 frequently required, the bed of the rolling table cannot be 
 made of a single slab of cast-iron, a number of carefully 
 jointed plates being, in fact, preferable, as they are less 
 liable to warp under the action of the hot glass. 
 
 In arranging the whole of the rolling plant, the chief 
 consideration to be kept in mind is that it is necessary to 
 produce a flat sheet of glass of as nearly as possible equal 
 thickness all over. The final thickness of the whole slab 
 when ground and polished into a sheet of plate-glass must 
 necessarily be slightly less than that of the thinnest part 
 of the rough rolled sheet. If, therefore, there are any 
 considerable variations of thickness, the result will be that 
 in some parts of the sheet a considerable thickness of glass 
 will have to be removed during the grinding process. This 
 will arise to a still more serious extent if the sheet as a 
 
EOLLED OE PLATE-GLASS. 135 
 
 whole should be bent or warped so as to depart materially 
 from flatness. The two cases are illustrated diagram- 
 matically in Fig. 11, which shows sectional views of the 
 sheets before and after grinding on an exaggerated scale. 
 
 While it is evident that careful design of the rolling table 
 will avoid all tendency to the formation of sheets of such 
 undesirable form, it is a much more difficult matter to 
 avoid all distortion of the sheet during the annealing pro- 
 
 
 FINIS HE D PLATE 
 
 GLASS GROUND AWAY 
 PLATE CAST WITH IRREGULAR SURFACE. 
 
 WASTE GLASS 
 FINISHED PLATE 
 
 WASTE GLASS 
 WARPED OR CURVED PLATE 
 
 FIG. 11. Sectional diagram illustrating waste of glass in grinding 
 curved or irregular plate. 
 
 cess and while the sheet is being moved from the rolling- 
 table to the annealing kiln. Owing to the great size of the 
 slabs of glass to be dealt with, and still more to the 
 stringent requirement of flatness, the continuous annealing 
 kiln, in which the glass travels slowly down a tunnel from 
 the hot to the cold end, has not been adopted for the 
 annealing of plate-glass, and a form of annealing kiln is 
 still used for that glass which is similar in its mode of 
 operation to the old-fashioned kilns that were used for other 
 
136 GLASS MANUFACTUBE. 
 
 kinds of glass before the continuous kiln was introduced. 
 These kilns simply consist of chambers in which the hot 
 glass is sealed up and allowed to cool slowly and uniformly 
 during a more or less protracted period. In the case of 
 plate-glass, the slabs are laid flat on the stone bed of the 
 kiln. This stone bed is built up of carefully dressed stone, 
 or blocks of fire-brick bedded in sand in such a way that they 
 can expand freely laterally without causing any tendency for 
 the floor to buckle upwards as it would do if the blocks were 
 set firmly against one another. The whole chamber is 
 previously heated to the requisite temperature at which the 
 glass still shows a very slight plasticity. The hot glass 
 slabs from the rolling table are laid upon the bed of this 
 kiln, several being usually placed side by side in the one 
 chamber, and the slabs in the course of the first few hours 
 settle down to the contour of the bed of the kiln, from 
 which shape and position they are never disturbed until 
 they are removed when quite cold. In modern practice 
 the cooling of a kiln is allowed to occupy from four to five 
 days ; even this rate of cooling is only permissible if care 
 is taken to provide for the even cooling of all parts of the 
 kiln, and for this purpose special air-passages are built 
 into the walls of the chamber and beneath the bed upon 
 which the glass rests, and air circulation is admitted to 
 these in such a way as to allow the whole of the kiln to cool 
 down at the same rate ; in the absence of such special 
 arrangements, the upper parts of the kiln would probably 
 cool much more rapidly than the base, so that the glass 
 would be much warmer on its under than on its upper surface. 
 When the slabs of plate-glass are removed from the 
 annealing kilns they very closely resemble sheets of rolled 
 
EOLLED OE PLATE-GLASS. 137 
 
 plate in appearance, and they are quite sufficiently trans- 
 parent to allow of examination and the rejection of the 
 more grossly defective portions ; the more minute defects, 
 of course, can only be detected after the sheets have been 
 polished, but this preliminary examination saves the 
 laborious polishing of much useless glass. 
 
 The process of grinding and polishing plate-glass consists 
 of three principal stages. In the first stage the surfaces of 
 the glass are ground so as to be as perfectly flat and parallel 
 as possible ; in order to effect this object as rapidly as possible, 
 a coarse abrasive is used which leaves the glass with a 
 rough grey surface. In the second stage, that of smoothing, 
 these rough grey surfaces are ground down with several 
 grades of successively finer abrasive until finally an 
 exceedingly smooth grey surface is left. In the third and 
 final stage, the smooth grey surface is converted into the 
 brilliant polished surface with which we are familiar by 
 the action of a polishing medium. 
 
 Originally the various stages of the grinding and polish- 
 ing processes were carried out by hand, but a whole series 
 of ingenious machines has been produced for effecting the 
 same purpose more rapidly and more perfectly than hand- 
 labour could ever do. We cannot hope to give any 
 detailed account of the various systems of grinding and 
 polishing machines which are even novv in use, but must 
 content ourselves with a survey of some of the more impor- 
 tant considerations governing the design and construction 
 of such machinery. 
 
 In the first place, before vigorous mechanical work can 
 be applied to the surface of a plate of glass, that plate must 
 be firmly fixed in a definite position relatively to the rest 
 
138 GLASS MANUFACTUKE. 
 
 of the machinery, and such firm fixing of a plate of glass is 
 by no means readily attained, since the plate must be 
 supported over its whole area if local fracture is to be 
 avoided. While the surface of the plate is in the uneven 
 condition in which it leaves the rolling-table, such a firm 
 setting of the glass can only be attained by bedding it in 
 plaster, and this must be done in such a manner as to 
 avoid .the formation of air-bubbles between plaster and 
 glass ; if bubbles are allowed to form, they constitute places 
 where the glass is unsupported. During the grinding and 
 polishing processes these unsupported places yield to the 
 heavy pressure that comes upon them, and irregularities in 
 the finished polished surfaces result. The most perfect 
 adhesion between glass and plaster is attained by spreading 
 the paste of plaster on the up-turned surface of the slab of 
 glass and lowering the iron bed-plate of the grinding table 
 down upon it, the bed-plate with the adhering slab of glass 
 being afterwards turned over and brought into position in 
 the grinding machine. When one side of the glass has 
 been polished, it is generally found sufficient to lay the 
 slab down on a bed of damp cloth, to which it adheres very 
 firmly, although sliding is entirely prevented by a few 
 blocks fixed to the table in such a way as to abut against 
 the edges of the sheet. In many works, however, the glass 
 is set in plaster for the grinding and polishing of the 
 second side as well as of the first. 
 
 The process of grinding and polishing is still regarded 
 in many plate-glass works as consisting of three distinct 
 processes, known as rough grinding, smoothing and polish- 
 ing respectively. Formerly these three stages of the process 
 were carried out separately ; at first by hand, and later by 
 
KOLLED OR PLATE-GLASS. 139 
 
 three different machines. In the most modern practice, 
 however, the rough and smooth grinding are done on the 
 same machine, the only change required being the substi- 
 tution of a finer grade of abrasive at each step for the 
 coarser grade used in the previous stage. For the polish- 
 ing process, however, the rubbing implements themselves 
 must be of a different kind, for while the grinding and 
 smoothing is generally done by means of cast-iron rubbers 
 moving over the glass, the polishing is done with felt pads. 
 The table of the machine, to which the glass under treat- 
 ment is attached, is therefore made movable, and when the 
 grinding and smoothing processes are complete, the table 
 with its attached glass is moved so as to come beneath a 
 superstructure carrying the polishing rubbers, and the 
 whole is then elevated so as to allow the rubbers to bear on 
 the glass. 
 
 The earliest forms of grinding machines gave a reciprocal 
 motion to the table which carries the glass, or the grinding 
 rubbers were moved backward and forward over the 
 stationary table. Kotary machines, however, were intro- 
 duced and rapidly asserted their superiority, until, at the 
 present time, practically all plate-glass is ground on rotating 
 tables, some of these attaining a diameter of over 30 ft. 
 The grinding "rubbers " consist of heavy iron slabs, or of 
 wood boxes shod with iron, but of much smaller diameter 
 than the grinding table. The rubbers themselves are 
 rotary, being caused to rotate either by the frictional drive 
 of the rotating table below them, or by the action of inde- 
 pendent driving mechanism, but the design of the motions 
 must be so arranged that the relative motion of rubber and 
 glass shall be approximately the same at all parts of the 
 
140 GLASS MANUFACTIJEE. 
 
 glass sheets, otherwise curved instead of plane surfaces 
 would be formed. This condition can be met by placing 
 the axes of the rubbers at suitable points on the diameter 
 of the table. The abrasive is fed on to the glass in the 
 form of a thin paste, and when each grade or " course " 
 has done the work required of it, the whole table is washed 
 down thoroughly with water and then the next finer grade 
 is applied. The function of the first or coarsest grade is 
 simply to remove the surface irregularities and to form a 
 rough but plane surface. The abrasive ordinarily employed 
 is sharp sand, but only comparatively light pressure can 
 be applied, especially at the beginning of this stage, since 
 at that period the weight of the rubber is at times borne by 
 relatively small areas of glass that project here and there 
 above the general level of the slab. As these are ground 
 away, the rubbers take a larger and more uniform bearing, 
 and greater pressure can be applied. The subsequent 
 courses of finer abrasives are only required to remove the 
 coarse pittings left in the surface by the action of the first 
 rough grinding sand ; the finer abrasive replaces the deep 
 pits of the former grade by shallower pits, and this is 
 carried on in a number of steps until a very smooth 
 ''grey" surface is attained and the smoothing process is 
 complete. The revolving table or " platform " is now 
 detached from the driving mechanism, and moved along 
 suitably placed rails on wheels provided for that purpose, 
 until it stands below the polishing mechanism. Here it is 
 attached to a fresh driving mechanism, and it is then either 
 raised so as to bring the glass into contact with the felt- 
 covered polishing rubbers, or the latter are lowered 
 down upon the glass. The polishing rubbers are large 
 
EOLLED OE PLATE-GLASS. 141 
 
 felt-covered slabs of wood or iron which are pressed against 
 the glass with considerable force ; their movement is very 
 similar to that of the grinding rubbers, but in place of an 
 abrasive they are supplied with a thin paste of rouge and 
 water. The time required for the polishing process 
 depends upon the perfection of the smoothing that has 
 been attained ; in favourable cases two or three hours are 
 sufficient to convert the " grey " surface into a perfectly 
 polished one ; where, however, somewhat deeper pits have 
 been left in the glass, the time required for polishing may 
 be much longer, and the polish attained will not be so 
 perfect. The mode of action of a polishing medium such as 
 rouge is now recognised to be totally different in character 
 from that of even the finest abrasive ; the grains of the 
 abrasive act by their hardness and the sharpness of their 
 edges, chipping away tiny particles of the glass, so that the 
 glass steadily loses weight during the grinding and smooth- 
 ing processes. During the polishing process, however, 
 there is little or no further loss of weight, the glass forming 
 the hills or highest parts of the minutely pitted surface 
 being dragged or smeared over the surface in such a way 
 as to gradually fill up the pits and hollows. The part 
 played by the polishing medium is probably partly chemical 
 and partly physical, but it results, together with the 
 pressure of the rubber, in giving to the surface molecules 
 of the glass a certain amount of freedom of movement, 
 similar to that of the molecules of a viscid liquid; the 
 surface layers of glass are thus enabled to " flow " under 
 the action of the polisher and to smooth out the surface to 
 the beautiful level smoothness which is so characteristic of 
 the surfaces of liquids at rest. This explanation of the 
 
142 GLASS MANUFACTURE. 
 
 polishing process enables us to understand why the proper 
 consistency of the polishing paste, as well as the proper 
 adjustment of the speed and pressure of the rubbers, plays 
 such an important part in successful polishing ; it also 
 serves to explain the well-known fact that rapid polishing 
 only takes place when the glass surface has begun to be 
 perceptibly heated by the friction spent upon it. 
 
 It has been estimated that, on the average, slabs of plate- 
 glass lose one-third of their original weight in the grinding 
 and polishing processes, and it is obvious that the erosion 
 of this great weight of glass must absorb a great amount of 
 mechanical energy, while the cost of the plant and upkeep is 
 proportionately great. Every factor that tends to dimmish 
 either the total weight of glass to be removed per square yard 
 of finished plate, or reduces the cost of removal, must be of the 
 utmost importance in this manufacture. The flatness of the 
 plates as they leave the annealing kiln has already been 
 referred to, and the reason why the processes of grinding and 
 polishing have formed the subject for innumerable patents 
 will now be apparent. The very large expansion of the use 
 of plate-glass in modern building construction, together 
 with the steady reduction in the prices of plate, are evidence 
 of the success that has attended the efforts of inventors 
 and manufacturers in this direction. 
 
 At the present time, plate-glass is manufactured in very 
 large sheets, measuring up to 26 ft. in length by 14 ft. 
 in width, and in thickness varying from T %th of an inch 
 up to 1J in., or more, for special purposes. At the 
 same time the quality of the glass is far higher to-day than 
 it was at earlier times. This high quality chiefly results 
 from more careful choice of raw materials and greater 
 
EOLLED OE PLATE-GLASS. 143 
 
 freedom from the defects arising during the melting and 
 refining processes, while a rigid process of inspection is 
 applied to the glass as it comes from the polishing machines. 
 For this purpose the sheets are examined in a darkened room 
 by the aid of a lamp placed in such a way that its oblique 
 rays reveal every minute imperfection of the glass ; these 
 imperfections are marked with chalk, and the plate is 
 subsequently cut up so as to avoid the defects that have 
 thus been detected. 
 
 Perhaps the most remarkable fact about the quality of 
 modern plate-glass is its relatively high degree of homo- 
 geneity. Glass, as we have seen in Chapter L, is not a 
 chemically homogeneous substance, but rather a mixture of 
 a number of substances of different density and viscosity. 
 Wherever this mixture is not sufficiently intimate, the 
 presence of diverse constituents becomes apparent in the 
 form of striae, arising from the refraction or bending of 
 light-rays as they pass from one medium into another of 
 different density. Except in glass that has undergone 
 elaborate stirring processes, such striae are never absent, 
 but the skill of the glass-maker consists in making 
 them as few and as minute as possible, and causing them 
 to assume directions and positions in which they shall 
 be as inconspicuous as possible. In plate-glass this is 
 generally secured in a very perfect manner, and to ordinary 
 observation no striae are visible when a piece of plate-glass 
 is looked at in the ordinary way, i.e., through its smallest 
 thickness ; if the same piece of glass be looked at trans- 
 versely, the edges having first been polished in such a way 
 as to render this possible, the glass will be seen to be full of 
 striae, generally running in fine lines parallel with the 
 
144 GLASS MANUFACTURE. 
 
 polished surfaces of the glass. This uniform direction of 
 the striae is partly derived from the fact that the glass has 
 been caused to flow in this direction by the action of the 
 roller when first formed into a slab, but this process would 
 not obliterate any serious inequalities of density which 
 might exist in the glass as it leaves the pot, so that success- 
 ful results are only attainable if great care is taken to 
 secure the greatest possible homogeneity in the glass 
 during the melting process. - 
 
 At the present time probably the greater bulk of plate- 
 glass is used for the purpose of glazing windows of various 
 kinds, principally the show windows of shops, etc. As used 
 for this purpose the glass is finished when polished and cut 
 to size. The only further manipulation tbat is sometimes 
 required is that of bending the glass to some desired 
 curvature, examples of bent plate-glass window-panes being 
 very frequently seen. This bending is carried out on the 
 finished glass, i.e., after it has been polished ; the glass is 
 carefully heated in a special furnace until softened, and is 
 then gently made to lie against a stone or metal mould 
 which has been provided with the desired curvature. It is 
 obvious that during this operation there are great risks of 
 spoiling the glass ; roughening of the surface by contact 
 with irregular surfaces on either the mould, the floor of 
 the kiln, or the implements used in handling the glass, can 
 only be avoided by the exercise of much skill and care, 
 while all dust must also be excluded since any particles 
 settling on the surface of the hot glass would be " burnt in," 
 and could not afterwards be detached. Small defects can, 
 of course, be subsequently removed by local hand-polishing, 
 and this operation is nearly always resorted to where 
 
ROLLED OR PLATE-GLASS. 145 
 
 polished glass has to undergo fire-treatment for the purpose 
 of bending. 
 
 In addition to its use for glazing in the ordinary sense, 
 plate-glass is employed for a number of purposes ; the most 
 important and frequent of these is in the construction of 
 the better varieties of mirrors. For this purpose the glass 
 is frequently bevelled at the edges, and sometimes a certain 
 amount of cutting is also introduced on the face of the 
 mirror. Bevelling is carried out on special grinding and 
 polishing machines, and a great variety of these are in use 
 at the present time. The process consists in grinding off 
 the corners of the sheet of glass and replacing the rough 
 perpendicular edge left by the cutting diamond by a smooth 
 polished slope running down from the front surface to the 
 lower edge at an angle of from 45 to 60 degrees. Since 
 only relatively small quantities of glass have to be removed, 
 small grinding rubbers only are used, and in some of the 
 latest machines these take the form of rapidly-revolving 
 emery or carborundum wheels. These grinding wheels 
 have proved so successful in grinding even the hardest 
 metals that it is surprising to find their use in the glass 
 industry almost entirely restricted to the " cutting " of the 
 better kinds of flint and "crystal " glass for table ware or 
 other ornamental purposes. The reason probably lies in 
 the fact that the use of such grinding wheels results in the 
 generation of a very considerable amount of local heat, this 
 effect being intensified on account of the low heat-conduct- 
 ing power of glass. If a piece of glass be held even lightly 
 against a rapidly-revolving emery wheel it will be seen that 
 the part in contact with the wheel is visibly red-hot. This 
 local heating is liable to lead to chipping and cracking of 
 
 G.M. T. 
 
146 GLASS MANUFACTURE. 
 
 the glass, and these troubles are those actually experienced 
 when emery or carborundum grinding is attempted on 
 larger pieces of glass. In the case of at least one modern 
 bevel -grinding machine, however, it is claimed that the 
 injurious effects of local heating are avoided by carrying 
 out the entire operation under water. 
 
 For the purpose of use in mirrors, plate-glass is frequently 
 silvered, and this process is carried on so extensively that it 
 has come to constitute an entire industry which has no 
 essential connection with glass manufacture itself; for 
 that reason we do not propose to enter on the subject here, 
 only adding that the nature and quality of the glass itself 
 considerably affects the ease and success of the various 
 silvering processes. Ordinary plate-glass, of course, takes 
 the various silvering coatings very easily and uniformly, 
 but there are numerous kinds of glass to which this does 
 not apply, although there are probably few varieties of 
 glass which are sufficiently stable for practical use, and to 
 which a silvering coating cannot be satisfactorily applied, 
 provided that the most suitable process be chosen in each 
 case. 
 
 While there is little if any use for coloured glass in the 
 form of polished plate, entirely opaque plate- glass, coloured 
 both black and white, is used for certain purposes. Thus, 
 glass fascias over shop-fronts, the counters and shelves of 
 some shops, and even tombstones are sometimes made of 
 black or white polished plate. From the point of view of 
 glass manufacture, however, these varieties only differ from 
 ordinary plate-glass in respect of certain additions to the 
 raw materials, resulting in the production of the white or 
 black opacity. The subsequent treatment of the glass is 
 
EOLLED OR PLATE-GLASS. 147 
 
 identical with that of ordinary plate-glass, except that these 
 opaque varieties are rarely required to be polished on both 
 sides, so that the operations are simplified to that extent. 
 
 Certain limitations to the use of all kinds of plate-glass, 
 whether rough-rolled, figured or polished, were formerly set 
 by the fact that under the influence of fire, partitions of 
 glass were liable to crack, splinter and fall to pieces, thus 
 causing damage beyond their own destruction and leaving 
 a free passage for the propagation of the fire. To overcome 
 these disadvantages, glass manufacturers have been led to 
 introduce a network or meshing of wire into the body of 
 such glass. Provided that the glass and wire can be made 
 so as to unite properly, then the properties of such rein- 
 forced or " wired " glass should be extremely valuable. In 
 the event of breakage from any cause, such as fire or a 
 violent blow, while the glass would still crack, the fragments 
 would be held together by the wire network, and the plates 
 of glass as a whole would remain in place, neither causing 
 destruction through flying fragments nor allowing fire or, 
 for the matter of that, burglar a free passage. The utility 
 of such a material has been readily recognised, but the 
 difficulty lies in its production. These difficulties arise 
 from two causes. The most serious of these is the consider- 
 able difference between the thermal expansion of the glass 
 and of the wire to be embedded in it. The wire is neces- 
 sarily introduced into red-hot glass while the latter is being 
 rolled or cast, and therefore glass and wire have to cool 
 down from a red heat together. During this cooling 
 process the wire contracts much more than the glass, and 
 breakage either results immediately, or the glass is left in a 
 condition of severe strain and is liable to crack spontaneously 
 
 L 2 
 
148 GLASS MANUFACTUEE. 
 
 afterwards. An attempt has been made to overcome this 
 difficulty by using wire made of a nickel steel alloy, whose 
 thermal expansion is very similar to that of glass ; but, as a 
 matter of fact, this similarity of thermal expansion is only 
 known to hold for a short range of moderate temperatures, 
 and probably does not hold when the steel alloy is heated 
 to redness. In another direction, greater success is to be 
 attained by the use of wire of a very ductile metal which 
 should yield to the stress that comes upon it during cooling ; 
 probably copper wire would answer the purpose, but the 
 great cost of copper is a deterrent from its use. A second 
 difficulty is met with in introducing wire netting into glass 
 during the rolling operation, and this lies in effecting a 
 clean join between glass and wire. Most metals when 
 heated give off a considerable quantity of gas, and when 
 this gas is evolved after the wire has been embedded in 
 glass, numerous bubbles are formed, and these not only 
 render the glass very unsightly but also lessen the adhesion 
 between the wire and the glass. This difficulty, however, 
 can be overcome more readily than the first, since the 
 surface of the metal can be kept clean and the gas expelled 
 from the interior of the wire by preliminary heating. On 
 the whole, however, wired glass is perhaps still to be 
 regarded as a product whose evolution is not yet complete, 
 and there can be no doubt that there are great possibilities 
 open to the material when its manufacture has been more 
 fully developed. 
 
CHAPTEK X. 
 
 SHEET AND CROWN GLASS. 
 
 IN the preceding chapter we have dealt with the processes 
 of manufacture employed in the production of both the 
 crudest and the most perfect forms of flat glass as used for 
 such purposes as the glazing of window openings. The 
 products now to be dealt with are of an intermediate 
 character, sheet-glass possessing many of the properties of 
 polished plate, but lacking some very important ones ; thus 
 sheet-glass is sufficiently transparent to allow an observer 
 to see through it with little or no disturbance in the best 
 varieties of sheet-glass the optical distortion caused by its 
 irregularities is so small that the glass appears nearly as 
 perfect as polished plate but in the cheap glass that is 
 used for the glazing of ordinary windows, sheets are often 
 employed which produce the most disturbing, and some- 
 times the most ludicrous, distortions of objects seen through 
 them. It is a curious fact that even in good houses the 
 use of such inferior glass is tolerated without comment, 
 the general public being, apparently, remarkably non- 
 observant in this respect. In another direction sheet-glass 
 has the great advantage over plate-glass that it is very 
 much lighter, or can at least be produced of much smaller 
 weight and thickness, although this advantage entails the 
 
150 GLASS MANUFACTURE. 
 
 consequent disadvantage that sheet-glass is usually much 
 weaker than plate, and can only be used in much smaller 
 sizes. In recent times the production of relatively thin 
 plate-glass has, however, made such strides that it is now 
 possible to obtain polished plate-glass thin enough and 
 light enough for almost every architectural purpose. 
 Finally, the most important advantage of sheet-glass, and 
 the one which alone secures its use in a great number of 
 cases in preference to plate-glass, is its cheapness, the price 
 of ordinary sheet-glass being about one-fourth that of 
 plate-glass of the same size. 
 
 The raw materials for the manufacture of sheet-glass are 
 sand, limestone, salt-cake, and a few accessory substances, 
 such as arsenic, oxide of manganese, anthracite coal or 
 coke, which differ considerably according to the practice of 
 each particular works. In a general way these materials 
 have already been dealt with in Chapter III., and we need 
 only add here that the sheet-glass manufacturer must keep 
 in view two decidedly conflicting considerations. On the 
 one hand the requirements made in the case of sheet-glass 
 as regards colour and purity render a rigorous choice of 
 raw material and the exclusion of anything at all doubtful 
 very desirable ; but on the other hand the chief com- 
 mercial consideration in connection with this product is its 
 cheapness, and in order to maintain a low selling price at a 
 profit to himself the manufacturer must rigorously exclude 
 all expensive raw materials. For this reason sheet-glass 
 works such as those of Belgium and some parts of Germany, 
 which have large deposits of pure sand close at hand, 
 possess a very considerable advantage over those in less 
 favoured situations, since sand in particular forms so large 
 
SHEET AND CEOWN GLASS. 151 
 
 a proportion of the glass, and the cost of carriage frequently 
 exceeds, and in many cases very greatly exceeds, the actual 
 price of the sand itself. The same considerations will apply, 
 although in somewhat lesser degree, to the other bulky 
 materials, such as limestone and salt-cake ; but both these 
 are more generally obtainable at moderate prices than 
 are glass-making sands of adequate quality for sheet 
 manufacture. 
 
