THE MANUFACTURE OF LEATHERTHE MANUFACTURE OF LEATHER BY HUGH GARNER BENNETT, M.Sc., F.C.S. MEMBER OF THE INTERNATIONAL ASSOCIATION OF LEATHER TRADES’ chemist; assistant lecturer and demonstrator AT THE LEATHER INDUSTRIES DEPARTMENT OF THE UNIVERSITY OF LEEDS NEW YORK D. VAN NOSTRAND COMPANY 25 PARK PLACE 1915Printed in Great Britainft ¿15 £4 3 k- PREFACE The manufacture of leather is now a chemical industry, and in this volume it has been the author’s aim to treat it as such. The book is written for the general student of this branch of chemical technology, and an attempt has therefore been made on the one hand to keep the phraseology from being too scientific, in order that it may be intelligible to the intelligent workman, and on the other hand to keep it from being too technical in order that it may be of use to the pure chemist who wishes to apply his knowledge in the leather industry. On the technical side the author has endeavoured in his descriptions of practical methods to give prominence to those processes which are at present most widely used, but as the details of these processes vary so very much in different parts of the country and change so rapidly under the march of modern progress, it has been considered useless to enter into a minute description of them. At the most they may be taken as to some extent typical of what is employed in this country, and in no case is it claimed that the methods described will produce the best possible results. This is particularly true with regard to the finishing processes that are given. The reader who scans the later chapters of this book in the hope of discovering new and wonderful recipes for “ seasons,” “ finishes,” etc., will search in vain. For the most part the recipes mentioned in the text are old and well known, and have been selected by the author merely from the standpoint of what was typical. On the scientific side it has been necessary to assume that the reader has some acquaintance with chemistry and with the common analytical methods; hence if any obscurity is encountered, reference should be made to the standard textbooks. Only those analytical processes have been given which are suitable for works control, and which in the author’sVI PREFACE experience have been found to combine tolerable accuracy with rapidity of manipulation, regard being paid also to the convenience and probable resources of the works chemist. Some attempt has also been made to meet the needs of the candidates for the examinations of the City and Guilds of London Institute, and for the Degree and Diploma examinations of the Leather students of the Leeds University. The author wishes to acknowledge his indebtedness to many kind friends for assistance in preparation of this book. For illustrations of leather-working machinery the author it indebted to Messrs. T. Haley & Go., Messrs. Farrar and Young, Messrs. Huxham and Browns, Messrs. The Turner Co. Ltd., Messrs. The Mcenus Machine Works, Ltd., Messrs-E. Wilson & Son, and Messrs. The Mirrlees Watson Co., Ltd. The author’s thanks are also due to Messrs. West, Newman and Co. for the loan of their block of the warble fly, to Messrs. E. J. Richardson, of Newcastle, and Messrs. Walker, Ltd., of Litherland for the illustrations representing work in their factories, and to Mr. H. Brumwell and Mr. C. H. Crabtree for their assistance in preparing other illustrations. In proof revision the author has received invaluable assistance from Prof. H. R. Procter, and desires to acknowledge here his kindness and helpful criticism.CONTENTS ---+--- CHAPTER I Historical and Introductory Outline PAGE Origin of leather manufacture ........ 1 Historical development of different tannages........2 The changes of this last century....................3 Brief outline of chief manufacturing processes......4—5 CHAPTER II The Nature of Skin Its physical structure......................6 Nature of epidermis.........................6 Growth of hair ........... 7 Nature of corium or true shin...............8 Microscopic examination of skin structure ..... 9 Its chemical composition ......... 10 General properties of proteids.............11 Gelatin and its decomposition.............. 16 CHAPTEE III Fermentation Simplest forms of life.....................19 Organised and unorganised ferments.........20 Yeasts.............................................. 21 Moulds and bacteria........................22 Putrefaction......................................... 24 Fermentations in leather manufacture........25Vili CONTENTS CHAPTER IV Hides and Skins Curing hides by drying . „ „ salting . „ „ dry salting Criteria for selection of hides Warbles .... Sources of supply of hides E. I. kips .... Sheep skins Goat skins Seal skins PAGE . 27 . 28 . 29 . 29 . 80 . 88 . 36 . 36 . 39 . 40 CHAPTER Y Water Nature of hardness ....... Softening by Clark process . 42 „ ,, Porter-Clark process .... . 42 ,, ,, Archbutt and Deeley process . 43 ,, „ caustic soda process .... . 43 Water analysis . 44 Suitability of waters for tanners5 purposes . 46 CHAPTER VI Soaking Mature of the process............................49 Danger of bacterial action.......................50 Mechanical treatment in drums, stocks, etc.......50 Chemical assistance in soaking ....... 52 Use of caustic soda and sodium sulphide. . . . .53 Practical treatment of market hides..............54 Soaking salted hides.............................54 Dried hides and dry-salted hides.................55 E. I. kips.......................................55 Calf skins ........... 55 Goat skins.......................................55 Seal skins..................................... 56 Chemical control of soaking . . • . ... 56CONTENTS IX CHAPTER YII Unh airing PAGE Depilation by sweating......................................57 Cold-sweat system...........................................57 Warm-sweat system ..........................................58 Depilation by lime..........................................58 Mode of action of lime......................................61 Use of sodium sulphide as depilatant........................62 „ arsenic sulphides in limes.............................63 „ caustic soda in limes..................................64 Bacterial action in limes...................................64 Action of ammonia...........................................65 Principles of liming........................................65 The mechanical operations of the lime-yard: hauling, unhairing, fleshing, rounding, etc.................................66 Unhairing machines..........................................69 Fleshing machines...........................................73 Systems of liming......................................... 74 Three-pit system............................................75 One-pit system . ......... 78 Pullman-Payne system ......... 79 Suspension systems..........................................80 Practical methods of liming.................................80 Hides for sole leather......................................80 Buffalo method..............................................81 Quick processes.............................................82 Hides for belting leather................................. 83 ,, harness leather......................................83 ,, dressing leather.....................................84 E. I. kips............................................ . 84 Calf skins..................................................84 Fellmongering sheep and lamb skins..........................85 Kid and lamb skins . 88 Goat skins..................................................89 Seal skins..................................................90 Chemical control of lime-yard...............................90 Principles of deliining Puering Bating CHAPTER VIII Deliming 92 93 97X CONTENTS CHAPTER VIII.—continued, PAGE Drenching........................................................98 Chemical deliming agents—boric acid, lactic acid, etc. . . . 100 Practical methods of deliming...................................102 Sole butts and offal............................................102 Strap butts.....................................................108 Harness backs ..................................................108 Dressing hides..................................................104 E. I. kips......................................................105 Calf skins......................................................105 Sheep skins.................................................... 107 Degreasing......................................................107 Pickling...................................................... 108 Defects in Sheep skins..........................................110 Kid skins..............................................• .111 Goat skins......................................................Ill Seal skins......................................................112 Chemical control of deliming processes..........................112 CHAPTER IX The Tannins Definition and properties of tannins . .... 113 Methods of purification of tannins . . . . . . .114 Classification................................... . . .115 Pyrogallol tannins and associated substances....................116 Gallic acid........................... .... 116 Ellagic acid (bloom).................................s . .117 Gallotannic acid................................................117 Ellagitannic acid...............................................119 Catechol tannins and associated substances......................119 Catechins (whites)..............................................120 Phlobaphenes (reds).............................................121 Catechutannic acid............................................12 L Quercitannic acid...............................................121 Quebrachotannic acid............................................121 Flavones........................................................121 CHAPTER X The Vegetable Tanning Materials Sources of tannin in plants................................... 123 Pyrogallol tanning materials....................................124 Valonia.........................................................124CONTENTS xi CHAPTER X.—continued PAGE Myrobalans . 126 Divi-divi . 129 Algarobilla ......... . 129 Sumach .......... . 129 Pistacia . 131 Oak wood extract ........ . 132 Chestnut extract ........ . 182 Willow bark ......... . 132 Knoppern ......... . 133 Turkish galls ......... . 133 Catechol tanning materials ...... . 133 Oak bark .......... . 133 Gambier .......... . 134 Quebracho and its extracts ...... . 135 Hemlock bark ......... . 136 Mangrove . 137 Mimosa .......... . 137 Mallet . • 138 Birch . . 139 Larch ........... . 139 Pine ,, . • 139 Canaigre .......... • 139 Cape sumach ......... • 139 CHAPTER XI \ The Analysis of Tanning Materials Lowenthal method for used liquors, spent tans, etc. # 140 Indirect gravimetric method ...... . 142 Sampling from bulk • 142 Preparation of infusion ....... * 143 Extraction for analysis . 144 Filtration • 145 Detannisation . 146 Filter bell method ........ • 147 Procter-Bennett shake method ..... • 148 Standardisation of hide powder ..... • 149 Evaporation and drying • 149 Statement of results . 150 I.A.L.T.C. official method . 151 Measurement of colour by tintometer and by pelt test . . 154 Barkometer • 155 Beaumé degrees * 156XU CONTENTS CHAPTER XII The Preparation of the Tanning Liquors PAGE Grinding ............ 157 Grinding mills...........................................158 Disintegrators...........................................158 Shredding mills . . . . . . . . . .159 Leaching . . . . . . . . . . . 160 Pumping system .......... 161 Press leach system .......... 162 Heating leach liquors . . . . . . . . .163 Extraction under pressure...............164 Sprinkler leaches.......................164 Extract manufacture.....................164 CHAPTER XIII The Principles of Vegetable Tanning Chemical and physical actions both probably involved . . . 168 Gradual increase in strength of liquors.168 Astringent and mellow liquors ........ 169 Free acid (sourness) in yard liquors . . . . . .170 Suspenders ........... 171 Paddles ............ 171 Handlers—dusters and floaters . . . . . . .172 How a round of handlers may be worked . . . . .173 Working liquor packs and dusters . . , . . . .176 Layers..................................177 Principles of drum tanning..............177 CHAPTER XIV The Tannage of Sole Leather The oak-bark tannage.......................'. . 179 The West of England tannage.............181 The modern mixed tannage................183 Working the layers...................185 Working the liquors..................188 American sole leather ..................189 Union tannage....................... 191 Drum tannages........................192CONTENTS CHAPTER XV The Tannage of Belting, Harness and Upper Leather, Belting leather........................................ Harness leather........................................ Dressing hides for upper leather....................... Dressing leather liquors ........ Dressing hides for legging leather..................... E. I. kips ............................................ Calf skins for upper leather........................... Hides for bag, portmanteau and bridle work, etc. . Picking band butts .... .... CHAPTER XVI The Tannage of Moroccos and Light Leathers Goat skins for moroccos.................... Seal skins for moroccos.................... Calf skins for bookbinding and fancy work . Sheep skins for basils, skivers, roans and roller leather, etc. . CHAPTER XVII The Tannage of Chrome Leather Historical development................. Nature of the mineral tannages ..... The two-bath process................... The chroming bath and its modifications.... The reducing bath...................... The reactions given by Eitner and Stiasny The one-bath process .....*. Principles of one-bath tannage ..... Mode of penetration of chrome salts .... Basic chrome liquors................... Chrome alum liquor..................... Glucose liquor ......... General qualities of chrome leather.... The tannage of calfskins and hides for “ box calf,” etc. . Goat skins for glacé................... Sheep skins for imitation glacé........ Chrome chamois ........ Picking band butts..................... Motor butts, strap butts, harness backs, sole butts, etc. Analytical control of the chrome tannages . xiii Etc. PAGE . 194 . 195 . 197 . 199 . 200 . 200 . 201 . 202 . 203 . 204 . 206 . 207 . 208 . 210 . 211 . 213 . 213 . 216 . 217 . 219 . 221 . 222 . 223 . 223 . 224 . 225 . 227 . 230 . 232 . 233 . 234 . 285 . 236XIV CONTENTS CHAPTER XVIII Alum and Combination Tannages PAC Principles of tawing.........................................2i 9 Combination tannages.........................................240 Lamb and kid skins for glove kid.............................2^1 Calf skins for calf kid......................................24 Goat skins for dongola.......................................24 CHAPTER XIX Fat, Oil and Aldehyde Tannages Fat tannages.............................................245 Helvetia and Crown leathers..................................246 Chamois leather .............................................247 Buff leather.............................................248 Buck leather.................................................249 Formaldehyde leather.....................................250 CHAPTER XX The Drying of Leather Principles of drying . . . . . . . . . .251 Wet and dry bulb thermometers ....... 252 Weather drying .......... 258 Screw fan ventilation........................................254 Centrifugal fan ventilation ........ 255 Downward ventilation ......... 255 Turret drying.......................... 255 Stove drying ............................................256 Samming..................................................256 Methods employed for heavy and light leathers .... 256 CHAPTER XXI The Finishing of Sole Leather Butts from a mixed tannage for “ scoured bends ” . . . . 25F Scouring.....................................................25* Vatting......................................... 261 Shedwork, oiling and drying.................................26S Striking out, pinning, etc...............................268 Rolling “ on ” and “ off ”..............................268 Brushing ........................................... . 266 Drum scouring offal.........................................267 Butts from a West of England tannage, for “ bloomed butts ” . 26b Butts from an oak bark tannage..............................269 American hemlock sides.................................. 269 Drum-tanned sole leather ...... , 271CONTENTS xv CHAPTER XXII The Currying and Finishing of Dressing Leather PAGE Nature of currying........................................274 ^paking.......................................... . .274 ¿Shaving, by hand and machine..............................274 ¿Splitting, by union, vibrating knife and band knife machines . . 277 Stoning or “jacking,” skiving, etc........................281 Scouring, by hand and machine ....... 282 Sumaching.................................................285 Principles of stuffing ...................................285 Hand stuffing ............................................288 Drum stuffing.............................................288 Burning in................................................292 Spueing of curried leathers...............................293 Currying and finishing strap butts........................295 'Black harness............................................297 Brown harness.............................................300 Waxed kip butts...........................................301 Waxed calf................................................308 Waxed splits..............................................303 Waxed shoe butts..........................................304 Army grains.............................................. 304 Satin leather.............................................304 Memel butts ........... 306 Levant....................................................307 Glove hide bellies........................................307 Lining leather ...........................................308 Bag hides.................................................308 Bridle leather ...........................................308 Kip bellies for straps....................................310 Legging leathers........................................ 310 Picking band butts........................................310 CHAPTER XXIII Leather Dyeing Introduction of coal tar colours....................................312 Classification of artificial dyestuffs into basic and acid colours . 312 Basic colours ......................................................312 Acid colours........................................................314 Commercial dyestuffs................................................316 The natural dyestuffs, logwood, etc. ...... '817 Mordants............................................................319XVI CONTENTS CHAPTER XXIII.-—continued. PAGE Bleaching........................................................320 Colour shades....................................................321 Methods of dyeing................................................324 Tray method......................................................324 Paddle method.................................................327 Drum method...................................................328 Brush dyeing or staining......................................329 Theory of dyeing.................................................330 CHAPTER XXIV The Finishing of Light Leathers Preparation for dyeing,—stripping, souring, sumaching, striking out, etc.................................................332 Graining or boarding..........................................336 Embossing and printing......................................... 338 Seasoning.....................................................340 Glazing ............................... 341 Fluffing......................................................343 Goat skins for hard-grain moroccos............................344 ,, ,, levant, blacks and colours......................345 Seal skin moroccos............................................346 Basils for linings, slipper, legging and gaiter leather . . . 348 Skivers for paste grain, straight grain, hat leathers, etc. . . 349 Roans............................................................350 Roller leather................................................350 E. I. sheep and goat for moroccos, linings, roller leather, glacé, bronzed leather, etc.............................................351 Hide splits for common upholstery................................354 CHAPTER XXV i The Finishing of Chrome Leather Neutralisation........................ ... 355 Fat-liquoring..................................................356 Staking........................................................358 Finishing box calf.............................................360 Box calf imitations............................................361 Glacé calf, dull calf..........................................362 Waterproof chrome upper leather ...............................362 Willow calf....................................................368 Glacé kid .....................................................364CONTENTS xvü CHAPTER XXV.—continued. Dull kid coloured glacé ...... Imitation glacé.................. Chrome chamois ....... Picking band butts ....... Harness backs ........ Strap, motor and sole butts .... PAGE 365 366 367 368 368 369 CHAPTER XXVI The Finishing of the Alum- and Combination-tanned Leathers Glove kid...................................... . 371 Calf kid....................................................373 Glazed dongola goat.........................................373 Combination chrome glace ........ 374 Semichrome box and willow calf ....... 375 CHAPTER XXVII The Finishing of Fat and Oil-tanned Leathers Helvetia and Crown leathers ...... . 376 Chamois for wash-leather....... . 376 Chamois for gloving ........ . 376 Bleaching chamois . 376 Dyeing chamois ........ . 378 Buff, buck and formaldehyde leathers .... . 379 CHAPTER XXVIII Japanned and Enamelled Leathers Nature of japanning and enamelling . 380 Preparatory processes ....... . 380 Manufacture of the japan ....... . 382 Application of the japan . 383 CHAPTER XXIX The Dressing of Wool Rugs Principles of rug dressing with sheepskins.385 Dressing blacks............................387 Dressing whites .......... 391 M.L. bXV111 CONTENTS CHAPTER XXIX.— continued. PAGE ..............................................391 .........................................392 ..............................................393 ..............................................394 ..............................................395 ..............................................396 ..............................................396 ..............................................396 ..............................................396 CHAPTER XXX The Analysis of Leathek Estimation of hide substance..........397 ,, ,, moisture and fat ....... 400 ,, ,, water soluble matter ....... 401 ,, ,, glucose.....................402 Estimation of mineral ash ........ 403 „ ., free mineral acid ....... 404 Typical analyses .......... 405 INDEX ........................................... 409-420 Dressing greys . „ browns „ walnuts „ crimsons ,, blues . „ greens China sheep sldns Angora goat skins Rabbit skinsLIST OF ILLUSTRATIONS VIG. PAGE 1. Vertical section of calfskin. ..... . 7 2. Structure of epidermis..................... 8 8. Cowhide, rough-tanned ........ 9 4. Cowhide, rolled .......... 10 5. Pigskin........................................11 6. E. I. goatskin .......... 12 7. Calfskin ........... 13 8. Welsh sheepskin. ...... 14 9. Yeast cells ........... 21 10. Mould (Pénicillium glaucum) ....... 22 11. Bacteria.......................................23 12. Warble fly (Hypoderma bovis) ....... 30 13. Tail-weights...................................31 14. Bounding a hide................................32 15. Stocks .......... 51 16. Scotch tumbler .......... 51 17. Latticed drum .................................52 18. Limeyard (Messrs. Walker, Ltd., Litherland) .... 60 19. XJnhairing by hand labour......................62 20. Eleshing by hand labour........ . 64 21. Beamhouse tools . ............................67 22. Drum beamhouse machine. . e e . , . . 68 23. The Conus unhairing machine ....... 69 24. The Leidgen unhairing machine................................70 25. Whitney unhairing and fleshing machine . . . . .71 26. Serial table unhairing machine...............................72 27. The “ Continua ” fleshing, unhairing and setting out machine......................................................73 28. The “ Cylindra” fleshing, slating, and setting out machine . 74 29. Wilson fleshing machine . . . . . . . .75 30. Bubber roll fleshing machine ....... 76 31. Scudding with a hand knife . . . . . , . 93 32. Puering paddle .......... 94 33. Smyrna and Greek valonia (camata) . . . . . .125 34. Myrobalans................................ , .127 35. Sicilian sumach (Rhus coriaria) . . . . . . .129 35a. Photo-micrographs of sumach, its adulterants and substitutes facing 130XX LIST OF IILLUSTEATIONS FIO. 86. American sumach (Rhus glabra) • 9 PAGE 131 37. Common oak ( Quercus robur) , , 133 38. Hemlock firs ( Pinus canadensis) .... » • 136 39. Golden Wattle (Acacia pycnantha) .... • • 137 40. Green Wattle (Acacia dealbata, variety of A. mollissima) . . 137 41. Black Wattle (Acacia binervata) .... . . 138 42. Students’ laboratory, Leather Industries Department of the University of Leeds .... . 141 43. Procter extractor ....... 144 44. Filtration apparatus with Berkefeld candle (upward filtration). 146 45. Filtration apparatus wit Berkefeld candle (downward filtration) ........ , . 146 46. Procter’s filter-bell apparatus # « 147 47. Shake machine for the Procter-Bennett method of tannin analysis ......... • • 148 48. Tintometer. ........ 154 49. Barkometer. ........ 155 50. Bark breaker ........ • 157 51. Grinding mill ........ 158 52. Myrobalans husking machine ..... . 159 53. Disintegrator ........ . 159 54. Battery of press leaches . . 161 55. Y ary an evaporator ...... . , 165 56. Tan yard (Messrs. Walker, Ltd., Litherland) . . . 184 57. Tanning drum ...... . 192 58. Double paddle for light leather tanning . , 205 59. Tanning paddle ........ r 226 60. Screw fan .... .... • . 254 61. Samming machine 257 62. Wilson six-arm butt scourer « . 260 63. Wilson butt striker # , 261 64. Wilson’s single-bed butt roller .... • « 264 65. Wilson’s twin-bed butt roller « ' 265 66. Brushing machine ....... • 266 67. Priestman striker . « 267 68. Offal roller 268 69. Pendulum roller ....... 270 70. Portion of leather warehouse (Messrs. Walker, Ltd., Lither- land) ......... 271 71. Loads of leather leaving tannery (Messrs. Walker Ltd., Litherland) • 272 72. Hand-shaving 275 73. Shaving machine ....... . . 276 74. Union splitting machine . 278 75. Band-knife splitting machine . . 280 76. Hand-scouring 282LIST OP ILLUSTRATIONS xxi FIG. PAGE 77. Burdon scouring machine (T. Haley & Co.) . . . .283 78. Circulum scouring and butt-striking machine . . . 284 79. Jackson scouring machine ....... 284 80. Scouring machine....................................285 81. Hand-setting .......... 286 82. Curriers’ tools ......... 287 83. Hot air stuffing drum (T. Haley & Co.)..............289 84. Hot air stuffing drum (Farrar & Young) .... 290 85. Hot air stuffing drum (Farrar & Young) .... 291 86. Currying shop, Leather Industries Department of the Uni- versity of Leeds ......... 294 87. Stoning jack (Turner & Co.) ....... 299 88. Pendulum whitening machine (Turner & Co.) . . . 305 89. Areameter (Farrar & Young) ....... 309 90. The chromatic circle ........ 322 91. Striking out after dyeing (Messrs. E. & J. Richardson, New- castle) ........... 325 92. Polygonal tumbler...................................329 93. Single table putting out and striking out machine (Turner & Co.)..............................................333 94. The “Tabula” setting out and scouring machine . . . 334 95. Serial table setting machine (Turner & Co.) .... 335 96. Morocco finishing shop (Messrs. E. & J. Richardson, New- castle) ........... 336 97. Embossing machine........................... . 337 98. The 4 ‘ Altera ” embossing machine.............338 99. Printing machine ......... 339 100. Graining machine ......... 340 101. Level bed glazing machine ..... . 341 102. Glassing machine...................................342 103. Fluffing machine (Farrar & Young) ..... 343 104. Currying seal skins (Messrs. E. & J. Richardson, Newcastle). 347 105. Slocomb staking machine........................ . 359 106. Bower’s chrome glazer (Huxham & Brown’s) .... 361 107. Preparing seal skins for enamelling (Messrs. E. & J. Richard- son, Newcastle)...................................381 108. Kjeldahl apparatus.................................398 109. Extraction of fat from leather by means of Soxhlet apparatus 401 M.L. cTHE MANUFACTURE OF LEATHER CHAPTER I HISTORICAL AND INTRODUCTORY OUTLINE It is always a fascinating occupation to trace the history id to note the origin of a great industry, to study the gradual development in the processes employed in connection with it, aud to notice the various outstanding steps in the evolution of more efficient methods. In connection with the leather industry, however, we are met at the outset with the difficulty tl at the origin of this most interesting occupation is altogether prehistoric, there being little doubt, in fact, that this practice was one of the earliest attempts of man to make use of his available materials. The treatment of animal skins to fit them for useful purposes is a practice which seems to exist wherever mankind has been found. In China the manufacture and use of leather dates back to the remotest eras of its very ancient history, in the more recently discovered continents of America and Australia the curing of skins was found to be well known and widely practised, and even in the most ancient of the Western civilisations the preparation of some kind of leather seems always to have had a place in human skill and knowledge. The Egyptians, who formed the first civilised nation of history, have left considerable evidence, both as relics and drawings, of their very early acquaintance with the dressing of skins, including vegetable tannage, the dyeing, painting, gilding, and embossing of lighter leathers, and the manufacture of shoes, straps, aprons, and other useful articles from the leathers so prepared. The methods of preparing such leathers were also well known to the early M.L. B2 THE MANUFACTURE OF LEATHER Babylonian and Persian civilisations, and at a later period were also practised by the Greeks. Under the great Homan World State the manufacture of leather, like many other useful industries, made considerable headway both in the various processes employed and the extent of their use. Pliny mentions that gall-nuts, bark, and sumac were used as tanning materials. The fall of the Roman Empire brought about a decided check on the development of all the arts and industries, including that of leather manufacture, but a good deal of the existing knowledge continued to spread, and gradually reached the Hun, the Russian, and the Teuton. The Moorish invasion of Spain also, in the eighth century, introduced into Europe again the manufacture of the more artistic leathers. Erom Cordovan or Spanish leather, famous in the eleventh century, our present “ morocco ” was evolved, and in the fourteenth century leather dyeing and embossing reached considerable perfection in Western Europe. Although it is certain that the process of vegetable tanning is exceedingly ancient, it is almost as certain that this was not the most primitive method, and that the curing of skins by smoke, by treatment with oils and fats, and by merely sundrying, are methods even more ancient. Processes such as these, still in use in many of the less civilised parts of the world, leave us in little doubt of the origin of leather manufacture, in spite of the absence of any written historical records. Leather, indeed, is often mentioned as one of the earliest materials for writing upon, though there is considerable doubt whether such leather was in any sense “ tanned,” and in many cases there is no doubt at all that the material referred to was very closely similar to our present parchment. The manufacture of u tawed ” or alumed leather was introduced at a much later date than the vegetable tannages, and was for a considerable period confined to a few localities in which the tawing materials were readily obtainable. It will be readily seen from the preceding historical outline that the manufacture of leather has been for many ages a purely empirical industry, and that improvements in the methods employed have been in consequence exceedingly few, and slowly introduced into general use. The last century,HISTORICAL AND INTRODUCTORY OUTLINE 3 however, has witnessed not only vast developments in pure science, but also the commencement of a new era in the history of the human industries, and just as science has applied itself to the manufacture of soaps, dyes, metals, etc., so also has the manufacture of leather become a branch of chemical technology. This transformation from an empirical art to a scientific industry has been, of course, slow and gradual, and confined almost to the last half-century, not being even yet by any means complete. It is fully a hundred years since it was first recognised by Sir Humphrey Davy and others that the process of tannage was a chemical matter, but the real application of chemistry to leather manufacture began much later, the first definitely scientific advances in the industry being mainly of a mechanical character, involving the introduction of machinery and other labour-saving appliances. The application of chemical science to the manufacture of leather has chiefly resulted in the introduction of new materials, in greater efficiency of working and economy of production, in quick process, in the analytical treatment of both raw materials and finished goods, and in the more accurate control of the various processes, rather than in any outstanding improvements in the quality of the goods made by the time-worn methods, and it is also responsible for a new class of leather altogether, viz., chrome leather, which has already taken its place in the front rank of present-day leathers. The writer has thought it very desirable that some brief outline should also be given in this chapter of the various stages and processes in the manufacture of the leathers which are spoken of in greater detail in the later parts of this volume. It has been stated already that “ leather ” consists of the animal skin prepared for application to useful purposes by treatment with various processes of empirical origin. All these processes have for their object the conversion of the readily putrescible skin into an insoluble and more lasting material, one which is not only less subject to putrefaction and decay, but which possesses also considerable strength and pliability. It will be readily understood also that the nature of the leather produced is varied very considerably by the b24 THE MANUFACTURE OF LEATHER character of the processes by which it is produced, and that these processes are varied deliberately according to the purpose for which the leather is required. The first process is generally that of “soaking” (Chap. VI.), which involves the cleansing of the skin from blood, dung, dirt, and temporary preservatives by treatment with water, any previous dehydration being now counteracted by a partial swelling. The next process, except for the dressing of furs and some other skins, is that of “ unhairing ” (Chap. VII.), by which the hair is removed from the skin by the root. This is brought about by first treating the goods with the hydrates and sulphides of calcium and sodium, milk of lime being most generally used, and causing in this way the destruction of the epidermis of the skin, including the hair root, and the “ plumping ” of the dermis or true skin, which brings about a separation of the hide fibres. A similar result can be obtained by “ sweating,” in which process the goods are kept moist and warm in an enclosed chamber, bacterial influence being in this case a more prominent factor. Whichever method has been used, however, both the epidermis and loosened hair are removed by scraping over the “beam” with a blunt two-handled knife. Any animal fat or flesh is removed also at this stage with a sharp knife or machine, and any other necessary trimming brought about. The “ deliming ” (Chap. VIII.) which follows varies largely in character according to the class of leather required. Heavy leathers are either treated with suitable dilute acids, or merely washed in water, whereas the finer leathers undergo a little-understood fermentative action, involving a more thorough removal of the lime and a complete relaxation or “ pulling down ” of the plumped skin, which treatment causes the production of a much softer leather. When an infusion of the dung of fowls is used for this purpose, the process is called “ bating,” but if dog-dung is used, as for the lightest leathers, the process is termed “ puering.” Either of these processes may be followed by “ drenching,” which is an immersion in a bran infusion, the organic acids formed by its fermentation completing the neutralisation of the lime, and evenHISTORICAL AND INTRODUCTORY OUTLINE o swelling the skins slightly; the bran itself acting also as a mechanical cleanser. The “ tanning” of the hide or skin follows immediately after the deliming processes, and in the case of the vegetable tannages (Chaps. IX.—XYI.) consists in soaking the goods in infusions of the vegetable tanning materials, of gradually increasing strength in “ tannin,” which is a name given to all those astringent phenolic colloidal bodies which bring about the desired conversion of “ pelt ” into “ leather.” This treatment extends over some months for the heavier leathers, and for several weeks in the case of light and dressing leathers. The finishing processes (Chaps. XX.—XXIY.) nearly always involve the use of a certain amount of oil or fat, and, with the exception of sole and fancy leathers, the process of u currying ” (Chap. XXII.) is undergone, in which the goods are stuffed with fatty matters, thus causing both pliability and impermeability to water. Many of the lighter leathers are dyed (Chap. XXIIIJ and then undergo special glazing and finishing processes. The mineral tannages (Chaps. XVII. and XVIII.) include, first of all, the “ tawing ” process, which consists in treating the goods with a mixture of alum and salt and subsequent finishing with oily filling materials. The chrome tannages also fall in this division, the tanning of the pelt being accomplished either by drumming with a “ basic ” chromic salt, as in the one-bath process, or by treatment first with chromic acid and then a reducing agent, as in the two-bath process. Chrome leather made by either of these processes is “ neutralised ” by the action of some weak alkali, and subsequently dyed and “ fat-liquored,” this last operation being a treatment with an emulsion of soap and oil. The aldehyde tannages (Chap. XIX.) include “ chamoising,” which is a treatment of the pelt with marine oils, the aldehydes formed by their oxidation causing a true tannage of the skin; the manufacture of fat tanned leathers by drumming the pelt with soft fats and greases, a less perfect tannage taking place, but a more complete coating of the hide fibres with oxidised oil products ; and also the direct use of aldehydes, as in the manufacture of formaldehyde leathers by Messrs. Pullman’s patent process.CHAPTER II THE NATURE OF SKIN Its Physical Structure.—All animals have an external covering for their flesh and fat which goes by the name of skin, and which forms the raw material for the manufacture of leather. The following description is primarily intended to set forth the characteristics of the ordinary ox hide, but the general structure of the skins of all mammals is so very similar that it applies almost equally well to them all. Although the skin is made up of many parts, it is readily divided into two distinct layers, the epidermis and the corium, which are both distinct in their functions and different in their structure. The Epidermis, epithelium, or cuticle, forms the outer layer of the skin and is much thinner than the corium, over which it lies. The inner portion near the corium is called the rete Mcdjjighi, and consists of minute nucleated cells, which increase rapidly by division into two separate nuclei when sufficiently grown. The multiplication of the cells in this way causes the older ones to be pushed nearer the outer surface, and as they approach this they gradually become flat and dry, and are finally transformed into a layer of laminated scales. This forms the outmost horny layer, the flattened scales of which are continually worn away and replaced from the rete Malpighi. From the epidermis the hair, horns, hoofs, and similar structures are evolved. The hair, which may be regarded as a type of these formations, sinks for some distance into the corium, but does not properly penetrate it, being completely surrounded by an epidermally constructed sheath, the inner layer of which grows outwards with the hair and forms its cuticle, being scaly in its outmost parts, like the epidermis of the skin. The inner part of the hair consists of spindle-shaped and pigmented fibres, which become soft and round at the hair root, and expand into a bulb. This bulbTHE NATURE OF SKIN 7 has within it the hair papilla, which is a projecting portion of the corium containing the blood-vessels by which the hair is nourished. Young hairs are developed from a knob of the epidermis which projects into the corium, or from a thickening glands. at the bottom of the epidermal sheath of an old hair, and being thus more deeply seated, are with difficulty removed in dépilation. Every hair possesses an involuntary muscle known as the arrector or erector pili, which passes from the epidermis to the hair bulb, and contracts under the influence8 THE MANUFACTURE OF LEATHER of fear or cold, causing the hair to assume a more perpendicular position. The corium, cutis, or derma, which forms the greater part of the skin, has a totally different structure, being chiefly composed of white connective tissue, which is fibrous and interlaced, the fibres consisting of extremely minute fibrils cemented together by a gelatinous substance. In between the connective tissue fibres, besides nerves and blood-vessels, there is a Fig. 2.—Structure of epidermis. A, epidermis; b, corium; c, hair with epidermal sheath; D, arrector; E, development of new hair ; e, duct of sudoriferous gland. variable amount of interjibrillar substance, which gradually changes into fibre, and also a quantity of minute elastic fibres, which possess a yellow colour, and are of a muscular character. The connective tissue fibres are the least compact in the central part of the corium, and become much more closely interwoven as the epidermis is approached. This closely interlaced portion is known as the pars papillaris, and is that part of the corium which surrounds the hair and its epidermal sheath, and which contains the arrector and the sebaceous or fat glands. These glands have a cellular or epithelial structureTHE NATURE OF SKLN 9 each cell embracing a nucleus and a considerable proportion of fatty matter, which passes through minute ducts into the hair sheath near the skin surface, and lubricates the hair. In the looser tissue near the hair roots are the sudoriferous or sweat glands, excretory organs that consist of connective tissue lined with an epidermal layer the cells of which secrete the perspiration. The ducts pass right through the epidermis to the skin surface, the opening being often at the orifice of the hair sheath. Between the corium and the epidermis is the hyaline or glassy layer, essential for the production of an even-coloured leather and easily damaged by either mechanical or bacterial action. This layer, together with the pars papillaris, constitutes the grain of the leather, the characteristic pattern of which forms a ready method for distinguishing the different classes of hides and skins (see Figs. 3—8). That part of the corium nearest the flesh also pos- Fig. 3—Cowhide, rough tanned, sesses a more compact structure than the centre, but is connected to the flesh proper by a loose network of adipose tissue, which is composed of hide fibres embracing masses of protoplasmic nuclei, each of which has secreted large quantities of fatty matter. These nuclei are also found to a considerable extent in the middle and lower portion of the corium tissue. For the microscopic examination of the skin structure small and perfectly fresh pieces of hide should be hardened by soaking in several changes of absolute alcohol, and exceedingly thin sections cut by hand with a razor in the plane of the hair roots. The section may be stained by a10 THE MANUFACTURE OF LEATHER twenty-four hours’ immersion in a diluted picrocarmine solution, which will give a red colour to the connective tissue and cell nuclei, and a yellow colour to the epidermal cells, gland cells, and elastic fibres. It may be then dehydrated in alcohol, mounted in a xylene solution of Canada balsam, and examined by transmitted light under a low power (1-inch objective). Temporary slides are best made by immersing the sections in concentrated ammonium sulphate solution, which shows up the white connective tissue fibres, or in a mixture of equal volumes of glacial acetic acid, glycerine, and water, which shows up the fat and sweat glands, elastic fibres, and hair bulbs, the connective tissue being rendered transparent. For high-power observations the hide pieces should be first hardened by immersion for a week in a solution of osmie and chromic acids and afterwards dehydrated with alcohol. 'With this treatment Fig. 4.—Cowhide, rolled. the fatty matter of the glands and adipose tissue appears black. The structure of the grain pattern of leathers may be readily studied under a low power with a good reflected light. Embossed leathers should be wet back and stretched before examination. Its Chemical Composition.—Although such a variety of constituents go to form the animal skin, they all belong to one great class of organic bodies, which are known by the general name of proteids, all of which are products of animal or plant life and possess an extremely complex chemical constitution. The ultimate analysis of these bodies indicates the same general constitution, and the following figures may be takenTHE NATURE OR SKIN 11 to represent roughly the lowest and highest limits in composition :— Element. Carbon Hydrogen . Nitrogen . Oxygen Sulphur Per cent. . 49—55 . 6-4—7'3 . 15—19 . 17—26 . -3—2-4 The variations noticed in the analyses of any one proteid and the certainty of excessively high molecular weights 1 make it evident that to calculate any empirical formulae would not only be entirely hypo-thetical, but altogether useless. The proteids resemble one another in their behaviour with certain reagents, one of which is “Millon’s reagent,” a solution of mercuric nitrate containing nitrous Fig. 5.—Pigskin, fumes, a curdy pink precipitate or red colour being obtained on warming the proteid with it. This reaction has been traced to a definite group in the proteid molecule due to tyrosine.2 A second characteristic test for proteids is the “ biuret3 reaction,” which is a pink colour obtained by adding a trace of cupric sulphate and excess of caustic soda, and which is due either to the biuret radicle or to similar diacidamide groups.4 1 Paal, Ber., 1892, 1202. 2 p.-oxy. a amidopheiiyl propionic acid. 8 NH (CONHa>2. 4 Malonamide, oxamide, and glycine amide -will give the biuret reaction.12 THE MANUFACTURE OE LEATHER Another general reaction is the precipitation of the so-called “ xanthoproteic acid ” by boiling with dilute nitric acid (1 : 2) for some days, a yellow floceulent precipitate being obtained which dissolves in ammonia and caustic alkalies with a brown or orange-red colour. The proteids are also extremely similar in their ultimate products of hydrolytic decomposition, breaking up first into simpler molecular complexes known as peptones, and afterwards into still simpler constituents, the chemical constitution of which has been thoroughly investigated in recent years.1 The peptonisation may be brought about by the prolonged action of hot water, but more quickly by the use of dilute acids or alkalies, by the hydrolytic ferments of the gastric and pancreatic juices, and by the action of the bacterial zymases, as in putrefactive fermentation. The more complete decomposition is accomplished by continued putrefaction and also by the action Fio. o.—E. I. goatskin. of concentrated alka- lies and acids. The most satisfactory methods for the isolation of these simpler products are due to Kossel and E. Fischer. The former hydrolyses the proteid with hydrochloric or sulphuric acid, which is afterwards removed by cuprous oxide or baryta, and precipitates the cystine,2 the xanthine bases,3 4 and indol derivatives4, with mercuric sulphate, mercury being removed 1 Cp. starches, dextrins, and carbohydrates. 2 An anhydride oí thioseriae. 3 Guanine and other purine derivatives. 4 E.g., tryptophane and skatol acetic acid. These compounds are the cause of the unpleasant odours in the putrefactive fermentation of proteids.THE NATURE OF SKIN 13 by sulphuretted hydrogen. Phosphotungstic and sulphuric acids are now added to the filtrate, and the “ hexone bases, ” ornithine,1 lysine,2 arginine,3 and histidine,4 thrown down and filtered off, the acids being removed from the filtrate with baryta. The solution after evaporation below 40° in vacuo then undergoes Fischer’s process, which consists in treating with absolute alcohol and dry hydrochloric acid gas, thus forming the hydrochlorides of the amido-acid esters. The amido esters are obtained from the hydrochlorides by reducing to a low temperature, treating with very cold caustic soda, and extracting quickly with ether. After the removal of the ether, the esters are fractionally distilled in vacuo. Below 100° C. the esters of glycine,5 alanine,6 amidovaleric acid,7 leucine,8 and pyrol-lidine carboxyllic acid are obtained, and between 100° C. and 160° C. the esters of Fig. 7.—Calfskin, aspartic acid,8 glutamic acid,9 phenylalanine, serine,10 tyrosine, and pyrollidine carboxyllic acid. Certain proteids have also been found to contain carbohydrate groups, and split off sugars when boiled with acids. ’1:4 diamidovaleric acid. 2 1: 5 diamidocaproic acid. 3 1 amido, 4 guanidinevaleric acid. 4 Unknown constitution. 5 Amidoacetic acid, glycoooll. 6 Amidopropionic acid. 7 Amidoisocaproic acid. 8 Amidosuccinic acid. 9 Amido-glutaric acid. 10 Hydroxyamidopropionic acid.14 THE MANUFACTURE OF LEATHER Mucine,1 for example, yields glucosamine. When proteids are subjected to dry distillation a mixture of pyridine C5H6N, pyrrol C4H5N, and their derivatives is obtained, but it is open to doubt whether these compounds exist to any extent in the original proteid molecule. There is a very large number of proteids, but, in the absence of any satisfactory criterion of purity for colloid bodies, their accurate analysis and scientific classification is even yet quite impossible. One or two have been obtained in a microcrystalline condition, but there appears to be little hope of progress in this direction, so that it is necessary to fall back on the rather unsatisfactory method of classifying them according to their behaviour with certain reagents, and in this way they may he divided into albumins, keratins, and gelatins. These are differentiated in the first place by the action of cold water, which dissolves Fig. 8.—Welsh sheepskin. the albumins, does not affect the keratins, and only swells the gelatin. When heated in water the gelatin dissolves, the albumins coagulate at rather over 70° C , and the keratins are dissolved only at temperatures above 100° C. On adding acetic acid and potassium ferro-cyanide to aqueous solutions a precipitate is obtained from the albumins and keratins, but not from gelatin. By boiling with alcohol, washing with ether, and heating with hydrochloric acid (specific gravity 1*2), a violet coloration is obtained with albumins, but not with keratins or gelatin. 1 From animal secretions.THE NATUKE OE SKIN 15 The albumins, of which the white of egg may be regarded as a type, occur to some extent in the corium of the skin, as the serum in the blood-vessels, and the lymph which permeates the connective tissue and nourishes the whole skin. , Their solutions are optically active and laevorotatory, and when quite fresh have a slightly alkaline reaction. The coagulation by heat, which is the most characteristic reaction, occurs usually about 72° C., and is assisted by the presence of some mineral salts. Concentrated mineral acids and alcohol coagulate them in the cold. Coagulated albumins behave in almost every respect like keratins. Globulins are albumins insoluble in water, but soluble in weak saline solutions, from which they coagulate at 70—75° C. The myosin of the muscle, fibrinogen of the blood, and vitellin of egg yolk are typical members of this class, but all except the last are insoluble in saturated brine. The albuminates are albumins soluble in acids and alkalies and insoluble in water, and are obtained by the action of dilute acids in the cold on the native albumins and globulins. They are precipitated from solution by exact neutralisation of the dissolving acid or alkali, or by the addition of common salt or magnesium sulphate. They are not coagulated by heat, and probably form the first stages of hydrolytic decomposition. The albumoses, or albumin peptones, are the next stages in this hydrolysis, being very soluble in water, acids, and alkalies, and not coagulated by heat. They are formed also by the digestive enzymes pepsin and trypsin, and are very similar to the gelatoses and keratin peptones. The keratins include all horny tissues, and therefore embrace the epidermis proper, and all the epithelial structures of the skin, hair, horns, hoofs, nails, claws, sebaceous and sudoriferous glands and ducts, and also the elastic fibres. They are softened somewhat by water, but not dissolved unless digested for a considerable time at a temperature over 100° C., by which means a slightly turbid non-gelatinous solution is obtained. They are dissolved by the hydrates of the alkali and alkaline earth metals, the rete Malpighi, and other soft keratins with great readiness, and the hair and horns with difficulty and only on heating. The elastic fibres are not16 THE MANUFACTURE OF LEATHER dissolved even by hot caustic potash, but all keratinous matters are dissolved with great ease by solutions of sulphides. They are also hydrolysed by the action of mineral acids, first into peptones and afterwards into amido-acids, etc. Gelatin1 is the proteid obtained by boiling the purified hide fibre with water and evaporating the solution. Whether the product is identical with the white connective tissue of the corium is doubtful, but the only certain difference between hide fibre and the best gelatin is that the former is distinctly less soluble in hot water. This perhaps might be accounted for by the peculiar physical condition of the corium tissue, but is more likely to be due to a difference in the degree of hydration, gelatin being a slightly hydrated form of the collagen of the tissue. This latter view is supported by the fact that gelatin dried at 180° C. becomes insoluble even in hot water. The following typical figures show that no striking difference can be demonstrated analytically :— Material. c. Per cent, of H. N. o. Analyst. Purified sheepskin corium. 