or. EORAVENTURE CHEN. ISTRY LIBRARY H THE CHEMISTRY OF _ LEATHER MANUFACTURE BY JOHN ARTHUR WILSON CHIEF CHEMIST, A. F. GALLUN & SONS CO., MILWAUKEE, WIS.; CHAIRMAN, LEATHER DIVISION, AMERICAN CHEMICAL SOCIETY American Chemical Society Monograph Series mek DE PAR TV NT The CHEMICAL CATALOG COMPANY, Jne. 19 EAST 24TH STREET, NEW YORK, U. S. A. 1923 CoPYRIGHT, 1923, BY The CHEMICAL CATALOG COMP “ All Rights Reserved Press ret J. J. Little & Ives Compas ¢ New York, U. S. A. GENERAL INTRODUCTION American Chemical Society Series of Scientific and Technologic Monographs By arrangement with the Interallied Conference of Pure and Applied Chemistry, which met in London and Brussels in July, 1919, the American Chemical Society was to undertake the production and pub- lication of Scientific and Technologic Monographs on chemical subjects. At the same time it was agreed that the National Research Council, in codperation with the American Chemical Society and the American Physical Society, should undertake the production and publication of Critical Tables of Chemical and Physical Constants. The American Chemical Society and the National Research Council mutually agreed to care for these two fields of chemical development. The American Chemical Society named as Trustees, to make the necessary arrange- ments for the publication of the monographs, Charles L. Parsons, Secretary of the American Chemical Society, Washington, D. C.; John E. Teeple, Treasurer of the American Chemical Society, New York City; and Professor Gellert Alleman of Swarthmore College. The Trustees have arranged for the publication of the American Chemical Society series of (a) Scientific and (b) Technologic Monographs _ by the Chemical Catalog Company of New York City. The Council, acting through the Committee on National Policy of the American Chemical Society, appointed the editors, named at the close of this introduction, to have charge of securing authors, and of considering critically the manuscripts prepared. The editors of each series will endeavor to select topics which are of current interest and authors who are recognized as authorities in their respective fields. The list of monographs thus far secured appears in the publisher’s own announcement elsewhere in this volume. The development of knowledge in all branches of science, and espe- cially in chemistry, has been so rapid during the last fifty years and 3 | 4 GENERAL INTRODUCTION the fields covered by this development have been so varied that it is difficult for any individual to keep in touch with the progress in branches of science outside his own specialty. In spite of the facilities for the examination of the literature given by Chemical Abstracts and such compendia as Beilstein’s Handbuch der Organischen Chemie, Richter’s Lexikon, Ostwald’s Lehrbuch der Allgemeinen Chemie, Abegg’s and Gmelin-Kraut’s Handbuch der Anorganischen Chemie and the English and French Dictionaries of Chemistry, it often takes a ereat deal of time to coordinate the knowledge available upon a single topic. Consequently when men who have spent years in the study of important subjects are willing to codrdinate their knowledge and present it in concise, readable form, they perform a service of the highest value to their fellow chemists. It was with a clear recognition of the usefulness of reviews of this character that a Committee of the American Chemical Society recom- mended the publication of the two series of monographs under the auspices of the Society. Two rather distinct purposes are to be served by these monographs. The first purpose, whose fulfillment will probably render to chemists in general the most important service, is to present the knowledge available upon the chosen topic in a readable form, intelligible to those whose activities may be along a wholly different line. Many chemists fail to realize how closely their investigations may be connected with other work which on the surface appears far afield from their own. These monographs will enable such men to form closer contact with the work of chemists in other lines of research. The second purpose is to promote research in the branch of science covered by the monograph, by furnishing a well digested survey of the progress already made in that field and by pointing out directions in which investigation needs to be extended. To facilitate the attainment of this purpose, it is intended to include extended references to the literature, which will enable anyone interested to follow up the subject in more detail. If the literature is so voluminous that a complete bibliography is imprac- ticable, a critical selection will be made of those papers which are most important. - The publication of these books marks a distinct departure in the policy of the American Chemical Society inasmuch as it is a serious attempt to found an American chemical literature without primary regard to commercial considerations. The success of the venture will depend in large part upon the measure of cooperation which can be secured in the preparation of books dealing adequately with topics of BOARD OF EDITORS Technologic Series :— IAM. DA. Noves, Editor, Harrison E. Howe, Editor, : (ERICK: WILLIAM HOSKINS, F, A. Lipsury, ARTHUR D. LITTLE, Cia REESE, C. P. TOWNSEND. American Chemical Society MONOGRAPH SERIES Other monographs in the series of which this book is a part now ready or in process of being printed or written. Organic Compounds of Mercury. By Frank C. Whitmore. 397 pages. Price $4.50. Industrial Hydrogen. By Hugh S. Taylor. 210 pages. Price $3.50. The Chemistry of Enzyme Actions. By K. George Falk. 140 pages. Price $2.50. The Vitamins. By H.C. Sherman and S. L. Smith. 273 pages. Price $4.00. The Chemical Effects of Alpha Particles and Electrons. By Samuel C. Lind. 180 pages. Price $3.00. Zirconium and Its Compounds. By F. P. Venable. Price $2.50. The Properties of Electrically Conducting Systems. By Charles A. Kraus. 415 pages. Price $4.50. The Origin of Spectra. By Paul D. Foote and F. L. Mohler. Price $4.50. Carotinoids and Related Pigments. By Leroy 8. Palmer. Price $4.50. The Analysis of Rubber. By John B. Tuttle. 155 pages. Price $2.50. Glue and Gelatin. By Jerome Alexander. 236 pages. Price $3.00. Thyrozin. By E. C. Kendall. : The Properties of Silica and the Silicates. By Robert B. Sosman. | Coal Carbonization. By Horace C. Porter. : The Corrosion of Alloys. By C. G. Fink. | Piezo-Chemistry. By L. H. Adams. Cyanamide. By Joseph M. Braham. Liquid Ammonia as a Solvent. By E. C. Franklin. Wood Distillation. By L. F. Hawley. Solubility. By Joel H. Hildebrand. Organic Arsenical Compounds. By George W. Raiziss and Joseph L. Gavron. Valence, and the Structure of Atoms and Molecules. By Gil- bert N. Lewis. Shale Owl. By Ralph H. McKee. Aluminothermic Reduction of Metals. By B. D. Saklatwalla. Absorptive Carbon. By N. K. Chaney. Refining Petroleum. By George A. Burrell, eé al. Extraction of Gasoline from Natural Gas. By George A. Burrell. The Animal as a Converter. By H. P. Armsby and C. Robert Moulton. Chemistry of Cellulose. By Harold Hibbert. The Properties of Metallic Substances. By Charles A. Kraus. The Structure of Crystals. By Ralph W. G. Wyckoff. Physical and Chemical Properties of Glass. By George W. Morey. Photosynthesis. By H. A. Spoehr. Colloid Chemistry. By The Svedberg. . Chemistry of the Treatment of Water and Sewage. By A. M. Buswell. The Chemistry of Wheat Flour. By C. H. Bailey. The CHEMICAL CATALOG COMPANY, Inc. 19 EAST 241TH STREET, NEW YORK, U.S. A. PREFACE The chemistry of leather manufacture is progressing more rapidly now than at any previous time. Much of the earlier work fasled to recognize the existence of important variable factors and has been rendered obsolete by recent investigations carried out under more highly refined conditions. In preparing this monograph, it was found necessary, for the purpose of correlating existing data, to conduct many special investigations and these are being reported here for the first time. Advance information on investigations under way in other laboratories has been obtained, wherever possible, so that the presenta- tion might be made reasonably complete to the close of the year 1922. The literature pertaining to leather manufacture is so vast and the views expressed so numerous and divergent as to make an impersonal compilation of all published papers encyclopedic in size, bewildering to the average reader, and an undertaking of questionable value. In order to fulfill the first purpose of this series of monographs, namely, to present the knowledge available in a readable form, intelligible to those whose activities may be along a wholly different line, the author has felt compelled to present the subject from his own viewpoint, making no attempt to discuss views which, in his opinion, fail to contribute anything to the development of leather chemistry. In so doing, the author is fully aware that there are others who do not share his opinions of the relative merits of various views, but he can only admit his inability to present adequately views which appear to him unsound. But, ina field so vast, there is ample room for as many volumes as there may be sides to the question worthy of presentation and it is in the preparation of additional volumes that criticism of this attitude may find its best expression. A considerable amount of space has been devoted to the histology of skin and to the physical chemistry of the proteins because of their fundamental bearing on the chemistry of leather manufacture. Descrip- tions of analytic methods and practical details of leather manufacture have been given only where they seemed necessary to make the subject clearer to chemists unfamiliar with tannery routine. Many of the ideas presented in this book were gained during a 7 8 PREFACE period of intimate association with Professor H. R. Procter, of the University of Leeds, England, who is affectionately known throughout the world as the “father of leather chemistry” and whose books on leather manufacture have ‘been the standard for the past thirty-five years. In the preparation of sections and photomicrographs, valuable assist- ance was rendered by Mr. Guido Daub, whose painstaking efforts are largely responsible for the success of this phase of the work. The sections and specimens of human skin were procured from Professor T. H. Bast, of the University of Wisconsin. Professor Arthur W Thomas, of Columbia University, supplied the skins of guinea pigs and albino rats fixed in Erlicki’s fluid. Leathers from the hides of the hippopotamus, walrus, and camel were furnished by Professor Douglas McCandlish, of the University of Leeds. Most of the remaining speci- mens were provided by the firm of A. F. Gallun & Sons Company, in whose laboratories the work was done. The interesting photographs illustrating the drying of gelatin blocks were furnished by Dr. beige Sheppard, of the Eastman Kodak Company. Grateful acknowledgment is made of the generous criticisms and suggestions given by Mrs. Marion Hines Loeb, of the University of Chicago, on the general histology of skin; by Dr. Jacques Loeb, of the Rockefeller Institute, on the physical chemistry of the proteins; and by Professor A. W. Thomas, Mr. Frank L. Seymour-Jones, and Miss Margaret W. Kelly, of Columbia University, on many important points throughout the book. The author is most deeply indebted to the late Arthur H. Gallun, whose devotion to the cause of leather chemistry has made arate a large portion of the data presented in this book. jae Milwaukee, Wisconsin, March 12, 1923. CONTENTS CHAPTER I.—INTRODUCTION it Sa Roe ee, eee CHAPTER 2.—HISTOLOGY OF SKIN Preparation of Sections and Photomicrographs for Study—General Histology of Skin—Cow Hide—Calf Skin—Sheep Skin—Goat Skin— Hog Skin—Horse Hide—Guinea Pig Skin—Fish Skins—Other Skins. CHAPTER 3.—CHEMICAL CONSTITUENTS OF SKIN . CHAPTER 4.—IONIZATION OF ACIDS AND BASES CoMMONLY USED IN THE EMERY.) oc. . Acids—Bases—Order of Strengths—Temperature—pH Values—Effect of Added Salts. CHAPTER 5.—PHYSICAL CHEMISTRY OF THE PROTEINS . + + + ¢ Donnan’s Theory of Membrane Equilibria—Swelling of Protein Jel- lies—The Acid-Protein Equilibrium—Repression of Swelling by Salts —The Alkali-Protein Equilibrium—Two Forms of Collagen and Gela- tin—Electrical Potential Difference between Protein Jelly and Aqueous Solution—Rhythmic Swelling of Protein Jellies—Structure of Gelatin Solutions and Jellies—Relation of the Osmotic Pressure and Viscosity of Gelatin Solutions to the Swelling of Gelatin Jellies—Osmotic Pres- sure and Membrane Potentials—Changes in Viscosity of Gelatin So- lutions with Time—Theory of Salting Out and the Stability of Colloidal Dispersions—Adsorption. CHAPTER 6.—PRESERVATION AND DISINFECTION OF She RCP ee ee bee ek ae Salting — Salt Stains — Drying — Salting and Drying — Pickling — Disinfection. OpArteR 7. SOAKING AND FLESHING . . - + + + + © © § CHAPTER 8.—UNHAIRING AND SCUDDING ...- .- Sweating—Liming—Plumping and Falling—Fresh vs. Mellow Lime Liquors—Unhairing by Means of Other Alkalies—Unhairing by Means of Acids—Unhairing by Means of Pancreatin—Combined Bating and Unhairing by Means of Pancreatin. CHAPTER 9.