 Ordinary " white " sheet-glass is now almost universally 
 produced in tank furnaces, and a very great variety of 
 these furnaces are used or advocated for the purpose. It 
 would be beyond the scope of the present book to enter in 
 detail into the construction of these various types of furnace 
 or to discuss their relative merits at length. Only a brief 
 outline of the chief characteristics of the most important 
 forms of sheet-tank furnaces will therefore be given 
 here. 
 
 Sheet tanks differ from each other in several important 
 respects ; these relate to the sub-division of the tank into 
 one, two, or even three more or less separate chambers, to 
 the depth of the bath of molten glass and the height of the 
 "crown" or vault of the furnace chamber, to the shape 
 and position of the apertures by which the gas and air are 
 admitted into the furnace, and the resultant shape and 
 disposition of the flame, and finally to the position and 
 arrangement of the regenerative appliances by which some 
 of the heat of the waste gases is returned into the 
 furnace. 
 
 Taking these principal points in order, we find that in some 
 sheet tank furnaces the whole furnace constitutes a single 
 large chamber. In this type of furnace the whole process 
 
152 GLASS MANUFACTUEE. 
 
 of fusion and fining of the glass goes on in this single 
 chamber, and an endeavour is made to graduate the 
 temperature of the furnace in a suitable manner from the 
 hot end where the raw materials have to be melted down 
 to the colder end where the glass must be sufficiently 
 viscous to be gathered on the pipes. It is obvious that 
 this control of the temperature cannot be so perfect in a 
 furnace of the single chamber type as in one that is 
 sub-divided. . Such sub-divided furnaces are, as a matter of 
 fact, much more frequent in sheet-glass practice ; but this 
 practice differs widely as to the manner and degree of 
 the sub-division introduced. In the extreme form the 
 glass practically passes through three independent furnaces 
 merely connected with one another by suitable openings of 
 relatively small area through which the glass flows from 
 one to the other. If it were possible to build furnaces of 
 materials that could resist the action of heat and of molten 
 glass to an indefinite extent, it is probable that this extreme 
 type would prove the best, since it gives the operator of the 
 furnace the means of controlling the flow of glass in such a 
 way that no unmelted material can leave the melting 
 chamber and enter the fining chamber, and that no 
 insufficiently fined glass can leave the fining chamber and 
 find its way into the working chamber. But in practice 
 the fact that this extreme sub-division introduces a great 
 deal of extra furnace wall, exposed both to heat and to 
 contact with the glass, involves very serious compensating 
 disadvantages the cost of construction, maintenance and 
 renewal of the furnace is greatly increased, while there 
 is also an increased source of contamination of the glass 
 from the erosion of the furnace walls. It is, therefore, in 
 
SHEET AND CROWN GLASS. 153 
 
 accordance with expectations to find that the most successful 
 furnaces for the production of sheet-glass are intermediate 
 in this respect between the simple open furnace and the 
 completely sub-divided one. In some cases the working 
 chamber is separated from the melting and fining chamber 
 by a transverse wall above the level of the glass, while fire- 
 clay blocks floating in the glass just below this cross wall 
 serve to complete the separation and to retain any surface 
 impurities that may float down the furnace. 
 
 As regards the depth of glass in the tank, practice 
 also varies very much. The advantages claimed for a deep 
 bath are that the fire-clay bottom of the furnace is thereby 
 kept colder and is consequently less attacked, so that this 
 portion of the furnace will last for many years. On the 
 other hand the existence of a great mass of glass at a 
 moderate heat may easily prove the source of contamina- 
 tion arising from crystallisation or " devitrification " 
 occurring there and spreading into the hotter glass above. 
 Also, if for any reason it should become necessary to 
 remove part or all of the contents of the tank, the greater 
 mass of glass in those with deep baths becomes a formi- 
 dable obstacle. On the whole, however, modern practice 
 appears to favour the use of deeper baths, depths of 
 2 ft. 6 in. or even 3 ft. being very usual, while depths up to 
 4 ft. have been used. 
 
 The question of the proper height of the " crown " or 
 vault of the furnace is of considerable importance to the 
 proper working of the tank. For the purpose of producing 
 the most perfect combustion, it is now contended that a 
 large free flame-space is required. The earlier glass-melting 
 tanks, like the earlier steel furnaces, were built with very 
 
154 GLASS MANUFACTURE. 
 
 low crowns, forcing the flame into contact with the surface 
 of the molten glass, the object being to promote direct 
 heating by immediate contact of flame and glass ; the 
 modern tendency, however, is strongly in the direction of 
 higher crowns, leaving the heating of the glass to be 
 accomplished by radiation rather than direct conduction of 
 heat. There can be little doubt that up to a certain point 
 the enlargement of the flame-space tends towards greater 
 cleanliness of working and a certain economy of fuel, but if 
 the height of a furnace crown be excessive there is a decided 
 loss of economy. Flame- spaces as high as 6 ft. from the 
 level of the glass to the highest part of the crown have been 
 used, but the more usual heights range from 2 ft. to 5 ft. 
 
 The " ports " or apertures by which pre-heated gas and 
 air enter the furnace chamber differ very widely in various 
 furnaces. In some cases the gas and air are allowed to 
 meet in a small combustion chamber just before entering 
 the furnace itself, while in other cases the gas and air enter 
 the furnace by entirely separate openings, only meeting in 
 the furnace chamber. The latter arrangement tends to the 
 formation of a highly reducing flame, which is advan- 
 tageous for the reduction of salt-cake, but is by no means 
 economical as regards fuel consumption. On the other 
 hand, by producing a perfect mixing of the entering gas 
 and air in suitable proportions, the other type of ports can 
 be made to give almost any kind of flame desired, although 
 their tendency is to form a more oxidising atmosphere 
 within the furnace. The latter type of ports, although 
 widely varied in detail, are now almost universally adopted 
 in sheet tank furnaces. 
 
 All modern tank furnaces work on the principle of the 
 

 SHEET AND CEOWN GLASS. 155 
 
 recovery of heat from the heated products of combustion as 
 they leave the furnace, and the return of this heat to the 
 furnace by utilising it to pre-heat the incoming gas and air ; 
 but the means employed to effect the application of this 
 "regenerative" principle differ considerably in various 
 types of plant. Perhaps the most widely-used form of 
 furnace is the direct descendant of the original Siemens 
 regenerative furnace, in which four regenerator chambers 
 are provided with means for reversing the flow of gas and 
 air in such a way that each pair of chambers serves 
 alternately to absorb the heat of the outgoing gases and 
 subsequently to return this heat to the incoming air that 
 passes through one, and the incoming gas that passes 
 through the other of these chambers. In these furnaces, 
 the regenerator chambers themselves are generally placed 
 underneath the melting furnace, and they are built of 
 fire-brick and filled with loosely-stacked fire-bricks, whose 
 function it is to absorb or deliver the heat. In the most 
 modern type of furnaces of this class, the gas- regenerators 
 are omitted entirely, the air only being pre-heated by means 
 of regenerators, while the gas enters the furnace direct from 
 the producer, thus carrying with it the heat generated in 
 the producer during the gasification of the fuel. While 
 this arrangement is undoubtedly economical, it has the 
 serious disadvantage, especially in the manufacture of sheet- 
 glass, that the gas, rushing direct from the producer into 
 the furnace, carries with it a great deal of dust and ash, 
 which it has no opportunity of depositing, as in the older 
 types of furnace, in long flues. 
 
 The most serious disadvantages of the ordinary types of 
 regenerative furnaces are due to the considerable dimensions 
 
156 GLASS MANUFACTUEE. 
 
 of the regenerative apparatus, necessitating a costly form of 
 construction and occupying a large space, while the 
 necessity of periodically reversing the valves so as to secure 
 the alternation in the flow of outgoing and incoming gases 
 requires special attention on the part of the men engaged 
 in operating the furnace, as well as the construction and 
 maintenance of valves under conditions of heat and dirt 
 that are not favourable to the life of mechanical appliances. 
 It is claimed that all these disadvantages are overcome to a 
 considerable extent in one or other of the various forms of 
 furnace known as " recuperative." In these furnaces there 
 is no alternation of flow, and the regenerator chambers are 
 replaced by the " recuperators." These consist of a large 
 number of small flues or pipes passing through a built-up 
 mass of fire-brick in two directions at right-angles to one 
 another ; through the pipes running in one direction the 
 waste gases pass out to the chimney, while the incoming 
 gas and air pass through the other set of pipes. A trans- 
 ference of heat between the two currents of gas takes place 
 by the conductivity of the fire-brick, and thus the outgoing 
 gases are continuously cooled while the ingoing gases are 
 heated the transference of heat being somewhat similar to 
 that which takes place in the surface condenser of a steam 
 engine. Theoretically this is a much simpler arrangement 
 than that of separate regenerator chambers, and to some 
 extent it is found preferable in practice, but there are 
 certain disadvantages associated with the system which 
 arise principally from the peculiar nature of the material 
 fire-brick of which the recuperators must be constructed. 
 In the first place, the heat-conductivity of fire-brick is not 
 very high, so that, in order to secure efficiency, the recupe- 
 
SHEET AND CEOWN GLASS. 157 
 
 rators must be large, and while the individual pipes must be 
 of small diameter, their area as a whole must be large 
 enough to allow the gases to pass through somewhat slowly. 
 Next, owing to the tendency of fire-brick to warp, 
 shrink and crack under the prolonged effects of high 
 temperatures, it becomes difficult to prevent leakage of 
 gases from one set of pipes into the other. If this occurs 
 to a moderate extent its only effect will be to allow some of 
 the combustible gas to pass direct to the chimney, and at 
 the same time a dilution of the gases entering the furnace 
 by an addition of products of combustion from the waste- 
 gas flues. This, of course, will materially reduce the 
 efficiency of the furnace and require a higher fuel con- 
 sumption if the temperature of the furnace is to be 
 maintained at its proper level. If, however, the leakage 
 should become more serious, a disastrous explosion might 
 easily result, particularly if the nature of the leakage were 
 such as to allow the incoming gas and air to mix in the 
 flues. It follows from these considerations that, although 
 the recuperative furnace is somewhat simpler and cheaper 
 to construct, it requires, if anything, more careful main- 
 tenance than the older forms of regenerative furnace. 
 
 Tank furnaces for the production of sheet-glass in this 
 country are generally worked from early on Monday morn- 
 ing until late on Saturday night, glass-blowing operations 
 being suspended during Sunday, although the heat of the 
 furnace must be maintained. On the Continent, and 
 especially in Belgium, the work in connection with these 
 furnaces goes on without any intermission on Sunday a 
 difference which, however desirable the English practice 
 may be, has the effect of handicapping the outpu of a 
 
158 GLASS MANUFACTUKE. 
 
 British furnace of equal capacity by about 10 per cent, 
 without materially lessening the working cost. 
 
 The process of blowing sheet-glass in an English glass- 
 works is generally carried out by groups of three workmen, 
 viz., a "pipe-warmer," a "gatherer" and a "blower," 
 although the precise division of the work varies according 
 to circumstances. The pipe-warmer's work consists in the 
 first place in fetching the blowing-pipe from a small sub- 
 sidiary furnace in which he has previously ^placed it for the 
 purpose of warmin up the thick "nose" end upon which 
 the glass is subsequently gathered. The sheet-blower's pipe 
 itself is an iron tube about 4 ft. 6 in. long, provided at 
 the one end with a wooden sleeve or handle, and a mouth- 
 piece, while the other end is thickened up into a substantial 
 cone, having a round end. Before introducing the pipe 
 into the opening of the tank furnace, the pipe-warmer must 
 see that the hot end of the pipe is free from scale or dirt 
 and must test, by blowing through it, whether the pipe is 
 free from internal obstructions. He then places the butt 
 of the pipe in the opening of the furnace and allows it to 
 acquire as nearly as possible the temperature of the molten 
 glass. When this is the case the pipe is either handed on 
 to the gatherer, or the pipe-warmer, who is usually only a 
 youth, may take the process one step further before handing 
 it on to the more highly skilled workman. This next step 
 consists in taking up the first gathering of glass on the 
 pipe. For this purpose the hot nose of the pipe is dipped 
 into the molten glass, turned slowly round once or twice 
 and then removed, the thread of viscous glass that comes 
 up with the pipe being cut off against the fire-clay ring 
 that floats in the glass in front of the working opening. 
 
SHEET AND CEOWN GLASS. 159 
 
 A small quantity of glass is thus left adhering to the 
 nose of the pipe, and this is now allowed to cool down 
 until it is fairly stiff, the whole pipe being meanwhile 
 rotated so as to keep this first gathering nicely rounded, 
 while a slight application of air-pressure, by blowing 
 down the pipe, forms a very small hollow space in the 
 mass of glass and secures the freedom of the opening of 
 the pipe. When the glass forming the first gathering has 
 cooled sufficiently, the gatherer proceeds to take up the 
 second gathering upon it. The pipe is again introduced 
 into the furnace and gradually dipped into the molten 
 glass, but this must be done with great care so as to avoid 
 the inclusion of air-bells between the glass already on the 
 pipe and the new layer of hotter glass that is now taken up. 
 This freedom from air-bells is secured by a skilful gatherer 
 by a gradual rotation of the pipe as it is lowered into the 
 glass, thus allowing the two layers of glass to come into 
 contact with a sort of rolling motion that allows the air 
 time to escape. When completely immersed, the pipe is 
 rotated a few times and is then withdrawn and the " thread" 
 again cut off. The mass of glass on the end of the pipe is 
 now considerably larger than before and requires more 
 careful manipulation to cause it to retain the proper, nearly 
 spherical, shape. During the cooling process which now 
 follows the pipe is laid across an iron trough, kept brim- 
 ful of water; this serves to cool the pipe itself, and also 
 allows the pipe to be readily rotated backwards and 
 forwards by rolling it a little way along the trough. When 
 the whole mass of glass has again cooled sufficiently to be 
 manipulated without risk of rapid deformation, a third 
 gathering of glass is taken up, in precisely the same manner 
 
160 GLASS MANUFACTUKE. 
 
 as that already described for the second gathering, and if 
 the quantity of glass required is large, or the glass itself is 
 so hot and fluid that only a comparatively small weight 
 adheres at each time of gathering, the process may be 
 repeated a fourth or even a fifth time, but as the weight of 
 pipe and adhering glass increases with each gathering, each 
 step becomes more laborious, while the hot glass, being 
 now held on a much larger sphere, tends to flow off more 
 readily, so that greater skill is required to avoid " losing" 
 the gathering. 
 
 The care and skill with which these operations of 
 gathering are carried out determine, to a large extent, the 
 quality of the resulting sheet of glass ; any want of 
 regularity in the shape of the gathering leads inevitably to 
 variations of thickness in different parts of the sheet, while 
 careless gathering will introduce bubbles or " blisters " and 
 other markings. During the intermediate cooling stages 
 the glass must be protected from dust and dirt of all kinds, 
 since minute specks falling upon the hot glass give rise 
 an evolution of minute gas bubbles which become painfulb 
 evident in the sorting room. 
 
 When the last gathering has been taken up and the mass 
 cooled so far as to allow of its being carried about without 
 fear of loss, the glass forms an approximately spherical 
 mass, with the nose-end of the pipe at or near the centre of 
 the sphere. The next stages of the process consist in the 
 preliminary shaping of this mass in such a way as to bring 
 the bulk of the glass beyond the end of the pipe, and then 
 in forming just beyond the end of the pipe a widened shoulder 
 of thinner and therefore colder glass, of the diameter 
 required for the cylinder into which the glass is to be 
 
SHEET AND CEOWN GLASS. 161 
 
 blown. This is done by bringing the glass into the succes- 
 sive shapes shown in Fig. 12, the forming of the glass 
 being effected by the aid of specially shaped blocks and 
 other shaping instruments in which the glass is turned and 
 blown. The final shape attained at this stage is a squat 
 cylinder containing the bulk of the glass at its lower end, 
 and connected to the pipe by the thinner and colder neck 
 and shoulder already mentioned. 
 
 At this point of the process the pipe with its adherent 
 glass is handed over to the blower proper. This operator 
 
 FIG. 12. Early stages in the formation of cylinders for sheet glass. 
 
 works on a special stage erected in front of small furnaces, 
 called " blowing holes," although in some works these are 
 dispensed with, and the stages are erected in front of the 
 melting furnace itself. The sheet-blower's stage is simply 
 a platform placed over or at the side of a suitable excava- 
 tion which gives the blower the necessary space to swing 
 the pipe and cylinder freely at arm's length. The blowing 
 process itself involves very little actual blowing, but depends 
 rather upon the action of gravitation and on centrifugal 
 effects for the formation of the large, elongated cylinder 
 from the squat cylinder with which the blower com- 
 mences. The process consists in holding the thick, lower 
 G.M. M 
 
162 GLASS MANUFACTURE. 
 
 end of the cylinder in the heating-furnace, and when suffi- 
 ciently hot, withdrawing it and swinging the pipe with a 
 pendulum movement in the blower's pit. The cylinder 
 thus elongates itself under its own weight, and any ten- 
 dency to collapse is counteracted by the application of air- 
 pressure by the mouth, the pipe being also, at times, 
 rotated rapidly about its own axis. The re-heating of the 
 
 FIG. 13. Later stage in sheet glass blowing. 
 
 lower end of the cylinder is repeated several times, until 
 finally the glass has assumed the form of a cylinder of equal 
 thickness all over, but closed with a rounded dome at the 
 lower end (Fig. 13). This rounded end is now opened. 
 In the case of fairly thin and light cylinders this is done by 
 holding the thumb over the mouthpiece of the pipe in such 
 a way as to make an air-tight seal, and then heating the 
 end of the cylinder in the blowing-hole. The heat both 
 softens the glass at the end and at the same time causes 
 
SHEET AND CROWN GLASS. 163 
 
 considerable expansion of the air enclosed in the cylinder, 
 with the result that the end of the cylinder is burst open. 
 After a little further heating, during which the glass at the 
 end of the cylinder becomes very soft, and takes a wavy, 
 curly shape, the blower withdraws the cylinder from the 
 furnace, and holding it vertically downwards in his pit, 
 spins it rapidly about its longitudinal axis. The soft glass 
 at the lower end immediately opens out under the centri- 
 fugal action, and the blower increases the speed of rotation 
 until the soft glass has opened out far enough to form a 
 true continuation of the rest of the cylinder, and in this 
 position it is allowed to solidify. With thick, heavy 
 cylinders, the first opening of the end is done in a different 
 way. A small quantity of hot glass is taken up by an 
 assistant on an iron rod, and is laid upon the centre of the 
 closed end of the cylinder. The heat of this mass of hot 
 glass softens the glass of the cylinder, and the operator, 
 with the aid of a special pair of shears, cuts out a small 
 circle of this softened glass, thus opening the end of the 
 cylinder. The final operation of straightening out the 
 opened end is carried out in the same way as described 
 above for lighter cylinders. 
 
 The completed cylinder, still attached to the pipe, is now 
 carried away from the blowing-stage and laid upon a 
 wooden rack ; then the blower takes up a piece of cold iron, 
 and placing it against the neck of glass attaching the 
 cylinder to the pipe, produces a crack ; a short jerk then 
 serves to completely sever the pipe from the cylinder. A 
 boy now takes the pipe to a stand where it is allowed to 
 cool and where the adhering glass cracks off from it prior 
 to passing it back to the pipe-warmer for fresh use. 
 
 M2 
 
164 GLASS MANUFACTUEE. 
 
 On the wooden rack the cylinder of glass is allowed to 
 cool to a certain extent, and then the remaining portion of 
 the neck and shoulder (see Fig. 13) are removed. This is 
 done by a boy who passes a thread of soft, hot glass 
 around the cylinder at the point where it is to be cut off ; 
 the thread of hot glass merely serves to produce intense 
 local heating, for as soon as it has become stiff, the thread 
 of glass is pushed off and a cold or moist iron is applied to 
 the cylinder at the point where it had been heated by the 
 thread. As a rule a crack immediately runs completely 
 round the cylinder along the line of the thread, and the 
 "cap" is thus removed. The glass is now in the form of 
 a uniform cylinder open at both ends, but it must be 
 opened out into a flat sheet before it can assume the familiar 
 form of sheet-glass. 
 
 The first stage in the opening-out process is that of 
 splitting. For this purpose the cylinders are carried to a 
 special stand, upon which they are laid in a horizontal 
 position, and here a crack or cut is made along one of the 
 generating-lines of the cylinder. This may be done either 
 by the application of a hot iron, followed, if necessary, by 
 slight moistening, or by the aid of a cut from a heavy 
 diamond drawn skilfully down the inside of the cylinder. 
 It will be seen from the account of the process so far given, 
 that the glass has as yet undergone no real annealing, 
 although the blower is expected to " anneal " his cylinder 
 during the blowing process, as far as possible, by never 
 allowing it to cool too suddenly, and this degree of annealing 
 is usually sufficient to save the cylinder from breaking 
 under its internal stresses when left to cool on the racks. 
 The surface of the glass, however, is left in a decidedly 
 
SHEET AND CKOWN GLASS. 165 
 
 hardened condition, especially on the outside, which has 
 necessarily been most rapidly cooled. For this reason 
 among others the splitting cut is always made on the 
 inside of the cylinder. The difference between the rates of 
 cooling of the outside and inside of the cylinder has a 
 further effect, which becomes evident as soon as the cylinder 
 is split. The outside having become hard while the inside 
 was still relatively soft, the outer layers of glass are in a 
 state of compression and the inner layers in a state of 
 tension in the cold cylinder. As soon as the cylinder is 
 split, however, these stresses are to some extent relieved, 
 the inner layers being then free to contract and the outer 
 layers to expand ; the result is an increase in the curvature 
 of the cylinder, which slightly decreases in diameter, the cut 
 edges overlapping. If the cylinder has been cooled rather 
 too quickly, or if the glass itself has a high co-efficient of 
 expansion, this release of internal stresses at the moment 
 of splitting becomes very marked, and each cylinder splits 
 with the sound of a small explosion, while if the internal 
 stresses are still more severe, the cylinders may even fly to 
 pieces as soon as they are cut. 
 
 The next stage in the manufacture of a sheet of glass is 
 the flattening and annealing process. For this purpose the 
 split cylinders are taken to a special kiln, generally known 
 as a "lear," or "lehr," where they are first of all raised to a 
 dull red-heat; they are then lifted, one at a time, on to a 
 smooth stone or slab placed in a chamber of the kiln where 
 the heat is great enough to soften the glass. Here the 
 cylinder is laid down with the split edges upwards, and by 
 means of a wooden tool the glass is slowly spread out, being 
 finally rubbed down into perfect contact with the slab or 
 
166 GLASS MANUFACTURE. 
 
 " lagre." From the flattening slab, the sheet as it now is 
 passes into the annealing kiln, which communicates with 
 the flattening chamber. This consists, similarly to other 
 continuous annealing kilns already described in connection 
 with other varieties of glass, of a long tunnel, heated to the 
 temperature of the flattening kiln at one end and nearly 
 cold at the other. The sheets are moved down this tunnel 
 at a uniform slow rate by the action of a system of grids 
 which, at intervals, lift the sheets from the bottom of the 
 kiln, move them forward by a short distance, and again 
 deposit them on the bottom, the grids themselves returning 
 to their former position by a retrograde movement made 
 below the level of the kiln -bottom, and therefore not 
 affecting the glass. 
 
 On leaving the annealing kiln the sheets of glass are 
 sometimes covered with a white deposit arising from the 
 products of combustion in the kiln and their interaction 
 with the glass itself. This deposit can be removed by 
 simple mechanical rubbing, but it is usual to dip the glass 
 into a weak acid bath, which dissolves the white film and 
 leaves the glass clear and bright, ready for use. 
 
 From the annealing kiln the finished sheets of glass are 
 taken to the sorting room, where they are examined in a 
 good light against a black background, and are sorted 
 according to their quality for different purposes. 
 
 The defects which are found in sheet-glass are of a very 
 varied nature, as would bo anticipated from the long and 
 complicated process of manufacture which the material 
 undergoes in the course of its transformation from the raw 
 materials into the finished sheet of glass. A full enumera- 
 tion of all possible defects, with their technical names, need 
 
SHEET AND CEOWN GLASS. 167 
 
 not be given here, but a description of the more important 
 and frequent ones will be useful. The defects may be 
 conveniently grouped according to the stage of the process 
 from which they originate. 
 