50*2 6-5 170 26 3 Yon Schroeder Paessler. and Purified oxhide corium. 50-2 6*4 17*8 25*4 Yon Schroeder Paessler. and Ash-free gelatin 50-0 6-7 18*3 25-0 Schutzenberger. The variations are within those due to the uncertain purity of materials analysed, so that for most purposes we may regard the two substances as identical. Pure gelatin is a colourless, transparent, and rather brittle solid, devoid of taste and odour, and having a specific gravity of 1’42. It melts about 140° C. with decomposition, and is insoluble in petroleum ether, ethyl ether, alcohol, and benzene. When placed in cold water it swells up very considerably, absorbing about ten times its own weight of water, and 1 Sometimes called glutin, in which, case it must not be confused with the gluten of cereals. 2 The connective tissue has been chemically termed collagen or ossein.THE NATUEE OE SKIN 17 forming a “ jelly,” which may be regarded as a solution of water in gelatin. The solution of the gelatin in water, on the other hand, is very slight, but the concentration of both these solutions increases very rapidly with rise of temperature, the surface tension between them diminishing, until the jelly suddenly disappears, and a perfectly homogeneous solution is obtained, which, however, sets again to a jelly on cooling if it contains more than 1 per cent, of gelatin. Aqueous solutions of gelatin are optically active and strongly lsevorotatory, but the specific rotation is very variable. Gelatin is precipitated from solution by various reagents, including alcohol, solutions of chromium and aluminium salts and of mercuric chloride, ammonium sulphate when concentrated, slightly acidified brine, metaphosphoric acid, quinone, alkaline or warm formaldehyde, and all vegetable tannins. Alcohol and ammonium sulphate will also dehydrate a water-swollen jelly, but the former will not if the jelly be swollen with acid. The significance of many of these reactions in the processes of leather manufacture will be noticed if it is borne in mind that hide fibre behaves in a precisely similar manner to gelatin, and that when pelt is treated with these reagents it is converted into some kind of leather. The reaction with tannin solutions is typical of the vegetable tannages, and is exceedingly delicate (p. 118), but the precipitate of “leather” is soluble in excess of gelatin solution. When the tannin is in excess the precipitate contains about 66 per cent, of gelatin,1 but the proportion is variable, no definite compound being known. Rendering gelatin insoluble by means of a chrome alum solution may also be regarded as a type of the mineral tannages, and the action of formaldehyde as representing the aldehyde tannages. The action of acidified salt, ammonium sulphate, and alcohol has its analogy in the pickling process (p. 108), which is a temporary tannage. Gelatin decomposes hydrolytically in a manner closely similar to other proteids, and yields under the influence of acids, alkalies, or enzymes compounds known as gelatoses or gelatin peptones, which are apparently little different from M.Xj* 1 Bottinger, A., ccxliv., p. 227. c18 THE MANUFACTURE OF LEATHER the albumoses or keratin peptones. Many preparations of gelatoses have their origin in attempts to obtain pure hide substance, but almost any treatment of the connective tissue results in a partial hydrolysis. By the action of hot water Hofmeister obtained hemicollin and semiglutin, which he differentiated by the precipitation of the latter by alcohol and platinic chloride. By the prolonged action of caustic lime solution hide fibre is gradually hydrolysed to a soluble substance which has been named coriin,1 and which is precipitated by careful neutralisation with acetic acid. It is slightly soluble in water and dilute acids, and readily in alkalies and a 10 per cent, salt solution. It is precipitated by tannin, basic lead and iron salts, but not by acidified ferrocyanide, mercuric chloride, copper sulphate, or lead acetate. By the action of hot dilute acids and alkalies, other soluble gelatoses are obtained, such as the collin of Parker and Payne. Generally speaking, the gelatoses are soluble in water, strongly basic, precipitated by tannins, and some of them by alcohol and metaphosphoric acid. They yield on more complete hydrolysis the same products as other proteid matters. Gelatin made from bones, tendons, ligaments, and cartilages possesses the same general properties, and yields the same products of decomposition. Glue, which is a mixture of gelatin and its peptones, is used as an adhesive agent. Isinglass is the fibrous gelatin from the swimming bladders of the sturgeon and other fish. Russian isinglass is the best, and is nearly colourless, semi-transparent, odourless, tasteless, and gives a very strong jelly. It is noteworthy that jellies can be made also from agar-agar, algin, and similar extracts of various lichens and seaweeds. Such bodies contain little or no nitrogen, and are not proteids. 1 Reimer, Ding, Polyt, Journ, ccv., p. 153.CHAPTER III FERMENTATION The importance of fermentation in the manufacture of leather will be readily understood when we not only remember that many of the materials used are highly organised matter, and therefore easily decomposable or fermentable, but also ^ recognise that nearly all the processes of the industry are dependent upon the action of various ferments for their satisfactory consummation. It will be seen, therefore, that it is extremely desirable some attempt should be made to understand something of the nature of the agencies at work, the conditions under which they operate for weal or woe, and the methods by which we are able to control and limit their action. The simplest form of organic life consists of the living proteid called protoplasm, which is a jelly-like mass capable both of independent existence and of motion, and is well illustrated by the lymph of the human system and the amoeba of ponds. This protoplasm, like all living things, requires nutriment to produce its heat and energy, and this it obtains from its immediate surroundings. It uses this food also to build up its system and usually to enclose itself in a wall of cellulose or keratinous matter, thus forming a cell. Every living cell has in its protoplasm a nucleus, which for purposes of reproduction is capable of division into two portions, each of which, associated with some of the surrounding protoplasm, goes to form a new cell, as already illustrated in the rete Malpighi of the skin. In multiplying thus the new cells may separate and become quite independent, or they may remain together in chains or masses. Differences also occur in shape, in the cell-wall substance, in the secretions and excretions, and masses of such various cells go to build up the higher forms of animal and plant life, in which every variety of cell has its c 220 THE MANUFACTURE OF LEATHER own specific functions in the maintenance of the whole structure. It has been pointed out that the cells which compose the animal system require food, but the substances which are ordinarily supplied are not usually in the form in which they can be readily absorbed, so that chemical changes are necessary to bring them into the desired condition, and these changes, which generally involve the liquefaction, solution, or emulsification of the nutriment and the breaking down of its components into molecules of simpler chemical structure, are brought about by enzymes, which are lifeless proteids of an albuminous nature, and which form the class of unorganised ferments. These enzymes, or zymases as they are often named, being devoid of life, possess no powers of reproduction. They are soluble in water, and are coagulated by heat (cp. albumin), being thereby rendered quite inoperative. Their activity is also destroyed by certain antiseptics (see below). Many of them may be precipitated from their aqueous solutions by means of alcohol, and after filtering off may be redissolved in water and again caused to act as active ferments. They are well illustrated by the ptyalin of the saliva, which converts starch into sugar, the pepsin of the stomach glands, and the trypsin of the pancreas, which dissolves fibrin. The mechanism of the chemical changes which these ferments accomplish is still very obscure, but it is known that very small quantities of them are capable of decomposing very large quantities of the bodies on which they operate, and that they are themselves unchanged at the end, so that some analogy is found in the so-called “catalytic actions” of chemistry. Much interesting work has been done in recent years on the selective action of these ferments, such as maltase, emulsin, invertase, and lactase, on the glucosides and carbohydrates, and it has been suggested that the enzymes are asymmetrical bodies which “fit in” with certain other asymmetrical1 substances, which are those they ferment, as a key fits in a lock, and where the key will not fit (e.g.9 1 In the stereochemical sense.FERMENTATION 21 invertase or maltase with milk sugar) no action can take place. It has also been shown that with comparatively small quantities of enzyme the decomposition is a linear function of the time, but that with sufficient enzyme it is a logarithmic function and follows the ordinary mass law. That enzymes are not always analytic and hydrolytic agents has been demonstrated by their synthetic production of amygdalin,1 disaccharoses,2 and esters.3 4 In plant life these unorganised ferments fulfil the same functions as in animal life (e.g., diastase and starch), but when green plants send forth their leaves they obtain most of their nourishment from the atmospheric carbon dioxide by means Fig. 9.—Yeast cells. of their chlorophyll and the sunlight. Certain plants, however, known as fungi* have no chlorophyll, and are therefore compelled to obtain their organic nourishment from their immediate surroundings, which they accomplish directly, or by means of the enzymes they secrete. The lower forms of these fungi (yeasts, moulds, and bacteria) constitute the organised ferments. Yeasts (saecharomycetes) are simple cells of nucleated protoplasm with a cellulose wall. They multiply by the formation 1 By maltase from glucose and the glucoside of mandelic nitrile. 2 Isolactose by lactase on glucose and galactose, and isomaltose by maltase on invert sugar. 3 By lippase. 4 Mushrooms, etc.22 THE MANUFACTURE OE LEATHER of “buds” on the side of the parent cells, which enlarge till they become full-grown cells and have themselves buds. This method of reproduction results in groups or chains of cells, but each one provides its own nutriment. The action of yeasts is well illustrated by the ordinary alcoholic fermentation of glucose. Moulds {ascomycetes and phycomycetes) are a rather higher form of plant life in which the cells are elongated and remain connected, thus forming stems and branches Qiyplice), upon which appear masses of spore-bearing cells. They are often Fig. 10.—Mould (Penicillmm glaucum). characteristically coloured, and the cells of which they are composed are not always independent of other cells for their nutrition. Some of these plants are capable of reproduction by sexual fructification. The action of moulds is illustrated often on decaying fruit. Bacteria (schizomycetes) are the lowest forms of the fission fungi. They are simple cells which multiply by lengthening and dividing, but certain forms are capable of producing spores, which appear at one end and grow into a new cell. There is an immense variety of species of these fungi, and our knowledge of them is as yet very incomplete. Classification,FERMENTATION 23 therefore, is difficult, but they are often divided into (1) micrococci, small spheres, connected or independent ; (2) bacilli, rods, dumb-bell-shaped, or ovals, often motile ; (3) spirilla, spirals, usually motile. They are all exceedingly small, requiring a ^ objective and oil immersion lens for proper examination. Even then this is difficult, for confusion may easily arise between a dumb-bell-shaped bacillus and a dividing micrococcus, and the spores of the spirilla and bacilli may be taken for cocci. They are recognised by their size, which varies usually between 0’5 and 5*0 ; by their Eigl 11.—Bacteria. A, micrococci; b, bacilli; c, spirilla. flagella, which are filaments used by the motile forms for motion; by their colour, and also their appearance on staining with various basic aniline dyes; by their method of reproduction and the range of temperature within which they can live. Bacteria may be “ aerobic,” in which case they thrive in the presence of oxygen (or air), which is absorbed by them in oxidising their food; or they may be “ anaerobic,” in which case they can live only in the absence of oxygen and derive what they need of it directly from their nutriment. The former tend to produce as waste products water, carbonic acid, nitrogen, nitrates, sulphates, etc., which are harmless and without odour, but the latter, being reducing agents, yield such compounds as the hydrides of carbon, nitrogen, sulphur 1 fx is a micromillimetre (= Q’001 mm.). Some bacteria, however, are ultra-microscopic,24 THE MANUFACTURE OF LEATHER and phosphorus and their organic derivatives, many of which are excessively poisonous to human beings. This is the case with many of the disease bacteria, and perhaps is the reason why bacteriology is better developed on its pathological side. Many bacteria are poisoned by the products of their own work and, unless these are removed, die out, their places being rapidly taken in putrefactive fermentations by organisms more suited to those surroundings. Some bacteria can thrive under both aerobic and anaerobic conditions, and produce different effects accordingly. The chemical effects and physiological actions of bacteria are extremely useful in identification. It is now wel] recognised that many of these organised ferments secrete enzymes, and hence that many of the effects noticed are only due indirectly to the fungi. Yeast, for example, cannot attack cane sugar until it is converted into monosaccharoses by the enzyme invertase, and it has been shown that the effects of puers are due to enzymes rather than the bacteria with which they are associated. It is in many cases exceedingly difficult to distinguish the two classes of fermentation, for both organised and unorganised ferments are destroyed by heat. They may, however, be differentiated by means of chloroform, which does not at all affect enzyme action, but which will paralyse or kill bacteria. The two classes of ferments may also be separated to a certain extent by filtration through very compact earthenware, such as the Chamberland “ candle,” which will not allow the passage of many bacteria. The complex fermentation of nitrogenous organic matter, usually accompanied by unpleasant odours, is generally termed putrefaction, and is brought about by bacteria, directly and indirectly, by moulds, and to some extent by the lower forms of animal life {monads, infusoria, etc.). No specific fermentation takes place, of course, under such circumstances, but an extremely complex action occurs, the course of which is never exactly repeated. The first stage of this involves the action of aerobic bacteria (.B. liquefaciens magna, etc.), solid matter being liquefied and but little odour produced. The second stage permits the action of the true putrefactive, and facultatively anaerobicEEBMENTATION 25 bacteria, of which Proteus vulgaris and P. mirabilis are the commonest. These do not secrete any enzyme, hut ferment by direct action. Bacillus prutrificus, which attacks fibrin, is also active. Other bacteria follow, Proteus zenkeri, etc., which only attack peptones, and the final stages of decomposition are extensively effected by flagellate monads. From recent investigations of such fermentation processes it has become “ very probable that specific organisms ferment the different albuminous compounds in the same way that different carbohydrates are each decomposed by specific ferments.” 1 All ferments are affected, in greatly varying degrees, by certain bodies called antiseptics or disinfectants such as mercuric chloride, salicylic, sulphurous, and boric acids, phenols, cresols, naphthols, and chloroform. These destroy the ferments or paralyse their activity, thus rendering their surroundings sterile. They are used extensively both to arrest and to prevent fermentation, or to allow only the class of fermentation which is desired. As moisture is essential and a high temperature fatal to fermentation, the processes of drying and heating are both extremely useful in preventing it. In the manufacture of leather various fermentations are encouraged and discouraged. Hides and skins are kept from putrefaction by drying and by the use of salt and other antiseptics (p. 28). In soaking an attempt is made to cleanse and soften the hide without fermentation unless it is extremely hard, in which case the bacterial solvent effect of a putrid soak is sometimes considered an advantage (p. 50). In depilation the hide is placed in a lime liquor, which, in addition to its chemical action, acts as an antiseptic to many of the putrefactive bacteria, but allows the fermentative action of those which attack the epidermis and hair root. This is also accomplished, less perfectly, in an ammonial atmosphere by the “ sweating ” process (p. 57). In bating and puering, the hide or skin is immersed in a nutrient medium which especially favours the growth of certain bacteria whose enzymes and products of decomposition produce the required effect. The temperature is also adjusted to assist in the same direction * Wood, J.S.O.I., 1906, 109,26 THE MANUEACTUEE OE LEATHEE (p. 97). The action of the drench is due to the weak acids and gases produced by the successive action of certain enzymes and bacteria (p. 99). In the tan-house bacteria are useful in acting on the sugars to produce the acids which neutralise the lime and swell the hides, whilst yeast and moulds are injurious, acting destructively both on the acids and on the vegetable tanning matters. Moulds and bacteria are also the cause of leather being destroyed in the sheds by “ heating,” and the former are also apt to commence their action upon leather which is being dried too slowly. Many defects in finished leathers (spots, weak grain, etc.) are due to,the injurious effects of bacteria which have been accidentally introduced into some of the processes employed (p. 50).CHAPTER IY HIDES AND SKINS The commercial distinction between hides and skins is based upon the size and age of the animals, and the class of leather for which they are intended to be used. Hides, the name given to the external coating of the larger and adult animals, are used for the manufacture of sole and heavy leathers, and are obtained from the ox, heifer, bull, cow, buffalo, horse,hippopotamus, walrus, and include also “kips’* (p. 36), but as the ox is the most commonly used, the word “hide” is often used as if the term were restricted to that animal. Skins are obtained from the smaller animals, such as the sheep, goat, seal, pig, deer, and the calf, and also from alligators, serpents, lizards, fish, etc. These are used for the manufacture of the lighter and fancy leathers. Hides of the ox, to which the following remarks chiefly apply, are procured largely from the slaughter houses of Great Britain, being sold by auction in the principal market towns, and in these cases no attempts are made to “ cure” or preserve them from putrefaction; but there are now immense quantities of hides which come from all parts of the world, and it will be clear from what was said in the previous chapter, that if they are to arrive in this country in a satisfactory condition for tanning, some precautions are absolutely necessary to keep them preserved from decomposition and decay. There are several processes of curing in operation, and an outline of the chief of these is now given. One of these consists in merely drying the hide, which in this way becomes hard and horny,1 the absence of moisture preventing the development of any putrefactive bacterial action. In places where preservatives such as salt are 1 This fact has given rise to the term “ flint hides,” which is sometimes applied to this class of goods.28 THE MANUFACTURE OF LEATHER dear, and where the distance for land transit is considerable and weight therefore of importance, this method is often the only one really practicable, but it is a process which is difficult to carry out satisfactorily. It is essential that the drying should be even and gradual, but rapid, and the best drying is done therefore by means of a good draught in the shade. If the drying is too slow putrefaction will begin; if it is too rapid the hide will be merely caked on the exterior whilst the interior is still moist and putrescible, and there is a strong probability also that in some parts the fibrous structure will be quite destroyed. With this method of curing there is always much more difficulty in soaking and softening the hides than there is with any other class of goods, and where the drying has been faultily carried out, the difficulties are much greater in the soaking, and the damaged parts are often irreparably ruined in the unhairing. The opportunity for bacterial attack can be reduced to a minimum by the use of antiseptics, such as carbolic acid. Salting is one of the most satisfactory methods of curing now in use. Common salt possesses considerable hygroscopic qualities and can withdraw from the hide with great readiness a large proportion of its moisture, and being also a mild antiseptic, hides well preserved in this way will keep almost indefinitely. The method is employed extensively for the preservation of “packer hides” in the stockyards of the United States. The hides are trimmed, and stored in cool cellars in packs of which the sides are kept higher than the middle by folding, so that the brine that is formed can only escape by passing through the goods. Each hide requires about 25 per cent, its weight of salt, which should be spread evenly, and chiefly on the flesh side. The cure should take about a fortnight, and when complete much of the salt is recovered and mixed with fresh salt for the cure of a fresh pack. If instead of white crystallised salt an impure rock-salt is used, the iron which is often present is very liable to cause “ salt-stains ” on the hides, and similar stains are produced by bacteria when the cure has not been efficient. Brined hides are cured by steeping in a strong solution of salt, and are much less perfectly preserved.HIDES AND SKINS 29 Dry salting is also a very efficient method of curing and is now used to a considerable extent. It is a combination of the drying and salting processes. The hides after flaying are hung up in a cool room and partly dried. They are then spread out on the floor and piled flesh side up with salt between, the edges being folded slightly inwards. When the brine has formed it is run off, the hides suspended again and, dried till in a rubbery condition and then resalted. The hides of the small Indian cattle come to this country in a dry salted condition (“kips ”), but the cure is accomplished by an earthy sodium sulphate instead of sodium chloride (see p. 36). Criteria for selection. The nature of the hide is much influenced by the breed of the animal, its food, and the conditions of its life. Those animals which are exposed to all winds and temperatures, especially those from hilly regions, and districts where there is opportunity to roam about, yield much the best hides for the leather manufacturer, being thick, compact, smooth in grain, and of good texture; whereas those which are bred for their milk-producing qualities, such as the Dutch animals, and others from lowland regions, give poor thin hides. Stall fed animals are objectionable from the tanner’s standpoint, being often very badly ribbed in the neck, and diseased animals also give hides of very poor quality. The age of the animal has also a very great influence on the resulting hide. Calf shin is obtained by killing the animal at the age of about six months; it is very soft and milky, and possesses a characteristic fine grain and compact texture. It is used extensively for the manufacture of leather for boot uppers. v The hide of the ox and heifer are intermediate in texture and substance and give much the best results for sole leather, being well supplied with interfibrillar substance. Bull and cow hides, on the other hand, are very rough and wrinkled, showing extremely coarse fibres and a lack of interfibrillar substance, producing therefore a thin non-waterproof leather. They are used for boot uppers, portmanteaux, etc. Bull hides are apt to be thin in the back and thick in the neck and belly. The quality of the hide is also much influenced by the care taken in flaying. This operation should take place immediately30 THE MANUFACTURE OF LEATHER the animal is killed, and should aim at detaching as much oi the flesh and fat as possible without gashing the hide itself. The process requires a certain amount of dexterity, and should be accomplished, if possible, by experienced persons. The possibility of blood stains, especially in hides to be cured, should be carefully guarded against. Where the hides are not to be cured they should be sold as soon as possible, or lightly salted, for delay involves putrefaction, and hides which are “slippy,” i.e., those in which the hair can be easily pulled out by hand, are of much less value to the leather manufacturer. Fig. 12.—Warble Fly (Uypoderma bovis). 1, egg ; 2, maggot; 4, chrysalis case ; 6, fly, magnified (Brauer); 3, 5, chrysalis and fly, natural size (B. Clark). Defects and injuries, however, unfortunately occur before the animal is killed, and have considerable bearing on the value of the hide. Scratches due to hedges and barbed wire, drovers’ goad marks, scabs, tar marks, and brands, all (especially the last) reduce considerably the worth of the hide to the tanner. Another serious defect in hides is the occurence of warble holes or marks. These are caused by the Ox Warble Fly, or Bot Fly (Hypoderma bovis) which is a two-winged fly, about half an inch in length, banded with thick hairs of different colour, somewhat like the humble bee. The female of the English species lays its eggs on the backs of the animals in the hair,HIDES AND SKINS 31 and the larva when hatched eats its way into the skin, making in this way a sore or swelling, and feeds on the matter caused by this irritation. As the maggot develops the sore “ripens,” and opens in the middle, showing the breathing spiracles of the nearly mature larva, which when fully grown comes out at this “ warble hole,” falls to the ground and creeps to some stone or shelter to pupate. It stays in the chrysalis state from twenty-six to thirty days, and then comes out as the imago or perfect insect, which continues the infestation. This pest sometimes gives rise to several hundred “ warbles” on one beast, causing much pain and occasionally death. The warbled hides also sell at a decidedly lower price than the undamaged, and leather made from the former is obviously useless where it is necessary that the material should be water or wind tight. Fia. 13.—Tail-weights (79 lbs.). It is therefore of some considerable importance to take some preventive action against the insect in the interests of the farmer and leather manufacturer alike, and this may be done by squeezing out the maggot by hand, or killing it by a small application of mercurial ointment, and also by applying a mixture of sulphur, tar oil, and train oil along the spine of the animal, the smell of this mixture driving off the flies and thus preventing the eggs being deposited. It has been demonstrated that if thorough measures are taken in this way, any district may in the course of a few seasons be completely cleared from the pest. As hides are bought and sold by weight, they are classed in the principal markets in this manner, the weight being marked on the tail end as shown in Fig. 13. The weight of obtainable leather being approximately proportional to the32 THE MANUFACTURE 0E LEATHER weight of “pelt,” which is the hide unhaired and swollen with lime, hides should be chosen that give a good yield of pelt, and it is clear that those with short hair, and those which have small horns, or which are weighed without horns, are best in this respect, and it is often economical to buy these at a nominally higher price. With ordinary market hides there may be a 90 per cent, yield of pelt, varying, however, with the time of year, but it is necessary to remember in buying that salted hides may yield 120 per cent, of pelt on account of the dehydration involved in the curing process. It is desirable, moreover, to keep an eye on the amount of salt which is being bought with the hide, for this may vary between 20 and 40 per cent. Brined hides do not come up to salted hides in yield of pelt. It is usual also to “round” the hides after soaking and unhairing, thus cutting them into “butts,” “shoulders,” and “ bellies ”; the butt, when cut down the middle of the back, gives two “bends,” as shown in figure, and as the butt is much the most valuable portion it will be readily understood thatHIDES AND SIONS 33 selection should be made to give the best percentage yield of this part. Prom this standpoint, animals which have short necks and legs, and which do not run thin in the flank are the best, and should give butts fully 50 per cent, of the pelt weight, especially in the autumn. The chief sources of supply are the home markets, South America, and the Continent. Amongst the English breeds the “Herefords ” yield some of the best hides. The animals have white faces, bellies, and tail tips, and carry medium-sized horns which are nearly horizontal. Their hides are obtained in the Midland markets, and unless fed for show purposes are very free from grease and of good texture. They yield usually quite 50 per cent, of butt pelt, and give stout level-grained shoulders. They are very suitable for sole and harness leather. “ Shorthorns’’ is a general name which covers many varieties of similar breeds in England, Scotland, and Ireland, and there is consequently great variety also in the quality and texture of the hides obtained from them. The animals are of a red, white, or roan colour, with small neat horns, but on account of their high feeding for show purposes, some are apt to be very greasy. The “Devons” (N. Devons, S. Hams, etc.) are beautiful cattle of medium size and dark red colour, excepting the white udder, tail tip, and nose. The hides are well grown and of good texture, but often very badly warbled, which spoils them for chrome and high class dressing leather. The “Sussex” cattle are really a cross of Devons with other cattle. They are larger, stronger, more darkly coloured, but less proportionate than the Devons. Their long necks and legs lower the percentage yield of butt. The “ Suffolk Bed Polls ” are a hornless red cattle found throughout East Anglia. The hides give a clean well-grown butt, and the cowhides are suitable for dressing leather and chrome work. The “ Channel Islands cattle ” (Alderneys, Guernseys, Jerseys) are well groomed and somewhat delicate cattle with short hair, small heads and legs. The hides are yellow and very thin, but usually possess a good undamaged grain with no warbles; they are therefore very suitable for some classes of chrome and dressing leather. The “Longhorns” (“Cravens”) are the largest British breed, but are dying M.L. D34 THE MANUFACTURE OF LEATHER. out. They have large down-curving horns, and in colour are dun deep red, roan, or pied. These hides yield good stout butts. The “ White wild cattle,” which are the ancient breed of Britain, are now nearly extinct; only a few degenerate herds now remain. Of the Welsh breeds the North Wales Black (Carnarvons, Cardigans, etc.), the South Wales Black (Pembrokes, etc.), the Castle Martin White, and the Glamorgan s are the principal varieties. They will all yield good stout butts. The Scotch breeds are also important. The ‘‘ West Highlanders ” (“Kyloes”) are the hardiest race in Britain, possessing long, thick, shaggy and glossy hair and a hairy head. They are chiefly black but are also dun, brown, and brindled. They have big, upright and wide spreading horns with black or red tips, and are great in body and short in leg. The “ Polled Aberdeen-Angus ” are a black hornless cattle with smooth coats. The “ Galloways,” another polled Scotch breed, are much like the Aberdeen-Angus, but have woolly coats of rough and rather curly hair. They have a white head, short neck, straight back, and short legs. All these three breeds yield good, well-grown hides. The “ Scotch shorthorns” are a cross with the Aberdeen-Angus, but are in colour red, pied, dirty white or brindled. Their hides are amongst the best procurable for sole leather purposes, being well grown, thick, short in neck and leg, small in head, and usually well flayed. They yield over 50 per cent, of butt and up to 24 per cent, of shoulder. The “ Ayrshires,” which are the best milkers in Scotland, are red, white, black, mixed, or patchy; their hides are not so good, being big in belly. The “Orkneys” and “ Shetlands ” are small hardy cattle with shaggy coats. Their hides are not of great commercial importance. The “ Kerrys ” are the only distinct breed of Ireland. They are small and hardy, usually black but sometimes brown or red and with white streaks on the belly and udder. They have short hair and thin wide-spreading horns. They give good light butts for sole and dressing leather. The “ Dexter-Kerry ” is somewhat smaller and stouter. Other hides (from Shorthorns, etc.) come from Ireland, but are mostly thin, light, coarse textured, and badly flayed. These are suitable only for cheap light sole bends,HIDES AND SKINS 35 The South American hides, which are salted or dry salted* are excellent in breed, carefully slaughtered, well flayed, thoroughly cleansed, and well cured, but they are objectionable on account of their brand, which is not only large and deep but is also on the most valuable portion of the hide. These brands, which it seems impossible to avoid, are however usually on one side only, so that one clear uninjured side can always be obtained. Those from Buenos Ayres are considered the best, whilst those from Uruguay are also good. Hides from the Bio Grande are often inferior and tick pitted. There are three classes of these hides: “ Saladeros,” which come from the large slaughtering establishments, and are well flayed and cured; “ Estancias,” which are killed at ranches up in the country, and are not so good; and “ Mataderos,” which are killed by town butchers and are often ill flayed and imperfectly cured. The Central American supply consists mostly of flints from Brazil, Texas, Mexico and other Central American States. They are difficult to soften satisfactorily, and are mostly used in the United States for sole leather (acid hemlock tannage). The Chicago packer hides, wet salted and well flayed, are not very thick, and are mostly used in the United States for dressing leathers. “ Natives ” are hides from native steers, and usually free from brands; they are chiefly used for harness and belting leathers, but if “spready” are used for the tops of carriages, and if from “ butt-branded steers ” they are used for sole leather (union tannage). “ Texans” are hides from steers from Texas, and make the best sole leather of the packer hides ; they are nearly always badly branded. “Colorados ” are branded also; they are used for upper leather. The Continental hides are usually salted and without horns and tail bones. Bavarian highland hides are celebrated for their thickness and evenness of growth. They are well grown, tightly built up, and possess a fine smooth grain. Usually, also, they are well flayed, and hence are especially liked for army work and for heavy belting. Hides from the Swiss Alps are similar to the Bavarians, but have shoulders not quite so thick. Italian hides are from a smaller breed of animal ; they are shorter in the butt, but thicker. They also d 2TI1E MANUFACTURE OF LEATHER 30 are hides of even growth, compact texture, fine grain, and are well flayed. Spanish and Portuguese hides are also good, but somewhat liable to scratches and spots. Scandinavian hides rank high, being well grown, well flayed, and of even texture. Hungarian hides are often very large, but are tight in texture, well flayed, and give good grain. Hides from lowland regions (Berlin, Cologne, Holland, etc.) are long in shank and not particularly well grown. They are mostly used as dressing hides. French hides are often badly flayed. “ E. I. kips,” which are almost a class to themselves, are obtained from a small breed of Indian cattle killed when one or two years old, and are largely imported into this country in a dry salted state. The cure is with a sodium sulphate earth,1 which is mixed into a paste with water and painted on the flesh with a brush, the hide being then dried in the sun. This is repeated several times before export. They are tanned for boot uppers, “ waxed kip butts,” satin kip, box calf imitations, etc. A large number are now imported in the rough-tanned state. Buffalo hides, obtained from the true buffalo (Bubalus buffalus), which is common throughout Asia and Eastern Europe are imported into this country for the manufacture of pickers, belt leather, and sole leather. Horse hides, obtained chiefly from South America, are tanned for boot uppers and enamelled leathers, the butts being made into “ crup ” leather. They are also “ tawed ” for whip thongs, aprons, etc. Hippopotamus and Walrus hides are imported to a small extent, and after a long tannage become exceedingly thick and porous. The leathers are principally used for polishing and burnishing implements, and for brakes. Skins are obtained from many animals, but most extensively from the common sheep. Much of what was said as to influence of food, health, breed, and surroundings on the quality of hides, applies also to these skins. Sheep exposed to adverse climatic conditions yield the best pelts, whilst those which are carefully bred for their wool give poor thin skins. Devons and Cheviot crosses, for example, yield poor wool, but 1 Procter and Towse, J.S.C.I., 1895, 1025,HIDES AND SKINS 37 give good pelts, whereas the Leicesters, Lincolns, and South Downs give excellent wool but often only poor skins. It has been noticed also that better pelts are obtained some little time after the animals have been shorn. The leather manufacturer usually obtains his sheep skins from the fdlmonger, whose business it is to remove the wool, which is the most valuable portion of the skin. The fellmonger divides British sheep into Long Wools, Short Wools, and Mountain Breeds. Of the Long Wools, the “ Leicesters ” are an important class, and are found extensively distributed throughout the north of England (Border Leicesters, Yorkshire Leicesters, etc.), and to some extent in the Midlands and in Ireland. They are great favourites with stockmasters for crossing. The “Lincolns” are found only in the Lincolnshire wolds. They have white faces and shanks and yield a big pelt of good grain. They are noted for their great weight of fleece. The “ Cotswolds,” which are the largest British breed, are found only in the Cotswold Hills, and have white faces and legs and no horns. They yield a very long and fine wool, but the pelts are often very greasy especially in the back. The “ Devons/’ found in the valleys of Devon, Somerset, and Cornwall, yield strong wool of fair length and somewhat peculiar colour. The pelts are good and quite white, but are not so big as those previously mentioned. “ Kents ” (Romney Marsh) are somewhat larger than Leicesters, with white faces and legs and no horns. They have also a tuft of wool on their heads. Good pelts are obtained from them. “ Roscommons ” are the principal Irish breed, but are much crossed with the Leicesters. They yield long silky wool and wide pelts, which are somewhat “ leggy.” Of the Short Wools, the “ South Downs ” are amongst the most important. They are small and well shaped, having grey faces, fine wool, and no horns. This variety yields fair pelts and the best mutton, but is a rather delicate sheep, and therefore has been much crossed with Cotswolds and other breeds. “ Suffolk Downs” are cross-bred Downs, found chiefly in the eastern counties, with black faces, heads, and legs. They yield fair pelts. “ Oxford Downs ” are similar crosses but larger. “ Hampshire Downs” are cross-bredm THE MANUFACTURE OF LEATHER downs, also similar to the above, but with heavy black faces. “ Shropshire Downs ” are cross-breds which are rather hardy, and are heavier in wool. “ Somerset and Dorset Horns ” are a very hardy and prolific short-woolled breed. They have white faces, nice horns, and wool and pelts of intermediate quality. “ Eyelands ” have short white wool and no horns. “ Clun Forest ” (“ Eadnors ”) have speckled tan faces, coarse wool, and yield fair pelts. Of the Mountain Breeds, the “ Cheviots ” yield very good pelts. They are a middle-woolled breed, which grow much hair amongst the wool, and possess white faces and legs and no horns. They are long in body and very hardy. “ Black Faces/' found in the mountainous parts of North England, Lancashire and the Scottish Highlands, grow long, open and coarse wool, but give good pelts. “ Herdwicks,” found in Cumberland and Westmoreland only, are rather similar “ Limestones ” are a good-sized white sheep with large horns, fine wool, and good pelts. “ Lonks,” which are the largest mountain breed, give a big pelt, good mutton, and coarse wool. They have horns and black faces. “ Exmoors” and “Dart-moors ” are small breeds, which yield tough pelts and strong wool. The “ Soft-Woolled Mountain sheep” produce white, fine soft wool, and a very small pelt. “ Shetlands ” are rather wild animals, with brown-grey fleeces, much hair, and good pelts. The “ Welsh Mountain sheep ” yield poor wool, but a small fine-grained pelt. Sheep skins are usually classified by the leather manufacturer according to their size. Small skins are obtained from the Welsh mountain sheep, from those of the mountainous districts of England, and also from young sheep and certain foreign sheep. These are mostly tawed and used for shoe linings, gloves, etc. The Welsh mountain sheep are liked for roller leather manufacture. Medium skins are from the North and South Downs and Scottish sheep, and are tanned to make “ roans ” (p. 208), a kind of imitation morocco leather. Large skins, from the Leicesters, Lincolns, Cotswolds, etc., are extensively used for the manufacture of “ skivers,” which are made by splitting the limed skins in two (p. 279) and tanning the grain with vegetable tanning materials, the flesh splitsHIDES AND SKINS 39 being made into chamois leather by treating with oil (p. 247). Skivers are made into hat linings, pocket-books, camera bellows, etc. A large number of these skins are also made into “basils” in the west of England by tanning with oak bark and extracts, and in Scotland and the north with larch bark. Basils are used for slipper leathers, etc. Some of these skins are also tawed for aprons and for druggists’ “ white leather,” and others dressed with the fur still on and used for mats, rugs, and similar articles. There are a considerable number of imported sheep skins, chiefly from South America, Mexico, the Cape, Australia, and New Zealand, but the two last-named are beginning to tan their own skins, and we now import them to some extent in the tanned state. Tanned sheep skins are also imported largely from the East. Lamb skins are very largely tawed for the manufacture of “ glove kid,” the skins being obtained both at home and from Italy and the south of France. These skins yield pelts with an exceedingly fine grain and delicate texture, and will dye very uniformly, but it is essential that the lambs should be slaughtered when about a month old, for these special characters gradually disappear as the animal gets older. Lambs killed whilst having still their characteristic short curly wool have their skins dressed for “ Astrakan.” Goat skins yield a leather which possesses a far greater durability and a much superior texture than that obtained from the sheep, and they are therefore in great demand, supplies coming from all parts of the world, usually in a dried or salted condition. Those obtained from the Cape goat are often larger, thicker, and stronger than the others, but, being imported in the dry state, they are not always in a perfectly sound condition. The skins of the Swiss goat are fairly large, clean fleshed, well grown, have a fine grain, and are made into a strong durable leather. The Abyssinian skins are exceedingly tough and compact, and yield a bold-grained leather. The Balkan and Bavarian skins are rather smaller, but are plump and give a fine close grain. Mexican skins have also a high character, but Scandinavian goats yield skins which are very poor and flat. Goat skins also40 THE MANUFACTURE OE LEATHER come from the East in a tanned state (E. I. goat). An immense quantity of these skins are manufactured into various kinds of morocco leathers, which have a very characteristic grain and are capable of taking up evenly some very brilliant colours. They are used for a great variety of purposes, bookbinding, upholstery, and many fancy articles. Indian dried goat skins are largely imported. They are rather small, and are used extensively for the manufacture of light chrome leathers. Kid skins are imported from Italy for the manufacture of glove leathers by tawing. The kid skins of France and Ireland are also held in high esteem. As with lamb skins, it is exceedingly important that the skin should be obtained before the animal begins to feed extensively on vegetable matter, for when this commences the extreme fineness of grain and delicacy of texture are rapidly impaired. Tawed kid is also used to some extent for the manufacture of light shoes and slippers. Seal skins are obtained from the true seal (Phoce Green-landica, Phoco barbata). They are imported to a considerable extent into this country for the manufacture of a special class of morocco leather. The skins, which are salted, and very oily, are obtained from North America, Newfoundland, the northern parts of Europe, and also from the Cape. They produce a leather which is exceedingly strong and which has an extremely compact texture. A good proportion of japanned and enamelled leather is also made from these skins. The fur-seals of Alaska and the Northern Pacific are sea-bears (Otaria). They are dressed for muffs, jackets, caps, and other articles of clothing. Pig skins are tanned to some extent in Austria-Hungary, and Scotland, and form a very light and tough leather, which is used largely for harness, saddle, and bag work. Porpoise skins are now much used for boot uppers and for laces, producing when tanned a very soft but strong and tough leather. Deer skins are tawed and oil-dressed to a considerable extent in the United States, and alligator and serpent skins are utilised for the manufacture of fancy leathers for purses, small bags, and similar articles.CHAPTER V WATER There is no material used in the manufacture of leather in such large quantities as water, and it is therefore clear that its quality is a matter of exceeding importance. As the use of pure distilled water is quite out of the question, and as all natural waters contain a certain amount of dissolved matter it is very desirable that the leather manufacturer should consider the nature of the commoner impurities, their influence in the various processes in the manufacture, the methods by which they can be conveniently removed and accurately estimated, and the most suitable sources for the supply of a water of satisfactory quality. The most common property of natural waters is that of “ hardness,” a term applied to such waters on account of the difficulty in obtaining a lather in the ordinary process of washing. This property is principally due to the salts of calcium and magnesium which react with the soap and cause the precipitation of the insoluble stearates, oleates, etc., of the same metals. In the water the metals are usually present as carbonates, sulphates and chlorides. The carbonates are kept in solution by the presence of dissolved carbonic acid, which forms soluble bicarbonates with them. When such waters are boiled, however, these bicarbonates are decomposed, carbonic acid is driven off, and the carbonates are precipitated, that of magnesium eventually changing into the hydrate. Ca (HC03)2 = Ca C03 + C02 + H20. Mg (HC03)2 = Mg (OH)2 + 2 C02. This expulsion of carbon dioxide and precipitation of chalk and magnesia is the cause of the “ furring ” of kettles, and its gradual diffusion in nature gives rise to stalactites and other calcareous deposits. Hardness due to the decomposable42 THE MANUFACTURE OF LEATHER bicarbonates of calcium and magnesium is therefore termed temporary hardness, whilst that due to the sulphates and chlorides, which is not removed by boiling is called permanent hardness. In many industries the presence of these dissolved salts is distinctly undesirable and in many of the processes of leather manufacture may be the cause of decidedly injurious effects; hence many suggestions have been made for “ softening ” the water by ridding it of these impurities, and outlines of some of the most successful and most widely used of these are now given. The Clark process, of which a great number of modifications have been suggested, involves the use of a solution of lime. This will remove the temporary hardness only. Calcium hydrate, being a strong alkali neutralises all the free carbonic acid and all that combined as bicarbonate, forming calcium carbonate, which is precipitated; and further, that calcium carbonate which had been kept in solution by this carbon dioxide is also rendered insoluble. The reaction may be expressed thus :— Ca (HC03)2 + Ca (OH)2 = 2 Ca C03 + 2 HaO. Temporary hardness due to magnesium bicarbonate is also removed in a very similar manner, but a further quantity of calcium hydrate is used, the magnesium being removed as hydrate thus:— Mg (HC03)2 *4“ 2 Ca (0H)2 = 2 CaC03 + H20 + Mg (0H)2. The insolubility of magnesium hydrate causes all the magnesium present as permanent hardness to be precipitated also, but it will be noticed that the permanent hardness is not reduced in this way. Mg S04 + Ca (0H)a = Mg (0H)a + Ca S04. The process as originally carried out consisted merely in mixing the requisite quantity of lime with the water in a vat, and running the mixture into large tanks, to the bottom of which the precipitated chalk and magnesia gradually settle. The Porter-Clark p'i'ocess is a modification of the above in which some of the water to be softened is used to makeWATER 48 saturated lime water, and so softened in the process, the chalk being deposited with the excess of lime as the solution passes slowly up a vertical cylinder. This lime water is now used to soften more of the hard water by running both of them into another cylinder in the required proportions. An agitator causes thorough mixing and the precipitated calcium carbonate is removed by means of the filter press. This process is arranged so that it may be continuously in operation. The Archbutt and Deeley process is also a modification of the Clark process in which the object is to shorten the time required for the precipitated chalk to subside and to reduce the space required for settling tanks. In this process the precipitated chalk is not removed but allowed to mix with subsequent charges of water and lime. Steam injectors, at the bottom of the tank cause the mixing of the contents, and also slightly raise the temperature. In this way it is found that the calcium carbonate from previous charges causes a much more rapid settling of that newly precipitated, and is especially helpful when the quantity of magnesia in the water is considerable. It is usual to pass the gases from burning coke into the exit pipe to “ carbonate ” the softened water, and so render soluble any unsettled carbonate of lime or magnesia, which might otherwise collect in and choke the pipes through which the water passes. The Caustic soda process, as implied in the name, involves the use of sodium hydrate instead of calcium hydrate. The principle of the process is closely similar to the lime processes, all the free carbonic acid is neutralised, and the dissolved chalk and magnesia precipitated. In this case, however, the carbon dioxide is neutralised to form soluble sodium carbonate, which is advantageous in that it causes the removal of some, at any rate, of the permanent hardness of the water, thus :— CaS04 + Na2C03 = CaC03 + Na2S04. If, however, the permanent hardness is less than the temporary, the softened water will be alkaline with sodium carbonate. The caustic soda solution is run into the hard water at a regulated rate, the temperature is raised by a steam injector, and the mixture passed through the tortuous paths of a conical44 THE MANUFACTURE OF LEATHER mixer, at the bottom and point of which a large part of the sludge rapidly collects and is occasionally run off by opening a cock. The mixture then passes into a settling tank, in which the rest of the precipitate is deposited. This process is also continuous but somewhat costly. It will be evident that the permanent hardness of waters softened by the lime processes and the permanent hardness in excess of that removed by the caustic soda process can be removed by using in addition to lime and caustic soda the requisite quantity of sodium carbonate. Where desirable, this can usually be arranged to work in with the apparatus used for the softening of temporary hardness only. The estimation of the hardness of water has been accomplished by titrating with a standard soap solution, the end point being a “ permanent lather,” but it can now be done much more accurately and scientifically by the methods of Hehner as subsequently modified by Pfeifer and others. Directions for these are now given. Temporary hardness.