—BATING Falling—Regulation of Hydrogen-lon Concentration—Deliming—Bac- terial Action—Enzyme Action and Elastin Removal—FEffect of Hydro- gen-Ion Concentration—Effect of Time of Digestion—Effect of Con- centration of Enzyme—FEffect of Concentration of Ammonium Chloride —Distribution of Elastin Fibers in the Skins of Different Animals— Effect of Elastin Removal on the Final Leather—Digestion of Col- lagen during Bating. 9 PAGE Il 15 65 76 04 133 142 151 173 10 CONTENTS CHAPTER 10——-DRENCHING AND PICKLING. . . ©. -«.” sunigula eee CHAPTER II.—VEGETABLE TANNING MATERIALS ... ba Classification—Sources of Tanning Materials—Leaching—Effect of Temperature—Effect of Hardness and Alkalinity of the Water—Effect of pH Value on the Color of Tan Liquors—Effect of pH Value on the Oxidation of Tar Liquors—Effect of pH Value on the Precipitation of Tan Liquors—Clarifying, Decolorizing and Drying. CHAPTER I2.—THE TANNINS nr Practical Definition of Tannin—The Gelatin-Salt Test for Tannin— The Determination of Tannin—A. L. C. A. Method—Wilson-Kern Method—Comparison of A. L. C. A. and Wilson-Kern Methods—Effect of Washing—Conversion of Nontannin into Tannin—Effect of Aging —Effect of pH Value—Modified Wilson-Kern Method—Potential Dif- ference of Tannin Solutions—Isoelectric Points of the Tannins—Pre- cipitation of Tan Liquors. CHAPTER 13—VEGETABLE TANNING . .. . ? The Structures of Tanned Skins—Rate of Diffusion of Tan Liquor into Gelatin Jelly—Rate of Tanning as a Function of Time and Con- centration of Tan Liquor—Rate of Tanning as a Function of pH Value—Stability of the Collagen-Tannin Compound at Different pH Values—Effect of Neutral Salts upon the Rate of Tanning—Degree of Plumping of Skin as a Function of Concentration of Acid and Salt in Tan Liquors—Rapid Tannages—Theory of Tanning—Procter-Wil- son Theory—Oxidation Theory. CHAPTER I14.—CHROME TANNING . Chromium Collagenate—Hydrolysis of Chromium Salts—Diffusion of Chromium Salts into Protein Jellies—The Time Factor in Chrome Tanning—The Concentration Factor in Chrome Tanning—Effect of Neutral Salts upon Chrome Tanning—Effect of Salts of Hydroxy- Acids upon Chrome Tanning—Comparison of Chrome and Vegetable Tanned Leathers—Theory of Chrome Tanning. CHAPTER I5.—OTHER METHODS OF TANNING Combination of Chrome and Vegetable Tanning—Alum Tanning—Iron Tanning—Tanning with Colloidal Silicic Acid—Miscellaneous Mineral Tannages—Tanning with Oils—Tanning with Aldehydes and Quinones —Tanning with Syntans. CHAPTER 16,—FINISHING AND MISCELLANEOUS OPERATIONS . .. . Bleaching—Stuffing and Fatliquoring—Penetration of Dispersions through Grain Surface—Fatty Acid Spews—Coloring—Finishing. 213 240 278 309 322 THE CHEMISTRY OF LEATHER MANUFACTURE Ghapter “I. Introduction. Leather chemistry is one of the most fascinating branches of indus- trial chemistry and also one of the most complex, dealing, as it does, with reactions between those poorly defined groups of substances, usually colloidal, whose compositions are still matters for speculation. The raw skin is composed largely of various kinds of protein matter and is complicated by a structure which varies considerably in differ- ent animals and even in different parts of the same skin. Conversion into leather involves the removal of some of these proteins by the action of alkalies, enzymes, or bacteria, and the interaction of the re- mainder with tanning materials, oils, soaps, emulsions, mordants, dye- stuffs, gums, resins, and other complex materials. During these reactions the structure of the skin must be carefully preserved, or improved, and highly developed technic is required to impart to the resulting leather certain necessary, but almost indefinable, properties, many of which it is an art even to appreciate fully. When one con- siders the vast amount of energy expended by organic chemists upon the materials involved in making leather and the uncertainty of our knowledge concerning the individual substances, the complexity of the whole problem becomes more apparent. Leather manufacture as an art probably antedates chemistry as a science. Well preserved specimens of leather from ancient Egypt bear testimony to the high state of development of the art over three thou- sand years ago. Its origin presumably dates back to the time when man first began to kill animals for food. The skins, not being palatable, were very likely discarded at first, but the value of dried skins for clothing, or protective covering, could hardly remain long undiscovered. Dried skins are hard and stiff, but would become considerably softer and more pliable after being bent and worked during use, and it was probably noticed very early that this softening action is more pronounced if the skins are worked while being dried, especially in the presence of fats, such as would naturally cling to the skins of animals crudely flayed. In rainy seasons, when the skins could not II 12 THE CHEMISTRY OF LEATHER MANUFACTURE be dried rapidly, putrefaction of the epidermal cells would cause the hair to slip and reveal the advantages of unhaired skins for certain purposes. The tanning and coloring actions of leaves, barks, and woods were probably also accidental discoveries of a prehistoric age. In fact, many of the tannery operations in use today are of ancient origin. | Secrecy and lack of accurate records make it difficult to follow the evolution of the art, especially in the matter of details essential to the production of the finer qualities of leather. But developments have not all been made by rule-of-thumb methods, as has often been supposed. The great success of a certain class of tanners, for example, has been due to the development of a science of leather manufacture, as distinct from the art, based upon a belief in the constancy of natural laws and involving the organization and classification of countless facts gained by experience or handed down from previous generations. This science, because of its high degree of specialization, has proved more powerful in a practical way than chemistry, so much so that chemistry must still be regarded as of value primarily in supplementing and not replacing the science of the tanner. Disillusionment has been common among chemists entering this industry, as the result of the unexpected intricacy of the application of chemistry to leather manufacture, of insufficient training, of false notions of superiority over artisans who had devoted their lives to the industry, or of failure to appreciate that the tanner’s own science is usually far more reliable than the chemistry of a beginner in the industry. ~In order to make substantial progress, the chemist must, asa rule, devote himself completely to a study and explanation of the mechanism of each step of a process already in successful operation and without in any way interfering with the operation of that process. Once avail- able, sound explanations of the mechanism of existing processes are of incalculable value in suggesting practical experiments leading to the elimination of unnecessary operations and to the improvement and development of others. That this procedure has not been more widely adopted is easily explained. Long and costly studies are required for which there is no immediate return, and whether there will ever be a return commensurate with the cost of the studies must depend upon the skill of the chemist, which it is difficult for the tanner to judge. Moreover, the qualifi- cations required of the chemist are extremely severe. He must have a broad, theoretical training, marked ability to advance the pure science, great skill in adapting delicate apparatus to crude, tannery conditions, and power to appreciate the viewpoint of a successful tanner. Previous contact with the industry, on the other hand, is not essential. That close codperation between the university and the industry would be highly profitable to both cannot be denied, but there is little chance of such cooperation being brought about until each acquires. a better understanding of the needs and potentialities of the other. The stumbling block has been either the failure to appreciate the value INTRODUCTION 13 of cooperation or the disinclination of one or the other to take the initiative. The university can derive at least three important lines of ad- vantage from cooperation with the industry, the most obvious of which is much needed financial support. But the laws af-the; con- servation of mass and energy hold in industry, as in everything else. The university cannot continue to receive from the industry with- out returning a like amount, although this may be of a different kind. The university has vast resources of potential wealth, but it suffers from having too little in liquid form. But industry constitutes a means of converting one form of wealth into another. The university can be assured a continuous financial support from the industry, but only by supplying the industry with the means of producing this wealth. Another advantage to be gained by the university is the view- point of the industry, which is necessary for the university to prepare its potential wealth so that it may be assimilated by the industry. This viewpoint will also help the university to train its students to make a greater success in industry. The third advantage lies in the fact that the industry offers a field of employment for the students of the university. The industry is always in need of men _ properly trained from its own viewpoint. But, too often, the training which men receive at the university does not equip them with a power of service to the industry that is in demand at all times. Through closer cooperation a system of training could be devised that would guarantee the opportunities of industry to men with initiative and ambition. | The source of wealth that codperation offers to the industry con- sists of fundamental data and of men trained to apply these data to practical production. The possibilities for increasing efficiency in the industry are almost unlimited and so are the profits to be derived by both the university and the industry from intelligent cooperation. Chemists within the industry have always been handicapped by lack of fundamental information. Many of the physical properties that determine the value of leather are determined by its microscopic structure, but very few tanneries have found themselves in a position to develop the means for studying the histology and chemical constitu- ents of skin and the structure of leathers made under different condi- tions. Such studies are expensive and time consuming and their devel- opment in each individual tannery would be very extravagant. The _ industry could well afford to finance a laboratory to be devoted solely to such studies, which would probably cover a period of many years. That a good start has been made will be evident from a perusal of the next chapter. The physical chemistry of the proteins is a subject of fundamental importance to leather chemistry, but, since it is also of fundamental importance to many other branches of chemistry, it should be pos- sible for some good university laboratory to establish a great scheme for co-operative work in this field, drawing financial support from 14 TITHE CHEMISTRY OF LEATHER MANUPACIU Re many different fields. A. university research laboratory is also an ideal place in which to study both the physical and organic chemistry of the proteins and the natural tannins. Physical chemistry offers much the better prospects for immediate application to manufacturing practice, but both should be developed simultaneously. There is hardly any fundamental work in leather chemistry that is not suitable for the university laboratory, but, in order to com- mand the financial support of the industry, it must be done in such | a way as to make it directly serviceable to the industry. Appreciating the inertia that must be overcome before any great research move- ment can gain sufficient momentum to make it practically self-support- _ ing, the late Arthur H. Gallun, with remarkable foresight and lofti- ness of purpose, established a research in the fundamentals of leather chemistry, under the direction of Professor A. W. Thomas, of Columbia University, with the proviso that all results be published freely for the henefit of the industry as a whole. The results obtained during the past few years compare favorably with all previous work done on the mechanism of chrome and vegetable tanning. It will be evident from the description of this work, in the later chapters, that it must ultimately prove of great practical value and it is difficult to see how it can fail to gain the support of the entire industry in due time. It is worthy of special mention here as a demonstration of a kind of cooperation that should prove very profitable to both the university and the industry. It is hoped that the following pages will give chemists in many fields a better understanding of the problems of the leather industry and of the opportunities for cooperative research, and also give the industry itself a clearer appreciation of the possibilities for further extending the application of pure chemistry to leather manufacture. j i oe j @hapter 2} Histology of Skin. Since animal skin is the basis of leather, the importance of its histology to the science of leather manufacture israpparent.