 The first class of defects accordingly embraces those that 
 arise from the condition of the glass as it exists in the 
 working-end of the furnace. Chief of these are white 
 opaque enclosures, known as " stones." These may arise 
 from a variety of causes within the furnace, such as an 
 admixture of infusible impurities with the raw materials, 
 insufficient heat or duration of melting, leading to a residue 
 of unmelted raw material in the finished glass, or from 
 defective condition of the interior of the furnace, leading to 
 contamination of the glass with small particles of fire- 
 brick. Further, if any part of the furnace has been 
 allowed to remain at too low a temperature, or if the 
 composition of the glass is unsuitable, crystallisation 
 may occur in the glass, and white patches of crystalline 
 material may find their way into the finished sheets. 
 Another defect that may arise from the condition 
 of the glass in the furnace is the presence of numerous 
 small bubbles, known as "seed" in the glass. By the 
 blowing process these are drawn out into pointed ovals, and 
 they are rarely quite absent from sheet-glass. They arise 
 from either incomplete fining of the glass in the furnace or 
 from allowing the glass to come into contact with minute 
 particles of dust during the gathering process. Another 
 possible defect to the glass itself may be found at times in 
 too deep a colour. This is only seen readily when a sheet 
 of some size is examined edgewise, as most varieties of 
 ordinary sheet -glass are too free from colour to allow this 
 
16S GLASS MANUFACTURE. 
 
 to be judged by looking through the sheet in the ordinary 
 way. It follows from this fact that for practical purposes, 
 where the light always traverses one thickness of the glass 
 only, a slight difference of colour should be regarded as a 
 very minor consideration, at all events as compared with 
 freedom from other defects. 
 
 The gathering process in its turn is responsible for 
 further defects of sheet-glass. Some of these, such as 
 defects arising from the use of a dirty pipe, are never 
 allowed to pass beyond the sorting-room, and are therefore 
 of no interest to the user of glass. Of those whose traces 
 are seen in the glass that passes into use, " blisters " and 
 " string " are the most important. " Blisters " are some- 
 what larger, flat air-bells, arising from the inclusion of 
 air between successive layers of the gathering. " String " 
 is a very common defect in all sheet-glass. To some 
 extent it may arise from want of homogeneity in the glass 
 itself. If this consists of layers of different densities and 
 viscosities, the gatherer will take these up on his gathering, 
 and ultimately they will form thickened ridges of glass 
 running around the cylinders and across the sheets. Such 
 strise, due to want of homogeneity in the glass, are much 
 more common in flint glass than in the soda-lime glasses 
 used for sheet manufacture, but are not unknown in the latter. 
 On the other hand, even if the glass be as homogeneous as 
 possible, the gatherer can produce these striae if he takes 
 up his glass from a place close to the side of the fire-clay 
 ring that floats in the furnace in front of his working 
 opening. Glass always acts chemically upon fire-clay, 
 gradually forming a layer of glass next to the fire-clay that 
 contains much more alumina than the rest of the contents 
 
SHEET AND CEOWN GLASS. 169 
 
 of the furnace. Such a layer is formed on the surface of 
 each ring in a sheet tank, but if the gathering is taken 
 from the centre of the ring, this thick viscous layer of 
 aluminiferous glass remains undisturbed. If, however, the 
 gatherer brings his pipe too near the side of the ring, the 
 glass will draw some of this different layer on to the 
 gathering, and this glass will form thick ridges and striae 
 running across the sheet in all directions. Another defect 
 for which the gatherer is generally responsible is that of 
 variation of thickness within the same sheet. The blower, 
 however, can also produce this defect. 
 
 During the blowing proper, a further series of defects 
 may be introduced, principally by allowing particles of 
 glass derived from certain stages of the process to fall upon 
 the hot glass of the cylinder and there become attached 
 permanently. More serious, and also more frequent, is the 
 greater or less malformation of the cylinder. If the glass 
 as it leaves the blower is of any shape other than that of a 
 true cylinder, it becomes impossible to spread it into a 
 truly flat sheet in the flattening kiln. Sometimes, in 
 practice, the "cylinder" is wider at one end than at the 
 other, or, worse still, it is of uneven diameter, showing 
 expanded and contracted areas alternately. When such a 
 cylinder comes to be spread out on the slab it cannot be 
 flattened completely, and various hollows and hillocks are 
 left, which mar the flatness of the sheet and interfere with 
 the regular passage of light through it when in use. 
 
 Finally, the process of flattening is apt to introduce 
 defects of its own. The most common of these are 
 scratches arising from marks left by the flattening tool ; 
 indeed, in all sheet glass it is quite possible to see, by 
 
170 GLASS MANUFACTUEE. 
 
 careful examination of the surfaces, upon which side the 
 flattening tool was used. Sheet-glass thus has one side 
 decidedly brighter and better in surface than the other, the 
 better side being that which rested upon the "lagre" during 
 the flattening process. On the other hand, if the slab itself 
 be not quite perfect, or if any foreign body be allowed to 
 rest upon it, that side of the glass will be marked in a 
 corresponding manner. 
 
 In the account of the manufacture of sheet-glass given 
 above, we have outlined one typical form of the process, 
 but nearly every stage is subject to modifications according 
 to the practice and particular circumstances of each works. 
 We will now describe one or two special modifications that 
 are of more general importance. 
 
 First, as regards the melting process, although the tank- 
 furnace has almost entirely superseded the pot furnace for 
 the production of ordinary sheet-glass, there are still some 
 special circumstances under which the pot furnace is 
 capable of holding its own. Thus, where for special 
 purposes it is desired to produce a variety of sheet-glass 
 which, as regards all defects arising out of the glass itself, 
 and especially as regards colour, is required to be as perfect 
 as possible, melting in pots is found advantageous, and for 
 some very special purposes even covered (hooded) pots are 
 used. For such special purposes, too, sulphate of soda is 
 eliminated from the raw materials and carbonate of soda 
 (soda ash) substituted. For the production of tinted 
 glasses also, whether they are tinted throughout their mass, 
 or merely covered with a thin layer of tinted glass 
 (" flashed "), manufacture in pot rather than tank 
 furnaces is generally adopted, the exact nature and com- 
 
 
SHEET AND CEOWN GLASS. 171 
 
 position of the glass being far better under control in the 
 case of pots. 
 
 The blowing process is also subject to wide variations of 
 practice. The most important of these variations concerns 
 the shape and dimensions of the cylinders. In English 
 and Belgian works the dimensions of the cylinders are so 
 chosen that the length of the cylinder constitutes the 
 longest dimension of the finished sheet, the diameter of the 
 cylinder forming the shorter dimension. In some parts of 
 Germany, however, the practice is the reverse of this, the 
 cylinders being blown shorter and much wider, so that the 
 circumference of the cylinder constitutes the longest dimen- 
 sion of the finished sheet. It is, however, pretty generally 
 recognised that the latter method has very serious dis- 
 advantages, although it is claimed that somewhat more 
 perfect glass can be obtained by its means. For the pro- 
 duction of a special variety of glass, known as " blown 
 plate glass," this method of blowing short wide cylinders 
 is still adhered to. This is a very pure form of sheet-glass, 
 blown into thick, small sheets which are subsequently 
 ground and polished in the same manner as plate-glass. 
 Here the great thickness of glass required seems to render 
 the blowing of long cylinders very difficult, and the other 
 form is therefore adopted. On the other hand, English 
 patent plate-glass, which is made by grinding and polishing 
 the best quality of ordinary sheet-glass, is made from glass 
 blown into long narrow cylinders in the manner described 
 in detail above. 
 
 The process of blowing described above is capable, with 
 slight modifications, of yielding glass with surfaces other 
 than the plain smooth face of ordinary sheet-glass. Thus 
 
172 GLASS MANUFACTURE. 
 
 fluted and " muffled " glass are produced in a very similar 
 manner to that described above for ordinary sheet, except 
 that the fluting or the irregular surface markings which 
 constitute the peculiarities of these two varieties of glass, 
 are impressed upon the surface of the cylinder at an early 
 stage in the process. 
 
 From the outline description given above of the usual 
 method of manufacture of sheet-glass, it will readily be seen 
 that this is a long, complicated, and laborious process, 
 involving the employment of much skilled labour, and 
 involving the production of a relatively complicated form, 
 viz., the closed cylinder, as a preliminary to the production 
 of a very simple form, viz., the flat sheet. It is therefore 
 by no means surprising to find that a great many inventors 
 have worked and are still working at the problem of a 
 direct mechanical method of producing flat glass possessing 
 a natural " fire polish " at least equal to that of ordinary 
 sheet-glass. The earlier inventors have almost uniformly 
 endeavoured to attain this object by attempting to improve 
 the process of rolling glass, with a view to obtaining rolled 
 sheets having a satisfactory surface. We have already 
 indicated why these efforts have never met with success 
 and what reasons there are for believing that they are 
 never likely to attain their object. A totally different line 
 is that taken by Sievert, to whose inventions we have 
 already referred in connection with the mechanical produc- 
 tion of blawn articles. This inventor has endeavoured to 
 utilise his process for blowing large articles of glass for the 
 direct production of sheets of flat glass. His method is to 
 blow, by the steam process described in another chapter, a 
 large cubical vessel, having flat sides, the flatness of these 
 
SHEET AND CEOWN GLASS. 173 
 
 sides being ensured by blowing the vessel into or against 
 a mould having flat sides. This flat-sided vessel is ulti- 
 mately to be cut up into five large sheets. This process 
 also appears to involve some of the main difficulties of 
 rolling as regards the means of transferring the glass from 
 the furnace to the plate of the blowing machine, and in 
 practice the inventor has not yet succeeded in producing 
 glass of sufficiently good surface for the purposes of sheet 
 glass. 
 
 Another class of processes entirely avoid all means of 
 transferring molten glass from the furnace to any machine, 
 by working on glass direct from the molten bath itself. 
 Some of these processes are in actual use in America, and 
 others are being experimented with in Europe, but their 
 complete technical and commercial success has yet to be 
 proved ; there can, however, be little doubt that they have 
 overcome the greatest of the many difficulties that stood in 
 the way of the mechanical production of sheet-glass, and 
 that they are therefore destined very shortly to solve the 
 problem completely, in which case they would, of course, 
 rapidly supersede the hand process. 
 
 One of the earliest of these direct processes proposed to 
 allow the molten glass to flow out from the furnace, down- 
 ward, through a narrow slit formed in the side or bottom of 
 the tank. The impossibility of keeping such a narrow 
 orifice open and at the same time regulating the flow of 
 glass made this proposal impracticable, although the use of 
 drawing orifices has been revived in one of the latest 
 processes. 
 
 The American process, which is said to be at work 
 under commercial conditions, is not entirely satisfactory 
 
174 GLASS MANUFACTUEE. 
 
 in this respect that it is a mechanical process for 
 the production of cylinders and not of flat sheets, so 
 that the subsidiary processes of splitting and flattening 
 still remain to be carried out as before. In this process an 
 iron ring is lowered into the bath of molten glass through 
 an aperture from above ; the glass is allowed to adhere to 
 the ring which is then slowly raised by mechanical means, 
 drawing a cylinder of glass with it. If left to itself, such a 
 cylinder, owing to the effects of surface tension in the glass, 
 would soon contract and break off, but the American inven- 
 tion avoids this action by chilling each bit of the cylinder 
 as soon as it is formed. This is done by the aid of air 
 blasts delivered upon both sides of the glass as it emerges 
 from the bath, and it is claimed that by this means cylinders 
 of any desired length and diameter may be drawn direct 
 from the bath. The obviously great mechanical difficulties 
 connected with these operations have probably been over- 
 come, but not without sacrificing much of the simplicity of 
 the arrangement, and the relative economy of this process 
 as a whole, compared with the hand process, has yet to 
 assert itself. 
 
 The inventions of Fourcault, which are at present being 
 developed on the Continent by a syndicate of glass manu- 
 facturers, aim at a much more direct process. Here also 
 the glass is drawn direct from the molten bath by the aid 
 of a drawing-iron that is immersed in the glass and then 
 slowly raised, but in this case the piece immersed is simply 
 a straight bar, and the aim is to draw out a flat sheet. In 
 this case the tendency, under surface tension, is to contract 
 the sheet into a thread, and apparently the simple device 
 of chilling the emerging glass is not adequate to prevent 
 
SHEET AND CROWN GLASS. 175 
 
 this in a satisfactory manner, and subsidiary devices have 
 been added. Those that have been patented include a 
 mechanism of linked metal rods so arranged as to be 
 immersed and drawn out of the glass continuously with the 
 emerging sheet, in such a manner as to support the vertical 
 edges of the glass and so aid in resisting the tendency of 
 the glass to contract laterally. Another device consists in 
 the use of a slit or orifice formed in a large fire-brick that 
 floats on the surface of the glass. Through this orifice the 
 glass is drawn, of the desired thickness and width. The 
 use of this orifice, however, interferes markedly with the 
 perfection of the product, and in fact all the glass produced 
 in this way shows quite plainly a set of longitudinal stria- 
 tions due to the inevitable irregularities in the lips of the 
 drawing slot. Further, it appears to be impracticable to 
 draw thin glass in this way, a thickness of from 2J to 3 milli- 
 metres (about ^ inch) being the least that is practicable, on 
 account of the large amount of breakage that occurs with 
 weaker sheets. This process, in its present stage of develop- 
 ment, however promising, does not appear to have solved 
 the problem of mechanical manufacture of sheet-glass, since 
 it is just in the thinner, lighter kinds of glass that the 
 advantages of sheet are most pronounced. On the other 
 hand, it is quite possible that this drawing process, or some 
 development arising from it, may shortly supplant the 
 casting process in the production of polished plate-glass, 
 although for the largest sizes of this product also, the 
 difficulty and danger of handling the weights involved may 
 prove a serious obstacle. 
 
 Crown Glass. Although this is a branch of manufacture 
 that is nearly obsolete, it deserves brief notice here, partly 
 
176 GLASS MANUFACTUKE. 
 
 because it is still used for the production of special articles, 
 and also because it illustrates some interesting possibilities 
 in the use and manipulation of glass. 
 
 The process of blowing crown glass may be briefly 
 described as that of first blowing an approximately 
 spherical hollow ball, then opening this at one side and 
 expanding the glass into a flat disc by the action of centri- 
 fugal forces produced by a rapid rotation of the glass in 
 front of a large opening in a special heating furnace. The 
 actual process involves, of course, the preliminary of 
 gathering the proper quantity of glass, much in the manner 
 already described in connection with sheet-glass manufacture. 
 This gathering is then blown out into a hollow spherical 
 vessel. This vessel is now attached to a subsidiary iron 
 rod by means of a small gathering of hot glass, applied at 
 the point opposite the pipe itself, the glass being thus, for 
 a moment, attached to both the pipe and the " pontil " or 
 "punty" (as the rod is called). The pipe is, however, 
 detached by -cracking off the neck of the original glass, 
 which now remains attached to the pontil in the shape of 
 an open bowl. This bowl is now re-heated very strongly in 
 front of a special'furnace, the open side of the bowl being 
 presented to the fire. The pontil is meanwhile held in a 
 horizontal position and rotated. As the glass softens the 
 rotation spreads it out, until finally the entire mass of 
 glass is formed into a simple flat disc spinning rapidly 
 before the mouth of the furnace. This flat disc or " table " 
 of crown glass is allowed to cool somewhat, is detached 
 from the pontil by a sharp jerk, and is then annealed in a 
 simple kiln in which the glass is stacked, sealed up, and 
 allowed to cool naturally. 
 
SHEET AND CROWN GLASS. 177 
 
 It is obvious that by this process no very large sheets of 
 glass can be produced ; tables 4 ft. in diameter are already 
 on the large side, and these can only be cut up into much 
 smaller sheets on account of the lump of glass by which 
 the table was originally attached to the pontil, and which 
 remains fixed in the centre of the finished disc. For certain 
 ornamental purposes, where an " antique " appearance is 
 desired, these bullions are valued, but for practical purposes 
 they interfere very seriously with* the use of the glass. As 
 a matter of fact, even several inches away from the central 
 bullion itself, crown glass is generally marked with circular 
 wavings, which render it readily recognisable in the windows 
 of older buildings, but which decidedly detract from the 
 perfection of the glass. On the other hand, crown glass is 
 still valued for certain purposes, such as microscope slides 
 and cover glasses, where entire freedom from surface 
 markings, such as those found in sheet glass as a result of 
 the flattening operations, is desirable. While, therefore, 
 the process has merely an historical interest so far as 
 ordinary sheet-glass purposes are concerned, it is still used 
 in special cases. 
 
 G.M. N 
 
CHAPTER XL 
 
 COLOURED GLASSES. 
 
 IN various chapters throughout the foregoing portions of 
 this book we have had occasion to refer to the colour of 
 glass and the causes affecting it, but these references have 
 chiefly been made from the point of view of the production 
 of glasses as nearly colourless as possible under the circum- 
 stances. While it is obvious that for the great majority of 
 the purposes for which it is used the absence of all visible 
 coloration is desirable or even essential in the glass 
 employed, there are numerous other uses where a definite 
 coloration is required. Thus we have, as industrial 
 and technical uses of coloured glass, the employment of 
 ruby, green and purple glasses for signalling purposes, as 
 in the signal lamps of our railways, the red tail-lights of 
 motor-cars, or even the red or green sectors of certain 
 harbour lights and lighthouses ; again, coloured glasses, 
 ruby, green, and yellow, are extensively employed in con- 
 nection with photography. Eather less exacting in their 
 demands" upon the correctness of the colour employed are 
 the architectural and ornamental uses to which coloured 
 glass is so extensively put in both public and domestic 
 buildings, while, finally, coloured glass is largely the 
 foundation upon which the stained-glass worker builds up 
 
COLOURED GLASSES. 179 
 
 his artistic achievements ; in another direction, coloured 
 glass is also utilised in the production of ornamental articles 
 and of some table-ware. While it must be admitted that 
 in a great many cases the colour-resources of the glass 
 maker are hopelessly misapplied, yet in really artistic hands 
 few other materials are capable of yielding results of equal 
 beauty. 
 
 By the " colour " of a glass is generally understood the 
 tint or colour which is observed when it is viewed, in com- 
 paratively thin slices, by transmitted light ; the actual 
 colour is thus a property, not so much of the kind or 
 variety of glass as of each individual piece, since thick 
 pieces out of the same melting will show a different tint 
 from that seen in thinner pieces. As we have already 
 pointed out, such glasses as sheet or plate, which appear 
 practically colourless when viewed in the ordinary way, 
 show a very decided green colour when viewed through a 
 considerable thickness. In the same way, a very thin layer 
 of the glass known as " flashing ruby " shows a brilliant red 
 tint, but a thickness of one-sixteenth of an inch is sufficient 
 to render the glass practically opaque, giving it a black 
 appearance by both transmitted and reflected light. Again, 
 cobalt blue glass, when examined with a spectroscope in 
 thin layers, is found to transmit a notable proportion of 
 red rays, but thicker pieces entirely suppress these rays. 
 These phenomena will be readily understood when we 
 recollect that colour in a transparent medium arises from 
 the fact that the medium has different absorbing powers for 
 light of different colours. All transparent substances, and 
 certainly glass, are only partially transparent : all light 
 waves passing through such a substance are gradually 
 
 N 2 
 
180 GLASS MANUFACTUEE. 
 
 absorbed, and the extent to which they are absorbed differs 
 according to the length of these waves. It always happens 
 that for some special wave-lengths the substance has the 
 power of absorbing the energy of the entering waves and 
 converting it into heat-vibrations of its own molecules or 
 atoms. In the most transparent and colourless glasses this 
 process, so far as the waves of ordinary light are concerned, 
 only goes on to a negligibly slight extent ; if, however, we 
 extend our view beyond the range of ordinary visible light, 
 and consider the region of shorter waves that lies in 
 the spectrum beyond the violet, we find that ordinary 
 colourless glass becomes strongly absorbent ; thus to waves 
 of about half the length of those which produce upon our 
 eyes the impression of yellow light, ordinary glass is as 
 opaque as is a piece of metal to white light. In this wider 
 sense, then, we may fairly say that all glasses are coloured 
 i.e., all have a power of selective absorption ; but in the 
 case of those which are nearly colourless in the ordinary 
 sense, this absorption takes place only for waves which are 
 either decidedly shorter or decidedly longer than those to 
 which our eyes are sensitive. Those glasses which appear 
 coloured in the ordinary sense, on the other hand, owe this 
 property to the fact that the power of absorption for 
 light-waves extends into the region of the visible spectrum ; 
 thus a blue or violet glass is practically opaque to red rays, 
 while a red glass is opaque to blue, green or violet rays. 
 This statement may be verified in a striking manner by 
 holding over one another a piece of deep blue or green 
 glass and a similar piece of ruby glass the combination 
 will be found to be very nearly opaque even when each 
 glass by itself is practically transparent. 
 
COLOUBED GLASSES. 181 
 
 The question which now naturally presents itself to us is, 
 what is the essential difference between, for instance, a 
 piece of red glass and a piece of " white " glass that confers 
 upon the former the power of absorbing blue light ? A 
 perfectly complete and satisfactory answer to this question 
 is not, in the writer's opinion, available in the present state 
 of our knowledge, but to a certain extent the difference 
 between the two kinds of glass can be explained. The 
 difference is produced, in the /first instance by introducing 
 into the colourless glass some additional chemical element 
 or elements, the substances in question being generally 
 known as " colouring oxides," although they are by no 
 means always introduced in the form of oxides, and are 
 frequently present in the glass in entirely different forms. 
 To a certain extent the colour of the glass may be ascribed 
 to a definite " colouring " property of the chemical elements 
 concerned ; thus most of the chemical compounds of such 
 elements as nickel, cobalt, iron, manganese and copper are 
 more or less deeply coloured substances, and it would seem 
 as if the atoms or " ions " of these elements had the specific 
 power of absorbing certain varieties of light-waves while 
 not materially affecting others. But this specific " colour- 
 ing" property is not so easily explained when we recollect 
 that the colours of iron compounds, for example, may be 
 green or red according to the state of combination in which 
 that element is present, and that iron has also the power of 
 imparting either a green or a yellow colour to glass accord- 
 ing to circumstances. The detailed discussion of these 
 questions, however, lies outside our present scope, and we 
 must confine ourselves to the broad statement that colour- 
 ing substance in glass may be roughly divided into two 
 
182 GLASS MANUFACTURE. 
 
 kinds or groups ; the first and probably the largest group 
 are those bodies which occur in glass in true solution, the 
 element itself being present in the combined state as a 
 silicate or other such compound (borate, phosphate, etc.) 
 which is soluble in the glass. In this class, the colouring 
 effect upon the glass is specifically that of the element 
 introduced, and is brought about in the same way as the 
 colouring of water when a coloured salt such as copper 
 sulphate is dissolved in it. The second class of colouring 
 substances, however, behave in a different manner; they 
 are probably present in the glass in a state of extremely 
 fine division, and held not in true solution, but really in a 
 sort of mechanical suspension that approximates to the 
 condition of what is known as a " colloidal solution." The 
 point which is known beyond doubt, thanks to the researches 
 of Siedentopf and Szigmondi on ultra-microscopical particles, 
 is that in certain coloured glasses, of which ruby glass is 
 the best example, the colouring substance, be it gold or 
 cuprous oxide, is present in the form of minute but by no 
 means atomic or molecular particles suspended in the glass. 
 The presence of these particles has been made optically 
 evident, although it can hardly be said that they have been 
 rendered visible, and it is at all events probable that these 
 suspended particles act each as a whole in absorbing the light- 
 waves characteristic of the colour which they produce in 
 glass. This being the case, it is easy to understand how 
 readily the colour of such glasses is altered or spoilt by 
 manipulations which involve heating and cooling at different 
 rates too rapid a rate of cooling producing a different 
 grouping of the minute particles, altering their size or 
 shape, or even obliterating them entirely by allowing the 
 
COLOUEED GLASSES. 183 
 
 element in question to go into or to remain in solution in 
 the glass. 
 
 While it would be entirely foreign to the purpose of this 
 volume to give in this place a series of recipes for the 
 production of various kinds of coloured glass, it will be 
 desirable to state in general terms the colours or range of 
 colours which can be produced in various kinds of glass by 
 the introduction of those chemical elements which are 
 ordinarily used in this way. In general terms it may be 
 said that the lighter elements do not as a rule tend to the 
 production of coloured glasses, while the heavier elements, 
 so far as they can be retained in the glass in either solution 
 or suspension, tend to produce an intense colouring effect. 
 The element lead appears to form a striking exception to 
 this rule, but this is due to the fact that while the silicates 
 of most of the other heavy elements are more or less 
 unstable, the silicate of lead is very stable, and can only be 
 decomposed by the action of reducing agents. When lead 
 silicates are decomposed in this way, however, the resulting 
 glass immediately receives an exceedingly deep colour, 
 being turned a deep opaque black, although in very thin 
 layers the colour is decidedly brown. On the other hand, 
 glasses very rich in lead are always decidedly yellow in 
 colour, and it has been shown that this coloration is due 
 to the natural colour of lead silicates and not to the presence 
 of impurities. What has just been said of lead applies, 
 with only very slight modification, also to the rare metal 
 thallium and its compounds, which have been introduced 
 into glass for special purposes. Leaving these two excep- 
 tional bodies on one side, we now pass to a consideration of 
 the elements in the order of their chemical grouping. The 
 
184 GLASS MANUFACTURE. 
 
 rare elements will not be considered except in certain cases 
 where their presence in traces is liable to affect results 
 attained in practice. 
 
 The Alkali Metals, sodium, potassium, lithium, etc., and 
 their compounds, have no specific colouring effect, although 
 the presence of soda or of potash in a glass affects the 
 colours produced by such substances as manganese, nickel, 
 selenium, etc. 
 