—100 cc. of the water is titrated in a hard porcelain basin with N/10 hydrochloric acid and alizarin as indicator. It is necessary to boil off the carbon dioxide continuously, as alizarin is affected by it; this being done, however, the end point, from violet to pale lemon-yellow, is very sharp. With very hard waters the indicator may be precipitated, but this may be overcome by a repetition of the titration in which the indicator is not added until near the end of the titration. Two drops of a i per cent, solution of the purest alizarin paste is used. No. of cc. N/10 HC1 X 5 = parts CaC03 per 100,000. Permanent hardness.—200 cc. of water are placed in a clean Jena flask and 25 cc. of N/10 caustic soda, and 25 cc. of N/10 sodium carbonate are then added. The mixture is boiled down to about 150 cc. and rinsed with hot distilled water into a 200 cc. flask, cooled, made up to mark, and mixed well. The solution is then filtered, rejecting the first 50 cc. 100 cc. are pipetted from the rest and titrated with N/10 hydrochloric acid and alizarin as before. (25 — No. of cc. used) X 5 = parts CaC03 per 100,000.WATER 45 If a minus quantity is obtained there is no permanent hardness, and the water contains sodium carbonate, which being thus estimated, should be deducted from the temporary hardness. Each part so deducted corresponds to 1*06 parts of anhydrous sodium carbonate. Magnesia hardness (whether temporary or permanent).— 100 cc. of the water are titrated to neutrality as in the determination of temporary hardness. 25 cc. of lime water (50 cc. if the quantity of magnesium is great) is placed into a 200 cc. flask with a long stoppered neck and the neutralised water run into it hot, rinsing the basin with hot carbonic acid-free distilled water. The mixture is mixed well, warmed for a short time on the water-bath, cooled, and made up to mark. It is now filtered, the first 50 cc. being rejected, and 100 cc. titrated with N/10 hydrochloric acid and phenol phthalein, in the cold. The process is repeated exactly with distilled water instead of the water being analysed. It being supposed that x cc. of N/10 acid were used for the first titration and y cc. for the second, 5(y —■ x) = magnesia hardness in parts CaC03 per 100,000. In any of these calculations “ parts CaC03 ” may, of course, be readily converted into “ parts” of Ca, CaO, Mg, MgO, etc., by factors obtained from the atomic weights. Lime required for softening will include any free carbon dioxide over and above that combined as bicarbonates, and may be determined by repeating the process for the determination of magnesia hardness with the unneutralised and unboiled water. Actual free carbonic acid can be estimated by titration of 100 cc. of the water with N/10 caustic soda and phenol phthalein. Other than these, the chief constituents of the water which it is desirable to estimate are chlorides, iron, and nitrogenous matter. Chlorides are easily determined by titrating 100 cc. of the water with N/10 silver nitrate and the chromate indicator, and iron can be readily determined colorimetrically with thiocyanate. Nitrogen is best determined by the Kjeldahl process (p. 398), and is important as indicating a possible contamination with sewage, and consequent bacterial infection.46 THE MANUFACTURE OF LEATHER There is no clonbt whatever that for the purposes of the leather manufacturer the best water is that which is most free from dissolved matter, both inorganic and organic, and it may be useful to note here the effect of some of the usual impurities on the various processes of the manufacture. In soaking hardness is of little moment, the most important matter being the freedom of the water from bacteria, which would have a solvent or other injurious effect on the hide or skin. In liming all temporary hardness is removed in making up the liquor, and any permanent hardness in the form of the sulphates of calcium and magnesium will help in plumping, though chlorides are said to act in the opposite direction. Any sodium carbonate, either naturally occurring or from the caustic soda softening process, will be converted into hydrate and assist in the operation. Injurious bacteria may also make their influence felt here, though the lime acts as an antiseptic to many of them. In the unhairer’s and flesher’s water pits the occurrence of any dissolved carbonates in the water is exceedingly injurious, for they interact with the caustic lime on the hides or skins and cause the precipitation of calcium carbonate which roughens the grain and which is not removed by the weak acids in any of the various deliming processes. This carbonate will also cause discolorations in the liquors (see below). Temporarily hard waters are exceedingly bad in this respect, for they not only fix the carbonate formed from the lime and carbonic acid, but also that contained in the water itself. The addition of a slight excess of lime to these pits is very desirable, for acids do not satisfactorily expel the carbonic acid they liberate. Water containing sodium carbonate may cause this effect also, which may be remedied as indicated above or by the addition of a slight excess of calcium sulphate. In either case a thorough mixing is necessary. In the deliming processes the same remarks are relevant, but the freedom from bacterial contamination is even more important. In leaching temporary hardness is very objectionable, for the metals combine with many times their own weight of tannin, making it quite useless by the formation of insoluble tannates, and, furthermore, these are readily oxidised to dark-coloured compounds and are a very common cause of discolorationsWATEB 47 and stains on tanned goods. The same objection applies with equal weight to the dissolving up of extracts. In these cases the difficulty can be got over quite satisfactorily by the use of oxalic acid, which precipitates the calcium entirely from solution as oxalate, or by the addition of sulphuric acid, which converts the temporary hardness into permanent, this latter being much less injurious in this way as the calcium is combined with a strong acid, and will therefore not react readily with the tannic acids. The introduction of iron into any of the processes will also result in the appearance of marks and stains of a very similar nature. The occurrence of appreciable quantities of chlorides in the liquors is also objectionable for sole leather tanning, as these have considerable power in preventing “ plumping.” In the mineral tannages hardness is of much less consequence, but in fat liquoring (p. 356) each part of hardness causes the destruction of more than twelve parts of soap by the precipitation of insoluble lime and magnesia soaps, which are also liable to stick to the leather and make trouble in the finishing processes. A very similar difficulty occurs in scouring. In dyeing, iron must be absent, and temporary hardness is injurious in that it reacts with and precipitates the basic dyes, and may cause unevenness of colour by the deposit of this precipitate. This difficulty does not arise with the acid dyes, and may be overcome in the former case by addition of acetic acid. For boiler purposes all hardness is very undesirable. Temporary hardness, as we have seen, is destroyed by boiling, but this involves the precipitation of the dissolved carbonate as a “ sludge,” which it is necessary to remove at intervals. Calcium sulphate is also insoluble in water at high temperatures, and is deposited as a very hard scale, whilst magnesium chloride hydrolyses, and the liberated hydrochloric acid may cause corrosion. Only mineral oils should be used for cylinder purposes, as those of animal or vegetable origin form sticky deposits of lime soaps which are very apt to cause the plates to be heated unevenly. A properly softened water, moreover, is much better than any boiler compositions. From these considerations it is clear that the purer the water48 THE MANUFACTURE OF LEATHER supply is the better are the results obtained by it. The best natural water is rain water, and where there is no other good supply it is strongly advisable to make some attempt to collect this and use it at least for the leaches and liquors. River water ranks next, wrhilst well water is often exceedingly hard.CHAPTER YI SOAKING The process of soaking is essentially one of softening and cleansing by means of water. As we have already noted (p. 17), both gelatine and hide substance are practically insoluble in water, though water is absorbed by them, so that there is no danger of the loss of our gelatinous raw material through a solvent action unless water at a high temperature be used. On the other hand the albuminous matters, such as the blood of the minute corium vessels, as well as that which has been allowed to adhere mechanically to the exterior, and the lymph, are soluble in water and are dissolved by it in the soaking process. Dung is also softened by the water and ceases to adhere to the hide or skin. It is desirable to remove both blood and dung because they are both liable to cause the appearance of stains in the later stages of the manufacture, and the latter is apt to counteract the plumping influence of the limes by acting as a mild bate. If therefore after the soaking process any dung still adheres to the hide, it should be mechanically removed by some blunt instrument before the goods enter the lime liquors. Earthy matters are also removed by soaking, and the possibility of “iron stains” from this source thereby eliminated. It is also necessary in the case of cured hides to remove salt and other materials which have acted as preservatives, and as these usually are readily soluble, this also is accomplished by soaking. Even for wet salted hides a considerable dehydration of the original skin material has occurred, and it is one of the functions of soaking to restore this moisture to the hide and to bring it back again into the condition in which it was before the cure was effected. All these operations are most efficiently accomplished by the plentiful use of water. M.L. E50 THE MANUFACTURE OE LEATHER Eacterial action.—For sole and heavy leathers it is highly desirable to accomplish these objects without putrefaction and consequent loss of truly gelatinous matter, and hence it is exceedingly important that the possibility of bacterial action should be reduced to a minimum. In the case of some of the lighter leathers, however, this is less important, for in these a partial peptonisation and loss of the gelatinous fibre cementing substance may even be desirable in order to produce the requisite pliability and softness. In bacterial action, however, the difficulty is to obtain that kind of decomposition which is desired, for indiscriminate putrefaction undoubtedly leads to ruin, and there is as yet little definite knowledge as to which are the ferments which will bring about the desired effect, and which are the organisms which attack the grain and more valuable portions of the hide or skin. There are a few defects, such as “stippen” (spots) “weak grain/’ etc., which have been identified as originating in the soak liquors, but as many of these injuries only become noticeable in the tanned or finished leather the matter is difficult to investigate. If these influences were less obscure, the hide might be introduced into an appropriate sterile medium and a pure culture of the desired bacteria added, but all that can be done as yet is to keep the putrefaction within limits, and by empirically discovered arrangements to encourage that bacterial influence which has been found to produce the desired effect. In the case of dried and dry-salted hides in which the difficulty of softening is much greater, it was once thought essential to allow a certain degree of putrefaction in order to produce the required softness, and “putrid soaks” were therefore used for this purpose, but it is now universally recognised that this is both undesirable and unnecessary; the danger of “ damaged grain ; due to uneven attack, the risk of extensive loss of pelt substance, by action at an irregular rate, and the very careful supervision always required for the process, all contributing to its overthrow. Mechanical treatment has been resorted to for assistance in both the softening and cleansing processes. It was once not uncommon to give hides, after some preliminary soaking, a “ breaking over ” by means of a blunt tool on the beam,SOAKING 61 which operation also assisted in the removal of the dung and dirt, and this process is still employed for some skins. The “fulling stocks” (Fig. 15) have also been very popular for Fig. 15.—Stocks (Farrar and Young). dried and dry-salted hides and skins. These consist of two heavy hammers, raised alternately by projections on a revolving wheel, and allowed to fall on the hides which are contained in a curved box below. This beating or kneading Fig. 16.—Scotch tumbler action is continued for rather less than half an hour, according to the quality and condition of the material, and is used only to assist in the softening by water. The American “hide mill” is a modification of this arrangement, and “crank52 THE MATSTTFACTURE OF LEATHER stocks * have been found useful for the smaller hides and skins. The possibilities of mechanical damage and of the unnecessary loss of hide substance, however, have been increasingly recognised, and these rather severe methods of treatment are now much less generally used. “Drums” or “tumblers” are now considered to give an efficient and safe mechanical action, and their use is very widely extended. There are many forms of these, but they are all large wooden cylinders capable of slow rotation about their axis. One common form, shown in (Fig. 16), is fitted in the interior with shelves or blunt pegs, as in the common churn. The goods are inserted through the hole in the side and a continuous stream of water may be run in during the operation, an exit being arranged by pulling a plug in the front of the drum. The direction of rotation is usually reversible. These drums are also employed for tanning, dyeing, and other operations. Another useful form consists merely in a staved cylindrical enclosure which rotates in a deep tray of water. A “lid” or opening is arranged in the staved part of the drum (Fig. 17). Like the stocks, these are chiefly used to assist in the ordinary pit soaking, and are useful at all stages, according to the class of goods. Chemical assistance is also extensively employed, the aim being to use a substance which will not only assist in Boftening and swelling but will also have some antiseptic action, and atSOAKING 53 the same time have no solvent or injurious effect on the hide substance and cause no complications when the hides are transferred to the lime liquors. Dilute mineral acids, which have considerable antiseptic effect, have been tried, but their swelling effect is rather drastic, and it is generally necessary to neutralise them before the goods are forwarded to the limes. Of these a 2 per cent, solution of sulphurous acid has been found to give the best results, ammonia or sodium hydrate being used to neutralise. Borax, boric acid, sodium carbonate, ammonia, salt, potassium nitrate, arsenic sulphide, and many other substances have also been suggested, their action being generally that of a mild antiseptic, and also in the case of the acid and alkaline substances, that of a weak swelling agent. They all seem open to some objection, however, and as some are rather costly, it is not surprising that they have never been widely adopted. A 0*1 per cent, solution of carbolic acid is excellent if it is desired to make or keep the soak liquor quite sterile, and the only objections to its use are that it tends to coagulate albuminous matter, and that it is liable to carry its antiseptic effect into the limes and so render the process of depi-lation decidedly slower. It is very useful however, for tainted goods as a preventive of further damage. Practically all that is desired in a chemical assistant to the soaking process is found in the use of a 0*1 per cent, solution of caustic soda. A solution of this concentration has very considerable softening and swelling effect, and acts rapidly on the most obstinate hides. It is a reagent which it is not necessary to neutralise or remove and which causes no complications when the goods enter the lime liquors, being in fact often one of the constituents of these. Its solvent effect on hide substance is very small and at this concentration has been shown to be practically nil; and whilst it is a fairly good antiseptic to all common putrefactive organisms, it is not so fatal to those which are useful in the unhairing process. A 0*2 per cent, solution of sodium sulphide has been also found to give good results, but the time required by it is rather longer, and its solvent effect is slightly greater. Both are now extensively employed for this purpose 1 and there is much to be said for them also from a sanitary standpoint.54 THE MANUFACTURE OE LEATHER The nature of the treatment which should be applied in practice naturally varies very widely with the class of goods and their condition when received. No hard and fast rules can therefore be given, but some idea can be gathered as to the extent to which this operation is desirable from the following outline of the methods used for the commoner classes of goods. Market hides, which are uncured and not dehydrated, require chiefly the cleansing effect, being already in a fairly soft and moist condition. The chief aim in this case should be to reduce" the time of soaking as far as possible ; and two fresh waters, in each of which the goods only remain a few hours, are all that are usually required, though a third water may be occasionally desirable. For “slippy” or even slightly tainted goods the use of the tumbler is necessary as a preliminary operation, with possibly the addition of carbolic acid. If a drum is not available the first water should be changed in about four hours. The use of the drum is also desirable in hot weather. It is not uncommon to use a weak lime liquor as a soak water for green hides. Salted hides require a decidedly longer soaking, for in this case a certain amount of dehydration has occurred, and hence some softening and swelling are necessary. It is also very desirable to get rid of the salt as far as possible, for if not the hides are said not to plump satisfactorily in the limes. Besides this flattening action it is to be borne in mind that common salt in 10 per cent, solution has a distinct solvent effect on the hide fibre, and as this concentration is soon reached with salted goods it is necessary to change the goods very quickly from the first water and fairly soon also from the second. It should be remembered that salt has a checking influence on bacterial growth and action so that a more prolonged soaking is admissible without sterilisation. At the very least three waters should be given, some American yards giving much more, and in any case it is decidedly advantageous to remove the last traces of salt by means of the tumbler and running water. No other mechanical treatment is necessary with such goods. It is permissible for these soaks to use the third water of one pack as the first water of a new pack, and gome economy of water is thereby effected,SOAKING 55 Dried hides and dry-salted hides are naturally softened with much greater difficulty than are wet-salted goods, and with regard to the removal of salt the remarks just made still hold good. In these cases it is often necessary to extend the soaking to a week or more. Dried hides can be left for a considerable time in their first water if carbolic acid or borax be used, but dry-salted hides should be changed more quickly on account of the solvent action of the salt. Drumming with slightly warmed water may occasionally be helpful after some preliminary soaking in the cold, but this is dangerous for goods which are not quite reliably sound, for then the bacterial damage would proceed further, unless antiseptics were added. Some tumbling in cold water is desirable in any case, but the use of “ putrid soaks,” stocks, and other drastic measures is to be condemned for regular use. It is for this class of goods that the use of caustic soda and sodium sulphide has been the greatest boon. With these, dry hides can be satisfactorily softened in one or two days, a short soak in fresh water or a slight drumming in running water being all that is further necessary. E. I. kips are treated much in the same way as ordinary dry-salted hides. It used to be common to give them a short soak and then put them in the stocks, but it is usual now to use chemical assistance. They are soaked over night, and next morning hauled, and swilled to remove the “ cure.” They are then placed in a sodium sulphide soak till next day, and finished by drumming in running water till soft. Flint dried kips may be given about a week’s soaking in a water made sterile with carbolic acid. Calf skins are usually uncured, but should receive three short soaks in fresh water. If salted, further treatment is necessary. G-oat skins, if salted, should soak over night and then be stretched, after which they should be soaked for a few hours in a fresh water and drummed. After further soaking they are worked over the beam. The treatment should be continued until the skins are thoroughly soft, and antiseptics used if the process be prolonged. Flint dried goods should have a rather longer preliminary soaking, and the second water should56 THE MANUEAOTUEE OF LEATHEE contain sodium sulphide or borax, the whole operation in this case taking about a week. Seal skins are treated with warm water (about 25° G.) to assist in removing the large quantity of oil they contain. After cooling in this water they are “ blubbered” or “brushed over” with a blunt knife on both sides, and the oil thus eliminated. This treatment may be repeated, with perhaps some drumming, until ready for the limes. As a rule the time required for soaking skins is less on account of the thinner character of the goods. It is a good maxim both from a bacterial and chemical point of view to make the first soak the shortest, and it is highly necessary that soak pits should be effectively disinfected at definite intervals. For this purpose “ creolin,** which is a tar oil containing phenols and cresols emulsified by soap, is very efficient, though carbolic acid alone is quite satisfactory. Another point to bear in mind is the undesirability of making up the last soak water of any pack in a pit which has just been used for a first water, unless the pit has been disinfected or cleaned. The chemical control of this process involves principally the determination of the nitrogenous matter which goes into solution. This can readily be determined, when necessary, by Kjeldahling. The nitrogen found includes that from ammonium salts, albuminous matter, and peptonised hide substance, but it is often convenient to state this result in terms of undecomposed hide substance, on the assumption that it contains 17*8 per cent, of nitrogen. Where caustic soda is used it can be easily estimated by titration in the cold with N/10 acid and phenol phthalein. The ammonia may render this result slightly inaccurate, but for control work all that is necessary is to keep the alkalinity approximately constant, and by this method a caustic soda soak can be repeatedly used for dried hides, but for dry-salted goods the rapid accumulation of salt must be considered. Salt can be estimated as in water analysis, the solution being first made neutral by the addition of a slight excess of formic acid and subsequent addition of magnesia.CHAPTER YII UN HAIRING The term unhairing or dépilation is usually understood to include not only the mechanical removal of the loosened hair of hides and skins, but also the means by which they are brought into a suitable condition for such treatment, and also those other trimming and cleansing processes which are associated with this operation in practice. Dépilation is brought about either by “ sweating55 or by “ liming.” In sweating the hides or skins are subjected to a regulated putrefactive process, which is stopped when the hair is sufficiently loosened to be removed by gentle mechanical treatment. In liming the hides or skins are immersed in milk of lime, occasionally assisted by other depilatants, and the hair and other epidermal structures similarly attacked. The lime also swells the corium fibres, and at the same time saponifies or emulsifies the greasy matters of the skin. Dépilation by Sweating.—This is the most ancient method of removing hair, and was first accomplished by merely piling the skins in a heap and allowing them to putrefy. Later it was discovered that the principal factors which controlled this fermentive action were the temperature and humidity of the surroundings, and hence it is now general to conduct the process in closed chambers in which these forces are within control. A series of such chambers together form the “ sweat pit.*’ The cold-sweat system is carried out largely in the United States of America for unhairing the thoroughly softened flint hides. The sweat pit is a structure which may be entirely above ground, and is built of stone and is protected from outside climatic influences by another wall and a lining of timber, earth, tan bark, etc. A passage about 7 feet wide runs between two sets of chambers, which have each tight58 THE MANUFACTURE OE LEATHER doors, ventilators, and windows. The pit is covered with a false bottom, under which are the pipes that deliver the steam for warming and moistening the atmosphere. By means of a cold water sprinkler also the temperature is kept between 60°—70° F. The hides are hung on hooks, and each chamber takes a pack. The sweating lasts usually four to five days, and, especially near the end, is very carefully watched by experienced men. When complete, the hides are thrown into either water or a weak lime liquor, which takes away the “ slimy ” feel and produces a slight swelling, so that the hair can now be conveniently scraped off with a blunt knife. The warm-sweat system is employed largely on the Continent both for hides and for sheep skins, in which latter case it is often termed “ staling.” The process is very closely similar to that just described, but takes place at a higher temperature (75°—80° F.) and is on the whole rather quicker. In this process the putrefaction makes rapid progress, and the operation requires even more care than that just described. A slight salting on the flesh side is occasionally practised. Both these processes are attended with very grave risks of injury through the bacterial action going too far or being uneven, and it is very common to find leathers with damaged grain when the goods have been unhaired by this method. The putrefactive bacteria attack first the hair root and epidermal structures, but afterwards will also attack the corium, and there is a further risk that some injurious bacterium may thrive under the conditions used. A considerable quantity of ammonia is produced by the ferments as a decomposition product, and this doubtless affords considerable assistance in loosening the hair. Procter, indeed, has shown that a sterile ammoniacal atmosphere will produce a similar effect. Further disadvantages of sweating are that grease is not “ killed ” as with lime, and there is little, if any, swelling, so that it is necessary to “raise” the pelt by subsequent liming or by treatment with acids before commencing the tannage. Dépilation by Lime.—The use of milk of lime for unhairing dates back many centuries, and the effects that it is capable of producing under varying conditions have therefore been longTJNH AIRING 59 known through the extensive experience and empirical observations of unscientific men. Although its mode of operation is now very much more clearly understood the whole process of depilation involves so many different actions that it has been difficult to find a material which will yield the same combined effect, and hence the principal modifications which have been introduced consist usually in usuing some other material in addition to lime and in that way assisting its influence in the required direction. Lime, at any rate, has the great advantage that it is a common and cheap material. It is obtained fiom natural limestone and chalk by “burning ” with coal in a kiln. In this way the calcium carbonate, which forms the principal part of these substances, is converted into calcium oxide by the elimination of carbon dioxide from the molecule. CaC03 = CaO + C02. The reaction may be represented by the given equation, but the process is really more complex, and a good proportion of carbon monoxide escapes with the furnace gases. Quicklime, or calcium oxide, is a white, amorphous, and infusible solid which possesses a very strong affinity for water, and which is therefore useful in many cases as a dehydrating agent. When a small quantity of water is poured upon it combination takes place with the evolution of a considerable amount of heat, some steam is evolved, and the mass swells to some extent and crumbles to a soft and apparently dry powder. This operation is known as the “ slaking ” of quicklime, and results in the production of “ slaked lime,” or calcium hydrate, CaO + H20 = Ca(OH)2. Ordinary quicklime is, however, very liable to contain magnesia as well as various silicates, and if the heating in the kiln has been too intense these latter substances fuse and cause the quicklime to have a much more compact texture. The lime is then said to be “ dead burnt,” and is found to slake with much greater difficulty. Heat is a considerable help to slaking, and by adding only a small quantity of water to the lime the heat evolved is utilised in assisting the reaction. If a large quantity of water is added this heat is merely used in60 THE MANUFACTURE OF LEATHER slightly warming the water so that both the reaction and the disintegration would proceed much more slowly. This should be remembered in making up the lime liquors. Calcium hydrate is soluble in water to the extent of one part in 778 parts water, but this varies slightly with the origin of the lime and the nature and extent of its impurities. It will be seen, however, that its solubility is very limited, its saturated solution being about N/20, and from the standpoint of the Fio. 18.—Lime Yard (Messrs. Walker, Ltd., Litberland). leather manufacturer this is an advantage, for even if a saturated solution be used it is only a comparatively mild reagent, and some variation in the time in which it is in contact with the goods makes, therefore, very little difference, and as it is usual to use it at its full strength the exact quantity to add does not seriously matter so long as there is an excess of undissolved hydrate. For the same reason, moreover, it is almost impossible to ruin goods by the accidental employment of too large a quantity, and all these areTlNHAlEIisra 61 no doubt further reasons for its continued popularity. As a considerable amount of hydrate is absorbed by hides and skins when they are in the lime liquors, it is indeed necessary to have some excess of undissolved lime to take the place of this, and hence it is quite general to use “ milk of lime ” rather than “ lime water ” for the lime liquors of the unhairing process. The lime may be kept in the yard in the form of oxide and the liquors made up by allowing the proper quantity to slake over night in the bottom of the lime pit and then making up the liquor by adding water and mixing well, or a large quantity of lime may be slaked in a shallow tank and afterwards mixing to a stiff paste and thus used as the source of supply for making up the liquors. It should never be tipped into the liquors in unslaked lumps, for this leads to local heating and the burning of the goods. The mode of action of the lime is entirely chemical, and takes place in several directions. Being a caustic alkali it attacks the softer keratinous structures of the hide (rete malpiglii, hair bulb, etc.), first softening and finally dissolving them ; but on the harder keratins it has practically no effect, so that the hair is little damaged by the ordinary lime liquor. This solvent action is the basis of the unhairing property of the reagent, for on the gelatinous hide fibre it has little solvent effect. It has, however, a vigorous physico-chemical action on the corium tissue, by which water and lime are absorbed by the hide in considerable quantity. This change may be distinctly separated into two distinct parts, swelling and plumping. Swelling may be defined as the extent to which the surrounding solutions is absorbed by the hide fibre and it can be measured by the increase in weight. This has been shown to be a function of the concentration of the hydroxyl ions of the solution,1 and is therefore obtained with other alkalies and is increased by the use of more strongly ionised solutions. It is, however, not directly proportional to this factor, but is influenced by the basic radicle from which the OH ions have dissociated. The plumping effect is the change by which the soft and flacid hide becomes firm and elastic. Both these actions are fairly vigorous with lime, but other 1 Stiasny, see Collegium, 1907, 128.62 THE MANUFACTURE OF LEATHER alkalies produce very different effects. Both ammonia and caustic soda will swell, but the ammonia-swollen hide will always be soft and unplumped, whilst the soda-swollen hide will be violently plumped. Baryta, like lime, is intermediate in both effects. The function of this physical change in the hides is to separate the fibres and to split them up into the finer fibrils of which they are composed. This separation is very helpful in rendering the tanning process both quicker and more complete. A certain small amount of the inter-fibrillar substance is also dissolved by the alkali and a certain amount of lime “fixed” in the fibres so that it is neutral to phenol phthalein but not to methyl orange. Lime also acts upon the natural fatty and oily matters of the skin and converts them into insoluble calcium soaps which are removed partly in unhairing and partly in scudding. Sodium sulphide, Na2S, 9 H20 is now used to a considerable extent as a depilatory agent. It may be obtained in pale brown deliquescent crystals which contain only about 30 per cent, of the actual sulphide. In solution it decomposes into NaOH and NaSII and it is to both these substances that it owes its characteristic effects. It rapidly attacks and dissolves keratinous matters, but the gelatinous tissue is much less affected by it and hence it was first used for unhairing by painting the hair side with a strong solution, the hair and epidermis being rapidly reduced to a pulp.TJNHAIBING m The disadvantages of the process were that the hair roots were not removed in this way and the hair was a total loss, and as, moreover, the sulphide is expensive when compared with lime it was necessary to make some modification in the method of treatment. It is now common to mix lime with its solution and paint the flesh side of certain skins with the mixture. In this way the hair root is attacked first and when softened the hair can be pulled away quite uninjured. It is also added in smaller quantities to the ordinary lime liquors, especially during the short hair season, and so assists in rapid and complete unhairing. Arsenic sulphide,1 realgar AS2S2 has been used as an assistant to lime for many years. It is found in nature but is now prepared on a large scale by distilling a mixture of iron and arsenical pyrites. It is a red solid, which is insoluble in water, and hence cannot be added directly to lime liquors. It is added to slaking lime, a complex chemical action then taking place, in which calcium sulphydrate Ca (SH)2 and a little calcium sulph-arsenite are formed. It is the former of these which produces the characteristic action of an arsenical lime, and the quantity which is formed depends upon the temperature reached in slaking and the proportion of realgar mixed with the lime, The higher the temperature, the more powerful is the product. It will be noticed that in this case no caustic soda is produced, and it is this fact which constitutes the difference between a lime liquor made up with realgar and one made up with sodium sulphide. Calcium sulphydrate with excess of lime has a solvent effect on the keratins as great as sodium sulphide, in equivalent quantity, and it has been shown by Stiasny that the solvent action is due to the joint action of the OH and SH ions, and is at a maximum when they are present in equal proportions, as it is found in the solution of sodium sulphide. Many suggestions have been made to obtain calcium sulphydrate and the effects it produces in other ways. “ Gas lime,” which contained the sulphydrate and sulphide of lime, was once used to make a paint for unwooling lamb skins. The “ tank waste” of the Leblanc process consists chiefly of calcium 1 Quite distinct from the yellow As2S3 and As2S5, which, however, may be employed quite similarly.64 THE MANUFACTURE OF HEATHER. sulphide; but by the action of atmospherical moisture sulphy-drate is formed, and the product has been used for unhairing. A very similar mixture was patented by Lufkin, who mixed sulphur with soda ash and added the mixture to slaking lime. Caustic soda, sodium hydrate, NaOH, is also used to “ sharpen ” the action of lime for certain classes of goods. It may be added directly to the lime liquor or produced there by the addition of carbonate of soda. The addition of “ wood ashes,” which are chiefly potassium carbonate, will, of course, produce a similar effect and has been long practised. Caustic soda is used on the Pullman - Payne process to produce lime in the skin by first using a bath of caustic soda and afterwards one of calcium chloride. It forms soluble soaps with the greasy matters of the skin and both swells and plumps the hide fibre with great vigour. Bacterial action is nearly always a prominent factor in the unhairing process and indeed it is a question whether, with some materials, the ordinary depilatory effect can be obtained without its aid. Although a new lime liquor is an almost completely sterile medium, the solvent effect it exerts on the various proteid matters which enter it causes the amount of organic matter which is in solution gradually to increase. In this way it is made possible for certain bacteria to thrive and then also assist in dissolving further quantities of both keratinous and gelatinous matter. If, therefore, a liquor be Fig. 20.—Fleshing by hand labour.UNHAIRING 65 repeatedly used, it is clear that there will be not only a rapid accumulation of both the hydrolytic and putrefactive decomposition products of the proteids (ammonia, amines, salts of amido acids, peptones, etc.), but also an almost proportionate multiplication of the fermentive organisms; and it is evident that if this be continued indefinitely the liquid may continue to unhair, but it will be by virtue of its bacterial rather than its chemical activity. It has been long thought that the ammonia formed in lime liquors by bacterial action was a powerful assistant in loosening the epidermal structures by its solvent effect, and this was apparently borne out by the rapid loosening of the hair which occurs when pieces of hide are placed in weak ammonia solutions. Stiasny, however, has shown that in an old lime liquor the ammonia forms complex compounds with the calcium salts present, of which the familiar Ga (8 NH3) CI2 may be regarded as a type, and that under these circumstances no unhairing effect is produced. The depilatory and solvent effects of old lime liquors therefore are due almost entirely to bacterial action and are prevented by the addition of antiseptics. From the above considerations, therefore, it is possible to judge the effect that various liquors will produce. An old or “ mellow” lime liquor, by virtue of its bacterial activity and solvent effect not only on the keratins but also on the gelatinous and interfibrillar substances, will tend to produce a leather which is soft or loose, and with a dull grain. It will unhair quickly from the same cause, but will not plump, on account of the ammonia present. A new lime liquor, especially if sharpened by the presence of caustic soda, will be pretty strongly antiseptic and have little solvent effect on the gelatinous matters, whereas the swelling and plumping effects will both be strong. With caustic soda, however, there is a tendency towards a rough grained leather. If arsenic sulphide has been used the purely depilatory effect is greatly assisted, and if slightly mellow the moderate amount of bacterial action will assist in producing a soft and flexible leather, but with a sound and silky grain. With sodium sulphide the unhairing is quickened, the plumping effect better, but the resulting M.L F66 THE MANUFACTURE OF LEATHER leather may be rather harsh grained unless the sulphide is present in small quantity. If an old sulphide liquor be used a certain amount of pliability may be thereby introduced. In this way it is possible to determine what liquors are most suitable for the class of leather we desire to make. In sole leather manufacture where weight and firmness are important, it is clear that a short liming in new liquors is very desirable to keep the solvent and fermentive actions at a minimum; and that caustic soda may be beneficial in order to get the full plumping effect. A significant fact, pointed out by Eitner,1 is that although the solvent effect of an old lime liquor is in the long run much greater than that of a new lime, it is decidedly less during the first two days of contact. Hence it is wise to place the hides first in the oldest lime to get the softening and partial swelling effect, and least solvent action; and afterwards into a newer lime where the solvent effect will then be least and the plumping effect at a maximum. This has long been a common mode of procedure. With harness and belting leathers a greater flexibility is required and hence it is desirable to have rather more mellow liquors; though it is possible to produce nearly the same effect by liming as for sole leather, with perhaps a little sodium sulphide, and allowing the solvent bacterial action to take place in the “ bating.” For upper leathers a somewhat long and mellow liming is desirable (as well as bating) in order to obtain the requisite “ feel.” For the light leathers where softness and pliability are important, a long mellow liming is desirable, and for “ glove kid,” etc., where a glossy grain is required, the use of arsenic sulphide is an advantage. The Mechanical Operations of the Lime-yard.—The liming of the hides and skins is usually carried out in pits of which the sides are built of masonry and the tops are level with the floor of the yard. They will usually take a “pack” of several dozen hides at once, or a proportionate quantity of skins. When the liquor is made the milk of lime is well “ plunged ” up with long poles, and the hides laid in horizontally one at once, pressing each one down with poles to ensure 1 Der Gerber, 1895, 157 and 169.UNHAIRENQ- 67 proper immersion. After remaining thus for the particular period required they are “hauled” up again by means of poles to which a blunt hook is attached, and piled one above the other D. E. E. Pig. 21.—Beamhouae tools. A, unhairing knife; b, scudding knife; c, fleshing knife; D, hauling hooks; E, scudding slate ; l', rounding knife. antil either that pit or another has been plunged up and prepared to receive the pack. On coming from the last liquor they are taken by the “ unhairers,” and laid over a sloping wooden “ beam,” which has a convex surface covered with f 268 THE MANUFACTURE OF LEATHER zinc. The hair is then removed by scraping with a blunt “ unhairing knife,” the motion being away from the operator. The hides are then immediately thrown into soft water, from which they are taken by the “fleshers” as required. These workmen lay the hides again over the beam, flesh-side up, and by means of a sharp “ fleshing knife ” cut off the pieces of adhering flesh, fat, tissue, etc. They are then thrown into water again to preserve them from the carbonation of the Fig. 22.—Drum beam-house machine. caustic lime on the grain by atmospheric carbon dioxide. Hides then go to the rounding table1 and are cut up as explained earlier (p. 32), and are then ready for the deliming process. Skins are not rounded, but both hides and skins may for certain purposes be “split” at this stage by machine to form “grains” and “ fleshes ” of equal area, which may afterwards be tanned in very different ways. 1 In America and sometimes in this country they are cut down the back into “ sides.”UNHAmnsfO- 69 The use of machinery for the beam-house work is also gradu. ally increasing, and slowly superseding hand labour. These machines are usually cumbersome, expensive, and require a good deal of power, but are constanly being improved and made more suitable to the conditions required for the treatment of any particular class of goods. Unhairing machines are the least satisfactory beam-house machines, and are gaining ground very slowly on hand labour for various reasons. Unhairing by hand is not difficult work and can be done at considerable speed by practised men, and Fig. 23.—The Conus unhairing machine. it is also a much cleaner process, for most goods unhaired by machines need to be gone over by hand, especially at the edges, after passing thorough the machine. Goods unhaired by hand are, moreover, much less liable to damage in the process, because the pressure that is applied can be varied as necessary on different parts of the hide. The Drum beam-house machine (Fig. 22), sometimes known as the “ Yaughn ” machine, but now made by the Turner Co., Ltd., represents one type of unhairing machine. The working tool is a cylindrical roll with spiral knife blades. These spiral blades vary considerably in sharpness of edge and steepness of pitch, according to the class of goods for which they are70 THE MANUFACTURE OF LEATHER intended, and are generally arranged to be left-handed in one half and right-handed in the other half, hence scraping the goods both sideways and in the direction of rotation. Another feature of the machine is the semicylindrical drum, covered with thick rubber, over which the goods are placed, half only on the outside. When this rotates the clamped hide is drawn under the spiral blade; the motion of the drum then automatically reverses and the roll rotates at a much higher speed. Fig. 24.—The Leidgen unhairing machine. The other half of the hide or skin is then treated similarly. This machine, with a suitable cylinder, also works satisfactorily for fleshing, scudding, striking, scouring, etc., and many modifications of it are now on the market for these purposes. The Conus unhairing machine (Fig. 23) (Moenus Machine Works, Ltd., Frankfort), is a machine of different type, the hide being thrown over a moveable cone which by rotation about its axis brings automatically the whole of the hide under the tools. These are fixed on an endless band, and work fromTJNHAIEING 71 the point of the cone outwards. No fixing arrangement is required, and the cone can be raised or lowered at will by the workman, thereby adjusting the pressure to the condition of the goods in various parts. Fig. 25.—Whitney unhairing and fleshing machine.72 THE MANUFACTURE OF LEATHER The Leiclgen unhairing machine (Fig. 24) (The Turner Co.), more closely resembles the Vaughn in that the tool is a spiral knife blade cylinder. In this case, however, it is pushed down the hide which rests on a thick rubber apron stretched and arranged on springs. The pressure here also can be adjusted to some extent by the operator, and it unhairs so that there is no need for further hand work. The Whitney unhairing machine (Fig. 25), represents another type of machine. In this also a spiral knife blade cylinder is the working tool, but unlike the Leidgen the roll is stationary except for its rotations, and the goods are fed to it by means of rollers. It is used to a considerable extentUNHATRING 73 in the United States. The Turner Serial Table unhairing machine (Fig. 26) also represents a distinct type of beam-house machine, designed particularly for unhairing calf, goat, and sheep skins. The machine is fitted with three, four, or five tables, which continuously rise and fall. During their circuit they pass two separate pairs of cylinders, which remove the hair. Between the lower and upper cylinders the table covering is automatically shifted, thus allowing the upper cylinders to work that part of the skin which was not touched Fig. 2i.—The “C'ontmua” ileshiug, uuUairmg, and setting out machine. Dy the lower rolls. This machine is quite similar to the serial table setting machines (p. 335). Drums and stocks have also been used for unhairing, but are not to be recommended. The former, however, are quite satisfactory for hides painted with sodium sulphide. Fleshing machines are in much more general use and produce better results than unhairing machines, especially for the lighter leathers. The greatest danger is the tearing of the flesh. Some unhairing machines will also work as fleshing machines if the working tool be suitably changed. The Turner drum machine and the Whitney machine are both effective fleshing machines, but the former is not very suitable for heavy hides. The Moenus “ Continua ” (Fig. 27) is a semi-cylindrical drum machine, which is very suitable for74 THE MANUFACTURE OF LEATHER fleshing sheep, goat, and calf; and the “ Cylindra ” machine (Fig. 28), recently introduced by the same firm, seems also to possess some advantages. The Wilson fleshing machine (Fig. 29) is one of the very few machines suitable for fleshing hides for sole leather; it produces a clean cut with no tearing and little pressure, but has the disadvantage that it will only flesh the butts, so that the operation has to be completed by hand labour. The Turner Rubber Roll fleshing machine somewhat resembles the Whitney machine. A rubber roll carries the hide against the working cylinder and the grip rolls pull it out. Fig. 30 represents this machine as especially Fig. 28.—The “ Cylindra ” fleshing, slating, and setting out machine adapted for skins. It takes considerably less power to run than the drum machines. Many of these machines, by the use of modified working tools, may be used satisfactorily for “ scudding ” (p. 93), striking out, setting, scouring, etc. (pp. 259, 282). In the United States it is often usual to flesh hides out of the soak liquors. There are many systems of working the goods through the lime liquors varying according to the class of leather which is being made. For reasons already noted it is quite general to place the goods in the more mellow liquors first and to arrange several neighbouring pits to work together and form a “ round.” In this way it is possible to arrange a continuous working, theUNH AIRING 75 tail ” liquor being discarded where necessary and the new “ head ” liquor being freshly made up in the now vacant pit. In this way the lime liquors can be worked much like the liquors in the tan-yard (see p. 17B). A deservedly popular method for liming hides for sole and heavy leather is the Three-Pit System which is worked on the principle just described. The goods go into three liquors, first into an “ old ” lime, then into a “ medium ” lime, and lastly into a “ new ” lime. After once using the “new ” lime becomes a “ medium ” liquor, and76 THE MANUFACTURE OE LEATHER after again being used is then the “old” liquor which receives the green pack. After being used three times the liquor is rejected and the new lime is made up in its place. The goods 1’To. 60.—Rubber roll fleshing machine.UNHAIEINGr 77 may be a few days in each liquor and are hauled as often as desirable. One great advantage of this system is that the top hides in one pit may become in the transfer the bottom hides in the next pit so that a regular treatment is assured. To make the system perfectly clear an example will now be given to show how the goods move through the liquors and how a “ shift ” may be brought about. 12 3 A B c N M o Pit 3 contains an old lime liquor (o) and the “ green ” pack (c), which has been in three days; pit 2 contains a medium lime (m) and the middle pack (b), which has been in this pit three days and in limes six days; pit 1 contains a new lime (n) , and the pack next to be unhaired (a), which has been in limes nine days. The procedure is then as follows: pack c in pit 3 is hauled up on to the slope ; pack a in pit 1 goes to unhair ; the lime liquor in pit 3 is run away and a new lime made up in the same pit; the middle pack (b) in pit 2 goes now into the new lime in pit 3 ; the green pack (c) on the slope of pit 3 now goes into pit 1, which contains now the medium lime ; the new green pack (d) goes into pit 2, which contains now the old lime. The position is therefore thus : 12 3 c D B M 0 N In three days the next shift is similarly brought about, and a new pack (e) brought into work. This results in the following position : 1 2 3 E c D 0 _ isr M78 THE MANUFACTURE OE LEATHER The goods remain in this position for another three days, hauling and setting occasionally, and precisely the same process is gone through, pack e going to unhair and a new pack (f) being placed in pit 8. This gives the original position again: 2 3 E M F o 1 D N In this way each pack receives a nine days’ liming, involving three days in each liquor. In winter, or whenever desired, one can give a shift after four days instead of three, and thereby obtain a ten, eleven, or twelve days’ liming. Another common mode of procedure is the One-Pit System or “ Bettering Method.’’ In this case the goods enter a used lime liquor, and after hauling and setting for a few days the liquor is strengthened or “ bettered ” by the addition of more fresh lime. The goods again enter the well-plunged liquor and are hauled and set till ready for unhairing. It is often usual to place the hides at first in the unplunged liquor and after the first hauling to plunge up well. The liquor also may be bettered more than once by the addition of more lime. There are several disadvantages to this method of liming, not the least of which is the rapid accumulation of lime and inorganic matter in the pits, necessitating a thorough clean out at regular intervals. It will be seen, moreover, that at the end of the process the goods come out of a liquor which is by no means new and fresh, which indeed is so mellow as to be used for the first liquor of the next pack. For sole leather goods this is clearly a disadvantage, but it should be borne in mind that in a regular system of working a new lime will have to be made up for every two or three packs. This, of course, being done when the liquor is due to be bettered, and hence half or one-third of the packs will finish up in a fresh lime. For dressing leather hides, moreover, it is often desirable that the goods should not finish up in an absolutely fresh lime, and in this case the newly made-up lime liquorsUNHAIRINGr 79 should be “ mellowed ” by innoeulation with part of the old lime which is being run away. A few inches of liquor may be left in the pit or a few bucketfuls of old lime liquor added. The exact mode of procedure may be, perhaps, best explained by examples as before. In one process we have hides receiving a nine days’ liming, and a regular cleaning out of the pits once in three weeks after two packs of goods have gone through. This system is carried out as follows :— Bettering Method (1). Pack I. Mon.—put into unplunged liquor. Tues.—hauled, plunged, and set. Wed.—hauled, plunged, and set. Thurs.—hauled, bettered, and set. (1-2 cwt. lime). Fri.—hauled, plunged, and set. Sat.—let lie. Sun.—let lie. Mon.—hauled, plunged, and set. Tues.—let lie. Wed.—hauled and unhaired. Pack II. Wed.—put into unplunged liquor. Thurs.—hauled, plunged, and set. Fri.—hauled, plunged, and set. Sat.—hauled, NEW lime made, and set. Sun.—let lie. Mon.—hauled, plunged, and set. Tues.—let lie. Wed.—hauled, plunged, and set. Thurs.—let lie. Fri.—hauled and unhaired. Of course there will be slight variations from this scheme, according to the time of year, and the class of hides, etc. In another one-pit process only a seven days’ liming is given, but the limes are either somewhat mellower or contain some sulphide. The process may be carried out in the following manner:— Bettering Method (2). Mon.—goods enter old unplunged liquor. Tues.—hauled, plunged, and set. Wed.—hauled, bettered with J cwt. lime, and set. Thurs.—hauled, plunged, and set. Fri.—hauled, bettered with 1| cwt. lime, and set. Sat.—hauled, plunged, and set. Sun.—let lie. Mon.—hauled and unhaired. The Pullman-Payne process already mentioned aims at quickening the operation of liming. The goods are placed first in a 1 per cent, solution of caustic soda for two days, and after some draining are passed into another pit containing a 1J per cent, solution of calcium chloride, in which they80 THE MANUFACTURE OF LEATHER remain also for two days, and then go into water. This process will not loosen the hair unless the goods have been previously steeped in a putrid soak for about 86 hours, and this undesirable and dangerous procedure constitutes the great disadvantage of the method. It is, however, useful for obtaining the true liming effect on skins which are not to be unhaired. The suspension systems involve the use of agitators in the lime pits which keep the milk of lime thoroughly mixed and save frequent hauling. Many patents have been taken out for such methods. In one variety the liquor is circulated between the suspended hides by means of pumps, which raise up the liquor and sludge from the bottom. In another method the circulation is caused by a screw propeller which is worked below a false bottom in the pit. Practical Methods of Liming.—The following account will illustrate the nature and extent of the liming which is given to the different classes of goods; but it will be understood that there is nothing cast iron about the figures given, partly because no two yards make the same goods, and partly because the duration of the liming is, of course, judged by the fitness of the goods for unhairing, and this varies considerably with the time of year, being distinctly shorter in the summer months. Hides for sole leather receive a short, “ sharp ” liming, but there are wide variations in treatment according to the class of hides used and the quality of leather being produced. For ordinary fresh market hides which are to made into “ scoured bends ” after a mixed tannage, the three-pit system of liming is now very general. They should receive about three days in each liquor, hauling every day in the first liquor and once in each of the others, the goods remaining piled for an hour or two to assist in removing creases. About 10 lbs. of lime for each 70 lb. hide is taken when making up a new liquor. If caustic soda be used to assist in plumping, about 2 ozs. per hide may be employed, and in this case 8 lbs. of lime per hide will be sufficient. In the short hair season it will be advisable to use a little sulphide to assist in complete depila-tion, and in this case about 9 lbs. of lime per hide is used toUNHAIRING 81 make up the new lime, and 2 ozs. of sodium sulphide per hide may be added to the medium lime, i.e., after the liquor has been used once. Each pack therefore passes through two sulphide limes and one fresh ordinary lime. The goods are best unhaired by hand labour, but the Wilson machine may be used to assist in fleshing. For high class oak-bark tanned sole leather, good Continental hides (Italian, Hungarian or Bavarian) are employed, or the best Scotch Shorthorns, and after appropriate soaking are limed by the three-pit system like market hides. The one-pit system, however, is also in use. For the West of England tannage of “ bloomed butts,” heavy South American salted hides are employed and are given a distinctly longer and stronger liming. The three-pit system may be used, but rounds of more than three pits are also convenient. The new liquor is made up by using 12 to 16 lbs. of lime per hide, and the liquors are plunged well when the goods are hauled. The hides remain in the liquors 14 to 18 days and in winter even up to 21 days. The liquors are sharp so that there is not much solution of hide substance, but it would probably be better to give a longer soaking and to use a little sulphide in both soaks and limes to reduce the time of liming. In rounding it is usual to cut long butts and narrow bellies. For the quick drum tannage in extract liquors (pp. 178,192) it is essential for the rapid penetration required that the hide fibres should be thoroughly split up into the finer fibrils of which they are composed, and hence a somewhat long and sharp liming is given of about 14 days. If the limes are kept sharp there will be little danger of any undue loss of hide substance. In the United States and on the Continent a common way of depilating sole hides is the so-called “Buffalo” method of liming. This involves the use of warm water, which largely increases the chemical and bacterial action, so that a quick process is obtained. The method can also be modified by the use' of the sulphydrates. One mode of procedure is to give one day in an old lime, one day in a medium lime, one day in a new lime, and to throw the goods into warm water at 40° C. M.L. G82 THE MANUFACTURE OE LEATHER They will then be ready to unhair in 6 to 8 hours. A large American finqa used the following process on packer hides. After 4 days’ soaking the goods were limed with 2 lbs. of lime and 2^ ozs. of sodium sulphide per “ side ” for 10 hours. They were then thrown into water at 85—48° C. and left overnight. Next morning they unhair easily. A Continental firm give a 2 to 8 days’ liming in liquors to which a little “ tank waste” has been added. The hides are then thrown into water at 82° C. for 6 to 8 hours, unhaired, returned to warm water, and then scudded. These examples are perhaps sufficient to illustrate the idea of the method. It will be seen that the chief advantages are the rapidity of the process, which not only is an advantage in itself but causes also a smaller loss of hide substance, and the washing of the hide free from lime. On the other hand, however, the “grease is not killed,” i.e., the fatty matters of the hides are imperfectly saponified during the short action of the lime, and the fibres are not sufficiently swollen and plumped, necessitating in some cases the subsequent “raising” of the pelt with vitriol. Short liming processes are also employed which depend upon the use of larger quantities of sodium sulphide to assist the lime than those suggested above. From 2 to 12 ozs. per hide may be employed with success in addition to 7 to 8 lbs. of lime per hide, but the larger quantities are liable to give harsh grain and act deleteriously upon the hair. Another type of quick unhairing methods is that in which sodium sulphide is used in strong solution and with possibly some lime as an assistant. In one process a 80 to 40 per ce nt. solution, thickened with lime, is painted on the hair side, using 2J lbs. of sulphide and about 1 lb. of lime per hide. This is well brushed into the roots of the hair with a cane brush and the hides are folded up and packed into a tub or pit. The rest of the “ paint ” is mixed up with water and poured into the pit, which is then filled up with water. The hair is reduced to a pulpy mass and can be brushed off in a few hours, the exact time varying with the strength of the solution. Another process used with some success is to employ a shallow tank, wide enough to take a hide flat, containing a 80 to 40 per cent, solution of sodium sulphide andUNHAIRING 83 possibly a little lime. The hides are drawn into this liquor by means of hooks or strings and brushed well whilst still in the solution. They are then drawn out to pile on a sloping floor, from which the liquor drains back to the tank. The hides unhair in a few hours as before. Yet another process is to suspend the hides for a few hours in a solution of sulphide of about the same strength and then to unhair as usual. To each of these processes, however, there are a number of disadvantages. The hair roots are apt to be left in, causing an unsightly appearance on the finished goods; the hair, which has some commercial value, is total loss; the grain is liable to be injured, being rendered temporarily tender by the sulphide; the goods often receive stains from the impurities in the commercial sulphide; the caustic action of the liquors on the hands and nails of the workmen makes it a nasty process for them; the short time admits of very little plumping and grease saponification, which causes 4‘bad weights ” and “greasy bends” respectively. It will be understood, therefore, that these processes are only suitable for the cheaper classes of goods, and are not very widely practised. Hides for belting leather usually receive much the same liming as sole hides, but often somewhat longer and mellower. Especially where no bating is given, as is now usual, an extra day or two in limes may be helpful in producing the requisite pliability. The quick processes with strong solutions of sulphide have also been employed successfully. Hides for harness leather receive a distinctly longer liming than sole leather goods, but the exact time depends upon the class of harness being produced; from 10 to 16 days, however, represents the range of variation. The liming process is also dependent upon the deliming process. For the cheaper classes of harness goods it is now usual to replace bating by a mere chemical deliming, and where such treatment is given a somewhat long liming should be given also in order to obtain a sufficient solution of hide substance to produce the flexibility required. Either the three-pit system or a bigger round of liquors can be used, with a little sulphide in the short hair season. Harness hides are rounded into “backs” or long butts. g 284 THE MANUFACTURE OF LEATHER Dressing hides (for upper leather, etc.) receive a 14 to 16 days’ liming in mellow liquors. The one-pit system is commonly employed, but a round can also be used with advantage, and of course the three-pit system can be quite well used, remembering to add always a certain amount of old lime liquor to the fresh lime as it is made up. For the cheaper classes of goods sulphide may be used and a shorter liming given (7 to 10 days). This is desirable for chrome work (“box sides”). Dressing hides are often not rounded till after tannage, though occasionally cut into sides, and hides for bag work are not rounded at all. E. I. kips receive very similar treatment in the limes, remaining 14 to 16 days in mellow liquors, hauling every day. A round of seven liquors is commonly employed, moving the goods forward every second day. No sulphide is added as a rule, but a little comes in from the soaks. Eed arsenic is sometimes used. Calf skins receive rather different treatment according to their destiny. For boot and shoe work (vegetable tannage) a mellow sulphide liming of 10 to 14 days is given, using 2 cwt. of lime per pack of 200 to 250 skins. In the tail liquors of the round the goods are hauled every day, and 1 per cent, on the lime of sodium sulphide (i.e., 2|- lbs. per pack) is used in these liquors. In the head liquors the goods are hauled and handled forward on alternate days, but no sulphide is employed. A mellow liming of 3 weeks can also be given without the use of any sulphide. For box calf and willow calf, etc. (chrome tannage), a very short liming is essential, 4 to 7 days being ample, and, of course, in this case a considerable amount of sulphide is employed. The minimum solution of hide substance is desired for this tannage, but not so much sulphide should be used as to cause harsh grain. For calf kid (alum tannage) the goods receive a not very mellow liming of about three weeks. No sulphide should be used unless it be red arsenic, and the goods should never enter a quite fresh lime liquor. For bookbinding and fancy purposes (sumach tannage), the skins receive a mellow liming of 18 to 21 days.UNH AIRING- 85 A round such as just described for shoe calf is successfully employed, slaking the 2 cwt. of lime with 20 to 30 gallons of old lime liquor, and using no sulphide. Sheep skins and lamb skins are fellmongered by methods which differ widely with the class of goods to be unwoolled. The sweating process (or “ staling” process, as it is often termed with these goods) is used to a considerable extent both on the Continent and in this country for unwoolling these skins, for although it is very liable to injure the pelt and give weak grain, it gives, on the other hand, a very satisfactory yield of wool in good condition. It is therefore employed where the wool is important, and also for dried and salted foreign skins. The fresh skins enter the “ first soaks ” until the next day, clean water being, of course, used. If the skins are salted or dried they are returned to a fresh water until sufficiently softened. The skins go direct from the soaks to the “ burring machine,” which consists essentially of a set of spiral blades rapidly revolving against the wool of the skin. The skin is drawn up and down on an inclined table by means of reversing geared rollers, and a good supply of clean water is constantly flowing over the skins during the process. All dirt is removed from the wool in this way. The goods now enter the “ second soaks,” remaining 24 hours or more in clean water. They are then drawn and laid flat to drain. The “ tainting ” or “ sweating ” stoves are now entered. These are air-tight chambers, of the type mentioned (p. 57). They are fitted up in this case with wooden rails and hooks attached. The well-drained skins are hung on the hooks by the fore shanks, flesh to flesh, and the tainting is given a “ start ” often by the injection of live steam. The skins are ready for pulling in times which vary from 6 to 8 days in summer and 12 to 14 days in winter. 1 No rules can be given for the control of the process ; the smell of ammonia in the stove, the condition of the goods, and the experience of any man with any one stove, are the best criteria. Long-woolled skins are sometimes “ slimed ” about halfway through the sweating process, i.e., they are worked over the beam on the flesh to remove fat and grease. When in proper condition the wool is pulled and the pelts are paddled in water or a weak86 THE MANUFACTURE OF LEATHER mellow lime. The skins now enter the lime liquors and are “ limed up ” from mellow to fresh liquors. This subsequent liming is necessary to kill the grease in the skins, for this grease is not saponified or removed in sweating. The skins are next unhaired, returned to fresh lime, and sold as soon as possible to the leather manufacturer. Another method used with success in this country involves the use of lime before sweating. The skins are first thrown into a soak pit of clean water. Clean water should be constantly running into this pit, and beneath a false bottom is an outlet with an eased plug to allow a constant escape of the soak liquor. After 24 hours or more the skins are taken out and carefully “broken over” the beam or put under the burring machine. They are then spread out, flesh side up, and painted over with a creamy mixture of lime and water by means of a mop. The mixture should be well up to but not past the edges. The skins are now placed in a pit, and sometimes “flooded” by filling up the pit with water. Difference of opinion exists as to whether it is better to flood at this stage or not, but it is probably better as a rule to do so, for although the wool is not so good the pelts are distinctly better and all heating or “specking” is prevented. The skins remain thus for about two days and are distinctly plumped. They are then drawn through fresh water to wash off the loose lime, laid in pile flesh to flesh, and again rinsed more thoroughly. They are now horsed up to “sipe” or drain, till ready for the sweat pit or tainting house. In this the proper “ pitch ” is got up by means of the steam pipes on the floor and the process is carried on as usual. The skins are then taken to the pulling shop. The “long wools” or “ wool skins ” are placed over the “ pulling beam ” with two or three pelts, flesh up, to act as bolster, and the true wool is pulled by hand, keeping separate wool of differing quality, according to the breed, age and sex of the animal and the part of the body from which the wool is taken. Thus are obtained ewe wool, wether wool, hog wool, lamb wool, etc. The “rubbing knife” is next carefully applied to the hairy parts on the shanks and faces, this hair being kept carefully separate from the wool, The pelts are then placed into water,UNHAIRING 87 then into a weak fresh lime for one day, and then into a strong fresh lime in which they are handled frequently until sold to the leather manufacturer. In unwoolling “ Downs ” and “short wools” the skins are bolstered as usual, and the rubbing knife applied first to the shanks, hairy parts and the coloured parts. The hair and wool thereby obtained are carefully cleared away and the whole skin now rubbed and pulled, keeping separate wools of different length. The unwoolling of “mountain breeds” is somewhat intricate because of the intermingled hair and differing quality of wool. In this case therefore the procedure is to remove first the best quality of wool, then the inferior qualities, and then to throw the skins into limes. They are limed four days in weak limes, paddling three hours each day, and four days in strong limes, paddling similarly. They are then unhaired and fleshed. In unwoolling shearlings or “ pelts ” the rubbing knife should be applied with great care so as not to damage the grain, and in this case also the true wool is kept separate from the hair and from the other wool. Shearlings are also sometimes limed in ordinary lime liquors for 9 to 14 days, hauling frequently. They are then unwoolled without sweating. Another type of fellmongering much used in America and now common in this country involves the use of sodium sulphide as a depilatory, a solution of this being made into a creamy mixture with lime and painted on the flesh side of the skins, which are folded up or piled flesh to flesh until ready for pulling. The skins, after efficiently soaking and cleansing, are drained over horses or put through the “ wringer ” to eliminate water. They are then painted on the flesh with a mop with the sulphide mixture, taking great care to keep the solution from coming in contact with the wool, and putting more of the paint on the neck and along the back as these places unwool less readily. The strength of the sodium sulphide solution determines the rapidity of the depilation. The solution may range in strength between 14° and 24° Beaume according to the class of skins, thin skins requiring the weak solutions, and thick skins or salted skins the strong solutions. The lime should be thoroughly slaked to a stiff paste and several pailfuls of this added to a barrel88 THE MANUFACTURE OF LEATHER full of the sulphide solution. The wool will be loose in a few hours, but it is thought better by some to leave the skins folded up until next day before pulling. It is desirable that the skins should not be allowed to heat in summer or to freeze in winter, and consequently the exact treatment differs somewhat with the strength of depilatory and with the season. Some firms use a 25 per cent, solution of sodium sulphide; this is made creamy with lime, painted on the flesh, and the skins placed in pairs flesh to flesh. When about three dozen skins have been painted the first pair are ready for pulling. Another way is to use sulphide only in strong solution, the goods are piled flesh to flesh for one hour and then pulled. All these methods of depilation depend purely on the chemical action of the sodium sulphide on the hair root, and the method of painting on the flesh side is efficient for sheep skins because the wool root is very deeply seated in the corium, being one-third to one-half the way through the pelt. The porous and spongy texture of the corium also assists in making this method of operation possible. After pulling the wool the skins are opened out and dropped into clean water and washed. The unhairing is now completed by liming in pits in the ordinary way, from mellow liquor to fresh, hauling frequently. They may be limed thus for 4 to 6 days. This plumps the pelt, separates the fibres and kills the grease. This liming is as necessary as if the skins were sweated, for the grease is very little affected by the sulphide depilatory, and sheep skins contain a very large amount of grease (5 to 15 per cent.) compared with ox hides (2 to 3 per cent.) and other skins. After liming and unhairing, the fellmongering is complete. Kid skins and lamb skins for “glove kid” are treated usually in rather special ways which are of great importance in producing the qualities desired in this class of leather. A certain amount of interfibrillar substance should be dissolved in order to obtain the property of “ stretch,” i.e., of extending in any direction without springing back. At the same time it is desirable to avoid old lines which tend to make a loose, porous leather with a dull grain. Liquor containing sodium sulphide should also be avoided on account of its harsh effectUNH AIRING- 89 on the grain. It is furthermore necessary that the fat should be fully saponified and removed or difficulties occur in dyeing the goods. Hence it is common to employ arsenic limes for these goods, using about 1 per cent, of arsenic on the weight of the lime when making up the liquors. The goods should remain in these liquors 7 to 10 days. Some firms use rather more arsenic : 5 per cent, of lime and 0*1 to 0*3 per cent, of red arsenic on the green weight of the skins. The liming then lasts only 4 to 5 days. Good results have also been obtained by liming for about 14 days without arsenic but with new limes. Another short method (Eitner) is to use 15 litres of lime paste for 500 skins, and place the skins in this liquor for one day. On the next day and also on the third day another five litres of lime paste are added, and on the fourth day five litres of paste which contain 10 per cent, of arsenic. Next day the skins are unwoolled and washed. It is now common, however, to unwool lamb skins by painting the flesh with mixtures of sodium and lime as described above. It is desirable that skins treated thus should be pulled as soon as possible and placed in cold water. Gas lime and other depilation paints have also been employed successfully. Whatever process has been employed the pelts after unhairing are thrown into a very weak lime for a short time and then “ levelled,” which includes fleshing and the trimming of the head, ears, shanks and flanks. They are then thrown into soft water or weak lime till ready for puering. Goat skins, for moroccos, glacé kid, etc., receive 16 to 21 days’ liming in summer and about 8^ weeks’ in winter. They are slightly more difficult to unhair than some other skins and require a fair amount of hide substance to be dissolved in order to produce the desired softness in the finished leather. They are given two rounds of liquors. The first series consists of mellow arsenic liquors, 6 per cent, of red arsenic being used on the weight of the lime. The goods should be hauled frequently, and remain in these liquors about ten days. The skins are then unhaired, flushed and placed in a second series of limes to plump. These are new limes containing up to 6 per cent, of caustic soda on the weight of the lime. The goods go through this round in about 10 days. Sodium90 THE MANUFACTURE OE LEATHER sulphide limes may be used in the first round instead of arsenic limes, and the skins may then be unhaired in 4 to 5 days. Seal skins, after soaking and blubbering, are fleshed before entering the limes. They receive a long liming in mellow liquors without any sulphides. This is necessary to kill the grease. About three weeks in limes is usually sufficient. A one-pit system is most convenient, but some firms finish up in new limes. The older animals are very liable to show “ ribs ” in the pelts which are difficult to remove. When the skins are sweated, however, this trouble does not occur. The Chemical Control of the Lime-yard.—The “ available lime ” in a sample may be determined by a 0*1 per cent, solution with hot distilled water, cooling, making up to mark, and titrating an aliquot portion of the filtrate with N/10 hydrochloric acid. Each cc. used corresponds to 0*0028 per cent. Sodium sulphide may be titrated with N/10 acid and methyl orange, as that indicator is unaffected by sulphuretted hydrogen. Any caustic soda will be included in this, so that to determine the amount of sulphide proper it is necessary to titrate the solution with N/10 zinc sulphate solution, using lead acetate as an outside indicator for the complete precipitation of the sulphide. Ammonia in lime liquors may be estimated by distilling 100 cc. in the Kjeldahl apparatus (p. 898), and collecting the ammonia in standard acid, or more accurately by the method of Procter and McCandlish.1 Dissolved hide substance, which is not only important in itself, but is also the best criterion of the “ mellowness,” of the age and of the bacterial activity of a lime liquor, can be determined by the Kjeldahl method after acidifying and evaporating, or by the formaldehyde method suggested by the Author.2 For mere control work, however, the best method is to estimate dissolved hide substance empirically by means of a simple titration with phenol phthalein and with methyl orange.3 1 J.S.C.I., 1906, 254. 2 Bennett, J.S.C.I., 1909, 291. 8 Bennett, J.S.O.I., 1909, 292,UNH AIRING- 91 The lime liquor should be filtered through the Berkefeld filter candle (p. 146), or through S. and S. “ 605 ” paper, and 25 cc. titrated in a porcelain basin with N/10 hydrochloric acid and phenol phthalein. Methyl orange is now added and the titration continued to a distinct red. The difference between the two titrations represents the acid consumed in neutralising the weak alkalies (amines, calcium salts of weak acids, etc.). These are proportionate to the amount of dissolved hide substance. For ordinary lime liquors 1 cc. titration difference corresponds to 0*0058 grams of hide substance. In cases where a small amount of sodium sulphide is used to assist in depilation a constant error is introduced, and a different factor will have to be used according to the amount of sulphide. The titration also must in this case be conducted in dilute solution. Caustic lime in a piece of hide may be determined by the direct titration of the finely sliced piece with N/10 acid and phenol phthalein, but the titration should be continued until a permanent discharge of the pink colour is obtained.1 The hide substance in a limed hide may be determined by weighing it when completely immersed in water. Weight in water X 8*88 = weight of hide substance. 1 Cp. Bennett, J.S.C.I., 1907, 455.CHAPTER VIII DELIMING It is intended in this chapter to give some account of those operations which are associated with the preparation of the limed hide or skin for the tanning liquors, for the necessity for some such preparation is clearly evident when it is observed how readily all vegetable tanning matters oxidise and darken in the presence of caustic alkalies. The neutralisation and removal of caustic lime is thus a matter of some importance and has therefore given the title to the chapter, but there are several other operations and changes associated with this process which for certain classes of leather are almost equally important and which are conveniently dealt with also at this stage. For the softer and finer leathers it is usually necessary not only to effect a complete removal of the lime, but also to reduce the swelling and plumping that has occurred in the limes, and to allow some bacterial action on the interfibrillar substance in order to produce the requisite softness and pliability. These changes are usually accomplished by puering and drenching. Puering consists in treating the goods for some hours with a warm fermenting infusion of dog-dung, which assists in the removal of the lime by both chemical and mechanical action, and which allows the bacterial enzymes to bring into solution a certain amount of hide substance. Drenching, which consists in steeping the goods in a fermenting infusion made with bran and hot water, is usually a subsequent process, and assists in cleansing the skin, in completing the neutralisation of the caustic lime, and perhaps also in slightly plumping the pelt with the organic acids it contains. For harness and belting leather, upper leather, etc., a muchdeliming 93 more moderate treatment is desirable; but a certain amount of flexibility is required so that the goods are generally bated. This process is similar in principle to puering, but the dung of pigeons or hens is used to make the infusion, which is used cold over a period of several days. These goods may be drenched or further delimed by means of acids. For sole and heavy leather, in which weight and firmness are primarily required, no softening or reducing of the pelt is desirable, and no bacterial action should be allowed. Hence it is usually quite sufficient to neutralise the surface lime by a bath of weak acid and to pass therein to the tan liquors, the natural acidity or “sourness” of which will complete the neutralisation and keep the material well plumped. For all goods the delimed or partially delimed state is a convenient stage for further cleansing from fatty matters, Fla 31.-Scudding with a hand knife. lime soaps, small hairs, hair sheaths, pigment, dirt, etc., and this is accomplished by working over the beam again (Fig. 31) with a “scudding knife,” which resembles the unhairing knife but is rather sharper (see Fig. 21). A hand knife is also often used. It is sometimes further desirable that some mechanical working and shaving of the pelts (“ bate shaving ”) should take place to assist in softening the thicker parts and in rendering the goods of even texture. Puering, which involves the use of a fermenting infusion of94 THE MANUFACTURE OF LEATHER dog-dung, has long been used in the manufacture of morocco and glove leathers. It has never been regarded as a satisfactory process, not only on account of the revolting nature of the material, but also because of its variable quality and the consequent uncertainty of the results obtained by it. The food of the dog, the accidental or even deliberate adulteration with water or sand, the age of the material and the manner Fio. 32.—Puering paddle. in which it has been kept, are all factors which extensively influence the puering process. The moisture content of different samples of dung has been observed to vary between 4 and 85 per cent., and some samples have been found to contain quite 50 per cent, of inorganic matter. An extensive analytical investigation, moreover, is not justified by the value of the results obtained, so that the only way is to endeavour to obtain as regular and fresh a supply as is possible. DungDELIMING 95 obtained from kennels, therefore, is far better than that from the streets, which latter is liable to contain a very indefinite proportion of dirt. Dry dung should be of a fairly light colour; dark or black dung has undergone an improper fermentation and is unsuitable for puering. Fresh dung is best kept in a dry condition and as little exposed to the air as possible. It has been discovered, however, that the puer bacteria are not present in the dung when excreted, so that it is necessary to mix it into a paste with water and allow it to ferment for some weeks before use, and it is often convenient to have a series of such pastes made up in casks from which they can be drawn when ready. It is stated that dung should never be kept over three months in this condition, and probably at least a fortnight is required for the growth of the true puer ferment, but both these times are no doubt dependent on the material, the season, and the manner of storing. - In making up the liquor it is usual to dilute the required amount of dung paste with warm water and strain the stirred infusion through coarse cloth into the paddle, and then make up with a further quantity warm water. The great variation in the quality of the material makes it quite impossible to specify any particular quantity to be used per weight of pelt, and it is therefore necessary to rely merely upon empirical observations of its rapidity of action in “ pulling down ” the skins ; but in any yard the number of “ pailfuls ” per pack is soon found out and varied as necessary. The temperature of the puer liquor is a very important point, as some skins are very liable to be “ scalded,” and the time for which the goods should be treated is also dependent on this, being longer for lower temperatures but varying also with the thickness and nature of the skin. It is also difficult to state precisely the condition of skins after sufficient puering. They should, however, be thoroughly relaxed and flaccid, and the swelling pulled down so that they are in a very soft and “fallen” condition; the springiness and elasticity of the plumped pelt should also have completely disappeared and the impression of the thumb or hand should be readily retained. The grain should appear white and feel soft and silky, and the flesh side tender and easily torn with the finger nail. The thoroughness96 THE MANUFACTURE OE LEATHER with which this change is accomplished varies, however, for different skins, and judgment based on practical experience is the only reliable guide in this matter. The extent to which lime is being removed can be readily tested in this or any other deliming process by moistening a freshly-cut section of the skin with two or three drops of phenol phthalein solution—the familar pink colour appearing where caustic alkali still remains. The nature of the process has been long understood as partly chemical and partly bacterial, but its precise mode of operation was quite obscure until cleared up by the researches of J. T. Wood. To find the cause of puering he investigated first of all the action of the digestive enzymes on the limed pelt, as there is little doubt that these occur in fresh dung. His results indicated that 'pepsin, if contained in the dung, has practically no influence in the process, and that trypsin only contributes to the action in a very minor degree, partly in pulling down the pelt and partly in emulsifying the fatty matters of the skin. He then examined the action of a fresh puer liquor freed from both organised and unorganised ferments by boiling, and found that although there was considerable puering action it was confined merely to the removal of lime, and that no loss of interfibrillar substance occurred under these conditions owing to the absence of all fermentive action. The deliming action was discovered to be principally due to the salts of the amines with organic acids, the caustic alkali displacing the weak organic bases. The hydrochlorides of organic amido compounds were found on experiment to give a similar result. Wood next investigated the bacterial side of the puering process and attempted to determine the species which would yield pelts of the requisite texture. None, however, that were isolated gave such a powerful puering effect as an ordinary puer liquor, and hence he concluded that the process demanded both the chemical and bacterial actions for satisfactory operation. He further discovered that the puer bacteria did not act directly on the pelt but that the action was really due to the enzymes they discharge; and these enzymes with the assistance of mixed amine hydrochlorides, and in the absence of any organisedDELIMINGr 97 ferments, were fonncl to pull down limed pelt like an ordinary puer liquor. Puering, therefore, is due to the enzymes of the bacteria which collect on fermenting and exposed dog-dung and which thrive under the ordinary conditions of the process; and these cause some of the gelatinous matter of the skin to be brought into solution. It is also due to the amido salts which occur in dung and its decomposition products and which act merely as deliming agents; and in a much smaller degree to the enzymes already present in fresh dung. In view of the uncertainty associated with ordinary dung and the disgusting nature of the material, it was very natural that Wood should proceed to make up an artificial puer by means of a suitable culture medium, which could be inoculated with prepared cultures of suitable bacteria before use. This he has been able to do with considerable success, so that both bacterial culture and culture medium are now on the market under the name of Erodin.1 In its use 1 per cent, of the solid medium on the weight of pelt is dissolved to form a 2 per cent, solution, which is heated in a cask by steam to a temperature of 40° C. The culture of Bacillus erodiens is then added and the solution kept above 25° C. for 8 days, warming up to 40° C. each morning. About half of this liquor is taken and the fermentation continued by the addition of more of the sterile 2 per cent, medium solution ; more Bacillus erodiens being also added occasionally. This process is now widely used both in this country and on the Continent, giving results of much greater certainty and safety, and yielding puered skins quite free from the objectionable stains which are often experienced with the less definite dung puer. Bating, which in principle is the same as puering, is usually carried out in the cold and applied to heavier goods which are to be made into pliable leather. It involves the use of the dung of hens and pigeons instead of that of dogs. In this case also it is usual to make up the infusion in a separate vessel by adding warm water and allowing the mixture to stand for a day or two. When the fermentation is thus started, the infusion is filtered from insoluble organic matter 1 Drs. Popp and Becker also worked independently on these lines. M.L. H98 THE MANUFACTURE OF LEATHER by passing through sacking into the pit prepared for the goods ; this prevents “ bate-burning.” The goods should now go in for several days, hauling frequently to ensure even action. It is found in practice that the bate liquor must be neither very alkaline nor very acid for satisfactory results. A bate liquor can be “mended’5 by the addition of a further quantity of dung infusion and by the precipitation of lime, as oxalate by the addition of oxalic acid, but this must not be carried too far, for the organic matter in solution and the bacterial activity would then rapidly increase and become dangerous. The nature of these bacteria has not yet been thoroughly investigated, but they are no doubt different from the Bacillus erodiens, which thrive in a warm puer, and seem to consist chiefly of micrococci. It is not uncommon, however, especially in the United States, to use warm bates. In this case it is a question whether the prominent bacteria are the same as for cold bating. In carrying out this process the hides are placed in a tumbler or large paddle and the bate kept at about 35° C., which brings about the desired effect in a few hours. It is obvious that an effect is obtained very similar to bating by a longer liming in mellower liquors, and cases have been already noted in which either alternative may be chosen with satisfactory results. Where the bate is not used in these cases, some deliming acid must be substituted. The action of the bate is generally understood to be less drastic than the puer, both in its bacterial and purely chemical effects; the process being slower, a more even result is obtained and the leathers resulting are not so loose and soft. Many artificial “ bates ” have been put on the market, but they are mostly deliming agents only and often even antiseptic! Some fermentive mixtures have, however, been suggested. An American “bate” involves the use of glue, glucose and blue cheese, and is perhaps good enough for belting offal, etc., but its action is more like that of the drench. Drenching, as a rule, follows puering or bating, but in some cases it is used as a substitute for these. In this process an infusion of bran is made by scalding it with hot water and allowing it to stand until it reaches a temperature of aboutDELIMINGr 99 35—40° C. A few buckets of an old drench liquor are then added to inoculate the infusion, and after mixing, the drench pit is brought to the required temperature by the addition of cold'water. The goods are then inserted for the required period, which varies largely with the temperature and class of skin or hide. The modus operandi of the drench has also been made clear by the investigations of Wood. The enzyme cerealin which occurs in the bran converts the bran starch into glucose, which then can form the food of the drench bacteria, jBacterium furfuris a and /3. By the action of these organisms, appreciable quantities of lactic, acetic and other acids are formed, together with some evolution of hydrogen, carbon dioxide, nitrogen, methane and sulphuretted hydrogen. The organic acids act as neutralising agents for any remaining Caustic lime and may even plump the pelts slightly and so fit them for tanning. The gases also are produced to a considerable extent within the pelt and have a peculiar opening effect on the corium fibres. Moreover, if the drench has been used subsequent to puering or bating, the bran acts as a mechanical cleanser of the skin from the noxious products with which it has just been in contact. The following is a typical analysis of the drench acids :— Lactic Acid Acetic Acid Formic Acid Butyric Acid 0*7907 gm. per litre. 0*2402 „ 0*0306 „ 0*0134 „ >> ?> If however, the temperature of the drench be too high, the proportion of butyric may very largely increase on account of a very vigorous butyric fermentation which is apt to take place, and in this case the skins are rapidly swollen to a considerable degree and may soon be irreparably ruined. If this by any chance occurs, the skins may be “ pickled ” by the addition of salt, which also checks the fermentation, or may be paddled in water to which borax, ammonia, or whitening is gradually added. This difficulty can be avoided and the activity of the drench much reduced by previously extracting some of the starchy matters from the bran by means of cold water. Conversely, the activity of the drench may be increased when h 2100 THE MANUFACTURE OE LEATHER necessary by the addition of rye meal, pea meal, etc., to the bran when making np the liquor. In all cases it is desirable to keep the temperature below 35° C., and in some cases perhaps even 10° C. is not too low, but the time of the process must then be correspondingly in creased. As with puering and bating, the requisite extent to which the process is to be carried can only be safely judged by experience in the treatment of any particular class of skin, but the skins should not be swollen till transparent, and should still be white and flaccid. If the drench is “ working ” too vigorously, the formation of the gases is apt to raise blisters on the grain, and, especially in the case of sheep skins, may thus separate the grain from the flesh and yield what is known as “ pipy grain.” The drench bacteria do not attack gelatin, so that if any damage occurs in this way it is probable that some injurious bacterium has been introduced from the puers or bates. Chemical deliming agents have been suggested in endless numbers, but these are often merely acids which neutralise the caustic lime on the hide and are therefore more closely similar to the drench than to the puer or bate. Boric (boracic) acid is now very widely used for this purpose and is popular for nearly all classes of leather. It has quite a mild effect on hide substance, and being difficultly soluble is harmless even in excess. It neutralises lime and in dilute solution forms calcium borates which are soluble in water. It is found in practice that by its use a beautiful silky grain may be obtained, in which respect it is in contrast with other deliming agents, and it is also found that it causes a decided improvement in colour during the early tanning liquors, and at the same time both quickens the tannage and prevents “ drawn grain.” For sole leather it is found very useful in removing the surface lime. It is also used to a considerable extent as a drench for bated hides, and being an antiseptic agent it stops all bacterial action. Light leathers often receive a bath of boric acid, even after drenching, to hasten the tannage and give a good grain and colour. It has been discovered that it is beneficial to keep the goods in motion in order to get a perfectly even action, so that skins are best treated in paddle, and “ rockers ” (p. 171) are found useful for the heavier goods.DELIMING- 101 Lactic acid is also found to be in certain respects a very suitable deliming agent. We have already noted that it is the chief active acid of the bran drench, and artificial drenches have been made up of similar compositions and found to be effective and rapid substitutes. A useful liquor of this nature consists of:— Lactic Acid . . „ 1*0 gm. per litre. Acetic Acid . . . *4 ,, „ Formic Acid. . . *2 ,, ,, It is observed, however, that such a reagent will not give the characteristic opening and inflating effect of the ordinary fermenting drench, because of the absence of any evolution of gases. It may also be used alone in 0*2 per cent, solution as a deliming agent, and is best used in the paddle like the puer at a temperature of 80—85° C. Lactic acid is one of the natural constituents of “ sour” tan liquors, in which it fulfils the same deliming function, and being one of the best plumping agents yet discovered, it is often added to “sweet” tan liquors to produce the requisite degree of acidity. It is made on a large scale by the *4lactic fermentation” of sugar residues, and is put on the market in solution of about 45 per cent, strength. Acetic acid, another product of the drench ferment, has also been suggested for deliming purposes, but the commercial article is apt to be very impure, and harsh grain has often resulted from its use. Formic acid is now manufactured synthetically from caustic soda and carbon monoxide, and is obtainable in 60 per cent, solution. It is an effective deliming agent if used in very dilute solution, but if present in large quantity is liable to give a very rough grain. Various inorganic acids have been suggested from time to time for deliming, but, except for special cases, they are not very extensively used. Sulphuric acid can be used if great care is taken and the acid added only gradually, but its lime salt is insoluble, and it has an exceedingly violent plumping action. Hydrochloric acid has similarly been employed for pulling down dressing goods, but is objectionable for sole leather on102 THE MANUFACTURE OF LEATHER account of the formation of chlorides. Sulphurous acid is also effective, but must be used in excess to dissolve the insoluble calcium sulphite. Carbonic acid has also been used in a similar way. “ Antiealcium,” “ C. T. bate,” and similar preparations are mixed sulphonic acids derived from commercial cresols and aromatic hydrocarbons, and are more suitable as drenches than bates. They are good antiseptics, but are apt to swell somewhat. Many neutral salts of weak bases have also been used as deliming agents, acting in a manner closely similar to the amido salts of the puer. Borax and ammonium chloride, sulphate, phosphate and oxalate have all been suggested, sometimes in addition to various weak acids. Common salt, as noted earlier, has a direct depleting action on the swollen skin, and when used along with even sulphuric acid may be very effective with goods for chrome tannage or in cases of “ lime blast.” An excellent preparation for any process of deliming skins is a good tumbling in soft warm water. Practical Methods.—There is probably more variation in the methods of the deliming processes than in any other part of leather manufacture, but the following outlines represent some of the commonest modes of treatment:— Sole butts should be kept as plump as possible, and hence all that is ever done is to neutralise the lime on the surface for the sake of colour, and to pass the goods immediately into tan liquors the natural acidity of which (‘‘sourness”) completes the neutralisation and keeps the goods well plumped. In yards where the tail suspender is very acid (see p. 170), the butts are merely washed with water. They are suspended in the water, which must be very soft (see p. 46), and either rocked or handled frequently for a few hours. Another way is to suspend in running water. The exact time depends upon the method employed and the acidity of the suspender liquors. This used to be quite the usual method for sole butts, but in those days the tan liquors were not so quickly worked down the yard, and in consequence had a good acidity (up to 10° lime water, see p. 154). The modern methods of working and the large proportion of “ extract ” (see p. 184) employed yields suspenders with comparatively little acidDELIMING 103 (3° to 5° lime water), and hence it is now usual to give a short bath of some acid to the goods before passing them to the liquors. Boric acid is generally preferred for this purpose. The butts are rocked in a dilute solution for 4 or 5 hours, using 10 to 15 lbs. boric acid per 100 butts. This, in the author’s experience, will remove all the surface lime and three-fifths of the total. If a hard water only is available the hardness should be “killed” with sulphuric acid before dissolving the boric acid, but an excess of the former should be carefully avoided. Good results can also be obtained with lactic acid. The acid used should be free from butyric acid and iron. About 12 lbs. commercial lactic acid should be used per 100 butts. Acetic acid has also been used successfully for this purpose, but is not popular. With better methods of manufacture, however, one can expect it to be increasingly used. Strong acids and mineral acids, such as formic acid, sulphuric acid and hydrochloric acid, can be quite well used if the principles of application are well understood. The solution must be dilute, the amount of acid used should never be in excess, and the quantity that is used should be gradually added in successive small quantities during the process. Shoulders and bellies are often merely washed in water, but it is better to give the former an acid bath. Scudding is desirable even in sole leather manufacture in order to ensure good colour, but the scudding should be light or bad weights will result. The operation should be carried out after deliming, when the surface of the hide is relaxed and scud loose; what is necessary can then be removed with little pressure. Strap butts are sometimes lightly bated, a few hours being sufficient, and then given a bath of boric acid, using up to 20 lbs. of boric acid per 100 butts. More often in these days the bating is quite omitted and the goods simply delimed like sole leather. After scudding they are sent to the tan liquors. Harness backs may be bated at 20° C. for 3 days, handling each day. Sometimes the thicker ones are softened mechanically by working with a graining tool. Any bate shaving next takes place, and the goods are suspended for 4 hours in104 THE MANUFACTURE OF LEATHER tepid water which contains 10 lbs. of boric acid per 100 backs. Scudding follows, and the goods then enter the tan yard. In some yards the goods are not bated so much ; they are put into the bate late in the afternoon and taken out next morning, or started in the early morning and taken out late at night. They are then delimed with 20 lbs. of boric acid per 100 backs. A cheaper method still, now very common, is to have a longer liming, not less than 14 days, and to omit bating altogether. The goods are placed in a liquor containing 50 lbs. of boric acid per 100 backs and left until next day. They are then scudded, drummed with water to clean, rescudded, shaved if necessary, and sent to the tan liquors. Dressing hides are generally bated at 25—80° C. but for some classes of goods 20° C. is sufficient. They are given 8 days in sets of 8 pits. When hauled the thicker ones are worked on the butt and neck to soften them. They are then sorted and scudded. Some tanners now drum them in tepid water (20—25° C.), again work them over the beam, and give them a bath of boric acid to complete the deliming, but others drum them straight away in boric acid, using 20 to 80 lbs. per 100 hides. This stops the action of the bate and gives good grain. When lactic acid is used for deliming bated hides, 2 per cent, on the pelt weight of the commercial acid is taken. The deliming may be done in drums, but is preferably brought about in vats, a three-vat system being convenient. One-third of the total quantity of lactic acid is placed in each vat with sufficient water to keep the strength of the solution below 0’2 per cent. After mixing the solutions well, the rinsed hides from the bates are placed into each of the three vats successively for 8 to 12 hours, and then into water for 5 to 10 hours. They are then scudded by hand or machine, and sent to the tan house. Where a soft grain is desired the goods are placed in a large paddle with weak boric acid, and paddled 2 to 5 hours. The liquor in the first vat contains now no free acid and is run away. A new liquor is made in its place with fresh water and lactic acid, and the next pack goes first into the second vat, then into the third, and finally into the new liquor.DELIMING- 105 E. I. kips are rinsed and scudded after fleshing and then bated for B to 5 days at about 30° C., handling frequently. About one bushel of hen-dung is used for every 100 skins. After rinsing through water, the skins are sometimes bate-shaved, and bate-grained, and are then lightly drenched with 6 percent, of bran on the pelt weight, scudded again or merely rinsed and placed in the tan liquors. Some bating is almost absolutely necessary for this class of goods in order to obtain the desired softness in the finished article. Lactic acid, however, is employed to some extent to complete the deliming after bating, i.e., as a substitute for the drench. The skins are put into plenty of warm water (30—35° C.) in a paddle and 2 per cent, lactic acid on the pelt is diluted considerably and added gradually over a period of 2 to 3 hours. There should be at least three batches, and in this case not less than 20 minutes should elapse between each addition. The solution, moreover, must never be stronger than 0*2 per cent. One of the chief differences between drenching and chemical deliming is that in the former the acid is gradually produced and neutralised. In the latter, therefore, the more gradually the acid is added the greater the approximation to the conditions of drenching. Calf skins for “shoe calf” (vegetable tannage) are treated very much in the same way as kips, but are bated more lightly and are preferably drenched. From 2 to 3 days in bates, and 6 per cent, of bran for drenching are quite sufficient. For book-binding leathers the skins are severely bated. Five days at 30° C. is sufficient. Occasionally book-binding calf are puered. They are bate-shaved and drenched with 8 to 9 per cent, of bran. For chrome calf neither bating nor puering is either necessary or desirable. The nature of the chrome tannage demands the full amount of hide substance in the skins for obtaining the requisite fullness. As, moreover, the tanning material is all inorganic, there would seem no obvious reason for deliming at all. This is certainly true with regard to the two-bath process (p. 213), the first bath of which will completely and effectually delime the goods by means of its free acid, hydrochloric or chromic. Sufficient acid, however, should be106 THE MANUFACTURE OF LEATHER added to the bath to compensate for this if the goods are not delimed. By this method the skins would be rinsed, scudded, rinsed again and inserted straight into the chroming bath. The one-bath process (p. 219) almost demands a surface deliming, with lactic acid for example, to avoid the formation of calcium sulphate on the grain, but otherwise no deliming would seem to be necessary. It is nevertheless quite common to delime calf skins for chrome tannage by puering, and this undoubtedly makes it easier to obtain that silky feel on the grain which is so much desired for “ box calf ” and “ box sides.’’ A common method is to delime with acids and drench lightly. Four parts of formic acid are mixed with one part of lactic acid (each being of about 40 per cent, strength), and about a pint of the mixture is used for every 100 lbs. of pelt. The skins are first rinsed and scudded, and then paddled for half an hour in water to get rid of the excess of lime. They then enter the deliming paddle, which contains half a pint of the acid mixture per 100 lbs. pelt. After half an hour’s running the other half pint is added and the paddling continued until the skins, when cut and tested with phenol phthalein, show only a very thin streak of red. The skins are then rinsed through water, drenched overnight at 25—30° with 6 per cent, bran on the pelt weight. After branning or rinsing they are ready for tanning. Another method to is drum with tepid water and delime by drenching with 10 per cent. bran. It is also common to place calf for chrome work into a solution of 5 per cent, of potash alum and 5 to 10 per cent, salt, before placing them in the chrome liquors. Often also the undelimed or merely surface delimed skins are completely delimed by this bath. It is extremely doubtful, however, whether this has any beneficial effect for either one-bath or two-bath leathers. For “ calf kid ” the skins are plumped in fresh limes for a few days after unhairing and fleshing, and afterwards gradually delimed by steeping in a series of waters softened by the addition of some of that previously used for the same process. No dung is used, but a certain amount of bacterial action undoubtedly takes place. The goods are next drenched in 3 to 4 per cent. bran.DELIMING 107 Sheep skins, whatever their destiny, have to be “ degreased” some time during the process of manufacture, and this is often brought about in the limed state by pressing out the grease from the warm skins. The skins are placed one by one into a hydraulic press with a layer of sawdust 01; bran between each and an iron plate between every hundred. Usually about 1,000 are put into the press each time. The pressure is applied very slowly indeed or the skins are ruined, being at this stage very liable to damage. Most of the grease is removed on pressing in this way, and the rest comes out in puering and scudding. Tanned skins are degreased by extracting with solvents. They are drummed or churned with benzine, to which acetone is often added; the vessels are kept carefully shut, and the solvent is recovered for subsequent use. For “skivers,” sheep skins are always split in two by machine before degreasing, giving grains (the “skivers”), and fleshes, which latter are often given an oil tannage and made into chamois leather (p. 247). After splitting the grains are degreased, thoroughly puered at 80° G. for about three hours in paddle or drum, and then worked over the beam to remove scud. They are next drenched for 2 to 3 days at 20° C., and, after again working over the beam, are rinsed and sent to the tan house. The fleshes are given a further thorough liming after splitting, using fresh limes, then drenched to remove caustic lime, and allowed to drip dry for oil dressing. For “roans” the skins are degreased and puered and scudded as above, and drenched overnight in 10 per cent, of bran at 30—35° C. After branning they pass to the tan house. For “roller leather ” the skins are machine-fleshed, short-haired, and are then ready for puering. In many of the smaller yards dog-dung is still used for this, but in the larger works either erodin or merely some weak deliming acids are used instead for pulling down. The skins are then drenched with bran in paddle pits, the paddles of which run only intermittently. The skins are then thoroughly scudded and pass into the tan liquors. For “ basils ” sheep skins are sometimes puered, but usually108 THE MANUFACTURE OF LEATHER bated for two days at 25—30° C. with constant handling. After scudding and drenching they are again worked over the beam and sent to the tan house. Another method is to delime with acids instead of bating. A mixture of four parts of formic and one part of acetic acid is used. After fleshing and short-hairing they are washed in paddle with water at 83—35° C., and the extent of this washing determines the amount of acid to be used in deliming. The acid “ bate” is prepared’in a paddle with clean water at 30° C., to wdiich one quarter of a pint acid mixture has been added per 100 skins. The skins are paddled 15 minutes, and another quarter of a pint of acid is added. One or two thicker skins are now tested with phenol phthalein, and if any red shows a further portion of acid is added and the paddling continued. This is repeated unti} the deliming is just complete, but care must be taken to avoid excess of acid or the skins will swell. The goods are now rinsed through water and drenched overnight with a half bushel of bran per 100 skins, made up as usual to be 30° C. Next morning the skins are removed, washed, scudded if necessary, and sent to the tan house. For chrome work, imitation glacé, etc., the skins must be degreased and are better puered. They are thoroughly scudded and then delimed by drenching with 6 per cent, bran or by means of lactic acid. They are also often delimed by pickling (see below) in acid and salt, and in some large works pickled skins are dried out for sorting. Pickling consists in treating the skins with common salt and a small quantity of sulphuric acid (or of other acids), which preserves the skins temporarily. The process is particularly useful where the skins are to be exported. Acid is of course readily absorbed by pelt, which swells up under its influence. The addition of common salt to the liquor, however, totally prevents any swelling and, indeed, pulls down the skins, but does not hinder the absorption of acid, and hence skins treated in this way undergo a temporary tannage into a kind of “ white leather.” Pickled skins can at any time be “ depickled,” and then tanned by the ordinary methods. Many recipes have been given for pickling in one bath, but there is little doubt that where the skins are to beDELIMINGr 109 thoroughly pickled and kept for some time in this state, the best method is to finish up in a saturated salt solution. The method given by Procter, based on theoretical as well as practical considerations, is the best. It is a two-bath process. The skins are first paddled for 30 to 85 minutes in the “ rising solution,” which is a 0*75 per cent, solution of sulphuric acid containing also 8 per cent, of salt to keep the swelling within limits. The amount of acid in this bath should be kept as nearly constant as possible for each pack of skins, being restored each time a pack comes out. About 15 cc. of N/l NaOH should always be required to neutralise 100 cc. of the bath to phenol phthalein. Some salt is also absorbed and removed by the skins, and the strength of this should also be kept approximately constant. This is done with sufficient accuracy by keeping the specific gravity of the liquor at 1*065, which is readily ascertained by means of the Barkometer (65°) (see p. 155). The skins are now stirred in the “ falling solution ” for several hours, until the thickness of the skins is quite reduced. This bath is a saturated salt solution containing excess of salt, the strength therefore being quite unaffected by any dilution due to the added skins. The skins are then either dried out or kept in contact with a saturated salt solution. Pickled skins must be rigidly kept away from water, as in contact with it they both swell violently and stain badly. Depickling may be brought about by wetting back the skins in a 10 per cent, salt solution to which an excess of whitening has been added. The salt restricts the swelling until the acid diffuses out and is neutralised by the calcium carbonate. The skins can then be washed and tanned by any of the ordinary methods. Other alkalies may be used besides whitening for depickling; borax, sodium carbonate and bicarbonate and other sodium salts of weak acids are effective, but should be carefully added to the solution so that no excess is obtained. Pickled skins may also be given a vegetable tannage without depickling if salted liquors are used. It has been pointed out by the Committee of the Society of Arts on book-binding leathers, that the action of even small quantities of sulphuric acid has a considerable deleterious influence on the permanence of book-binding and furniture110 THE MANUFACTURE OF LEATHER leathers, so that it is becoming increasingly common to adopt the suggestion of Seymour-Jones and substitute formic or acetic for sulphuric acid in pickling. By this method the skins after deliming and drenching are paddled in a 0*25 per cent, solution of formic acid for a time, and then left 24 hours, and are afterwards transferred to a saturated salt solution for 24 hours. For chrome work pickling is more of a deliming process and preparatory treatment for tannage, and therefore can be quite well carried out in one bath. The quantities are usually reckoned on the pelt weight; 100 lbs. pelt require 2 to 4 lbs. hydrochloric or sulphuric acid and 8 to 10 lbs. of salt dissolved in 15 to 20 gallons of water. The goods are drummed in this for perhaps 15 minutes, during which the diluted acid is slowly added, and then allowed to stand for a few hours. Sheep skins are particularly difficult to deal with in the wet work, largely no doubt on account of their greasy nature, but partly also because they are somewhat liable to bacterial and mechanical damage, and because the animals are subject to various skin diseases, all of which make it a matter of considerable difficulty to prepare leathers of good appearance and quality. The following are some of the common sheep skin troubles. “ Colt ” is a spring disorder in the animal resulting in a curdling in the fat glands, which causes lime soaps to be fixed in the skin even till after tannage. “ Cockle 99 is a somewhat mysterious sheep skin disorder which occurs when the animal has a heavy coat of wool. Pimples appear which are full of pus or matter. This disease also appears in spring, but soon disappears when the animal is shorn. “Lime speck” is a trouble which occurs when the skins have been too long in contact with lime paste, or occasionally when they have been too long in the lime liquors. It appears as a spot or even as a circle up to a quarter inch in diameter, and is probably due to the deposition of some calcium compound by one of the putrefactive bacteria. “ Pin holes ” are mostly found in skivers and are due to over-sweating, but sometimes to overliming. They are minute spots which are often not noticeable unless the light is appropriately reflected. “ Pitted ” is aDELIMING 111 name given to a very similar appearance but with rather larger holes. It is caused by leaving the skins too long in the soaks, especially in warm weather. The last three defects mentioned are of course cases of bacterial damage. Lamb and kid skins for glove leathers are puered at 18°— 20° C. for some hours. About 1 or 1^ pailfuls of dung paste are sufficient for 200 skins. The process should be complete in 2 to 8 hours for these skins, but may take 12 to 14 hours for strong skins, paddling 10 minutes each hour for 5 to 6 hours, and then allowing to lie undisturbed. After careful scudding they are drenched at 85° G. for 1 to 8 hours. Goat skins for morocco leathers are puered at 27—80° C., afterwards worked over the beam, rinsed, and again worked ovei the beam. They are then drenched overnight at 80—85° C. with 10 per cent, of bran. After scudding they are forwarded to the tan house. For chrome glacé goat the skins should, for the best leathers, be thoroughly puered in order to obtain a smooth grain, and are then usually drenched at 15—20° C. for 2 to 8 days; they are then rinsed and placed in the chroming bath. It is usual in many of the large American factories to omit the drenching altogether. Many processes have been devised to do away with puering for this class of chrome leather, but without complete success. Clean skins and a plump, soft, fine-grained leather are said to be obtained, however, by the following method. The skins are washed in warm water, and puered in the ordinary manner at 80—88° C. for three-quarters of an hour, till the skins just begin to fall ; they are then rinsed through water and scudded. The deliming process is now completed by paddling in a solution at 30° C. in which 8 lbs. of calcium chloride and a half pint of a mixture of four parts formic and one part lactic acid are dissolved for every ten dozen skins. The skins are then rinsed through water and placed in the chroming liquor. Another method, only suitable for cheaper classes of goods and poor skins, is carried out as follows : A paddle liquor is made up at 80—85° C. as above with a half pint of the formic-lactic mixture. The skins after washing in warm water are paddled in this twenty minutes, and more acid is gradually added until the skins are just delimed. They are112 THE MANUFACTURE OF LEATHER then washed through water, and drenched overnight with a half bushel of bran per ten dozen skins. Next morning they are washed, scudded, and sent to the tan house. This process is apt to give a rather harsh grain, doubtless an effect of the acids used. Seal skins are heavily puered at about 85° C. for about three hours. They are then scudded by machine or by hand, and drenched overnight at 20° C. with 10 per cent, of a mixture of bran and pea meal. They are then sent to the tan yard. Chemical Control of Deliming Processes.—Little can be done in the case of bating and puering, as already pointed out, but the acidity of the drench liquor may be determined by titrating with N/10 caustic soda and phenol phthalein. Boric acid alone gives no definite end point with phenol phthalein, but can be accurately estimated in the presence of glycerin. To 50 cc. of boric solution (if about 1 per cent.) add 80 cc. of neutral glycerin and titrate, with N/10 caustic soda and phenol phthalein. Each cc. used corresponds to 0*062 gm. H3BO3. Lactic acid is apt to contain some anhydride, so that it is necessary to boil with a measured excess of N/10 caustic soda and titrate back with N/10 acid. The total acid may thus be determined, and the first appearance of the pink colour in direct titration gives a means of estimating the anhydride also.CHAPTER IX THE TANNINS Although the term “tanning” now includes the treatment of hides with mineral salts, oils and aldehydes, as well as with infusions of the vegetable tanning materials, the term “ tannin ” (or tannic acid) is still restricted to these latter, and is used as a class name for the essential constituents of them. The tannins may therefore be defined as a group of organic compounds which can be extracted from many vegetable materials by water, giving infusions which all possess, the property of converting the hide tissue of animals into an imputrescible and permanent material known as leather. A similar reaction occurs when the tannins are added to a dilute gelatin solution, a curdy precipitate of “ amorphous leather ” being obtained. A delicate test, therefore, for the presence of tannin in solution is to add two drops of a 1 per cent, gelatin, 10 per cent, salt solution to the liquor, a turbidity being produced if the solution contains more than one part of tannin in 100,000. The tannins are also all precipitated by a solution of basic lead acetate, and many of them more or less completely by solutions of many metallic salts, alkaloids and basic colouring matters. In dilute solutions ferric salts give dark colorations. Alkalies rapidly cause the tannins and many associated compounds to darken, oxidation taking place, and a deep red colour is also obtained by the addition of potassium ferricyanide and ammonia. The tannins are soluble in water, alcohol, acetone and ethyl acetate, but insoluble in benzene, chloroform, carbon disulphide, petroleum ether, dilute sulphuric acid and pure ether, though some quantity may be dissolved by ether containing water or alcohol. They all contain only carbon, hydrogen and oxygen, and belong to the aromatic division of M.L. I114 THE MANUFACTURE OE LEATHER organic compounds, being derivatives of the dihydric and trihydric phenols and their carboxyllic acids. Our knowledge of their chemical constitution, however, is as yet very incomplete, and this is chiefly due to the impossibility of obtaining these bodies in a pure condition. They are all uncrystallisable and non-volatile in air or steam. They are very unstable, and hence in many cases are confused with their decomposition products. Most plants also contain mixtures of more than one tannin, and the impossibility of separation makes the evidence of experimental investigation very inconclusive. The methods of purification of the tannins are thus very limited, and as each tanning material contains very different soluble non-tannin bodies, no general process of purification can be described. The following methods, however, with modifications according to the material, have been found useful either alone or in conjunction. Fractional solution in different liquids is perhaps the most generally useful method. The material may be extracted with alcohol or acetone and the solution evaporated to dryness, the later stages under reduced pressure. An aqueous infusion is now made, and insoluble impurities (phlobaphenes, resins, etc.) filtered off. The filtrate may now be agitated with ethyl acetate which is immiscible with water and which takes up most of the tannin. This process may sometimes be assisted by the presence of common salt. The ethyl acetate solution of the purified tannin may now be evaporated to dryness. Water, of course, may be used for the extraction, and the clear infusion treated with salt and ethyl acetate direct. The employment of ether is also extremely valuable at many stages of these processes, various impurities (gallic acid, etc.) being soluble in it. Being only partially miscible with water, it may be either shaken with aqueous solutions or used upon the dry tannin at any stage. Fractional precipitation of aqueous solutions by means of lead acetate, the middle fractions only being taken, gives the lead salts of the tannins. These may be decomposed by an insufficient quantity of dilute sulphuric acid or by passing sulphuretted hydrogen through water in which the lead salt isTHE TANNINS 115 suspended. The filtrate contains the purified tannin and may be evaporated to dryness. It is often uncertain, however, whether the tannin has not been changed by this process. Organic derivatives of the tannins, such as the crystalline acetyl compounds, have been used for purification purposes,1 but these are not so reliable as the less drastic methods. The classification of the tannins is as yet very imperfect owing to our ignorance of their chemical constitution. When heated carefully to about 200° C. all tannins yield either pyrogallol or catechol, and occasionally phloroglucol, together with other products, and this forms the basis of their division into pyrogallol tannins (tannins derived from pyrogallol) and catechol tannins (tannins derived from catechol), which division is further justified by other marked differences between the two groups. These are summarised in the following table:— Test. (Use dilute aqueous solution.) Pyrogallol tannins. Catechol tannins. Boil with dilute sulphuric acid. Yellow ppt. of ellagic acid, insol. in cold alcohol and hot water (not given by some). Eed ppt. of phloba-phenes, soluble in cold alcohol and hot water. Add iron alum solution. Blue-black colour. Green-black colour. Add bromine water.2 No ppt. Precipitate. Add diazo benzene chloride.3 No ppt. Precipitate. Add to one drop of solution cone. H2S04. Brown or yellow colour. Crimson colour. The coloration tests are naturally not completely satisfactory in the presence of mixtures of tannins from both groups, but the precipitation tests are still reliable. Another significant difference between the two groups is that the 1 Yournasos, “Le tannin de la noix de galle, sa constitution,” Baillère et Fils, Paris, 1903. 2 Procter, J.S.C.I., 1894, 488. 3 Nierenstein, Collegium, 1906, 376. i 2116 THE MANUFACTURE OF LEATHER catechol tannins invariably contain about 60 per cent, of carbon, whereas the pyrogallol tannins contain only about 52 per cent. Pyrogallol tannins and associated substances. — Pyrogallol (jjyrogallic acid, 1:2:8 trihydroxy benzene) C6H3(OH)3, the parent substance of this group, is a trihydric phenol which melts at 182° C. and' sublimes at 210° C. It is soluble in water, alcohol, ethyl acetate, ether and acetone, but insoluble in chloroform and petroleum ether. Its aqueous solution gives a red colour with ferric chloride, and a blue colour with ferrous sulphate which is turned red by acids and restored by ammonia. In the presence of alkalies oxygen is very rapidly absorbed and the mixture darkens quickly. It does not precipitate gelatin, but reduces the salts of silver, gold, platinum and mercury, and also Fehling’s solution. It is precipitated by lead acetate and some other metallic salts. Many oxidising agents cause it to yield purpuro-gallin, a napthalene derivative. Metagallic acid is formed by heating rapidly to 250° C. This is a black amorphous substance soluble in alkalies but not in water, and is also formed from gallic and gallotannic acids when treated in a similar way. Gallic acid (8:4:5 trihydroxy benzoic acid), C6H2(OH)3COOH is found associated with many pyrogallol tannins, and is formed by the hydrolysis of gallotannic acid by acids, alkalies or ferments. It is difficultly soluble in cold water but readily in hot, from which it crystallises on cooling in pale yellow needles containing a molecule of water of crystallisation. It is soluble in alcohol and ether, and by the latter may be removed from aqueous solution. When carefully heated to 210° C. it loses carbon dioxide, and yields a sublimate of pyrogallol, but if raised quickly to 250° C. metagallic acid is formed. Rufigallic acid (hexa-oxyanthraquinone) is obtained by heating with concentrated sulphuric acid to 140° 0. and diluting with water. Alkaline solutions of gallic acid darken and absorb oxygen. Ferric chloride gives a blue colour, and pure ferrous sulphate a white precipitate which rapidly darkens. Lime water gives a precipitate which turns blue by oxidation (ctr. pyrogallol). Potassium cyanide gives a red colour, and sodium arsenate aTHE TANNINS 117 green colour. A hot solution reduces potassium permanganate, and salts of silver and gold, but not Fehling’s solution. It does not precipitate gelatin but is absorbed to a considerable extent by hide powder1 (p. 147). Ellagic acid (“ bloom”) Ci4H608 is a double lactone of a hexa-hydroxy-diphenyldicarboxyllic acid, and has the following constitution2 :— It is obtained by the decomposition of ellagitannic acid as a yellow amorphous powder, and is therefore yielded by any material containing this tannin either by the action of acids or ferments. It may be purified by crystallisation from pyridine or absolute alcohol. It is insoluble in cold alcohol or hot water, but soluble in alkalies, being decomposed by hot caustic potash solution and further still by fusion with potash, the lactone groups opening up and the carbonyl groups being eliminated. It may be prepared from gallic acid by oxidation of its 10 per cent, solution in acetic acid with potassium persulphate and a little sulphuric acid, and pouring into water. It is a feeble colouring matter. Nitric acid, containing nitrous acid, gives with it a crimson colour. Flavellagic (hydroxy-ellagic) acid Ci4H609, catellagic acid Ci4H606 (a dihydroxy compound) and metellagic acid Ci4H605 (a monohydroxy compound) are very similar in properties and are obtained in similar ways. Gallotannic acid {cligallic acid) CnHioOg is the principal tannin of oak galls and of sumac. It also occurs with ellagitannic acid in many other materials (myrobalans, divi-divi, etc.). Its chemical constitution has received more attention than has been given to any other tannin, but is still not satisfactorily elucidated. Many syntheses of gallotannic acid have been claimed, but doubt has been thrown upon 1 Parker and Bennett, J.S.C.I., 1906, 1193. 2 A. G. Perkin, J. 0. S. 1905, LXXXVII., 1412, 1418, 1426, and LXXXIX., 251.118 THE MANUFACTURE OF LEATHER them all. It is usually accepted to be an anhydride of gallic acid, and the most commonly used formula is the following1:— OH oh/ \. OH OH -\ / \ y—CO * 0—/ y \_____/ OH /COOH There is some evidence, however, for the view that it exists in the plant originally as a glucoside, and it has been pointed out, moreover, that the purified tannin is optically active and dextro-rotatory, so that many other formulas containing an asymmetric carbon atom have been suggested, amongst which that of Dekker may be mentioned here:— The presence of a carboxyl group is doubtful, and the number of hydroxyl groups in the molecule is as yet indeterminate. Various acetyl and benzoyl derivatives have been described, but their composition seems to vary somewhat widely. Commercial gallotannic acid is a buff-coloured powder with a characteristic odour, and containing varying quantities of gallic acid, glucose, starch, etc. It is somewhat hygroscopic and contains usually 10 to 12 per cent, of moisture. When hydrochloric or sulphuric acid is added to a strong solution a white precipitate is obtained, which when boiled with excess of acid is completely converted into gallic acid. Ferments and warm alkalies (in the absence of oxygen) accomplish the same hydrolysis. Ellagic acid is not obtained from gallotannic acid under any of these conditions. The aqueous solution gives precipitates with gelatine, various alkaloids and other organic bases, with lead nitrate, lead acetate and acetic acid, ammoniacal copper and zinc solutions and with tartar emetic 1 Vournasos (see footnote p. 115).THE TANNINS 119 and ammonium chloride, and by these reactions may be distinguished from gallic acid, from which, however, it cannot be quantitatively separated. It gives white precipitates with baryta and lime water1 which rapidly oxidise and turn blue, purple, and finally black. EUagitcmnic acid is a pyrogallol tannin often occurring naturally with gallotannic acid. It occurs most plentifully in myrobalans, valonia, divi-divi and algarobilla. Almost nothing is known of its constitution, but its presence is always recognisable by the formation of ellagic acid (bloom) from its aqueous infusion, by the action of ferments as in practice, or by hot dilute mineral acids. The yield of bloom may be taken as a measure of the quantity of ellagitannic acid. Oakivood tannin (C15H12O9 * 2 H20 ?),2 which is used extensively in practice in the form of oakwood extract, has been thought to be a methyl derivative of gallotannic acid, but it is probably a mixture, and certainly is capable of yielding a considerable quantity of bloom. Babool tannin (pods) is a pyrogallol tannin which does not give a precipitate with lime water. Chestnut wood, divi-divi and algarobilla contain pyrogallol tannins which resemble both gallotannic and ellagitannic acids. Catechol tannins and associated compounds. Catechol (pyro-catechin, 1: 2 dihydroxy benzene) CeELi (OH)2 the basis of the tannins of this group, is a dihydric phenol which melts and sublimes at 104° C. It is soluble in water, alcohol and ether. It gives a dark green colour with a few drops of ferric chloride solution, which is turned red by alkalies and restored by acids. It does not precipitate gelatin or alkaloids, but darkens in alkaline solution. Silver salts are reduced by it in the cold, and Fehling’s solution on heating. It is precipitated by lead acetate (ctr. resorcinol and quinol). Protocatechuic acid (8 :4 dihydroxy benzoic acid) C6H3(OH), COOH is a decomposition product of the catechol tannins 1 Procter and Bennett, J.S.O.I., 1906, 251. 2 Bôttinger, Ber. XX., 1887, 761—766.120 THE MANUFACTURE OF LEATHER catechins and phlobaphenes. It crystallises from hot water in white needles containing a molecule of water of crystallisation. It strongly resembles gallic acid in its properties and reactions, but its barium and calcium salts are more soluble.1 Vanillic and iso-vanillic acids are methoxy derivatives. Phloroglucol (phloroglucin 1:3:5 trihydroxy benzene) C6H3(OH)3 is an isomer of pyrogallol. It melts at 220° C. and sublimes. It is soluble in water, alcohol and ether. When pure it gives no colour with iron salts and is only precipitated by basic lead acetate. Bromine water gives a white precipitate of the tribrom derivative. When a deal shaving is moistened with its solution and concentrated hydrochloric acid added, a deep violet colour is formed which is not given by catechol and pyrogallol. It is obtained from some catechol tannins by dry distillation, but more usually on fusion with potash. The catechins are thought to be the parent substances of the catechol tannins, which are supposed to be their first anhydrides. They are white crystalline bodies occurring naturally with the catechol tannins. They are sparingly soluble in cold water, but freely in alcohol, ether and hot water. Aqueous solutions give precipitates with lead acetate, mercuric chloride and albumin, but not with gelatin, alkaloids or tartar emetic. When heated with dilute mineral acids they are first converted into tannins and afterwards into phlobaphenes, probably by dehydration in successive steps. They are often deposited as “ whites ” on the sides of pits containing gambier liquors. Gambier catechin, Ci5Hu(V4 H20, has been thoroughly investigated by Perkin.2 It melts at 176—177°, gives the deal shaving reaction for phloroglucol a green colour with ferric chloride and yields acetyl, benzoyl and azobenzene derivatives which indicate five hydroxyl groups. An isomer has been isolated from the same material, but with no water of crystallisation and melting at 235—237° C., but otherwise similar in properties. 1 Procter and Bennett, J.S.O.L, 1906, 251. a A. Gr. Perkin, J. O. S., LXXXL, 1892, p. 1160.THE TANNINS 121 Acacia catechu yields a catechin melting at 204—205° 0., which is also isomeric and similar in properties to the above. The phlobaphenes or reds are the condensation products (anhydrides) of the catechol tannins, from which they are derived by the continued boiling of solutions of these tannins or by boiling in the presence of mineral acids. They are usually found to some extent associated naturally with the parent tannin. They are sparingly soluble in cold water, but freely in alcohol and hot water. They are soluble in dilute alkalies, alkaline carbonates, and in sulphites, borax, etc., and have been thereby made available for tanning. Each catechol tannin yields a series of these “ reds,” the solubility of which decreases as the degree of condensation and dehydration increases. The more soluble members are the “ difficultly soluble tannins,” which form the principal colouring matters of the materials containing them, and which combine wdth hide to form leather. The more insoluble members form the red sediment often found in tan pits. On fusion with caustic potash they all yield protocatechuic acid, and some yield phloroglucol. Catechutannic acid is the name given to the catechol tannins of gambier, cutch, kino and other plants. It appears to be formed by the condensation of the catechins, and yields bromine derivatives containing about 50 per cent, of bromine. It probably represents several chemical individuals, of which the tannin of gambier is the least astringent and combines most loosely with hide fibre. Quercitannic acid is the principal tannin of oak bark, which also contains the associated phlobaphene. It contains 59*8 per cent, carbon, but the evidence for its constitution is somewhat contradictory. Its bromine derivative contains 28*4 per cent, of bromine. Quebrachotannic acid is the principal tannin of quebracho wood and extract. It contains phloroglucol and also readily yields phlobaphenes by condensation ; its bromine derivative contains about 43 per cent, of bromine. The catechol tannins of mimosa and chestnut barks resemble quercitannic acid. The Jlavones are natural colouring matters found associated122 the manufacture of leather with the tannins. They are hydroxy derivatives of flavone, of, which the constitution is as follows : Quercetin, myricetin, fisetin and morin are some of the principal members. It is noteworthy that phosphorus oxychloride, arsenic acid, sulphuric acid, aldehydes, and the halogens, all act upon the higher phenols and their carboxylic acids to form compounds which have many of the reactions of tannins.CHAPTER X THE VEGETABLE TANNING MATERIALS The tannins described in the previous chapter are very widely distributed in the vegetable kingdom, but exist in very varying quantities in the different plants. A vast number of plants contain a certain amount of tannin in some of their parts, but only a limited number contain sufficient to be of commercial importance in the leather industry, and it is with these materials only that this chapter will in any way attempt to deal. The tannins are found in almost all parts of plants, and the vegetable tanning materials may therefore be barks, wood, twigs, leaves, fruit, or even excrescences {e.g., galls). Generally speaking, barks and fruits are found to possess the greater tannin strength in trees, but the wTood, though containing less tannin, is often found to be a cheaper source. Hence it is that fruits are usually imported in the natural state, whilst wood is converted into “ extract” by the careful evaporation of its aqueous infusion. As so many different parts of various plants contain tannins of commercial importance, it is outside the scope of this work to give any account of the structure of these plants, and the reader is therefore referred to works on botany. It should perhaps be pointed out, however, that in barks the tannin is contained chiefly in the bast layer which lies just under the epidermis, in roughly cubical cells with rather thick walls. These often contain starch granules of characteristic forms, which can be examined microscopically by staining sections with iodine solution. Tannin is similarly detected by staining with an alcoholic solution of ferric chloride. The “ reds ” and dark colouring matters are usually contained in the outer and dead part of the bark, which as a rule contains little tannin. Although it is known that the natural tannins are often124 THE MANUFACTURE OE LEATHER mixtures of more than one chemical individual, and that different tannins occur in different parts of the same plant, it has been shown, on the other hand, that it is usual to find one of the two main groups of tannins in predominance, so that the natural materials generally yield infusions which contain chiefly pyrogallol tannins, or which consist almost entirely of catechol tannins. As the difference between these two groups is practical as well as theoretical, this method of classification has been adopted also in this chapter. Broadly speaking, the group of pyrogallol materials produce rather soft and porous leathers when used alone (though valonia is an exception), but yield “ sour ” liquors and plump well on account of the fermentable sugars naturally associated with them, which yield various organic acids. Many of them also deposit “ bloom ” (p. 117). The catechol group, on the other hand, deposit “ reds” (p. 121), which often occur as sediment at the bottom of the pits; and some yield “ whites,” a crystalline deposit of catechins (p. 120) on the sides of the pits. This class of materials often contains little or no sugar, and hence with catechol tanning materials alone hides cannot usually be satisfactorily tanned. For most leathers it is desirable to combine the two classes, but if the catechol tannins are alone employed it is necessary to resort to an artificial acidification of the liquors1 and possibly also a “ raising ” of the pelt with acids. Pyrogallol Tanning Materials. Yalonia is the calyx or acorn cup of the Turkish oak (Quercus aegilo'ps), though probably obtained also from other species (Q. macrolepis, Q. graeca, etc.). It grows extensively in Asia Minor and Greece. The fruit in the former district ripens in August, and is gathered and forwarded to Smyrna, which gives rise to the term “ Smyrna valonia.” This is stored at Smyrna in special chambers, in layers of about one foot deep, until dry. The drying process is fermentive and in a few weeks the acorn, which contains but little tannin, falls away from the cup. The cups are then sorted by hand ; the largest and finest cups being exported to Trieste and used 1 Bennett, J.S.C.I., 1908, 1193.THE VEGETABLE TANNING MATERIALS 125 in Austria, Germany and France ; the second quality and much of the remainder comes to England. The “ Greek valonia ” is obtained from many parts of the Grecian Archipelago as well as the mainland. The best quality is obtained when the fruit is still immature and the acorn well enclosed. This is known as camatina if the acorn is quite covered, and as camata if slightly exposed. It gives excellent colour, and is therefore used in dyeing. The second quality (rhabdisto) is beaten off in September and October, and the charcala, which falls after the first rains, is valueless and not collected. The tannins contained are a mixture of both groups, but chiefly from the pyrogallol section, including a Eig. 33.—Smyrna and Greek valonia (camata). considerable proportion of ellagitannic acid (p. 119). This makes the material extremely suitable for the tannage of sole leather goods, the deposition of bloom in the material making a firm compact leather of good weight. If the bloom be allowed to deposit before use tannin is of course lost, but a tannage very suitable for dressing leathers is obtained, and in conjunction with gambier and other materials, it is often thus used. The relative value of Greek and Smyrna valonia to the tanner has been investigated by Parker and Leach1 with interesting results. Samples of average quality were taken with great care and submitted to an analytical investigation. 1 J.S.C.I., 1903,1184.126 THE MANUFACTURE OF LEATHER The cup and beard were analysed separately and also together, taking two-thirds cup and one-third beard to represent the bulk. The analyses are given in the following table, together with the tintometer (p. 154) results Material. Tannin. ISTon- tannins. Insoluble. Water. Red. Yellow. Smyrna valonia . 32*43 12-50 43-07 12-00 1-6 5"6 „ cup 30-99 12-79 44-12 12-10 1-7 4-6 ,, beard 43*61 14-45 29*93 12-01 1-2 4-1 Greek valonia 32-07 12-96 42-97 12-00 1*5 5-0 ,, cup . 27*37 12-92 47-71 12-00 2-0 6-7 ,, beard 41-03 13-96 33 -0 1 12-00 1-3 4-4 This shows a slight superiority in strength and colour on the part of the Smyrna variety. The bloom-yielding properties were also investigated, and it was found that in each case about 75 per cent, of the total was deposited in a fortnight. The Smyrna valonia yields a greater amount than the Greek valonia, and in each case the cup more than the beard. It was found also that the Greek variety produces more acid than the Smyrna, and in both cases the acid is more quickly formed from the beard. The valonia tannins have only moderate affinity for hide substance and penetrate it very slowly, but the material has excellent weight-giving properties, and leather tanned with it offers considerable resistance to water. The internal deposition of bloom accounts for the last two properties. Myrobalans1 is the name given to the dried fruit of the Indian Terminalia chebula, though other species are sometimes used. These trees grow to a height of 40 to 50 ft., and are valuable also for their timber. The nuts are about the size of a pigeon’s egg, but some are elongated and wrinkled. As they ripen on the trees certain of these are 1 Sometimes called “myiobalams/' ‘‘myrabolams ” or even “ myrabs.’THE VEGETABLE TANNING MATERIALS 127 picked by hand before the sun has had time to darken them in colour. These form the “ picked ” varieties, and usually fetch a higher price in the market. Afterwards, as the fruit ripens, the trees are knocked or shaken and the fruit falls to the ground and is collected. On reaching the warehouse it is sorted by hand according to colour into “ No. 1,” “ No. 2,” and sometimes “ No. 3 ” varieties. It has been shown that the riper the fruit the greater is the tannin strength and the Fig. 34.—Myrobalans. darker the external colour, but that this has no effect on the colour of the liquors or the leather made from it. Myrobalans gives a rather mellow tannage, and when used alone is said to give a spongy leather. Its affinity for hide substance is exceedingly small, and it penetrates very slowly. It contains a large proportion of ellagitannic acid which ferments rapidly and deposits a light-coloured “ bloom.” It is not considered, however, a good weight-giving material. The tannin strength is not largely different from valonia, but the latter is much dearer. It has been found that when blended in small quantity with certain other materials the ordinary colour produced by these is greatly improved, a much brighter effect being obtained. One of the greatest uses of myrobalans is in producing acids in the liquors, in which128 THE MANUFACTURE OF LEATHER respect it excels most other materials on account of the large percentage of sugars which it contains. From these considerations it is clear that myrobalans can be best used in the earlier stages of tannage, giving mellow liquors, good colour, and satisfactory plumping. The relative value of the chief commercial varieties has been investigated by Parker and Blockey,1 who worked with carefully selected average samples. The results are given in the following table, the varieties being named after the district from which they are obtained :— Material. Tannin. Non- -tannins. In- soluble. Water. Red. Yellow. Black. Picked Bhimley . 33-0 13*1 41-7 12*0 0*8 2*5 No. 1 38*4 16T 33-5 12*0 0*3 1*8 — No. 2 ,, * 35'2 14*2 38-6 12*0 1-0 5-1 — Picked Rai pore . 32-2 13*0 42-8 12-0 1-1 3-0 — No. 1 3o#4 12T 40-5 12-0 0-9 4-0 01 No. 2 27*6 12*7 47-7 12-0 2*5 7*4 — Picked Jubblepore 28*9 12*7 46*4 12*0 0-8 2*2 — No. 1 36-5 14*4 37*1 12*0 0-8 3*4 — No. 2 27-3 14-1 46*6 12-0 1-3 5*9 — Vingorlas 31*5 9*5 47-0 12*0 1*2 3*0 — Pair Coast Madras 34*8 15*4 37-8 12-0 1*2 3*9 — It will be noticed that “No. 1 ” varieties are better than “ picked,” and that “ B.’s ” show the greatest strength, followed by “ J.’s ” and afterwards by “ B.’s.” It is also clear that the colour of the fruit is no indication either of the tannin strength or of the colour of the liquors produced therefrom. The weight-giving properties were also investigated, “J.’s” being distinctly the best, followed by “B.’s” and “R.’s” which were about the same. “No. 2” varieties were better than “No. 1,” and “No. 1” better than “picked.” In examining the bloom-yielding properties it was found that “J.’s” and “ V.’s ” gave much larger deposits than the rest, and that more than half was deposited in the first week ; whereas with the rest the reverse was true. “B.’s” were shown to be the best acid-producing variety, the “ J.’s ” developing least. 1 J.S.O.I., 1903, 1183.THE VEGETABLE TANNING MATERIALS 129 Prom these considerations it is evident that “ J.’s ” are most suitable for the layers and “ B.’s ” for the early liquors. Divi-divi is the seed pod of a South American tree (Gaesalpina coriaria) and contains 40 to 50 per cent, of tannin which lies almost entirely in the husk of the pod. It contains a large proportion of ellagitannic acid, and is somewhat similar to myro-balans but much more prone to fermentation and to the sudden development of a red colouring matter. It has been reported as yielding great weights in sole butt tanning, but is used chiefly as a gambier substitute for dressing leather and for the rapid drum tannage of light leather. Antiseptics prevent to some extent its rapid fermentation. Algarobilla is the seed pod of several other species of Caesal-pina, but chiefly C. brevifolia. It contains 45 to 50 per cent, of tannin, mostly ellagitannic acid, which is readily extracted and fermented quickly. It Fiq 35._Sicilian sumach {Rhus roriaria, is said to give better weight and firmness than divi-divi and is less prone to discoloration; hence its use as a myrobalans substitute in the tannage of offal. After fermentation it gives a leather of very bright colour. When algarobilla is employed the leather is very prone to mould in the sheds. It is often blended with myrobalans, divi-divi and quebracho. Sumach1 (Sicilian) is the leaves and small twigs of Rhus coriaria, which is a bushy shrub cultivated in Italy for the extensive production of this tanning material. Suckers from older plants are planted in rows during spring and begin to bear the following year, though the older plants give better 1 “ Sumac ” and “ shumac ” are also used, the latter being the invariable pronunciation. M.L. X130 THE MANUFACTURE OF LEATHER tannin strength. The leaves are picked by hand, or the shoots pruned from July to September, and dried in the sun in the fields or under cover. If the leaves only are collected the plants are pruned in winter. If pruned when gathered the dried shoots are beaten to separate leaves and stems. This is “ leaf sumach,” and may be exported in this state. Some, however, is packed by hydraulic press, sent to Palermo and there ground to a fine powder in stone mills. For this to be done properly there must be left in with the leaves a certain proportion of stems. The product is “ ground sumach,” and yields “ ventilated sumach ” by winnowing over a screen, the light leaves passing over and leaving the heavy stems and sand behind. “ Mascolino ” is the best sumac ; feminella is weaker and contains a greater proportion of stems. The unpulverised portions are re-ground, and the coarse residue from this either ground again or used for fuel. Good sumach should contain 26 to 28 per cent, of tannin, which is nearly all gallotannic acid, but it is now often adulterated with inferior materials. It is an extremely valuable tanning material, giving a soft tannage, excellent colour and a durable leather. The report of the committee1 of the Society of Arts on book-binding leathers shows that sumach tannages are much less affected by light, heat, gas fumes, wear, and are less liable to decay, than any other tannage. It has one disadvantage, the infusion rapidly ferments, and the tannin becomes hydrolysed into gallic acid. It is extensively used for the tannage of moroccos, roans, skivers, etc., and also for bleaching the goods from, darker tannages, as in the finishing of sole leather. The common adulterants of sumach contain catechol tannins, of which pure sumach contains none, so that they may be detected by the reactions for catechol tannins (p. 115) ; but by far the best way is to submit the sample to a microscopic examination. The best mode of procedure is that of Lamb and Harrison as modified by Priestman,2 in which a small amount of the suspected sample is placed in a test tube and 1 J. Soc. Arts, 1901, 14. 2 J.S.O.I., 1905, 231.2. Rhus coriaria, upper cuticle (dyed). 1. Rhus coriaria, upper'cuticle (undyed). 3. Rhus coriaria, lower cuticle (dyed). 4. Rhus coriaria, stem cuticle. f>. Pistacia leutiscas, upper cuticle. 6. Pistacia Icntiscus, lower cuticle. 7. Tamerix africana, stem cuticle. 8. erniaria my rii folia, upper cuticle. 9. Colpoon compressum, upper cuticle. 10. Rhus glabra, upper cuticle. Fig. 35a.—Photo-micrographs of sumach, its adulterants and substitutes.THE VEGETABLE TANNING MATERIALS 131 subjected to the action of strong nitric acid. The temperature is gradually raided by means of a water bath. This destroys the internal structure of the leaves, and the acid may then be neutralised with a slight excess of ammonia, the cuticles washed somewhat with distilled water and examined under a low power with transmitted light. The leaf cuticles so obtained exhibit very characteristic structures, and in this way are easily recognised. Those of sumach are characterised by numerous small hairs which are especially prominent if the cuticle be dyed with safranine. Pistacia lentiscus (“ lentisco,” “ schinia ”), which is perhaps the commonest adulterant, contains only 12 to 15 per cent, of tannin. It gives a darker tannage than sumach, and the leather reddens on exposure to light. Its cuticles have no hairs and are more résistent to the acid and less transparent than those of sumach. Tamerix africana (“ brusca ”), as small twigs, dried and ground, is imported into Sicily from Tunis for adulterating sumach. It contains 10 to 15 per cent, of tannin, produces a thin harsh leather, though of good colour. The cellular structure of the stem cuticle is rather elongated in the direction of growth, and there are no hairs. Coriaria myrtifolia (French sumach, “ stinco ”) contains about 15 per cent, of tannin in its leaves, which are used to adulterate sumach. It is a poisonous plant. Its cuticles have structures somewhat resembling those of pistacia, but easily distinguishable from them by a slight waviness in the cell walls. Many other species of the Rhus family have been suggested Eig. 36. -American sumach [Rhus i/labra).132 THE MANUFACTURE OF LEATHER as tanning materials, but none are so good as R. coriaria, In tlie United States, jR. glabra (American sumach) is somewhat extensively cultivated for this purpose, but its tannin content is distinctly less (22 to 24 per cent.), and it produces also a much more yellow and dark leather, though this can be remedied somewhat—at the expense of the tannin content— by collecting the leaves earlier than is usual. R. copallina, which contains only about 17 per cent, of tannin, is also grown in America and used as a sumach substitute; and R. cotinus (Venetian sumach) is about the same strength and is also used for tanning though more important as a dyeing material. Oakwood extract is now an exceedingly important and valuable tanning material, and is manufactured very extensively in Slavonia from the wood of the common oak (Quercus pendunculata) ,x which, however, only contains 2 to 4 per cent, tannin. The extract contains usually 26 to 28 per cent, tannin, and has a specific gravity of about 1*2. The tannin yields bloom and has a good combining affinity for hide substance. Its penetrating power is excellent, and its weightgiving properties and water-resisting powers are also good, so that it is widely used for making up the layer liquors of sole and dressing leathers. It is sometimes mixed with quebracho and sold as oak-bark extract, but the true oak-bark tannin belongs to the catechol group (see p. 188), and gives different results in practice. Chestnut extract is extensively manufactured from the wood of the Spanish chestnut (Castanea vesca), which contains 8 to 6 per cent, of a tannin rather similar to oakwood. Its astrin-gency is not so great as oakwood tannin; but its penetrating powers are even better, and its weight-giving and water-resisting powers are equally good on account of the bloom it deposits. It is used largely in a similar way to oakwood extract, and is similarly adulterated. It is a very suitable material for drum tannages. Willow bark, from Salix arenaria and S, Russeliana, contains 7 to 11 per cent, tannin, and is used for tanning in i A sub-species of Q. robur, see p. 133.THE VEGETABLE TANNING MATERIALS 133 Russia and Denmark. It imparts to leather a characteristic odour which, when combined with that ol birch tar oil, produces the well-known scent of “Russia” leather. Knoppern are oak-galls from Q. cerris and other species. They form on the unripe acorn and contain up to 85 per cent, of gallotannic acid. They were at one time extensively employed in Austria for tanning, but are now largely replaced by valonia. Turkish or Aleppo galls are excrescences from the young shoots of Q. infectoria. The tannin content is at its highest just before the gall-insect escapes, and may be nearly 60 per cent. These galls are the source of commercial “tannin,” (gallotannic acid). Catechol Tanning Materials. Oak bark, from Q. robur (embracing Q. pendunculata and Q. sessiliflora as subspecies), has been used as a source of tannin for many centuries. It con- Fig. 37.