> =Never- theless scientific advancement, especially in regard to the preparation of skin for tanning, has been retarded by an insufficient knowledge of the histology of the skins of animals used in making leather. This has been due less, perhaps, to lack of appreciation of the value of histology than to the high degree of refinement of equipment and technic required for its study. Much of the complexity of the structure of the skin is due to the manifold purposes it serves. As a means of protection for the under- lying organs, it is so constructed as to act as a buffer against shocks or blows, while not interfering with the operation of any organs. It is an organ of sense, equipped with nerves sensitive to touch, pain, heat and cold, and, as an organ of secretion and excretion, it is supplied - with glands, ducts, muscles, and blood vessels. It serves also as a regulator of the body temperature, which it controls by regulating the evaporation of water from its surface and the secretion of oil to cover the surface in order to prevent too great a loss of heat. The degree to which each constituent part of the skin is developed depends upon the extent to which it is needed by the body and also upon the amount of available nourishment. The structure of a single skin varies considerably in different regions of the body. Nerve papilla, for example, are very numerous in regions where the sense of touch is most needed, as in the finger tips, and widely scattered in other regions. The skin structure is developed to meet sudden changes of temperature to the greatest extent in the regions of. the body most exposed to such changes and to resist friction and blows where these are most frequent. In fact, the skin tends to develop a structure at each point designed to be of greatest service to the body at that point. This results in a large number of types of skin structure that must be studied, depending upon the species of animal, the general nature of its feeding, the climatic conditions under which it lived, and the region of the body from which the specimen is taken. The number of possible types appears formidable, but tanners have learned from experience how to classify them in a general way accord- ing to the properties of the leather they yield. It seems reasonable to. believe that histologists may be able to develop a similar classifica- tion, based upon histology, that will prove extremely valuable in supple- menting the tanners’ information. 15 16 ‘THE CHEMISTRY OF LEATHER MANU FAG ee Because of the large number of highly trained investigators in the medical sciences, considerable progress has been made in the histology of human skin. This work is invaluable as a guide to the student of the histology of the skins of lower animals because most skins possess a common basic structure. But the several types of skin exhibit such marked differences in details of structure of vital importance in leather manufacture that a knowledge of the histology of a few specimens of human skin alone might actually be misleading. In order to apply histology intelligently to leather manufacture, separate studies must be made of the structure of each type encountered. Although much yet remains to be learned of the histology of skins used for leather manufacture, substantial progress has been made, the most notable work being that of Alfred Seymour-Jones.1 Sys- tematic studies have also been under way in the author’s laboratories, for several years, dealing with the structure of the skins of different animals and the changes which they undergo during the conversion of the skin into leather. In this book much of this work is presented for the first time. Preparation of Sections and Photomicrographs for Study. Since much of the information given in this chapter was obtained by direct observation of sections prepared in the author’s laboratories, a description of the methods employed will probably assist in making the presentation clearer, particularly so in view of the fact that work of this kind appears not to have been general in tannery laboratories. The description, however, will be limited to the methods used in the production only of the photomicrographs appearing in this book. The subjects of microscopy, microtomy, and photomicrography are too vast in scope for adequate treatment here and the reader desiring to pursue these subjects further is referred to the several excellent works available, such as those of Gage? and Lee.’ Sampling.—In studying the entire skin of an animal, strips of about 0.5 x2 inches were cut from different parts of the skin so as ta show, not only the general structure, but also its variation throughout the skin. Care was taken, in cutting the strips, so that the later section- ing could be done in definite planes, as, for example, that including a hair follicle and erector pili muscle. It was found important that the plane selected be uniform for any given series of sections show- ing changes taking place during the passage of a skin through the tannery processes. Fixing.—After a tissue dies, the structure undergoes a gradual change unless it is immediately fixed. According to Lee, the word fixing implies two things: “first, the rapid killing of the element, so that it may not have time to change the form it had during life, but is fixed in death in the attitude it normally had during life; and 1 Physiology of the Skin. Alfred Seymour-Jones. J. Soc. Leather Trades’ Chem. serially 1917-21. ‘ The Microscope. S. H. Gage. Comstock Publishing Co., Ithaca, N. Y * ee Microtomist’s Vade-Mecum, A. B. Lee. P. Blakiston’s’ Son & Co., Philadel- phia, Pa. HISTOLOGY OF SKIN 17 second, the hardening of it to such a degree as may enable it to resist without further change of form the action of the reagents with which it may subsequently be treated. Without good fixation it ts impossible to get good stains or good sections, or preparations good in any way.” | The photomicrographs shown in this book are from sections fixed either in Erlicki’s fluid or in alcohol. Numerous other fixing agents were tried, but they did not answer so well for the specific purposes in view. E[rlicki’s fluid is made simply by dissolving 25 grams of potassium dichromate and 10 grams of copper sulfate in a liter of water. The strips of fresh skin were placed directly into this solu- tion without previous washing. ‘They were transferred to fresh solu- - tions daily for the first 3 days and then kept in the last bath until the solution had thoroughly penetrated them. The period of contact was usually from 5 to 7 days, after which they were washed in running tap water for about 20 hours and then dehydrated with alcohol. The skins of the sheep, cow, calf, and guinea pig, whose sections are shown in this book, were fixed immediately after the animals were killed. Their sections may, therefore, be regarded as showing the normal structure of the living skin. The other sections exhibit structures of skins as the tanner usually receives them. A duplicate series of strips of fresh skin was fixed in dilute alcohol, in each case, for comparison with those fixed in FErlicki’s fluid. Sec- tions from the Erlicki fixer generally showed various details more sharply than those from alcohol alone. All specimens of skin from the unhairing and bating processes were fixed in alcohol in order to avoid possible complications due to the reactions of the Erlicki fixer with the tannery liquors. Samples of air-dry leather were not fixed, but were imbedded in paraffine either directly or after soaking in santal- wood oil and then in molten paraffine. Dehydrating and Imbedding.—All specimens of skin, after fixing, were kept for the stated lengths of time in the following baths: 6é 66 66 t day 50 per cent alcohol “« “absolute alcohol fresh absolute alcohol Y% “ alcohol-xylene : carbol-xylene xylene fresh xylene molten paraffine. 66 66 The mixture of alcohol and xylene consisted of equal volumes of the two. The carbol-xylene is known as a clearing agent and has for its object the removal of alcohol from the specimen; it is prepared by mixing 25 cubic centimeters of melted phenol with 75 cubic centimeters of xylene. Very thick specimens had to be left in the molten paraffine for a much longer time, the object of this bath being to replace the 1% THE CHEMISTRY OF LEATHER MANUFPAGEGRe xylene by paraffine. The strips from the paraffine bath were sus- pended in aluminum beakers, having a capacity of 100 cubic centi- meters, and covered with molten paraffine. The beakers were then plunged into cold water and kept there until the paraffine had com- pletely solidified. The beakers were then heated just sufficiently to loosen the paraffine blocks, which were pulled out and cut into the proper size and shape for placing in the microtome. Sectioning.—Really good work in preparing sections is possible only when the microtome knife is free from nicks and extremely sharp, the sharper the better. The thickness to which it is desirable to cut . the sections depends upon the particular part of the skin to be studied. For a general picture of the whole skin, a thickness of 20 microns is satisfactory. In the sections of skin taken from the unhairing processes, it will be noticed that there would be nothing to hold the loose epidermis and hair in place, if these were not securely fastened to the slide in some way. In order to prevent the loss of important material from the sections, the entire paraffine ribbons from the microtome were fastened to the slides by means of Mayer’s albumen fixative. This is made by mixing equal parts of glycerin and well-beaten white of egg, adding 2 per cent of sodium salicylate, and filtering. A tiny drop of this fixative was spread evenly over the middle of a slide with the finger and was then covered with water. A ribbon, con- taining a section of skin, was then floated onto the water, which was heated over an alcohol lamp carefully so as not to melt the paraffine. This causes the ribbon to spread out flat and it was then worked into place and smoothed out with a camel’s-hair brush. Slides prepared in this way were left to dry for at least one day, the sections meanwhile becoming securely fastened. They were then freed from paraffine by flooding the slides with xylene, after which they were. washed with absolute alcohol in preparation for staining. | Staining.—Six stains were used in preparing the sections shown in this book, with the exception of those of the human heel and scalp. These two sections were prepared in Professor Bast’s laboratory and were stained with Delafield’s hematoxylin and eosin. Where an aqueous stain was to be employed, the section was soaked, for sev- eral minutes, successively in the following strengths of alcohol: 95 per cent, 75 per cent, 50 per cent, 25 per cent, and then in water. After the staining, it was worked up through the series of solutions of alco- hol in the reverse order, finally being rinsed with absolute alcohol. The six stains used were prepared as follows: (1) Van Heurck’s logwood: 6 grams of powdered logwood ex- tract and 18 grams of alum were ground together in a mortar and 300 cubic centimeters of water added slowly. The mixture was then filtered and 20 cubic centimeters of alcohol were added to the filtrate. The solution was kept exposed to air for several weeks, water being added to replace that lost by evaporation. The sections were kept in this stain for 3 minutes, rinsed in tap water until they turned blue, and then passed through the series of alcoholic solutions of increasing — fist OLOGY OFS KIN 19 strength. Sections were transferred from the 95-per cent alcohol to the picro-indigo-carmine solution, where this was used for counter- staining. But where the counterstaining was done with bismarck brown, the sections were transferred from the 95-per cent alcohol to a 0.1-per cent solution of HCl in absolute alcohol, where they were kept until they turned pink and no more color was seen to wash away, after which they were rinsed with fresh alcohol and put into the bismarck brown stain. (2) Friedlander’s logwood: 2 grams of powdered logwood ex- tract dissolved in 100 cubic centimeters of alcohol were mixed with 2 grams of alum dissolved in 100 cubic centimeters of water and 100 cubic centimeters of glycerin. This was used like Van Heurck’s stain. (3) Picro-indigo-carmine: To 100 cubic centimeters of 9o-per cent alcohol was added 1.0 cubic centimeter of absolute alcohol sat- urated with picric acid. This solution was then saturated with indigo carmine and allowed to stand with an excess of indigo carmine, with occasional shaking, for several weeks. The decanted solution was used. Sections were kept in this stain from 3 to 4 hours. (4) Picro-red: 5 cubic centimeters of absoltite alcohol saturated with picric acid were added to 55 cubic centimeters of go-per cent alcohol saturated with the dye Leather Red-X. This solution was diluted with alcohol to 10 times its volume before using. Sections re- mained in this stain for 2 minutes. (5) Weigert’s resorcin-fuchsin: 2 grams of basic fuchsin and 4 grams of resorcin in 200 cubic centimeters of water were boiled for IO minutes, 25 cubic centimeters of a 30-per cent solution of ferric chloride were then added and the boiling was continued for 5 minutes. Then a saturated solution of ferric chloride was added until all of the color was precipitated. The mixture was allowed to stand over night to cool and settle and the supernatant liquor was decanted off and discarded. The residue was dissolved in 200 cubic centimeters of boiling 95-per cent alcohol and the hot solution was filtered into a bottle. After it had cooled, 5 cubic centimeters of concentrated HCl were added. For staining, this solution was diluted with an equal volume of alcohol and sections were left in it from 60 to 90 minutes, after which they were rinsed with alcohol. (6) Daub’s bismarck brown: To 95 cubic centimeters of absolute alcohol were added 5 cubic centimeters of saturated lime water and then more bismarck brown than would dissolve and the whole was shaken and allowed to settle, the solution being decanted off after standing for several days. Fifteen cubic centimeters of alcohol were added to the solution to replace any lost by evaporation, which would otherwise cause a precipitation of some of the dye. Sections were kept in this stain for 1 day. Mounting.