Chemical Monographs 
 
 SB 272 3^0 
 
 L^BBBHHHIB 
 
 CHEMISTRY OF DYEING 
 
 JOHN KERFOOT WOOD 
 
 , Van Nostrand Company 
 
MAR 27 1922 
 
 
CHEMICAL MONOGRAPHS 
 
 EDITED BY A. C. GUMMING, D.Sc. 
 
 No. II 
 
 The Chemistry of Dyeing 
 
CHEMICAL MONOGRAPHS 
 
 EDITED BY A. C. GUMMING, D.Sc. 
 
 THE progress of Chemistry is so rapid that it is 
 becoming a matter of ever-increasing difficulty to 
 keep abreast of the modern developments of the 
 science. The volume of periodical literature is so 
 enormous that few can hope to read, far less 
 assimilate, all that is published. The preparation 
 of summaries has therefore become a necessity, and 
 has led to the publication of various well-known 
 journals devoted to the abstraction of original papers. 
 For obvious reasons, however, these do not fully 
 supply the wants of advanced students and research 
 workers, and it is now generally recognised that 
 monographs on special subjects are also needed. 
 
 This series of monographs is intended primarily for 
 Advanced and Honours students. As each mono- 
 graph is written by an author with special knowledge 
 of the subject, and copious references are given, 
 it is hoped that the series will prove useful also to 
 those engaged in research. 
 
 The following volumes are ready or in active 
 preparation : 
 
 THE ORGANOMETALLIC COMPOUNDS OF ZINC AND MAGNESIUM. 
 By HENRY WREN, M.A., D.Sc., PH.D., Head of the 
 Department of Pure and Applied Chemistry at the Muni- 
 cipal Technical Institute, Belfast. Ready. 
 
 THE CHEMISTRY OF DYEING. By JOHN KERFOOT WOOD, D.Sc., 
 Lecturer on Chemistry, University College, Dundee. 
 
 Ready. 
 
 THE CHEMISTRY OF RUBBER. By B. D. PORRITT, F.I.C., B.Sc., 
 Chief Chemist to the North British Rubber Company. 
 
 In the Press. 
 
 THE FIXATION OF ATMOSPHERIC NITROGEN. By JOSEPH KNOX, 
 D.Sc., Lecturer on Inorganic Chemistry, University of 
 Aberdeen. In Active Preparation. 
 
 Other Volumes to follow. 
 
THE CHEMISTRY 
 OF DYEING 
 
 BY 
 
 JOHN K. WOOD, D.Sc. 
 
 Lecturer in Chemistry, University College, Dundee 
 
 NEW YORK 
 D. VAN NOSTRAND COMPANY 
 
 EIGHT WARREN STREET 
 1913 
 
 
Kuf :-. ' 
 
 W* 
 
PREFACE 
 
 IN writing this little book my principal aim has been 
 to give a concise and connected account of the work 
 which has been carried out, particularly during the 
 last thirty years, with the object of throwing light 
 on the nature of the dyeing process. As will appear 
 from a perusal of the book itself, many of the 
 common practices of dyeing and the phenomena 
 connected with the process are found, on examination, 
 to be in agreement with and easily explained by the 
 general principles of Physical Chemistry, with which 
 the reader is supposed to be familiar. The book may 
 have, therefore, the effect of widening the student's 
 outlook, by showing him that the principles which 
 govern many of the operations of the laboratory 
 apply with equal force to a large industry. Such 
 information respecting the textile fibres and the 
 dyestuffs as is necessary for a complete understanding 
 of the principles of dyeing has been included in the 
 book. 
 
 I hope that the work may also appeal to some of 
 those actively engaged in the Dyeing Industry 
 as well as to the student, and that it may have the 
 effect of arousing in them a greater interest in the 
 theoretical side of their work. 
 
 J. K. W. 
 
 DUNDEE, January 1913. 
 
 f 
 
 603813 
 
CONTENTS 
 
 I'AUB 
 
 INTRODUCTION ...... 1 
 
 SECTION I. THE CHEMICAL COMPOSITION AND PRO- 
 PERTIES OF THE TEXTILE FlBRES . . 3 
 
 SECTION II. DYES AND THEIR PROPERTIES . . 12 
 
 SECTION III. THE NATURE OF THE DYEING PROCESS 30 
 
 BIBLIOGRAPHY ...... 75 
 
 INDEX . 79 
 
 Vil 
 
The Chemistry of Dyeing 
 
 INTRODUCTION 
 
 BY the term "Dyeing" we mean the colouring of 
 various materials, especially textile fabrics, in such 
 a manner that the colour is not readily removed by 
 washing or rubbing the article ; moreover, the colour 
 must be distributed right through the whole material, 
 and not lie simply on the surface as with a painted 
 article. 
 
 The art of dyeing dates from prehistoric times 
 and is of Eastern origin. Pliny gives a short account 
 of the methods employed in Egypt in the first 
 century, but in even earlier times dyeing operations 
 were carried on in India, China, and- Persia. From 
 Egypt knowledge of the art travelled in a westward 
 direction, but it was not until towards the end of 
 the fifteenth century that the Dyers' Company was 
 incorporated in London. 
 
 Previous to the middle of the last century, all 
 the materials used as colouring agents were of natural 
 origin, being chiefly obtained from various portions 
 of trees and plants. Probably in the early stages of 
 the development of dyeing, the colours produced were 
 of a fugitive character and little better than stains, 
 
 A 
 
2 THE CHEMISTRY OF DYEING 
 
 but as ' fame wnt. ! pn methods were discovered by 
 means : of .which, the colours could be made more 
 permanent* ; 'the .'Egyptians, for example, were well 
 acquainted with the use of alum for this purpose. 
 
 In 1856 the first artificial dyestuff was manu- 
 factured, and this marked the beginning of a new 
 epoch in the dyeing industry. During the last fifty 
 years thousands of other artificial colouring matters 
 have been prepared, with the result that those of a 
 natural origin have been almost completely dis- 
 placed. The artificial dyes are, for the most part, 
 more easily applied than the natural ones, as well as 
 being more reliable; a much greater variety of 
 colour is now possible than was the case prior to the 
 introduction of the artificial colouring matters. 
 
 As regards the materials which are dyed, these 
 consist principally of goods to be used for clothing, 
 upholstery, etc., composed of products of an animal 
 or vegetable origin. These animal and vegetable 
 substances differ very much in their chemical char- 
 acters and in their behaviour towards dyes and other 
 chemicals. In addition to this, the dyes also show 
 great diversity of constitution and properties. It 
 will be at once apparent, therefore, that before 
 studying the dyeing process and endeavouring to 
 ascertain the nature of the union between material 
 and dye, it is necessary to be familiar with the' 
 properties of the more important textile fibres and 
 with the different groups of colouring matters. 
 
SECTION I 
 
 The Chemical Composition and Properties of the 
 Textile Fibres. 
 
 THE chief textile fibres with which the dyer comes 
 into contact are cotton, linen, jute, wool, and silk. 
 It is customary to divide these into two groups, the 
 first three being classed as vegetable fibres and the 
 remaining two as animal ones; as will be shown 
 shortly, there are marked differences between the two 
 kinds of fibres. 
 
 A microscopical examination of the fibres reveals 
 the fact that they are structurally very different. 
 The wool fibre has the most complex structure, being 
 made up of cells of three distinct kinds ; silk, on the 
 other hand, might be said to be devoid of structure, 
 the fibre, as it issues from the spinneret in the head 
 of the worm, consisting simply of a long double 
 cylinder. The vegetable fibres are composed of 
 hollow cells, each cell having a central canal or 
 lumen running through it. In the case of cotton 
 the fibre consists of a single cell, whilst with linen 
 and jute the cells are grouped together in bundles to 
 form the fibres. 
 
 It is with the differences of a chemical nature 
 shown by the fibres that we are, however, principally 
 
4 THE CHEMISTRY OF DYEING 
 
 concerned, for it is largely upon these that the 
 difference in the behaviour towards dyestufFs depends. 
 
 Cotton and Linen. 
 
 Cotton and linen consist essentially of the polysac- 
 charose cellulose, and exhibit in general the properties 
 of that substance; instances of peculiar behaviour, 
 such as that of the cotton fibre towards concentrated 
 solutions of alkalis, are traceable to the structure of 
 the fibre and have no connection with its chemical 
 composition. Solutions of strong acids have a hydro- 
 lysing action on the fibres, which are converted 
 ultimately into dextrin and glucose. Even with very 
 dilute solutions of such acids gradual disintegration 
 of the fibre is produced if the acid solution is allowed 
 to dry upon the fibre. Weak acids such as acetic 
 and formic acids have no appreciable action upon 
 cotton and linen. As is well known, concentrated 
 nitric acid either alone or in conjunction with 
 sulphuric acid converts cellulose into various nitrates, 
 often incorrectly called nitro-celluloses ; this property 
 is now made use of for preparing from cotton and 
 other forms of cellulose certain kinds of the so-called 
 artificial silks (see later). From the foregoing state- 
 ments it will readily be concluded that great care 
 must be taken when dyeing cotton and linen to 
 avoid the use of dyebaths containing considerable 
 amounts of strong acids, whilst even when the 
 proportion of such acids is very small the treatment 
 in the bath must be followed by thorough washing 
 of the material in order to remove the small amount 
 of acid present in the cloth, and so prevent the- 
 
MERCERISATION 5 
 
 destruction of the fibre which would result from 
 the gradual concentration of the acid. 
 
 Dilute solutions of alkalis have no appreciable 
 action on cotton or linen, but cold concentrated solu- 
 tions of sodium or potassium hydroxide have a 
 remarkable effect upon the former fibre. The cotton 
 fibre is naturally flat and twisted spirally, but after 
 treatment with the alkali it is found to be cylindrical 
 and straight. The lumen practically vanishes during 
 this treatment, whilst the fibre becomes translucent 
 and has a superior attraction for colouring matters 
 as compared with the natural fibre. This behaviour 
 of cotton was noticed first by Mercer, and is now 
 taken advantage of for the manufacture of the so- 
 called mercerised cotton. 
 
 Solutions of hypochlorites, especially when warm, 
 convert cotton and linen into a substance to which 
 Witz has given the name of oxycellulose. This 
 substance possesses distinct acid properties, and has 
 a greater attraction for dyes of the basic class than 
 have the natural fibres. 
 
 Jute. 
 
 Jute, although resembling cotton and linen in its 
 behaviour towards acids, differs from those fibres in 
 its chemical composition. Cross and Bevan, 12 who 
 have made a study of the subject, have given the 
 name of bastose to the substance of which jute is 
 composed. An insight into the nature of this sub- 
 stance is afforded by the fact that on treatment with 
 alkalis it yields cellulose, together with substances 
 
6 THE CHEMISTRY OF DYEING 
 
 related to the tannins. The researches of Cross and 
 Be van (loc. cit.) have shown that bastose, which is 
 one of the ligno-celluloses, may be regarded as a 
 complex made up of ordinary cellulose, a penta- 
 cellulose containing an aldehyde group and yielding 
 furf urol on hydrolysis, and a substance of the nature 
 of a quinone, which, on chlorination and reduction, 
 yields derivatives of the trihydric phenols. Owing to 
 the presence of this latter constituent, jute behaves 
 towards basic dyes in the same manner as cotton 
 which has been mordanted with tannic acid. 
 
 Wool. 
 
 Wool and silk differ very materially in their 
 chemical composition from the vegetable fibres. 
 Wool is chemically, as well as structurally, the most 
 complex of the common textile fibres; it contains 
 nitrogen and sulphur in addition to carbon, hydrogen, 
 and oxygen. The amount of sulphur is only small 
 and varies in different samples. It appears probable 
 that this amount of sulphur does not all enter into 
 the composition of the fibre itself, but that the bulk 
 of it is present as a loosely combined compound, since 
 most of the sulphur can be removed from wool by 
 the agency of alkalis without causing any apparent 
 change in the structure of the fibre. The small 
 amount of the element not so removed, and forming 
 about 0-5 per cent, of the total weight of the fibre, 
 probably enters into the composition of the wool- 
 substance. The name of keratin has been given to 
 the substance of wjrich wool is composed. It is of 
 
HYDROLYSIS OF WOOL 7 
 
 the nature of a protein, and like all such substances 
 is amphoteric* in its reactions. Hydrolysis with 
 solutions of alkalis breaks up the keratin into 
 simpler substances. One such substance is lanuginic 
 acid, which was isolated by Champion, 11 and after- 
 wards more thoroughly examined by Knecht and 
 Appleyard. 37 It is prepared by dissolving purified 
 wool in a moderately concentrated solution of barium 
 hydroxide, and then passing carbonic anhydride 
 through the liquid to precipitate the barium; the 
 precipitated barium carbonate is filtered off and the 
 filtrate mixed with a solution of lead acetate. The 
 precipitate obtained contains the lanuginic acid; it 
 is washed, suspended in water, and the lead thrown 
 down as sulphide by means of sulphuretted hydrogen. 
 On evaporating the filtrate from the lead sulphide 
 a dirty yellow residue remains, which is the lanuginic 
 acid. Knecht and Appleyard (loc. cit.) found this 
 substance to give the ordinary protein reactions; 
 it contains sulphur and dissolves in water, forming 
 a solution which is not coagulated by heat. Further 
 
 * An amphoteric compound is one which 'is so constituted 
 that it is capable of acting either as an acid or a base. The 
 simplest organic substance of this kind is glycine ; by virtue 
 of the carboxyl group which it contains, it is able to react with 
 alkalis and form salts in which it takes the part of the acid, 
 while the presence of the amino group makes it possible for 
 glycine also to form salts by union with strong acids, and in 
 the latter case the glycine is playing the part of a base. Some 
 metallic hydroxides, such as, for example, those of aluminium, 
 zinc, lead, etc., also possess the power of forming two classes 
 of salts, in one of which they act as a base and in the other as 
 an acid. 
 
8 THE CHEMISTRY OF DYEING 
 
 reference to this compound will be made when 
 dealing with the theories of dyeing. 
 
 The presence of aromatic amino groups in keratin 
 is indicated by the behaviour of wool towards nitrous 
 acid, a diazo compound being formed which can be 
 afterwards coupled with a phenol in the usual 
 manner. The fact that wool, when treated with 
 dilute solutions of sulphuric or hydrochloric acid, 
 absorbs and holds tenaciously part of the acid, is 
 further evidence of the possession of basic properties 
 by the fibre. 
 
 As regards their effect upon the physical properties 
 of the wool fibre, acids are nothing like so destructive 
 as in the case of the vegetable fibres. Dilute, hot 
 solutions of strong acids, even, have little effect upon 
 the strength of the fibre, but concentrated solutions 
 gradually destroy it. Nitric acid, as with many 
 proteins, turns wool yellow owing to the formation 
 of xanthoproteic acid. 
 
 On the other hand, alkalis are much more severe 
 upon wool than upon cotton and linen. Dilute 
 solutions of alkaline hydroxides, even in the cold, 
 weaken the fibre, whilst on heating the wool 
 gradually dissolves. 
 
 Silk. 
 