—Common oak (Quercus rdbur', tains up to 12 or 14 per cent, of tannin, chiefly from the catechol group, but gives a blue-black (p. 115) with iron alum, and will yield considerable bloom and gallic acid. Laevulose (fruit sugar) is also present in the bark, though uncombined with the tannin. English bark (Hants and Sussex) is considered the best, though Belgian is occasionally good. Other Continental barks are distinctly inferior. Trees under 25 years old, growing in rich soils and warm situations, yield the best bark. The bark is usually peeled in April to June, when the sap is rising and the cells are multiplying, but at other seasons may be steamed off the newly-felled tree with little tannin loss. It is then piled134 THE MANUFACTURE OE LEATHER in the woods for a preliminary drying, and afterwards stored to complete the process. Oak-bark tannin has always been noted for the good colour, soundness, and durability of the leather it produces. Its combining and penetrating powers are also good and its weight-giving properties excellent. Its use has nevertheless largely decreased in recent years on account of its weakness in tannin compared with other available materials, and its bulky storage. No genuine and satisfactory extract has been as yet made from it. Gambier is a solid or pasty extract from the leaves and twigs of Nauclea gambir, an Eastern shrub. Like oak bark, it is one of the oldest tanning materials known in this country, and is imported to a considerable extent from Singapore, Straits Settlements, Malay Archipelago, etc. It is worked chiefly by Chinese and Malays, often with no white supervision. Shoots from older plants are planted in rows, and begin to bear in their third year. The prunings are taken to the factory, bruised, and boiled in an open boiler. The infusion is strained, concentrated until syrupy, and poured into special vessels, in which it cools and grows pasty. In the manufacture of “ cube gambier,” which is the best quality, the syrup is run into trays 2 inches deep and dried in the sun. When in a sticky condition it is cut into 1^-inch cubes and dried further on cocoanut matting till the pieces occupy about 1 cubic inch. It is imported thus into this country and contains 40 to 56 per cent, of tannin. “ Block gambier,” an inferior quality, comes in large oblong blocks and in a more pasty condition, and when wrapped in cocoanut matting forms 1-cwt. or 2-cwt. bales. It contains 25 to 40 per cent, of tannin. It gives a peculiar tannage, mellow and soft, and hence is used largely for sole leather in the early stages, but must be used along with other materials. There is some evidence to show that the cafcechutannic acid goes into the hide and is afterwards substituted by other tannins. It is an excellent thing to add to an astringent tan liquor, stopping “ drawn grain.” It does not give weight. It is used for the tannage of harness and dressing hides, shoe-calf and kip. It is thought that gambier tannages carry grease better thanTHE VEGETABLE TANNING MATERIALS 135 some other tannages, and it is therefore much liked for curried work. Quebracho is the wood of a South American tree, Loxopteryngkmi Lorenzii (Span. Quebracho Colorado). This contains about 20 per cent, of a tannin (p. 121), which is not very soluble in water. The wood contains in addition a considerable quantity of phlobaphenes, which are also difficultly soluble, and a quebracho tannage therefore is only economical in weak liquors. The wood, which is very hard, is imported in logs and chipped into small pieces for extraction. The tannin is red, very astringent, and quickly penetrates the hide, but the material is not noted for its weight-giving properties. It produces a firm red leather, which, however, becomes distinctly darker on exposure to light. Practically no sugars are associated with it, and hence either acids or acid-forming materials must be used along with it. Its specific gravity being about 1*8, it sinks in water, and a few inches of quebracho shavings are sometimes put at the bottom of a leach (see p. 160) and used as a straining material. Mellow liquors also are sometimes sharpened by pumping them over quebracho. Though a very cheap source of tannin, it is not very popular in this country, and where used at all is generally blended with myrobalans, valonia, divi-divi or pine bark. Quebracho extract is now commercially important and appears cheap. It is usually solid and contains about 60 per cent, of tannin and an exceedingly small proportion of nontannins. Only weak infusions should be made up of this material also, and it should not be used for the layer liquors. Solublised quebracho extracts are now extensively manufactured in which the difficultly soluble tannins (phlobaphenes) are brought into solution by treatment with sulphites and other alkalies, and thereby made available for tanning.1 Such extracts are unsuitable for use in making up the layer liquors, as the deposition of the reds would be retarded by the excess of sulphites; and they have also been found to produce bad results if used in any considerable 1 Lepetit Hollfus and Gansser, E., patent 8582, 1896,136 THE MANUFACTURE OF LEATHER quantity in the early stages, so that if used at all they are probably best added in very moderate quantities to the handler liquors. Hemlock bark is obtained from the hemlock fir (Pinus canadensis) which is abundant in the north and west of the United States of America and in Canada. It forms the staple Fig. 38.—Hemlock firs (Pinus canadensis). tanning material of America, though other materials are now increasing in relative importance. It contains 8 to 11 per cent, of tannin and much of the phlobaphenes. It yields a red leather which darkens on exposure to light. It contains appreciable quantities of sugars, and has been successfully used alone. It is used for the tannage of Bole and dressing leather, and for the manufacture of hemlock extract, in w’hich latter case other materials are often mixed with it.THE VEGETABLE TANNING MATERIALS 137 Mangrove bark, from Rhizuphora mangle, and sometimes other species, comes from West Africa, Borneo, and other places near the coasts of tropical countries, in the tidal swamps of which it grows freely. It gives a harsh tannage of dark red colour, and is therefore not very popular, but when blended with myrobalans the colour it produces is greatly improved. Its strength is very variable, but is Fig. ■ Golden Wattle (Acacia pycnantha). sometimes over 40 per cent, in tannin. Extracts are now extensively made from it, and are decolorised and sometimes sulphited. They are apt to contain common salt derived from ' the swamps in which the tree grew, and hence unless sparingly used they are liable to produce a rather flat leather. Mimosa bark forms the principal tanning material of Australia and the Cape, and is obtained from various species of the Australian wattles (acacias). The Adelaide Golden Wattle (Acacia pycnantha) is the best and contains up to 50 per cent, tannin ; the Fig. 40.—Green Wattle (Acacia dealbata, Green Wattle ' variety of A. ?nollissima). TtiollissVYYMl) COIlt&ins 86 to138 THE MANUFACTURE OF LEATHER 40 per cent., and the Black Wattle (A. binervata) about 30 per cent, of tannin, but many other species contain considerable quantities. The Green Wattle has been transplanted to Natal, and its bark is now largely exported to England1 *. Though an astringent tannin is yielded by the bark it is not so harsh as quebracho, and does not deposit so many “ reds.” It penetrates fairly quickly, and gives good weights. It yields better colour than quebracho, but this is liable to darken much more rapidly on exposure to light. It is useful in this country in freshening up liquors which are too mellow, and makes a good blend with myrobalans. It also prevents too much “ souring” in summer, but if used too freely in the early liquors it is liable to draw the grain.3 A. catechu, another member of the mimosa family, grows in India, and the solid extract of its wood (cutch) contains about 60 per cent, of tannin, but the material is used chiefly for dyeing. A. arabica, babool, also grows in India, and its bark is one of the chief Indian tanning materials. It contains 12 to 20 per cent, of tannin and much red colouring matter. Its pods, which are used for bating, contain a pyrogallol tannin of which the lime salt is soluble in water. Mallet bark from Eucalyptus occidentalis, a South Australian gum, is one of the newer tanning materials and contains 45 to 55 per cent, of tannin. In some respects it resembles Fio. 41.—Black Wattle (Acacio binervata). 1 Hence the name “ NataliBaTk.” 4 See Bennett, J.S.C.I., 1908, 1193,THE VEGETABLE TANNING MATERIALS 139 mimosa, but is not quite so harsh and gives light-yellow colour instead of red. It yields a fairly firm and plump leather and gives some weight. Birch hark (inner bark of Betula alba, the common birch) contains about 5 per cent, of tannin. It is used with larch bark for tanning Scotch basils, and also in Russia with willow bark (p. 182). The outer bark is the source of birch tar oil, which assists in the production of the characteristic smell of Russia leather. Larch hark, from Pinus larix, contains 9 to 18 per cent, of tannin of mild astringency, and also some sugary matters which assist in plumping. It is used in Scotland for tanning basils. Pine hark, from Pinus abies (Norway spruce) is used extensively in Austria, and also in Russia, Germany and Denmark. It contains 10 to 14 per cent, of tannin, and a considerable proportion of sugars, and hence, in contrast with most other members of this group, will give good results alone. It gives a light-yellow leather with a characteristic pine oil smell. It is the source of the so-called “ larch extract.” Canaigre is the tuberous root of a Central American red dock. The air-dried roots contain 25 to 80 per cent, of a tannin somewhat similar to that of mimosa bark. The plant grows freely in wet sandy soil and is harvested in the second year after planting. The roots are cut into thin slices and either dried at once at a low temperature or immediately converted into extract. A considerable percentage of starch is contained, and hence the extraction must be effected at temperatures between 80° and 50° C. It is used to some extent for the finishing of harness hides and light leathers. Cape sumach consists of the leaves of Colpoon compressum, which contain 28 per cent, of a tannin somewhat resembling that of gambier. It is used as a substitute for Sicilian sumach.CHAPTER XI THE ANALYSIS OP TANNING MATERIALS In probably no branch of analytical chemistry has there been suggested such an enormous variety of processes as in that section of technical analysis which deals with the estimation of tannin in the vegetable tanning materials. It is therefore quite impossible in this book to give any account of them, but it should be pointed out that the matter is one of exceptional difficulty, not only because the tannins are complex organic substances, but because they are also bodies of unknown constitution and molecular weight, and as a rule occur naturally in the form of mixtures one with another and with bodies which bear considerable resemblance to them in their reactions, but which are not tannins. The Lowenthal Method at one time extensively used, is a process in which very dilute infusions of tannin are slowly titrated in the presence of an excess of indigo with a *05 per cent, solution of potassium permanganate. The indigo itself consumes at least three-fifths of the permanganate, and a blank experiment must be made to determine the correction; but it also regulates the extent of the oxidation and acts as an indicator for the attainment of this desired limit. Any variations in the methods of manipulation of different analysts are overcome by standardising the permanganate solution against a per cent, solution of gallic acid, in which precisely the same method is used as with the solution of unknown strength. As the rate of addition and admixture of the permanganate has a very appreciable influence on the result, this point is one of considerable importance. The “unknown” solution is afterwards treated with a gelatin solution in the presence of salt and sulphuric acid, and the separation of the precipitate of “ amorphous leather ” is assisted by the addition of a little kaolin. It is probably better to use hide powder, prepared as describedFig. 42.—Students’ laboratory, Leather Industries Department of the University of Leeds. THE ANALYSIS OF TANNING MATERIALS142 THE MANUFACTURE OF LEATHER below, for detannising. An aliquot part of the filtrate is then titrated as before, and the amount of permanganate consumed by oxidisable non-tanning bodies is thereby determined and correction made accordingly. This method can be made to give fairly concordant results, and where a large proportion of free acids and of gallic acid and other oxidisable non-tanning matters are present, e.g., in tan-yard liquors it will probably yield also the most accurate results.1 The Indirect Gravimetric Method, however, has been for many years the most important method, and has been employed by the members of the International Association of Leather Trades Chemists (I. A. L. T. C.) since its formation in 1897. Its principal operations are those of filtration and detannisation, measured volumes of the unfiltered, filtered, and detannised infusion being evaporated to dryness in tared basins and the residues weighed. The amount of matter removed by filtration and detannisation is thus indirectly weighed, and the proportion of ‘‘ insoluble matter” and “tanning matter” thereby determined, whilst those substances still unremoved from solution are calculated as “ soluble non-tanning matters.” 2 The detannisation, involving the removal of tannin from the solution, is accomplished by means of “hide powder,” which is merely powdered hide substance specially prepared for this process. The method will now be described in greater detail. Sampling the material for analysis is the first important consideration, and must be very carefully done if a correct judgment is to be formed of the material, and if concordance between more than one chemist is desired. In weighing out the sample for analysis equal care should be taken to obtain a representative portion as in sampling from bulk. The directions of the I. A. L. T. C. are as follows :3 Sampling prom Bulk. 1. Liquid extracts.—In drawing samples, at least 5 per cent, of the casks must be taken, the numbers being selected as far apart as possible. 1 See Procter and Hirst, J.S.C.I., 1909, 293 and 294. 2 These determinations are often spoken of as “ insolubles,” “ tannin,” and “non-tannins.” 3 See London Report, pp. 22—29, and 124.THE ANALYSIS 0E TANNING- MATERIALS 143 The heads must be removed, and the contents mixed thoroughly by means of a suitable plunger, care being taken that any sediment adhering to sides or bottom shall be thoroughly stirred. All samples must be drawn in the presence of a responsible person. 2. Gambler and pasty extracts.—Gambier and pasty extracts should be sampled from not less than 5 per cent, of blocks, by a tubular sampling tool, which shall be passed completely through the block in seven places. Solid extracts shall be broken and a sufficient number of portions drawD from the inner and outer parts of the blocks to fairly represent the bulk. 3. General tanning materials.—Yalonia, algarobilla, and all other tanning materials containing dust or fibre, shall be sampled, if possible, by spreading at least o per cent, of the bags in layers one above another on a smooth floor, and taking several samples vertically down to the floor. Where this cannot be done, the samples must be drawn from the centre of a sufficient number of bags. While valonia and most materials may be sent to the chemist ground, it is preferable that divi-divi, algarobilla, and other fibrous material shall be unground. Bark in long rind and other materials in bundles shall be sampled by cutting a small section from the middle of 3 per cent, of the bundles with a saw. 4. Other directions.—All samples shall be rapidly mixed and enclosed at once in an air-tight bottle or box, sealed and labelled. Samples to be submitted to more than one chemist must be drawn as one sample, well mixed, then divided into the requisite number of portions, and at once enclosed in suitable packages. Preparation eor Analysis, 1. Liquid extracts.—Liquid extracts shall be thoroughly stirred and mixed before weighing, which shall be rapidly done to avoid loss of moisture. Thick extracts, which cannot be otherwise mixed, may be heated to 50° 0., then stirred and rapidly cooled before weighing; but the fact that this has been done must be stated in the report. 2. Solid extracts.—Solid extracts shall be coarsely powdered and well mixed. Pasty extracts shall be rapidly mixed in a mortar, and the requisite quantity weighed out with as little exposure as possible to avoid loss of moisture. Where extracts are partly dry and partly pasty, so that neither of these methods is applicable, the entire sample shall be weighed and allowed to dry at the ordinary temperature sufficiently to be pulverised, and shall then be weighed, and the loss in weight taken into calculation as moisture. In such cases as gambier, in which it is not possible to grind or by other mechanical means to thoroughly mix the constituents of the sample, it is permissible to dissolve the whole, or a large portion of the sample, in a small quantity of hot water, and immediately after thorough mixing to weigh out a portion of the strong solution for analysis. 3. General tanning materials.—The whole sample, or not less than 250 gms., shall be ground in a mill until it will pass through a sieve of 5 wires per centimetre. Where materials, such as barks and divi-divi, contain fibrous materials which cannot be ground to powder, the ground sample shall be sieved, and the respective parts which do and do not pass through the sieve shall be weighed separately, and the sample for analysis shall be weighed so as to contain like proportions. Preparation of the infusion.—In the case of solid materials like sumach, valonia, etc., extraction is necessary and is144 THE MANUFACTURE OE LEATHER usually accomplished by means of Procter’s apparatus (Pig. 48). The principle of this is that the requisite quantity of material is digested for some hours in cold distilled water, and subsequently percolated with further quantities of water, the temperature of which is gradually increased to boiling point. The extraction apparatus consists of an ordinary open beaker, which is placed in a water bath. A common thistle-head funnel is bent twice at right angles to form a syphon. The head, covered with silk gauze, rests on the bottom of the beaker, the other end being lengthened by a piece of slender glass tube, connected by indiarubber tube fitted with a screw pinch-cock, the whole being held in position by a clamp. The beaker is filled with sand and tanning material, water is poured in, and the whole raised to the requisite temperature in the water bath, when percolation is started by sucking, and warmed water added as required.1 The percolate is collected in a litre flask. In this way a rapid and complete extraction of the material is obtained with less than one litre of water, and if the greater part of the infusion be obtained below 50° C. (see Regulations, p. 150), the destruction of tanning matter is quite negligible. The precautions to be taken in dissolving extracts, etc., are given in the official regulations (p. 151). In all cases the volume, after cooling, is made up to 1 litre and the infusion thoroughly mixed. As practically all materials contain substances which are partially soluble, and other bodies whose degree of absorption by hide powder is dependent upon the concentration of the solution, the whole method of analysis is quite empirical, and it has been found necessary for concordance to limit the tannin content of the infusion within ‘35 and *45 per cent. To obtain such 1 L.I.L.B., 2nd ed., p. 185.THE ANALYSIS OF TANNINO MATERIALS 145 infusions the strength of the material to be analysed must be roughly known in order to determine the quantity which should be weighed out, for if the strength of solution should be afterwards found to be outside the specific limits the whole analysis must be repeated. The approximate amounts of different materials to be taken for 1 litre of infusion are as follows :— Unextracted materials. Extracts. Sumach. . .15 gins. Pistacia . . 20—30 Yalonia. . . 14—15 Yalonia beard . 10—11 Myrobalans . 15 gms Divi-divi . . 9 gms. Algarobilla. . 8—9 gi Oak bark . . 30—36 Oak wood . * 50—10( Hemlock bark . 32—36 Mimosa bark . 11 gms, Quebracho wood. 20-Mangrove bark . Pine bark . Willow bark Chestnut wood . 45 Canaigre . .15 Spent tans . . 50 Oakwood . .16 gms. Ohestnut (liquid) . 14 gms. ,, (solid) . 7 gms. Quebracho (liquid). 9—13 gms. ,, (solid) . 6 gms. Mimosa D. . . 10—12 gms. Myrobalans (liquid) 16 gms. Gambier (cube) . 7—9 gms. ,, (block) . 10—12 gms. Mangrove (liquid) . 9 gms. „ (solid) . 7 gms. Cutch . . .7 gms. Pine bark . .16 gms. Hemlock . . 10—14 gms. Filtration.—This is sometimes a matter of considerable difficulty, for many infusions of tanning materials (e.g., that of quebracho wood or extract) contain very finely-divided particles of the phlobaphenes, etc., and for such solutions all ordinary filter papers are quite inefficient. Schleicher and SchulFs “No. 590” paper will be found to answer for a great many tanning materials, and S. & S. “ No. 605 extra hart” will work even with the most obstinate cases. The filtration is often assisted to a considerable degree by the addition of 1 or 2 gms. of kaolin to the infusion, but both kaolin and filter papers absorb very appreciable amounts of tannin and other colloidal matters from the solution ; so that whatever paper is employed, and whatever amount of kaolin is used along with it, a correction must be always determined M.L. L146 THE MANUEACTTJKE OE LEATHEB Eig. 44. — Eiltration apparatus with Berkefeld candle (upward filtration). for that particular process and applied to every analysis in which that process is used. A further disadvantage of all filter-paper methods is that they are always exceedingly slow, and a very useful alternative method is that introduced by Parker and Payne in 1904,1 involving the use of the Berkefeld “ filter candle,” a cylinder of kieselgahr about 7 cms. long. This is open at one end, having an external diameter of 3 cms. and an internal diameter of 1 cm. Into the open end is fitted a rubber bung through which passes a glass tube connected with an ordinary vacuum flask. The air is exhausted from this by a water pump or syringe and the liquor sucked through the thick layer of siliceous earth, which thereby acts as the filtering medium (see Pigs. 44 and 45). If a quantity of the filtrate which is first obtained be rejected no correction is necessary and a comparatively rapid filtration of the liquor is obtained. With some materials, however, the filtrates are turbid and need to be repeatedly re-filtered in order to obtain them “ optically clear both by reflected and transmitted light’’(see Regulations, p. 150). Detanmsation. The manner m tug with Berkefeld ^ndle which hide powder has been (downward filtration). 1 Collegium, 1904, 249,THE ANALYSIS OF TANNINO MATERIALS 147 used for removing the tannin from solution varies very considerably, but from 1897 to 1907, the official method of the I. A. L. T. C. was that of Procter, in which a “filter bell” (Fig. 46) was carefully filled with the hide powder and the liquor allowed to soak up into this by capillary attraction, and afterwards slowly syphoned over into a receiving vessel, the first 30—35 cc. being rejected and 50 cc. pipetted off from the next 60 cc. for evaporation to dryness. This method was a great improvement on earlier maceration processes in rapidity, accuracy and concordance, and for nearly twenty years was quite generally used throughout Europe. Later experiment, however, showed that it was open to various sources of error, among which were the imperfect removal of soluble matter in the powder by the mere rejection of the first 30 cc. of percolate, the successive change in the composition of the filtrate according to the amount which had previously come over, the difficulties experienced in the satisfactory “ packing ” of the filter bell with the powder, which operation required considerable care and some skill, and the impossibility of carrying out the process when any quantity of free acid was present in the infusion to be detannised. One of the greatest objections to the filter bell was that the solution is almost completely detannised by the first portions of the hide powder it meets, and hence that this detannised solution has to flow through a certain amount of fresh and very absorptive hide powder which readily takes out of solution many soluble non-tanning matters and so causes them to be reckoned as tannins. Some of these sources of error were partially overcome by the use of chromed powder, but still more so by the method adopted by the leather chemists of America in which the l 2 Fig. 46.—Procter’s filter-bell apparatus.148 THE MANUFACTURE OP LEATHER powder is first lightly chromed with chrome alum, afterwards washed free from soluble matter, and then immediately introduced into the tannin infusion and vigorously shaken for ten minutes. All these methods were very carefully compared on the same solutions in an extensive investigation by Dr. Parker and the Author,1 in which artificial mixtures of known composition were used, as well as commercial samples, so that the extent of the error with each method could be accurately determined. The results showed that although the concordance obtained by the use of lightly-chromed powder in the Fig. 47.—Shake machine for the Procter-Bennett method of tannin analysis. filter bell was on the whole better than with unchromed powder, the extent to which soluble non-tanning matters were absorbed was distinctly greater, whereas with the “ shake method” the concordance was decidedly better and the absorption of non-tannins less. An investigation of the “shake method ” as used in America was made by Prof. Procter and the Author,2 its defects being pointed out and suggestions made for a modified and improved process. These suggestions were afterwards crystallised into definite proposals3 in 1 Parker and Bennett, J.S.C.I., 1906, 1193. 1 Procter and Bennett, J.S.C.I., 1906, 1203, * Procter and Bennett, J.S.O.I., 1907, 79.THE ANALYSIS OE TANNING MATEElALS 149 the “ basic chloride method,” which was soon adopted by the I. A. L. T. C. (see Regulations, below), as their official method of analysis. The “ shake machine ” used by Procter and Bennett is shown in Fig. 47. In this method the powder is chromed rather more heavily than in [the American method and in a very much shorter time, and the process of washing the powder is also subjected to a definite chemical control. The correction necessary for the dilution of the infusion by the moisture in the prepared powder is also made easy to apply and is the same for every analysis. The amount of hide powder for each analysis is also made definite and the conditions of experiment so arranged that the chances of concordance are at a maximum. In a later paper1 the Author pointed out the importance of the texture and acidity of the original unchromed hide powder and gave results showing the influence of these factors. He therefore suggested that liide powder should be “ standardised ” like any other chemical reagent, and specified the limits within which this might be done. These proposals were also embodied in the official method of the I. A. L. T. C. Evaporation and Drying.—This is best accomplished by pipetting 50 cc. of the unfiltered, and filtered infusions and 60 cc. of the detannised solution into tared porcelain basins and evaporating to dryness on a steam bath. The basins with their residues are then dried to constant weight in the vacuum oven. Where no vacuum oven is available, a steam or water oven is quite efficient, though somewhat slower. An air oven at 105° C. was often used, but is very liable to cause oxidation of the residues, and is now not used in official analyses. Where materials are being analysed in which extraction has been necessary, there is nothing gained by evaporating any of the unfiltered percolate to dryness, for most of the insoluble matter still remains in the extractor with the sand. In this case, therefore, it is necessary to take a separate portion of the original sample and determine the total amount of dry solid matter (and, of course, water also) in it by drying in the oven until constant. As the amount of “ soluble solids ” is determined 1 Bennett, J.S.C.I., 1901, 455.150 THE MANUFACTURE OE LEATHER from the filtered solution, the amount of “ insolubles” is then obtained by difference. With extracts the amount of water is determined indirectly by the weight of the residue from the unfiltered infusion (“total solids”), and hence in every case the analytical result is stated example:— as in the following Oakwood Extract. Tannin . 25-4 Soluble Non-Tannins 18*1 Insoluble in water at 17*5° G. . o-o Water 56-5 100*0 per cent. Spent tanning materials are usually stated in terms of the dry material, but may be calculated in terms of the unleached material by means of the “ insolubles,” which are assumed to be unaffected by the leaching. The actual regulations of the I. A. L. T. C. for their official method of analysis are as follows:— General Regulations. The Executive Committee have decided that any method which conforms to the conditions of sections 1 to 4 of the following statement may he regarded as conforming to the recommendations of the International Commission on tannin analysis, hut, that members of the International Association must work according to the detailed directions contained in sections 5 to 8. 1. The solution fon' analysis must contain between 3*5 and 4*5 gms. of tanning matter per litre, and solid materials must be extracted so that the greater part of the tannin is removed at a temperature not exceeding 50° C.,but if the Teas Extractor be used, the first portion of the extract shall be removed from the influence of heat as soon as possible. 2. “ Total solubles” must be determined by the evaporation of a measured quantity of the solution previously filtered till optically clear both by reflected and transmitted light; that is, a bright object such as an electric light filament must be distinctly visible through at least 5 cms. thickness, and a layer of 1 cm. deep in a beaker placed in a good light on black glass or black glazed paper must appear dark and free from opalescence when viewed from above. Any necessary mode of filtration may be employed, but if such filtration causes any appreciable loss when applied to a clear solution, a correction must be determined and applied as described in section 6. Filtration must take place at a temperature between 15° 0. and 20° 0. Evaporation to dryness shall take place between 98*5° C. and 100° 0. in shallow flat-bottomed basins which shall afterwards be dried until constant at the same temperature, and cooledTHE ANALYSIS OE TANNING- MATERIALS 151 before weighing for not less than twenty minutes in air-tight dessicators over dry calcium chloride. 3. “ Total solids ” must be determined by drying a weighed portion of the material, or a measured portion of its uniform turbid solution at a temperature between 98*5° 0. and 100° 0. in shallow flat-bottomed basins which shall afterwards be dried until constant at the same temperature and cooled before weighing for not less than twenty minutes in air-tight dessicators over dry calcium chloride. “Moisture” is the difference between 100 and the percentage of total solids, and “insoluble” the difference between “ total solids” and “ total solubles/’ 4. “Non-tannins—The solution must be detannised by shaking with chromed hide powder till no turbidity or opalescence can be produced in the clear solution by salted gelatine. The chromed powder must be added in one quantity equal to 6*0 to 6'6 gms. of dry hide per 100 cc. of the tanning solution, and must contain not less than 0*2 and not more than 1 per cent, of chromium reckoned on the dry weight, and must be so washed that in a blank experiment with distilled water, not more than 5 mgr. of solid residue shall be left on evaporation of 100 cc. All water contained in the powder should be determined and allowed for as water of dilution. I. A. L. T. O. Ofeicial Method. The following sections give the detailed method of carrying out the analysis adopted by the I. A. L. T. G. for the use of its own members. 5. Preparation of infusion.—Such a quantity of material shall be employed as will give a solution containing as nearly as possible 4 gms. of tanning matter per litre, and not less than 3'5 or more than 4*5 gms. Liquid extracts shall be weighed in a basin or beaker and washed with boiling distilled water into a litre flask, filled up to the mark with boiling water, and well mixed, and rapidly cooled to a temperature 17*5° 0., after which it shall be accurately made up to the mark, again well mixed, and filtration at once proceeded with. Sumach and myrobalans extracts should be dissolved at a lower temperature. Solid extracts shall be dissolved by stirring in a beaker with successive quantities of boiling water, the dissolved portions being poured into a litre flask, and the undissolved being allowed to settle and treated with further portions of boiling water. After the whole of the soluble matter is dissolved the solution is treated similarly to that of a liquid extract. Solid tanning materials, previously ground till they will pass through a sieve of 5 wires per centimetre, are extracted in Koch’s or Procter’s extractor with 500 cc. of water at a temperature not exceeding 50° 0. and the extraction continued with boiling water till the filtrate amounts to 1 litre. It is desirable to allow the material to soak for some hours before commencing the percolation, which should occupy not less than three hours, so as to extract the maximum of tannin. Any remaining solubles in the material must be neglected, or reported separately as “difficultly soluble” substances. The volume of liquid in the flask must after cooling be accurately made up to 1 litre. 6. Filtration.—The infusion shall be filtered till optically clear (see section 2). No correction for absorption is needed for the Berkefeld candle, or for S. & S. 590 paper if a sufficient quantity (250—300 cc.) is rejected before measuring the quantity for evaporation ; and the solution may be passed through repeatedly to obtain a clear filtrate. If other methods of152 THE MANUFACTURE OF LEATHER filtration are employed the average correction necessary must be determined in tbe following manner. About 500 cc. of the same or a similar tanning solution is filtered perfectly clear, and after thorough, mixing 50 cc. is evaporated to determine “ total soluble No. 1.” A further portion is now filtered in the exact method for which the correction is required (time of contact and volume rejected being kept as constant as possible) and 50 cc. is evaporated to determine “total soluble No. 2.” The difference between No. 1 and No. 2 is the correction sought, which must be added to the weight of the total solubles found in analysis. An alternative method of determining correction, which is equally accurate and often more convenient, is to filter a portion of the tanning solution through the Berkefeld candle till optically clear, which can generally be accomplished by rejecting 300 or 400 cc. and returning the remaining filtrate repeatedly; and at the same time to evaporate 50 cc. of clear filtrate obtained by the method for which correction is required, when the difference between the residues will be the correction sought. (Note. It is obvious that an average correction must be obtained from at least 5 determinations. It will be found that this is appromixately constant for all materials, and amounts in the case of S. & S. 605, 150 cc. being rejected, to about 5 mgr. per 50 cc. and where 2 gms. of kaolin are employed in addition, to 7J mgr. The kaolin must be previously washed with 75 cc. of the same liquor, which is allowed to stand 15 minutes and then poured off. Paper 605 has a special absorption for a yellow colouring matter often contained in sulphifced extracts.) 7. Detannisation.—Hide powder shall be of woolly texture, thoroughly delimed, preferably with hydrochloric acid, shall not require more than 5 cc. or less than 2*5 cc. of N/10 NaOH or KOH to produce a permanent pink with phenol phthalein on 6J- gms. of the dry powder suspended in water. If the acidity does not fall within these limits, it must be corrected by soaking the powder before chroming for twenty minutes in 10—12 times its weight of water to which the requisite calculated quantity of standard alkali or acid has been added. The hide powder must not swell in chroming to such an extent as to render difficult the necessary squeezing to 70—75 per cent, of water, and must be sufficiently free from soluble organic matter to render it possible in the ordinary washing to reduce the total solubles in a blank experiment with distilled water below 5 mgms. per 100 cc. The powder when sent out from the makers shall not contain more than 12 per cent, of moisture, and shall be sent out in air-tight tins. The moisture in the air-dried powder is determined and the quantity equal to 6*5 gms. actual dry hide powder is calculated, which will be practically constant if the powder be kept in an air-tight vessel. Any multiple of this quantity is taken according to the number of analyses to be made, and wet back with approximately ten times its weight of distilled water1 (2 gms. per hundred of dry powder of crystallised chromic chloride (Cr201612H20)2) is now dissolved in water and made basic with 0*6 gms. Na2C03 by the gradual addition of 11*25 cc. of N/l solution, thus making the salt correspond to the formula Cr2013 (OH)3. This solution is added to the powder and the whole churned slowly for one hour. In laboratories when analyses are continually being made it is more convenient to use a 10 per cent, stock solution, made by dissolving 100 gms. of 0r2Cl612H2O 1 Very woolly powders require slightly more than 10 times their weight of water. 2 Kahlbaum.THE ANALYSIS OF TANNINO MATERIALS 153 in a little distilled water in a litre flask, and very slowly adding a solution containing 30 gins, of anhydrous sodium carbonate, with constant stirring, Anally making up to mark with distilled water and well mixing. Of this solution 20 cc. per 100 gms. or 1*3 cc. per 6*5 gms. of dry powder should he used. At the end of one hour the powder is squeezed in linen to free it as far as possible from the residual liquor, and washed and squeezed repeatedly with distilled water, until on adding to 50 cc. of the filtrate, 1 drop of 10 per cent. B^CrCb and four drops N/10 AgN03, a brick-red colour appears. Four or five squeezings are usually sufficient. Such a filtrate cannot contain more than 0*001 gm. of NaOl in 50 cc. The powder is then squeezed to contain 70—75 per cent, water, and the whole weighed. The quantity Q, containing 6*5 gms. dry hide is thus found, weighed out, and added immediately to 100 cc. of the unfiltered tannin infusion along with (26*5—Q) of distilled water. The whole is corked up and agitated for 15 minutes in a rotating bottle at not less than 60 revolutions per minute. It is then squeezed immediately through linen, 1 gm. of kaolin added to the filtrate, stirred and filtered through a folded filter of sufficient size to hold the entire filtrate, returning till clear, and 60 cc. of the filtrate is evaporated and reckoned as 50 cc., or the residue of 50 cc. is multiplied by f. The non-tannin filtrate must give no turbidity with a drop of a 1 per cent, gelatine 10 per cent, salt solution. The kaolin may be used by mixing it with the hide powder in the shaking bottle. 8. The analysis of used liquors and spent tans shall be made by the same methods as are employed for fresh tanning materials, the liquors or infusions being diluted, or concentrated by boiling in vacuo or in a vessel so closed as to restrict access of air, until the tanning matter is if possible between 3*5 and 4*5 gms. per litre, but in no case beyond a concentration of 10 gms. per litre of total solids, and the weight of hide powder used shall not be varied from 6J gms. The results shall be reported as shown by the direct estimation, but it is desirable that in addition efforts shall be made, by determination of acids in the original solution and in the non-tannin residues, to ascertain the amount of lactic and other non-volatile acids absorbed by the hide powder, and hence returned as “ tanning matters.” In the case of tans it must be clearly stated in the report whether the calculation is on the sample with moisture as received, or upon some arbitrarily assumed percentage of water; and in that of liquors whether the percentage given refers to weight or to grams per 100 cc. ; and in both cases the specific gravity shall be reported. Reports of analyses shall only be sent out on a mean of at least two separate analyses which closely correspond. 9. All evaporation shall be rapidly conducted at steam temperature in shallow flat-bottomed basins of not less than 6*5 cms. diameter to apparent dryness ; and shall be subsequently dried between 98° and 100° C. in a water or steam oven until of constant weight, and shall be afterwards cooled in small air-tight dessicators over dry calcium chloride for at least twenty minutes, and then weighed rapidly. Not more than two basins shall be placed in one dessicator, and the basins must not be wiped after removal from the dessicator. The measurement of colour in tanning materials is also a matter of considerable importance, and is usually accomplished154 THE MANUFACTURE OF LEATHER by means of “ Lovibond’s tintometer.” In this instrument a layer of the filtered solution (as used in analysis) is matched in colour by slips of coloured glasses (red, yellow and blue), which are carefully graduated in units and decimals of colour and arranged so that equal numerical quantities in these colours will always produce a neutral tint. The liquor is contained in a cell 1 cm. thick, and the results are calculated to J per cent, solution of tanning matter. Another method is actually to tan a piece of calf or sheep grain carefully delimed with 2 per cent, boric acid, 1 per cent, phenol solution. Such a process will give comparable results if its manipulative details are kept always the same. In the analysis of tan-yard liquors an important determination is that of the amount of free acid present. This, however, is exceedingly difficult to bring about, although many methods have been suggested. The author has tested the most important of these1 and found that the most generally useful is that due to Procter, in which 10 cc. of filtered tan liquor is measured into a beaker and titrated with a saturated solution of lime water until a trace of permanent precipitate is obtained. The free acids are then neutralised and the calcium tannates begin to be precipitated. For all ordinary yard control the number of cc. saturated lime water so consumed may be taken as the result, but the lime water can be accurately standardised by means of N/10 hydrochloric acid and phenol phthalein, and the result calculated in terms of any particular acid. An exceedingly useful instrument in the tan yard for determining roughly the strength of the liquors is the “ barkometer,” Fig. 48.—Tintometer. 1 Bennett and Wilkinson, J.S.C.I., 1907, 1186.THE ANALYSIS OP TANNINO MATERIALS 155 which is a glass or brass hydrometer whose readings may be converted into specific gravity by dividing by 1,000 and adding 1, and therefore indicate the second and third decimal places of ordinary specific gravities, i.e.9 a liquor of barkometer strength 48° has a specific gravity of 1*048. The barkometer strength (or S.G-.) of a tan-yard liquor is, however, no guide at all to the actual tanning strength of the liquor, as this is also dependent upon the amount of non-tannins in the liquors, and these vary very considerably with different tanning156 ÌHE MANtfEACÌURE ÓE LEATHEft materials. A quebracho liquor 20° Bkr. and an oakwood liquor 40° Bkr. might have the same tannin strength. The barko-meter is nevertheless widely used in tanneries, for although its readings bear no direct relation to the tannin content, it will at any rate give in the tannage of any particular class of goods a fairly accurate indication of the loss in tannin strength of the liquors as the goods are passed through them, and it is for this purpose that it is so extensively used. The reading should be taken where the curved surface of the water touches the barkometer stem. Extracts are often sold as having a certain strength in Beaume degrees. This is quite an arbitrary scale of specific gravity, but its readings may be roughly judged from the following table of the 4 4 Rational Scale.” Beatjmé Hydrometer Scale. Beaumd Degrees. Specific Gravity. JBeaumé Degrees. Specific Gravity. 0 1-0000 40 1 3834 5 1-0358 45 14531 10 1-0745 50 1-5301 15 1-1160 55 1-6158 20 1-1608 60 1-7116 25 1*2095 65 1-8195 30 1-2624 70 1-9090 35 1-3202 75 1-9600 Degrees Twaddell are hydrometer readings in which each degree corresponds to 5° Bkr.CHAPTEE XII THE PREPARATION OF THE TANNING LIQUORS Grinding.—Nearly all solid materials, whether they are intended for leaching or for dusting material, require reducing to a finer state of division in order to allow a more complete and easy extraction of the tannin. In doing this a certain amount of crushing as well as cutting is desirable to destroy the cellular structure in which the tannin is contained. With oak bark, some preliminary preparation for true grinding is often desirable, and the material is “hatched” into Eia. 50.—Bark breaker. pieces a few inches long by chopping by hand, or by cutting with a machine having a fly wheel with radial knives, somewhat like a chaff cutter. In place of hatching “ bark breakers ” (Fig. 50) are often used, in which two toothed rollers, made to fit each other, crush and break the material. The bark is then158 THE MANUFACTURE OF LEATHER ground in some variety of that large class of mills which consist essentially of two toothed cones (see Fig. 51), one of which is usually fixed whilst the other rotates about its axis. One of these cones is within the other, but the axes may be vertical or horizontal, according to the other arrangements of the machine. The bark breaker may be attached to the mill and arranged to feed it. Yalonia for dusting purposes should be ground in a similar mill, but for leaching the result is too fine a dust to allow a thorough percolation of the leach liquor, and may also cause a caking of the material, in which case the internal part is scarcely leached at all. Hence a better way is to crush the cups by passing through two fluted rollers, the cellular structure being thereby sufficiently destroyed to allow a thorough extraction, and the product not too finely divided to permit a free permeation of the liquor in the leaches. Myrobalans for leaching should also be put through this machine (often called a myrobalans crusher), but when required for dusting purposes should not be ground, for although it is wanted in a fine state of division, the stones of the nuts are very hard and may injure the mill’s teeth, and the softer part of the nut is very apt to cause clogging in the ordinary mill, so that the usual plan is to use some form of “ disintegrator.” The principle of these machines is that very rapidly revolving beaters, either radial or concentric steel barTHE PREPARATION OF THE TANNING LIQUORS 159 cylinders, dash the material against a serrated surface and knock it to a powder. Into the serrated case is fixed a grating which allows the exit of the dust. Hard and tough barks, which are not brittle (e.g., mimosa bark), can also be satisfactorily pulverised in these machines. Their disadvantage is the great proportion of exceedingly fine dust which they make, necessitating its collection in silk bags, and their liability to cause fire. Fig. 52.—Myrobalans husking machine. Hard and brittle materials can be reduced by the use of toothed saws or rasps, and many American mills have been made on this principle, and have proved particularly suitable for hemlock bark. With such materials, also, “shaving” or “shredding” mills are quite efficient. They consist essentially of knived cones, wheels, or cylinders which are rotated at a high speed and have the material (quebracho wood, hemlock bark, oakwood, etc.), fed against them by toothed rollers. Divi - divi and algarobilla do not need either grinding or Fig. 53.—Disintegrator.160 THE MANUFACTURE OF LEATHER extracting, as their 'tannin is very loosely held and readily fermentable. Leaching was first used to complete the extraction of solid materials used in the later stages of tanning, and afterwards to make strong liquors for these stages, but the introduction of extracts has now made the preparation of strong liquors a matter of comparative ease, whilst at the same time the greater variety of tanning materials now employed and the better knowledge of the chemistry of some of the more unstable tannins, has assisted largely in the return to the old system of leaching only the partially spent material used in the “ layers ” (p. 177), technically termed “ fishings.” Yalonia, for example, whic 1 is extensively used for making up sole leather layer liquors, should never be leached for this purpose, because as mentioned earlier (p. 126), its bloom deposits rapidly and would merely form an insoluble precipitate in the leaches, and be lost to the tanner. What is desired is that the ellagitannic acid should be extracted, go into the hide and there deposit its bloom, giving the desired results of firmness, water-proofness, and weight. For some dressing leather tannages, however, the more astringent ellagitannic acid is not wanted, and the valonia should then be leached to allow its decomposition before the liquors are made up. These considerations have also to be borne in mind in deciding whether it is desirable to pump used liquors back again over the leaches, or to work them down the yard. In an ordinary mixed tannage of sole and heavy leathers the latter course is clearly the best, in order to use the fermentable tannin whilst it is in solution, but where only stable tannins are employed, such as in a pure oak bark yard, the old and weakened liquors might then be strengthened by passing them again through the leaches. Similar considerations have to be taken into account in the case of those materials which rapidly deposit reds. Leaching is in this country usually done in square pits sunk in the ground and built of brick and cement, or Yorkshire flagstone; though also sometimes of wood. A series of such pits is employed and worked on some systematic process of a continuous extraction of the materials with water, in which it is arranged that the fresh material is acted upon by theTHE PREPARATION OF THE TANNING LIQUORS 161 strongest liquor of the series, and the nearly “spent” material with fresh water or the weakest liquor. In this way, the liquor is gradually strengthened until it becomes the strongest liquor of the series, and is used as the source of the strong tanning liquors which are then worked down the yard to form the “ sour ” and “ mellow ” liquors (pp. 169—171), used in the early stages of tanning. Each leach pit is provided with a Fig. 54.