 Copper, as would be anticipated from the deep colour of 
 most of its compounds, produces powerful colouring effects 
 on glass. Cupric silicates produce intense green, to greenish- 
 blue tints. Copper, either as metal or oxide, added to glass 
 in the ordinary way, always produces the green colour ; but 
 when the full oxidation of the copper is prevented by the 
 presence of a reducing body, and the glass is cooled slowly, 
 or is exposed to repeated heating followed by slow cooling, 
 an intense ruby coloration is produced. In practice this 
 colour is produced by introducing tin as well as copper into 
 the mixture, and so regulating the conditions of melting as 
 to favour reduction rather than oxidation of the copper. 
 Under these circumstances the copper is left in the glass in 
 a finely divided and evenly suspended state ; if exactly the 
 right state of division and suspension is arrived at, a 
 beautiful red tint is the result, although the coloration of 
 the glass is so intense that it can only be employed in very 
 thin sheets, being " flashed " upon the surface of colourless 
 glass to give it the necessary strength and thickness for 
 practical use. It is further very easy to slightly alter the 
 arrangement of the copper in the glass, with the result of 
 producing an opaque, streaky substance resembling sealing- 
 wax in colour and appearance, this product being, of course, 
 
COLOURED GLASSES. 185 
 
 useless from the glass-maker's point of view. Finally, by 
 exceedingly slow cooling, and under other favouring condi- 
 tions which are not really understood, the particles of 
 suspended colouring-material be it metallic copper or 
 cuprous oxide grow in size and attain visible dimensions, 
 appearing as minute shimmering flakes, thus producing the 
 beautiful substance known as " aventurine." 
 
 Silver is never introduced into glass mixtures, the reason 
 being that it is so readily reduced to the metallic state 
 from all its compounds that it cannot be retained in the 
 glass except in a finely-divided form, causing the glass to 
 assume a black, metallic appearance resembling the stains 
 produced by the reduction of lead in flint glasses. On the 
 other hand, silver yields a beautiful yellow colour when 
 applied to glass as a surface stain, and it is widely used for 
 that purpose. 
 
 Gold is introduced into glass for the production of brilliant 
 ruby tints ; its behaviour is very similar to that of copper, 
 except that the noble metal has a great tendency to return 
 to the metallic state without the aid of reducing agents. 
 No addition of tin is therefore required, but the rate of 
 cooling, etc., must be properly regulated, since rapidly 
 cooled glass containing gold shows no special colour, the 
 rich ruby tint being only developed when the glass is 
 re-heated and cooled slowly. The colouring effect of gold 
 is undoubtedly more regular and uniform than that of 
 copper, and it is accordingly possible to obtain much lighter 
 shades of red with the aid of the noble metal. " Gold ruby " 
 can therefore be obtained of a tint light enough to be used 
 in sheets of ordinary thickness, and the process of " flashing " 
 is not essential. 
 
186 GLASS MANUFACTURE. 
 
 The elements of the second group, such as magnesium, 
 calcium, strontium, barium, zinc and cadmium, exert no 
 strong specific colouring action on glass, with perhaps the 
 exception of cadmium, and that element only does so to 
 any considerable extent in combination with sulphur, 
 sulphide of cadmium having the power of producing rich 
 yellow colours in glass. The sulphur compounds of barium 
 also readily produce deep green and yellow colours, and the 
 formation of these tints is, indeed, very difficult to avoid in 
 the case of glasses containing much barium. A colouring 
 effect has sometimes been ascribed to zinc, but this is not 
 in accordance with facts. 
 
 Of the elements of the third group, only boron and 
 aluminium are ever found in glass in any notable quantity. 
 Boron is present in the form of boric acid or borates, and as 
 such produces no colouring effect, nor does there seem to be 
 any tendency for the separation of free bor.on. The com- 
 pounds of aluminium also possess no colouring effect, 
 although certain compounds of this element are utilised for 
 imparting a white opacity to glass for certain purposes 
 such glass being known as "qgal." 
 
 The elements of the fourth group are of greater importance 
 in connection with glass. Carbon is capable of exerting 
 powerful colouring effects when introduced into glass. 
 These effects are of two kinds, viz., indirect in consequence 
 of the reducing action of carbon on other substances 
 present, and direct from the presence of finely-divided 
 carbon or carbides in the glass. The latter are similar in 
 kind to those produced by the presence of other finely- 
 divided elementary bodies (copper, gold, lead, etc.) except 
 that the lightness of the carbon particles tends to the 
 

 COLOITEED GLASSES. 187 
 
 production of yellow and brown colours rather than of red 
 and black, while the chemical nature of carbon renders the 
 glass in which it is suspended indifferent to rapid cooling, 
 so far as the carbon tint is concerned. The indirect effects 
 of carbon, in reducing other substances that may be present 
 in the glass, become evident with much smaller proportions 
 of carbon than are required to produce visible direct effects. 
 As we have seen above, carbon, in the form of coke, char- 
 coal or anthracite coal, is regularly introduced, as a reducing 
 medium, into glass mixtures containing sulphate of soda. 
 If even a slight excess of carbon be used for this purpose, 
 the formation of sulphides and poly-sulphides of sodium 
 and of calcium results, and these bodies, like all sulphides, 
 impart a greenish-yellow tint to the glass, at the same 
 time bringing other undesirable results in their train. 
 
 Silicon, in the form of silicic acid and its compounds, is a 
 fundamental constituent of all varieties of glass, and in this 
 form is in no sense a colouring substance ; on the other 
 hand, there is no doubt that under some conditions silicon 
 may be reduced to the metallic state at temperatures which 
 normally occur in glass-furnaces, and it is practically 
 certain, that if present in glass in this condition, silicon 
 would colour the glass. It is just possible that some of the 
 colouring effects produced in ordinary glass by powerful 
 reducing agents, such as carbon, either in the solid form or 
 as a constituent of furnace gases, may be due to the reduction 
 of silicon in the glass. 
 
 Tin by itself does not appear to have any colouring effect 
 upon glass, except that its oxide, in a finely suspended 
 state, produces opalescence and, in large quantities, white 
 opacity. Tin, however, is used in conjunction with copper 
 
188 GLASS MANUFACTURE. 
 
 in the production of copper-ruby, to which reference has 
 already been made. 
 
 Lead and Thallium have already been dealt with, and it 
 only remains to add that their presence in the glass, 
 although not in itself producing any intense colouring 
 action, increases the colouring effects of other substances. 
 This is probably merely a particular case of the fact that 
 dense glasses, of high refractive index, are more sensitive 
 to colouring agencies than the lighter glasses of low 
 refractive index ; this applies to barium as well as to lead 
 and thallium glasses. 
 
 Phosphorus occurs in some few glasses in the form of 
 phosphoric acid, and this substance, as such, has no colouring 
 effect. Calcium phosphate, however, is sometimes added to 
 glasses for the purpose of producing opalescence. Its 
 action in this respect is probably similar to that of tin 
 oxide and aluminium fluoride, these substances all remaining 
 undissolved in the glass in the form of minute particles in 
 a finely divided and suspended state. 
 
 Arsenic does not exert a colouring effect on glass, and 
 owing to its volatile nature it can only be retained in glass 
 in small quantities and under special conditions. A 
 " decolourising " action is sometimes ascribed to arsenic, 
 but if this action really exists it can only be ascribed to the 
 fact that arsenic compounds are capable of acting as carriers 
 of oxygen, and their presence thus tends to facilitate the 
 oxidation of impurities contained in the glass. A further 
 reference to this subject will be found below in reference to 
 the compounds of manganese. 
 
 Antimony, although frequently added to special glass 
 mixtures, does not appear to produce any very power- 
 
COLOUEED GLASSES. 189 
 
 ful effects, except possibly in the direction of producing 
 white opacity if present in large proportions. The sulphide 
 of antimony, however, exerts a colouring influence, although 
 its volatile and unstable character renders the effects 
 uncertain. 
 
 Vanadium, owing to its rarity, is probably never added 
 to glass mixtures for colouring purposes, although it is 
 capable of producing vivid yellow and greenish tints when 
 present even in minute proportions. On the other hand, 
 vanadium occurs in small proportions in a number of fire- 
 clays, including some of those of the Stourbridge district, 
 and glass melted in pots containing this element is liable 
 to have its colour spoilt by taking up the vanadium from 
 the clay. 
 
 Sulphur is an element whose presence in various forms 
 is liable to affect the colour of glass in a variety of ways. 
 The colouring effects of sodium-, calcium-, cadmium-, and 
 antimony -sulphides have already been referred to. Sulphur 
 probably never exists in glass in the uncombined state 
 at all, but sulphur and its oxides, which are often 
 contained in furnace gases, sometimes exert a very marked 
 action upon hot glass. The presence of sulphur gases 
 in the atmospheres of blowing-holes and annealing kilns is 
 liable to produce in the glass a peculiar yellowish milkiness 
 which penetrates for a considerable depth into the mass of 
 the glass and cannot be removed by subsequent treatment. 
 Glass vessels, particularly if made of glass produced from 
 raw materials among which salt-cake has figured, are also 
 affected by contact with fused sulphur or its vapour, the 
 effect being a gradual disintegration of the glass. The 
 precise mechanism of these actions is not known at present, 
 
190 GLASS MANUFACTUKE. 
 
 but they probably consist in the formation of sulphur com- 
 pounds within the glass, possibly giving rise to an evolution 
 of minute bubbles of gas. 
 
 Selenium, which is chemically so closely related to 
 sulphur, is a relatively rare element, which is, however, 
 finding some use in glass-manufacture as a colouring and a 
 decolouring agent. The introduction of selenium or of its 
 compounds under suitable conditions into a glass mixture 
 produces or tends to produce a peculiar yellowish-pink 
 coloration, the intensity of the colour produced being 
 dependent upon the chemical nature of the glass as a whole 
 and, of course, upon the amount of selenium left in the 
 glass at the end of the melting process, this latter in turn 
 depending upon the duration and temperature of the 
 process in question. The pini colour of selenium glass is 
 best developed in those containing barium as a base, but it 
 is also developed in lead glasses, while soda-lime glasses do 
 not show the colour so well. As a " decolouriser " the 
 action of selenium is entirely that of producing a comple- 
 mentary colour which is intended to " cover" the green or 
 blue tint of the glass ; where the depth of the tint to be 
 "covered" is small, selenium can be used very successfully 
 in this way, although it is a relatively costly substance for 
 such a purpose. No oxidising or "cleansing" action can 
 be ascribed to selenium or its compounds. 
 
 Chromium is one of the most intensely active colouring 
 substances that are available for the glass-maker, and it is 
 accordingly used very extensively. It has the advantage of 
 relative cheapness, and can be conveniently obtained and 
 introduced into glass in the form of pure compounds whose 
 colouring effect can be accurately anticipated ; the colours 
 
COLOURED GLASSES. 191 
 
 produced by the aid of chromium have the further advantage 
 of being very constant in character, being little affected by 
 oxidising or reducing conditions, and only very slightly by 
 the length or temperature of the melting process. The 
 rate of cooling, in fact, appears to be the only factor that 
 materially affects the colours produced by compounds of 
 chromium. The colours produced by chromium alone are 
 various depths of a bright green, the depth varying, of 
 course, with the proportion of chromium that is present in 
 the glass and with the purity of the glass itself. Very 
 frequently, chromium is used in conjunction with either 
 iron or copper to produce various tints of "cold blue" 
 and " celadon green " respectively. This element is most 
 usually introduced into the glass mixture in the form of 
 potassium bichromate ; although other compounds might 
 be employed, this substance presents several advantages to 
 the glass maker. In the first place, since the colouring 
 effect of chromium is very intense, it must be used in very 
 small quantities, and if chromic oxide itself were used, the 
 weighing would have to be carried out with extreme 
 care ; potassium bichromate, however, contains a much 
 smaller proportion of the effective colouring substance, so 
 that much larger weights can be employed, and the accuracy 
 of weighing required is proportionately reduced. A further 
 consideration arises from the fact that chromic oxide is 
 itself an extremely refractory body, and is therefore com- 
 paratively difficult to incorporate with glass, while its 
 presence tends to make the glass itself more viscid and 
 refractory; the simultaneous introduction of the alkali, 
 as provided by the use of the bichromate, is thus an 
 advantage in restoring the fluidity and softness of the glass 
 
192 GLASS MANUFACTURE. 
 
 when finished, while also facilitating the solution of the 
 chromium in the glass during the fusion process ; this 
 process of solution, however, takes some time, chromium 
 glasses being liable to appear patchy if insufficient time is 
 given to the "founding." 
 
 Uranium is one of the rarer and more costly elements, 
 but is nevertheless used in glass-making for special purposes 
 on account of the very beautiful fluorescent yellow colour 
 which it imparts when added in small proportions. This 
 yellow is quite characteristic and unmistakable, so that 
 none of the other varieties of yellow glass can ever be used 
 as a substitute for uranium glass, but the great cost of the 
 latter prevents its extended use. Uranium is usually 
 introduced into glass mixtures in the form of a chemical 
 compound, such as uranyl-acetate or uranyl-nitrate, both 
 these substances being obtainable in the form of small, 
 intensely bright yellow crystals. 
 
 Fluorine occurs in a number of glasses in the form of 
 dissolved or suspended fluorides, principally fluoride of 
 aluminium. The element is not essentially a colouring 
 substance, and is only mentioned here because the fluoride 
 named is the most frequently used means of producing 
 "opal" glass. The fluoride is most frequently introduced 
 into the glass mixtures as calcium fluoride, used in 
 conjunction with felspar, or as cryolite, a natural mineral 
 which consists of a double fluoride of sodium and 
 aluminium. 
 
 Manganese is one of the most important colouring 
 elements used by the glass-maker. When introduced into 
 glass in the absence of other colouring ingredients, 
 compounds of manganese produce a range of colours lying 
 
COLOURED GLASSES. 193 
 
 in the region of pinkish-purple to violet, according to the 
 chemical nature of the glass. The exact colour produced 
 varies according as the glass has lead, lime or harium as 
 its base, and it also depends upon the presence of soda or 
 potash as the alkaline constituent. The nature and 
 intensity of the colour, however, which the addition of a 
 given percentage of manganese will produce depends upon 
 other factors besides the chemical composition of the bases 
 used in the mixture. The heat and duration of the 
 " found " and the reducing or oxidising conditions of the 
 furnace in which it has been carried on very materially 
 affect the result. Thus, a glass having a slight tinge of 
 pink or purple derived from manganese can be rendered 
 entirely colourless by the action of reducing gases or by intro- 
 ducing into the glass a reducing substance, such as a piece of 
 wood. It will thus be seen that while manganese is a most 
 useful element for the glass-maker, its employment requires 
 much skill and care, and generally involves some troublesome 
 manipulations before the desired result is attained. 
 
 In practice, manganese is most frequently used with 
 other colouring ingredients for the production of what may 
 be called " compound " colours, the function of the 
 manganese being to provide the " warm " element, i.e., the 
 pink or purple component, required. One of the most im- 
 portant uses of manganese coming under this head is its use 
 as a " decolouriser." By a " decolouriser " the glass-maker 
 understands a substance which can be used to improve the 
 colour of a glass which, from the nature of its raw materials 
 and conditions of melting; would have a greener colour than 
 is thought desirable for the product in question. It may 
 be said at once that the most perfect and satisfactory 
 
 G.M. o 
 
194 GLASS MANUFACTUBE. 
 
 method of obtaining the better colour required is to adopt 
 the use of purer raw materials and methods of melting less 
 liable to lead to contamination of the glass. On the other 
 hand, this radical course is often impossible on the ground 
 of expense, and the less satisfactory course must be adopted 
 of covering one undesirable colour by another complementary 
 colour which would, in itself, be equally undesirable. The 
 rationale of this procedure depends upon the fact that a 
 slight amount of absorption of light is not readily detected 
 by the human eye if it be uniformly or nearly uniformly 
 distributed over the whole range of the visible spectrum, 
 i.e., if the colour of the resulting light is nearly neutral, 
 while an equally slight absorption in one region of the 
 spectrum, while actually allowing more light to pass through 
 the glass, is at once detected by the eye owing to the colour of 
 the transmitted light. Now it has been found that the colour 
 produced in glass by the addition of very small proportions 
 of manganese is approximately complementary to the 
 greenish-blue tinge of the less pure varieties of ordinary 
 glass ; the addition of manganese in suitable proportions to 
 such glass therefore results in the production of a glass 
 which transmits light of approximately neutral, usually 
 slightly yellow, colour, the increased total absorption only 
 becoming noticeable in large pieces. This ''covering" of 
 the greenish tinge is generally most completely successful 
 in the case of soda-flint glasses, but the method is also used 
 to a certain extent in the case of the soda-lime glasses used 
 for sheet and plate-glass manufacture. Manganese added 
 to glass for this purpose is generally introduced into the 
 mixture in the form of the powdered black oxide (manganese 
 dioxide), which is available as a natural ore in a condition 
 
COLOURED GLASSES. 195 
 
 of sufficient purity. Added in this form, the manganese 
 compound exerts a double action, the decomposition of the 
 dioxide resulting in the liberation of oxygen within the 
 mass of melting glass, and this oxygen itself exerts a 
 favourable influence on the resulting colour of the glass, 
 since it removes organic materials whose subsequent reducing 
 action would be deleterious, and it also converts all iron 
 compounds present into the more highly-oxidised (ferric) 
 state in which their colouring effects are less intense. The 
 actual colouring effect of the manganese itself is, of course, 
 afterwards developed, and produces the effects discussed 
 above. 
 
 The "covering" of the greenish tints due to iron and 
 other compounds is only possible when these are present 
 in very small proportions. When larger quantities of 
 these substances have been introduced into the glass the 
 addition of manganese modifies the resulting colour, but is 
 no longer able to neutralise it. A very large range of 
 colours can be obtained by using various proportions of 
 iron and manganese, the best-known of these being the 
 warm brown tint known as "hock-bottle," while all shades 
 between this and the bright green of iron and the purple of 
 manganese can be obtained by suitable mixtures. What 
 has been said above as to the sensitiveness of manganese 
 colours applies with even greater force to these mixed tints, 
 since here both the iron and the manganese compounds 
 are liable to undergo changes of oxidation. Copper- 
 manganese and chromium-manganese colours are also 
 used, as indeed almost any number of colouring ingredients 
 may be simultaneously introduced into a glass mixture, 
 the resulting colour being, as a rule, purely additive. 
 
 o 2 
 
19H GLASS MANUFACTURE. 
 
 Iron is so widely distributed among the materials of the 
 earth's crust that it is exceedingly difficult to exclude it 
 entirely from any kind of glass, although the purest 
 varieties of glass contain the merest traces of this element. 
 Cheaper varieties of glass, however, always contain iron 
 in measurable quantity, while the cheapest kinds of 
 glass contain considerable proportions of this element. 
 The colouring effects of iron have already been alluded to 
 at various points in the earlier chapters as well as in the 
 section on manganese just preceding. Little further 
 remains to be said here. Just as the less highly oxidised 
 compounds of iron i.e., the " ferrous " compounds always 
 show a decided green tint, so glasses containing iron when 
 melted under the usually prevalent reducing conditions of 
 a glass-making furnace, show a decided green tint whose 
 depth depends upon the amount of iron present, provided 
 no manganese or other " decolouriser " has been introduced. 
 " Ferrous ; ' compounds are, however, readily converted 
 into the more highly oxidised or " ferric " state by the 
 action of oxidising agents, and this change can also be 
 brought about in molten glass by the action of such sub- 
 stances as nitrates or other sources of oxygen. The ferric 
 compounds, however, show characteristic yellow tints which 
 are much less intense and vivid than the corresponding 
 green colours of the " ferrous " series, and a similar result 
 is brought about by the oxidation of iron compounds con- 
 tained in glass ; hence the "washing " or cleansing effects 
 ascribed to oxidising agents introduced in the fusion of 
 glass. It should, however, be borne in mind that the 
 oxidation of other substances besides iron compounds, viz., 
 organic matter, carbon and sulphur compounds, may, and 
 
COLOURED GLASSES. 197 
 
 probably does, play a most important part in this process 
 in the case of most varieties of glass. 
 
 Nickel exerts a powerful colouring influence on glass, in 
 accordance with the fact that most of the other compounds 
 of this element are also deeply coloured. The exact colour 
 produced in glass depends upon the nature of the glass 
 and on the condition of oxidation in which the nickel is 
 present. The colours, however, are usually of a greenish- 
 brown tint, although brighter colours can be produced by 
 nickel under special conditions. This element is not, 
 however, much used as a colouring agent in practice, 
 although it has been advocated as a " decolouriser." The 
 writer is not, however, aware that it has ever been success- 
 fully used for this purpose, and, in fact, the colours to which 
 it gives rise do not appear to be even approximately com- 
 plementary to the ordinary green and blue tints which 
 " decolourisers " are intended to cover. 
 
 Cobalt is one of the most powerful colouring agents in 
 glass, and is very largely used in the production of all 
 varieties of blue glass. The blue colour produced by 
 cobalt is, in fact, probably the most "certain" of the 
 colours available to the glass-maker, this tint being least 
 affected by all those circumstances that lead to variations 
 in other tints. Almost the only difficulty involved in the use 
 of cobalt is the great colouring power of this element, which 
 requires that for most purposes only very small quantities 
 may be added to the glass mixture. Formerly cobalt was 
 added to glass mixtures in the form of " zaffre," which was 
 a very impure form of cobalt oxide. At the present time, 
 however, the more expensive but much more satisfactory 
 pure oxide of cobalt is in almost universal use. This 
 
198 GLASS MANUFACTURE. 
 
 substance shows a perfectly constant composition and, by 
 means of accurate weighing, enables the glass-maker to 
 introduce precisely the right amount of cobalt into his 
 batch. 
 
 The range of colours which are available to the modern 
 glass manufacturer are, as will be seen from a considera- 
 tion of the list of colouring elements given above, practi- 
 cally unlimited, particularly as these substances can be 
 used in almost any combination to produce mixed or inter- 
 mediate tints. This practically infinite variety of possible 
 tints, indeed, involves the principal difficulty encountered by 
 the manufacturer of coloured glass, i.e., that of matching 
 his tints, or of keeping the colour of any particular variety 
 of glass so constant that pieces produced at various times 
 can be used indiscriminately together. This ideal is, 
 perhaps, never entirely realised, but in the case of glasses 
 intended for special technical uses the ideal degree of 
 constancy is very closely approached. 
 
 In addition to being called upon to produce a large 
 variety of different tints, the glass-maker is also called 
 upon to produce various depths of the same tint. In many 
 cases this can be readily done by the simple means of 
 varying the amount of colouring material added to the 
 glass. Where the colouring effect of small quantities of 
 these substances is not excessively powerful there is no 
 very great difficulty in doing this, but in certain cases this 
 mode of regulating the intensity of the colour is not 
 available. Thus copper-ruby glass cannot readily be made 
 of so light a tint as to appear of reasonable depth when used 
 in sheets of the thickness of ordinary sheet-glass. As has 
 already been indicated, the desired tint is obtained by the 
 
COLOUBED GLASSES. 199 
 
 process of " flashing," i.e., of placing a verythin layer of deep 
 ruby-coloured glass upon the surface of a sheet of ordinary 
 more or less colourless glass of the usual thickness. This is 
 generally accomplished by having a pot of molten ruby 
 glass available close to a pot from which colourless glass is 
 being gathered. A small gathering of ruby glass is first 
 taken up on the pipe, and the remaining gatherings required 
 for the production of the sheet are taken from the pot of 
 colourless glass. When such a composite gathering is 
 blown into a cylinder in the manner described in the 
 previous chapter, the ruby glass lies as a thin layer over 
 the inner face of the cylinder, but special care and skill on 
 the part of the gatherer and blower is required to ensure 
 that this layer shall be evenly distributed and of the 
 right thickness to produce just the tint of ruby required. 
 Since the whole layer of red glass is so thin, a very slight 
 want of uniformity in its distribution leads to wide varia- 
 tions of tint, and in practice these are often seen in the 
 less successful cylinders of such glass. 
 
 The chemical composition of the ruby and the colourless 
 glass which are to be employed for this purpose must also 
 be properly adapted to one another in order to produce two 
 glasses which shall have as nearly the same coefficient of 
 thermal expansion as possible. If this requirement is not 
 met, the resulting glass is subjected to internal strains 
 which may lead to fracture, while, if the ruby glass has the 
 higher co-efficient of expansion, the sheet after flattening 
 tends to draw itself up on the " flashed " side and cannot 
 be passed out of the annealing kiln in a properly flat con- 
 dition. 
 
 Although most usually applied to copper-ruby glass, the 
 
200 GLASS MANUFACTURE. 
 
 flashing process is often used with other colours also. 
 Coloured glass of this kind is at once recognised when 
 looked at through the edges. Thus examined the glass 
 simply shows the greenish tint of ordinary sheet-glass 
 which constitutes practically the entire thickness of the 
 sheet. In the same way, if such " flashed " glass be cut or 
 etched in such a way that the layer of coloured glass is 
 removed in places, the resulting pattern appears in white 
 on the coloured ground a feature which is utilised for 
 certain decorative purposes. The flashing process just 
 described, it should be noted, is applicable to any form of 
 glassware which is blown from a gathering, and the 
 coloured layer can be applied either upon the inside or 
 outside of any object thus produced. 
 