 Silk bears a considerable resemblance to wool 
 in its chemical properties. It is also a protein, but, 
 unlike wool, contains no sulphur. Raw silk consists 
 of two substances, sericin and fibroin. The former 
 constitutes about 25 per cent, of the whole; it is 
 
SILK AND ITS PROPERTIES 9 
 
 sometimes called silk glue, and has the effect of 
 making the fibre stiff and harsh in feeling. Before 
 being manufactured into silk goods the raw fibre is 
 scoured by means of a solution of soap; this has 
 the effect of removing the sericin, leaving the 
 ordinary well-known glossy fibre which consists of 
 fibroin. Fibroin gives the usual reactions of proteins, 
 and on hydrolysis is resolved into a mixture of 
 amino acids; like keratin it contains the aromatic 
 amino group. 
 
 Silk resembles wool in its behaviour towards acids 
 and alkalis, but is rather less sensitive to the action 
 of these substances. Concentrated solutions of 
 mineral acids and hot solutions of alkaline hydroxides 
 dissolve the fibre. 
 
 Artificial Silks. 
 
 In addition to the previously described substances, 
 some account must be given of certain artificial 
 products now in considerable use for textile purposes. 
 Numerous attempts have been made to prepare 
 artificial products which should have the properties 
 of silk but be capable of being produced at a lower 
 cost. Of these so-called artificial silks, three have 
 proved commercially successful up to the present. 
 The starting-point in the preparation of all these 
 varieties of artificial silk is some form of cellulose 
 (a variety of artificial silk has been prepared from 
 gelatin, the threads being treated with formalin to 
 render them insoluble, but this process has met with 
 no appreciable success). About half the artificial 
 
10 THE CHEMISTRY OF DYEING 
 
 silk used at the present is prepared from the cellulose 
 nitrates; solutions of these substances in alcohol, 
 ether, and other solvents are squirted through capil- 
 lary tubes into the air. A thread is in this way 
 produced which is, however, of too inflammable a 
 nature to be fit for use in the making of textiles ; by 
 denitration with various mixtures a product is ulti- 
 mately obtained which does not exceed cotton in 
 inflammability. A second process for the preparation 
 of artificial silk, giving rise to the product known as 
 Glanzstoff, consists in dissolving cellulose by means 
 of an ammoniacal solution of copper oxide, and forcing 
 the liquid through fine tubes into a solution of 
 sulphuric acid or some other coagulating medium. 
 
 Viscose silk is another form of artificial fibre now 
 being manufactured in considerable quantities. Cellu- 
 lose is treated with a 15 per cent, solution of sodium 
 hydroxide, and the resulting mass, after being squeezed, 
 is then submitted in a closed vessel to the action of 
 carbon bisulphide. After several hours a product 
 is obtained which is known as viscose, from the fact that 
 when dissolved in water it gives rise to an extremely 
 viscous liquid. By squirting this product through 
 platinum jets into solutions of ammonium salts threads 
 are obtained which can be made up into textiles. 
 
 It will be evident from the description which has 
 been given of the methods of preparation of artificial 
 silk, that these products are chemically more allied 
 to cotton than to silk. Saget and Silvern have stated 
 that whereas natural silk contains about 17 per cent, 
 of nitrogen, the various makes of artificial fibre all 
 contain less than 0-25 per cent, of that element. 
 
ARTIFICIAL SILKS 11 
 
 Consisting essentially of cellulose, the artificial silks 
 behave towards reagents in the same way as cotton 
 does. The artificial fibres also, for the most part, 
 resemble cotton in their behaviour towards dyestuifs, 
 the principal point of difference being that the silk 
 from the cellulose nitrates can be dyed directly with 
 dyes of the basic group. 
 
SECTION II 
 
 Dyes and their Properties. 
 
 As was stated in the introduction, thousands of 
 artificial colouring matters are now known. It is 
 not within the scope of the present work to enter 
 into a full consideration of the constitution and 
 methods of preparation of the colouring matters ; for 
 information on these subjects the reader may be 
 referred to Cain and Thorpe's work on The Synthetic 
 Dyestuffs. 
 
 It has been found that these substances belong to 
 certain classes of organic compounds, so that it is 
 evident that the question of constitution enters into 
 the problem as to whether a particular substance 
 shall be a dyestuff or not. Witt came to the con- 
 clusion that every dyestuff must contain one or more 
 of certain groups which determined the character of 
 the dye, and which he called chromophores. Amongst 
 such groups may be mentioned the azo group 
 N = N , the para quinone group =<^ \=, etc. 
 The simplest substance containing a chromophore 
 was called a chronaogen. In the case of the azo 
 group, azo-benzene would be the chromogen. Azo- 
 
CLASSIFICATION OF DYES 13 
 
 benzene, however, though highly coloured, is not 
 a dyestuff, and cannot be used for the dyeing of 
 textile fabrics. The presence of a second group, 
 called an auxochrome group, is necessary before the 
 chromogen can become a dyestuff. The auxochrome 
 confers salt-forming properties on the compound, 
 and is in general either the amino or the phenolic 
 group. It will be seen, therefore, that all substances 
 which act as dyestuffs must possess either basic or 
 acidic properties. 
 
 From the point of view of the dyer it is much 
 better to classify the colouring matters according 
 to their methods of application to the textile fibres 
 rather than according to their chemical constitution, 
 although, of course, the method of application is 
 dependent on the constitution. When the practical 
 method of classification is adopted it is found that 
 the dyes fall into six or seven different groups. 
 
 The Basic Dyes, as their name indicates, are basic 
 compounds, and are employed in combination with 
 hydrochloric acid or zinc chloride. Dyes of this 
 class can be applied directly to wool and silk, but 
 an acid mordant such as tannic acid or a fatty acid 
 is necessary in order to fix them upon cotton or linen. 
 Jute, owing to its different chemical composition, 
 behaves towards dyes of this group differently from 
 cotton, and can be dyed directly. Many derivatives 
 of triphenyl-methane (such as Magenta, Methyl 
 Violet, Malachite Green, etc.), and similar substances, 
 amino-azo compounds, acridine derivatives and certain 
 phthaleins, such as Rhodamine, belong to this group 
 of colouring matters. It must be noted that the 
 
14 THE CHEMISTRY OF DYEING 
 
 introduction of the sulphonic group into any of the 
 above types of compound destroys the basic character 
 of the dye. 
 
 The Acid Dyes consist of the sodium salts of 
 sulphonic acids of all kinds. Dyes of this group 
 are of particular importance for the dyeing of the 
 animal fibres, to which the colouring matters can 
 be applied directly without the use of a mordant 
 being necessary. The acid dyes are seldom used for 
 the dyeing of the vegetable fibres; when employed 
 for this purpose, a basic mordant such as alum is 
 required. Nitrophenols also belong to this class of 
 dyes. 
 
 The Direct Cotton Dyes form a very important 
 group of colouring matters, since they possess the 
 property of dyeing cotton and linen, as well as wool 
 and silk, without requiring the aid of a mordant. 
 They are salts, and are azo compounds derived from 
 diamines such as benzidine, tolidine, etc. 
 
 A number of other colouring matters capable of 
 direct application to cotton have come into extensive 
 use of late years. These are the dyes known as 
 sulphur or sulphide colours, and they are prepared 
 by fusing certain aromatic amines, etc., with sulphur 
 and sodium sulphide. The products obtained are in- 
 soluble in water and cannot be purified by recrystal- 
 lisation, so that the actual composition of these dyes 
 cannot be readily determined ; probably the majority 
 of these colouring matters consist of complex mixtures. 
 Although insoluble in water, the dyes dissolve in a 
 solution of sodium sulphide, and the solution to be 
 used in the dyebath is prepared in this manner, 
 
CLASSIFICATION OF DYES 15 
 
 sodium sulphate and soda ash being added as assistants. 
 It has been supposed that the solution contains the 
 dye in the form of its leuco compound, the actual 
 colouring matter being reformed on exposure to the 
 air. 
 
 Mordant Dyes. In distinction from the last group 
 of colouring matters, we have a fourth class known 
 as the mordant dyes, which are distinguished from 
 other dyestuffs by the fact that the use of a mordant 
 is invariably necessary in order to fix the dye, no 
 matter what the nature of the material of which 
 the fabric is composed. The mordants usually used 
 are salts of aluminium, chromium, and iron. The 
 majority of the natural dyestuffs belong to this class, 
 and many valuable wool dyestuffs also belong to it. 
 They are of an acidic character, and contain phenolic 
 or carboxyl groups. The most important dye of this 
 group is probably Alizarin, which is used very 
 extensively for the dyeing of cotton as well as being 
 employed in wool dyeing. 
 
 Vat Colours. Another group of colouring matters 
 form the class known as the vat colours. Up to 
 a few years ago Indigo was practically the only 
 member of this group, but during the last few years 
 a number of other dyestuffs have been prepared, all 
 of which are applied in the same way as the important 
 colouring matter mentioned above. 
 
 The dyestuffs of this class are all capable of reduction, 
 and it is the reduction product which is actually 
 absorbed by the fabric. Some of the dyes of this 
 group, as, for example, the Algole and Indanthrene 
 colours, are only suitable for cotton, as they require 
 
16 THE CHEMISTRY OF DYEING 
 
 a strongly alkaline bath for their application. The 
 solutions of the reduction products of dyes of this 
 character are darker in colour than the original 
 substance. 
 
 The Indigo, Thio-indigo, Helindone, and Ciba dyes, 
 on the other hand, only require at the most a slightly 
 alkaline bath, and can therefore be applied to the 
 animal as well as to the vegetable fibres; the 
 solutions of the reduced dyes are of a lighter colour 
 than the original compounds. After the leuco com- 
 pound has been absorbed by the fabric the original 
 substance is reformed in the pores of the fibre by 
 exposing the wet material to the oxidising influence 
 of the atmosphere. 
 
 Developed Dyes. The last group of colouring 
 matters to be mentioned may be described as the 
 developed dyes, since they are actually formed on 
 the fibre by the interaction of the substances necessary 
 for their preparation. The so-called mineral colouring 
 matters, such as Chrome Yellow, belong to this group, 
 the colour just alluded to being formed within the 
 pores of the fibre as a result of the interaction of 
 a lead compound with a solution of a bichromate. 
 
 Aniline Black, a well-known and very fast colouring 
 matter for cotton, is also included in this class, as 
 it is formed by oxidising an aniline salt with which 
 the fabric has been impregnated. (In the more 
 modern method the cotton is not first treated with 
 the aniline salt, but is immersed directly in a bath 
 containing the aniline salt and the oxidising agent; 
 on heating the mixture the colouring matter is formed 
 and deposited on the fibre.) 
 
CLASSIFICATION OF DYES 17 
 
 A number of azo dyes are also developed on the 
 fibre; these are sometimes known as ice colours 
 because of the fact that ice is frequently necessary 
 to prevent the decomposition of the diazo compound, 
 which is one of the reacting substances. The 
 substance applied to the fabric may be a dyestuff 
 itself or not; the only essential is that it must 
 contain a primary aromatic amino group and so be 
 capable of diazotisation. After the treatment with 
 nitrous acid the colour is developed by treating the 
 material with a solution of a phenol or an amine, 
 when coupling takes place in the usual manner with 
 the diazo compound. 
 
 Many of the direct cotton dyes which are not 
 themselves very fast can be converted by diazotising 
 and developing into very fast products. One of the 
 most useful developers is /3-naphthol. Of the amino 
 compounds employed in this manner which are not 
 themselves dyes, mention may be made of _p-nitrani- 
 line, a-naphthylamine, benzidine, etc. ; when these 
 substances are made use of the phenol is usually 
 applied to the cloth, which is then introduced into 
 the cooled solution of the diazotised amine. 
 
 Application of the Dyes. 
 
 As regards the methods of application of the various 
 types of dyestuffs to the different fibres, these have 
 been sufficiently indicated in principle in the fore- 
 going pages. Details as to the mode of working in 
 the case of individual dyes are best obtained from 
 the pattern books and notices issued by all the 
 
 B 
 
18 THE CHEMISTRY OF DYEING 
 
 leading manufacturers of dyestuffs. The principles 
 underlying certain of the practices commonly followed 
 may, however, be discussed here. 
 
 It is a common practice to add to the dyebath 
 a substance known as an assistant. Two of the 
 most commonly used assistants are sulphuric acid 
 and sodium sulphate, while in the dyeing of silk 
 boiled-off liquor or soap is frequently added to the 
 bath. Sulphuric acid is used in conjunction with 
 the acid dyestuffs. It acts upon the dyestuff which, 
 as has been stated, is a salt of a sulphonic acid, and 
 liberates the free colour acid which is the substance 
 probably taken up by the fibre. The added acid 
 (this is not necessarily sulphuric; acetic and formic 
 acids are employed to a considerable extent in this 
 connection) also has the effect of diminishing the 
 solubility of the colour acid. This is because of the 
 fact that the two substances give rise to a common 
 ion, hydrion, and the law of mass action applies to 
 this as to all such cases. 
 
 The question of the amount of acid to be added 
 depends on the character of the dye. The more 
 rapidly a colouring matter is absorbed by a fibre, 
 the greater is the danger of the dyed material being 
 uneven in colour. With a dyestuff, therefore, which 
 is readily absorbed by wool, an excess of acid is to be 
 avoided, so as to prevent the solubility of the colour- 
 acid from being sensibly diminished; too rapid 
 absorption of the dye will in this way be obviated. 
 The same end can be attained by an addition 
 of sodium sulphate in conjunction with the sul- 
 phuric acid; the sodium hydrogen sulphate formed 
 
THE USES OF ASSISTANTS 19 
 
 from the two substances is a weaker acid than the 
 sulphuric acid itself, and has therefore less action 
 upon the dyestuff, as it cannot compete so strongly 
 for the base of the dyestuff, which means that a 
 smaller amount of colour acid will be liberated. 
 Owing also to sodium hydrogen sulphate being a 
 weaker acid than sulphuric acid, an excess of the 
 former has much less effect on the solubility of the 
 colour acid than is produced by an excess of sulphuric 
 acid. A too rapid deposition of colouring matter is 
 also avoided by working at a low temperature. 
 
 When acid dyes are employed for dyeing silk an 
 acid is again added to the bath for the same reasons 
 as have been mentioned in the case of wool, and the 
 same considerations are necessary in determining the 
 quantity of acid to be added. When it is necessary 
 to regulate the rate at which the dyestuff is absorbed, 
 an addition of soap or of boiled-off liquor may be 
 made instead of sodium sulphate. 
 
 As regards the other classes of dyestuffs, the 
 members of the direct cotton group are the ones for 
 which the use of an assistant is most necessary. The 
 dyes of this group, it has been already stated, have 
 the character of salts, and it is customary to add 
 along with them to the dyebath some other salt, 
 such as sodium sulphate or chloride. The effect 
 of this addition is exactly similar to that caused 
 by the addition of an excess of acid to the acid dyes, 
 that is, by increasing the active mass of one of the 
 ions to which the dyestuff gives rise, the solubility 
 of the dye is diminished. Too rapid deposition of 
 the dye upon the fibre is in this case prevented by 
 
20 THE CHEMISTRY OF DYEING 
 
 adding a substance of an alkaline character, such as 
 soda ash, along with the sodium sulphate, the alkali 
 tending to keep the colouring matter in solution. 
 When silk goods are being dyed, the sodium sulphate 
 is replaced by boiled-off liquor or soap. 
 