—Since the sections of untanned skin were fastened per- manently to the slides before staining, the mounting of these was a very simple operation. After the sections were stained, they were rinsed successively with absolute alcohol, alcohol-xylene, carbol-xylene, and xylene. Each section was then covered with a drop of Canada 20 THE CHEMISTRY OF LEATHER MANUPACI es balsam followed by a cover glass. Sections thus prepared are permanent and ready for study or photographing. Sections of leather, from the microtome, were uncurled on a piece of smooth paper and fastened by pressing on the paraffine surrounding the sections. They were then removed from the paper in a flattened condition by means of tweezers, dipped into a I-per cent solution of parlodion in equal volumes of alcohol and ether, and then transferred to slides previously coated with a thin film of santalwood oil. They were carefully smoothed out, covered with santalwood oil, and allowed to stand exposed to air until the alcohol and ether had evaporated, usually about 30 minutes. They were then washed with xylene and covered with balsam and cover glasses. As a rule the staining’ of leather sections for study is unnecessary, but a stain often assists in getting sharper photographs. Where a stain was employed on leather, the fact is noted under the photomicrograph. Photographing.—All photographs were taken with a standard type of photomicrographic apparatus. Wratten and Wainright “M” plates — were used and developed according to the directions which accompany them. The source of light transmitted through the sections was a 6-volt mazda lamp having a concentrated filament. The light was filtered through appropriate color screens, consisting of the standard Wratten filters. The stains on the sections, together with the color screens, generally furnished all the detail or contrast necessary, but where this was not entirely satisfactory, the plates, after developing, were treated with standard intensifying or reducing agents as needed. Certain precautions were necessary in photographing grain surfaces for comparison. The hair follicles run obliquely to the surface, in consequence of which the lights and shadows depend upon the angle at which the light strikes the openings of the follicles. This was made uniform for all skins with nearly straight follicles by using for reference the plane including the line of the follicle and intersecting the plane of the grain at right angles. A beam from a powerful arc lying in the plane perpendicular to these two planes was made to strike the grain surface at an angle of 45 degrees. ; General.—Halftones were used to reproduce all photomicrographs excepting those in Figs. 20 to 27 inclusive, which were printed from line etchings. Because of the low magnification and consequent fine- ness of the fibrous tissues, a good reproduction could not be obtained by means of halftones; even a very fine screen produced an ap- preciable blurring effect. Line etchings, made without a screen, gave a much better result, the resulting increase in sharpness of the fine lines more than compensating for the loss in shading. All vertical sections are shown with the outer surface of the skin upward. Under each photomicrograph is given the location or region of the body from which the specimen was taken, where this was known, the thickness at which the section was cut in the microtome, the stains applied to the section, the eyepiece, objective, and filter used in photographing, and the final magnification of the section as it appears in the book. sts TSR Ramtascasvarates se 58 ag s, ipa e ee re ae ee; 2 coset SAO OATS Ore ote. ness oY +X f-) : ce e p ty 2 VAS ep = 1n. none, iece: ion of Human Sk Eyep —Vertical Sect I ig. F scalp Thickness of sect Location een. e eT blue 20 diameter =f e4 = ky 25 2 =. eee a ie 25s ap SO Ses Cpe Ys op Ses in, roo Til; hematoxyl 10n Delafield’s Stains Eosin. 21 Fig. 2.—Vertical Section of Human Skin. Location: lower part of back. Thickness of section: 20 wu. Stains: Van Heurck’s Daub’s bismarck brown. Eyepiece: none. Objective: 32-mm. Wratten filter: H-blue green. Magnification: 32 diameters. logwood, 22 HISTOLOGY OF SKIN | 28 General Histology of Skin. It is not: uncommon to find in the literature descriptions of skin structure that apparently clash. Occasionally an author will present what purports to be a general description, but which is actually based upon the examination of a single type of skin structure. Figs. 1, 2, and 3 all represent vertical sections of human skin, but the first was taken from the scalp, the second from the lower part of the back, and the third from the heel. A detailed description of one would give a very misleading picture of either of the others. In the section from the scalp, fat cells make up the greater portion of the whole, while in that from the back there are relatively very few fat ‘cells, but a great abundance of fibers of connective tissue. In the section from the heel, fat cells and connective tissues are both very prominent, but no hairs are seen. Practically, these sections represent very differ- ent types, yet all three conform to a common basic structure. A structure common to all skin may be greatly exaggerated in one type and scarcely detectable in another. All four general classes of tissues, epithelial, connective, muscular, and nervous, are present in the skin, as well as those of the blood. These tissues either consist of cells or are the product of cells. The epithelial tissues consist of layers of cells, which cover all the free surfaces of the animal body. The connective tissues are distinguished from the other fundamental tissues of the body by the fact that their cells lie imbedded in extracellular material which appears to be the result of their activity. The various types of connective tissues are distinguished among themselves by the kind of extracellular tissue which they produce, such as bone, cartilage, etc. The muscular tissues have a well developed power of contracting, apparently without change of volume, the decrease in length being compensated by an increase ‘n diameter. The cells of the striated or involuntary muscles are long in relation to their width and are marked with transverse bands, while those of the nonstriated or involuntary muscles are spindle shaped, without transverse striations. The nervous tissue found in the skin is a protoplasmic prolongation of cells lying in the central nervous system, or in the ganglia closely associated with that system. The skin is divided sharply into two layers, distinct both in structure and origin: a relatively very thin outer layer of epithelial tissue, the epidermis, and a much thicker layer of connective and other tissues, the derma. Raw skin, as an article of commerce, has also a third layer, the superficial fascia, known to the tanner as the adipose tissue or, more commonly, the flesh. In keeping with the nomenclature of the leather trade, the word flesh will be used only in this con- nection, although in anatomy flesh really means muscle tissue. In life, the adipose tissue, or flesh, connects the skin proper very loosely to the underlying parts of the body. The derma lies between the epidermis and adipose tissue. In the preparation of skin for tanning, except in special cases, such as the tanning of fur skins, the adipose tissue and the entire Fig. 3.—Vertical Section of Human Skin. Location: heel. Eyepiece: none. Thickness of section: 30 w. Objective: 48-mm. Stains: Delafield’s hematoxylin, Wratten filter: H-blue green. Eosin. Magnification: 20 diameters. 24 Hist OLOGY OF SKIN 25 epidermal system must be removed intelligently and with extreme care, leaving the derma to be converted into leather. The epidermal system, adipose tissue, and derma will be described in turn. The epidermis is made up of a cellular strata originating from the ectoderm, the outer layer of the young embryo, and independently of the derma, which is derived from the mesoderm, or middle layer. These two layers grow independently throughout life and differ ma- terially in both chemical and physical properties. In Fig. 2 the epi- dermis can be seen as a dark band forming the upper boundary of the skin and constituting only about 1 per cent of the total thickness. So far as its growth is concerned, the epidermis may be looked upon as a parasite, although it is a most important part of the body. It has no blood vessels of its own, but rests upon the upper surface of the derma and draws its nourishment from blood and lymph supplied by the blood vessels of the derma. It grows only through the reproduc- tion of its own cells. The portion of epidermis in contact with the derma is a layer of living epithelial cells, rather elongated in shape. It may be men- tioned that a cell consists of a nucleus suspended in protoplasm en- closed between very thin walls acting as a semi-permeable membrane. Nourishment from the lymph and blood streams diffuses through the cell walls, and after a certain period of growth the cell divides mitotically, forming two cells. ‘This change appears to be initiated by the nucleus. In the deepest layer of the epidermis, each cell increases in height and then subdivides, forming two cells, one above the other. _ This process is repeated indefinitely. As the older cells are pushed outward, they become flattened by dehydration and other changes. During this process, the protoplasm dries up and the cells lose their power of reproduction. In the outermost layer, the cells are very dry and scaly and are gradually worn away. ‘This scaling is often very noticeable on the scalp in the form of dandruff, which, in itself, is not the result of a disease, but rather is evidence that the epidermal cells are functioning and reproducing vigorously. Where the epidermis is very thick, as on the heel, the gradual transition which the cells undergo in their outward course gives the epidermis the appearance of having several distinct layers. The por- tion of the epidermis shown in the upper left hand corner of Fig. 3 is shown at a very much higher magnification in Fig. 4. Now the several strata can be seen very plainly. The layer marked E is the uppermost part of the derma and numerous protuberances of its surface, called papillae, can be seen extending upward into the epidermis, giving the boundary between epidermis and derma a serrated appearance. D is the Malpighian layer of the epidermis, or stratum mucosum. It is built up of several rows of living epithelial cells, whose nuclei appear in the picture as dark spots or rods. ‘Tiny fibers, often called prickles, pass from cell to cell, holding them together and securing them to the derma. Extending between these prickles, which look as if they were walls in section, are protoplasmic processes and it is supposed that food passes upward } } \ is. derm f Human Epi ion o 1 Sect Stratum corneum ig 4.—Vertica F dum. Stratum luci A B ters vo ie ve tt ° 76 ga. Seay GPx o ee on) 2 op os - ‘= OD - os Ave Nn O'm SW £3 .. 9528 eres) Hoss a Orr So pig foe x seers — EEs > a rh Son Ae a a : 3 yr =e AIG, - - Sc OA .