 In addition to the palette of colours which the glass- 
 maker is able to supply, the artist in stained glass has a 
 fnrther range of colours at his disposal in the form of 
 stains and transparent colours which can be applied to the 
 surface of glass and developed and rendered more or less 
 permanent by being properly " fired." The colours pro- 
 duced in this way are also, in one sense, coloured glasses, 
 or rather glazes, whose raw materials are put upon the 
 glass by the brush of the painter, and only subsequently 
 caused to combine and melt by suitable heating. The 
 degree of heat applicable under these circumstances is, 
 however, very limited by the necessity of avoiding any 
 great softening of the substratum of glass, while many of 
 the colours themselves are composed of materials which 
 could not resist very high temperatures. The fluxes used 
 in the composition of these colours must for this reason be 
 of a very fusible kind, with the inevitable result of a greatly 
 
COLOURED GLASSES. 201 
 
 reduced chemical stability as compared with the glass 
 itself. 
 
 The whole subject of painting on glass, even from the 
 purely technical as apart from the aesthetic point of view, 
 is a very wide one, and lies outside the scope of the present 
 volume. Only one further technical point in connection 
 with glass-painting and stained glass work will therefore 
 be touched upon here. This is an example of the fact that 
 the more technically "perfect" modern product is not 
 always preferable for special purposes which have been 
 well served by older and far less " perfect " products. The 
 production of technically excellent coloured glass in modern 
 times was, somewhat surprisingly at first, accompanied by 
 a very marked decline in the artistic beauty of stained glass 
 windows produced with this modern material ; the ancient 
 art of stained glass was, therefore, for a time regarded as a 
 " lost art," and glass-makers were blamed for being unable 
 to produce the brilliant and beautiful tints which had been 
 formerly available. More careful study, however, revealed 
 the fact that while the actual colour of modern glass was at 
 least as brilliant and varied as that of ancient glass, the 
 difference lay in the fact that the modern glass was practi- 
 cally entirely free from such imperfections as air-bubbles, 
 striae, and other defects which improved appliances and 
 methods had enabled the glass-maker to eliminate from his 
 products. Finding the beauty of his wares greatly improved 
 by this increased purity of the glass in the case of window 
 glass and table ware, it was natural for the glass-maker to 
 endeavour to produce the same "improvement" in the 
 coloured glasses intended for artistic purposes and, indeed, 
 it is more than likely that the stained-glass workers them- 
 
202 GLASS MANUFACTUEE. 
 
 selves pressed this line of improvement upon him by a 
 demand for "better" glass. It turned out, however, on 
 close examination, that this very perfection of modern 
 glass rendered it less adapted for these artistic purposes. 
 A perfect piece of glass, having smooth surfaces and no 
 internal regularities, allows the rays of light falling upon 
 it to pass through undeflected in direction, and merely 
 changed in colour, according to the tint of the glass in 
 question. On looking at the glass, external objects can be 
 quite clearly seen, and much of the interest and mystery of 
 the glass itself is lost. On the other hand, when falling 
 upon a piece of glass having an irregular surface, and con- 
 taining all manner of irregularities such as striae, air bells, 
 and even pieces of enclosed solid matter, the light is 
 scattered, refracted, and deflected into all manner of 
 directions until it almost appears to emanate from the 
 body of the glass itself, which thus appears almost to shine 
 with an internal light of its own ; the eye can hardly 
 perceive the presence of external objects, and the whole 
 window appears as a brilliant self-luminous object. 
 
 Once their attention had been drawn to these facts, 
 modern glass-makers endeavoured, and with much success, 
 to reproduce the desirable qualities of the ancient glass, 
 while still availing themselves of modern methods to pro- 
 duce more stable glasses and a wider range of colours. 
 The irregular surface of the old glass is imitated by using 
 rolled or " muffed " instead of ordinary blown glass, while 
 the internal texture is rendered non-homogeneous by the 
 deliberate introduction of solid and gaseous impurities and 
 by manipulations so arranged as to leave the glass in layers 
 of different density, which appear in the finished glass as 
 
COLOUEED GLASSES. 203 
 
 " striae." As a consequence, it is probably not too much to 
 claim that the modern workers in coloured glass have 
 materials at their disposal which are at least as suitable 
 for the purpose as those that were available in the best days 
 of the ancient art. 
 
 Some reference has already been made to the technical 
 uses of coloured glass, but one or two further points in that 
 connection remain to be discussed. For such technical 
 purposes as railway and marine signals, the consensus of 
 practical experience has decided in favour of certain colours 
 of glass, such as red and green of particular tints. On the 
 other hand, for various purposes in connection with photo- 
 graphy, the glass-maker does not appear to have been able 
 to meet the new requirements, with the result that flimsy 
 and otherwise unsatisfactory screens made of gelatine or 
 celluloid stained with organic dyes are employed in place 
 of coloured glass in such cases, for example, as the covering 
 of lamps for use in photographers' "dark" rooms, and for 
 the light-filters used for orthochromatic and tri-chromatic 
 photography. In all these cases it is necessary to use a 
 transparent coloured medium which transmits only light of 
 a certain very definite range of wave-lengths, and there is 
 no doubt that for the glass-maker, who is confined to the 
 use of a number of elementary bodies for his colouring 
 media, it is b}^ no means easy to comply with these require- 
 ments of exact transmission and absorption. On the other 
 hand, the field of available coloured glasses has not been 
 fully explored from this point of view, the only extensive 
 work on the subject having been done in connection with 
 the Jena firm of Schott, who have put upon the market a 
 series of coloured glasses of accurately-known absorbing 
 
204 . GLASS MANUFACTUEE. 
 
 power. There is, however, little doubt that a much greater 
 extension of this field is possible, and that it will be opened 
 up by a glass-maker who undertakes the exhaustive study 
 of coloured glasses from this point of view, although it 
 must be admitted that there is considerable doubt whether 
 the results obtainable by the aid of aniline and other dyes 
 as applied to gelatine can ever be equalled by coloured 
 glasses. 
 

 CHAPTEK XII. 
 
 OPTICAL GLASS. 
 
 OPTICAL glass differs so widely from all other varieties of 
 glass that its manufacture may almost be regarded as a 
 separate industry, to which, indeed, a separate volume 
 could well be devoted. In the present chapter we propose 
 to give an outline of the most important properties of 
 optical glass, and in the next chapter to describe the more 
 important features of the processes used in its production. 
 
 The properties which affect the value of optical glass may 
 roughly be divided into two groups. The first group com- 
 prises the specifically "optical" properties i.e., those 
 directly influencing the behaviour of light in its passage 
 through the glass, while the second group covers those 
 properties of a more general nature, which are of special 
 importance in glass that is to be used for optical purposes. 
 
 Optical Properties of Glass. The most essential property 
 of glass in this respect is homogeneity. We have already 
 indicated that glass can never be regarded as a definite 
 chemical substance or compound, but that it usually con- 
 sists of mutual solutions of various complex silicates, 
 borates, etc. Solutions being of the very nature of 
 mixtures of two or more different substances, it follows 
 that they can only become homogeneous when complete 
 
206 GLASS MANUFACTURE. 
 
 mixing has taken place. We have a familiar example of 
 the formation of such a solution when sugar is dissolved in 
 water. The water near the sugar becomes saturated with 
 sugar and of different density from the remaining water ; 
 if the liquid is slightly stirred a very characteristic 
 phenomenon makes its appearance the pure water and 
 the dense sugar solution do not at once mix completely, 
 the denser liquid remaining for a time disseminated through- 
 out the whole fluid mass in the form of more or less fine 
 lines, sheets, or eddies, and these are visible because the 
 imperfectly mixed liquids have different effects on the light 
 passing through them. In the case of sugar-water we are, 
 however, dealing with a very mobile liquid, and a few turns 
 of a tea-spoon suffice to render the mixture complete, and 
 the liquid, which for a few moments had appeared turbid, 
 becomes homogeneous and transparent. In the case of 
 glass, when the raw materials are melted together, a 
 mixture is formed of liquids of differing densities similar 
 to that which was temporarily formed in the sugar-water 
 solution. Molten glass, however, is never so mobile a liquid 
 as ordinary water, nor is it in the ordinary course of 
 manufacture subjected to any such thorough mixing action 
 as that which is produced by a spoon in a glass of water. 
 In glass as ordinarily manufactured, therefore, it is not 
 surprising to find that the lack of homogeneity which 
 originates during the melting persists to the end. Its effects 
 can be traced whenever a thick piece of ordinary glass is 
 carefully examined, when the threads or layers of differing 
 densities can be recognised in the form of minute internal 
 irregularities in the glass. These defects are known as 
 striae or veins, and their presence in glass intended for the 
 
UNIVERSITY 
 
 OF 
 
 OPTICAL GLASS. 
 
 207 
 
 better kind of optical work renders the glass useless. As 
 will be seen below in the production of optical glass, special 
 means are adopted for the purpose of rendering it as 
 homogeneous as possible ; in fact, the early history of 
 optical glass manufacture is simply the history of attempts 
 to overcome this very defect. The problem is, however, 
 beset by chemical and physical difficulties of no mean 
 order, and even in the best modern practice only a small 
 proportion of each melting or crucible full of glass is 
 
 FIG. 14. Diagram of striae-testing apparatus. 
 
 L, source of light ; S, slit; A and Z?, simple convex lenses; G, glass under 
 test ; E, eye of observer. The arrows indicate the paths of light-rays. 
 
 entirely free from veins or striae. In many cases these 
 defects are very minute, and sometimes escape observation 
 until the stage of the finished lens is reached. At that stage, 
 however, their presence becomes painfully evident from the 
 fact that they interfere seriously with the sharp definition 
 of the images formed by the lens in question. It will be 
 seen that in such a case time and money has been wasted 
 by grinding and polishing what turns out to be a useless 
 piece of glass. Methods are, therefore, used for examining 
 the glass before it is worked, whereby the existence of the 
 smallest striae can scarcely escape detection. These methods 
 
l>08 GLASS MANUFACTUEE. 
 
 depend upon the principle that a beam of parallel light 
 passing through a plate of glass will meet with no dis- 
 turbance so long as the glass is homogeneous, but if striae 
 are present, they will cause the light to deviate from 
 parallelism wherever it falls upon them. Under such 
 illumination, therefore, the stride will appear as either dark 
 or bright lines, when they can be readily detected. One 
 form of apparatus used for this purpose is illustrated in 
 Fig. 14. 
 
 Transparency and colour are obviously fundamentally 
 important properties of glass. In one sense homogeneity 
 is essential to transparency, but the aspect of the subject 
 which we are now considering is that of the absorption of 
 light in the course of regular transmission through glass. 
 It may be said at once that no glass is either perfectly 
 transparent or, what comes to nearly the same thing, per- 
 fectly free from colour. In the case of the best optical 
 glasses it is true that the absorption of light is very slight, 
 but even these, when considerable thicknesses are viewed, 
 show a greenish-yellow or bluish colouring. On the other 
 hand, certain optical glasses which are used at the present 
 time for many of our best lenses absorb light so strongly or 
 are so deeply coloured that a thickness of a few inches is suffi- 
 cient to reveal this defect. To some extent public taste or 
 opinion which objects to the use of even a slightly greenish 
 glass in optical instruments of good quality is to blame for 
 the tint of these glasses. In man}' cases glass-makers 
 could produce a very slightly greenish glass, but in order to 
 overcome this colour they deliberately add to the glass a 
 colouring oxide imparting to the glass a colour more or less 
 complementary to the natural green tint. The result is a 
 
OPTICAL GLASS. 209 
 
 more or less neutral-tinted glass which, however, absorbs 
 much more light than the naturally green glass would have 
 done. Since such glass is frequently used for photographic 
 lenses, it is interesting to note that the light rays whose 
 transmission is sacrificed in order to avoid the green tint 
 are those lying at or near the blue end of the spectrum, so 
 that the photographic rapidity of the resulting lenses is 
 decidedly reduced by the use of such glass. 
 
 Refraction and Dispersion. The quantitative properties 
 of glass, governing its effect upon incident and transmitted 
 light, are, of course, of fundamental importance in all its 
 optical uses. The fundamental optical constant of each 
 variety of optical glass is known as its refractive index ; 
 this number really represents the ratio of the velocity with 
 which light waves are propagated through the glass to the 
 velocity with which they travel through free space. Not 
 only does this ratio vary with every change in the chemical 
 composition and physical condition of the glass, but it also 
 varies according to the length of the light waves them- 
 selves. In other words, the short waves of blue light are 
 transmitted through glass with a different velocity from 
 that with which the longer waves of red light are trans- 
 mitted. The consequence is that when a beam of white 
 light is passed through a prism it is split up and spread 
 out into a number of beams representing all the colours of 
 the spectrum in their proper order, the blue light suffering 
 the greatest deflection from its original path, while the red 
 light suffers least deflection. Both the actual and relative 
 amount by which light rays of various colours are deflected 
 under such circumstances depends upon the nature of the 
 glass in question ; therefore, to fully characterise the 
 
 G.M. p 
 
210 GLASS MANUFACTURE. 
 
 optical properties of a given kind of glass it is necessary to 
 state not only its refractive index but to specify the refrac- 
 tive indices for a sufficient number of different wave-lengths 
 of light, suitably distributed through the spectrum. For 
 this purpose a number of well-marked spectrum lines have 
 been chosen, the systematic use of the particular set of 
 lines which is now usually employed being due to the 
 initiative of Abbe and Schott at Jena, who initiated the 
 system of specifying the optical properties of glass in this 
 way. The actual lines chosen are the line known as A' , 
 corresponding to a wave-length of 0'7677 micro-millimetres, 
 and the lines known as C, D, F, and G', whose wave- 
 lengths, in the same units, are 0*6563, 0*5893, 0'486'2, and 
 0'4341 respectively. The A' line, however, lies so near the 
 extreme red end of the spectrum that the data concerning 
 it are seldom required. 
 
 As a matter of fact, the actual refractive index is only 
 stated in most tables of optical glasses for sodium light 
 (D line), the dispersive properties of the glass being indicated 
 by tabulating the differences between the refractive indices 
 for the various lines, the table thus containing columns 
 marked C-D, D-F, F-G'. These figures are usually 
 described as the ** dispersion " of the glass from C to D, D 
 to F, etc. In addition to these figures it is usual to tabulate 
 what is called the " mean dispersion " of the glass, which 
 is simply the difference between the refractive indices for 
 C and F lines ; this interval is usually taken as repre- 
 senting that part of the spectrum which is of the greatest 
 importance for visual purposes. A further constant which 
 is of great importance in the calculations for achromatic 
 lenses is obtained by dividing the mean dispersion into the 
 
OPTICAL GLASS. 
 
 211 
 
 refractive index for the D line minus one (usually written 
 
 f-\ -p 
 
 -=v). This term, for which no satisfactory name 
 
 has yet been suggested, characterises the ratio of the dis- 
 persive power of the glass to its total refracting power. It 
 is usually denoted hy the Greek letter v. The following 
 table (taken from the Catalogue of the Optical Convention, 
 1905) gives a list of optical glasses produced by Messrs. 
 Chance, of Birmingham. This list, although it is not 
 nearly so long as that issued by the French and German 
 firms who manufacture optical glass, contains examples of 
 the most important types of optical glass which are avail- 
 able at the present time. Those, however, who wish to use 
 the data for the purpose of lens calculation are advised to 
 consult the latest issues of the optical glass-makers' cata- 
 logues, since the range of types available, and even the 
 actual figures for some of the glasses, are liable to variation 
 from time to time. 
 
 In the table on p. 212 the first column contains the 
 ordinary trade names by which the various types of glass are 
 known. These names, while somewhat arbitrary, indicate 
 in a rough way the chemical nature of the glass concerned. 
 Thus the word " flint " always implies a glass containing 
 lead and therefore having a comparatively high refractive 
 index and low value of v, while the word " crown," originally 
 applied only to lime-silicate glasses, is now used for all 
 glass having a high value of v. In the next column of the 
 table are given the refractive indices of the glasses, while 
 the third column contains the values of v. It will be seen 
 that the glasses are arranged in descending order of magni- 
 
212 
 
 GLASS MANUFACTUEE. 
 
 O O Tt< t- CO O 
 
 ^D CO rH 
 ^ OS <7<1 
 
 1C O O CO CO 
 
 8888888888 
 
 O O O T I CO 
 
 T IT IT I r I T I 
 
 ^OCOTjHOODTFT (i ltTHrHCC 
 OOOOOOOOi lOT-iTHT-i 
 
 TJH CO O OS CO rfl O 
 
 pppppppppppppppppppppp 
 
 ^t-OOCO(NCO<MCOX(M(M^OT-ICOT I O 
 - O5OST-l(MTticO^TtHt-OOt-O5OS^t- 
 
 O fc-<N OS t> 
 
OPTICAL GLASS. 213 
 
 tude in respect of this constant. An inspection of the 
 figures in these two columns will reveal the fact that for 
 the majority of the glasses contained in this table the 
 value of v decreases as the refractive index increases. 
 The glasses which are an exception to this rule are indicated 
 by an *. Asa matter of fact this rule applied to all glasses 
 that were known or were at all events commercially avail- 
 able prior to the modern advances in optical glass manufac- 
 ture which were initiated by Abbe and Schott of Jena. It 
 was Abbe's insight into the requirements of optical instru- 
 ment design that led him to realise the importance of 
 overcoming this limitation in the ratio between the disper- 
 sive and refractive powers of glass. With the collaboration 
 of Schott he succeeded in producing a whole series of 
 previously unknown varieties of optical glass in which the 
 relation between n and v is not that of approximately 
 
 simple inverse proportionality which holds for the older 
 crown and flint-glasses. Most valuable and in many ways 
 most typical of these new glasses are those known as the 
 " barium crown " glasses, which combine the high refractive 
 index of a light flint or even a dense flint-glass with the 
 high v value of an ordinary crown glass. It would lead 
 too far into the subject of lens construction to explain in 
 detail the possibility opened up to the optician by the use 
 of these newer varieties of glass. We must content our- 
 selves with pointing out that the great forward strides 
 marked by the production of apochromatic microscope 
 objectives, of anastigmatic photographic lenses, and the 
 modern telescope objectives are all based upon the employ- 
 ment of these new optical media; and although optical 
 
214 GLASS MANUFACTURE. 
 
 glasses of these newer types are at the present time pro- 
 duced in the optical glass manufactories of France and 
 England, in quality and quantity at least equal to the out- 
 put of the Jena works themselves, these great optical 
 achievements stand as a lasting monument to the pioneer 
 work of Abbe and Schott in this field. 
 
 The last six columns of the table of optical glasses given 
 above contain figures which define the manner in which 
 each of the glasses named distributes the various sections 
 of the spectrum. The columns C-D, D-F, and F to G' 
 give as already indicated the differences between the 
 refractive indices for the C, D, F and G' lines respectively ; 
 the smaller figures in the intermediate columns indicate the 
 ratio of each of these differences to the mean dispersion of 
 the glass. If all kinds of glass distributed the various 
 portions of the spectrum in the same proportionate manner, 
 merely differing in the total amount of dispersion produced, 
 these figures would be identically the same for all glasses. 
 In actual fact it will be seen that the figures differ very 
 widely from one type of glass to another. A moment's 
 consideration will show that when two glasses are used in a 
 lens for the purpose of achromatising one another, i.e., when 
 one is used to neutralise the dispersion of the other, such 
 achromatisation can only be perfect if these ratios (the 
 relative partial dispersions) are the same for both glasses. 
 To put the same statement in more concrete terms, if the 
 spectrum produced by one glass is comparatively long 
 drawn out at the red end, relatively compressed at the blue 
 end, while in the other glass the opposite relation holds 
 between the two ends of the dispersion spectrum, it is 
 evident that the two spectra can never be superposed in 
 
OPTICAL GLASS. 
 
 215 
 
 such a way as to entirely neutralise one another the 
 spectrum produced by the one glass will predominate and 
 leave a residual colour at the blue end, while the other will 
 predominate at the other end. In the case of lenses 
 achromatised by the use of such glasses, there will always 
 be a slight fringe of colour around the borders of the 
 images which they produce. One of the aims which Abbe 
 and Schott set themselves in the production of new varieties 
 of optical glass was to obtain one or more pairs of glasses 
 in which the relative partial dispersions should be as nearly 
 alike as possible while the actual values of v should differ as 
 widely as possible. Some success in this direction was at 
 first claimed by the Jena workers, but unfortunately some 
 of the most promising glasses in this respect were found to 
 be too unstable for practical use and had ultimately to be 
 abandoned. At the present time the only pair of really 
 perfectly achromatic glasses offered by the Jena firm is 
 that tabulated below, and it will be seen that although the 
 relative partial dispersions are very closely alike, the v 
 values of the two glasses only differ by 10, and at least one 
 
 
 
 
 
 
 c-d 
 
 
 d-f 
 
 
 f 
 
 Name. 
 
 llD 
 
 v 
 
 C-F. 
 
 C-D. 
 
 <TT. 
 
 D-F. 
 
 
 F-G'. 
 
 tt 
 
 Telescope Crown 
 
 1-5254 
 
 61-7 
 
 00852 
 
 00250 
 
 202 
 
 00602 
 
 707 
 
 00484 
 
 568 
 
 Telescope Flint . 
 
 1-5-211 
 
 51-8 
 
 001007 
 
 00297 
 
 294 
 
 00710 
 
 705 
 
 00577 
 
 573 
 
 of these glasses is not readily obtainable in really satisfac- 
 tory optical quality. On the other hand, practically per- 
 fectly achromatised lenses (generally known as " apochro- 
 matic "). have been produced, especially by Zeiss of Jena, 
 for microscopic purposes, by the careful selection of glasses 
 
216 GLASS MANUFACTUEE. 
 
 suited to each other in this respect. Such a solution of 
 the problem is further facilitated by the fact that in these 
 lenses more than two varieties of glass can be used to 
 neutralise one another, while a natural mineral (fluorite) is 
 also employed. From the glass-maker's point of view, 
 however, the problem of producing a satisfactory pair of 
 glasses capable of entirely achromatising one another has 
 yet to be solved. 
 
 The table of optical glasses given above, although brief 
 as compared with the lists issued by French and German 
 optical glass-makers, fairly covers the range of practically 
 available glasses, and a rapid inspection will at once show 
 how extremely limited this range really is. Thus the 
 refractive index varies only between the limits 1'49 and 
 1*71, and even if we admit; as practical glasses such extreme 
 types offered by some makers as would extend this 
 range to 1*40 in one direction and to 1*80 in the other, 
 this does not affect the present argument. Of course, a 
 glass of a refractive index as low as I/O, or even I'lO, is 
 not theoretically possible, since the mere density of any 
 substance enters into the factors that affect its refractive 
 index, and a glass having a density lower than that of 
 water (whose refractive index is about 1/8) is scarcely con- 
 ceivable. In the other direction, however, the limits met 
 with in the case of glass are considerably exceeded by 
 certain natural mineral substances. Thus the diamond has 
 a refractive index of 2*42, while the garnets show refractive 
 indices from 1/75 to 1*81. The values of v found in the 
 table of optical glasses are still more narrowly restricted, 
 lying between 67 and 29, while such a mineral as fluorite 
 ghows a value of 95'4. These facts show that it is physi- 
 
OPTICAL GLASS. 217 
 
 cally possible to obtain transparent substances having 
 optical properties lying far beyond the limited range 
 covered by our present optical glasses, and it scarcely 
 needs showing that if such an extended range of materials 
 were available greatly increased possibilities would be 
 opened up to the designer of optical instruments. It is 
 consequently interesting to inquire as to the actual causes 
 which limit the range of optical glasses at present available. 
 It will be found that these limits are set by the properties 
 of glass itself. ' While the more ordinary kinds of glass, 
 having average optical properties and showing dispersive 
 powers roughly conforming to the law of inverse propor- 
 tionality with refractive index which governs the older 
 varieties of optical glass, are chemically stable substances, 
 showing little tendency to undergo either chemical changes 
 or to crystallise during cooling, the more extreme glasses 
 exhibit these undesirable features to an increasing extent 
 the more nearly the limit of our present range is approached. 
 As the chemical composition of a glass is " forced " by the 
 addition of special substances intended to affect its optical 
 properties in an abnormal direction, so the chemical and 
 physical stability of the glass is rapidly lessened. The more 
 extreme glasses, in fact, behave as active chemical agents 
 readily entering into reaction or combination even with 
 relatively inert substances in their environment they act 
 vigorously upon the fire-clay vessels in which they are 
 melted, and they are readily attacked by acids, moisture or 
 even warm air, when in the finished condition, while many 
 of them can only be prevented from assuming the condition 
 of a crystalline (and opaque) agglomerate by being rapidly 
 Cooled through certain critical ranges of temperature. 
 
218 GLASS MANUFACTURE. 
 
 A limit to the possibility of production is set by these 
 tendencies when they exceed a certain amount a point 
 being reached where it ceases to be practicable to overcome 
 the tendency of the glass to self-destruction. On the lines 
 of our present glasses, therefore, it does not appear hopeful 
 to look for any considerable extension of the range of our 
 optical media. On the other hand, as the known optical 
 properties of transparent crystalline minerals show, a much 
 greater range of optical constants would become available 
 if it were possible to manufacture artificial mineral crystals 
 of sufficient size and purity for optical purposes, and the 
 author believes that in this direction progress in optical 
 materials is ultimately bound to lie 1 . 
 