 No addition is made to the bath with the basic 
 dyes unless it is desired to retard the rate of absorp- 
 tion, in which case a small amount of sulphuric or 
 other acid is added ; this has a solvent action on the 
 dye and brings about the desired effect. 
 
 The Condition of Dyes in Solution. 
 
 Of late years a considerable amount of work has 
 been done in connection with the mode of existence 
 of dyestuffs in aqueous solution, and as the subject 
 is of interest because of its bearing on the principles 
 underlying the dyeing process, an account of the more 
 important work in this direction will now be given. 
 
 As has been indicated in the earlier part of the 
 section, the dyes of the acidic, basic, and direct 
 cotton groups are all of the nature of salts, and 
 assuming, therefore, that they form true solutions 
 when added to water, they should be largely ionised, 
 diffuse through a parchment membrane and, in fact, 
 exhibit all the phenomena of salts in general ; probably 
 in some cases where the acid or the base, or both of 
 these, is weak, a certain amount of hydrolysis would 
 take place. It will be evident that a great deal 
 depends on whether the dyes exist in a state of 
 true solution or not, and most of the recent workthas 
 been directed to the settlement of this problem. 
 
DIALYSIS OF DYE-SOLUTIONS 21 
 
 Amongst the earliest observations dealing with this 
 subject may be mentioned those of Pfeffer, 47 who 
 noticed that some of the basic colouring matters, such 
 as Methylene Blue and Methyl Violet, are able to 
 pass through the wall of a living cell and colour 
 the protoplasm. Obviously, therefore, dyes such as 
 those mentioned must exist in true solution, for unless 
 such were the case no dialysis could take place. 
 Later investigations go to show that this property 
 of existing in a state of true solution is not one 
 which applies to dyes in general. 
 
 True and Colloidal Solutions. By means of 
 dialysis experiments and also as a result of examining 
 solutions with the ultra-microscope, Freundlich and 
 Neumann 22 have come to the conclusion that dye- 
 stuffs should be divided into three classes, and a 
 similar conclusion was arrived at by Biltz and 
 Pfenning. 6 
 
 The first group consists of those colouring matters 
 which diffuse readily through parchment paper, and 
 which are therefore present in true solution ; amongst 
 the dyes of this class may be mentioned Chrysoidine, 
 Bismarck Brown, Auramine, Eosine, Methylene Blue, 
 Safranine, Picric Acid, etc. 
 
 The second group contains dyes which form solu- 
 tions of a semi-colloidal nature, that is to say, dialysis 
 takes place, but at a very slow rate. Magenta, Methyl 
 Violet, Capri Blue, and Nile Blue are a few of the 
 dyes which fall into this group. 
 
 The third class contains the dyes which form 
 colloidal solutions proper, as indicated by their non- 
 diffusibility through a membrane, and by the fact 
 
22 THE CHEMISTRY OF DYEING 
 
 that the solutions, when examined with the ultra- 
 microscope, are not optically void. Many of the 
 dyes of the direct cotton group, such as Congo Red, 
 Benzopurpurin, Benzoazurin, etc., are of this type, 
 but the property of forming colloidal solutions is not 
 confined to the direct cotton colours; Night Blue, 
 Induline, Alkali Blue, etc., all form solutions of this 
 nature. 
 
 Freundlich and Neumann (loc. cit.) divide the dyes 
 of the third group into two sections, according to 
 whether they have a marked influence or not on the 
 properties of the solvent. The first section contains 
 the dyes which form solutions of the same nature as 
 that given by arsenious sulphide. They are called 
 " suspensoids," and are precipitated by the addition 
 of small quantities of salts. As regards the dyes of 
 the other section, called "emulsion colloids," they 
 require the addition of considerable amounts of salts 
 before precipitation takes place, and influence very 
 considerably the surface tension and other physical 
 properties of the solvent; the solutions formed by 
 dyes of the latter type bear a resemblance to solutions 
 of gelatine. Night Blue is a colloid of the latter kind, 
 while Congo Bed is one of the colouring matters 
 included in the first sub-section. 
 
 Molecular Complexity of Dyestuffs. Biltz and 
 Pfenning (loc. cit.) have drawn the conclusion that 
 the dialysing power of a dyestuff depends on its 
 molecular complexity. If the molecule of the dye 
 contains less than forty-five atoms the substance will 
 diffuse rapidly through parchment or through a 
 collodion membrane, but as the number of atoms in 
 
DIALYSIS OF DYE-SOLUTIONS 23 
 
 the molecule increases, the rate of diffusion diminishes ; 
 with dyes containing between fifty-five and seventy 
 atoms in the molecule the velocity of dialysis is 
 very small, while when the number of atoms exceeds 
 seventy dialysis ceases altogether. 
 
 This relationship between the complexity of the 
 substance and the rate of dialysis is, however, 
 influenced to a certain extent by the composition 
 and constitution of the substance. It was found, 
 for example, that the introduction of the sulphonic 
 acid group into a compound has a marked effect in 
 increasing the dialysing capacity of the substance; 
 dyes of the Malachite Green series containing two 
 or three sulphonic acid groups pass readily through 
 a membrane even when the number of atoms in the 
 molecule exceeds seventy. Dyes having a constitution 
 allied to that of Alizarin, on the other hand, dialyse 
 less readily than would be expected from the number 
 of atoms contained in the molecule. 
 
 Pelet- Jolivet and Wild 45 agreed with the previously 
 mentioned investigators in coming to the conclusion 
 that dyes can be divided into three classes according 
 to the character of the solutions they form, but did 
 not always agree as to the class in which certain 
 dyes should be placed. Their methods of investigation 
 included determinations of electrical conductivity and 
 the use of the ultra-microscope. They also determined 
 the effect of dye solutions upon diazo acetic ester 
 and noticed no catalytic action, from which they 
 naturally concluded that the colouring matters 
 examined did not undergo hydrolysis in aqueous 
 solution. This is opposed to the views commonly 
 
24 THE CHEMISTRY OF DYEING 
 
 held, and it would be of interest to ascertain whether 
 the results can be confirmed. Biltz and Pfenning 7 
 have made determinations of the electrical conductivity 
 and osmotic pressure of solutions of dyestuffs which 
 had been freed from admixed inorganic salts by the 
 process of dialysis. The conductivity results seem 
 to show that the substances examined behave as 
 normal electrolytes, although they do not obey 
 Ostwald's rule regarding the dependence of the 
 molecular conductivity on the basicity of the acid. 
 The results of the determinations of osmotic pressure 
 showed that a dye solution has in many cases a 
 complex character, as products of association, ionisa- 
 tion, and hydrolysis may be existing together in a 
 state of equilibrium. With solutions of mono- 
 sulphonates the association and hydrolysis products 
 are the most important, and the molecular weight 
 calculated from the osmotic pressure is much higher 
 than the theoretical value. With the disulphonic 
 derivatives ionisation practically balances association, 
 so that the molecular weights which are determined 
 agree on the whole with the theoretical values. 
 When the number of sulphonic acid groups exceeds 
 two, the question of ionisation becomes of most 
 importance, and owing to the extent to which this 
 takes place the observed values for the molecular 
 weights are considerably smaller than the theoretical 
 values. 
 
 Determinations of the electrical conductivity of 
 solutions of dyestuffs have also been made by Knecht 
 and Batey, 38 some of the dyes experimented with, as 
 for example, Benzopurpurin, being amongst those 
 
EXPERIMENTS WITH CONGO RED 25 
 
 classed as colloids by other investigators. They 
 found the solutions to be good conductors, and the 
 results obtained with dilute solutions were such as 
 to indicate a high degree of ionisation. The results 
 of the conductivity experiments were supported by 
 the values obtained for the molecular weights of 
 the dyes by the boiling-point method. With Benzo- 
 purpurin, Soluble Blue, and Chrysophenine, the 
 observed elevation of boiling-point of the aqueous 
 solution was such as to indicate a considerable amount 
 of ionisation. The authors mentioned also found 
 Benzopurpurin to have a high rate of dialysis, and 
 they are consequently opposed to the view that it 
 and many other dyestuffs, especially those of high 
 molecular weight, exist in the state of colloidal 
 solution. 
 
 Results of an interesting and important character 
 have been obtained from the investigation of solutions 
 of Congo Red and allied colouring matters. Bayliss 3 
 showed that although this colouring matter will not 
 pass through a parchment membrane and also 
 exhibits other properties associated with colloids, 
 yet it exerts an osmotic pressure equal to that which 
 would be expected if- it was present in a state of 
 true solution in the unassociated form. This result 
 was only obtained, however, when the outside vessel 
 was filled with distilled water; if solutions of acids, 
 bases, or salts were used instead of water, the values 
 of the osmotic pressure determined were of consider- 
 ably lower magnitude. Bayliss considered the electro- 
 lyte to have the effect of causing the molecules of 
 dye to collect together to form aggregated particles. 
 
26 THE CHEMISTRY OF DYEING 
 
 Similar experiments were carried out by Biltz and 
 Vegesack, 8 making use of Congo Red, Benzopurpurin, 
 and Night Blue. The results obtained with Congo 
 Ked, previously freed from admixed salts by pro- 
 longed dialysis, confirmed those obtained by Bayliss. 
 With water in the outer vessel the value calculated 
 for the molecular weight was 602, the theoretical 
 value being 696, whilst the result obtained when 
 the outer vessel was filled with a salt solution of 
 the same conductivity as the dye solution was 2333. 
 From a consideration of the equilibrium between 
 the ions of Congo Red and those of sodium sulphate, 
 the conclusion was drawn that if the solute is not 
 polymerised the apparent molecular weight of Congo 
 Red should be three times the normal value, that 
 is, 2088 ; as this figure is only slightly different from 
 the one actually determined, Biltz and Vegesack 
 came to the conclusion that Congo Red only under- 
 goes slight polymerisation in solution. Results of 
 a similar character were obtained with Night Blue, 
 but in this case complications were introduced because 
 of the fact that a certain amount of hydrolysis takes 
 place. 
 
 The investigation was extended to the examination 
 of commercial preparations of certain dyestuffs. 
 Many of these products contain a certain quantity 
 of salts, chiefly sodium sulphate; in Congo Red 
 the amount of impurity is about 26 per cent. The 
 molecular weight of Congo Red, as determined from 
 this sample, was found to be about 7380, from 
 which the conclusion was drawn that association 
 takes place in the presence of salts. 
 
EXPERIMENTS WITH CONGO RED 27 
 
 This view of the effect of electrolytes on the 
 molecular complexity of dyes such as Congo Eed, 
 Night Blue, and Benzopurpurin, received additional 
 support from ultra-microscopic examinations of the 
 solutions. The effect of the electrolyte appears to 
 become more pronounced when the solution is kept, 
 and the polymerisation is greater in concentrated 
 than in dilute solutions. The temperature of the 
 solution also appears to have a marked influence on 
 the degree of association; Biltz and Vegesack, from 
 the determination of the osmotic pressure at different 
 temperatures, came to the conclusion that in a solution 
 of Night Blue the degree of association at was 
 6-7, at 25 it was 3-05, while at 70 the value was 
 only 1-9. 
 
 Anomalous Behaviour of Dyestuffs on Dialysis. 
 An important paper dealing with this subject was 
 published recently by Donnan and Harris. 15 Several 
 of the observations they made confirmed those of 
 the previous workers on this subject, as, for example, 
 that Congo Red gives an osmotic pressure when 
 measured against distilled water which agrees approxi- 
 mately with the value which would be obtained for 
 a true solution in which the dye was present 
 as single molecules. From the determinations of 
 electrical conductivity they drew the conclusion that 
 the dye exists to a very considerable extent in the 
 ionised condition. 
 
 Amongst the most important of their observations 
 were those dealing with the dialysis of solutions of 
 dyes of the Congo type. It was found that both with 
 Congo Red and Benzopurpurin 4B a peculiar 
 
28 THE CHEMISTRY OF DYEING 
 
 " membrane hydrolysis " takes place, sodium ions in 
 company with hydroxyl ions diffusing out of the 
 dialyser, while the free dye acid, an acid salt, or some 
 other insoluble phase remains behind. It was found 
 possible to prevent this hydrolysis by adding to the 
 dye solution an alkaline hydroxide, the quantity of 
 alkali required depending on the temperature and 
 concentration of the solution of colouring matter. It 
 is evident, therefore, in the light of these observations, 
 that the osmotic pressure observed with a solution of 
 a colouring matter like Congo Red does not corre- 
 spond with the ordinary state of osmotic equilibrium. 
 Donnan and Harris also confirmed the fact that 
 the osmotic pressure of Congo Red is lowered in the 
 presence of certain electrolytes, but they threw a 
 considerable amount of new light on this matter, and 
 were led to conclusions which must, if they are 
 accepted, and the author sees no reason for adopting 
 any other attitude, lead to the abandonment of some 
 of the views previously held. 
 
 It was found that when Congo Red and sodium 
 chloride were introduced into a dialyser, the sodium 
 chloride distributed itself unequally on the two sides 
 of the parchment membrane, and a reversible ionic 
 equilibrium was ultimately established, the concentra- 
 tion of sodium chloride at equilibrium being higher 
 on the side opposite to that occupied by the dyestuff. 
 It was found that this membrane equilibrium is 
 necessary from thermodynamical considerations, and 
 that it is one of a group of phenomena of a general 
 character. It is obvious that because of the unequal 
 distribution of the salt on the two sides of the 
 
EXPERIMENTS WITH CONGO RED 29 
 
 membrane an opposed osmotic pressure will be set 
 up, and this is probably the cause of the smaller 
 values obtained with solutions of dyes containing 
 dissolved salts, or where the outer vessel contains 
 salts. 
 
 All the conclusions which have been drawn, there- 
 fore, regarding the molecular complexity of dyes in 
 solution from measurements of the osmotic pressure 
 are valueless when another electrolyte is present, 
 unless account be taken of the unequal distribution 
 of the foreign electrolyte. The subject is one of 
 great interest, and no doubt further experiments will 
 be made in the near future which will shed additional 
 light on the condition of dyestuffs in solution. 
 
 While at present, in view of the results obtained 
 by Donnan and Harris, it would be unwise to lay too 
 much stress upon the results of dialysis experiments 
 with solutions of dyes, it is yet quite clear from the 
 evidence gained by the use of the ultra-microscope 
 that many colouring matters in aqueous solution 
 exist to a greater or lesser extent in the colloidal 
 condition. On the other hand, the aqueous solutions 
 of other dyes are optically void, and appear, from 
 determinations of the electrical conductivity, to be 
 ionised to a considerable extent; in such cases we 
 are justified in believing that the dye is present 
 in a state of true solution. 
 
SECTION III 
 
 The Nature of the Dyeing Process. 
 
 IT is not surprising that in the case of an industry 
 so old and so universal as is that of Dyeing many 
 speculations should have been made as to the nature 
 of the process by which the colouring matter becomes 
 fixed on the fibre in an insoluble form. An attempt 
 will be made in the following pages to give some 
 account of the different ideas which have from time 
 to time been held on the subject, together with the 
 experimental evidence on which these theories have 
 been based, and the criticisms which have been 
 brought against them. As can readily be imagined 
 when one considers the diversity of substances 
 involved, both the dyes and the fibres, the subject is 
 one fraught with great difficulty, and even yet 
 different views are held by chemists as to the 
 mechanism of dyeing. 
 