- B Ee fe Ov = as rs peo Ot wv mm OF) sees 2g BM Locat onic Stains Eos 26 HISTOLOGY OF SKIN 27 between the cells and waste from the upper layers downward. From this food the cells derive the nourishment necessary for reproduction. This layer contains no blood vessels, but very fine nerve fibers pass from the derma into this layer, forming a network between the cells and terminating in bulbous swellings or undergoing a gradual breaking up into nerve granules. As the new cells are formed, the older ones are pushed outward where nourishment is no longer available and the protoplasm of the cells gradually dries up. Upon staining, the cells then appear as though they contained coarse granules and form the layer shown at C, which, from its appearance, has been called the stratum granulosum. The cells also contain a pigment, which is at least partly responsible for the color of the skin. This pigment, known as melanin, is thought to be a derivative of hematin containing iron and sulfur. It is very concentrated in the skin of the negro and almost entirely absent from the skin of a blonde. Apparently the pigment is formed as a pro- tection against strong sunlight, both for the skin and the underlying tissues. The pigmented layer may thus be looked upon as a color filter. When the pigment-containing cells are collected in spots, they appear as freckles. The pigment in the negro skin is found in the deepest cells of the stratum mucosum, in the connective tissue cells of the upper part of the derma, and in the wandering cells of the lymph, found in the lymph spaces or between the cells of the epidermis or connective tissues. The pigment granules are found only in cells. As the cells are pushed still further outward, the cell granules break down, yielding a material, called eleidin, which resists staining and gives the epidermis in this region a transparent appearance, from which it has derived the name stratum lucidum. This layer is shown at B. The cells continue to undergo changes during their outward course, becoming drier and flatter, and finally form the very thick layer shown at A, the stratum corneum, in which the cells tend to break away from each other and to scale off. This layer is being worn away continually and is replaced by the newer cells from below. The corneous layer is a very poor conductor of heat and the waxy material usually present on its surface makes it water repellent. In the photomicrograph a duct can be seen taking a spiral course up through the corneous layer. This is the outlet of a sudoriferous or sweat gland seated in the derma. Its opening at the surface of the corneous layer is called a pore. All of the strata noted above can be detected only where the epidermis is very thick. Elsewhere only the stratum mucosum and stratum corneum are visible. In no case have we yet observed a sec- tion of skin used for making leather where more than these two layers could be recognized in the epidermis. The independent growth of the epidermis and derma involves a number of important appendages of the skin. In the epidermal sys- tem, the reproduction of epithelial cells produces, not only the epi- dermis, but also the hair and the sebaceous and sudoriferous glands. These cellular structures are composed of proteins of the class known itu. im si Bulb from Hog, ir a —Vertical Section of e 5 Fig red ation: 225 diameters. 1 ge + fr Ss es WP 5 Cs ir oe co 8 owet gy CES G+ Sa emt O 3 4 Os og OS a < @) Oo 4 on pe = ro) Mae 4 oD) O 24.8 oo & ols ua ¢ erred a meet ie te air SE:-0 ‘5 Ba BU 8.8 5 Oy ahead Hew 28 HISTOLOGY OF SKIN 29 as keratins as distinct from the collagens and elastins of the derma. Where a portion of the epidermis is lost, through accident, it can be regenerated only by the surrounding epithelial cells spreading over the bare spot, by reproduction. The necessity for removing the epidermal system completely before tanning and without any injury to the derma makes the difference in chemical composition between the two systems a matter of great importance to the tanner. In the class with hair belong also nails, claws, hoofs, scales, and feathers, which are all special growths of the epidermis. To the naked eye, the hair appears to pierce the skin, but actually it does not do so. An examination of Fig. 2 will show that the epidermis dips down into the body of the derma, forming a pocket, or follicle, in which the hair grows. The follicle is complex in structure because it is made up of the epidermal layers on the hair side and of the layers of the derma on the other. At its bottom, the follicle is penetrated by a projection coming from the derma and known as the hair papilla, which is supplied with both nerves and blood vessels. A good example of a hair papilla is shown in Fig. 5 in the hair bulb from the skin of a hog. The bottom end of the bulb appears like a pair of pincers with the jaws slightly open and facing downward. A similar structure may be seen in the hair bulbs of the scalp shown in Fig. 1. Passing through the opening in the jaws into the large open space above and resembling a candle flame in shape is the papilla, which contains tiny nerves and blood vessels which supply nourish- ment. Lining the lymph space surrounding the papilla are numerous epithelial cells, which derive from the blood and lymph the nourish- ment necessary for reproduction. As new cells are formed, the older ones are pushed outward through the follicle, forming the hair. The rate of growth of the hair is determined by the rate at which the cells surrounding the papilla reproduce. The newly formed cells of the hair, like those of the Malpighian layer of the epidermis, are very soft. As they are pushed upward, they become elongated in shape and harder. In forming the hair, they assume the shape of the follicle; if this happens to be curved, the hair will be curly. In the negro, the follicles often have a curvature of nearly go degrees, which accounts for the tightness of the curls. The portion of the hair showing above the surface of the skin is called the shaft and the lower portion the root, which enlarges ‘nto a bulb at its lower extremity, where it is penetrated by the hair papilla. The shaft is made up of a central medulla, or pith, of rounded cells, containing eleidin granules, surrounded by a much thicker por- tion composed of long fibrillated cells, containing pigment, and en- closed by an outer layer of cells which become hardened in the form of overlapping scales. These scales, which give fur and wool their felting properties, open outward so as to resist the pulling out of the hair. Unless the lighting is properly adjusted and the magnification sufficiently great, the scales are not easily discernible. In Fig. 6 may be seen the scales of a tiny piece of wool. The scales of one side and the shadows of those on the other both show because the wool ing oot ne: Silane ed nanmrpti be gaat,» As TT BE octet A Fig. 6—Segment of Sheep Wool. Stain: none. Wratten filter: H-blue green, Eyepiece: -7.5X. Magnification: 1260 diameters. Objective: 4-mm. 30 Pie OLOGY CGP SKIN es was photographed with transmitted light. The same general structure can be seen on most hair, but it is not always so pronounced. . When a hair is shed, after reaching the limit of its existence, the epithelial cells left surrounding the hair papilla keep on multiplying and soon another hair is formed to replace the one shed. Baldness results from the failure of the blood vessels of the papilla to furnish the required nourishment or from the destruction of the epithelial cells in some other way. Any serious attempt to grow hair on a bald head must be accompanied by some means of introducing living epi- thelial cells into the hair follicles, of which there are something like a thousand to the square inch. In other words, we cannot grow a crop without seeds or seedlings. In old age, pigment is no longer available for the hair cells and the new hairs, containing no pigment, appear gray in color. Hair containing pigment, however, may look white by reflected light, due to the presence of tiny air bubbles among the cells. Each hair follicle is supplied with sebaceous glands with ducts emptying into the upper portion of the follicle. A group of these glands can be seen in Fig. 2. They are lined with epithelial cells which secrete from the blood the materials required for the synthesis of the oils which they produce. When they become charged with oil, the protoplasm disappears and the cell breaks down, discharging the oil into the duct. New cells are continually being formed to re- place the old ones. The oil is forced into the follicle, where it coats and lubricates the hair, and finally to the surface of the skin, which it softens and protects against the cold. In contact with air, this: oil thickens to the consistency of ear wax, to which it is related. When the ducts become clogged with dirt, the pressure behind them causes them to become distended, giving rise to blackheads. Sebaceous glands are sometimes found also in parts of the skin free from hair. Attached to each hair follicle, just below the sebaceous glands, and extending obliquely upward through the derma, almost to the sur- face, is a bundle of nonstriated muscle tissue, known as the erector pili muscle. In Fig. 2 one of these muscles forms a V with the hair follicle, and the sebaceous glands may be seen within the angle so formed. The nerves supplying these muscles are known as the pilo- motor nerves. These muscles contract under the influence of emotions, such as fear, surprise, anger, or other disagreeable states, or in re- sponse to cold or grazing tactile stimuli. Among the commoner visible effects are the roughening of the skin called goose-flesh and the effect of the hair standing on end, very pronounced in a frightened cat. _ The real purpose of the erector pili muscles 1s apparently to pro- tect the body against sudden changes of temperature by their con- trol over the operation of the glands; they seem to act as effectively as a thermocouple in a good thermostat. Their contraction puts a pres- sure on the glands which causes the cells to give up their oil to the hair follicle and, in the process, the cells are destroyed. The oil is then forced up through the follicle to the surface of the skin, where Fig 7.—Vertical Section of Calf Adipose Tissue. Location: butt. Thickness of section: 20 uw. Stains: Van Heurck’s Daub’s bismarck brown. logwood, 32 Eyepiece: none. Objective: 16-mm. Wratten filter: H-blue green. Magnification: 70 diameters. al lle tne ie me OLOGY Ot SKIN 33 it tends to stop the action of the sudoriferous glands and _ the evaporation of water from the surface of the skin. The sudoriferous or sweat glands are coiled sacs with spiral ducts leading to the surface of the skin. In Fig. 3 several of these ducts can be seen winding up through the epidermis and terminating at the surface as pores. Often the ducts seem to lead into the hair follicles above the openings of the ducts of the sebaceous glands. The sacs of the sweat glands are lined with epithelial cells, which are continuous with the cells of the Malpighian layer of the epidermis, and which secrete water, salts, urea, and other wastes from the blood and pass them out through the ducts. Where no sebaceous glands are present, the sudoriferous glands also provide an oily fluid to keep the surface of the skin soft. These glands serve the dual purpose of disposing of waste products and of permitting control of the body temperature through the regulation of the rate of evaporation of water. ; This entire epidermal system, including the epidermis, hair, and sebaceous and sudoriferous glands, must be removed from the skin in such manner that the derma suffers no injury that can be detected in the finished leather. The skin is connected to the underlying parts of the body very loosely by means of fibers of connective tissue, usually called adipose tissue because it is so frequently the seat of fat deposits, most numerous in the vicinity of the abdomen, which serve to protect the body against cold. The looseness of connection allows the skin very free movement and, incidentally, makes flaying a much simpler matter than it would otherwise be. ‘The adipose tissue, while not a part of the skin proper, is of importance to the tanner because much of it remains adhering to the skins received at the tannery and must be removed prior to tanning. If left on the skins, it greatly impedes the progress of tanning. In Fig. 7 is shown a vertical section of adipose tissue from the butt of a calf skin along with the lower portion of the derma. The top quarter of the picture shows a portion of derma bound on its under side by strands of elastin fibers, appearing as compact masses of black threads; actually they are of a pale yellow color. The fat cells of the adipose tissue are arranged in layers and are held together by fibers of connective tissue. The light colored tissues are the white fibers, composed of collagen, and the dark ones are the yellow fibers of elastin. Large arteries, nerves, and veins which supply the derma traverse the adipose tissue in many places and can often be seen heavily protected with connective tissue. ‘This region is sometimes supplied also with striated muscle fibers to permit the voluntary twitching of the skin. The removal of the adipose tissue of the skin, preparatory to tanning, is an operation known as fleshing. This is done efficiently when all of the tissues underlying the derma are cut away, leaving the derma itself entirely intact. It is the derma, or true skin, that is actually used to make leather Fig. 8.