 In addition to possessing the requisite optical constants, 
 a good colour and perfect homogeneity, certain other 
 properties are essential in good optical glass. These are 
 the general physical and chemical qualities which are 
 essential in all good glass, but especially emphasised by 
 the fact that the requirements for optical glass are more 
 stringent than for any other variety of the material. Thus 
 chemical stability is of the greatest importance, for the best 
 lenses would soon become useless if the action of atmo- 
 spheric moisture were to affect them appreciably the 
 polished surfaces would rapidly become dull and the whole 
 lens would soon be rendered useless. The conditions 
 governing the chemical stability of glass and the methods 
 of testing this quality have already been indicated (Chapters 
 I. and II.) . The harder varieties of optical glass, such as 
 
 1 See a Paper by the present author on " Possible Directions of 
 Progress in Optical Glass " Proceedings of the Optical Convention, 
 London, 1905. 
 
OPTICAL GLASS. 
 
 219 
 
 the glasses quoted in the above table under the names of 
 " Hard Crown " and Boro-Silicate Crown, are probably 
 among the most durable and chemically resistant of all 
 varieties of glass, but as we have already indicated, when 
 extreme optical properties are required, the necessary 
 chemical composition of the glass always entails a sacrifice 
 of this great chemical stability, until a limit is reached 
 where valuable optical properties no longer counterbalance 
 the serious disadvantage of a chemical composition which 
 renders the glass liable to rapid disintegration. In certain 
 special cases it is, perhaps, possible to protect lenses made 
 of such unstable glass by covering them with cemented-on 
 lenses of stable glass, but this device entails concomitant 
 limitations in the design of the optical system and is, there- 
 fore, rarely used. In any case, however, it is well for the 
 lens-designer to consider the relative stability of the glasses 
 employed when arranging the order in which they are to 
 be used, since it is obviously preferable to put a hard, 
 durable glass on the outside of his system, where it is most 
 directly exposed to atmospheric moisture, and is also subject 
 to handling and " cleaning " by inexpert hands. This latter 
 factor is a very important one for the life of any lens. In 
 the first place, a glass surface is very seriously affected by 
 the minute film of organic matter which is left upon it 
 when it has been touched with even a clean finger ; unless 
 the glass is of the best quality in this respect, such finger- 
 marks readily develop into iridescents pots and may even 
 turn into black stains. Particles of dust allowed to settle 
 on the surface of the glass will affect it in the same way, so 
 that the protection afforded by mere mechanical enclosure 
 in the tube of an instrument is of decided value in 
 
220 GLASS MANUFACTURE. 
 
 preserving a glass surface. It should, however, be noted 
 that in some instances the interior metal surfaces of optical 
 instruments are varnished with substances that give off 
 vapours for a long time after the instrument is completed, 
 and in that case the inside lenses are apt to be tarnished in 
 consequence. On the other hand, outside lenses are also 
 exposed to direct mechanical injury from handling and 
 "cleaning." As far as the latter operation is concerned, it 
 frequently happens, particularly in glasses containing soda, 
 that a slight surface dimming is formed on the glass when 
 it has been left in a more or less damp place for a long 
 time. This dimming is chiefly due to the formation on the 
 surface of a great number of very minute crystals of 
 carbonate of soda, which are hard and sharp enough to 
 scratch the glass itself if rubbed about over it. If such a 
 lens be wiped with a dry cloth, however clean and soft, the 
 effect is a permanent injury to the polished surface, which 
 could readily be avoided by first washing the lens with clean 
 water, or even by using a wet cloth instead of a dry one 
 for the first wiping. 
 
 The mechanical hardness of the glass is an important 
 factor in determining its resistance to such injurious treat- 
 ment or to the effects of accidental contact with hard, sharp 
 bodies. The subject of the hardness of glass has already 
 been discussed in a general way in Chapter II., and little 
 remains to be added here. Broadly speaking, a high degree 
 of hardness and a low refractive index are found together. 
 This statement is certainly true where any considerable 
 difference of hardness is considered, as, for example, in 
 comparing a hard crown glass with a dense flint ; but where 
 the difference of refractive index or of density is small, it 
 
OPTICAL GLASS. 221 
 
 is not at all certain that the lighter glass will also be the 
 harder. 
 
 The properties involved in the quality known as " hard- 
 ness " also affect in a very marked manner the behaviour 
 of glass when subjected to the grinding and polishing 
 processes. The ease with which a good polish can be 
 obtained varies very much in different kinds of glass, both 
 the hardest and the softest glasses showing themselves 
 difficult in this respect. The harder glasses are certainly 
 less liable to accidental scratching during the polishing 
 operations, and generally work in a cleaner manner; but 
 the time required to produce a satisfactory polish is much 
 greater owing to the resistance to displacement offered by 
 the molecules. Both the speed of working and the pressure 
 exerted during the polishing operation have, in fact, to be 
 carefully adapted to the quality of the glass in this respect 
 if the best possible results are to be obtained. 
 
 Another property which is essential in optical glass of 
 the highest quality is that of freedom from internal strains. 
 This subject will be again referred to later in connection 
 with the annealing processes used in the manufacture of 
 optical glass, and it need only be mentioned here that the 
 presence of internal strain is readily recognised in glass, 
 by the aid of the polariscope. Perfectly annealed 
 glass, entirely free from internal strains, produces no effect 
 upon a beam of polarised light passing through it, while 
 even slightly strained glass becomes markedly doubly- 
 refracting. For many purposes of optics this double 
 refraction becomes undesirable or even inadmissible, 
 especially as it is accompanied by small variations in the 
 effective index of refraction of various portions of the mass 
 
222 GLASS MANUFACTUEE. 
 
 of glass. Further, if the amount of double refraction 
 observed is at all serious it indicates a state of strain which 
 may easily lead to the fracture of the whole piece, particu- 
 larly when undergoing the earlier stages of the grinding 
 process or if exposed to shocks of any sort. As will be 
 seen below, perfectly annealed glass is obtainable, but very 
 special means are required for its production, and the 
 optician should for that reason avoid making unnecessarily 
 extreme demands in this direction. The very small amount 
 of double refraction frequently found in the better class of 
 optical glass is entirely harmless for most purposes. 
 
CHAPTER XIII. 
 
 OPTICAL GLASS. 
 
 THE process of manufacturing the best qualities of 
 optical glass may be briefly described as consisting in 
 obtaining a crucible full of the purest and most homo- 
 geneous glass, and then allowing it to cool slowly and to 
 solidify in situ. From the resulting mass of glass the best 
 pieces are picked and moulded into the desired shape for 
 optical use. It will be seen at once that in this process 
 there is an essential difference from all others that have 
 been described in this book viz., that the glass is never 
 removed from the melting-pot while molten, and that none 
 of the operations of gathering, pouring, rolling, pressing, or 
 blowing are applied to it. The reason for this apparently 
 irrational mode of procedure lies in the fact that the perfect 
 homogeneity essential for optical purposes can only be 
 attained by laborious means, and can then only be retained 
 if the glass is left to solidify undisturbed ; any movement 
 by the introduction of pipes or ladles would result in the 
 contamination of the glass by striae and other objectionable 
 defects. 
 
 The choice and proportion of raw materials used in the 
 production of any given quality of optical glass is governed 
 by the chemical composition which experiment has shown 
 
224 GLASS MANUFACTUKE. 
 
 to be necessary to yield the desired optical properties. The 
 composition of optical glass mixtures cannot therefore be 
 varied to suit the conditions of the furnace or to facilitate 
 ready melting and fining, so that many of the usual 
 resources of the glass-maker cease to be available in the 
 very case where their aid would be most welcome to 
 facilitate the production of technically perfect glass. On the 
 other hand, the manufacturer has a certain amount of 
 choice as to the precise form in which the various chemical 
 ingredients are to be introduced into the mixture, and he 
 makes his choice among oxides, carbonates, nitrates, and 
 hydrates, according to the behaviour that it is desired to 
 impart to the mass during the earlier stages of fusion. 
 The state of purity in which the various substances are 
 commercially obtainable also enters largely into the 
 question, since the greatest possible degree of purity in 
 the raw materials is essential to the production of glass of 
 good colour, or rather freedom from colour. 
 
 Since homogeneity is so essential in the finished product, 
 very thorough mixing of the raw materials is necessary in 
 the case of optical glass, and the ingredients are for this 
 purpose generally used in a state of finer division than is 
 necessary with other varieties of glass. As a rule the 
 quantities of mixture of any one kind that are required are 
 not large enough to justify the use of mechanical appliances, 
 and very careful hand-mixing is carried out. 
 
 Although it is quite possible to obtain successful meltings 
 from raw materials alone, it is preferable to mix with these 
 a certain proportion of " cullet " or broken glass derived 
 from a previous melting of the same sort. The broken 
 glass used for this purpose is first carefully picked over for 
 
OPTICAL GLASS. 225 
 
 the purpose of rejecting pieces that contain visible impuri- 
 ties, although pieces showing striae are not usually rejected. 
 The greater part of this cullet is generally mixed as 
 evenly as possible with the raw materials, but a certain 
 proportion is reserved for another purpose, as explained 
 below. 
 
 The furnaces used for the production of optical glass vary 
 very much in type in different works. In some the old- 
 fashioned conical coal furnaces are still used, the disadvan- 
 tages attached to their employment being outweighed in 
 the opinion of the manufacturers by their simplicity and 
 ease of regulation. In other works gas -fired regenerative 
 furnaces of the most recent type are installed, and in these 
 also optical glass of the highest quality can be produced. 
 As a rule, however, optical glass furnaces differ from other 
 pot-furnaces found in glass-works in this respect that the 
 former are usually constructed to receive one pot or crucible 
 only, while in other glass furnaces from four to twelve or 
 even twenty pots are heated at the same time. The reason 
 for this restriction in the capacity of the furnaces lies in 
 the fact that since the mixtures used for optical glass can- 
 not be adjusted to suit the furnace, the latter must be 
 worked as far as possible in such a way as to suit the 
 mixture to be melted in it, and this implies that every pot 
 will require its own adjustment of times and temperatures, 
 and this it would be difficult, if not impossible, to secure if 
 more than one pot were heated in the same furnace. It is 
 further to be remembered that the amount of care and 
 attention required during the melting of a pot of optical 
 glass is out of all proportion to that needed with other 
 varieties, so that little would be gained by having a number 
 
 G.M. Q 
 
226 GLASS MANUFACTUEE. 
 
 of pots in one furnace, since several sets of men would 
 be required to tend them. 
 
 In addition to the single-pot melting furnace, a very 
 important part of the equipment of the optical glass works 
 is formed by a number of kilns or ovens which are used for 
 the preliminary heating, and sometimes for the final cool- 
 ing of the various crucibles or pots. Similar kilns are 
 used in other branches of the industry, but in those cases 
 the pots, once introduced into the furnace, are expected to 
 last for a number of weeks, or even months. In optical 
 glass manufacture, on the other hand, a pot is used once 
 only, so that fresh pots are required for every new melting. 
 The kilns in which these pots are heated up before being 
 placed in the melting furnace are thus in very frequent use. 
 As a rule they are simply fire-brick chambers provided 
 with sufficient grate-room and flue-space to be gradually 
 raised to a red heat in the course of four or five days, 
 while for the purpose of gradual cooling they can be sealed 
 up like the annealing kilns used for polished plate-glass. 
 
 The pots or crucibles in which optical glass is melted are 
 usually of the same shape as the covered pots used for flint- 
 glass as illustrated in Fig. 2. The optical glass pots, 
 however, are made considerably thinner in the wall, since 
 they are not required to withstand the prolonged action of 
 molten glass in the same way as pots used for flint-glass 
 manufacture. On the other hand, the fire-clays used for 
 this purpose must be chosen with special care so as to avoid 
 any contamination of the glass by iron or other impurities 
 which might reach the glass from the pot. For the pro- 
 duction of certain special glasses, in fact, pots made of 
 special materials are required, since these glasses, when 
 
OPTICAL GLASS. 227 
 
 molten, produce a rapid chemical attack upon ordinary fire- 
 clays. A certain amount of the aluminiferous material of 
 the pot is, in fact, always introduced into the glass by the 
 gradual dissolving action of glass on fire-clay which we 
 have already described. The glass contaminated with 
 these aluminiferous substances is generally more viscous 
 than the rest of the contents of the pot, and therefore 
 ordinarily remains more or less adherent to the walls of the 
 crucible, but the inevitable disturbances which accompany 
 the processes of melting and fining lead to the dissemina- 
 tion of some of this viscous glass through the entire pot in 
 the form of veins or striae, which are only removed during 
 the stirring process. On the other hand, more of this 
 viscous glass is constantly being formed so long as the 
 glass remains molten, and if disturbances are not sufficiently 
 avoided during the later stages of the process fresh veins 
 may easily be formed. 
 
 The actual operations of producing a melting of optical 
 glass begin by the gradual heating-up of the pot in the 
 kiln just described. When the pot has reached a full red. 
 heat the doors of the kiln are opened and the pot drawn 
 out by means of a long heavy iron fork running on wheels ; 
 this implement is run into the mouth of the kiln and the 
 tines of the fork are pushed under the pot, and the latter is 
 then readily lifted up and withdrawn from the kiln. Mean- 
 while the temperature of the furnace has been regulated in 
 such a manner as to be approximately equal to that 
 attained by the heating kiln, so that the pot, when trans- 
 ferred as rapidly as possible from the kiln to the furnace, 
 is not subjected to any very sudden heating ; were it 
 attempted to place the new pot in a furnace at full melt- 
 ed 2 
 
228 GLASS MANUFACTUKE. 
 
 ing heat the fire-clay would shrink rapidly and the entire 
 vessel would fall to pieces. Even under the best conditions 
 it is not possible to avoid the occasional failure of a pot by 
 cracking either at this or a slightly later stage of the 
 process. The latter occurrence is apt to be particularly 
 disastrous, as the pot may then be full of molten glass, 
 which runs out and is lost. 
 
 As soon as the empty pot has been put into place, the 
 melting furnace is carefully sealed up by means of tempo- 
 rary work built of large fire-bricks, the whole being so 
 arranged that the mouth of the hood of the pot is left 
 accessible by means of an aperture in the temporary 
 furnace wall. This aperture can be closed by one or more 
 slabs of fire-clay, and when these are removed an opening is 
 left by which the raw materials are introduced, and through 
 which the other manipulations are carried out. 
 
 When this stage of the process is reached, the wagons 
 containing the mixed raw materials are usually wheeled 
 into place in front of the furnace, but the introduction of 
 the materials themselves into the pot is not begun until 
 several hours later, when the furnace has been vigorously 
 heated and an approach to the melting heat has been 
 attained. 
 
 When the furnace and pot have attained the necessary 
 temperature, but before the raw materials are introduced, 
 a small quantity of the cullet, which has been reserved for 
 this purpose, is thrown into the pot and allowed time to 
 melt, and then only is the first charge of mixture put into 
 the pot. The object of this proceeding is to coat the bottom 
 and part of the walls of the pot with a layer of molten glass 
 which serves to protect it from the chemical and physical 
 
OPTICAL GLASS. 229 
 
 attack of the raw materials during the violent action which 
 takes place when they are first exposed to the furnace heat. 
 
 The gradual filling of the pot with molten glass is now 
 carried out by the introduction of successive charges of raw 
 material ; as the mixture not only occupies more space than 
 the glass it forms, but also froths up a good deal during 
 melting, the quantities introduced each time must be care- 
 fully adjusted so as to avoid an overflow of half-melted glass 
 through the mouth of the pot. As the pot is more and 
 more nearly filled, the space left for the raw materials is 
 proportionately diminished, and the later charges are there- 
 fore much smaller than the first few. 
 
 When, finally, sufficient material has been introduced to 
 fill the pot completely, the next stage of the process 
 commences. When the last charge of raw materials has 
 melted, the glass in the pot is left in the state of a more or 
 less viscous liquid full of bubbles of all sizes ; it is essential 
 that these bubbles should escape and leave the glass pure 
 and "fine," and this result can only be achieved by raising 
 the temperature of the furnace and allowing the glass to 
 become more fluid, while the rise of temperature also causes 
 the bubbles to expand owing to the expansion of the gas 
 contained in them. In both ways, rise of temperature 
 facilitates the escape of the bubbles, and the furnace is 
 therefore heated to the full, and this extreme heat is 
 maintained until the glass is free from bubbles. Jn the 
 case of the more fusible glasses the temperature required 
 for this purpose is not excessively high, and, indeed, in the 
 case of these glasses care is taken to avoid too high a 
 temperature, as it entails other disadvantages. In the case 
 of the harder crown glasses, however, the difficulty lies in 
 
230 GLASS MANUFACTUBE. 
 
 producing an adequately high temperature without at the 
 same time endangering the life of furnace and crucible. 
 The difficulty of freeing the molten glass from bubbles 
 constitutes one of the causes that limit the range of our 
 optical glasses in one direction still harder glasses could 
 be melted, but it would not be feasible to maintain a 
 temperature high enough to render them fluid enough 
 to "fine." 
 
 In the case of other kinds of glass, again, it becomes 
 impossible to entirely remove the bubbles from the molten 
 mass even when very hot and very fluid. The exact cause 
 is not known, but in some kinds of glass the bubbles 
 formed are so minute that even when the glass is perfectly 
 mobile the bubbles show no tendency to escape, while in 
 other kinds of glass there appears to be a steady evolution 
 of minute bubbles as soon as the temperature is raised with 
 a view to removing those already in the glass. As this 
 property attaches to some of the most valuable of the newer 
 varieties of optical glass, opticians and the public have 
 learnt to put up with the presence of minute bubbles in the 
 lenses and prisms made of these glasses. These bubbles 
 are, however, very minute and do not interfere with the 
 optical performance of the lenses, &c., except to the extent 
 of arresting and scattering the very small proportion of 
 light that falls upon them ; their presence is therefore to 
 be regarded as a small but unavoidable drawback to the 
 use of glasses which offer advantages that completely out- 
 weigh this defect. 
 
 Returning to the melting process, we find that the 
 extreme heating required for the purpose of " fining" the 
 glass is continued for a considerable period of time, as long 
 
OPTICAL GLASS. 231 
 
 as thirty hours in some cases, the glass being examined from 
 time to time to test its condition as regards freedom from 
 bubbles. This is done by taking a small sample of glass 
 out of the pot and examining it to see if it still contains 
 bubbles. In some works this test is made by taking up a 
 very small gathering of glass on the end of a small pipe and 
 blowing it into a spherical flask ; on looking at such a flask 
 in a suitable light the presence of even minute bubbles is 
 readily detected. In other works a simpler process is 
 adopted, a small quantity of glass being ladled out of the 
 pot on the surface of a flat iron rod. It is allowed to cool 
 on the rod, and when pushed off forms a small bar of glass 
 some eight or ten inches long and about an inch wide ; in 
 this also the presence of bubbles is easily detected. These 
 test pieces are known among glass-makers as " proofs." 
 
 When proofs, taken as just described, have shown that 
 the glass is free from bubbles, the extreme heat of the 
 furnace is allowed to abate, and the fire-clay slabs in front 
 of the mouth of the pot are removed. The next step is 
 that of skimming the surface of the glass. Since most of 
 the materials liable to contaminate the contents of a pot 
 are specifically lighter than the molten glass, they will be 
 found floating on the surface, and the surface glass is there- 
 fore removed with a view to ridding the glass of anything 
 that may have been accidentally introduced and that has 
 not melted and become incorporated with the molten mass. 
 
 The next steps in the process are those of stirring the 
 molten glass with a view to rendering it homogeneous and 
 free from striae. The stirrer used for this purpose is 
 usually a cylinder of fire-clay, previously burnt and heated. 
 This is provided with a deep square hole in one end, and it 
 
232 GLASS MANUFACTURE. 
 
 is held at first by means of a small iron bar passed into 
 this hole. By this means the red-hot cylinder of fire-clay 
 is introduced into the open mouth of the pot, and when it 
 has attained approximately the temperature of the molten 
 glass it is dipped into the glass itself, in which it ultimately 
 floats. When stirring is to begin, the square, down-turned 
 end of a long iron bar is introduced into the corresponding 
 square hole in the upper end of the stirrer, and by this 
 means the fire-clay cylinder is held in a vertical position in 
 the glass and given the steady rotatory movement which 
 constitutes the stirring process. For this purpose the long 
 iron bar just mentioned is made to pass over a swivel- 
 wheel, while a workman moves it steadily by the aid of a 
 large wooden handle. This operation is always laborious 
 and trying ; the workman is necessarily exposed to the 
 intense heat radiated from the open mouth of the crucible, 
 so that men have to relieve each other at frequent intervals. 
 
 During the earlier stages of the stirring process the glass 
 is very hot and mobile, but the stirring is continued, with 
 short intervals, until the glass is so cold and stiff that the 
 stirrer can scarcely be moved in it at all, so that the work 
 of moving the stirrer becomes heavy towards the end of 
 the operation. The actual amount of stirring required 
 varies according to the nature of the glass, and the size of 
 the pot or crucible in question. Some meltings are found 
 to be satisfactory after as little as four hours' stirring, while 
 for others as much as 20 hours are required. 
 
 When the glass has stiffened to such an extent that it is 
 no longer possible to continue the stirring, preparations are 
 made for the final cooling-down of the pot of glass. The 
 fire-clay stirrer is sometimes withdrawn from the glass, but 
 
OPTICAL GLASS. 233 
 
 this is laborious, and entails dragging a considerable 
 quantity of glass out of the pot with the clay cylinder ; 
 more usually, therefore, the stirrer is simply left embedded 
 in the glass. 
 
 The next object to be accomplished is that of cooling the 
 glass as rapidly as safety will permit until it has become 
 definitely " set " the purpose being to prevent the recru- 
 descence of striae as a result of convection currents or other 
 causes which might disturb the homogeneity of the glass. 
 This rapid cooling is obtained in various ways; in one 
 mode of procedure the furnace is so arranged that by 
 opening a number of apertures provided for the purpose 
 cold air is drawn in and the pot and its contents chilled 
 thereby without being moved. This method has the 
 advantage that the pot containing the viscous glass is 
 never moved or disturbed in any way, but on the other 
 hand the cooling which can be effected within the furnace 
 itself is never very rapid, and the furnace as well as the pot 
 is chilled. Further when the glass has been chilled down 
 to a certain point this rapid rate of cooling must be 
 arrested, as otherwise the whole contents of the pot would 
 crack and splinter into minute fragments. Where the pot 
 has been left in the furnace this can only be done by 
 sealing up the whole furnace with temporary brickwork 
 and lutings of fire-clay, leaving it to act as an annealing 
 kiln until the glass has cooled down approximately to the 
 ordinary temperature, a process that occupies a period 
 of from one to two weeks according to the size of the 
 melting. Such enforced idleness of a melting furnace is of 
 course very undesirable from an economical point of view, 
 and it is generally avoided by adopting the alternative 
 
234 GLASS MANUFACTUKE. 
 
 method of drawing the pot bodily out of the furnace as 
 soon as the stirring operation is ended. For this purpose 
 the temporary brickwork forming the front of the furnace 
 is broken down, and with the aid of a long crow-bar the 
 bottom of the pot is levered up from the bed or siege of the 
 furnace to which it adheres strongly, being bound down by 
 the sticky viscous mass of molten glass and half-molten 
 fire-clay which always accumulates on the bed of the 
 furnace. The pot being temporarily held up by the inser- 
 tion of a piece of fire-brick, the tines of a long and heavy 
 iron fork running on a massive iron truck are introduced 
 beneath the pot ; an iron band provided with long handles 
 is then passed around the pot, and the latter is then drawn 
 forward by the aid of suitable pulley blocks. The tines of 
 the fork are then raised, and the pot is wheeled out of the 
 furnace and deposited upon a suitable support. Here it is 
 allowed to cool to the requisite extent, when it is again picked 
 up on the tines of the fork and deposited in an annealing 
 kiln which has been previously warmed to a suitable tempera- 
 ture. It will be seen that this handling of a heavy mass of 
 intensely hot material involves much labour, while there is 
 also a risk of losing the glass if the pot should break before 
 the glass has set sufficiently. Every care is taken to 
 prevent such an accident, the pot being wrapped round 
 with chains or otherwise supported in such a way that 
 a small crack could not readily develop into a large gap. 
 
 When such a melting of glass has cooled sufficiently, 
 either in the furnace or in the annealing kiln, to be safely 
 handled, the whole pot is drawn out, and the fire-clay shell, 
 which is generally found cracked into many pieces, is 
 broken away by the aid of a hammer. Under favourable 
 
OPTICAL GLASS. 235 
 
 circumstances the whole of the glass may have cooled 
 intact as one solid lump sometimes weighing over half 
 a ton. Unless special care is taken, however, it is more 
 usual to find the glass more or less fissured, a number of 
 large lumps being accompanied by a great mass of small 
 fragments. These are now picked over, and all those which 
 are free from visible imperfections or which can be readily 
 detached from such imperfections by the aid of a chipping 
 hammer are put upon one side for further treatment. 
 