 Although unanimity of opinion has not yet been 
 arrived at, there are certain ideas, which will be 
 discussed at a later stage, which may be brought into 
 harmony with all the different opinions, and so it is 
 not impossible, unlikely as such a thing may at first 
 sight appear, to arrive at a theory which shall apply 
 to all cases of dyeing. 
 
 80 
 
THEORIES OF DYEING 31 
 
 The Mechanical Theory. 
 
 The earliest idea respecting the dyeing process was 
 that it was of a purely mechanical character. Ex- 
 planations of the process on these lines were given 
 about the middle of the eighteenth century by Hellot, 
 Le Pileur d'Apligny, and others. Hellot stated that 
 the heat of the dyebath causes the pores of the 
 fibre to open so that the particles of colouring matter 
 can enter and be deposited ; when the fibre is removed 
 from the dyebath the pores contract, and so the dye 
 is retained in position. As regards the various sub- 
 stances used in preparing the material for dyeing, 
 these were also considered to be retained in the 
 pores of the fibre, and were thought to coat the 
 particles of dye with a kind of varnish. 
 
 The difference in the behaviour of different fibres 
 towards the same dyestuff was a phenomenon with 
 which the early dyers were thoroughly familiar, and 
 this was also explained on a purely mechanical basis, 
 Le Pileur d'Apligny suggesting that it was to be 
 attributed to the difference in size of the pores of 
 the various fibres, so that in some cases the particles 
 of dye were too large to enter the pores, or, if they 
 entered, the contraction on cooling was insufficient 
 to permit of the dye being retained. 
 
 Although the majority of chemists at the present 
 day are of the opinion that the dyeing of a piece of 
 wool or cotton is not to be accounted for in such a 
 simple manner as the above ideas would suggest, 
 yet there are some who still adhere to the mechanical 
 theory of dyeing, and there are certain cases where 
 
32 THE CHEMISTRY OF DYEING 
 
 the fibre certainly does appear to act in a mechanical 
 manner. Amongst the later supporters of this theory 
 may be mentioned Hwass, 33 von Perger, 46 and Spohn. 55 
 
 Stress is laid on the fact that no definite compound 
 of a fibre and a dyestuff has ever been actually shown 
 to be produced during the dyeing process, and also on 
 the retainment of many of its original properties by 
 a colouring matter after it has been fixed upon the 
 fibre. The rubbing off of the colour which is fre- 
 quently observed with dyed goods, and the possibility 
 in some cases of separating the dye from the fibre 
 by the process of sublimation, are also considered to 
 favour the mechanical conception of the character of 
 the dyeing process. 
 
 A property common to all dyestuffs is, that if a 
 fibre is introduced into a very dilute solution of the 
 colouring matter practically the whole of it is taken 
 up and is firmly retained, even when the dye is one 
 for which the fibre in question may naturally have 
 little affinity. This phenomenon is attributed to the 
 forces of capillarity and adhesion, and it is argued 
 by the supporters of the theory that similar forces 
 must come into play and be responsible for all the 
 results which follow the introduction of a fibre into 
 a more concentrated solution of a colouring matter, 
 such as is actually employed for the dyeing of a 
 fabric. 
 
 While it is probable that the forces of adhesion 
 and capillary attraction do come into play to a con- 
 siderable extent, it is scarcely justifiable to conclude 
 that all the phenomena of dyeing can be explained 
 as resulting from the operation of those forces. 
 
THEORIES OF DYEING 33 
 
 Reference may also be made to the views of Rosen- 
 stiehl, 50 who pointed out that solids, under the influence 
 of pressure, can be made to adhere rigidly to one 
 another. He considered the fixation of dyes to be 
 brought about in this manner, the pressure necessary 
 to make the dye adhere to the fibre being the osmotic 
 pressure of the solution, this osmotic pressure being 
 increased by the addition to the dyebath of the 
 assistants, such as acids and salts, commonly added 
 to the bath. 
 
 The experiments of Dreaper and Wilson 17 go to 
 show that to regard the dyeing process as a purely 
 mechanical one is to take up an untenable position. 
 It was shown that when Night Blue is absorbed by 
 silk at a temperature of 15 the whole of the colour- 
 ing matter can be subsequently removed from the 
 fabric by means of alcohol or by treatment with a 
 boiling solution of soap; if, however, dyeing takes 
 place at a temperature of 40 and upwards a portion 
 of the dye appears to be taken up in a different 
 manner, and cannot be removed by means of the 
 above-mentioned agents. In all these experiments 
 the total quantity of the dye upon the fibre was 
 maintained constant, so that the results must really 
 be due to a difference in the mode of existence of the 
 colouring matter when fixed under the different 
 conditions of temperature. Similar results were 
 obtained in the case of a number of other colouring 
 matters. In these cases, therefore, it appears obvious 
 that something in addition to mere mechanical action 
 is necessary to explain the fixation of the dye. 
 
 There is one sense, however, in which the 
 
 C 
 
34 THE CHEMISTRY OF DYEING 
 
 mechanical theory may be accepted, and that is in 
 connection with the fixation of mordant colours, the 
 so-called mineral colouring matters and the ingrain 
 colours (that is, the azo dyes produced on the fibre). 
 Spohn (loc. cit.) examined with the aid of the micro- 
 scope cotton which had been dyed with lead chromate, 
 and noticed the crystals of the colouring matter 
 adhering to the colourless fibre. In such a case the 
 colour is simply deposited in the pores of the fibre 
 as it is formed by the interaction of the two reacting 
 substances, and under such conditions the part played 
 by the fibre is the purely mechanical one of a 
 pigment carrier. 
 
 This view of adjective or mordant dyeing was 
 supported by Weber, 59 who pointed out the difference 
 in behaviour /shown by Night Blue when applied 
 to cotton in the absence of a mordant and in the 
 presence of tannic acid. Under the former conditions 
 the colouring matter retained its original properties 
 and at once reacted with Naphthol Yellow, but when 
 fixed in the presence of tannic acid no reaction 
 ensued on subsequently treating the dyed cotton with 
 the yellow dye. 
 
 The Chemical Theory. 
 
 Some little time after the period of Hellot, etc., 
 Bergmann advanced the view that the process of 
 dyeing is to be regarded as being of a purely chemical 
 character, an interaction taking place between the 
 fibre and the dye. Berthollet and others supported 
 this view, which has persisted to the present day. 
 
THEORIES OF DYEING 35 
 
 There is much to be said in favour of such an 
 explanation of the dyeing process. With regard to 
 the dyestuffs themselves, it has already been pointed 
 out that the presence of groups of an acid or basic 
 character is necessary before a substance can behave 
 as a dye. 
 
 When we turn to the fibres, we find that the 
 animal fibres possess an amphoteric character by 
 virtue of which it would be possible for them to 
 unite either with a base or an acid with the formation 
 of a salt. This is in fact the way in which the 
 advocates of the chemical theory of dyeing explain 
 the substantive dyeing of wool and silk with the 
 basic and acid dyes. 
 
 As regards cotton, the fact that it is practically 
 devoid of acid or basic properties makes it an im- 
 possibility for any salt to be formed by the interaction 
 of fibre and dyestuff, and the comparative inertness 
 of cotton towards colouring matters is thus accounted 
 for. 
 
 When we consider jute, we find that the fibre can 
 be dyed directly with the dyes of the basic class, 
 whereas such dyes can only be fixed upon cotton 
 after the fibre has been mordanted with tannic acid 
 or some other acid mordant. The difference in com- 
 position of cotton and jute makes it possible to easily 
 explain this difference, for as has been already stated, 
 jute is composed of ligno-cellulose, which is not un- 
 like, in properties, cellulose which has been mordanted 
 with tannic acid. Here again, therefore, the difference 
 in behaviour towards colouring matters can be easily 
 explained by taking into account the chemical nature 
 
36 THE CHEMISTRY OF DYEING 
 
 of the fibre. Certain of the objections against the 
 theory have been mentioned in dealing with the 
 mechanical theory. 
 
 A large amount of experimental work has been 
 carried on during the last twenty-five years in support 
 of the ideas embodied in the foregoing general state- 
 ment. Knecht 34 showed that when wool w r as boiled 
 with a moderately concentrated solution of sulphuric 
 acid it gradually dissolved and gave rise to a light 
 brow T n solution ; if solutions of dyestuffs were added 
 to the acid solution richly coloured precipitates were 
 formed. By the careful neutralisation of the acid 
 solution a substance was precipitated which, on 
 drying, yielded an amorphous brown powder which 
 was only sparingly soluble in acids, but dissolved 
 readily in solutions of alkalis; when the alkaline 
 solution was mixed with solutions of colouring matters 
 and an acid then added, precipitates were formed 
 similar to those which were obtained from the 
 original acid solution. From these results Knecht 
 drew the conclusion that the principal reason why 
 sulphuric acid is added to the dyebath in the dyeing 
 of wool is to act upon the fibre so as to produce 
 a compound having properties similar to those possessed 
 by the substance isolated; the compound so formed 
 would then combine with the dye to form an 
 insoluble product. 
 
 Reference has been made in an earlier chapter to 
 the experiments made by Knecht and Appleyard 37 
 with lanuginic acid which they prepared from wool. 
 When solutions of basic colouring matters were 
 added to a solution of lanuginic acid, precipitates 
 
THEORIES OF DYEING 37 
 
 were at once produced, and it was considered that 
 this kind of change probably took place during the 
 dyeing of wool with dyes of the basic class, a certain 
 quantity of the fibre first undergoing decomposition 
 with the formation of lanuginic acid, which would 
 then react with the colouring matter. 
 
 Amongst other results obtained by Knecht and 
 Appleyard (loc. cit.\ some of a quantitative character 
 may be referred to. When wool was dyed with 
 certain acid dyes, it was found that if the quantity 
 of one colouring matter, Picric Acid, absorbed by 
 a given quantity of wool, was taken as the unit, 
 then the amounts of two other dyes, Naphthol Yellow 
 S and Tartrazine, absorbed by the same weight of 
 wool, corresponded to one and to three-quarters of 
 a molecule respectively ; and the obtaining of these 
 quantitative results was thought to be a strong piece 
 of evidence in support of the chemical theory. 
 Further quantitative results were given by Knecht 36 
 in a later paper. 
 
 Gelmo and Suida 2456 also came to the conclusion 
 that in the substantive dyeing of the animal fibres 
 it is necessary for the fibre to first undergo a more 
 or less profound hydrolysis, as a result of which 
 products are formed which contain active salt-forming 
 groups. This hydrolysis is greater in the case of 
 wool than with silk, and is promoted by the presence 
 of acid, and is necessary, according to the authors 
 in question, for the satisfactory dyeing of the fibres. 
 As regards the fixation of the dye, Gelmo and Suida 
 considered that basic dyes are fixed by salt formation 
 with the acid groups of the fibre substance, and acicj 
 
38 THE CHEMISTRY OF DYEING 
 
 dyes by means of guanidyl or imidazole groups of 
 the fibre. 
 
 It will be noticed from the preceding statements 
 that by some supporters of the chemical theory, at 
 any rate, the salt formation which is supposed to 
 take place is not one involving particular groups of 
 the fibre itself, but groups present in some product 
 formed by the hydrolysis of the fibre. If this is 
 really the nature of the action, then, according to 
 Witt, 60 the dyeing of wool and silk with the acid 
 and basic colouring matters should not be regarded 
 as examples of substantive dyeing, but as instances 
 of adjective dyeing, the products of the hydrolysis 
 of the fibre acting as the mordant. 
 
 Other chemists, however, appear to regard the action 
 as one in which the salt-forming groups of the fibres 
 themselves are involved, and if such were the nature 
 of the process then the operation would undoubtedly 
 be one of substantive dyeing. Amongst those who 
 may be said to have favoured this view was Weber, 59 
 who carried out some experiments of an interesting 
 character. He argued that if the process of the 
 substantive dyeing of wool and silk is one in which 
 the fibre may be regarded as playing the part of 
 an amphoteric compound, then even although an acid 
 dye has been fixed upon the fibre it should still be 
 possible to fix a basic one upon the same material, 
 seeing that the acid group in the wool by means 
 of which the basic dye is fixed would still be free. 
 This was actually found to be possible in practice, 
 a skein of wool being dyed with a large excess of 
 Scarlet B, and then, after thorough washing, intrq- 
 
THEORIES OF DYEING 39 
 
 duced along with a white skein of equal weight 
 into a solution of Magenta. It was found that the 
 same weight of Magenta was absorbed by both skeins, 
 thus showing the acid colouring matter to have no 
 influence on the quantity of basic dye taken up. 
 Of course it might be argued against this experiment 
 that union had taken place between the two dyestuffs, 
 but any such objection was confuted by the behaviour 
 of the dyed material when treated with alcohol. 
 The substances formed by the union of acid and 
 basic dyes are readily soluble in that liquid, but 
 it was found that only a small amount of the 
 Magenta and none of the Scarlet was removed from 
 the material by the solvent. 
 
 A number of experiments were carried out by 
 Vignon, 57 which appeared to support the chemical 
 theory of dyeing. Cotton, it is well known, has 
 practically no affinity for the acid dyes, and this 
 is attributed to the absence of groups of a basic 
 character in the fibre. Vignon showed, however, 
 that when cotton is heated in sealed tubes with an 
 aqueous solution of ammonia or with the compound 
 of ammonia with calcium chloride; its properties 
 undergo considerable modification. After the material 
 has been thoroughly washed with water and acids 
 it is found to contain nitrogen, the amount of which 
 may reach 3 per cent., and when a piece of the 
 modified cotton is placed in a solution of an acid 
 dye in company with a piece of untreated cotton, 
 a considerable amount of the colouring matter is 
 taken up by the modified fibre, while the ordinary 
 Cptton is scarcely stained. On the other hand, oxy- 
 
40 THE CHEMISTRY OF DYEING 
 
 cellulose, which has acid properties, has a greater 
 attraction than ordinary cotton for basic dyes. 
 
 Vignon also measured the amount of heat which 
 is generated when different fibres are immersed in 
 solutions of acids and alkalis. The heat effect pro- 
 duced by cotton under such conditions is very small 
 when compared with the effect produced with wool 
 or silk, and this was considered as pointing to a 
 certain amount of chemical action taking place with 
 the animal fibres, as against practically no action 
 with the cotton. This conclusion was strengthened 
 
 e 
 
 by the observation that when the ammoniated cotton 
 already referred to was immersed in sulphuric acid 
 a considerable amount of heat was generated. 
 
 Physico-Chemical Objections to the Chemical 
 Theory. With the rise and development of physical 
 chemistry, some of the arguments formerly brought 
 forward in favour of this explanation of the dyeing 
 process have been shown to have little bearing on 
 the subject. It was, for example, noticed by 
 Knecht 35 that when wool is dyed by means of a 
 basic dye such as Magenta, the whole of the hydro- 
 chloric acid originally present in the colouring matter 
 remains in the dyebath, the colour base alone being 
 taken up by the fibre. 
 