—Battery of press leiches. false bottom, and with a wooden shaft or “ eye,” which passes down the side of the pit in one corner and communicates with the space below the false bottom, and also with a plug which allows the leach liquors to be run at any time into a deep well. In one method of leaching, the strong liquor is run off and the next strongest liquor pumped over the material left behind, the third strongest pumped over the next pit, and so on, water being pumped on the last and nearly spent pit. This system M.L. M162 THE MANUFACTURE OE LEATHER requires a good deal of pumping, and when the liquors are run off, the materials settle, and often will not rise again satisfactorily. These difficulties are only imperfectly overcome by forcing the liquor in at the bottom. Hence a deservedly popular method of leaching is now the press leach system, in which the liquor is pressed round the “ battery ” of pits by virtue of its own weight and constantly increasing specific gravity, a certain amount of fall being allowed. The liquor passes downward through the leach and up a vertical spout connected with the next stronger leach by a short trough or pipe, which must be of ample size to allow easy percolation, and which can be closed when necessary. When the battery is in actual work, the strong liquor of the “ head leach ” is run off to make up the tanning liquors, and water is pumped on to the “ tail leach’’ containing the nearly spent material, which causes the liquor to press round into the next pits until the head leach is again full. The material of the tail leach is now exhausted, so that the liquor is run to the well, and the spent tan cast, pressed, and burnt as fuel. Into the now empty leach pit is placed the fresh batch of material for extraction, and this pit is now made into the head leach by pumping the weak liquor in the well on to the next pit, which contains the weakest material, and causing the liquor to press round into the newly made up leach, which then becomes the source of the tanning liquors. The arrangement of the pits is illustrated in Fig. 54, and it will be readily understood that a circular arrangement is most convenient. , If more than eight pits are employed in the same series, it is necessary to assist the pressing round by a certain amount of pumping, either at some definite intermediate stage or by a series of pumps on the Holbrook system in which each vertical spout is provided by a simple pump driven by power. Another arrangement sometimes convenient is to have two distinct press leach batteries, the strongest liquor of one being the source of the weak liquor for the other, but this system involves both more pumping and the removal of the material being extracted from one battery to the other. Circular vat leaches are sometimes used instead of pits, especially in the United States, but are not so durable.THE PREPARATION OF THE TANNINO LIQUORS 163 The efficiency of the leaching arrangements determines the amount of tannin going to waste in the so-called spent tan, and hence the matter is one of considerable economical importance. With care the tannin content can be reduced below 1 per cent., and with some materials {e.g., oak bark) it is desirable for economy to do this, but where the materials are mixed, and many contain unstable tannins, it becomes a question whether it may not be better to have a smaller system of leaches, which gets the liquors quickly forward and into the yard, thus saying a considerable amount of decomposable tannin and allowing the loss of a less amount of difficultly extractable tannin, by letting the spent tan go with 2 or 3 per cent, more tannin than is usually considered desirable. The extent to which this is done depends almost entirely upon the arrangements and facilities of each particular yard and the class of leather being made. The same policy may be adopted in' those cases where a material improvement in colour may be gained by not pushing the extraction too far, and in the leaching of fairly strong materials. The question of heating the leach liquors is also important. Just as in analytical work, where cold water was used to extract the first part of the tannin, and hot water to remove the last traces, so also is it desirable in practice, for heated infusions are apt to give dark colour, yield insoluble matter, and decrease in tannin strength. When only used in the extraction of the last portions, however, this effect of heat is of such small proportion that it is usually quite negligible. The heating may be done by means of steam-heated copper coils below the false bottoms—an expensive way—or by the direct but careful leading in of steam by means of special jets, but for all ordinary purposes hot water is quite sufficient. It is not essential to have the highest possible temperature, and indeed it has been shown1 that many materials (e.g., valonia, mimosa bark, sumach, and canaigre) are mostly easily extracted with only warm water (50° to 60° C.), whilst, on the other hand, it is pretty general that the colouring matters are more largely extracted at higher temperatures. Hence this 1 Procter and Parker, J.S.O.I., 1895, 635. M 2164 THE MANUFACTURE OF LEATHER matter is determined by the nature of the materials being infused. Oak bark, gambier, myrobalans, quebracho, and mangrove will do better with higher temperatures (80° to 90° C.), but with the last two the question of colour should be considered. In Germany extraction under pressure is often made in large copper pans and at a raised temperature. After being in contact with the material for the desired length of time the liquor is pumped into the next pan, and in this way it is possible to obtain by only three waters liquors up to 90° Bkr., and a spent tan with only 1 per cent, tannin. The plant is very expensive, however, and only pays for large factories. Sprinkler leaches, once used to some extent in the United States, are so arranged that the material for extraction is placed in a vat and sprayed on the top at the same rate as the liquor is removed from the vat, but the complete exhaustion of the material in this way is exceedingly difficult, and the loss in tannin due to oxidation is very considerable. Extract Manufacture.—In this industry the extraction is carried distinctly further than in ordinary leaching, especially when the material being extracted contains only a comparatively small amount of tannin. Hence it is often necessary to decolorise afterwards in order to remove the colouring matters which have been formed and extracted by the higher temperatures and the more thorough infusion employed. Where wood is the source of the tannin, the larger branches are stripped of bark and stored for a few months until the resinous matters become insoluble, and, after cutting into small logs, are shredded in a mill. The extraction is made in large deep circular vats, which have a copper coil below the false bottom running round three times and passing several inches up the centre of the vat, where it is covered by a copper funnel. The vats have thick oak removable lids, protected against spurting by a copper bell. After two hours’ boiling on the first lot of wood, the liquor is pumped on to a frerii lot in another vat, and so on. When the liquor comes out of the last vat it is generally about 8° Be, and very dark, because of the metallic “tannates” it contains. Sulphuric acid isTHE PREPARATION OF THE TANNING LIQUORS 165 sometimes used in the extraction to obviate this, but excess is carefully avoided. Fresh waters come over the partially spent wood, and after one or two changes it is quite spent. After extraction, the liquor is allowed to settle in tanks, and the clear liquor run off from the reds, fibre, resinous matters, etc., Fio. 55.—Yarvan evaporator. into a brewer’s cooler, which brings the temperature down to about 55° C. The liquor is now run into the decolorising plant, which consists of a deep vat with a false bottom, a coil of pipes beneath this to raise the temperature, and a mechanical stirrer, attached to power. Many substances, such as lead acetate, alum, casein, potassium ferrocyanide, oxalic acid, etc., have been suggested as decolorising agents, but166 THE MANUFACTURE OF LEATHER none are so good as the originally used material—blood albumin. The bullock’s blood, or “ blood crystals,” dissolved in water, is run into the vat and well mixed. The vat is now heated up to 70° C.; the albumin coagulates and carries down with it much of the colouring matter and some little tannin. After standing for an hour or two, the clear liquor is then gently run off by means of taps fixed at different heights in the sides of the vats, and passed into the evaporating plant. The sediment is run off separately and passed through filter presses which yield more liquor for evaporation, and solid cakes of albuminous matter which can be sold for manure. The evaporation is carried out in steam-heated vacuum pans, in order to keep the temperature low and to prevent oxidation. The Yaryan apparatus (Fig. 55) is very suitable for this purpose, the evaporation being quickened by conversion of the liquor into spray by admission into steam-heated copper tubes, these passing into a separating chamber at the other end, kept at a low pressure by an air pump. The steam formed from the evaporation of the first liquor, after straining from any spray, is often used to heat up a second arrangement of pipes maintained at a lower pressure, and so double, triple, and multiple effects may be obtained. It is not possible to concentrate much beyond 25° Be with this apparatus, so that it is necessary to finish off in ordinary vacuum pans, and where solid extracts are being manufactured these last pans should be broad and shallow and have a wide exit, as the evaporation is continued until the extract will only just run out into the cooling boxes, and may be accidentally taken too far. Sulphited extracts are made, chiefly from quebracho and hemlock (see p. 186), by heating with sodium bisulphite in closed vessels. Sulphurous acid is evolved, and the base combines with the reds, which are thereby made soluble and available for tanning. The use of extracts in the tannery has very largely increased in recent years in spite of much early prejudice. They save much cost and trouble in preparing the liquors, and strong liquors can be made from them without difficulty, and of definite strength. Better colour is also obtained from many extracts than by the ordinary leaching of the materialTHE PBEPAEATION OE THE TANNING- LIQUOKS 167 from which they are made, and many new materials, such as oakwood and chestnut wood extracts, are now on the market which the tanner would never have thought of preparing himself, on account of their poor strength in tannin. Extracts should always be bought on analysis by the standard method, and are best dissolved in warm water (40° to 60° C.) when making up the liquorsCHAPTEB XIII THE PRINCIPLES OF VEGETABLE TANNING The principles involved in the vegetable tanning processes require very careful consideration, for they largely determine both the nature of the resulting product and the economical manufacture of that product. All vegetable tannages consist in immersing the goods in aqueous infusions of the vegetable tanning materials, and in allowing the penetration of the tannins into the hides, on the corium fibres of which they act so as to give an insoluble and imputrescible product. The penetration involves probably both chemical and physical actions, and has often associated with it also the mere mechanical deposition of other insoluble matters, e.g., bloom, reds, etc., which are in no way chemically fixed in the hide. The completeness of this penetration is a matter in which there is considerable variation; in some cases the hide fibres are merely coated with the tannin, whilst in others the penetration is thorough and even. In all tannages the most important maxim is to start the tanning in weak infusions and afterwards to move the goods through other liquors in which the tannin strength is gradually increased. With hides, the penetration is practically only due to the diffusion of the tanning liquor, and as the exterior is first tanned the interior of the hide is somewhat protected, hence the necessity arises of increasing the concentration of the diffusing liquor so that the penetration may be maintained at an appreciable rate. The rate of this increase in tannin strength is an excessively important factor in determining the nature of the leather which will be obtained. If the strength of the liquors be increased rapidly the penetration through the hide will be considerably quickened, but the penetration of the tannin through the hide fibres is very liable to be incomplete, and the grain tends to be rough and harsh, and in extreme casesTHE PBINCIELES OE VEGETABLE TANNING 169 will be “ drawn ” and wrinkled. When the difference in the strength of the liquors is very great the exterior may be tanned so hard that it is quite impossible, even with longer time, to penetrate into the untanned portion. All these points are well illustrated in many drum tannages. If, on the other hand, the rate of increase in tannin strength is very small, a longer time is needed for the tannin to “ strike through ” the hide, and indeed for the whole tannage, but the fibres are more completely tanned and a smooth grain can be obtained with very much less difficulty. This is well illustrated in a long oak bark tannage in weak liquors. If goods are placed in liquors of less strength than those from which they have just been taken, the gradually decreasing concentration of the liquor in the hide is considerably disturbed and with difficulty re-established. The nature of the tannins being used has also a great influence. The astringent tannins (e.g., quebracho) are very liable to draw the grain in the early stages if used in too great a concentration, but if used in weak solution will strike quickly through the hide, but not so thoroughly through the fibre. The less astringent tannins (e.g., myrobalans) can be used in stronger solutions and the rate of increase may be greater; their penetration through the hide is much more even, but usually slower. From these considerations it will be clear that to obtain a thorough and quick tannage the best way is to use the less astringent materials in the earlier stages and to assist their penetration by a certain amount of motion, and afterwards, when the grain is fixed, to use stronger liquors containing the more astringent tannins. This is usually accomplished in practice by working the liquors gradually down the yard. As the goods pass through the liquors their astringency as well as their strength decreases, because the more astringent tannins are in preference absorbed by the hides, and hence, by employing such used liquors for the next earlier stage in the tanning, a series of liquors are obtained where tannin is gradually being absorbed and whose action is becoming increasingly “ mellow.” The increasing quantities of calcium and other salts in these liquors and their greater proportion to the weak acids present has also been demonstrated to be170 THE MANUFACTURE OE LEATHER an important factor in producing mellowness. The usual procedure, therefore, is to place the hides at first in old and nearly exhausted liquors and gradually to move the goods through newer and stronger liquors. It is obvious that this method of manipulation is economical as well as scientific. Another point lor an economical tannage is the desirability of keeping the volume of the tanning liquor in as small a ratio to the quantity of hides as is consistent with easy manipulation and good results. The quantity of free acid in the tanning liquors is a matter of the first importance, for practically all tanning liquors contain these acids in some degree. They are weak organic acids (lactic, acetic, etc.), derived from the sugars associated naturally with the vegetable tanning materials and play an exceedingly important part in the tannage. They tend to swell the hide fibres and thus act in the opposite direction to the tannins, so that the important matter is to have a correct ratio of acid to tan throughout the whole yard, but especially in the earlier stages. This ratio differs widely for different classes of goods, and is largely influenced by the condition of the hides when entering the tan liquors, which is determined by the nature and extent of the deliming process. Hides in a plumped state tan more slowly than in a fallen condition, but absorb a greater amount of tanning matter and yield a firmer and more inflexible leather. Hence butts for sole leather practically always enter the tanning liquors with a certain amount of caustic lime in them, this being later neutralised by the acids in the liquors which are kept in excess, not only to prevent the formation and oxidation of lime tannates (cp. p. 46), but also to maintain the goods in a plump condition. If the butts are delimed merely by washing in water for a short time, the acidity of the first suspender liquor (which in a regularly worked tannery indicates generally the degree of sourness of the whole yard) should correspond to nearly 10 cc. of lime water when determined by Procter’s method (p. 154); but if a bath of boric acid is used to remove all the surface lime and half the total the acidity may safely go down to that corresponding to 4—5 cc. of lime water, though this matter is somewhat influenced byTHE PRINCIPLES OF VEGETABLE TANNING 171 the nature and strength of the liquor. With dressing leather goods and skins, which are delimed uniformly and complete^, the acidity is, generally speaking, less than in the sole leather tannages, because more flexible leathers are required, and the more unswelled and fallen the goods are in the early stages of tanning, the softer the product obtained. Where extracts are much used and the liquors worked quickly down the yard, the natural acidity or “ sourness ” of the first liquors may not be so great as is desirable. These “ sweet ” liquors must then be artificially acidified by the addition of lactic and acetic acids, or by an increased use of myrobalans or other acidforming materials. The amount of motion given to the goods in tanning influences not only the rate of penetration, but the nature of the resulting grain, and the tan-yard liquors are usually divided up into distinct sections in which the mechanical operations are different and distinct. The first pits entered by the goods containing the old mellow liquors are called the “ suspenders.” The goods are hung vertically in these liquors by means of hooks or strings, or by placing over poles. In these pits the acid penetrates into the hide, neutralising any remaining lime and plumping the pelts, thus preparing the way for the more slowly penetrating tannins. In these pits evenness of action of both acid and tannin and a good light colour are some of the chief points to be aimed at, and these are best obtained by a frequent shifting of the goods. Hence each pack is usually shifted once each day, and in the first suspender liquors the goods are often kept in constant motion by suspending in wooden frames, which are rocked gently either by a horizontal to and fro motion or by a rotatory up and down motion, in which latter case each half of the pack is alternately partly removed from the liquor. The frames are worked with overhead shafting attached to power. A series of such pits are often termed “ rockers.” When a quick and even colouring is desired, as in the case of some dressing leathers and many skins, the goods may be paddled in the first liquors; but this should not be done when firmness is important, as in sole leather, or where a smooth grain finish is desired, as the constant bending of the goods works up the172 THE MANUFACTURE OF LEATHER grain pattern in a manner somewhat similar to some of the finishing processes. Where a “ grain ” is afterwards to be worked up, however, it is sometimes desirable to start the tannage in a rather strong liquor to draw the grain slightly. This may be done in paddle or in pit, the goods afterwards entering ordinary weak suspenders. A method of working these liquors which is sometimes convenient is to arrange them like a press leach, except that the liquors should press upwards. The weakest liquor is each day run down the drain, and liquor from the later stages is pumped into the top pit of the battery which presses on and refills the tail pit. Usually, however, the suspenders are worked in “ rounds ” in which the goods receive a definite number of liquors of definite barkometer strength. The fresh liquor is made up each day from the weakest liquors from the handlers, and is gradually worked through the round until it becomes the tail liquor and is run away. The handlers are the next liquors, and are also worked in rounds. In these pits the goods are laid horizontally one above the other, grain side uppermost, and allowed to lie perhaps two or three days without being shifted. The strong liquor is made up from the old layer liquors or leaches, and after working through the round till it forms the weakest liquor, is then used as the source of the head suspender liquors. In the forward handlers ground bark, myrobalans, algarobilla, etc., are often “dusted down” with the goods as they are placed in the liquors, a little being put in between each hide or butt, or sometimes a separate round of pits is used in which this treatment is given. Such pits are termed dusters, while the term floaters is applied to those pits in which clear liquor only is used. It is obvious that the arrangement of all these pits and rounds is a matter in which there is considerable scope for variation, and for adjustment to the local convenience and the character of the tannage, but it might be instructive to show the working of a round of handlers in order to illustrate generally the nature of the mechanical operations in the tan yard. If a round of six pits are in use, let A be the pack in the strongest liquor in pit 1, and F the “green” pack whichTHE PBINCIPLES OF VEGETABLE TANNING 173 has last come from the suspenders and lies in the weakest liquor in pit 6. 3 2 1 c B A D E F 4 5 6 The procedure is then as follows: Pack A is hauled and taken to the dusters or layers, pack F is next hauled and the liquor pumped away to form the head suspender. In pit 6 is now made up the new head handler liquor, and pack B is hauled from pit 2 and put into it; pack C is put into pit 1, which is now the second best liquor; pack D is shifted into pit 2, pack E into pit 8, pack P into pit 4, and the new green pack Gr, from the suspenders, is placed in the weakest liquor in pit 5. The arrangement is now thus:— 3 2 1 E D c i i F G B 4 5 6 If on the other hand no green pack enters and no tan pack leaves the round, and the goods are to be shifted and a new liquor made, the pack F in the weakest liquor in pit 6 is hauled, the new liquor made in that pit, and the goods moved174 THE MANUFACTURE OF LEATHER forward merely into the next pit, the tan pack A into the new liquor in pit 6, pack B from pit 2 into pit 1, and so on, the green pack first hauled going into pit 5. The position is then thus:— 3 2 1 D c B E F A 4 5 6 It is evident that either of these changes may be worked once a day or once every two days, and that they may be worked on alternate days. In cases where a round of handlers contains both floaters and dusters, it is desirable that the goods in the latter should lie undisturbed for a longer period than those in the former. If, for example, in a round of six handlers, the goods are dusted down in the head liquor, and if a green pack enters and a tan pack leaves the round every two days, one liquor being given also every two days on the day on which no pack is received, let the position be the following :— Position I. 3 2 i c A* B* D E F 4 5 6THE PEINCIPLES OF VEGETABLE TANNING 175 The mode of procedure is then as follows : the “tan” pack A comes out of its duster and is taken to its layers, the next pack B (which is in its duster) is not disturbed, but the remaining packs C, D, E and F are given a single shift forward, and the new green pack G from the suspenders is placed in the weakest liquor in pit 6. The arrangement is now as follows:— Position II. 3 2 1 D c B* E F G 4 5 6 On the next day the green pack G is hauled, the liquor in pit 6 run to the suspenders, and a new liquor made in that pit. Pack G is now hauled and dusted down in pit 6. The rest are given a single shift forward, thus :— Position III. 8 2 1 E D B* F G c* i 4 5 Ö176 THE MANUFACTURE OF LEATHER If it be desired to save handling and tan-yard labour, both these changes may be carried out on the same day, and the goods left undisturbed on alternate days. The new liquor is given and the duster made on the same day as the tan pack leaves and the new green pack enters the round. Pack C is then given a treble shift and enters its duster immediately in pit 6, the rest receive a double shift, and the green pack G enters pit 5, which now contains the weakest liquor. The goods, therefore, change from Position I. directly to Position III. In this case the round always contains two dusters, but each pack only receives half the number of liquors. If one pack is received and lost each day, and a new liquor given each day, the change is also direct from Position I. to Position III., but takes place each day, and in this case the goods pass through the round in half the time taken by the preceding cases. In the case where one pack is received and lost every two days, and a new liquor given each day, there are two liquors for every pack. The method of working is then to give the change from Position I. to Position II. on one day, and the double change every second day, similar to the change from Position I. to Position III. Similar methods of working can be of course used for rounds of eight, ten, twelve or more pits, with three or more dusters in the round, the general principle being that the tan pack is always in the pit next to the strongest floater. These methods of working a round of handlers are often common in dressing leather yards, but no dust is given, and the “ dusters” are therefore termed “ liquor packs.” The handling forward of goods is best done by hand labour; but “ reels ” have been employed, in which case the goods are strung together and wound over into the next pit. This is rather quick, but is really only suitable treatment for offal. An overhead crane has also been used to some extent. Goods are worked through handlers to flatten and straighten them by virtue of their own weight. It is a less convenient method for handling the goods than suspension, and the goods also do not “feed” so well. In many modern yards therefore it is found more convenient to put the goods in handlers for aTHE PKINClPLES 0E VEGETABLE TANNING 177 sufScient time to set the fibres flat and to ensure the absence of creasing and buckling, and then to put the goods again in suspension. Hounds of such pits may be worked like a handler round, and are often termed “ suspender-handlers.’’ The “layers” or “layways” are pits used for the heavier leathers in which much stronger liquors are used, a greater amount of dusting material employed, and in which the goods are allowed to lie undisturbed for a decidedly longer time. They are made up from the leaches, strengthened where desired with extract and solid material, and are afterwards used as the source of the head handler liquors. The principal function of these pits is to give time for a complete tannage of the hide fibres, a thorough penetration of the liquor, and to give weight and firmness to the leather by the deposition of bloom, reds, etc., in the interior of the hides. In this stage only one or two pits are necessary, the goods being hauled at certain intervals and dusted down again in the same or a new liquor with further quantities of fresh material. Generally speaking, the shoulders and bellies of rounded hides have their own suspenders, handlers and layers, but are sometimes taken with the butts through the whole or part of the suspender stages, and occasionally through the handlers. In the tannage of skins for lighter leathers the general principles as explained^above still apply. The suspenders are largely replaced by paddles, which ensure even penetration and colour. The handler liquors need not be of such great strength as in tanning hides, because the goods are thinner and the complete penetration therefore quicker, whilst at the same time the iveight of the resulting product is usually of no consequence. For the same reasons the long layers are not needed, the skins being often finished in paddle in order to obtain softness and good colour. Where a quick tannage is desired the bottle or bag tannages are sometimes employed, in which the skins are sewn up to form a closed vessel in which tanning liquor is placed and is forced through either by mechanical pressure or by the pressure of its own weight. In drum tannages also the general principles of tanning are the same, the particular object in drumming being to bring M.L. N178 THE MANUFACTURE OE LEATHER about a quick penetration of the liquor and a rapid tannage. The tanning strength of the liquors is increased at a much more rapid rate than in the pit tannages, the final liquors being often neat extract. It is usual to pass the goods through suspenders for a few days before entering the drum liquors in order to fix the grain, but some processes claim to avoid this by short suspensions in solutions of formaldehyde, boric acid and other materials. In all cases, even for sole leather, the goods should be completely delimed and thoroughly scudded, the former because the acidity of extract liquors is so small, and the latter to assist in quick penetration. The tannage is essentially with extracts, but that from chestnut wood is about the only suitable one, being a mellow, quickly penetrating and fair weight-giving tannin. Treated quebracho extracts, hemlock extracts, and oakwood extracts, have, however, been employed in conjunction with chestnut extract in the stronger drum liquors. Both hides and skins are often “ split ” (see p. 277) when half tanned or “ struck throughthe “grains” then go forward with the ordinary tannage, whilst the “ splits ” receive a much cheaper and inferior tannage, often in drums. “ Degreasing ” may also be done before the tannage is complete.CHAPTER XIV THE TANNAGE OP SOLE LEATHER In the tannage of any clsss of leather it is impossible to specify a method which is in quite general use, for the methods employed differ in almost every tan yard. Hence the practical methods which are given in this and the following chapters must be taken merely as typical processes, which have been found to yield good results, but which may be varied in many directions according to local circumstances. Broadly speaking, there are in this country three classes of methods which are in common use for the tannage of sole leather—the oak-bark tannage, which is the oldest process, the West of England tannage, of which valonia is the chief material, and the modern mixed tannage in which many tanning materials are blended together. In the United States also there are three chief varieties of sole leather tannage— the oak tannage, the hemlock tannage (acid and non-acid), and the union tannage, in which a combination of oak and hemlock is used. Another different class of tannage employed in both Europe and America is the quick drum tannage in extracts. These will now be dealt with in more detail. The oak-bark tannage, in which the best Continental and Scotch hides are mostly employed, lasts about twelve months and is brought about in comparatively weak liquors. The barkometer strength of these is very variable, according to whether the used liquors are re-strengthened by pumping again through the leaches or worked down the yard. In the former case there is a gradual accumulation of non-tannins in the liquors, so that although the tanning strength may not be widely different from the other method, the barkometer strength appears greater. . In a pure oak-bark tannage, where the liquors are worked down the yard, about the strongest liquors obtainable are of 20° Bkr. strength, but where the liquors are constantly strengthened up in the leaches the strongest liquors may go up to about 35° Bkr. In most n 2180 THE MANUFACTURE OF LEATHER modern oak-bark tannages a certain proportion of valonia, gambier and sometimes otheb materials are used and stronger liquors obtained. This is illustrated in the following typical tannage. The goods (e.g. 100 butts) after deliming and scudding go into the suspenders, in which they are hung by strings over poles in liquors, which run usually from 10—18° Bkr., though the weakest may be taken as low as 6° Bkr. Here they remain 16 to 18 days, being handled forward each day to the next pit, and hauled frequently or rocked in the first two days. They then go into the handlers (floaters) for a month, being moved every day in the first fortnight and every second day in the second fortnight. A series of 12 pits are used, ranging from 18—24° Bkr. The head handler is a 20° Bkr. liquor from the dusters or leaches made up to 24° Bkr. with gambier. The butts now go into the dusters, a series of seven pits, in each of which the goods lie for one week. The first four dusters are about 24° Bkr., and the goods receive cwt. of bark dust; the next three pits are about 26° Bkr., and each pack is dusted down with 3 cwt. bark and 1 cwt. myrobalans. The goods now go to the layers, which are shown as follows:— Layer. Bkr. strength. Time in weeks. Material, etc. 1st 30° 3 4 cwt. bark. 2nd 33° 4 4 cwt. bark (light butts now go to sheds). 3rd 38° 4 4 cwt. bark. 4th (for mediums) 40° 4 4 cwt. bark and J cwt. valonia (medium butts now go to sheds). 4th (for heavies) ' 40° 4 4 cwt. bark and J cwt. valonia. 5th (for heavies) 45° 6 4 cwt. bark and 1 cwt. valonia (heavies now go to sheds). 6th (for extra heavies) 50° 6 4 cwt. bark and 1 cwt. valonia (extra heavies now go to sheds). Mimosa bark may be added to the leaches, and a small amount of extract employed where it is necessary to bring up the strength of the new liquors. The shoulders go throughTHE TANNAGE OF SOLE LEATHEE 181 the suspenders and handlers with the butts, but have their own layers as follows :— Layer. Bkr. strength. Time in weeks. Material, etc. 1st 26° 1 3 cwt. bark. 2nd 27° 1 ^ >> j> 3rd 30° 2 4 ^ 5 > >> 4th 34° 4 4 * } > J > The bellies have a set of liquors of their own, made by leaching the fishings from the dusters and layers. They are 14 days in suspenders, ranging from 10—20° Bkr., 14 days in handlers, ranging from 20—25° Bkr., and then go (250 pairs being taken) to the layers, which are as follows :— Layer. Bkr. strength. Time in weeks. Material, etc. 1st 25° 1 4 cwt. bark. 2nd 26° 2 4 ^ ) 1 5 > 3rd 28° 3 4 4 th 32° 4 An old butt layer liquor and 4 cwt. bark. A little gambier is used in making up the first three belly layers. The rate of tannage by this oak-bark process has been investigated,1 and the results obtained by the analysis of the butts at various stages in tanning are shown in the Table on p. 182. The West of England tannage of heavy South American salted hides lasts about 9 months and is carried out as follows :— The butts (100) are in suspenders for three weeks; the liquors range from 20—40° Bkr., but consist largely of non-tannins. They then go to the handlers for four weeks, the best liquor of which is obtained from the old dusters, and is made up to strength with J ton of gambier. The pits run from 40— 55° Bkr. Pour dusters at 60° Bkr. are now given, and the packs allowed to lie a week in each, handling twice. These liquors are made up either from the old hemlock liquors or 1 1902, 839.182 THE MANUFACTURE OF LEATHER Oak Bark Tankage; 10 months. — Hide Sub- stance. Tan- ning. Mineral Ash. Mois- ture. Tannin absorbed per 100 parts of Hide Substance. Per Per Per Per Cent. Cent. Cent. Cent. End of 24 hours 80-9 2*8 2*3 14*0 3*4 \ „ 2nd day 79-0 0*1 1*9 14*0 6*4 „ 4th „ 76-6 7*8 1-6 14*0 10-1 [ Suspenders. „ 8th „ 71-6 13*5 0*9 14*0 18*8 „ 18th „ 60*0 25*4 0*6 14*0 42*3 ) 5, 2 5 th ? j 56*3 29-1 0*6 14*0 51*6 „ 32nd „ 55-1 31*3 0*6 14*0 56*8 f Handlers. „ 46th „ 53-4 32-0 0-6 14*0 59*9 J „ 88th „ 49-0 36*3 0*7 14-0 74-6 End of 6 weeks in dusters. „ 102nd „ 47-3 38-0 0*7 14*0 80*3 1st layer. „ 132nd ,, 46-5 39-9 0*6 14-0 87*6 2nd layer. „ 162nd ,, 44T 41-2 0*7 14*0 93*4 3rd layer. „ 192nd „ 43*4 42-0 0-6 14-0 96'7 4th layer. „ 234th „ 42*6 42*8 0*6 14*0 100*4 5th layer. „ 278th „ 41*6 43*8 0-6 14*0 105-2 6th layer. „ 290th „ 45*3 40*1 0-6 14*0 88-5 Washed up in weak liquor. 46-5 38*9 0*6 14-0 83-6 Scoured by ma- chine. 47-2 39*2 0*6 14*0 83-0 Brushed and pinned by hand. „ 310th „ 47*0 39-4 0*6 14-0 83-8 Finished leather. from a 40° Bkr. fresh myrobalans leach liquor and several casks of hemlock extract. The latter course gives the better colour. The goods are dusted down with 1J cwt. bark and cwt. myrobalans. The packs now receive a hemlock round of four liquors at 60° Bkr., made up entirely from hemlock extract. They lie a week in each liquor and then enter the layers, which are as follows:— Layer. Bkr. strength. Time in weeks. Material, etc. 1st 60° 2 2 cwt. oak bark and 2 cwt. valonia. 2nd 65° 2 4 cwt. yalonia. 3rd 70° 3 4 ^ J J 5 > 4th 75° 4 5 „ 5th 80° 4 6 5 J J > 6th 90° 6 6 » >>THE TANNAGE OF SOLE LEATHEE 183 Sometimes an extra layer is given and the valonia partially substituted by myrobalans in the early layers. It is usual also to employ a certain amount of oakwood extract in making up the strong layers. The rate at which the butts are tanned by this process has also been investigated1 analytically, and the following results obtained:— West of England Tannage; South American Salted Hides; 10 Months. — Hide Sub- stance. Tan- ning. Mineral Ash. Mois- ture. Tannin absorbed per 100 parts of Hide Substance. — Per Per Per Per Cent. Cent. Cent. Cent. End of 1st day 78-4 4*2 3*4 14*0 o*3 > „ 2nd „ 75-8 8*2 2*0 14*0 10*8 „ 4th „ 74*0 10*4 1*6 14*0 14*0 -Suspenders. „ 8th „ 65*9 19*1 1*0 14*0 28*9 „ 21st „ 58-3 26*8 0*9 14*0 45*9 J „ 28th „ 53*8 31*3 0*9 14*0 58*1 ) >5 35th ,, 51-8 33*4 0*8 14*0 64*4 Handlers. „ 56th „ 49*2 36*0 0*8 14*0 73*1 J „ 88th „ 44*2 41*0 0*8 14*0 92*7 Dusters. „ 112th „ 41*0 44*2 0*8 14*0 107*8 Hemlock round. „ 126th „ 40*8 44*4 0*8 14*0 108*8 1st layer. „ 147th „ 39*6 45*6 0*8 14*0 115*1 2nd layer. „ 177th „ 38*0 47*1 0*9 14*0 123*9 3rd layer. „ 208th „ 37-2 47*9 0*9 14*0 129*0 4th layer. „ 238th „ 36*5 48-7 0*8 14*0 133*4 5th layer. „ 270th „ 35*9 49*2 0*9 14*0 137*0 6th layer. „ 300th „ 34*7 50*4 0*9 14*0 144*9 7th layer. „ 302nd „ 38*9 46*2 0*9 14*0 118*4 Washed in weak liquor to cleanse. 39*7 45*5 0*8 14*0 114-5 Scoured under machine. 39*6 45*5 1-1 14-0 114*6 Finished. Pro- duct after oil- ing, colouring, roiling and dry- mg. The modern mixed tannage of salted Continental, Scotch and English marked hides last only about 4—5 months, and may be carried out in the following ways:— 1 J.S.O.I., 1902, 839.184 THE MANTJEACTUBE OE LEATHEE Process (I.).—Tbebutts (100)go into suspenders(20—40°Bkr.) for about a week, rocking in tbe first liquors. The goods are handled forward to a new liquor each day. Tbe handlers (40—55° Bkr.) are now entered, rounds of eight pits being employed, tbe two most forward of which are dusters. Tbe best liquor is obtained from the layers, and the goods are dusted down with 1J cwt. of myrobalans (or a mixture of myrobalans and algarobilla). Tbe goods remain a fortnight in these liquors and should then he struck through. They now proceed to the suspender handlers (55—65° Bkr.) in which they remain 2 or 3 weeks, handling forward on alternate days.THE TANNAGE OF SOLE LEATHER 185 The head liquor is made up from fresh leach liquors and chestnut extract. Four layers are given as follows : Layer. Bkr. strength. Time in weeks. Material, etc. 1st 70° 1 4 cwt. myrobalans. 2nd 75° 2 4 cwt. yalonia. 3rd 80° 3 4 cwt. of yalonia and 3 casks chestnut extract. 4th 90° 4 4 cwt. yalonia and 4 casks oakwood extract. The third and fourth layers are made up of fresh leach liquors (65° Bkr.) made from the fishings ; the second layer is an old fourth layer liquor and the first layer an old third liquor. The fishings go to the leaches, and the clear liquors from the old first and second layers are pumped chiefly to the handlers but partly to the offal pits. As chestnut extract and myrobalans are used in one pit, and oakwood extract and valonia in the other, it is desirable to mix these in a large pit before pumping to the handlers, and if the liquor is too strong it should be weakened with a 40° Bkr. liquor from the leaches, and any other materials (e.g., quebracho extract) may also be added at this stage. A little mimosa dust may be added to the first layer, the valonia partially replaced by myrobalans in the second, and a blend of oakwood and chestnut extracts (2 : 1) may be used in the third and fourth layers. Another method for working the layers is as follows:— Layer. Bkr. strength. Time in weeks. Material, etc. 1st 80° 2 4 cwt. myrobalans. 2nd 85° 2 4 cwt. myrobalans and chestnut extract. 3rd 90° 3 4 cwt. valonia and 4 casks oakwood extract. 4th 100° 4 4 cwt. valonia and 4 casks oakwood extract. The first layer liquor is an old fourth layer liquor in which the goods are handled three times in the first 10 days and186 THE MANUFACTURE OF LEATHER dusted down with the myrobalans for the rest of the time. The second liquor is an old first liquor strengthened with extract, the goods being dusted down at once for the whole time. The third liquor is made up from the leaches with the help of extract, and the fourth liquor is an old third liquor re-strengthened with extract. In this case therefore only one pit is used, and the old second liquor used entirely for the butt handlers, and the fishings are leached for the offal. Yet another method is as follows :— Layer. Bkr. strength. Time in weeks. Material, etc. 1st 70° 1 Old fourth liquor, no dust, liquor goes to handlers. 2nd 75° 2 New liquor from leaches, and extract. 3rd 85° 3 Old second strengthened with extract. 4th 95° 4 Old third strengthened with extract. The offal is usually taken through separate liquors, though the shoulders sometimes go through the suspenders with the butts. A common method is to colour the shoulders and bellies in latticed drums, using old butt suspender liquors. They then go into suspenders (18—40° Bkr.) for 4 to 5 days, and the shoulders are now put into the shoulder-handlers (40—55° Bkr.) The packs (200 in each) are handled forward each day for three weeks, and in the head liquor, which is made up each day, 2 cwt. of myrobalans are used as dusting material. The shoulder layers are as follows :— Layer. Bkr. strength. Time in weeks. Material, etc. 1st 60° 2 3 cwt. myrobalans. 2nd 65° 3 3 cwt. myrobalans and chestnut extract. 3rd 80° 4 2 cwt. myrobalans, 1 cwt. valonia and chestnut extract. The first liquor is an old third liquor, the second a new leach liquor strengthened with extract, and the third liquor is an old second liquor, fished and raised to 80° with chestnut extract.THE TANNAGE OF SOLE LEATHER 187 The bellies (250 pairs) go through the shoulder handlers for a fortnight and then go to three layers of one week each, being 60°, 70° and 80° Bkr. strength, and having myrobalans as dusting material. In another method they receive 4 days in suspenders, 4 days in 4 handler pits (40—60° Bkr.), and then go to the first layer (70° Bkr.), for one week, using 4 cwt. myrobalans. They are then drummed in the first layer liquor made up to 80° Bkr. with chestnut extract. A little mimosa dust is added and the drumming continued for four hours in different parts of the day. The goods are then firmed up by a second layer (100° Bkr.) for 8 or 4 days. In leaching for a mixed tannage such as the one just described, only the fishings from the layers should be used. These will seldom yield a liquor of more than 65° Bkr. strength, and it is usual to employ the head leach as the source for making the yard liquors (ctr. dressing leather, p. 198). The liquors are worked down the yard as shown in the diagram on p. 188. Process {II.).—In this tannage the butts were delimed with 20 lbs. boric acid per 100 butts. The tannage consists of one-third extract, one-third valonia, the remaining third being chiefly myrobalans, but with smaller quantities of mimosa bark, quebracho, algarobilla, etc. The butts (100) are in rockers (20°—45° Bkr.) for 4 to 6 days, being of course in suspension. The handlers (45°—60° Bkr.) are large rounds including several dusters, and the goods remain in these liquors 4 to 5 weeks. In the dusters the butts receive 2—8 cwt. myrobalans. The layers are as follows:— Layer. Bkr. strength. Time in weeks. Mateiial, etc. 1st 2nd 3rd 4th oth (“ heavies ” only) 65° 70° 80° 90° 100° 1 2 3 4 6—7 4 cwt. myrobalans and chestnut extract. 2 cwt. myrobalans, 2 cwt. mimosa bark and chestnut extract. 4 cwt. valonia \ j i j , 1 and oakwood - ” ” | extract. o „ „ > Mix the quebracho and gambier in the handlers,Best Leach Liquor (65° Bkr.). i 2nd or 3rd Leach Liquor (60—65° Bkr.). QO 00 \ 4th Butt Latee (90° Bkr.). 3bd Butt Latee (80° Bkr.). 4 2nd Butt Latee (75° Bkr.). 1st Butt Latee (70° Bkr.). \ / Mixing Pit (+ 40° Bkr. Leach Liquor). \ \, \ \ \ \ \ \ _ y \ \ Bellt Latees (60—80° Bkr.). Shouldeb Latees (60—80° Bkr.). / Handlers (40—55° Bkr.). \ \ / \ \ ✓ \ Shoulder Handlers (40—55° Bkr.). Suspenders (20—40° Bkr.). / \ / Latticed Drums (18° Bkr.). Offal Suspenders (18 —40° Bkr.). (For colouring offal.) / \ s Drain. THE MANUFACTURE OF LEATHERTHE TANNAGE OF SOLE LEATHER 189 Process (III.).—Another mixed tannage may now also be outlined in order to illustrate further the possible variety in methods of procedure. In this case the tannage only lasts about four months. The butts are given 8 days in suspenders (10— 25° Bkr.) handling twice a day and handling forward each day, and then pass into a round of handlers (80—45° Bkr.) for four weeks. The two head liquors of this round act as dusters, 1 cwt. of myrobalans dust being used. If necessary the head liquor may be strengthened with quebracho. The layers are as follows :— Layer. Bkr. strength. Time in weeks. Material, etc. 1st 60° 1 Mimosa bark and myrobalans. 2nd 70° 2 Myrobalans and valonia. 3rd 75° 3 Yalonia. 4th 90° 4 Yalonia. In “ vatting ” (see p. 261), these goods receive a full week in a liquor (100° Bkr.) made from bright chestnut extract, and are afterwards washed through sumach. The analysis of the butts at different stages in tanning1 by this process is given on p. 190. American sole leather is not rounded but tanned in “ sides,” or the hides are cut into sides after tannage. The oak tannage, lasting about six months, most resembles the British tannages and is chiefly used with native hides. The hides after a short liming (Buffalo method) are delimed and plumped in rockers, and then receive 8 to 14 days in handler liquors. Five layers are given on an average ranging from 80—45° Bkr., and lasting five and a half months altogether. Oak bark (Q. prinus) is used as dusting material, and this is also leached^o form the liquors. Oak-bark extract is used to make up the strength of the stronger liquors. The non-acid hemlock tannage is when the only acids used are those formed by natural fermentation. South American dried hides are chiefly used, and after sweating and unhairing are placed in water overnight, and then swollen with acid. They 1 J.S.O.I., 1902, 840.190 THE MANUFACTURE OE LEATHER Modern Mixed Tannage eor Scoured Bends; 4 Months. i Tannin — Hide Sub- stance. Tan- ning. Mineral Ash. Mois- ture. absorbed per 100 parts of Hide Sub- — stance. Per Per Per Per Cent. Cent. Cent. Cent. End of 1st day 77*5 5*5 3*0 14*0 7*0 \ 5) 55 2nd „ 4th „ 74-5 73-2 10-0 11*6 1*5 1*2 14*0 14*0 13-4 15-8 - Suspenders. 55 8th „ 61-1 24-0 0*9 14*0 39*2 / 55 16th „ 53*2 32*1 0*7 14*0 60*3 55 23rd „ 50-7 34-7 0*6 14*0 68*4 ^Handler round. 55 37th „ 45*6 39*8 0*6 14*0 87*2 J 55 45th ,, 41-6 43-6 0*8 14*0 104-8 1st layer. 55 60th „ 39-3 46*1 0*6 14*0 117*3 2nd layer. 55 81st „ 37-8 47-5 0*7 14*0 126-8 3rd layer. 55 lllth „ 37-5 47*8 0*7 14*0 127*4 4th layer. 38*1 47-3 0*6 14*0 1241 Scoured in ma- chine. 55 119th „ 35-4 49*9 0*6 14*0 140 0 After 1 week in liquor with chestnut extract at strength 100° Bkr. 55 120th „ 38-5 46*7 0*8 14*0 121*2 After washing through sumach hath to brighten colour. »5 136th „ 40*8 44-7 0*5 14-0 109*5 Finished in ware - house. then go through the handlers (10—16° Bkr.) and then to the layers, which run from 28—86° Bkr. and last about five and a half months altogether. The latter liquors are obtained from the leaches in which hemlock bark is used, and the stronger liquors are made up with hemlock extract. The acid hemlock tannage is also made chiefly with dried hides—buffalo hides from China being a common material. After sweating and unhairing they are soaked and put into a “ colouring vat ” containing a weak tanning liquor for \ hour to 1 hour. They are afterwards placed in a 0*1 to 0*3 per cent, solution of sulphuric acid until plumped to the required extent, and then may be put safely into the first layer liquors, the strength of which differs widely in different yards, but may be even 38° Bkr. The goods' are about sixTHE TANNAGE OF SOLE LEATHER 191 months in the five or six layers, hemlock bark dust and a little hemlock extract being used, and the strongest liquors being nearly 50° Bkr. A few native hides are limed (and occasionally bated), and afterwards given the acid hemlock treatment. This is called the “ slaughter-hemlock ” tannage. The union tannage is by means of both oak and hemlock barks, and yields leather of better texture, colour, and firmness than the hemlock tannages. Salted hides are largely used, and after liming, unhairing, fleshing, and soaking overnight, are put through the handlers. The layers usually receive oak bark only as dusting material and the fishings are leached with fresh hemlock bark to make the liquors. Much “ Union ” leather is sold as oak, though tanned with 75 per cent, hemlock. As, moreover, the supply of hemlock bark is now less plentiful, the American methods of tanning are in a transition stage. Quebracho is now very largely employed (sometimes even alone) for making “ hemlock’’ leather. Some tanners employ a mixture of quebracho and mangrove, sometimes along with chestnut extract. Union leather is also tanned with quebracho extract entirely, then bleached and drummed with hemlock extract and glucose. The imports of myrobalans, valonia and mimosa bark into the United States are also rapidly increasing. The barkometer strengths of all liquors in which hemlock bark is used are very varied in different yards, according to the thoroughness of the leaching. If the leaching is less “ close ” the liquors have a less barkometer strength; some loss in tannin is incurred, but the colour of the resulting leather is thereby appreciably improved. In the use of both oak and hemlock barks, the used liquors are very often passed again through the leaches to be strengthened, and this also produces considerable differences in the barkometer strengths of the liquors. In all these tannages also the liquors contain a greater proportion of tannin to non-tannins than in the British methods, and hence the differences in the strength of the layer liquors when the packs are put in and taken out are much greater than in British tannages. The difference in barkometer strength, which is called “ sappage,” is used as a rough192 THE MANUFACTURE OF LEATHER means of indicating the degree to which tannins have been absorbed, and runs from 20—40° Bkr., according to the strength and purity of the liquors. Drum tannages, differ considerably in the first stages of the tanning, according to the deliming and preparatory processes that are used, the great aim being to make these processes as short and inexpensive as is consistent with good results. Many patents have been taken for these processes, notably by Messrs. Fratelli Durio, but practically all have Fig. 57.—Tanning drum. proved unsatisfactory in the hands of some experimenter, probably owing to insufficient knowledge or attention to the details of the processes in the critical stages, hence many modifications of these patent processes have been made in which the difficulties experienced have been wholly or partially overcome. The following process has been found to give satisfactory results in some hands. The goods after deliming are suspended for about a week in a round of pits, the liquors ofTHE TANNAGE OE SOLE LEATHER 193 which have come from the drums. The head suspender is a 80° Bkr. liquor, and is the used liquor from the weakest drums. The goods are struck through in these liquors and then enter the drums in a 7—8° Be liquor (50° Bkr.). The drums should be quite half full of liquor, and should revolve four to five times per minute, the temperature (which rises because of the friction) being about 80—88° C. The pack should remain in this liquor for twelve hours, and be similarly treated with a stronger liquor, and afterwards with a still stronger liquor, and finally in neat extract 25° Be. In these later liquors the rotation need not continue more than half the total time of immersion. In another process, the goods are delimed and somewhat plumped with acetic acid. They are then taken through a series of suspenders ranging from 25—50° Bkr., which involves 10 to 14 days. The goods are then drummed in a 70—80° Bkr. extract liquor for twelve hours, and afterwards in neat extract, 200° Bkr. (25° Be), for thirty-six hours. In another method the goods are suspended after deliming in a weak (about *8 per cent.) and slightly acid solution of formaldehyde for sixteen hours, and then placed directly into a 70° Bkr. extract liquor, finishing off as usual in liquors of gradually increasing strength. Great difficulties have been experienced in many hands in obtaining a thorough and even tannage, the thicker parts of the hides being very liable to yield patches quite untanned. Prolonged drumming sometimes completes the tannage of these portions, but is apt to cause the rest of the hide to dry out hard and brittle. The essential conditions necessary for the successful drum tannage of heavy leather have not been as yet completely investigated, and where empirically discovered have not been fully made known. M.L. OCHAPTEB XY THE TANNAGE OF BELTING, HARNESS AND UPPER LEATHER, ETC. The Tannage of Belting Leather is in many respects similar to the tannage of sole leather, so much so, indeed, that some manufacturers give the same tannage to sole and strap butts, and sort them before finishing. As a somewhat softer result is desired, however, greater quantities of gambier, myrobalans, etc., are usually employed. The layers also are generally not so strong as for sole leather on account of the nature of the finish. After scudding, the butts enter the suspenders (8—80° Bkr.), in which they remain two or three weeks. These liquors should not be too strongly acid, as a rather pliable leather is required, and excess of acid produces crackiness. The goods afterwards enter the handlers (80—45° Bkr.), in which they remain four weeks. As gambier tannages carry grease well, the butts are now given a gambier round (50—55° Bkr.), in which liquors they are suspended for 14 to 18 days, handling every day. Occasionally a little myrobalans or chestnut extract is used to make up the head liquor. The weak liquors are run to the handlers and worked down the yard. The goods now enter the layers, which are as follows :— Layer. Bkr. strength. Time in weeks. Material, etc. 1st 50° 1 4 cwt. myrobalans. 