—Vertical Section of Reticular Layer of Calf Skin. Location: butt. Thickness of section: 20 wp. Stains: Van Heurck’s Daub’s bismarck brown. logwood, 34 Eyepiece: 5X. Objective: 16-mm, Wratten filter: B-green. Magnification: 170 diameters. Brew LOGY (OF SKIN 35 and the chief leather-forming constituent of the derma is collagen, the substance of the white fibers of connective tissue. Sound leather can be produced only from skins in which these fibers are well de- veloped and abundant. The three contrasting structures in Figs. 1, 2, and 3 are typical of the extremes found in the skins of the lower animals. A skin composed chiefly of fat cells is of little value in making leather and one in which large groups of fat cells are interspersed between the collagen fibers will yield only a spongy leather because of the empty spaces left after the fat cells have been destroyed in the processes preparatory to tanning. The tendency toward one ex- treme or the other depends largely upon the habits and feeding of _the animal as well as upon its species. In considering the general structure of skin, one should look upon the major portion of the derma as consisting of both fat cells and connective tissues, either of which may be very abundant or relatively scarce. Unlike the epithelial tissues, the major portion of the connective tissues is not made up of cells, but results from the activity of migra- tory cells very much smaller in size than the extracellular material. The relation of these cells to the collagen fibers of calf skin can be seen in Fig. 8. The cells stain more deeply than the fibers and appear in the picture as black specks having a diameter of about 1 millimeter, which means that the actual cells have a diameter of about 1/170th of this. In the sections we have examined, the abundance of these cells diminishes with increasing age of the animal. By examining the cross sections of fibers running pefpendicular to the plane of the page, the arrangement of the fibers, or fibrils, in bundles can be seen very plainly. Seymour-Jones regards the fibers as enclosed in very thin sheaths of what he terms “fiber sarcolemma.” ‘While we have not been able, as yet, to detect such a sheath micro- scopically, the investigations of Wilson and Gallun, described in Chap- ter 8, seem to indicate that the surfaces of the collagen fibers are very much more resistant to tryptic digestion than the material just under the surface. Of the two kinds of fibers composing the connective tissues, the collagen fibers are very much thicker and more abundant than the elastin fibers. ‘There is usually a dense layer of elastin fibers at the lower surface of the derma, where it is attached to the adipose tissue, as shown in Fig. 7, and another in the region of the erector pili muscles. But the greater portion of the derma seems to contain relatively few elastin fibers and these are generally to be found surrounding the blood vessels and nerves traversing the derma. The main trunk lines of blood vessels and nerves supplying the derma run parallel to the surface just above the lower elastin layer. From these trunk lines branches shoot upward and are distributed to all parts of the derma. A network of lymph ducts also is distributed throughout the skin. Cross sections of the arteries and veins show three distinct layers: an outer layer of collagen and elastin fibers, a middle layer of non- striated muscle tissue and elastin fibers, and an inner membrane of Fig. 9.—Grain Surfaces of Tanned Skins. - Wratten filter: K2-yellow. Magnification: 7 diameters. Eyepiece: none. Objective: 48-mm. 36 Vin ea Horned Toad Fig. 10.—Grain Surfaces of Tanned Skins. Eyepiece: none. - Wratten filter: K2-yellow. Objective: 48-mm. Magnification: 7 diameters. of clonal owe: CHEMISTRY OF LEATHER MANUFACTURE flattened cells. All three layers are pronounced in the arteries, but in the veins the outer layer is very much thicker than the inner layers, which are much less developed and collapse when the vein is empty. The veins are also equipped with semilunar valves in order to prevent backflows of blood. A cross section of an artery can be seen at the top (ot gHig.n7 It is the large circular body just to the left of the midline. To the right of the artery is a vein, which has collapsed. The circular mass just under the artery is a cross section of a bundle of nerves. Three more sections of nerve bundles are prominent, elongated in shape, two just below the vein and one to the extreme left of the artery. In those parts of the body where the sense of touch is well de- veloped, as in the fingers, there are numerous protuberances of the sur- face of the derma into the epidermis, called papille. These are very pronounced in the section of skin from the human heel shown in Fig. 3. They are arranged in definite patterns which do not change throughout life. The design of the thumb print is produced by the papillae. They seem to be absent entirely from some parts of the body, particularly where the sense of touch is not well developed and where the epidermis is very thin. They are of two kinds, one containing blood vessels furnishing-lymph to the active epithelial cells in their vicinity and the other containing the nerves sensitive to touch, pain, heat and cold. The epidermis above the papillz is thinner than at other points, the papillze serving the purpose of bringing the nerve ends nearer to those surfaces where they are most needed. The portion of the derma immediately in contact with the epidermis has been called the “grain membrane” by Seymour-Jones because it forms the grain surface of the finished leather. Although its boundary on the side in contact with the epidermis is very sharp, on the other side it blends into the rest of the derma with no sharp change of properties. The fibers of connective tissue grow finer as they near the grain surface, in which the fibers are extremely fine and generally run parallel to the surface. They can be seen very plainly in the hori- zontal section of tanned calf skin shown in Fig. 150, of Chapter 16. Whether or not the fibers of the grain surface are continuous with those of the connective tissues of the derma, they seem to possess somewhat different properties. When unhaired skin is kept in boiling water, the fibers of the grain surface remain as a thin sheet, although - somewhat changed, long after the larger collagen fibers below have passed into solution as gelatin. The outer surface is then very sharp, but the inner side, facing the remnants of the collagen fibers, appears jellylike and heterogeneous, indicating a gradual change in properties of the fibers as they pass from the derma into the grain surface. ; It is of great importance that no damage be done to the grain surface in removing the epidermis, because it determines the appear- ance of the finished leather. It is therefore fortunate for the tanner that the fibers in this surface are more resistant to the action of alkalies than the epidermis above it and more resistant to the action of tryptic enzymes than the elastin fibers below it. The grain surface ee a ee at a WISHOLOGY OF SKIN 39 is readily attacked by proteolytic bacteria under certain conditions, however, resulting in what is known to the tanner as pitted grain. The design of the grain surface, as seen on the skin after unhairing and tanning, is distinct for each species of animal, while the fineness of the pattern is an indication of the age of the animal. It is due to the arrangement of the hair follicles and pores, and of the papillz where these are present. The grain surfaces of the tanned skins of a number of different animals are shown in Figs. 9 and 10. They are all magnified to exactly the same extent and are directly com- parable. It will be noted that the cow and calf have the same pattern, but that it is much coarser in the older animal. These designs can be used to identify different species of animal. We shall now turn from considering the general histology of skin to the more detailed structures shown by definite types of skins used in making leather. Cow Hide. In selecting skin for the production of heavy, sound and durable leather, the tanner usually chooses the hide of the steer or cow. In Fig. 11 is shown a vertical section of cow hide taken from the thickest part of the butt. The specimen was fixed in Erlicki’s fluid immediately after the death of the animal. This is the type of skin suitable for manufacture into sole leather or heavy belting or harness leather. Over 80 per cent of the total thickness of. the hide is made up of heavy, interlacing bundles of collagen fibers, the chief leather-forming con- stituent of skin, and very few of the fat cells that tend to make the leather spongy are to be found among these fibers. The epidermis appears as a thin, dark line forming the upper boundary of the section and occupying barely one-half of one per cent of the total thickness, the rest being the derma, the adipose tissue having been removed from this portion of the hide in flaying. The epidermis can be seen to dip down into the derma in many places, forming the follicles in which the hairs grow. The presence of the muscles, glands and follicles in the top fifth of the derma give this region the appearance of a layer quite distinct from the lower part of the derma. Indeed, it is advantageous, in leather manufacture, to look upon the derma as divided into two distinct layers. The dividing line might conveniently be taken as that formed by the deepest points of the sudoriferous, or sweat, glands. The lower, fibrous region of the skin is often referred to as the reticular layer because of the network appearance of the collagen fibers. This name might well be accepted for most skins suitable for leather manufacture, although it might seem somewhat strained for skins in which the derma is made up largely of fat cells. The chief func-— tion of the upper layer seems to be that of a thermostat for the body and the writer, therefore, proposes the name thermostat layer as indicating its structure as well as its chief function. In Fig. 11 the thermostat layer occupies the top fifth and the Fig. 11.—Vertical Section of Cow Hide. Location: butt. Eyepiece: none. Thickness of section: 20 u. Objective: 48-mm. Stains: Van Heurck’s logwood, Wratten filter: F-red. Daub’s bismarck brown. Magnification: 19 diameters. 40 Fig. 12.—Vertical Section of Thermostat Layer of Cow Hide. Location: butt. Eyepiece: 5X. Thickness of section: 20 uw. Objective: 16-mm. Stains: Van Heurck’s logwood, Wratten filter: H-blue green. Daub’s bismarck brown. Magnification: 85 diameters. 4I 42 THE CHEMISTRY OF LEATHER MANUFAGEORe reticular layer the remaining four-fifths of the section. The advantage of dealing with these layers separately is made clear by the fact that the structure of the reticular layer determines the physical properties of the leather such as tensile strength, solidity, resilience, etc., while the thermostat layer determines more particularly the appearance of the leather. In making the finer grades of leather, a great deal of attention must be paid to the thermostat layer. It is a matter of considerable importance that this layer is almost as thick in a small skin as in a large one; in the thinner skins and even in the thinner parts of the same skin, this layer occupies a greater proportion of the total thickness. The section in Fig. 11 is magnified only 19 diameters. In order to show the structure of the thermostat layer in greater detail, the upper left hand corner of this section was magnified to 85 diameters. At this greater magnification, it is shown in Fig. 12. The Malpighian and corneous layers of the epidermis can now be clearly differentiated, the latter becoming extremely thin where it lines the hair follicle. The stratum granulosum and stratum lucidum do not appear to be present in the epidermis. Attached to the base of the hair follicle and weaving its way upward to the right is the erector pili muscle. Just above this muscle and emptying into the hair follicle is a group of sebaceous glands. The empty space near the lower left hand corner is that formerly occupied by a sweat gland whose duct has wandered out of the plane of the section, reappearing as a pore to the right of the hair just at the entrance to the hair follicle. The fine, black, threadlike lines running roughly parallel to the surface and to be found throughout the thermostat layer are the elastin fibers, or yellow fibers of connective tissue. In this layer, the collagen fibers are very much finer than in the reticular layer and appear to be broken up into individual fibrils. The grain surface appears only as portions of tiny fibrils with no sharp line of division from the rest of the derma. No papillz are to be seen in this section; in fact, we found no papillz in any part of the cow hide, except in the region of the legs. In order to present a still clearer picture of the important thermo- stat layer, we prepared series of sections parallel to the surface of the hide. Strips of hide imbedded in paraffine were placed in the microtome and sections, each 20 microns thick, were cut in succession from the corneous layer to a point in the reticular layer, every section being kept in order and mounted. The five horizontal sections shown in Figs. 13 to 17 were prepared from a strip of hide taken from the thigh so as to include the papillz, which were not present in the other regions. Fig. 13 is a section cut through the epidermis. In the center is the opening of a hair follicle. The circular mass just above - the center is the cross section of a hair. The stringy lines forming an oval shaped mass about the hair are the part of the corneous layer of the epidermis which dips down into the hair follicle. The heavy dots seen throughout the rest of the picture are the nuclei of the cells of the Malpighian layer of the epidermis. The irregularly shaped, light-colored patches are cross sections of the papille of the derma HISTOLOGY OF SKIN 43 Fig 13.