 At first sight this appears to be conclusive evidence 
 in favour of the chemical theory, but it was found 
 by von Georgievics 25 that if the operation is carried 
 out at a temperature below the boiling-point of water, 
 the whole of the halogen of the dyestuff is not present 
 in the bath at the conclusion of the dyeing process, 
 but a portion is taken up by the fibre, Moreover, 
 
CRITICISM OF CHEMICAL THEORY 41 
 
 if glass beads or pieces of unglazed earthenware are 
 introduced into a solution of Magenta at the ordinary 
 temperature, a certain quantity of colour is acquired 
 by such articles, and the chlorine, at the end of the 
 process, is present quantitatively in the bath; in 
 cases such as these there is no possibility of chemical 
 action taking place between the dyed object and the 
 colouring matter, and some other explanation of the 
 result must be sought for in place of that given by 
 Knecht. 
 
 Such an explanation was suggested by Zacharias, 61 
 who pointed out that the dyes have the character of 
 salts, and may therefore be more or less hydrolysed 
 in aqueous solution into free colour base and free 
 acid. As in all such cases, there will be a certain 
 degree of hydrolysis depending on the concentration 
 of the solution and on the temperature. As long as 
 nothing happens to disturb the equilibrium, no further 
 change will take place. The introduction of a piece 
 of wool into the solution has, however, a disturbing 
 effect, since it leads to the removal of one of the 
 products of hydrolysis, the colour base, and in 
 accordance with the laws of chemical equilibrium 
 a further amount of the salt must be hydrolysed to 
 restore the condition of equilibrium in the solution ; 
 it is at once evident that this process, if continued, 
 will lead eventually to the complete decomposition 
 of the dyestuff. The effect of temperature noticed 
 by von Georgievics can also be explained quite easily 
 on the same basis, for in the majority of cases the 
 degree of hydrolysis of a salt increases with the 
 temperature, so that at temperatures below the boiling- 
 
42 THE CHEMISTRY OF DYEING 
 
 point the process of hydrolysis might not be complete, 
 the products of hydrolysis being a basic salt and some 
 free acid ; the basic salt would be absorbed by the 
 fibre, which would therefore contain a certain amount 
 of halogen. 
 
 Coloured and Colourless Modifications of Dye- 
 bases. Emphasis was also laid by the advocates of 
 the chemical theory on the fact that when wool is 
 introduced into water in which the colourless base 
 of Magenta is suspended, the fibre acquires the 
 magenta colour characteristic of solutions of the salts. 
 It was pointed out by von Georgievics (loc. cit.) that 
 it did not necessarily follow from this experiment 
 that salt formation had taken place between the 
 fibre and the colour base; the coloration of the 
 fibre might be due simply to a change in the con- 
 stitution of the colour base. The ordinary colour 
 base has the carbinol form and is colourless, but von 
 Georgievics obtained a second variety in which' the 
 substance has a quinonoid structure, and this modifica- 
 tion has a magenta colour, and is probably the base 
 to which the dye salts really correspond. The 
 existence of this second form of the base was also 
 indicated by electrical conductivity measurements 
 made by Hantzsch and Osswald, 31 who were of the 
 opinion that the bases of other dyes of the triphenyl- 
 methane series existed also in two different modi- 
 fications. 
 
 / C 6 H 4 NH 2 / C 6 H 4 NH 2 
 
 HO C C 6 H 4 N H 2 C C 6 H 4 N H 2 
 
 \C 6 H 4 NH 2 ^C 6 H 4 = NH 2 OH 
 
 Cojourjesg form (para-rosaniline). Coloured fprm (para-rosanjline), 
 
CRITICISM OF CHEMICAL THEORY 43 
 
 In view of the existence of this second modification 
 of the colour base, it is obvious that the production 
 of colour when a colourless form of a base is taken 
 up by a fibre does not necessarily indicate the 
 formation of a salt. 
 
 Other Objections to the Chemical Theory. One 
 of the difficulties in the way of the chemical theory is 
 that of explaining the behaviour of fibres towards 
 the colouring matters of the direct cotton group. 
 Both cotton and the animal fibres are dyed directly 
 by these colouring matters, a fact which cannot be 
 explained by any theory which involves the chemical 
 nature of the fibre. 
 
 The recent experiments of Dreaper and Wilson, 18 
 who showed that acid dyes can be applied to silk 
 from an alkaline bath, are also scarcely favourable 
 to the chemical theory of dyeing. 
 
 The behaviour of dyed articles towards certain 
 solvents was also considered by Witt 60 to be difficult 
 to explain by means of the chemical theory. Silk, 
 for example, which has been dyed with Magenta 
 requires to be treated with a moderately concentrated 
 soap solution before it gives up any of the colouring 
 matter which has been fixed upon it, and one might 
 therefore argue that the Magenta and the silk form 
 a compound of considerable stability. When, how- 
 ever, the dyed silk is placed in absolute alcohol the 
 dye is removed almost instantaneously from the 
 fibre. In this latter case no chemical operation has 
 been involved, the relations between the dye and 
 the alcohol being simply those of solute and solvent. 
 On the addition of water to the alcoholic solution, 
 
44 THE CHEMISTRY OF DYEING 
 
 a certain quantity of the colouring matter, the actual 
 amount depending on the degree of dilution of the 
 alcohol, returns to the silk. 
 
 Problem of the Unexhausted Dyebath. A matter 
 of a similar nature to the foregoing is the impossibility 
 often experienced in practice of completely exhausting 
 the dyebath. If, after the material has been dyed 
 as fully as possible in a bath of this type, it is 
 removed from the bath and a second portion intro- 
 duced, some of the remaining colouring matter will 
 be taken up by the new material, but not the whole 
 of it, and this process can be repeated a number of 
 times without completely exhausting the dyebath. 
 
 It is difficult to explain results such as these by 
 means of the chemical theory, for if the process of 
 dyeing simply consists in the formation of a com- 
 pound between the fibre and the colouring matter, how 
 is it that the introduction of a further quantity of 
 the former does not result in the complete removal 
 of the latter from the dyebath ? Of course, behaviour 
 of this kind is not incompatible with the formation 
 of a compound of fibre and dye ; it might simply be 
 a case of the establishment of equilibrium in a 
 heterogeneous system made up of the undyed fibre, 
 the dyed fibre, and the dyestuff in solution. But in 
 such a system, where two of the substances are 
 practically insoluble and the third, the dye, soluble, 
 the condition of equilibrium, as arrived at from 
 the application of the Law of Mass Action, is that 
 for each temperature there will be a certain con- 
 centration of dye solution with which the dyed 
 fibre can be in equilibrium. If the concentration of 
 
CRITICISM OF CHEMICAL THEORY 45 
 
 the solution exceeds this equilibrium value, then dye 
 will be taken up by the fibre until the concentration is 
 reduced to the necessary degree, after which no 
 further dyeing will take place ; if, on the other hand, 
 a piece of undyed fibre should be placed in a solution 
 of a dye, the concentration of which is lower than the 
 equilibrium value, no dyeing will ensue. 
 
 The experiments of Walker arid Appleyard 58 showed 
 that these requirements were fulfilled by diphenyl- 
 amine when dyed with picric acid, but in no known 
 case of ordinary dyeing are the theoretical require- 
 ments satisfied, for no matter how dilute a solution of 
 a colouring matter may be, a certain amount of the 
 dye will be taken up from it on the introduction of 
 a fibre. The question of the unexhausted dyebath 
 remains therefore a difficult one to explain by means 
 of the chemical theory. 
 
 Experiments with a Liquid as an Artificial 
 Substitute for a Fibre. Prud'homme 48 thought he 
 might obtain information ds to the part played by 
 acid and basic groups of the fibre in fixing dyestuffs 
 by carrying out experiments using an artificial 
 substitute for a fibre in the form of a liquid. He 
 chose a neutral substance which was immiscible with 
 water, such as benzene, chloroform, or amyl alcohol, 
 and in this liquid dissolved an organic acid such as 
 salicylic acid, a weak base, preferably an imide 
 like acetanilide, or a mixture of acid and base; 
 these solutions corresponded to the fibre. Small 
 quantities of the bases of various basic dyes were 
 dissolved in a dilute solution of sodium hydroxide, 
 and two equal quantities of the different solutions 
 
46 THE CHEMISTRY OF DYEING 
 
 were then measured and mixed, the one with amyl 
 alcohol and the other with the same substance 
 containing 10 per cent, of salicylic acid. The 
 mixtures were rapidly shaken, when it was invari- 
 ably found that the neutral solution had the natural 
 colour of the base, while the other solution became 
 coloured like the salts of the base. 
 
 As silk when dyed with the same colouring matters 
 acquires the same colour as the acid amyl alcohol, it 
 was considered that these results showed that the 
 essential feature in the dyeing of silk with basic 
 colouring matters is the union of the base with the 
 acid group of the fibre. In the same manner it was 
 - found that when acidified solutions of acid colouring 
 matters were employed, the colour which resulted 
 when such a solution was shaken with a solution of 
 acetanilide in amyl alcohol was identical with that 
 acquired by wool and silk when treated with the 
 same dye, and it was accordingly considered as proved 
 that the essential feature of the dyeing of animal 
 fibres with acid dyes was salt formation between 
 the acid of the dye and the basic group of the fibre. 
 
 Exception was taken by Gillet 30 to the use of sali- 
 cylic acid and acetanilide in Prud'homme's experiments, 
 on the ground that those substances are appreciably 
 soluble in water, so that the solution in amyl alcohol 
 can scarcely be compared with wool or silk, the acid 
 and basic groups of which are not soluble in water. 
 Gillet employed a 5 per cent, solution of /3-naphthol 
 in amyl alcohol in place of the solution of salicylic 
 acid, and obtained results with the aqueous solutions 
 of colour bases which confirmed those arrived at by 
 
ABSORPTION OF DYES BY LIQUIDS 47 
 
 Prud'homme. The results with acid dyes, using a 
 solution of /3-naphthylamine, were not very conclusive. 
 Gillet doubted whether the acid dyes were fixed by 
 virtue of their acid groups, but thought it more likely 
 that it was through the feebly basic groups often 
 present in acid dyes that union with the fibre took 
 place. 
 
 Sisley 53 criticised the results obtained by Prud'- 
 homme, Gillet, etc., and stated that amyl alcohol 
 itself becomes coloured when warmed with alkaline 
 solutions of colour bases, the intensity of the colour 
 depending on the alkalinity of the solution. These 
 results obtained by Sisley were attributed by his 
 opponents 29 49 to the presence of small amounts of 
 acid impurities in the amyl alcohol used by him, 
 but he maintained that such was not the case, and 
 that even after thorough purification of the alcohol, 
 the colour was obtained on warming with the solution 
 of the colour base. In view of this observation, 
 no great support for the chemical theory can be 
 found in the results arrived at by Prud'homme and 
 others. Sisley (loc. cit.) was of the opinion that 
 these results were better explained by a change in 
 the structure of the colour base, such as had been 
 proved to happen with Magenta, rather than by salt 
 formation with the dissolved acid. 
 
 Nature of the Reactive Group in the Fibre. It 
 will have been noticed that several ideas have been 
 expressed regarding the kind of groups in the fibre 
 with which acid dyes in particular are supposed to 
 combine to form a salt-like product. Experiments 
 conducted by Bentz and Farrell 4 appear to show 
 
48 THE CHEMISTRY OF DYEING 
 
 that the union is not at any rate with the aromatic 
 primary amino group contained in the fibre. As 
 was mentioned in connection with the fibres, both 
 wool and silk contain a group of this description, 
 which can be diazotised in the usual manner, and 
 the diazo compound then coupled with phenols. If 
 the diazo compound is boiled with water, alcohol, or 
 a solution of cuprous chloride, the nitrogen originally 
 present as the aromatic amino group is removed. 
 Bentz and Farrell showed that wool and silk which 
 had been treated in this manner behaved towards 
 acid dyes in just the same manner as the normal 
 fibres, the dyebaths being equally exhausted and the 
 dyeings equally fast. 
 
 Binz and Schroeter 9 suggested that a union between 
 fibre and dye might take place altogether different 
 from the salt formation which most advocates of the 
 chemical theory of dyeing regard as the nature of 
 the action. They pointed out that if the chemical 
 theory in its ordinary form is true, the affinity of 
 dyes for the fibre would be a function of their salt- 
 forming power. This does not agree with many 
 observations, for it has been noticed that the introduc- 
 tion into a chromogen of a sulphonic or carboxyl 
 group does not confer any marked dyeing power 
 on the substance, whereas the introduction of the 
 amino and phenolic groups, whose salt -forming power 
 is much smaller, leads to the production of some of 
 our most powerful dyes. A number of experiments 
 were made with derivatives of azobenzene, and the 
 conclusion was drawn that when amino, sulphonic, 
 and other groups were introduced into the meta 
 
REACTIVE GROUPS IN FIBRES 49 
 
 position with regard to the azo group, the substance 
 formed had a certain degree of dyeing power, the 
 mechanism of dyeing appearing to be of the nature 
 of salt formation. When, however, the substituting 
 group entered in the para position to the azo group, 
 the substances formed were powerful dyes, and the 
 process of dyeing appeared to be different in character, 
 since it was not reversible according as acid or alkali 
 was present. Many of the fast wool and silk dyes 
 can be represented as having a quinonoid structure, 
 and Binz and Schroeter considered it probable that 
 they are fixed by a kind of ring condensation between 
 the fibre and the dye, salt formation only coming 
 in as a secondary factor. 
 
 The Solution and Adsorption Theories. 
 
 It has already been shown that Witt 60 considered 
 the chemical theory in many respects unsatisfactory, 
 and, in 1890, he brought forward another explanation 
 of the dyeing process which he considered to be more 
 in harmony with the observed facts. The new theory, 
 according to which the dye was supposed to exist 
 in the fibre in a state of solution, may be regarded 
 as occupying a position intermediate between the 
 two older theories. In support of his contention, 
 Witt pointed out that in the case of many basic dyes 
 of the triphenylmethane series, such as Methyl Violet 
 and Magenta, the colour acquired by wool or silk 
 when immersed in a solution of the colouring matter 
 is not that of the dry solid but that of the solution ; 
 the bronzy green colour characteristic of the solid 
 
 D 
 
50 THE CHEMISTRY OF DYEING 
 
 dyes is missing from the dyed fabric. If shellac 
 varnish is dissolved in alcohol, and some Magenta 
 is then added, the resultant liquid has a red colour 
 as long as any alcohol remains; when, however, the 
 solvent has all evaporated, the residue has the bronzy 
 colour characteristic of the solid dyestuff. The in- 
 ference is that the colour is entirely due to the 
 existence of the dye in the state of solution, Magenta 
 being soluble in alcohol but insoluble in shellac. 
 Then, again, Rhodamine, when dissolved in alcohol, 
 gives rise to a fluorescent solution. The solid dye- 
 stuff shows no fluorescence, but silk which has been 
 dyed with Rhodamine does exhibit this property, and 
 Witt therefore concluded that the Rhodamine fixed on 
 the silk must be present in a state of solution. 
 
 As to why some fibres can be dyed directly with 
 certain colouring matters, while in other cases the 
 agency of a mordant is necessary to effect dyeing, 
 this was explained by Witt by saying that in those 
 cases where a mordant is required the solubility 
 of the dyestuff in the fibre substance is too small 
 to permit of effective dyeing. It is not that the 
 dye is actually insoluble in the fibre, but the 
 solubility is of such small dimensions when com- 
 pared with the solubility of the colouring matter in 
 water that practically no colouring matter is taken 
 up by the fibre. It will be seen, therefore, that 
 according to the solution theory of the process, the 
 substantive dyeing of a fibre is akin to the extraction 
 of a substance from an aqueous solution by means 
 of an immiscible solvent in which the solubility is 
 greater than in water. An explanation of this kind 
 
SOLUTION THEORY OF DYEING 51 
 
 certainly provides a reason for the non-exhaustion 
 of the dyebath, which was one of the difficulties of 
 the chemical theory. 
 