 The next step of this treatment consists in moulding the 
 rough broken lump into the shape of plates, blocks, or 
 discs according to the purpose for which the- glass may be 
 required by the optician. The plant used for the moulding 
 process varies widely, but in all cases the operation con- 
 sists in gradually heating the glass in a suitable kiln until 
 it is soft enough to adapt itself to the shape of the mould 
 provided for the purpose. In some cases these moulds are 
 made of fire-clay, and the glass is simply allowed to settle 
 into them by its own weight ; in other cases iron moulds 
 are used, and the glass is worked into them by the aid of 
 gentle pressure from wood or metal moulding tools. In 
 yet other cases, particularly where the glass is required in 
 the form of small thin discs or where it is to be formed 
 into the approximate shape of concave or convex lenses, the 
 aid of a press is sometimes invoked. 
 
 In all cases the moulding process is followed by the final 
 annealing, which consists in cooling the glass very gradually 
 from the red heat at which it has been moulded, down to 
 the ordinary temperature. The length of time occupied by 
 such cooling depends very much upon the size of the object 
 and also upon the degree of refinement to which it is 
 
236 GLASS MANUFACTURE. 
 
 necessary to carry the removal of small internal strains in 
 the glass. For many purposes it is sufficient to allow it to 
 cool down naturally in a large lain in the course of six or 
 eight days. For special purposes, however, where perfect 
 freedom from double refraction is demanded, much greater 
 refinements are required, and special annealing kilns, whose 
 temperature can be accurately regulated and maintained, 
 are employed. In these the annealing operation can be 
 carried out so gradually that a rate of cooling in which a 
 fall of 1 C. occupies several hours can be maintained, so 
 that very perfectly annealed glass can be produced even in 
 discs or blocks of large size. 
 
 When removed from the annealing kiln the plates or 
 discs of optical glass are taken to a grinding or polishing 
 workshop, where certain of their faces or edges are ground 
 and polished in such a way as to permit of the examination 
 of the glass for bubbles, striae and other defects in the 
 manner indicated in the previous chapter. As the amount 
 of sorting that can be done while the glass is still in rough 
 fragments is necessarily very limited, it follows that a con- 
 siderable proportion of the glass which has been moulded 
 and annealed must be rejected as useless when thus finally 
 examined. A yield of perfect optical glass, amounting to 
 10 or at most 20 per cent, of the total contents of each 
 pot, is therefore all that can be expected, and smaller yields 
 are by no means infrequent a consideration that will 
 serve to explain the relatively high price of optical as 
 compared with other varieties of glass. 
 
 A consideration of the various factors that are involved 
 in the production of a piece of perfect optical glass will 
 make it apparent that the cost and difficulty of its pro- 
 

 OPTICAL GLASS. 237 
 
 duction increases rapidly with the weight of the piece to be 
 produced, so that it is not surprising to find that the price 
 of very large discs of perfect optical glass such as those 
 required for large astronomical telescopes, reaches figures 
 which become prohibitive when very large sizes are con- 
 sidered. Thus, while it is quite possible to obtain say 100 
 pounds of good glass from a single melting if the glass is 
 to be used in the form of pieces not weighing more than 
 five or six pounds each, it is only rarely that a single block 
 of perfect glass can be found weighing 100 pounds. In the 
 former case the best pieces can be picked, the worst defects 
 can be eliminated by chipping the rough fragments, and at a 
 later stage other defective pieces can be cut off or ground 
 away ; not so where a large single block is required. A single 
 fine vein, perhaps too small to be visible to the unaided eye, 
 may be found to run through a whole block in such a way 
 that it cannot be removed without breaking or cutting up 
 the whole piece, and it will be seen that the frequency with 
 which this is liable to occur increases with the volume of 
 the piece required. The difficulties of re-heating and 
 moulding are also increased enormously with the size of 
 the individual pieces of glass that have to be dealt with, 
 and where very large pieces have to be heated and cooled 
 accidental breakage becomes a serious risk. In view of 
 these difficulties it is not surprising to find that the 
 dimensions of our astronomical refractors appear to have 
 approached their limit, but rather are we led to admiration 
 of the skill and enterprise that has pushed this limit so far 
 as to produce discs of optical glass measuring as much as 
 one metre in diameter. 
 
CHAPTEE XIV. 
 
 MISCELLANEOUS PKODUCTS. 
 
 THE field of glass-manufacture is so wide and the 
 number and variety of its products so great, that in the 
 limited compass of this volume it is impossible to fully 
 enumerate them all ; there are, however, a certain number 
 of these products which, while of considerable importance 
 in themselves, yet do not fall readily under any of the 
 headings of the preceding chapters. A short space will 
 therefore be devoted to some of these in this place. 
 
 Glass Tubing. A widely-useful form of glass is that of 
 tubes of all sizes and shapes, ranging from the fine 
 capillary tubes used in the construction of thermometers to 
 the heavy drawn or pressed pipes that have been employed 
 for drainage and other purposes. The process of manu- 
 facture employed varies according to the size and nature of 
 the tube that is required. Thus lamp-chimneys are really 
 a variety of tube, used in short lengths and made of 
 relatively wide diameter and thin walls. These are not, 
 however, ordinarily made in the form of long tubes cut 
 into short sections, but as has already been mentioned 
 they are blown into moulds in the form of a thin-walled 
 cylindrical bottle, whose neck and bottom are subsequently 
 removed. By this process the various forms of chimneys 
 
MISCELLANEOUS PEODUCTS. 239 
 
 for oil-lamps, having contractions at certain parts of their 
 length, can be readily produced. 
 
 The articles more strictly described as glass tubes are, 
 however, produced by a process in which actual blowing 
 plays only a very minor part. A gathering of suitable size 
 is taken up on a pipe, a very small interior hollow space is 
 produced by blowing into the pipe, and then the gathering 
 is elongated by swinging the pipe in a suitable manner. 
 The end of the elongated gathering furthest from the pipe 
 is then attached to a rod or " pontil " held by a second 
 workman, and the two men then proceed to move apart, 
 drawing out the gathering of glass between them. Accord- 
 ing to the bore and thickness of wall required in the tube, 
 the men regulate the speed at which they move apart; the 
 thinner the tube is to be the more rapidly they move, in 
 order to draw the glass out to a sufficient extent before it 
 hardens too much. The rate of drawing must, of course, 
 also be adapted to the nature of the glass in question, and 
 this will vary very widely. For the production of the 
 smaller bored tubes the men find it necessary to separate 
 at a smart trot, while heavy tubes such as are used for 
 gauge-glasses, are drawn of hard glass by a very gradual 
 movement. In some cases, the setting of the glass, when 
 the tube has attained the desired thickness, is hastened by 
 the aid of an air-blast, or in more primitive fashion by 
 boys waving fans over the hot glass. In any case, suitable 
 troughs are provided for receiving the tube when drawn, 
 and from these the tube is taken to an annealing kiln to 
 undergo this necessary operation. 
 
 The glass used for the production of tubing varies very 
 widely according to the purpose for which the product is 
 
240 GLASS MANUFACTUKE. 
 
 intended. Almost any of the more usual varieties of glass 
 can be readily drawn out into tubes, and the choice of the 
 kind of glass to be employed is therefore left to other 
 considerations. Tubing required for the use of the lamp- 
 worker, i.e., for the production of instruments or other 
 articles by the aid of the glass-blower's blow-pipe, must 
 have the capacity of undergoing repeated cooling and heat- 
 ing without showing signs of crystallisation (devitrification), 
 while reasonable softness in the flame is also required. For 
 this purpose, also, glass containing lead is not admissible, 
 since this would blacken under the influence of the blow- 
 pipe flame. Soda-lime glasses rather rich in alkali are 
 most frequently used for these purposes ; one consequence 
 of their chemical composition, however, is that such glass 
 tends to undergo decomposition when stored for any length 
 of time, more especially in damp places. Frequently this 
 decomposition only manifests itself on heating the glass in 
 a flame, when it either flies to pieces or turns dull and 
 rough on the surface. Such glass is sometimes said to 
 have " devitrified," but this is not really the case; what 
 has actually happened is that the atmospheric moisture 
 has penetrated for some little distance into the thickness of 
 the glass, probably hydrating some of the silica ; on heat- 
 ing, this moisture is driven off, with the result that either 
 a few large cracks, or innumerable fine ones, are formed. 
 In the latter case these do not readily disappear when the 
 glass is softened and the dull, rough surface is left at the 
 end of the operation. 
 
 For purposes where the glass is to be exposed to high 
 temperatures, tubing made of so-called " hard glass " is 
 employed. This is practically a form of Bohemian crystal 
 
MISCELLANEOUS PBODUCTS. 241 
 
 glass, the chemical composition being that of a potash-lime 
 glass rather rich in lime. To some extent this Bohemian 
 hard glass has been superseded by the special "combustion 
 tube " glass manufactured by Schott, of Jena. This is a 
 very refractory borosilicate glass containing some mag- 
 nesia; it certainly withstands higher temperatures than 
 hard Bohemian glass, and is rather less sensitive to changes 
 of temperature ; on the other hand, it has the inconvenient 
 property of showing a white opalescence when it has once 
 been heated, and this, after a time, renders the glass 
 completely opaque. 
 
 For many purposes, where heat-resisting qualities are 
 chiefly required, ordinary glass has now a formidable rival 
 in the shape of vitrified silica, which is now available as a 
 satisfactory commercial product. This substance offers the 
 great advantage that for most ordinary purposes it may be 
 regarded as entirely infusible, since the intense heat of an 
 oxygen-fed flame is required to soften or melt the silica. 
 Further, vitreous silica has an extremely low coefficient of 
 expansion, and appears also to have a rather high coefficient 
 of thermal conductivity. The result is that tubes and other 
 articles made of this material possess an astonishing 
 amount of thermal endurance (see Chapter II.). 
 
 A white-hot tube or rod of this material can be plunged 
 into cold water with impunity, and no special care need be 
 exercised in heating or cooling articles made of this sub- 
 stance, unless articles of great size and thickness are 
 involved, and even with these only little caution is needed. 
 The only disadvantages which must be balanced against the 
 great advantages just named lie in the relatively high cost 
 of the articles and in their somewhat sensitive behaviour to 
 
 G.M. R 
 
242 GLASS MANUFACTUBE. 
 
 certain chemical influences. As regards cost, vitreous silica 
 is at present available in two different forms ; in the first 
 form it resembles ordinary glass very closely in appearance, 
 the shape and finish of the tubes and vessels of this kind 
 having undergone very great improvements quite recently. 
 This silica glass has, in fact, been worked from molten 
 silica in a way more or less analogous to that in which 
 ordinary glass is worked, the great extra cost of the silica- 
 ware being due, in part, at all events, to the extremely high 
 temperature required for melting and working this material ; 
 ordinarily, in the production of the class of silica ware now 
 referred to, this heat is generated by the liberal and 
 therefore expensive use of oxygen gas. In great contrast 
 to this glass-like, transparent silica ware is the other form 
 in which this material is available. This is a series of 
 products obtained from the fusion of silica in special forms 
 of electric furnace ; in this ware the minute bubbles so 
 readily formed in the fusion of all forms of quartz are not 
 even partially eliminated, and by their presence often in 
 the form of long-drawn-out, capillary hollows they impart 
 to this ware its very characteristic milky appearance. The 
 price of this product, which is mostly used in the form of 
 tubes, although such articles as basins, crucibles, and even 
 muffles of considerable size are available, is much lower 
 than that of the transparent variety, being in fact decidedly 
 lower than that of the best porcelain ; on the other hand, 
 even this price is considerably above that of the best glass 
 tubing. 
 
 Apart from the question of cost, the use of silica ware is 
 further limited by its sensitiveness to all forms of basic 
 materials. Thus alkaline solutions cannot be allowed to 
 
MISCELLANEOUS PRODUCTS. 243 
 
 come into contact with this substance, since they attack it 
 vigorously, especially when warm. At high temperatures 
 all basic materials produce a rapid attack on silica ware, the 
 silica, in fact, behaving as a strongly acid body at and above a 
 red heat. The attack which occurs when such a substance as 
 iron or copper oxide is allowed to come into contact with 
 heated vitrified silica is, in fact, so rapid that a tube is 
 completely destroyed in a few minutes, the formation of 
 silicates resulting in the cracking and disintegration of the 
 whole piece. While, therefore, silica ware, especially in its 
 cheaper forms, undoubtedly possesses great advantages and 
 possibilities, its use must be carried on with careful reference 
 to its chemical nature. 
 
 Vitreous silica, in addition to the uses and advantages 
 just named, has also an interest from the optical point of 
 view ; this arises from the fact that it is transparent to 
 short (ultra-violet) light waves to which all ordinary 
 varieties of glass are completely opaque. Quite recently, 
 the Jena works have produced special glasses which are 
 more transparent to these ultra-violet rays than ordinary 
 glass, but even these fall far short of silica in this respect. 
 This property of transparence to ultra-violet light is utilised 
 in two widely different directions. One of these is in the 
 production of ultra-violet light when required for medical or 
 other special purposes ; a most energetic source of such 
 rays is available by the use of tubes of vitrified silica within 
 which the mercury-vapour arc is produced. In another 
 direction the employment of quartz lenses makes it possible 
 to take advantage of the optical properties of ultra-violet 
 light, in connection with microscopy ; for the purpose of 
 constructing a perfect optical system, crystalline quartz 
 
 B 2 
 
244 GLASS MANUFACTURE. 
 
 would be useless, since its property of double refraction 
 would interfere hopelessly with the performance of the 
 lenses. This is now overcome by the use of vitreous silica 
 lenses, in the case of the " ultra-violet microscope," as 
 made by Carl Zeiss, of Jena. So far, however, it has only 
 been possible to produce quite small. pieces of vitreous silica 
 sufficiently free from bubbles to be used for optical 
 purposes. The great difficulty lies not so much in merely 
 melting the quartz down as in freeing it from the air- 
 bubbles enclosed within it ; the course usually adopted with 
 glass, of raising the temperature and allowing the bubbles 
 to rise to the surface, becomes impossible in this case, 
 because the silica itself begins to vapourise and even to boil 
 vigorously at temperatures not very far above its melting 
 point. Quite recently, however, two American workers 
 have claimed to be able to overcome this difficulty by the 
 use of both vacuum and high pressure applied at the earlier 
 and later stages of the fusion process respectively, so that 
 it may shortly be possible to produce vitreous silica in large 
 and perfectly clear blocks. 
 
 We have already indicated that glass tubing and rod 
 form the basis upon which the glass-worker, with the aid 
 of the blow-pipe or ' " lamp," fashions his productions, 
 which, of course, include a great number of scientific 
 instruments and appliances used more especially in the 
 field of chemistry. In another direction also glass tubing 
 serves as a basis for a branch of the glass industry ; this 
 is the manufacture of certain classes of glass beads, which 
 are formed by cutting up a heated glass tube of suitable 
 diameter and colour into short, more or less spherical 
 sections. In some cases the colour of the beads is secured 
 
MISCELLANEOUS PBODUCTS. 245 
 
 by using glass of the desired tint, but in other cases the 
 beads are made of colourless glass, and a colouring substance 
 is placed in the interior of the bead. 
 
 Solid glass rods are also employed for a variety of 
 purposes ; their mode of manufacture is exactly analogous 
 to that of tubing, except that the gathering is drawn out 
 without having first had a hollow space produced at its 
 centre by the blower. In its most attenuated form glass 
 rod becomes glass thread or fibre; this is produced by 
 drawing hot glass very rapidly, the resulting thread being 
 wound on a large wheel. At one time this material found 
 considerable use, since it was found possible to spin and 
 weave the thinnest glass fibres into fabrics which could be 
 used for dress purposes. It is not, however, to be regretted 
 that this fashion has neither extended nor survived, since 
 it was certainly liable to produce serious injury to health. 
 It is a well-known fact that there are few more injurious or 
 even dangerous substances to be inhaled into the human 
 throat and lungs than finely-divided glass; glass fibre, 
 moreover, when subjected to constant bending and wear, is 
 bound to undergo frequent fracture, and the atmosphere of 
 a ball-room, for example, in which several such dresses 
 were worn would soon be contaminated with innumerable 
 fine, sharp particles of glass which would produce an 
 injurious effect on those inhaling them. At the present 
 time glass fibre is used for little else than the " glass wool " 
 required for certain special purposes in chemical laboratories. 
 
 Fused quartz or silica fibres, of extreme tenuity, but of 
 relatively very great strength, are employed in many 
 scientific instruments, where their extreme lightness and 
 perfect elasticity and freedom from what is known as 
 
246 GLASS MANUFACTURE. 
 
 " elastic fatigue " renders them of very great value. These 
 fibres are not drawn from a mass of molten silica, as is 
 done with glass, but are produced by attaching a nail or 
 bolt to a small bead of fused silica produced by the aid of 
 an oxygen-fed blowpipe ; the nail or bolt is then suddenly 
 shot away down a long passage or similar space by means 
 of a cross-bow, drawing a very fine fibre of silica with it ; 
 the most difficult part of this operation, however, consists 
 in finding and handling the fibres thus produced. 
 
 Artificial Gems. The fact that pieces of suitably-coloured 
 glass can be made to show a superficial, but sometimes 
 more or less deceptive, resemblance to precious stones, has 
 led to the manufacture of imitation jewels of all descriptions. 
 The glass used for this purpose is usually a very dense flint- 
 glass whose high refractive index facilitates the imitation 
 which is aimed at. The external shapes of gems are, of 
 course, readily imitated by cutting and grinding the glass, 
 while the requisite colours are attainable by means of the 
 colouring materials described in Chapter XI. To a casual 
 observer the difference in sparkle and brilliance which 
 arises from the difference between the refractive index of 
 the heavy flint-glass (about 1*8) and that of minerals (which 
 ranges from 1*7 to 2*2) is not readily apparent, but closer 
 examination will at once reveal the difference. The deter- 
 mination of the optical constants by means of a refracto- 
 meter would at once reveal the true character of the 
 imitation, but an even readier test is that of hardness. The 
 dense flint-glass is naturally soft, and is readily scratched 
 by most of the harder minerals, while the precious stones, 
 more particularly garnets, rubies and diamonds, are very 
 hard. If an attempt is made to scratch an ordinary sheet 
 
MISCELLANEOUS PEODUCTS. 247 
 
 of window-glass, it will be found that most real precious 
 stones will do so readily, while flint-glass imitations will 
 fail to make more than a slight mark, which is more smear 
 than scratch. The test by determining the specific gravity 
 is also obviously applicable, since the flint-glass will readily 
 betray its presence by its high density (over 4). 
 
 In quite a different class from the imitation gems made 
 of cut flint-glass are the artificial gems, which in nature 
 and composition are exact reproductions of natuial gems, 
 but which have been produced by artificial processes. As 
 far as the writer is aware these are only found in any large 
 numbers in the case of the ruby, but in that case, at all events, 
 it is said that the production of the artificial crystals is 
 at least as costly as the purchase of the natural stones. 
 There can, however, be very little doubt that as the pro- 
 cesses of fusion and crystallisation become better known 
 and understood, and the chemistry of silicate minerals is 
 developed, the artificial production of mineral crystals in, at 
 all events, moderate sizes will become increasingly possible ; 
 it is even to be hoped that their production will be so far 
 perfected as to place their really valuable properties at the 
 service of man. 
 
 Chilled Glass. In all the processes of glass manufacture 
 described in the present book, annealing has always played 
 an important part. The glass, after it has undergone its last 
 treatment under the influence of heat, is subjected to a 
 gradual cooling process with the object of freeing it from 
 the internal strains which it would otherwise retain, and 
 which would, ordinarily, endanger its existence and inter- 
 fere with its use. It is, however, well known that surfaces 
 of glass subjected to such internal strains as result in a 
 
248 GLASS MANUFACTUEE. 
 
 compressive stress on the glass near the surface, are less 
 liable to injury, and are apparently stronger than when 
 the glass is annealed and the stresses are removed. On 
 the other hand, glass surfaces under tension are extremely 
 delicate and fragile. In some respects, therefore, glass 
 which has not been annealed may appear to be stronger 
 than the annealed product. The well-known case of the 
 Kupert's drop is an example of this kind. Rupert's drops 
 are produced by dropping molten glass into water ; they 
 generally take the form of a more or less spherical body 
 having a long tail, tapering off into a thread, attached to it. 
 Such a Rupert's drop may be struck with a heavy hammer, 
 and will safely resist a blow that would splinter a similar 
 body made of annealed glass. If, however, the surface be 
 scratched, or the tip of the tail be broken off,' the entire 
 " drop " breaks up, sometimes with a violent explosion, into 
 minute fragments. Numerous inventors, among whom 
 De la Bastie and Siemens figure most conspicuously, have 
 endeavoured to utilise these properties of chilled glass, not 
 exactly by endeavouring to produce that extreme degree of 
 internal strain which is characteristic of the Rupert's drop, 
 but by producing what they describe as " tempered " glass, 
 in which the internal strains have been reduced by less 
 violent cooling to such an extent as to retain some of the 
 advantages of the hardened, internally strained condition 
 while approximating more or less to the safer state of 
 annealed glass. At one time articles of this kind were 
 frequently seen as curiosities, such as tumblers that could 
 be dropped on the floor without breaking, etc., but these 
 articles generally ended by receiving a slight scratch or 
 chip and promptly falling into fragments. As a matter of 
 
MISCELLANEOUS PEODUOTS. 249 
 
 fact, however, some tempered glass is actually manufactured 
 by the firm of Siemens at the present time for special 
 purposes. De la Bastie's process was tried in England, and 
 some success was claimed for it ; but it is not in commercial 
 operation at the present time, and never appears to have 
 attained any great importance. 
 
 Massive Glass. Enthusiasts for the extension of the use 
 of glass have endeavoured to apply it to a great variety of 
 purposes, including the construction of buildings and the 
 paving of streets. In the former case, which was exempli- 
 fied at the Paris Exhibition of 1900, advantage was taken 
 of the light-transmitting power of the material, but 
 although the buildings erected with large blocks of cast 
 glass were not displeasing in effect, this use has not found 
 any considerable extension. For paving purposes, the 
 hardness and durability of glass are the only useful 
 qualities, and here also although several trials have been 
 made in France no signs of any considerable application 
 of the new products are as yet visible. What has been 
 said above with reference to the injurious character of 
 glass dust applies, further, to glass pavements, since their 
 natural wear would result in the formation of considerable 
 quantities of this dust. The advocates of glass paving, how- 
 ever, suggest that the hardness of glass would greatly 
 reduce the actual amount of wear, and that consequently 
 the dust would be reduced considerably. This is a matter 
 which prolonged experience alone can decide, but it does 
 not seem obvious that glass blocks should wear more 
 slowly than stone setts made of good granite, for example. 
 On the other hand, the glass blocks could probably be pro- 
 duced more cheaply, since the labour of cutting to size 
 
250 GLASS MANUFACTURE. 
 
 would be obviated by casting the blocks to the desired 
 dimensions. 
 
 Water-glass, or silicate of soda or potash is perhaps 
 scarcely to be classed under the heading of " Glass Manu- 
 facture " at all, but it bears a certain relationship to glass 
 in several ways. Thus one of the modes of manufacturing 
 water-glass is by the fusion of sand and alkali in tank 
 furnaces somewhat resembling those used for glass pro- 
 duction ; the fused silicate, moreover, solidifies as a vitreous 
 mass, in which respect it also resembles such substances as 
 borax, etc. The uses of silicate of soda and potash are, 
 however, so far removed from the field of glass-manufacture 
 that we cannot enter into them here. 
 
 In concluding this chapter, we wish to describe one more 
 product of the glassworks, and this includes some of the 
 most impressive and splendid examples of the glass-maker's 
 art. These are the great mirrors and lenses by whose aid 
 our lighthouses and searchlights send forth their powerful 
 beams of light. Although these objects are called " mirrors " 
 and "lenses," since they fulfil the functions of such optical 
 organs, yet in their nature and mode of manufacture they 
 are so far removed from the glass used for the production 
 of other kinds of lenses that they could not be included 
 under the heading of " optical glass." 
 
 The characteristic feature in the manufacture of optical 
 glass is the manner in which each separate pot or melting 
 is allowed to cool down and to break up into irregular 
 fragments which are subsequently moulded to the desired 
 shape. Were it attempted to manufacture the large glass 
 bodies required for lighthouse purposes in this manner, 
 the cost would approximate to that of the large discs used 
 
MISCELLANEOUS PEODUCTS. 251 
 
 for telescope objectives, and this would of course be entirely 
 prohibitive. The requirements as regards colour, homo- 
 geneity and freedom from other defects, which must be met 
 in lighthouse lenses, are further not nearly so stringent as 
 those which are essential in ordinary optical work of good 
 quality. The reason for this difference arises from the 
 fact that lighthouse lenses and searchlight mirrors are used 
 merely to impart a desired direction to a beam of light, and 
 not for the purpose of producing sharply-defined images ; 
 slight irregularities in the glass are therefore not of such 
 serious importance. 
 