—Horizontal Section of Cow Hide. (Through epidermis.) Location: thigh. Eyepiece: 5X. Thickness of section: 20 uw. Objective: 8-mm. Stains: Van MHeurck’s logwood, Wratten filter: H-blue green. Daub’s bismarck brown. Magnification: 200 diameters. which protrude into the epidermis and are made up chiefly of nerves and blood vessels. | | Fig. 14 represents a section cut 0.30 millimeter below the upper surface of the corneous layer. It marks the plane of the derma where the ducts of the sebaceous glands empty into the hair follicles. In 44 THE CHEMISTRY OF LEATHER MANUFACTURE Fig. 13.—Horizontal Section of Cow Hide. (0.30 mm. below upper surface.) Location: thigh. Eyepiece: 5X. ; Thickness of section: 20 wu. Objective: 8-mm. Stains: Van MHeurck’s logwood, Wratten filter: H-blue green. Daub’s bismarck brown. Magnification: 200 diameters. the lower part of the middle of the picture can be seen the cross sec- tion of a hair and of two ducts emptying into the follicle, just above the hair, to right and left. Both the ducts and the follicle are lined with epithelial cells which are continuous with the Malpighian layer of the epidermis and of which they are appendages. The dark, thread- HISTOLOGY OF SKIN 4S Fig. 15.—Horizontal Section of Cow Hide. (0.54 mm. below upper surface. ) Location: thigh. Eyepiece: 5X. Thickness of section: 20 Objective: 8-mm. Stains: Van Heurck’s logwood, Wratten filter: H-blue green. Daub’s bismarck brown. Magnification: 200 diameters. like structures are elastin fibers. The tiny collagen fibers of this region, being stained more lightly, are not prominent. The section in Fig. 15 forms the plane 0.24 millimeter below that of Fig. 14. The hair whose cross section is shown in the lower part of the middle of Fig. 15 is the same as that shown ite bigs dee Loe 46 THE CHEMISTRY OF LEATHER MANUFACTURE Fig. 16.—Horizontal Section of Cow Hide. (0.54 mm. below upper surface. ) Location: thigh. Eyepiece: none. Thickness ot section: 20 ,. Objective: 16-mm. Stains: Van Heurck’s logwood, Wratten filter: H-blue green. Daub’s bismarck brown. Magnification: 48 diameters. hair follicle at this point has a much thicker wall of epithelial tissues and is more thickly bound by elastin fibers. Above the follicle, to the right and left, are the two groups of sebaceous glands whose ducts can be seen emptying into the follicle in Fig. 14. These glands re- semble bunches of grapes. Each dot is a cell nucleus and the fine HISTOLOGY OF SKIN 47 a oe » ee Ge es a , LI OM CM A IS Fig. 17.—Horizontal Section of Cow Hide. (0.84 mm. below upper surface.) Location: thigh. Eyepiece: 5X. Thickness of section: 20 wn. Objective: 8-mm. Stains: Van Heurck’s logwood, Wratten filter: H-blue green. Daub’s bismarck brown. Magnification: 200 diameters. lines are the thin walls bounding the cells. A portion of the erector pili muscle is visible at the midpoint of the top of the picture. It is passing obliquely upward through the plane of the section and away from the hair follicle. The contraction of this muscle exerts a pressure upon the cells and their oily contents are forced up through the ducts 48 THE CHEMISTRY OF LEATHER MANUFACTURE and into the hair follicles at the openings shown in Fig. 14. Between the two groups of glands and the hair follicles is a mass of muscle tissue of the same kind as that constituting the erector pili muscle. Apparently the muscle extends also into this region and exerts its pressure upon the cells by a sort of pinching action. Fig. 16 is a photomicrograph of this section taken at lower mag- nification so as to show the general arrangement of follicles and glands. The portion appearing in Fig. 15 can now be recognized just below the center of the picture. Associated with the hair we have been fol- lowing are three others, and this tendency for the hairs to group themselves in threes and fours is very noticeable. Some of the follicles are not so deeply seated as others and have their sebaceous glands in a plane higher up. This explains why no glands are to be seen in the vicinity of some of the follicles. The short, thick lines appearing here and there are arteries or veins wandering in and out of the plane of the section. In Fig. 17 is shown the section forming the plane 0.30 millimeter below that of Fig. 15, or a total distance of 0.84 millimeter from the upper surface of the corneous layer. A cross section of the same hair as that shown in Figs. 14 and 15 appears in the center of the picture, but this time we have cut right through the hair bulb. The black mass is the bulb and the light patch at its center is the hair papilla. To the right and left and above the hair bulb are the sweat glands. They appear as large, empty sacs, with portions of their linings of epithelial cells showing like leopard spots. In this plane the elastin fibers are much less numerous than in the regions higher up and the collagen fibers are now much larger and grouped in bundles. At a distance of 0.12 millimeter below this plane, we encounter the last of the epithelial cells of the sweat glands and therefore the lower boundary of the thermostat layer. : The reticular layer consisted almost entirely of collagen fibers, elastin fibers being present only in the lowest region and surrounding the blood vessels and nerves traversing other parts of the reticular layer. Calf Skin. A calf skin, very naturally, appears much like a cow hide in minia- ture. In Fig. 18 is shown a vertical section from the skin of a healthy young heifer calf, which had been fixed in Erlicki’s fluid immediately following the slaughter and flaying of the animal. As a rule, the skin of a heifer calf has greater solidity and fineness of appearance than that of a steer calf and is, consequently, to be preferred for leather making. In comparing Figs. 11 and 18, it should not be overlooked that the section of calf skin is magnified more than twice as highly as that of the cow hide. In fact, in making comparisons of any photomicrographs in the book, erroneous conclusions may be drawn, if the magnifications are not taken into consideration. The relatively greater thickness of the thermostat layer in the calf Fig. 18.—Vertical Section of Calf Skin. Location: butt. Eyepiece: none. Thickness of section: 20 wu. Objective: 32-mm. Stains: Van Heurck’s logwood, Wratten filter: F-red. Daub’s bismarck brown. Magnification: 40 diameters. 49 50 THE CHEMISTRY OF LEATHER MANUFAC skin is noticeable. This fact is doubly interesting because the structure of this layer is of much greater importance for calf skin than for cow hide; calf skins are generally used to make dressing and other leathers where fineness of appearance of the grain surface is highly valued, while cow hides more often are used for sole, belting, and harness leathers. Another point to be noted in comparing Figs. 11 and 18 is that the sections were cut from exactly corresponding parts of the skins of the two animals. The importance of this point will be made clear from a study of Figs. 20 to 27. It is well known that a tanned skin is not uniform in structure throughout its entire area. The ‘butt is usually much thicker and has greater solidity than any other part. The ~ shanks are firm, but thin, while the flanks are thick, but spongy. In order to show how the structure of the skin varies in different regions, 8 strips were cut from the locations indicated in the diagram shown in Fig. 19. The skin was the same as that whose vertical section is shown in Fig. 18. Vertical sections of these 8 strips are shown in Figs. 20 to 27. In comparing the sections, it will be noted that the thickness of the thermostat layer is uniform through- out the skin, but that both ~ the thickness and texture of Fic. 19—Diagram of Calf Skin showing loca- the reticular layer vary tions of sections whose photomicrographs widely. are shown in Figs. 20 to 27. The reticular layer is - ee shank ; ee ae shank ; nearly 3 times as thick in the seuencuidert PAs ete butt as in the hind shank. In 7: butt; 8: tail. the shoulder, the reticular layer is thinner than that of the butt and its fibers are somewhat finer. In the belly, the collagen fibers run nearly parallel to the grain surface and offer little resistance to any tendency to pull them apart in a vertical direction, whereas many of the fibers in the butt run nearly vertically, with some running in almost any direction, making this region very resistant to distortion. The grain surface appears less serrated on the butt than elsewhere. In fact, most of the differences observable in the various parts of finished leather may be attributed to initial differences in structure of the living skin. In studying Fig. 18, use may be made of practically the entire description of cow hide given above. The bottom fifth of the picture shows the adipose tissue, consisting of rows of fat cells held together mimeo LOG SLOP eS KIN 51 by strands of connective tissues. The thick band forming the lower boundary of the derma is closely interwoven with elastin fibers, but between this region and the thermostat layer, as in the cow hide, there are very few elastin fibers. A better view of the fibers of the reticular layer may be had by referring to Fig. 8, which shows some of the fibers appearing at the left hand side of Fig. 18, but at a much higher magnification. Sheep Skin. Fig. 28 shows a vertical section of the skin of a healthy sheep, fixed in Erlicki’s fluid immediately after the death of the animal. Its ' structure is very different from that of the calf skin, both in the thermostat and reticular layers. A comparison of Figs. 18 and 28 indicates very plainly why sheep skin cannot be substituted for calf skin, where firmness and substance are desired. The collagen, or leather-forming, fibers of the sheep are extremely thin and not closely interwoven and tend to run parallel to the skin surface, which in itself makes for looseness of texture. Moreover, in the thermostat layer there are numerous sweat glands and fat cells, which leave empty spaces in the finished leather and make it very spongy The proportion of fat cells to collagen fibers in sheep skins varies considerably according to the feeding of the animal, and there is often to be: found an almost continuous layer of fat cells separating the two main layers of the skin. In such cases, it 1s desirable to separate the skin into its two layers before tanning and to tan each separately rather than to try to keep them together. Usually the skins are split into two parts after the liming process and the thermostat layers, called grains, are tanned with sumac or other tanning extract to make leather © suitable for bookbinding, hat bands, etc., while the reticular layers are converted into chamois leather, for which they are particularly suitable, by means of a tannage with cod oil. The dark, curved mass, very prominent in the upper, right hand of the picture and the smaller masses of similar appearance are por- tions of hair follicles. Unlike the follicles of the calf, those of the sheep turn and twist in every direction. We were unable to find one follicle lying wholly in a single plane. The curvature of these follicles is responsible for the curliness in the wool of the sheep. In _ the cow and calf, the hair is straight because the follicles are straight. The twisting of the follicles makes the study of the structure of sheep skin more difficult than that of the calf. But the examination of several sections is sufficient to show that the general mechanism of the two skins is the same. Running from the top of the portion of hair follicle showing in the upper right part of the picture is a part of an erector pili muscle. The sebaceous glands appear to be very near the surface, while the sweat glands occupy much of the lower portion of the thermostat layer. Sections from this skin at different stages of the tanning processes are shown in Chapters 8, 9, and 13 and should be examined in con- if y we "he. " GER Bete ee, Fic. 20.—Fore Shank.. Fic. 21.—Hind Shank. . Fic. 22.—Neck. Fic. 23.—Belly. Vertical Sections of Calf Skin. : Locations: as noted. Eyepiece: none. Thickness of sections: 15 wu. Objective: 32-mm. Stains: Van MHeurck’s logwood, Wratten filter: F-red. Picro-indigo-carmine. Magnification: I5 diameters. 52 SON pgp Sys ERR Fic. 24.—Shoulder. Fic. 25.—Backbone. Fic. 26.—Butt. Fic. 27.—Tail. Vertical Sections of Calf Skin. Locations: as noted. Eyepiece: none. Thickness of sections: 15 wu. Objective: 32-mm. Stains: Van MHeurck’s logwood, Wratten filter: F-red. Picro-indigo-carmine. Magnification: 15 diameters. 53 Fig. 28.—Vertical Section of Sheep Skin. Location: butt. Eyepiece: none. Thickness of section: 20 p. Objective: 16-mm. Stains: Van MHeurck’s logwood, Wratten filter: C-blue. Daub’s bismarck brown. Magnification: 50 diameters. 