 Witt only considered the chemical character of the 
 fibre to be of importance in so far as it affected the 
 solvent power of the substance. Silk was considered 
 to be dyed more readily than other fibres because 
 the solubility of dyes in fibroin is greater than in 
 keratin or in cellulose ; the solubility of many dyes 
 in cellulose is so small that sodium sulphate or some 
 other salt is added in order to diminish the solubility 
 in the liquor of the dyebath. 
 
 As regards adjective dyeing, Witt was of the 
 opinion that the question of solution still had to 
 be considered. In such cases it was, however, the 
 mordant which was dissolved by the fibre, and the 
 solution so obtained then reacted with the colouring 
 matter to form an insoluble compound. 
 
 There are certain cases where the dyed material 
 has a colour different from that belonging to the 
 aqueous solution, and Witt anticipated that this 
 might be brought forward as an argument against 
 his theory. He pointed out, however, that phenomena 
 of this kind are by no means unknown in cases 
 of undoubted solution, as for instance with iodine, 
 the aqueous solution of which is brown in colour, 
 while the solution in chloroform has a violet colour. 
 The explanation of these differences of colour is that 
 the iodine exists in the two solutions in different 
 conditions, a feeble union between solvent and solute 
 probably taking place in those liquids from which 
 brown solutions are obtained; the differences ob- 
 
52 THE CHEMISTRY OF DYEING 
 
 served with dyestuffs might be due to a similar 
 cause. 
 
 Objections to Witt's Solution Theory. It must 
 be admitted that the position taken up by Witt, 
 and the arguments advanced in support of his ideas, 
 strike one at first in quite a favourable manner, but 
 on further consideration certain objections suggest 
 themselves. It was pointed out by von Georgievics 25 
 that the phenomenon of fluorescence is shown by a 
 number of solid substances, and is not confined to 
 solutions. Moreover, although silk which has been 
 dyed with Fluorescein is fluorescent, yet wool which 
 has been dyed with the same substance shows no 
 fluorescence, and we should therefore, according to 
 the solid solution theory, have to conclude that the 
 Fluorescein is dissolved by the silk and not by the 
 wool, although both fibres are in the dyed state. 
 
 The arguments of Witt, based on the colour of 
 Magenta, were also shown to be untenable, for von 
 Georgievics demonstrated that when solid Magenta 
 is rubbed between two glass plates it loses its bronze 
 colour and becomes red. Bronziness, on the other 
 hand, is sometimes observed on dyed goods, as when 
 wool is dyed with a concentrated solution of Magenta. 
 But one of the most apparently destructive pieces 
 of criticism brought by von Georgievics 2527 against 
 the solid solution theory was that the dyeing process 
 is not reversible. If a dyed fibre is simply a solution, 
 then one would expect that when such a fibre was 
 placed in fresh water it would give up a portion 
 of the colouring matter which it had absorbed, no 
 matter what the nature of the dyestuff might be ; 
 
CRITICISM OF SOLUTION THEORY 53 
 
 this, however, is by no means the general behaviour 
 of dyed goods, for many dyes are quite fast to 
 washing. 
 
 Another anomaly arises from the fact that wool 
 is more readily dyed from a boiling than from a 
 cold solution of a dye, and takes up a larger quantity 
 of colouring matter under the former than under the 
 latter conditions; it might therefore be supposed 
 that the colouring matter is more soluble in the fibre 
 at the higher temperature, and that wool which had 
 been dyed at a high temperature would give up a 
 portion of the colour which it had absorbed when 
 it cooled down again, but this is a mode of behaviour 
 not met with in practice. 
 
 Now, if Witt's theory correctly represents the 
 nature of the dyeing process, we should expect the 
 laws governing the distribution of a substance between 
 two immiscible solvents to be obeyed. The work 
 of Nernst and others has shown us that when a 
 substance has the same molecular weight in two 
 solvents the ratio of concentrations of the two 
 solutions after distribution of the substance between 
 the solvents is, at any given temperature, independent 
 both of the quantity of solute and of solvents, and 
 depends only on the solubilities in the individual 
 solvents. If the molecular weight is not the same 
 in the two solvents, this simple ratio is departed 
 from, the ratio of the concentrations of the two 
 solutions varying with the amounts of substances 
 used. Even in such a case it is, however, possible 
 to obtain an expression for the ratio of distribution 
 of the substance between the two solvents. Suppose 
 
54 THE CHEMISTRY OF DYEING 
 
 the solvents be A and B, and that the molecular 
 weight of a substance when dissolved in the latter 
 solvent be n times as great as when dissolved in the 
 
 C 
 former; then A- will be constant in value, C A and 
 
 C B being the concentrations of the dissolved sub- 
 stance in the two solvents. These are the laws 
 which should be obeyed if the dyeing process is 
 simply one of the formation of a solution in the 
 fibre/ 
 
 In order to put the question to a practical test, 
 Walker and Appleyard 58 conducted experiments on 
 the dyeing of silk with picric acid. It was found 
 that, for given quantities of silk, water, and picric 
 acid, it was immaterial how the acid was distributed 
 at the beginning of the experiment, the same ultimate 
 equilibrium being arrived at whether the picric acid 
 was only dissolved in the water, all contained in the 
 silk, or partly present in both media at the commence- 
 ment of the experiment. This result was in harmony 
 with the requirements of the solid solution theory, 
 but would accord equally well with any other 
 theory involving the establishment of a condition 
 of equilibrium. It was also found that the ratio of 
 distribution of the acid between the water and the 
 silk varied with the quantities involved, but that 
 constant values were obtained when a formula was 
 employed of the type already given, the result 
 obtained being 
 
 Concentration in silk 
 
 = 35'5- 
 
 \/ Concentration in water 
 
SOLUTION THEORY UNTENABLE 55 
 
 A moment's consideration will show that this result 
 is, as far as the dyeing of silk by picric acid is 
 concerned, fatal to the solid solution theory of 
 dyeing as proposed by Witt. If we accept the 
 theory as true, then we are forced to the conclusion 
 that the molecular weight of picric acid in aqueous 
 solution is nearly three times as great as when the 
 acid is contained in silk. Such an idea cannot, of 
 course, be entertained for a moment, for we know 
 that, owing to ionisation, the molecular weight of 
 picric acid in aqueous solution is even less than 
 corresponds with the simple formula C 6 H 2 (N0 2 ) 3 OH, 
 and it is out of the question for the substance to 
 have a smaller molecular weight in a less active 
 solvent such as silk. We must therefore conclude 
 that picric acid does not exist in the silk in a state 
 of simple, homogeneous solution. 
 
 Previous to the publication of the paper to which 
 reference has just been made, several other investiga- 
 tions of a similar character had been carried out with 
 varying results. Yon Georgievics 26 concluded that 
 when silk is dyed with Indigo-carmine the process 
 is analogous to solution, but his results really lead 
 to conclusions of the same character as those deduced 
 from W^alker and Appleyard's experiments ; in a later 
 paper, published in conjunction with Lowy, 28 dealing 
 with the distribution of Methylene Blue between 
 water and mercerised cotton, von Georgievics came 
 to the conclusion that his results were incompatible 
 with the solid solution theory. Schmidt 52 could 
 obtain no constant distribution ratio, and considered 
 dyeing to be an absorption phenomenon. 
 
56 THE CHEMISTRY OF DYEING 
 
 Adsorption Theory. In actions of this kind where 
 one substance is absorbed by another, but where it 
 is evident that the absorbed substance cannot be 
 in a condition of homogeneous solution, it is con- 
 sidered that the surface of the absorbing substance 
 plays most part in the process of absorption, and 
 the term "adsorption" is made use of to describe 
 such cases of absorption. According to the results 
 of Walker, von Georgievics, etc., we should conclude 
 that in a dyed fibre the colouring matter is not 
 uniformly distributed throughout the fibre, but is 
 collected at the surface ; in other words, the process 
 is one of adsorption.* The fact that the surface of 
 a fibre appears to play an important part in the 
 dyeing process may be taken as showing that a certain 
 amount of what may be called mechanical action 
 does really enter into the operation. As has been 
 indicated, however, we cannot explain the whole 
 phenomena of dyeing on a mechanical basis. Many 
 investigations have been carried out during the last 
 fifteen years on this branch of the subject of dyeing, 
 and the results obtained support, for the most part, 
 the view that dyeing cannot be regarded simply as 
 the formation of a homogeneous solution of the dye 
 
 * To speak of dyeing as a surface phenomenon may appear 
 to be contrary to the definition of "dyeing" given in the 
 introduction. In this connection it is only necessary to point 
 out that & fabric is made up of innumerable individual fibres, 
 and that, owing to the fabric being more or less porous, the 
 solution of the dye can penetrate into the material, so that 
 colour can be absorbed by the surface of a fibre in the interior 
 of the fabric; the material will not, therefore, be coloured 
 only on its surface. 
 
SURFACE ACTION 57 
 
 in the fibre. Amongst those to whom we are in- 
 debted for further knowledge on the subject may 
 be mentioned Biltz, 5 Hiibner, 32 Freundlich and 
 Losev, 21 Pelet and Grand, 42 Schaposchnikoff, 51 and 
 Brown and M'Crae. 10 
 
 An interesting feature of Hubner's experiments 
 was that attention was particularly directed to the 
 influence exerted on the degree and rate of dyeing 
 by the degree of division of a fibre. In the case 
 of cotton a quantity of yarn was first thoroughly 
 scoured and a portion then disintegrated by treatment 
 in a beating engine. On afterwards introducing 
 equal weights of the beaten and unbeaten fibre into 
 a solution of Night Blue, it was found that while 
 the unbeaten fibre went on steadily absorbing the 
 colouring matter during a period of seventy-two 
 hours, the disintegrated cotton took up the dye very 
 rapidly during the first hour, and very little more was 
 absorbed during the succeeding seventy-one hours. 
 Not only was the rate of absorption greatly accelerated 
 by reducing the state of division of the fibre, but the 
 actual quantity of dye absorbed was also affected, 
 about twice as much colour being taken up by the 
 disintegrated fibre as was absorbed by the unbeaten 
 cotton. These results clearly indicate the part played 
 by the surface of the fibre in the absorption of a 
 colouring matter from aqueous solution, for of course 
 the extent of surface is much greater in the case 
 of the disintegrated fibre than with the unbeaten 
 cotton. On carrying out similar experiments with 
 wool it was found that the rate of absorption was 
 very considerably increased, but there was no increase 
 
58 THE CHEMISTRY OF DYEING 
 
 in the total amount of dye absorbed. The influence 
 of surface was further shown by experiments with 
 threads of artificial silk of different diameters and 
 with fine- and coarse-grained emery; in both cases 
 the proportion of dye absorbed by the fine sample 
 was greater than that absorbed by the coarser one. 
 
 The experiments of many of the later workers 
 on the subject have been partially made with the 
 idea of comparing the absorption of colouring matters 
 by textile fibres with absorption by inorganic sub- 
 stances such as animal charcoal, sand, China clay, 
 aluminium hydroxide, etc. The results clearly show 
 that there is no essential difference between the 
 absorption of dyes by fibres and by inorganic 
 substances, and that in both cases the process is 
 subject to the same laws and appears to be an 
 instance of adsorption. Freundlich and Losev (loc. 
 cit.) found that the extent of adsorption is inde- 
 pendent of the nature of the adsorbent, a dye which 
 was strongly adsorbed by charcoal also being strongly 
 adsorbed by the textile fibres. Basic dyes were 
 decomposed into acid and base both in the presence 
 of charcoal and of fibres, the acid remaining in 
 solution and the colour base being adsorbed, probably 
 in a polymeric form. In the case of mordant dyeing 
 the adsorption theory can still be applied, the only 
 difference being that it is the mordant which under- 
 goes adsorption. 
 
 Although the bulk of the evidence of a quantitative 
 character has been shown to be unfavourable to the 
 simple solid solution theory, there have been several 
 cases put on record of experimental results which 
 
ANOTHER SOLUTION THEORY 59 
 
 appear to be in harmony with that theory. Brown 
 and M'Crae (loc. cit.) showed that when wool is 
 dyed with Acid Magenta in the presence of sulphuric 
 acid and with Chrysoidine FF, the ratio of distribu- 
 tion of the colouring matter between the fibre and 
 the water is practically independent of the concentra- 
 tion of the solution, so that in the case of these 
 dyes it would appear as if the dyeing process is of 
 a nature similar to solution. Sisley 54 also obtained 
 results which he considered to be favourable to 
 Witt's theory. 
 
 In concluding this section reference must be made 
 to another idea regarding solution to which expression 
 was given by Weber. 59 It has already been explained 
 that this chemist regarded the dyeing of fibres with 
 the acid and basic dyes as a process of a chemical 
 nature. He recognised that the absorption of the 
 direct cotton dyes by fibres of different chemical 
 natures could not be explained in the same manner, 
 and he was led to the conclusion that in the case 
 of colouring matters of this class the process was one 
 of solution. Whereas, however, Witt considered the 
 dyestuff to be dissolved by the substance of the fibre 
 itself, Weber was of the opinion that the colouring 
 matter dissolved in the water contained in the pores 
 or intercellular spaces of the fibre. In support of 
 this view he pointed out that cellulose dinitrate 
 can be manufactured in such a manner that it shows 
 little structural difference, when examined by means 
 of the microscope, from ordinary cotton, and these 
 nitrated fibres can be dyed with the benzidine colours 
 in just the same way as ordinary cotton. If the 
 
60 THE CHEMISTRY OF DYEING 
 
 nitrated cotton is dissolved in acetone, and the solvent 
 allowed to evaporate, a film is left which is devoid 
 of structure and which contains no water. In this 
 form the nitrated cotton cannot be dyed with the 
 benzidine colouring matters. Additional support for 
 this view was obtained by Weber from a microscopical 
 examination of dyed fibres, when in many cases the 
 cell walls appeared colourless, while the lumen was 
 filled with colouring matter. 
 
 Other Theories of Dyeing. 
 