 Lighthouse glass can therefore be produced by rather 
 less elaborate means ; although every care is taken to make 
 the glass as perfect as possible, it is brought into approxi- 
 mately the desired form by casting tbe molten glass in iron 
 moulds of the proper shape. When removed from these 
 moulds and annealed, the glass is fixed on large revolving 
 tables and ground and polished to the final shape of lenses 
 and annular lens-segments as required for the various 
 types of Fresnel lighthouse lenses. In this way complete 
 rings, forming annular lenses, are produced up to 48 inches 
 diameter. Kings of larger size are usually built up of a 
 number of segments, and these built-up rings sometimes 
 have a radius as large as 7 feet. For the majority of 
 lighthouse lenses, ifc should be added, a hard soda-lime 
 glass having a refractive index of 1*50 to 1*52 is used, but 
 for special purposes a dense flint-glass having a refractive 
 index of 1'63 is employed. 
 
 Mirrors for searchlight purposes are of very varied forms 
 and sizes, the shape depending largely upon the particular 
 form of beam which they are designed to project. For 
 
252 GLASS MANUFACTURE. 
 
 many purposes a parabolic form is required, while in 
 others, where a flat, fan-shaped beam is to be produced, a 
 form having an elliptical section in a horizontal plane and 
 a parabolic section in the vertical plane is required. In 
 most cases these mirrors are produced by bending plates of 
 glass, previously raised to the necessary degree of heat, 
 over suitably shaped moulds, the surface being subsequently 
 re-polished to remove any roughness resulting from the 
 bending process. Another type of mirrors is that known 
 as " Mangin," which has two spherical surfaces placed 
 eccentrically in such a way that the centre of the mirror 
 is considerably thinner than the periphery ; in this type of 
 mirror the reflecting action of the back surface is modified 
 by the refracting action of the front surface, but both are 
 spherical, and can therefore be accurately ground and 
 polished by the usual mechanical means. Such mirrors 
 are manufactured of single pieces of glass up to 6 feet in 
 diameter. 
 
APPENDIX 
 
 BIBLIOGEAPHY. 
 
 THE existing literature of glass manufacture is so limited that a 
 complete bibliography could almost be given on a single page ; in the 
 English language, in particular, there are exceedingly few books and 
 papers on the subject. The French and German literature of the 
 subject is a little more extensive. In giving a list of the works, and 
 more particularly in referring to those which he has consulted in the 
 preparation of the present volume, the author thinks it will be an 
 advantage to indicate their scope, and, to some extent, what he 
 believes to be their value, in order to save the student the trouble of 
 seeking out comparatively inaccessible works only to find that they 
 contain little that is of value for his purpose. 
 
 English Boohs and Papers on Glass Manufacture. 
 
 The Principles of Glass Making (George Bell & Sons). By 
 Powell & Chance. An elementary book giving a clear and concise 
 account of the older processes, more especially in connection with flint 
 and plate-glass. 
 
 Glass. Articles in 9th Edition of Encyclopaedia Britannica. A 
 detailed account of processes, more or less covering the entire subject, 
 but the processes described are mostly obsolete at the present time. 
 
 Glass. Article in Supplement to 9th Edition of Encyclopaedia 
 Britannica. By Harry J. Powell. A brief summary of more recent 
 developments. Particularly valuable in reference to artistic English 
 flint-glass. 
 
 Jena Glass. By Hovestadt, translated by J. D. and A. Everett. 
 
254 APPENDIX. 
 
 Con bains a full account of the scientific work on glass and its practical 
 application, done in connection with the Jena Works of Schott. 
 Particularly interesting in connection with the subjects of Chapters I., 
 II., XII., and XIII. As the title indicates, the book is written from 
 the Jena point of view, and scarcely does justice to work done 
 elsewhere. The book has gained considerably at the hands of the 
 translators. 
 
 Some Properties of Glass. By W. Rosenhain. (Transactions of 
 the Optical Society of London, 1903.) Gives a brief account of the 
 properties of glass as affecting its optical uses. 
 
 Possible Directions of Progress in Optical Glass. By W. Rosenhain. 
 (Proceedings of the Optical Convention, London, 1905.) Has been 
 referred to in the text of this book (Chapter XII.). 
 
 Catalogue of the Optical Convention Exhibition, London, 1905. 
 Contains historical and general notices of optical and lighthouse glass, 
 glass-working machinery, etc. 
 
 Glass for Optical Instruments. By E. T. Glazebrook. (Cantor 
 Lectures to the Society of Arts.) Gives an account of modern optical 
 glass manufacture. 
 
 Old English Glasses. By Albert Hartshorne. Gives an account of 
 the history of glass-making in England. 
 
 The Methods of Glass Blowing. By W. Shenstone. Describes 
 the manipulation of glass-blowing for experimental purposes, i.e., 
 lamp work. 
 
 French Books on Glass Manufacture. 
 
 Guide du Verrier. By G. Bontemps. A classical work by one of 
 the greatest experts of his day. Much of the contents of the book 
 is, however, entirely out of date at the present time. The book is 
 interesting as being the work of the man who introduced optical glass 
 manufacture into England. 
 
 Verres et Emaux. By L. Comgnal. Chiefly of interest in connec- 
 tion with the subjects of Chapter VIII. 
 
 Le Verre et le Crystal. By J. Henrivaux. (P. Vicq Dunod et Cie., 
 Paris.) A lengthy book profusely illustrated and giving a great wealth 
 of detailed information. The writer was for some time the general 
 manager of one of the largest plate-glass manufactories in Europe ; 
 his account of plate-glass manufacture is, therefore, especially valuable. 
 Much space in this book is devoted to historical and aesthetical 
 matter. 
 
APPENDIX. 255 
 
 La Verrerie au XXi eine Siecle. By J. Henrivaux. (Paris, K. 
 Bernard et Cie., 1903.) Practically a supplement to the preceding ; 
 some of the processes and products described are, however, not of a 
 practical nature. Chiefly valuable for recent developments in plate- 
 glass and bottle-glass manufacture. 
 
 German Books on Glass Manufacture. 
 
 Die Glasfabrikation. By E. Gerner. (A. Hartleben's Verlag, 
 Vienna and Leipzig, 1897.) A concise and clear account of most of 
 the more important processes of glass manufacture. Very practical 
 in character. The information given appears to be reliable, although 
 far from complete. 
 
 Die Herstellung Grosser Glaskoerper and Die Bearbeitung Grosser 
 Glaskoerper. By C. Wetzel. (Hartleben's Verlag, Vienna and 
 Leipzig, 1900 and 1901 respectively.) Describes numerous special 
 processes and appliances devised for use in connection with large 
 glass objects. Some of these descriptions, however, appear to be 
 little more than transcripts from patent specifications. 
 
 Glasfabriken und Hohlglasfabrikation. By E. Dralle. (Leipzig, 
 Baumgaertner, 1886.) Looked upon as a classic in Germany. Gives 
 detailed plans and drawings of entire bottle-works, including furnaces 
 and all accessories. Deals principally with bottle manufacture. 
 
 Die Glasfabrikation. By Dr. E. Tscheuschner. (Weimar, B. H. 
 Voigt, 1888.) A full detailed account of all processes known at the 
 time. The rapid progress of modern practice has, however, already 
 rendered this book to some extent obsolete. 
 
 Jenaer Glas. By Hovestadt. Already referred to in respect of the 
 English translation. 
 
 Der Sprechsaal. (Schmidt, Weimar.) A trade journal devoted to 
 the discussion of technical matters relating to the glass and ceramic 
 industries. Occasionally contains articles and abstracts of technical 
 or scientific interest iri connection with glass manufacture. 
 
 In addition to the books and papers named in the above list, a 
 great number of scientific papers, notes, etc., are to be found scattered 
 throughout the technical and scientific publications of the world ; 
 those that have proved of real interest and importance have, however, 
 left their mark on the industry, and will be found described or referred 
 to in connection with the various branches of manufacture described 
 n the present volume or in the books named above. 
 
INDEX 
 
 A. 
 
 ABBE, 8, 10, 210, 213 
 Absorption of light in glass, 82, 
 
 179 
 Acid, action of, on glass, 11 
 
 boric, action of, on glass, 
 
 11, 186 
 carbonic, action of, on glass, 
 
 12 
 hydrofluoric, action of, on 
 
 glass, 12 
 phosphoric, action of, on 
 
 glass, 11 
 
 Air, compressed, 91, 105, 117 
 Alkali chlorides, use of, in glass 
 
 manufacture, 41 
 content of hygroscopic 
 
 glass, 6 
 metals, 184 
 nitrates, 44, 78 
 sources of, 40 
 Alkaline liquids, action of, on 
 
 glass, 11 
 
 Aluminium, 51, 186 
 Ammonia soda, 41 
 Anastigmatic photographic lenses, 
 
 213 
 
 Ancient windows, colours of, 16, 
 202 
 
 G.3I. 
 
 Annealing bottles, 103 
 kiln, 103 
 
 for optical glass, 
 
 235 
 for plate glass, 
 
 135 
 for rolled plate 
 
 glass, 127 
 
 Anthracite coal, 42, 53, 79 
 Antimony, 188 
 Apochromatic objectives, 213 
 Arsenic, 52, 78, 105, 117, 188 
 Artificial gems, 246 
 Auerbach, 22 
 Aventurine, 185 
 
 B. 
 
 BACTERIA, action of, on glass, 13 
 Barium compounds, 47, 186 
 
 crown glass, 212 
 
 glass, 7 
 Barytes, 48 
 Bases other than alkalies, sources 
 
 of, 45 
 
 Beads, 244 
 
 Behaviour, chemical, of glass, 6 
 Bending plate glass, 144 
 Bevelling, 145 
 Black ash, 41 
 
258 
 
 INDEX. 
 
 Blisters in sheet glass, 160, 168 
 Blocks, fire-clay, 58 
 
 tank, 59 
 
 Blower, sheet glass, 158 
 Blower's chair, 111 
 Blowing glass, 89 
 
 holes, 91, 161, 189 
 sheet glass, 161 
 Blown glass, decoration of, 114 
 
 plate glass, 171 
 Bohemian glass, 109, 240 
 ] foiling up, 81 
 Bottles, annealing of, 103 
 
 blowing, improvements 
 
 in, 99 
 
 machines, 100 
 colour of, 96 
 manufacture, furnace for, 
 
 97 
 
 moulds for blowing, 98 
 production of, by hand, 
 
 98 
 
 raw materials for, 95 
 strength of, 18 
 Boric acid, 11 
 Boron, 186 
 
 Boro-silicate crown, 212 
 Boucher's bottle - blowing 
 
 machine, 101 
 Bricks, fire-clay, 58 
 
 silica, 60 
 Bubbles in optical glass, 230 
 
 removal of, 81 
 Burning, pots, 58 
 
 C. 
 
 CADMIUM, 186 
 Calcium carbonate, 46 
 
 oxide, 45, 186 
 
 sulphate, 47 
 Carbon, 53, 79, 186 
 
 Carbonate of soda, 41 
 
 Carbonic acid, action of, on glass, 
 
 12 
 
 Carboys, blowing of, 104 
 Casting plate glass, 132 
 Chair, glass-blower's, 111 
 Chalk, 46 
 Chamotte, 57 
 Chance, 211 
 Charcoal, 42, 53, 79 
 Charging furnaces, 75 
 Chemical behaviour of glass, 6 
 
 composition of glass, 5 
 
 of optical 
 
 glass, 217 
 
 reactions during fusion, 
 
 76 
 
 Chilled glass, 247 
 Chimneys, gaslight, 23 
 
 lamp, 238 
 Chromium, colouring effect of, 
 
 190 
 
 Cleaning of lenses, 220 
 Coal, anthracite, 42, 53, 79 
 Cobalt, colouring effect of, 197 
 Coke, 42, 53, 79 
 Colour of ancient windows, 16 
 glass, 32 
 
 theory of, 181 
 optical glass, 208 
 sheet glass, 167 
 Coloured blown glass, 113 
 glass, 178 
 
 technical uses of, 
 
 203 
 
 Combustion tubing, 7, 241 
 Compressed air for glass blowing, 
 
 91, 105, 117 
 Conductivity, electrical, of glass, 
 
 30 
 
 thermal, of glass, 
 24, 29 
 
INDEX. 
 
 259 
 
 Copper, colouring effect of, 184 
 
 ruby, 184, 188, 198 
 Corrosion of glass, 11 
 Covered pots, 56, 109 
 Crown, boro-silicate, 219 
 
 glass, 175, 211 
 
 hard, 212, 219 
 
 soft, 212 
 
 telescope, 215 
 Crowns, furnace, 60 
 Crucibles, manufacture of, 56 
 
 for glass melting, 54 
 Crushing strength of glass, 19 
 Cryolite, 52 
 
 Crystallisation of glass, 3 
 Crystals, mineral, 218 
 Gullet, 74 
 
 for optical glass, 224 
 Cutting rolled plate glass, 128 
 Cylinders, sheet glass, 161, 171 
 
 D. 
 
 DECOLONISATION of glass, 52, 
 
 188, 190, 193, 197 
 Decoration of blown glass, 114 
 Defects in rolled plate glass, 129 
 
 sheet glass, 166 
 Definition of glass, 1 
 De la Bastie, 248 
 Devitrification, 3, 11 
 Diamond, refractive index of, 216 
 Dimming of glass surfaces, 12 
 Dinas bricks, 61 
 Dipping of sheet glass, 166 
 Dispersion of optical glass, 209 
 
 partial, 214 
 Double refraction in optical glass, 
 
 221 
 
 rolling machine, 130 
 Drawing tubes, 239 
 Ductility of glass, 20 
 
 Durability of glass, tests for, 14 
 Dust, action of, on lenses, 220 
 glass, 245 
 
 E. 
 
 ELASTICITY of glass, 20, 24 
 Electrical properties of glass, 29 
 Epinal, 39 
 Etching of glass, 12 
 Expansion, co-efficient of thermal, 
 24, 25 
 
 F. 
 
 FELSPAR, 40, 44 
 Fibres, glass, 245 
 silica, 245 
 
 Figured rolled plate glass, 87, 130 
 cutting 
 of, 131 
 
 Finger-marks on lenses, 219 
 Fining of glass, 81 
 
 optical glass, 229 
 Fire-clay, action of, on glass, 6 
 for pots, 55 
 wetting up, 57 
 Fire-polish, 117 
 Flashed glass, 25, 199 
 Flint, 40 
 
 boro-silicate, 212 
 dense, 212, 246 
 densest, 212 
 extra dense, 212 
 glass, 7, 49, 78, 108, 211 
 light, 212 
 soda, 212 
 telescope, 215 
 
 Flu,orite, refractive index of, 216 
 Fontainebleau, 38 
 Founding of optical glass, 227 
 Fourcault process, 174 
 
260 
 
 INDEX. 
 
 Fresnel, 251 
 Furnace crowns, 60 
 
 gas, 63 
 Furnaces for bottle manufacture, 
 
 97 
 
 glass melting, 54, 62 
 optical glass, 225 
 plate glass, 133 
 rolled plate glass, 
 
 122 
 
 sheet glass, 151, 170 
 ports, 67 
 
 recuperative, 66, 156 
 regenerative, 66, 155 
 tank, 59, 69 
 
 economy of, 72 
 Fusion, process of, 73 
 
 temperature of glass, 5 
 Freezing of glass, 2 
 
 G. 
 
 GASLIGHT, chimneys for, 23 
 
 Gas producers, 62, 64 
 
 Gatherer, 158 
 
 Gathering of glass, 85, 88, 158 
 
 Gauge tubes, 10, 18, 23, 26 
 
 Gems, artificial, 246 
 
 Ghosts, photographic, 16 
 
 Glauber's salt, 43 
 
 Gold, colouring effect of, 185 
 
 Grinding plate glass, 137 
 
 Gypsum, 47 
 
 H. 
 
 HARDENED glass, 20 
 Hardness of glass, 21 
 
 tests for, 22 
 Heavy spar, 48 
 Henrivaux, 19 
 Hertz, 22 
 
 Hock-bottle colour, 195 
 Hohenbocka, 38 
 Hollow glassware, 108 
 Horseshoe flame, 69 
 Hydrofluoric acid, action of, on 
 
 glass, 12 
 Hygroscopic glass, alkali content 
 
 of, 6 
 
 r. 
 
 INDENTATION modulus, 22 
 
 Index, refractive, 216 
 
 Insulating properties of glass, 29 
 
 Iron, 96 
 
 colouring effect of, 196 
 oxidation of, in glass, 195 
 
 Irregularities caused by rolling, 86 
 
 J. 
 
 JENA, 7, 10, 14, 26, 29, 203, 210, 
 213, 241 
 
 KELP, 40 
 Kowalski, 19 
 
 LABORATORY ware, 10, 23 
 Ladling glass, 85 
 
 rolled plate glass, 124 
 Lagre, 166 
 
 Lamp-chimneys, 110, 238 
 Lamp-work, 240, 244 
 Large vessels, production of, 105 
 Lead, 49, 183, 188 
 Lear for rolled plate glass, 127 
 
 sheet glass, 165 
 Leighton, 39 
 
INDEX. 
 
 261 
 
 Lenses, cleaning of, 220 
 
 finger-marks on, 220 
 pressing small, 94 
 
 Light, action of, on glass, 15 
 
 Lighthouse glass, 178. 250 
 
 Lime, slaked, 45 
 
 Lime-stone, 46 
 
 Limited range of vitreous bodies, 4 
 
 Lippe, 38 
 
 Lynn, 39 
 
 M. 
 
 MACHINES, bevelling, 145 
 
 double rolling, 130 
 
 grinding, 139 
 
 polishing, 141 
 Magnesia, 48, 186 
 Manganese, 15, 52, 80 
 Mangin mirrors, 252 
 Marver, 111 
 Massive glass, 249 
 Mechanical properties of glass, 18 
 Metal, attachment of, to glass, 26 
 Minerals, crystalline, 217 
 Mirrors, 145 
 
 searchlight, 251 
 Mixing of materials, 73 
 Moulds for glass-blowing, 90, 110, 
 116 
 
 pressed glass, 119 
 Muffled glass, 172 
 Muranese glass, 123 
 
 N. 
 
 NICKEL, 96 
 
 . colouring effect of, 197 
 
 steel, 27, 148 
 Nitrates, alkali, 44, 78 
 
 O. 
 
 OBJECTIVES, apochromatic, 213 
 telescope, 213 
 
 Opal glass, 45, 52, 186 
 
 Opaque plate glass, 146 
 
 Open pots, 56 
 
 Optical glass, annealing, 235 
 
 chemical composi- 
 tion of, 217 
 cooling of, 233 
 cost of, 237 
 fining, 229 
 founding, 227 
 furnaces for, 225 
 hardness of, 220 
 moulding, 235 
 pressing, 93 
 range of, 216 
 raw materials for, 
 
 223 
 
 sorting, 235 
 stability of, 219 
 strain in, 221 
 stirring, 231 
 yield of, 236 
 properties of glass, 205 
 
 1'. 
 
 PAINTING on glass, 201 
 Parason, 102 
 Patent plate glass, 171 
 Paving stones, glass, 249 
 Pearl ash, 43 
 Phosphoric acid, 11 
 Phosphorus, 188 
 Photographic ghosts, 16 
 
 lenses, anastigma- 
 
 tic, 213 
 colour of, 
 
 209 
 
262 
 
 INDEX. 
 
 Pipe, glass-maker's, 89 
 
 sheet-blower's, 158 
 warmer, 158 
 
 Plate glass, annealing kiln for, 135 
 bending of, 144 
 blown, 171 
 casting, 132 
 colour of, 33 
 figured rolled, 87 
 flatness of, 134 
 furnaces for, 133 
 grinding machines, 
 
 139 
 
 of, 137 
 mirrors, 145 
 opaque, 146 
 polishing machines, 
 
 141 
 
 of, 137 
 
 raw materials for, 132 
 rolled, 86, 123 
 silvering, 146 
 sizes of, 143 
 strength of, 15 
 striae in, 143 
 wired, 27, 147 
 Platinum, 27 
 Polishing, theory of, 141 
 Pontil, 98, 176, 239 
 Potash, 43 
 
 Potato, use of, in glass melting, 81 
 Ports, furnace, 67 
 Pots, burning of, 58 
 covered, 56 
 drying of, 58 
 for flint glass, 109 
 
 optical glass, 226 
 manufacture of, 56 
 open, 56 
 
 Pouring of glass, 85, 87 
 Pressed glass, 92, 118 
 
 composition of, 120 
 
 Presses for glass, 119 
 Proofs, 82, 231 
 Purity of materials, 36 
 
 QUARTZ, 40 
 
 Q. 
 
 E. 
 
 RANGE, limited, of vitreous 
 
 bodies, 4 
 
 Recuperative furnaces, 66, 156 
 Red lead, 49 
 Refraction, double, in optical 
 
 glass, 221 
 of light in optical 
 
 glass, 209 
 
 Refractive index, 216 
 Regenerative furnace, 66, 155 
 Reichsanstalt, 10 
 Resistance to crystallisation of 
 
 glass, 4 
 
 Rings for lighthouse lenses, 251 
 Rod, glass, 245 
 Rolled plate glass, 86, 123 
 
 annealing, 127 
 cutting, 128 
 defects of, 129 
 figured, 130 
 furnaces, 123 
 ladling, 124 
 raw materials 
 
 for, 124 
 rolling, 126 
 sorting, 129 
 surface of, 122 
 Rolling of glass, 86 
 Rubies, artificial, 247 
 Ruby, copper, 184, 188, 198 
 flashed, 184 
 gold, 185 
 Rupert's drops, 248 
 
INDEX. 
 
 263 
 
 S. 
 
 SALT-CAKE, 37, 42, 79, 189 
 
 Sand, 38 
 
 Sandstone, 39 
 
 Schott, 8, 19, 203, 213, 241 
 
 Scratches on sheet glass, 169 
 
 Searchlights, "250 
 
 Seed in sheet glass, 167 
 
 Selenium, colouring effect of, 
 190 
 
 Sheet glass, 70 
 
 blisters in, 160, 168 
 blowing, 161 
 colour of, 33, 167 
 compared with plate, 
 
 149 
 
 cylinders, 161, 171 
 defects of, 166 
 dipping. 166 
 flattening, 165 
 furnaces, 151, 170 
 lear, 165 
 
 mechanical produc- 
 tion of, 173 
 raw materials for, 
 
 150 
 
 sorting, 166 
 splitting, 164 
 strength of, 18 
 
 Siedentopf, 182 
 
 Siege blocks, 59 
 
 Siemens, 248 
 
 Sievert, 92, 105, 117, 172 
 
 processes, 105, 117 
 
 Signal glasses, 203 
 
 Silica bricks, 61 
 
 glass, 5, 26, 241 
 sources of, 37 
 
 Silicon, colouring effect of, 187 
 
 Silver, colouring effect of, 185 
 
 Silvering plate glass, 146 
 
 Sizes of plate glass, 142 
 Soda ash, 41 
 
 carbonate, 41 
 sulphate, 37, 42, 79 
 sulphide, 80 
 sulphite, 79 
 
 Solidification of glass, 1 
 Solutions, analogy of, with glass, 
 
 206 
 
 Sorting rolled plate glass, 129 
 Specific heat of glass, 25. 29 
 
 inductive capacity of 
 
 glass, 29 
 
 Stains, coloured, 200 
 Stassfurth, 44 
 Stones in rolled plate glass, 129 
 
 sheet glass, 167 
 Storage of materials, 37 
 Strain in optical glass, 221 
 Strength of glass, 19 
 Striae in coloured glass, 203 
 
 optical glass, 206, 227 
 plate glass, 143 
 testing apparatus, 207 
 String in sheet glass, 168 
 Strontium, 86 
 Structure of glass, 1 
 Sulphur, colouring effect of. 
 
 189 
 Surfaces, chemical behaviour of 
 
 glass, 8, 10 
 S/igmondi, 182 
 
 T. 
 
 TABLE, rolling, 126 
 Tank blocks, 59 
 
 furnaces, 59, 69 
 
 economy of, 72 
 for sheet glass, 
 152 
 
264 
 
 INDEX. 
 
 Telescope objectives, 213 
 Temperature of fusion of glass, 
 
 5 
 
 Tempered glass, 20, 248 
 Tensile strength of glass, 19 
 Thallium, 183, 188 
 Theory of colours in glass, 181 
 
 polishing, 141 
 
 Thermal endurance of glass, 23 
 properties of glass, 23 
 Thermometer glass, 7, 8, 28 
 Tin, colouring effect of, 187 
 Tonnelot, 7 
 Transparency of glass, 31 
 
 optical glass, 208 
 Trautwine, 19 
 Tubing, 238 
 
 combustion, 7 
 drawing of, 239 
 Tumblers, 111 
 
 U. 
 
 ULTKA-VIOLET microscope, 243 
 
 V. 
 
 VANADIUM, colouring effect of, 189 
 Veins in optical glass, 206, 227 
 
 w. 
 
 WATEE, action of, 011 glass, 10 
 
 glass, 250 
 
 Wetting up clay, 57 
 Winkelmann, 19 
 Wired plate glass, 27, 147 
 Witherite, 48 
 Wool, glass, 245 
 
 Y. 
 
 YOUNG'S modulus, 20 
 
 Z, 
 
 ZAFFRE, 197 
 
 Zeiss, 213, 244 
 
 Zinc, colouring effect of, 49, 186 
 
 Zschimmer, 14 
 
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