2nd 55° 2 2 cwt. myrobalans and 2 cwt. mimosa bark. 3rd 60° 4 2 cwt. myrobalans and 2 cwt. valonia. (for “heavies”) 4 cwt. valonia. (4th) 70° 4 (5th) 80° 4 (for “ extra heavies”) 4 cwt. valonia. In making up the strong layer liquors some amount of chestnut and quebracho extract is employed. German belting hides are given a six weeks’ suspensionTANNAGE OF BELTING, HARNESS LEATHER, ETC. 195 starting in a 7° Bkr. liquor. The liquor is changed twice a week for a slightly stronger infusion, the goods remaining in the same pit. They receive twelve liquors in all, the last being 24° Bkr. Occasionally the suspenders only run up to 14° Bkr., in which case a handler round is also given. The goods are now rounded and the butts are given two layers. In the first liquor (24° Bkr.) they are dusted down with 10 cwt. pine bark, 10 cwt. oak bark and 4^ cwt. valonia, and are allowed to lie six weeks. In the second liquor (28° Bkr.) they are dusted down with lli| cwt. of oak bark, cwt. valonia, and 4J cwt. pine bark, and remain in this eight to ten weeks. The leaches are made up from a blend of § pine bark, J quebracho and J myrobalans. The Tannage of Harness Leather differs somewhat according to the quality of the goods and whether light or heavy harness is being made. Light harness backs (high class) are entered into suspenders at 8° Bkr., and are handled forward every day for three weeks until they reach the head liquor (20° Bkr.). They then pass into the handlers (20—85° Bkr.) for six weeks, two of which are dusters, \ cwt. myrobalans and J cwt. oak bark being used for every 100 backs, and sufficient gambier to bring up the strength of the head liquor to 88° Bkr. The layers are as follows:— Layer. Bkr. strength. Time in weeks. Material, etc. 1st 35° 1 4 cwt. oak bark. 2nd 40° 2 * 4 cwt. oak bark and 1 cwt. myrobalans. 3rd 50° 3 3 cwt. oak bark and 1 cwt. extract. The extract is found to counteract somewhat the influence of the myrobalans. Heavy harness backs (high class) are put through suspenders in about the same time, but in rather stronger liquors (8— 80° Bkr.), and next go into the handlers (80—40° Bkr.), a series of nine pits. In these they remain only a month, but they are given a duster round (40—45° Bkr.) of three pits, the goods being handled each day. The layers are now entered :— o 2196 THE MANUFACTURE OF LEATHER Layer. Bkr. strength. Time in weeks. Material, etc. 1st 50° 2 4 cwt. oak hark. 2nd 55° 3 4 cwt. oak bark. 3rd 60° 4 4 cwt. oak bark. 4th 65° 4—6 4 cwt. oak bark and 1 cwt. valonia or mimosa. A shorter method, with more tannin added as dust, is as follows:— Layer. Bkr. strength. Time in weeks. Material, etc. 1st 50° 1 2 cwt. oak bark and 2 cwt. myro- balans. 2nd 55° 2 2 cwt. oak bark and 2 cwt. mimosa. 3rd 60° 3 2 cwt. oak bark and 2 cwt. valonia. 4th 65° 4-6 3 cwt. valonia and 1 cwt. myrobalans. The fourth layers are only given when the tanner is not also the currier. The leach liquors are made from oak bark and myrobalans. Cheap harness backs are given the following tannage: — Having received an excess of boric acid they enter a 16° Bkr. suspender liquor and work up to 30° Bkr. in 14 to 18 days, and then enter the handlers (30—45° Bkr.) for one month. The head handler is an old layer liquor made up from 40—45° Bkr. with gambier, and in this pit the goods are dusted down with 1 cwt. myrobalans. The layers are as follows :—■ Layer. Bkr. strength. Time in weeks. Material, etc. 1st 05 o / 1 2 cwt. oak bark and 2 cwt. myrobalans. 2nd 51° 2 3 cwt. myrobalans, 1 cwt. mimosa bark, J cask chestnut extract. 3rd 55° CO 2 cwt. myrobalans, 2 cwt. mimosa bark, § cask chestnut extract. 4th 60° 4 1 cwt. myrobalans, 3 cwt. mimosa bark, f cask chestnut extract.TANNAGE OE BELTING, HARNESS LEATHER, ETC. 197 The fourth layer is only given to the heavy goods, and the layer fishings are used for the leaches, together with some valonia and mimosa. If the tanner does not curry his own goods it is often profitable to give a fifth layer of 75° Bkr. German harness hides are given a week in paddle after scudding, starting with a very weak liquor and gradually strengthening by the addition of oak bark and pine bark in equal quantities. The rest of the tannage is done in suspenders (7—21° Bkr.), in which the goods remain in the same pit all the time, the liquors being pumped over into the next pit. They remain three days in the first liquor and a week in each of the rest. The liquor for the suspenders is made up from six parts pine bark and two parts each of quebracho, myro-balans and mimosa bark. The Tannage of Dressing Hides for Upper Leather is largely carried out by oak bark and valonia, assisted by the liberal use of gambier and a moderate proportion of mimosa bark. The hides may be tanned whole, but are sometimes cut into sides and occasionally rounded. They are first coloured off in paddle in a 12° Bkr. liquor obtained from the second handlers or leaches. This liquor may be strengthened up again with gambier for a second or third pack, but is afterwards run away. The goods are paddled 2 to 5 hours, and then enter the first handlers (8—18° Bkr.) for a fortnight. These consist of a round of six pits, which receive a liquor each day from the second handlers, and a new pack on alternate days. A single shift is given, therefore, on the day no pack is received, and a double shift on alternate days when the green packs. The hides next pass through the second handlers (18—88° Bkr.), which are also rounds of six pits. They remain in these liquors for a month, a new pack entering the round every fourth day, and a new liquor being given on every other day. The goods are, therefore, given a single shift forward each day. The head liquors are obtained from the fourth leach, and the strength made up with about 2 cwt. of gambier. Any spare liquors are returned to the sixth leach. The goods now go into dusters (40—48° Bkr.) for six weeks. Bounds of six pits may be used, in the two most forward of which the goods are dusted down with 1 cwt.198 THE MANUFACTURE OF LEATHER mimosa bark. A little gambier may be used also in this round. The goods are then drawn, sumached and dried out. In leaching for dressing leather, it is important to bear in mind that heavily bloomed goods are not desired, and it is, therefore, usual to leach slowly, so that the bloom deposits in the leaches. With this object it is customary not to use the head leach as a source of liquor for the yard, at any rate for a few days, and it is a common plan to return used liquors to the leaches, after passing them through a round, before working them down the yard. For the tannage described, a system of eight press leaches will be found convenient, the second leach being used as the source of the liquors, which after passing through the dusters are returned to the third leach. A suitable blend for leaching is J valonia, \ oak bark, •J mimosa bark. The liquors are worked down the yard as shown on opposite page. The Tannage of Hides for Army and Navy Uppers is a soft mixed tannnage, in which gambier, myrobalans, valonia and a little oak bark are used. They are started (“grained”) in paddle, which receives an old handler liquor and a bucket of strong gambier infusion for every dozen hides. The goods run in this liquor for one hour. Sometimes the graining is done in pits, the pack being handled three times and the liquors strengthened each time with one-third the above proportion of gambier. After draining for a short time they go to the handlers, a series of six pits of gradually increasing strength, and are finished off in five new leach liquors strengthened with gambier (2 lbs. per hide), and a single layer of one week, dusting down with valonia and mimosa bark. They are, therefore, not split until the tannage is complete. The offal is largely worked up for coloured boot uppers. The Tannage of American “ Split Hides ” is chiefly with hemlock, but sometimes with pure gambier liquors. In the former case the sides after bating are suspended in the “ handlers,” which may be a series of fourteen pits, handling forward on alternate days, or a round of six or seven pits, remaining in each four days, so that in either case about a month is taken. The goods are tacked on laths or racks with copper nails, and handled very frequently in the early liquors,TANNAGE OE BELTING, HARNESS LEATHER, ETC. 199 Dressing Leather Liquors, Water. Dusters (40—43° Bkr.). h2o. In the ordinary hypo bath, however, another reaction takes place alongside it involving the precipitation of sulphur.218 THE MANUFACTURE OF LEATHER (b) K2Cr207 + 6 HOI + 3 Na2S203 = 2 KC1 + 4 NaCl + Na2S04 + 2 Cr(OH) S04 + 2 H20 + 3 S. These reactions do not take place in any fixed proportion, the extent of each varying with the conditions of the reducing bath. It is clear also that the amount of sulphur deposited depends on the same conditions. It has also been shown that wdiether hydrochloric or sulphuric acid is used in the two-bath process the tanning agent is basic chromium sulphate, and that well-washed two-bath chrome leathers contain sulphates and no chlorides. Hence another equation must be given to show completely the changes of the second bath. (c) Cr(OH)Cl2 + Na2S04 = Cr(0H)S04 + 2 NaCl. In consequence of the rather complicated changes in the hypo bath the best practical mode of procedure is therefore only to be determined from empirical observations, but it is probably best in all cases to insert the goods in the hypo and then to add the acid slowly. If added quickly, the rate of the reduction is increased, but much sulphur dioxide escapes to the air, and much thiosulphate and hydrochloric acid is therefore wasted. There is also a liability to form too acid chrome salts and consequently to cause an undertannage. Where the acid chroming bath has been used no addition of hydrochloric acid should be made to the hypo bath for some time, and then a diluted solution should be added in successive portions. It is nevertheless customary in many factories to add all the acid to the hypo bath and to insert the goods quickly when sulphur begins to be precipitated. The ordinary colour changes are from yellow to olive-brown, green, and finally to a uniform blue throughout their substance, and when this has been accomplished the tannage may be considered complete, but it is always advisable to stop the motion but leave the goods for some hours, preferably overnight, in the excess of undecomposed thiosulphate. This not only ensures complete reduction and deposition of the basic chrome salt, but also permits the thiosulphate to act as a “ neutralising ” agent (p. 355) in virtue of its weakly alkalineTHE TANNAGE OF OHEOME LEATHER 219 character. Occasionally a fresh bath of thiosulphate without acid is used for this purpose, and it will be seen at any rate that an excess of thiosulphate is always an essential for this bath. The concentration of the bath is also a matter o£ rather more importance than in the case of the chroming bath, and greater dilution is desirable to prevent the loss of sulphur dioxide to the air. For the same reason also the temperature should be kept low. The exact quantity taken varies in different works from 10 to 20 per cent, of the pelt weight, according to the mode of application and nature of the goods. In the original Schultz bath the quantities specified were 10 per cent, thiosulphate and 5 per cent, hydrochloric acid, and if the acid is added gradually and the solution dilute, this is quite sufficient for light goods, but where the acid is added quickly, and in the case of heavier skins and hides, it is desirable to use from 12 to 15 per cent., and even more, of thiosulphate, with sufficient acid to complete thoroughly the colour changes mentioned above. This can generally be accomplished by a weight of acid about half the weight of the thiosulphate used, but is somewhat less when the acid chroming bath is employed. It is often usual to employ as a preliminary “dip” a solution of thiosulphate (occasionally acidified if the acid chroming bath has not been used) through which the goods are drawn and then piled or horsed to drain. The advantage of this procedure is in both preventing “ bleeding ” and drawn grain. If the former is chiefly feared a somewhat strong solution is used, if the latter, a weak solution is desirable. The goods are subsequently reduced in a hypo bath of ordinary strength. Many substitutes for sodium thiosulphate have been suggested or patented for use in this bath, including sulphuretted hydrogen, acidified sulphides and polysulphides, sulphites, bisulphites, hydrogen peroxide, nitrous acid, lactic acid, etc.; but none are so easy to manipulate or so satisfactory as the thiosulphate. The one-bath process bears some resemblance to the vegetable tannages, not only in its fundamental basis, but also in the principles of its practical application. In the first place the220 THE MANUFACTUEE OF LEATHEE results differ considerably according to the basicity of the salt employed, and this effect of basicity was considered analogous to the varying astringency of the vegetable tannins. The more acid salts penetrate rapidly and evenly through the goods without drawing the grain, and also swell considerably; but a very light tannage is given and much of the chrome salt will wash out again. The more basic salts on the other hand tan more slowly, the acid portion of the hydrolysed salt rapidly colouring through as before, but the full tannage proceeding slowly from the surface. The tannage in this case, however, is both heavier and more permanent. If very basic salts are employed the surface of the pelt becomes overtanned, an even more basic salt being deposited, and only a more acid salt penetrates into the interior. The tannage, therefore, is uneven, and the resulting leather has a brittle or tender grain. The very basic salts, like the astringent tannins, have also the effect of drawing the grain. The degree to which the salts of chromium should be made basic in order to produce the fairly quickly penetrating, even and irreversible tannage varies with the nature of the acid radicle. In the case of chrome alum, which is one of the commonest salts used for this process, the best results are obtained by adding sufficient soda to form the so-called “ normal basic salt” Cr(0H)S04, but in the case of chromic chloride the basicity must be made distinctly greater to produce the same results, the salt Cr2Cl3(OH)3 in which half the acid has been neutralised being found satisfactory for many purposes. It will be readily understood, therefore, that the addition of common salt to a sulphate liquor has the effect of reducing the apparent basicity of the solution and of causing a decreased absorption of chrome, and that similar effects, one way or the other, will be caused by the presence of other neutral salts. In the cases of tartrates and lactates this apparent reduction in basicity has been found to be specially marked. The influence on basicity of the condition of the green goods is also a point to be considered. It is generally considered best to have the pelt in a neutral or slightly acid condition ; but when a somewhat firm result is desired the deliming need only be on the surface, and a more basic tannage will then be obtained without any danger to theTHE TANNAGE OF OHEOME LEATHER 221 grain. It is common also for the prevention of drawn grain to use a bath of alum, or of alum and salt, and in some cases flour is also employed and the goods dried out and “aged” as in tawing (p. 240), If incompletely delimed goods are placed in such a bath a slight basic alum tannage will be given. Pickling is also often practised, but it is perhaps most usual in this case to depickle in order to give the alum and salt treatment. They may be depickled with salt and whitening, salt and soda, alkaline O.T. bate, or basic alum salts. Goods may also be tanned without depickling, in which latter case the effect of the pickling acid in reducing the basicity of the solution will have to be annulled by a corresponding increase in the basicity of the chrome liquors. Another respect in which one-bath chrome tanning resembles the vegetable tannage is in the necessity for a gradual increase in the strength of the liquor, immersion in a strong liquor producing drawn grain and overtannage on the surface, which causes both brittleness and slow penetration. The comparative rate of increase may be much greater, however, in the case of the mineral tannage owing to its powers of quick penetration, the whole tannage often involving only a few hours, and never more than a few days. In the case of light goods (goat, sheep, etc.), the tannage is most conveniently brought about in paddles, though perhaps more economically in slowly revolving drums. Drums are usually used for heavier goods (calf, hides, etc.), but for the heaviest work (sole and strap butts, etc.) the tannage is often in pits or vats by suspension. Whichever class of goods are being tanned, and whichever method is employed, there are two methods of adding the chrome solution; either the same liquor may be used throughout the tannage, and its concentration increased gradually by the slow addition of a strong stock solution, or a series of different liquors may be given whose concentration is increasingly greater. Where the first method is adopted some care and economy is necessary in exhausting the liquor that is left when the tannage is complete. If the second method is employed with paddles a three-paddle system may be worked as in the sumach tannage of goat skins (p. 204); if used with drums, a three-drum222 THE MANUFACTURE OF LEATHER system may also be utilised, but the goods should remain in the same drum during the whole of the tannage and the liquors run off and pumped on to another pack of goods in another drum; if used in pits, the best plan is to work the liquors like a round of suspender-handlers as in vegetable tanning. In many cases it is convenient to combine these two methods, and it is clear also that systems of more than three paddles or drums may be employed. The mode of penetration of the chrome salts through the pelt has given rise to much discussion, but the following view was held and taught by Procter for many years, and has recently been confirmed by the experimental work of Stiasny. The chromic salts are well known to be hydrolysed in aqueous solution, and the work of Denham has shown that one can express the course of the hydrolysis by the following typical equations:— (1) Blue chromic sulphate—moderately diluted. Cr2(S04)3 + H20 ■---> 2 Cr(0H)S04 + H2S04. (2) Blue chromic sulphate—largely diluted. Cr2(S04)g + 2 H20 ——* Cr2(0H)4S04 + 2 H2S04. (8) Green chromic sulphate—moderately diluted. ^Cr4(S04)r| (S04)2 + 2 Ha0^ZZ^.rCr4(S04)T| (0H)2S04 + H2S04. (4) Green chromic sulphate—largely diluted. Cr4(S04)4"j (S04)2 + 4 H20 Cr4(S04), j (OH)4 -f- 2 H2S04. The solutions contain, therefore, a basic chromium salt and free acid, and the view of Procter and Stiasny is that these act upon the hide fibres in different and totally unconnected ways. The free acid is in crystalloidal solution, penetrates quickly into the fibres, and is reversibly adsorbed by them. The basic salt changes by polymerisation into colloid complexes which, however, possess still considerable diffusive power, though much less of course than the free acid. ThisTHE TANNAGE OF CHROME LEATHER 223 basic part is also adsorbed gradually and undergoes the change of condition from “ sol ” to “ gel ” form, by which the irreversibility of the adsorption is caused. The separation from the solution of both products of hydrolysis effects a further hydrolysis in the sense of the above equations and further adsorption. Hence in the beginning of the tannage much free acid and little basic salt is taken up by the pelt, but in the further course of tannage the reception of the basic part becomes always relatively stronger, so that in practice the basicity of the tanning liquor at first quickly increases and then gradually decreases again. The reversibly adsorbed free acid can be removed by washing, but this is usually brought about in practice by “ neutralising ” (p. 355) with weak alkalies. Chrome liquors are also made “ basic ” before use by the addition of alkalies, which effects also a neutralisation of the free acid formed by hydrolysis. Basic chrome liquors may be made in a variety of ways, the simplest perhaps of which is to add a solution of soda crystals to a solution of chrome alum. If the soda is added quickly chromium hydrate Cr(OH)3 will, of course, be precipitated, but if the solution is poured in slowly and with constant stirring any precipitate of hydrate completely redissolves and a clear liquor is obtained. Sufficient alkali should be added to produce the salt Cr(0H)S04—i.e., 1 molecule of chrome alum K2SO4, Cr2(S04)324H20 (mol: wt. 998) will require 1 molecule of soda crystals Na2C03, H20 (mol. wt. 286). Hence for every ten parts by weight of chrome alum used, 2*86 parts of soda crystals (1*06 parts of anhydrous carbonate) must be employed. It is convenient in practice to make a 10 per cent, stock solution of this liquor—i.e., 10 lbs. of chrome alum are dissolved in 8 gallons of water, the soda solution added, and the total volume made up to 10 gallons. The soda may be dissolved in a small quantity of boiling water and the solution afterwards diluted to about a gallon with cold, but the chrome alum should be dissolved only in cold or tepid water, as in hot solution it splits up into a basic salt, and an acid salt, and complications in the tannage would arise therefrom. Each 100 lbs. of pelt requires fully 10 lbs. of chrome alum for a satisfactory tannage, and hence will224 THE MANUFACTURE OF LEATHER take 10 gallons of the stock solution. Eitner recommends neutralising the alum with hypo and boiling off the sulphur dioxide liberated. In the addition of soda there is no need to boil off the carbon dioxide, and this process is therefore easier and safer as well as cheaper. Procter’s glucose liquor is made up by dissolving 8 lbs. of potassium dichromate in a convenient amount of water, adding 2*5 lbs. of concentrated sulphuric acid (or 6 lbs. hydrochloric) and then glucose of good quality until the reduction is complete and a green solution obtained free from any yellowish tinge. About 3*5 lbs. of the sugar are usually required. A brisk effervescence occurs in the reduction due to the evolution of carbon dioxide, and it is necessary therefore to use a vessel of fair capacity. Other sugars—dextrin, glycerin, alcohol, organic acids and other substances may be used in place of glucose. The disadvantage of this liquor is that oxidation products of the glucose are of an uncertain nature, and are liable to affect the tanning properties of the solution to a varying extent. Practice has shown, however, that this bath will yield a plump and mellow leather from calf and hide. If the quantities mentioned above are taken, and the liquor made up to 10 gallons, the liquor will be of approximately the same strength as the basic chrome alum liquor before described—i.e.9 the 10 gallons will be sufficient for 100 lbs. of pelt. Many concentrated solutions of basic chrome salts are now on the market as “ chrome extracts,” that of the Martin Dennis Co.—“ Tanolin ”—being, of course, the oldest. It originally consisted merely of a basic chrome chloride, but now contains sulphates and other salts. Other preparations are also made by this firm. About 3 gallons of “ Tanolin ” are required for 100 lbs. pelt. Other preparations are “ Vul-canochrom,” “ Corin,” “ Chromalin,” “ Progress tan liquor,” and “ Chromatine.” They are of somewhat similar concentration, but contain usually more sulphates and chlorides, and often also aluminium salts. Eitner has also made some preparations, “ Cromast,” “ Cromul,” and “ Cromar,” containing organic matter, which is supposed to modify the annage and render it suitable for heavy leathers, upperTHE TANNAGE OF CHROME LEATHER 225 leathers, and light leathers respectively. In this country bought liquors are not very much used. General qualities of chrome leather.—On this subject much might be said, but perhaps one of the greatest advantages of the chrome tannage lies in the quickness of the process when compared with the vegetable tannages. This enables the tanner to have a large turnover. Its waterproofness (after fat-liquoring, etc.) and compactness of substance have also assisted in making it extremely popular for upper leathers (box calf, glacé kid, etc.). Another great advantage of chrome leathers is in their great tensile strength as compared with the vegetable tanned leathers. This quality makes the tannage exceedingly suitable for picking bands, belt and strap leathers. On the other hand, the tannage is “ empty,” i.e., it is apt to yield not merely light weight, but leathers devoid of solidity and firmness, and as a rule, therefore, there should be as little loss of hide substance as possible in soaks and limes, and the fermentation processes of deliming should, generally speaking, be substituted by the mere neutralisation of the lime with acids. Goat skins for chrome work, however, should be thoroughly puered to produce smooth grain. The isolation of the fibres is usually greater with chrome than with the vegetable tannages, and this causes a woolliness on the flesh side, which makes the goods unsuitable for finishing on this side. It also causes an unfortunate tendency to stretch, which is awkward in the case of belt and boot leathers. This defect can be minimised by using quick liming processes with sulphides, in which the hide fibres are not so much split up into their smaller constituent fibrils. The isolation of the fibres and contraction of the skins makes the substance more compact, however, and this is an advantage for sheep leathers. Different kinds of skin take up the chrome tannage in different degrees. Sheep skins, horse hides, and kips are apt to fix little chrome, but goat skins, ox hides, and especially calf skins, distinctly more. The basicity of the one-bath chrome liquors should therefore be modified accordingly. In the two-bath process the differences in absorption are less marked. The avoidance of drawn grain is also a difficulty, more especially with goat skins, and when formed M.L. Q226 THE MANUFACTURE OF LEATHER it can never be removed by mechanical treatment. Loose grain is a further difficulty which can only be satisfactorily overcome by a combination tannage of chrome and vegetable materials. Equally good leather is produced by “ two-bath ” and “one-bath ” processes, but the latter is the cheaper process, and is usually more convenient to manipulate. It has also the advantage that it does not give rise to the painful chrome sores which are very liable to occur with those who work in chromic acid liquors. The two-bath process is usually considered to give a more mellow tannage and a better colour than the one-bath, and this is generally accounted for by the Fig. 59.—Tanning padille. presence of free sulphur in the leathers tanned by the former process. Broadly speaking, the two-bath process is more suitable for the lighter leathers. It is unsuitable for leathers that are to be japanned (p. 880). Combined one-bath and two-bath processes have been suggested in which chrome alum and potassium dichromate are employed together and thiosulphate used later. These processes, however, are not very widely used. It is more important for all chrome tannages to have the raw materials free from defects, warbles, scratches, etc., as these show more in the finished goods than with the vegetable tannages. A brief description of some chrome tannages will now be given, all percentages having reference to 100 lbs. of wet untreated pelt, except where stated otherwise.THE TANNAGrE OF CHROME LEATHER 227 Calf skins and hides (for box calf, box sides, willow calf, enamelled calf, etc.) may be tanned by any of the following methods, but in this country the one-bath processes are more generally employed. Hides are cut into sides after unhairing, and are often split whilst full of lime (“green-splitting”), the grains being then tanned with chrome, whilst the fleshes are vegetable tanned and finished for waxed splits (see p. 30B). It s, however, common to split after chrome tanning or colouring through, and the fleshes are then often re-tanned in the drum with extracts before currying. (1) The following one-bath process is as cheap and satisfactory as any. The skins—after puering and drenching—are tanned by the basic chrome alum liquor. The liquor is made up with 10 per cent, of chrome alum on the pelt weight of each pack, to which has been added 3 per cent, of soda crystals (p. 223). The three-drum system mentioned above may be used for tanning. The green pack should receive first a liquor which has been used for two previous packs. It is then treated with a once-used liquor, and afterwards with a new liquor made up from chrome alum as described. A new liquor is therefore made for each pack of goods and exhausted by two following packs. A period of three to four hours in each liquor should be quite sufficient. By lengthening the time of drumming in the last and new liquor, and by adding some of the chrome solution after a time, it is quite practicable to work a two-drum system effectively. If, however, it is desired to work with one drum only, the goods should be started in sufficient water with a portion only of the chrome liquor, the rest of which should be gradually added at intervals of an hour or two. In this case also it is necessary to use 20 per cent, of chrome alum and 6 per cent, of soda to obtain the necessary strength of liquor for complete tannage. The liquor which is left over, strengthened with 10 per cent, chrome alum and 3 per cent, soda, may be used for the next pack in a similar manner. This tannage is also satisfactory for goods which have been delimed by puering only, by drenching only, or by pickling. Sometimes goods are placed in a bath of alum and salt (5 per cent, of each) just before tannage, and, indeed, are occasionally q 2228 THE MANUEACTUBE OF LEATHER delimed by means of it, but it is doubtful whether there is any advantage gained by such treatment. The process is also satisfactory for goods which have just been delimed on the surface with lactic acid, but is apt to give a rather hard feel to the grain. (2) Excellent chrome calf is also made by tanning with Procter’s glucose liquor (p. 224), using 5 per cent, sulphuric acid, 6 per cent, potassium dichromate, and 7 per cent, of glucose on the pelt weight. This corresponds really to the use of about 20 per cent, of chrome alum. The liquor is more troublesome to prepare, and requires more care, and is therefore not so much used as formerly. The mechanical operations of this tannage may be just the same as described for the basic chrome alum liquor. ' (3) In another one-bath method the skins are placed in a paddle with sufficient water to cover, and 5 per cent, alum and 10 per cent, salt are added. The goods are run for half an hour, and rather less than a gallon of the 10 per cent, basic chrome alum stock solution is added to the paddle liquor and the goods run another half-hour. Another gallon of stock solution is now added, and the goods run for one hour. Two gallons of stock solution are now added, and the paddling continued again for an hour. This last procedure is repeated until the tannage is complete, which will not be satisfactorily accomplished in less than 12 hours. The liquor left over may be used for the next pack of green goods if some alum and salt are added to it, but this must not be done more than twice. (4) A two-bath process can be carried out in the following manner: The skins, after drenching, are placed in a solution of 10 per cent, salt in 15 gallons per cent, of water, paddled for 15 minutes, a diluted solution of 2 per cent, hydrochloric acid gradually added, and the paddling continued another quarter of an hour. The pickled skins are drummed half an hour in a solution of 2 per cent, potassium dichromate in 12 gallons per cent, water, and a solution of 4 per cent, potassium dichromate and 2J per cent, salt in 15 gallons per cent, water is added to the drum and the drumming continued till the goods are struck through, which will be in about four hours. They are then horsed upTHE TANNAGE OE CHROME LEATHER 229 overnight, struck out on the vertical table machine (p. 888), and passed through the hypo dip. This is made by dissolving 81 per cent, thiosulphate in 18 gallons per cent, water. The main reduction bath is made up with 10 per cent, thiosulphate in 25 gallons per cent, water, and 5 per cent, hydrochloric acid is added. When the liquor turns milky the skins are quickly thrown in and the paddling continued for the rest of the day. The goods are left overnight in the liquor. (5) Another method is to put the drenched skins without pickling into a paddle liquor of 5 per cent, potassium dichromate, 2| per cent, hydrochloric acid, and 5 per cent, salt in 12 gallons per cent, water, and reduce as before. The chroming should require about six hours’ paddling and the reduction about four hours’ paddling. If chromic acid is employed 4 to 5 per cent, is taken and no hydrochloric acid is added, but the salt may be increased with advantage up to 10 per cent. (6) Where the skins have been merely delimed with acid the first bath may advantageously consist of 5 per cent, dichromate, 5 per cent, hydrochloric acid, 1 to 8 per cent, aluminium sulphate, and 5 to 10 per cent. salt. The reduction bath should then contain 12 to 15 per cent, thiosulphate, and after the insertion of the goods and some paddling 4 to 5 per cent, of hydrochloric acid should be added slowly. Pickled goods may also be placed directly in the above chroming bath, if the quantity of salt be increased somewhat; and if the quantity of acid be increased, goods which have not been delimed may be placed in this bath. (7) A further two-bath method is to give a pickle of 5 per cent, aluminium sulphate, 7J per cent, salt, and 8 per cent, sulphuric acid in 6 gallons per cent, of water. The goods are then dried out for any sorting and trimming. They are wet back by handling for 15 minutes in a solution of 5 per cent, salt in 6 gallons per cent, water, and are then passed in the neutral chroming bath, in which 6 per cent, of dichromate is employed. The second bath should contain 15 per cent, thiosulphate and 4J per cent, hydrochloric acid. (8) Combined one-bath two-bath processes have also been used with this class of goods with some success. The author has made good chrome calf with a chroming bath of 20 per230 THE MANUFACTUEE OF LEATHEE cent, chrome alum, made basic with 6 per cent, soda, to which 2 per cent, of potassium dichromate had been added. The goods are started in a little water and the chroming liquor added gradually over a period of six hours, and after remaining in the bath altogether for 10 to 12 hours they are horsed up overnight in a brownish condition. They are then reduced with 5 per cent, thiosulphate and 2^ per cent, hydrochloric acid. Good results are also obtained if the glucose liquor be employed instead of the chrome alum liquor. Two per cent, of dichromate are added to a liquor made from 10 per cent, sulphuric acid, 12 per cent, dichromate and 14 per cent, glucose. The goods are chromed with this and reduced as before. The chroming liquor may be strengthened with half quantities, and used again for the next pack, but it is desirable to control the chroming process by frequent analysis of the liquor. Goat skins (for glacé kid, dull kid, etc.) are perhaps most usually tanned by the two-bath process, but any of the following methods will give satisfactory results. (1) After puering, they are washed for a short time in boric acid, drained, and weighed. They are then pickled in hydrochloric acid and salt and chromed in drum precisely as in process (4) for chrome calf, horsed up overnight to drain, and struck out by machine. The hypo dip through which they are next passed consists of 4 per cent, thiosulphate in 15 gallons per cent, water. After being again horsed to drain for a short time, they are put into a reduction bath of 10 per cent, thiosulphate in 20 gallons per cent, water. The skins are inserted quickly when the liquor turns milky, and, after paddling for some hours, are left overnight in the liquor. (2) In another two-bath method the skins are both puered and drenched, but not pickled. A paddle is then filled with water, and a solution containing 1 per cent dichromate and J per cent, hydrochloric acid is added, and the goods inserted and paddled for some hours. The bath is then strengthened with 5 per cent, dichromate and 2J per cent, hydrochloric acid slowly added over two hours, the paddling is continued another six hours, and the goods then horsed up overnight. After passing through a somewhat strong hypo dip, they areTHE TANNAGE OE CHROME LEATHER 231 reduced with 18 per cent, thiosulphate and 6 per cent, hydrochloric acid. The goods are inserted when the liquor turns milky, and more acid is added later if necessary. (8) Where the skins are received in the pickled state they may be depickled in salt and whitening and possibly a little bicarbonate of soda, and afterwards washed, puered, and scudded. They may then be chromed with 6 per cent, dichromate and 8 per cent, hydrochloric acid, or with 5 to 6 per cent, chromic acid and 5 to 8 per cent, salt, and then reduced with 12 to 15 per cent, thiosulphate and 6 to 8 per cent, hydrochloric acid, the skins being inserted when the liquor turns milky. (4) If the acid chroming bath is desired the skins should be first well depickled in salt and whitening. They are then placed in sufficient water in a paddle, and a solution of 4 per cent, potassium dichromate and 4 per cent, hydrochloric acid slowly added. In reducing the bath should consist of 12 per cent, thiosulphate, and the goods should be paddled in this for 20 to 80 minutes, then 5 per cent, hydrochloric acid should be added gradually in 8 to 10 small portions, more rapidly for small light skins and more slowly in the latter half of the operation. (5) Another good two-bath tannage for goat skins is to chrome with 5 per cent, dichromate, 4 per cent, hydrochloric acid, and 2 J per cent, aluminium sulphate. This is practically an acid bath, and hence reduction is with a bath of 20 per cent, thiosulphate, to which 5 per cent, hydrochloric acid is slowly added. (6) In using the one-bath process it is often advisable to give a preparatory weak alum tannage. The skins are drummed in a liquor containing 8 per cent, aluminium sulphate, 4 per cent, sodium sulphate, and 5 per cent, salt1, for three-quarters of an hour, and then horsed to drain for a few days, or possibly even dried out. Flour may be used in this bath, and in some cases also egg yolk and olive oil, and the goods dried out and “aged ” as in tawing (p. 240). They are now wet back and drummed in water for three-quarters of an hour. The chroming liquor (10 gallons per cent, basic chrome alum liquor, or 1 8 to 9 per cent, of salt may be used, and the sodium sulphate omitted.232 THE MANUFACTURE OF LEATHER 8 gallons per cent. “ Tanolin,” etc.), is now diluted somewhat and divided into three portions. One portion is added to the drum, the drumming continued for half an hour, a second portion added, and the drumming continued for another half-hour. The last portion is now added and the goods drummed until the tannage is complete, which will be in a few hours. (7) In some cases it may have been desirable to pickle1 with salt and acid, and if so the skins may be first wet down by drumming in a solution of 10 per cent, salt in 5 gallons per cent, water for five minutes. The depickling is then brought about by a solution made by dissolving 8 per cent, aluminium sulphate in 5 gallons per cent, water, and slowly adding to it, with constant stirring, a solution of 8 per cent, sodium carbonate in 5 gallons per cent, water. This solution is then added to the drum, and the goods drummed half an hour. The chrome liquor may then be added to the goods in the same drum and liquor as in the one-bath process just described. When the tannage is complete it is often customary to add J per cent, of sodium bicarbonate, drum another half-hour, and leave the goods in the liquor overnight. (8) In many cases, however, it is sufficient to drum the skins for 20 minutes in a solution of 10 per cent, salt in 5 gallons per cent, water, and to add the chroming liquor gradually over a space of three hours. When the tannage is complete, the drumming is stopped, and the goods left overnight in the liquor. Sheep skins (for imitation glacé, etc.) are treated somewhat similarly to goat skins, but it is essential to degrease these skins by pressure or by extraction. (1) If tanning by the two-bath process the skins may be placed in the drum with sufficient water, and a solution of 5 per cent, dichromate and 2J per cent, hydrochloric acid and up to 5 per cent, salt is gradually added to the drum. Motion is given till the chrome has evenly struck through, which occurs in a comparatively short time with these goods. Chromic acid may also be used, 4 to 5 per cent, being taken 1 Pickles for chrome may be made fairly strong ; in some American factories 5 per cent, of vitriol and 20 to 2o per cent, salt are employed, whereas for dongola 1 per cent, acid and 10 per cent, salt is sufficient.THE TANNAGE OE CHEOME LEATHEE 233 with no acid but somewhat more salt. After horsing to drain and striking out they are reduced with 12 to 15 per cent, thiosulphate, and 3 per cent, hydrochloric acid is gradually added to the paddle liquor. If the skins are pickled, they may be depickled with salt and whitening or with the basic aluminium sulphate solution as described for pickled goatskins. (2) If it is desired to use an acid chroming bath the following will be found to give good results : 5 per cent, dichromate, 5 per cent, hydrochloric acid, and 10 per cent, salt in 40 gallons per cent, water. When the skins are thoroughly struck through they are horsed up overnight and reduced in 20 per cent, thiosulphate to which 5 to 7 per cent, hydrochloric acid may be slowly added after about half an hour’s paddling. (3) If tanning by the one-bath process the skins are nearly always tawed first in alum and salt. They are depickled therefore with salt and whitening or C.T. bate and soda.1 Then they are drummed in a solution of 3 per cent, aluminium sulphate, 6 to 8 per cent, salt (and possibly 4 per cent, sodium sulphate) for three-quarters of an hour, and horsed up to drain as long as possible. They are then tanned by the gradual addition of the basic chrome liquor as described for goatskins. It is also possible to taw as described and proceed immediately with the chrome tannage by adding the chrome to the drum in the usual way. If the skins have been wet down in a solution of 10 per cent, salt in 5 gallons per cent, water, and depickled with the basic alum solution, the chrome tannage may be commenced after 30 minutes’ drumming. This one-bath process is considered to give the best results for this class of goods. Sheep skin splits for “ chrome-chamois ” may be given the following tannage. After puering heavily, they are placed in a pickle of 6 per cent, sulphuric acid and 24 per cent. salt. They are paddled in this for three-quarters of an hour and 4 per cent, of potassium dichromate is added in two portions, the goods being run till well struck through. After horsing up overnight they are reduced with 15 per cent, thiosulphate, to which is 1 1 lb. of soda to every 2 lbs. bate.234 THE MANUFACTURE OF LEATHER added, after one hour’s paddling, 1 per cent, of hydrochloric acid. The neutralisation (p. 855) is with 1 per cent. soda. Picking band butts are now extensively chrome tanned, and have largely substituted the vegetable product on account of their greater durability, tensile strength and cleanliness. (1) After a very short sulphide liming they are bated (or delimed with acids), drenched, and pickled in 10 per cent, salt and 10 per cent, sulphuric acid. The pickle should be about 6° Be., and the goods are drummed in it for three to four hours, left overnight, drummed again for two hours, and horsed up. The goods then receive the acid chroming bath at 1° Be., in which 2 per cent, dichromate and 4 per cent, hydrochloric acid are employed together with some salt. They are drummed in this two hours, and the penetration of the chrome completed by giving two hours on the horse. The hypo bath consists of a vat at 10° Be., which may be used repeatedly for a week or two if strengthened up with 8 per cent, thiosulphate for each pack. The' goods remain in this bath about six hours, and no further neutralisation (p. 855) is needed. (2) Another two-bath process is to delime with lactic acid and chrome immediately in 2 to 5 per cent, dichromate, 2 to 5 per cent, hydrochloric acid, 4 per cent, aluminium sulphate, and 15 per cent, salt, and reduce in 5 per cent, thiosulphate to which 2 per cent, hydrochloric acid is slowly added. (8) A heavier tannage may be given by chroming the delimed goods in 6 per cent, dichromate, 6 per cent, hydrochloric acid, and 10 to 15 per cent. salt. The goods are inserted in the hypo bath (15 per cent, thiosulphate) for three hours, and 8 to 5 per cent, hydrochloric acid is gradually added. (4) A one-bath process may be carried out in the following manner: The butts are bated fairly low, scudded well, and bate shaved. The tannage is accomplished by suspension in a round of four large vats, giving three days in each vat. The liquors should be kept up to their strength by the addition of basic chrome liquor at the end of the first and second days. The barkometer may be used to judge this. At the end of the third day a shift is given; the tail liquor is run down the drain and a new liquor made up in the same vat, and theTHE TANNAGE OF CHROME LEATHER 235 goods shifted round, the old second liquor being used as the first without any strengthening. The tannage therefore lasts 12 days, but an extra vat may be given if necessary. If the basic chrome alum liquor is employed, the strongest vat should never take above 16 gallons of this per 100 gallons of water. A little aluminium sulphate may be beneficial, but salt should be avoided. If “ Tanolin ” is employed, the head liquor should be a 5 per cent, solution, and the other liquors of 4, 8, and 1A per cent, strength respectively. (5) The combined one-bath and two-bath process can be used for picking band butts. According to Eitner, the liquor is made with kilos. (7% lbs.) chrome alum and 150 grams (5 ozs.) dichromate per butt. The goods are drummed in a good quantity of water, half this liquor added, and the drumming continued for three hours, and then stopped for an hour. The second half of the liquor is now added, and the goods run three hours and stopped one hour as before. Further drumming is given if necessary. The butts are olive green, and the reduction is completed with 800 grams (If lbs.) of thiosulphate, and 500 grams (just over 1 lb.) of whitening per butt. Motor butts, strap butts, and harness backs, are delimed with acids, or very lightly bated, and are tanned out in suspension as in the one-bath process for picker band butts. They may also be tanned in drums, but the two-bath process is not found to give the best results with these leathers. It has been found an excellent plan with these goods to give them a bath of normal chrome alum of 1° Bkr., in which they are handled twice a day for a week, and then to proceed with the one-bath tannage. Sole butts1 should be only surface delimed, in drum or pit, and then inserted into a solution of 6 per cent, aluminium sulphate, and when this has penetrated they are placed in a solution of 8 per cent, thiosulphate for 24 hours, which not only fixes the previous alum tannages, but causes a deposition of sulphur which is beneficial both to the quality and colour of the goods. The butts are then tanned out with 1 The one-bath process is usually employed for these goods, but where colour is of importance, e.g., for tennis boot soles, &c., the two-bath process is an advantage.236 THE MANUFACTURE OF LEATHER the one-bath process. If this is done in drums, the basic liquor (10 gallons per cent.) is added in three portions, one in the early morning, a second in the late afternoon, and the third next day, the tannage being complete before that day is over. It is, however, better to give a longer time. Another method is to have a series of five pits of 10°, 20°, 30°, 40°, and 50° Bkr. respectively. The goods should be 2 days in the first two pits and 1 day in each of the rest. The Analytical Control of the chrome tannages is much simpler than the vegetable tannages on account of the definiteness of 1 he materials. In the two-bath process the composition of the liquors may be readily determined as follows :— (1) 10 cc. of the liquor are placed in a \ litre stoppered bottle, 5 cc. concentrated hydrochloric acid, and 10 cc. of 10 per cent, potassium iodide solution are added. After shaking and allowing to stand a few minutes, the liquor is then titrated with N/10 sodium thiosulphate in the presence of 1 cc. freshly made starch infusion until the intense blue colour due to the liberated, iodine has given place to the pale green of the chromic chloride. Each cc. of thiosulphate solution corresponds to 0*0033 grams CrOs, 0*0049 grams K2Cr207, or 0 00647 grams K2Cr04. (2) 10 cc. of the liquor are titrated with N/10 caustic soda in the presence of phenol-phthalein. Potassium chrolnate is neutral to phenol-phthalein, and chromic acts therefore as a dibasic acid. If the acid chroming bath is being used the excess of hydrochloric acid will also be determined here. More indicator should be added in the course of the titration on account of its oxidation by the chromic acid. Each cc. of soda solution corresponds to 0*005 grams free CrOg, 0*01 grams “ half bound ” Cr03 (t.e. present as K2O2O7), 0*0147 grams K2Cr207, or 0*00365 grams HG1. If determination (1) takes a cc. N/10 thiosulphate, and determination (2) b cc. N/10 caustic soda, the nature of the liquor can be gathered from the table on opposite page. If aluminium salts are present these calculations will not apply accurately, and if chromic salts are present the analysis becomes exceedingly complicated.THE TANNAGE OE CHROME LEATHER 237 (3) 10 cc. of the liquor are made neutral by the addition of magnesia and titrated with N/10 silver nitrate until the brick-red of silver chromate permanently appears. Each cc. of silver nitrate solution corresponds to 0*00745 grams KC1, 0*00585 grams NaCi or 0*00365 grams of hydrochloric acid. if— Liquor consists of— Hence 10 cc. liquor contain— b is less than £ a potassium chromate and potassium dichromate (b x 0*0147) gms. K20r207, and I (a X 0-0033) - (b x 0-01) j 1*94 gms. K20r04. b is equal to J a potassium dichromate only (a X 0*0049) gms., or (b X 0*0147) gms. K2Cr207. b is greater than a, but is less than § a potassium dichromate and chromic acid (b — £ a) X 0*01 J gms. Cr03, and (a X 0-0093) - — la) 0-01J j 1*47 gms. 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Treatise on Optics....................................i2mo, 1 50 O’Connor, H. The Gas Engineer’s Pocketbook............i2mo, leather, 3 50 —— Petrol Air Gas.................................................nmo, *0 75 Ohm, G. S., and Lockwood, T. D. Galvanic Circuit. Translated by William Francis. (Science Series No* 102.)..............i6mo, o 50 Olsen, J. C. Text-book of Quantitative Chemical Analysis..........8vo, *4 00 Olsson, A. Motor Control, in Turret Turning and Gun Elevating. (U. S. Navy Electrical Series, No. 1.)................i2mo, paper, *0 50 Ormsby, M. T. M. Surveying.................................... i2mo, 1 50 Oudin, M. A. Standard Polyphase Apparatus and Systems.............8vo, *3 00 Owen, D. Recent Physical Research.................................8vo, *1 50 Pakes, W. C. C., and Nankivell, A. T. The Science of Hygiene . .8vo, *1 75 Palaz, A. Industrial Photometry. Trans, by G. W. Patterson, Jr. . 8vo, *4 00 Pamely, C. 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Ledeboer..........................................8vo, *1 50 Pettit, Lieut. J. S. Graphic Processes. (Science Series No. 76.)... i6mo, o 50 Philbrick, P. H. Beams and Girders. (Science Series No. 88.)... 16mo, Phillips, J. Gold Assaying.......................................8vo, *2 50 —— Dangerous Goods...............................................8vo, 3 50 Phin, J. Seven Follies of Science..............................12010, *1 25 Pickworth, C. N. The Indicator Handbook. Two Volumes.. i2mo, each, 1 50 —— Logarithms for Beginners............................12 mo. boards, o 50 ----The Slide Rule.............................................12 mo, 1 00 Plattner*s Manual of Blow-pipe Analysis. Eighth Edition, revised. Trans. by H. B. Cornwall.......................................8vo, *4 00 Plympton, G. W. The Aneroid Barometer. (Science Series No. 35.) i6mo, 050 —— How to become an Engineer. (Science Series No. 100.).........i6mo, o 50 ----Van Nostrand*s Table Book. 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(Outlines of Industrial Chemistry).................................................8vo, *2 on Practical Compounding of Oils, Tallow and Grease................ 8vo, *3 50 Pratt, K. Boiler Draught.......................................12010, *1 25 ——High Speed Steam Engines.......................................8vo, *2 00 Pray, T., Jr. Twenty Years with the Indicator....................8vo, 2 50 ----Steam Tables and Engine Constant.............................8vo, 2 00 Prelini, C. Earth and Rock Excavation............................8vo, *3 00 ----Graphical Determination of Earth Slopes................... 8vo, *2 00 ----Tunneling. New Edition.....................................8vo, *3 00 ----Dredging. A Practical Treatise........................... 8vo, *3 00 Prescott, A. B. Organic Analysis................................8vo, 500 Prescott, A. B., and Johnson, O. C. Qualitative Chemical Analysis. . .8vo, *3 50D. VAN NOSTRAND CO.’S SHORT TITLE CATALOG 23 Prescott, A. B., and Sullivan, E. C. First Book in Qualitative Chemistry. i2mo, *1 50 Prideaux, E. B. R. Problems in Physical Chemistry..................8vo, *2 00 Primrose, G. S. C. Zinc. (Metallurgy Series.)..............{In Press.) Pullen, W. W. F. Application of Graphic Methods to the Design of Structures................................................i2mo, *2 50 ----Injectors: Theory, Construction and Working....................i2mo, *1 50 ----Indicator Diagrams ............................................8vo, *2 50 ----Engine Testing .............................................. 8vo, *4 50 Pulsifer, W. H. Notes for a History of Lead........................8vo, 4 00 Putsch, A. Gas and Coal-dust Firing................................8vo, *300 Pynchon, T. R. Introduction to Chemical Physics....................8vo, 3 00 Rafter G. W. Mechanics of Ventilation. (Science Series No. 33.). i6mo, ----Potable Water. (Science Series No. 103.).....................i6mo, ----Treatment of Septic Sewage. 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