 An electrical explanation of the attraction of 
 fibres for colouring matters was given by Gee and 
 Harrison. 23 In the case of basic and acid dyes, the 
 colour base of the former and the acid of the latter 
 carry a positive and negative charge respectively 
 derived from the ionisation of the dyestuff. When 
 wool or silk is immersed in water the fibre becomes 
 negatively charged, and it is quite natural therefore 
 that when such a fibre is immersed in a neutral 
 solution of a basic dye there will be an attraction 
 between the negatively charged fibre and the positively 
 charged colour base. When the fibres are placed 
 in an acid solution instead of in pure water they 
 become positively charged; such a change in the 
 kind of electrification with the nature of the solution 
 in which the substance is immersed is frequently 
 noticed with substances of a colloidal character. 
 Under these changed conditions the fibre will now 
 have an affinity for the negatively charged acid of 
 acid colouring matters, while the attraction for basic 
 
AN ELECTRICAL THEORY 61 
 
 dyes will now be less than before in view of the 
 fact that the two are now similarly charged. This 
 latter deduction is quite in keeping with the results 
 obtained in practice, for it is well known that the 
 basic dyes are much less readily absorbed from acid 
 solution than from a neutral one, and this device 
 is sometimes resorted to in order to regulate the 
 rate at which a basic colouring matter is absorbed. 
 This theory also accounts in quite a satisfactory 
 manner for the much smaller attraction shown for 
 dyestuffs of the acid and basic groups by a fibre 
 such as cotton. The potential difference between 
 wool and water is equal on the average to 0-91 volt, 
 while with cotton the average value of the potential 
 difference is only 0-06 volt. Owing to this much 
 smaller charge upon the cotton, it will be readily 
 understood why the fibre should have much less 
 attraction for colouring matters than is shown by 
 wool. While this theory gives an apparently satis- 
 factory explanation of the causes underlying the 
 affinity of fibres for dyestuffs, it does not explain 
 how the colouring matter becomes fixed upon the 
 fibre; for of course after the dye and fibre come 
 into actual contact, it must naturally be assumed 
 that their electrical charges will neutralise each other. 
 A somewhat similar view was taken by Feilmann, 20 
 according to whom the coloured ion formed from 
 the dyestuff was attracted by the oppositely charged 
 fibre, and penetrated the latter more or less deeply. 
 The absorbed ion was supposed to be retained, either 
 because the fibre acted as a protective colloid, or 
 because of chemical action taking place between the 
 
62 THE CHEMISTRY OF DYEING 
 
 ion and the fibre. It will be seen that this idea 
 goes a little further than that of Gee and Harrison, 
 inasmuch as it gives a possible explanation of the 
 means of fixation of the dye on the fibre. 
 
 Colloidal Theory. Of late years a large and ever- 
 increasing amount of attention has been given to the 
 properties and behaviour of colloids. The fibres 
 themselves are of course substances of a colloidal 
 character, and it has been shown in an earlier portion 
 of the book that many dyes when in solution probably 
 exist in the colloidal form. In view of these facts 
 it is not surprising that attempts have been made 
 to explain the dyeing process as a colloidal 
 phenomenon. One of the first to take up this view 
 of the matter was Krafft. 39 He pointed out that the 
 substances commonly employed as mordants, such as 
 the hydroxides of iron, aluminium and ' chromium, 
 tannic acid, soap, etc., are all of a colloidal nature, 
 and suggested that the use of such substances was 
 in many cases necessary in order to combine with 
 dyes of low molecular weight, which under normal 
 conditions only existed in solution to a small degree 
 in the colloidal condition, so as to form a more highly 
 colloidal compound capable of being fixed on the 
 fibre. As for the direct cotton dyestuffs, these were 
 supposed to exist in the colloidal condition to a much 
 greater extent than the dyes of the acid and basic 
 groups, and no assistance was therefore necessary in 
 order to fix the colouring matter. Of course this 
 explanation of the dyeing process, according to which 
 dyeing is simply a precipitation of colloidal substances 
 on or in the fibre, only applies to the dyeing of cotton, 
 
THE COLLOIDAL THEORY 63 
 
 for wool and silk do not require any mordant in 
 order to fix the acid or basic dyestufFs. Pelet- 
 Jolivet and Andersen 4344 were also in favour of a 
 colloidal theory of dyeing. 
 
 The view was expressed by several of the supporters 
 of the adsorption theory that the substance actually 
 fixed on the fibre was in the colloidal condition. 
 Freundlich and Losev, 21 for example, considered that 
 in the case of wool dyed with a basic colouring 
 matter, the colour base was adsorbed by the fibre 
 in a colloidal form insoluble in water ; and they also 
 considered that in some instances dyeing might be 
 due to the formation of a colloidal compound between 
 the fibre and the dyestuff. Pelet and Grand 42 also 
 considered that dyeing is due to the precipitation of 
 colloids on the fibre, and that the salts added to the 
 dyebath assist in this precipitation. Linder and 
 Picton 41 from experiments made with Methyl Violet, 
 Magenta, Soluble Blue, etc., were led to connect the 
 phenomena of dyeing with the electrical charges 
 which, as is well known, many substances carry 
 when existing in a state of colloidal solution. They 
 found that whereas a colloidal solution of ferric 
 hydroxide is precipitated by the addition of a 
 solution of Soluble Blue, no such action takes place 
 on the addition of Methyl Violet to the ferric 
 hydroxide solution; on the other hand, a colloidal 
 solution of arsenious sulphide is precipitated by 
 solutions of basic dyes like Methyl Violet and 
 Magenta but not by Soluble Blue, an acid colouring 
 matter. The reason is that the charges on the ferric 
 hydroxide and the basic dyes are of a similar nature, 
 
64 THE CHEMISTRY OF DYEING 
 
 these substances carrying positive charges, while the 
 arsenious sulphide and the Soluble Blue carry negative 
 charges; as is well known, when two oppositely 
 charged colloidal solutions are mixed together they 
 precipitate each other. A difference was noticed, 
 however, between the precipitation of the ferric 
 hydroxide by the acid dye and by means of an 
 electrolyte such as ammonium sulphate. In the 
 latter case a definite amount of the salt is required 
 to completely precipitate the colloid, and any excess 
 which may be added remains in solution. With /the 
 dye, however, a different result was observed, for 
 even after the ferric hydroxide had been completely 
 thrown out of colloidal solution it continued to take 
 up the colouring matter as a whole. Linder and 
 Picton explained this on the supposition that a 
 certain portion of the original charge was retained 
 by the ferric hydroxide even after it had been 
 coagulated by the addition of the dyestuff, and that 
 by virtue of this residual charge it continued to take 
 up additional quantities of the dye. The same kind 
 of action was considered to take place during the 
 ordinary process of dyeing, the place of the inorganic 
 colloid being taken by the fibre. They were accord- 
 ingly of the opinion that the first stage of the dyeing 
 process consisted in the separation of insoluble 
 derivatives of the dye having a feeble charge, these 
 substances being produced as the result of interaction 
 between the dyestuff and the fibre ; the second part of 
 the process was an attraction between this coagulum 
 and the remaining dyestuff, the particles of which 
 were retained as a whole. 
 
EFFECT OF PROTECTIVE COLLOIDS 65 
 
 It will be noticed there is a certain degree of 
 similarity between these views and those expressed 
 later by Gee and Harrison (loc. cit.), inasmuch as both 
 pairs of observers consider the electrical condition 
 of the fibre and the dyestuff to play an important 
 part in the process ; the differences between the two 
 were that Linder and Picton regarded the matter 
 exclusively from the point of view that the substances 
 with which they were concerned all existed in the 
 colloidal condition, and that they were led to regard 
 the taking up of a dye as proceeding in two stages. 
 
 Alexander l showed that various protective colloids, 
 such as gelatin and gum arabic, when added to a 
 solution of Benzopurpurin had the effect of causing 
 the mixture to behave in exactly the same manner 
 when treated with acid as wool or silk which had 
 been dyed with the same colouring matter. The 
 addition of a dilute solution of hydrochloric acid to 
 a solution of Benzopurpurin causes the colour to 
 change from red to blue, while on the addition of 
 a more concentrated solution of acid the coagulated 
 dark blue colour acid separates out. If a similar 
 solution of Benzopurpurin is first mixed with one 
 of the above-mentioned colloids the colour is only 
 changed to claret red or chocolate brown on the 
 addition of hydrochloric acid, the colour depending 
 on the concentration of the added acid, and even 
 when concentrated acid is used no precipitate is pro- 
 duced. These changes were investigated by means 
 of the ultramicroscope ; it was found that no change 
 resulted in the presence of the colloid, unless the 
 added acid was of sufficient concentration to cause 
 
 E 
 
66 THE CHEMISTRY OF DYEING 
 
 agglutination of ultramicrons into small groups. As 
 regards the protective action of different colloids, 
 it was found that gelatin was much more effective 
 than gum arabic, and that starch had very little 
 protective action. In the light of these results, it 
 was considered that the difference in colour changes, 
 which result when different fibres dyed with Benzo- 
 purpurin are immersed in dilute acid, is due to the 
 difference in protective action of the fibre on the 
 adsorbed colouring matter. 
 
 Alexander pointed out that in considering the 
 nature of the combination between the fibre and 
 the dye, it is necessary to bear in mind the state 
 of subdivision of both substances. The usual effect 
 of increase of temperature and of the presence of 
 dilute alkalis is to cause subdivision of the particles 
 of a substance, and it is not surprising therefore that 
 agencies such as these should bring about a closer 
 union between the fibre and the dye, such as Dreaper 
 and Wilson 1718 had shown to exist. Dreaper and 
 Wilson, 18 in fact, consider that the temperature of 
 the dyebath is the main factor in the actual fixation 
 of the dye on the fibre, and that though in the 
 application of acid dyes the presence of acid in the 
 bath may lead to a larger amount of dye being taken 
 up than would be the case from a neutral solution, 
 yet the acid plays no part in the fixing of the colour. 
 Dreaper and Wilson have shown that it is possible 
 to apply acid dyes from an alkaline solution ; under 
 such conditions, of course, the substance actually 
 absorbed by the fibre can scarcely be the free colour 
 acid. They consider that the sodium carbonate used 
 
EFFECT OF ALKALI ON A DYE 67 
 
 in the bath may act as a salt rather than an alkali, 
 and that it may affect the state of aggregation of 
 the dissolved colouring matter and bring into play 
 conditions more favourable to colloidal action. 
 
 Alexander (loc. cit.) studied by the aid of the ultra- 
 microscope the effect produced when alkali is added 
 to a solution of Acid Anthracene Red. The appear- 
 ance of the aqueous solution of this colouring matter 
 would lead one to conclude that a portion of the 
 dye at any rate is in a very fine state of subdivision, 
 if not in true solution. When the dye was dissolved 
 in a solution of sodium carbonate of N/20 concentra- 
 tion a certain amount of coagulation took place ; but 
 this only represented part of the total quantity of 
 dye actually present, for the alkaline solution, on 
 standing, was found to give only a small amount of 
 sediment, and it also gave a marked Tyndall effect 
 when examined with the ultramicroscope. When 
 a more concentrated solution of sodium carbonate 
 was employed, the bulk of the dye was precipitated ; 
 but this precipitate dissolved on being heated, and 
 appeared again when the liquid was allowed to cool. 
 One would infer from this experiment that the state 
 of a dye in solution is modified both by alkali and 
 by change of temperature, and Alexander considered 
 that these factors might explain the fixation of the 
 dye which would, in accordance with the ordinary 
 conditions of dyeing, be absorbed at a high temperature 
 and might be precipitated again on cooling, or be 
 flocculated by adsorbed alkali. 
 
68 THE CHEMISTRY OF DYEING 
 
 The Divided Nature of the Dyeing Process. 
 
 It may have been noticed that in most of the ideas 
 to which expression has been given in the preceding 
 pages, the dyeing process has been regarded as simple 
 in character, and the supporters of the different 
 theories have, for the most part, attempted to explain 
 all the phenomena of dyeing as resulting from a 
 single operation. This it is which has led to the 
 various difficulties which have confronted the 
 supporters of the different theories, and which have 
 led some chemists such as Weber 59 and Brown and 
 M'Crae 10 to the conclusion that no one theory can 
 possibly explain all cases of dyeing, and that it may 
 be necessary to adopt one theory to explain the 
 dyeing of one fibre, and another theory to explain 
 the dyeing of a second fibre. This would certainly 
 appear to be the inevitable conclusion if we are to 
 regard the dyeing process as simple in character, 
 that is, as involving one operation only. All the 
 theories have a certain amount of experimental 
 evidence which can be cited in support of them ; 
 but they are more or less antagonistic in character, 
 and no one of them will explain the whole of 
 the phenomena associated with the subject of 
 dyeing. 
 
 It is the aim of the author in the present section 
 to show that all these opposing theories may be 
 true as partial explanations of certain cases of dyeing, 
 and that they may be linked together in one general 
 theory of the dyeing process. In order to arrive 
 that such a result it is necessary to assume at the 
 
FIRST STAGE OF DYEING 69 
 
 operation of dyeing takes place in two distinct stages. 
 Dreaper, 16 Cross and Bevan, u Lewis, 40 Zacharias, 61 and 
 Fahrion 19 have all expressed themselves in favour 
 of regarding the dyeing process as having a dual 
 character, and in the opinion of the author it is 
 the most rational view to take of the matter. 
 
 In the first part of the process, which may be 
 called the absorption stage, the dyestuff is simply 
 absorbed from solution. This absorption was regarded 
 by Zacharias as being brought about by the diffusion 
 of the dissolved dye from the aqueous solution into 
 the fibre, and no particular attraction between the 
 fibre and the dye needs to be invoked to explain this 
 diffusion, although the process may be assisted by 
 electrical attraction. It is the process which takes 
 place naturally when a layer of water is placed over 
 a solution of a dissolved salt, or when a liquid, 
 immiscible with water, is placed in contact with an 
 aqueous solution of a substance which is also soluble 
 in the second solvent. Just as in those cases diffusion 
 is a slow process so that homogeneous solutions are 
 only obtained after the lapse of a considerable time, 
 so also the process of diffusion into the fibre will 
 take place slowly, and although the dye may be said 
 to be dissolved by the fibre, yet, generally speaking, 
 the solute will not be uniformly distributed through- 
 out the fibre but, because of this slow diffusion, will 
 be present in largest quantity at the surface ; in other 
 words, the process will appear as an adsorption 
 phenomenon. 
 
 The fibres are hygroscopic colloids, and absorb 
 other substances in accordance with certain general 
 
70 THE CHEMISTRY OF DYEING 
 
 laws. All colloidal substances absorb others accord- 
 ing to the law embodied in the expression 
 
 where C 1 and C 2 represent the concentration in the 
 aqueous and in the other phase respectively at the 
 end of the absorption, k is a constant and v a constant 
 coefficient, which may have a value either greater 
 or less than or equal to unity. 
 
 It was pointed out by Zacharias that this formula 
 may be deduced mathematically from the laws of 
 diffusion, and accordingly the dyeing of textile fibres 
 and also of other substances such as charcoal, alu- 
 minium hydroxide, etc., and the quantitative results 
 obtained in these processes can all be explained on 
 this basis of diffusion of the dissolved dye. The 
 formula is identical with that representing the 
 distribution of a substance between immiscible 
 solvents where homogeneous solutions are formed, 
 but it has already been shown what difficulties are 
 met with if we regard the matter from this latter 
 standpoint. 
 
 So far we have only considered the first stage of 
 dyeing; the second part of the process is the fixation 
 of the colouring matter. There could be no permanent 
 dyeing without this second operation. Here we 
 have to recognise the fact that dyes may be fixed 
 on fibres as the result of the operation of different 
 forces. 
 
 In some cases it seems probable that a certain 
 amount of chemical change takes place, either between 
 the fibre and the dyestuff or in the structure of the 
 
SECOND STAGE OF DYEING 71 
 
 latter compound. Bayliss 2 has put on record a case 
 of dyeing, where a process of absorption was un- 
 doubtedly succeeded by one of chemical action. 
 When a dilute colloidal solution of the blue colour 
 acid of Congo Red is mixed with well-washed 
 aluminium hydroxide, the latter absorbs the colour 
 acid and acquires a blue colour; this is adsorption. 
 On suspending the blue precipitate in water and 
 warming the mixture, the colour changes to red 
 owing to the formation of an aluminium salt, 
 chemical action taking place between the colour 
 acid and the basic hydroxide. 
 
 No doubt similar chemical changes take place in 
 the dyeing of textile fibres. Cross and Bevan 13 have, 
 in fact, described a very good example of a fibre 
 being dyed as the result of a double process. If jute 
 is immersed in a solution of ferric ferrocyanide it 
 very soon becomes dyed blue owing to the production 
 of Prussian Blue. In this case the substance absorbed 
 is the soluble ferric salt; this then reacts with the 
 jute substance, the unsaturated lignone groups of 
 which are oxidised, and as a result of this action 
 insoluble ferrous ferricyanide is produced and fixed 
 in the fibre. 
 
 In other cases the dye may have been originally 
 present in the dyebath in a condition of colloidal 
 solution, and the fixation of the colouring matter 
 may be due to the precipitation of the colloid. 
 Possibly, also, a certain amount of colour may be 
 fixed as the result of adhesion between the fibre 
 and dyestuff. 
 
 This theory of the dual nature of the dyeing 
 
72 THE CHEMISTRY OF DYEING 
 
 process applies to cases of mordant dyeing, to the 
 developed dyes, and those, such as Indigo, which 
 are applied by the vat method, equally well as to 
 all cases of substantive dyeing. In all cases we have 
 a first process of absorption; in the case of the 
 mordant dyes it is usually the mordant which is 
 taken up in this manner, and the mordant is then 
 fixed in an insoluble form. The colouring matter is 
 then absorbed from its solution and reacts with the 
 previously fixed mordant to form the colour lake. 
 The dyeing of cotton with lead chromate and with 
 the azo dyes like paranitraniline red formed directly 
 on the fibre also takes place in two stages, while in 
 the case of the vat dyes we have first the absorption 
 of the soluble leuco compound, and second the fixation 
 of the dye in the insoluble form owing to the 
 oxidation of the leuco compound. 
 
 As for the fact that with some colouring matters 
 the dyebath is practically completely exhausted, while 
 with others this is never the case, the probable 
 explanation is that the degree to which the bath is 
 exhausted depends both upon the rate of absorption 
 and on the rate of fixation of the dye. With a dye 
 which is fixed readily and which diffuses into the 
 fibre with considerable velocity, there appears no 
 reason why the dyebath should not be practically 
 exhausted in the period usually occupied by the 
 dyeing process. 
 
 This method of regarding the dyeing process also 
 provides an answer to the objections levelled against 
 the solid solution theory with respect to the non- 
 reversible character of the dyeing process. Were 
 
CAUSE OF PAST CONFUSION 73 
 
 dyeing nothing but an absorption, an extraction of 
 an aqueous solution by means of the immiscible 
 solvent, the fibre, there would be no reason why the 
 process should nob be reversible. When, however, 
 the dye becomes fixed, we change its properties either 
 by forming an insoluble compound or by convert- 
 ing it into a modification insoluble in water, and so 
 reversibility of the process ceases to be possible. 
 
 In concluding this study of the dyeing process, the 
 author would point out that some at least of the 
 diversity of opinion expressed in the past has been 
 due to the fact that a certain amount of confusion 
 prevailed as to what really constituted the dyeing 
 process. Some workers appeared only to regard the 
 method of fixation of the dye on the fibre; others 
 paid more attention to the process of absorption. 
 As has been shown, the dyeing process embraces both 
 absorption and fixation, and every example of dyeing 
 can be shown to include these two operations. This 
 may therefore be called a general theory or explana- 
 tion of the nature of the dyeing process. It is true 
 that different methods of fixation of the dyes are 
 recognised, and some may regard this as an objection 
 to a general theory, and as not much of an advance, 
 if any, over the position that no one theory of dyeing 
 can be applied to all cases, but that in some instances 
 the phenomena are to be explained on one basis, and 
 in other cases on a different one. This difference 
 in the mode of fixation is, however, a matter of minor 
 importance due to the great differences in properties 
 of the fibres and dyestuffs ; to have reached the stage 
 of being able to say that no matter what the fibre, 
 
74 THE CHEMISTRY OF DYEING 
 
 no matter what the dyestuff may be, the process of 
 dyeing the one with the other can always be divided 
 into the two stages of absorption and fixation is 
 certainly to have made a very considerable advance 
 in our knowledge of the dyeing process. 
 
BIBLIOGRAPHY 
 
 1 Alexander, Journ. Soc. Chem. Ind., 1911, 30, 517. 
 
 2 Bayliss, Zeit. Chem. Ind. Roll., 1908, 3, 224. 
 
 3 Bayliss, Proc. Roy. Soc., 1909, 81, B, 269. 
 
 4 Bentz and Farrell, Journ. Soc. Chem. Ind., 1897, 16, 406. 
 
 5 Biltz, Chem. Zentr., 1905, 2, 524. 
 
 Biltz and Pfenning, Gedankboek aan van Bemmelen, 1910, 
 108. 
 
 7 Biltz and Pfenning, Zeit. physikal. Chem., 1911, 77, 91. 
 
 8 Biltz and Vegesack, Zeit. pbysikal. Chem., 1910, 73. 481. 
 
 9 Binz and Schroeter, Ber., 1902, 35, 4225 ; 1903, 36, 3008. 
 
 10 Brown and M'Crae, Journ. Soc. Chem. Ind., 1901, 20, 1092. 
 
 11 Champion, Compt. rend., 72, 330. 
 
 12 Cross and Bevan, Researches on Cellulose. 
 
 13 Cross and Bevan, Journ. Soc. Chem. Ind., 1893, 12, 104. 
 
 14 Cross and Bevan, Journ. Soc. Chem. Ind., 1894, 13, 354. 
 
 15 Donnan and Harris, Chem. Soc. Trans., 1911, 99, 1554. 
 
 10 Dreaper, Journ. Soc. Chem. Ind., 1894, 13, 95 ; 1905, 24, 
 223. 
 
 17 Dreaper and Wilson, Journ. Soc. Chem. Ind., 1907,26, 667. 
 
 18 Dreaper and Wilson, Journ. Soc. Chem. Ind., 1910, 29, 
 
 1432. 
 
 19 Fahrion, Chem. Zeit., 1908, 32, 357. 
 
 20 Feilmann, Journ. Soc. Dyers and Col., 1909, 25, 158. 
 
 21 Freundlich and Losev, Zeit. physikal. Chem., 1907, 59 
 
 284. 
 
 22 Freundlich and Neumann, Zeit. Chem. Ind. Roll., 1908, 
 
 3,80. 
 
 23 Gee and Harrison, Trans. Faraday Soc., 1910, 
 
76 BIBLIOGRAPHY 
 
 2i Gelmo and Suida, Monatsh. Chem., 1905, 26, 855 ; 1906, 
 27, 1193. 
 
 25 Georgievics, G. von, Mitt. Tech. Gew. Museum in Wien, 
 
 4, 205 and 349. 
 
 26 Georgievics, von, Monatsh. Chem., 1894, 15, 705. 
 
 27 Georgievics, von, Zeit. angew. Chem., 1902, 16, [24], 574. 
 
 28 Georgievics, von, and Lowy, Monatsh. Chem., 1895, 16, 
 
 345. 
 
 29 Gillet, Kev. Gen. Mat. Col., 1900, 4, [6], 183. 
 
 30 Gillet, Rev. Gen. Mat. Col., 1900, 4, [46], 305. 
 
 31 Hantzsch and Osswald, Ber., 1900, 33, 303. 
 
 32 Hiibner, Chem. Soc. Trans., 1907, 91, 1057. 
 
 33 Hwass, Lehne's Farb-Zeit., 1890-91, 224. 
 
 34 Knecht, Journ. Soc. Dyers and Col., 1888, 104. 
 
 35 Knecht, Ber., 1888, 21, 1556. 
 
 36 Knecht, Ber., 1904, 37, 3479. 
 
 37 Knecht and Apple-yard, Journ. Soc. Dyers and Col., 
 
 1889, 71. 
 
 38 Knecht and Batey, Journ. Soc. Dyers and Col., 1909, 
 
 25, 194. 
 
 39 Krafft, Ber., 1899, 32, 1608. 
 
 40 Lewis, Phil. Mag., 1908, [vi], 15, 499. 
 
 41 Linder and Picton, Chem. Soc. Trans., 1905, 87, 1906. 
 
 42 Pelet and Grand, Rev. Gen. Mat. Col., 1907, 11, 225. 
 
 43 Pelet-Jolivet, Compt. rend., 1907, 145, 1182. 
 
 44 Pelet-Jolivet and Andersen, Compt. rend., 1907, 145, 1340. 
 
 45 Pelet-Jolivet and Wild, Zeit. Chem. Ind. Koll., 1908, 3, 
 
 174. 
 
 46 Perger, von, Lehne's Farb-Zeit., 1890-91, 356 and 371. 
 
 47 Pfeffer, Chem. Zeit., 10, 1259. 
 
 48 Prud'homme, Rev. Gen. Mat. Col., 1900, 4, [6], 189. 
 
 49 Prud'homme, Rev. Gen. Mat. Col., 1900, 4, [41], 156. 
 
 50 Rosenstiehl, Compt. rend., 1909, 149, 396. 
 
 61 Schaposchnikoff, Zeit. physikal. Chem., 1911, 78, 209. 
 52 Schmidt, Zeit. physikal. Chem., 1894, 15, 56. 
 63 Sisley, Rev. G&i. Mat. Col., 1900, 4, [6], 180. 
 54 Sisley, Bull. Soc. Chim., 1902, 27, 901. 
 r * Spohn, Dingl. polyt. J., 1893, 287, 210. 
 
BIBLIOGRAPHY 77 
 
 56 Suida, Z. Farb.-Ind., 1907, 6, 41 ; Z. physiol. 'Chexn., 1910, 
 
 68, 381. 
 
 57 Vignon, Compt. rend., 112, 487 and 623 ; 148, 1195 ; Bull. 
 
 Soc. Ind. Mulhouse, 1892, 563 ; 1893, 407. 
 :>8 Walker and Appleyard, Chem. Soc. Trans., 1896, 69, 1334. 
 ;>!) Weber, Journ. Soc. Chem. Ind., 1894, 13, 120. 
 
 00 Witt, Farberzeitung, 1890-91, 1. 
 
 01 Zacharias, Farberzeitung, 1901, 12, 149 and 165. 
 
 Readers who desire to study the subject in greater detail, 
 but who do not wish or have not the facilities to consult the 
 original papers, may be referred to the following books, where 
 they will find more extended accounts of the results embodied 
 in the above and other papers than were possible in a mono- 
 graph of the dimensions of the present work : 
 
 " The Chemistry and Physics of Dyeing," Dreaper. 
 
 " Die Theorie des Farbeprozesses," - L. Pelet-Jolivet. 
 
INDEX 
 
 ABSOKPTION of dyes, regulation 
 
 of rate of, 18 
 
 Absorption stage of dyeing, 69 
 Adhesion of dyes and fibres, 33, 
 
 71 
 
 Adjective dyes, 34 
 Adsorption theory of dyeing, 56 
 Amphoteric substances, 7, 35 
 Assistants, 18 
 Auxochrome groups, 13 
 
 BASTOSE, 5 
 
 CELLULOSE, 4, 51 
 Chemical theory of dyeing, 34 
 arguments in support of, 
 
 35, 45 
 
 objections to, 40, 43 
 Chromogen, 12 
 Chromophore, 12 
 Colloidal theory of dyeing, 62 
 Colloids, precipitation of, by 
 
 dyes, 63 
 protective influence of, on 
 
 benzopurpurin, 65 
 Colours, ice, 17 
 mineral, 16 
 sulphur, 14 
 
 Condition of dyes in solution, 20 
 Cotton, 4, 35 
 action of acids on, 4 
 action of alkalis on, 5 
 action of hypochlorites on, 5 
 
 DIALYSING power, influence of 
 molecular complexity on, 
 22 
 
 Dialysis of dye solutions, 21 
 anomalous behaviour of dye- 
 stuffs on, 27 
 Distribution of dyes between 
 
 fibre and dyebath, 54, 59 
 Dyeing, absorption stage of, 69 
 definition of, 1 
 development of, 1 
 divided nature of, process, 68 
 examples of, occurring in two 
 
 stages, 71 
 
 fixation stage of, 70 
 influence of state of fibre on 
 
 rate of, 57 
 non-reversibility of, process, 
 
 52,' 72 
 
 of inorganic substances, 58 
 theories of, 31, 34, 49, 56, 60, 
 
 62, 71 
 
 uses of assistants in, 18 
 Dye-bases, two modifications 
 
 of, 42 
 
 Dyes, acid, 14, 35, 46, 60, 62 
 basic, 13, 35, 45, 60, 62 
 developed, 16, 72 
 direct cotton, 14, 59, 62 
 distribution of, between 
 fibre and dyebath, 54, 59 
 mineral, 16 
 molecular complexity of, 22,29 
 
80 
 
 INDEX 
 
 Dyes, mordant, 15, 51, 72 
 sulphur, 14 
 vat, 15, 72 
 
 ELECTRICAL attraction between 
 fibres and dyes, 60, 63, 69 
 
 FIBRES, 3 
 
 absorption of dyes by in- 
 organic substances and 
 by, 58 
 nature of the reactive groups 
 
 in, 47 
 
 Fibroin, 8, 51 
 Fixation stage of dyeing, 70 
 
 GLANZSTOFF, 10 
 
 HYDROLYSIS, membrane, 28 
 of dyes in solution, 20, 23, 41 
 products of, of wool, 7, 36 
 
 Hypochlorites, action of, on 
 cotton, 5 
 
 IONISATION of dyes in solution, 
 20, 25, 29 
 
 JUTE, 3, 5 
 KERATIN, 6 
 
 LANUGINIC acid, 7, 36 
 Linen, 3, 4 
 
 MECHANICAL theory of dyeing, 
 31,34 
 
 objections to, 33 
 Membrane hydrolysis, 28 
 Mercerised cotton, 5 
 Molecular complexity,innuence 
 
 of, on dialysing power, 22 
 
 Mordant dyes, 15, 34 
 Mordants, 14, 15 
 
 OXYCELLULOSE, 5, 40 
 
 KATE of dyeing, 18, 57, 61 
 Reactive groups in fibres, 47 
 
 SERICIN, 8 
 Silk, 8 
 
 action of acids on, 9 
 action of alkalis on, 9 
 artificial, 9 
 viscose, 10 
 Solution, condition of dyes in, 
 
 20 
 
 theory of dyeing, 49 
 arguments in favour of, 50 
 objections to, 52 
 results favourable to, 59 
 another, 59 
 Surface action of fibres, 56, 69 
 
 TEMPERATURE, effect of, on 
 condition on dyes in solu- 
 tion, 27, 66 
 
 Theories of dyeing, 31, 34, 49, 
 56, 60, 62, 71 
 
 UNEXHAUSTED dyebath, prob- 
 lem of the, 44, 51, 72 
 
 Union between fibre and dye, 
 nature of, 35, 48 
 
 VISCOSE silk, 10 
 
 WOOL, 6 
 
 action of acids on, 8 
 action of alkalis on, 8 
 
 PRINTED BY OLIVER AND BOYD, EDINBURGH. 
 
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