34 E Fig. 29.—Vertical Section of Kid Skin. Location: butt. Eyepiece: none. Thickness of section: 25 w. Objective: 16-mm. Stains: Van MHeurck’s logwood, Wratten filter: H-blue green. Picro-indigo-carmine. Magnification: 50 diameters. 55 56 -THE CHEMISTRY OF LEATHER MANUFACTURE nection with the study of the raw skin. The epidermis can be differ- entiated more clearly by comparing Fig. 28 with Figs. 66 and 67 of Chapter 8. The general arrangement of the elastin fibers is best shown in Fig. 83 of Chapter 9. 3 The specimen of sheep skin shown was unusually free from the fat cells that tend to separate the skin into two layers. We were able to tan it into a reasonably firm piece of leather, A section of this leather is shown in Fig. 104 of Chapter 13. The leather was soft and somewhat spongy, but is probably a good example of the type of skin often substituted for kid skin in the manufacture of glove leather. Goat Skin. In many respects the skin of the goat may be regarded as having a structure intermediate between that of the calf and the sheep. The fibers are fuller and firmer than those of the sheep, but are hardly equal to those of the calf. The glands and fat cells, which are re- sponsible for the sponginess of sheep leather, are very much less abundant in goat skin, although it must be admitted that this is largely dependent upon the animal’s feeding. Both the goat and the sheep skins of the general market vary widely in quality and substance, a fact which warrants a considerable extension of the study of their structures. Calf skins, on the other hand, do not vary in quality nearly so widely. | Like the calf, the goat has straight follicles, and, consequently, straight hair. The surface of goat skin is very much coarser than that of calf skin. A glance at Fig. 9 will show that the pattern of the calf grain is considerably finer, even than that of the kid. Rough- ness of grain, however, is sometimes desirable and the grain surface of goat skins is often made still coarser by mechanical means, A vertical section of kid skin is shown in Fig. 29. This was just an average domestic skin in the condition in which fresh skins are usually received at the tannery. The epidermis is the very thin dark line forming the upper boundary of the skin. It dips down into the derma, forming a nearly straight follicle, in which the hair grows. The erector pili muscle is the thin fine running upward to the right from the base of the follicle. The opening of the sebaceous glands into the follicle can be seen just above the erector pili muscle. The fact that the collagen fibers run nearly parallel to the surface gives this skin, in its most solid part, a softness and looseness found only in the flanks of the calf skin. | Bounding the lower surface of the derma is a layer of striated muscle tissue, which permits the animal to twitch its skin. Muscles of this kind are often found on most of the various kinds of skins used for making leather. A typical section of chrome tanned goat skin is shown in Fig. 146 of Chapter 14. It is interesting to compare its general structure with those of the calf and sheep. MISPLOLOGY OF SKIN 57 Hog Skin. The comparatively low value of hog skin for leather manufacture can be appreciated by studying the section shown in Fig. 30. The Fig. 30.—Vertical Section of Hog Skin. Location: butt. Eyepiece: none. Thickness of section: 20 uw. Objective: 48-mm. Stains: Friedlander’s logwood, Wratten filter: C-blue. Daub’s bismarck brown. Magnification: 14 diameters. reticular layer is composed chiefly of fat cells, which have practically no value in making leather. We have here a case where the general use of the term reticular is apt to be misleading. ‘The fat cells extend 58 THE CHEMISTRY OF LEATHER MANUFACTURE even up into the thermostat layer. The close relation of this structure to that of the human scalp, shown in Fig. 1, should be noted. The epidermis, as well_as the upper surface of the derma, is very rough and irregular in appearance. As in other skins, the epidermis dips down into the derma, forming the follicles in which the hairs, or rather bristles, grow. The hair bulbs are imbedded in the mass of fat cells which make up the reticular layer. These fat cells ex- tend higher up into the thermostat layer in the region of each hair follicle, about which the fat cells form cone-shaped masses. The structure of a hair bulb from the hog is shown in Fig, 5. The erector pili muscle belonging to the follicle shown in Fig. 30 did not lie in the plane of the section. A portion of one of these muscles can be seen in Fig. 84 of Chapter 9, which, because of its very much higher magnification, also shows the arrangement of the elastin fibers of the thermostat layer. The hog has relatively much fewer elastin fibers than the cow, calf, or sheep. : The roughness of the surface of the derma is further accentuated by the presence of papillae, which seem to be rare in the skins of most of the lower animals studied. In the cow hide, papille were found only in the region of the legs, while in the calf, sheep, and goat skins, no papillae were found at all. It would be interesting to determine whether the abundance of papille makes the hog more sensitive to touch and pain than the other lower animals. The ex- treme roughness of the grain surface of tanned hog skin is very noticeable in Fig. 9. After the skin has been unhaired and prepared for tanning, only a portion of the thermostat layer remains. The follicles then are simply pockets lined with the grain membrane, the lower portions protruding out from the under side of the skin. When the tanned skin is shaved down on the under side to make it smooth, the bottoms of these pockets are cut away, leaving holes wherever there were bristles in the original skin. This serves further to lower the value of leather made from hog skin. A section of tanned hog skin is shown in Fig. 107 of Chapter 13. Horse Hide. The outstanding peculiarity of horse hide lies in the reticular layer. In the region of the butt there is a dense mass of collagen fibers in the reticular layer so compact as to render leather made from the butt naturally waterproof and nearly air tight. A section of horse hide taken from the butt is shown in Fig. 31. The dense mass of fibers, often called the glassy layer, can be seen running horizontally across the middle of the picture and appearing much darker than the remaining fibers. The portion of the hide containing the glassy layer is known as the shell and is used to make the leather sold under the name of cordovan. The rest of the hide not only does not have this glassy layer, but the fibers of the reticular layer are very loosely inter- } Fig. 31.—Vertical Section of Horse Hide. Location: butt. Eyepiece: none. Thickness of section: 20 wu. Objective: 32-mm. Stains: Van MHeurck’s logwood, Wratten filter: C-blue. Daub’s bismarck brown. Magnification: 25 diameters. 59 60 THE CHEMISTRY OF LEATHER MANUFACTURE woven, giving the leather made from it a spongy substance that limits 1US “115e; The thermostat layer of horse hide resembles that of cow hide. The general arrangement of the hair follicles, the erector pili muscles, and the sebaceous glands can be seen in Fig. 31, but the full detail shown in the sections of cow hide is lacking because the specimens of horse hide were not fixed immediately after the death of the animal, as in the case of the cow hide. The section, however, represents a hide in probably the usual condition in which horse hides are received at the tannery. Figs. 105 and 106 of Chapter 13 show a comparison of leather made from the shell and that made from the portion of hide immediately adjoining the shell. In splitting the leathers to a nearly uniform thickness, the knife of the splitting machine cuts through the lower part of the glassy layer. ‘lhe greatest contrast between the two specimens is thus shown in the lower portions. Guinea Pig Skin. A section of guinea pig skin is shown in Fig. 32 as an example of very small skins. Such skins can be made into fairly good leather, but their diminutive size limits the demand for them and it is question- able whether such leather could be sold at a profit. A point worthy of note is that the thermostat layer of the guinea pig skin is of practically the same thickness as that of a calf skin, which is very much larger. As shown in the description of the different parts of the calf skin, when nature provides a thinner skin, she does so almost entirely at the expense of the reticular layer, and not of the thermostat layer. It is possible that a mininium thickness for any size of animal is required for the proper operation of this important layer. The corneous layer of the epidermis appears like a few strands of delicate threads just above the Malpighian layer, the dark line bounding the upper side of the derma. ‘The collagen fibers of the reticular layer are so fine that they appear only as thin threads even at a magnification of 70 diameters. The dark band crossing the bottom of the picture is a mass of striated muscle tissue. Fish Skins. The detailed structure of fish skins is very different from those of mammals. Nevertheless fish skins yield a leather comparing favor- ably with some of the more common types of commercial leathers. Fish leather is very tough, as a rule, and is suitable for many purposes where great strength is required. Sturgeon leather used for lacing heavy belts together has been known to outwear the belts. Tt je said that the people of New England, in the old days, made shoes and gloves from the skin of the cod fish. Other fish skins are sometimes used for making fancy leathers. In Pigs.°33, 345° andeahaare photomicrographs of sections of the Fig. 32.—Vertical Section of Guinea Pig Skin. Location: butt. Eyepiece: none. Thickness of section: 30 p. Objective: 16-mm. Stains: Van Heurck’s logwood, Wratten filter: F-red. Picro-indigo-carmine. Magnification: 70 diameters. 61 Fig. 33.—Vertical Section of Halibut Skin. Fig. 34.—Vertical Section of Cod Fish Skin. Fig. 35.—Vertical Section of Salmon Skin. Location: side. Eyepiece: none. Thickness of sections: 20 u. Objective: 32-mm. Stains: Friedlander’s logwood, Wratten filter: H-blue green. Picro-indigo-carmine. _ Magnification: 17 diameters, 62 / Ve a ae ee ee | . see a iy, Made Sika -5.CH,.CH(NH,)-COOH. 10. Aspartic acid, aminosuccinic acid, HOOC.CH,.CH(NHz2). COO. 1Cf. Chemical Constitution of the Proteins. R. H. A. Plimmer. Longmans, Green & - Co., London. 65 1. Glycine, aminoacetic acid, NH,.CH,.COOH. 2. Alanine, a-aminopropionic acid, CH,.CH(NH,).COOH. 3. Valine, a-aminoisovalerianic acid, (CH,;)2:CH.CH(NH,). COOH. A. Leucine, a-aminoisocaproic acid (CH,)2:CH.CH,.CH(NH,). PGOOH. vp 6. 66 THE CHEMISTRY OF LEATHER MANUFACTURE 11. Glutamic acid, a-aminoglutaric acid, HOOC.CH,.CH,.CH (NH,).COOH. 12. Arginine, a-amino-5-guanidinevalerianic acid, HN:(C.NHz). NE UCH. Cre Ghee Ch UNE Cer aise 13. Lysine, o-e-diaminocaproic acid, NH,..CH,.CH,.CH,.CHz2. CH(NH,).COOH: 14. Caseinic acid, diaminotrioxydodecanic acid, Ci,2H..N,O;. 15. Histidine, B-iminazolyl-a-aminopropionic acid, CH Un N NH Rae! HC — ClCH CE CNT pisos 16. Proline, a-pyrrolidinecarboxylic acid, EEG dab (CH. [eae HG c GH GOOCH: as NH 17. Oxyproline, oxypyrrolidinecarboxylic acid, C;H,NO,. 18. Tryptophane, B-indole-a-anunopropionic acid, NH C.CH,.CH(NH,).COOH. Under suitable conditions, amino acids can be made to combine with each other by removing the elements of water, the amino group of one combining with the carboxyl group of another, thus CH,.CH(NH,) .COOH + NH,.CH,.COOH = CH,.CH(NH,). CO.NH.CH,.COOH--H.O. A combination of two amino acids is called a dipeptide, one of three a tripeptide, etc. Fischer * succeeded in preparing the octadecapeptide NH, . CH(C,H,) . CO. [NHCH,CO], . NH . CH(C,H,) . CO. [NHCH,CO],.NH.CH(C,H,).CO.[NHCH.CO],.NH . CH,.COOH, which contains 15 glycine and 3 leucine residues and has a molecular weight of 1213. It gives the biuret test for protein, is precipitated from solution by tannin, and would have been classed as a protein had it been found in nature. Later Abderhalden and Fodor? succeeded in ? Synthesis of Polypeptides. Emil Fischer. Pr. Chem. Soc. 23, 82; C. A. 1 (1907), 1545. 3; ® Synthesis of Polypeptides of High Molecular Weight from Glycocoll and 1-Leucine. E. Abderhalden and A. Fodor, Ber, 49 (1916), 561; C. A. 10 (ome f sexing CHEMICAL CONSTITUENTS OF SKIN 67 preparing a polypeptide containing 15 glycine and 4 leucine residues and having a molecular weight of 1326. The close resemblance of the more complex polypeptides to the natural proteins and to their first products of decomposition, the pro- teoses and peptones, and the fact that all proteins yield amino acids upon complete hydrolysis have established the view that the general structure of proteins is at least similar to that of the polypeptides. The above list of amino acids indicates the tremendous number of pos- sible combinations to form proteins and of the isomeric forms that any individual protein may have. The generally accepted methods of classifying proteins are based upon differences in solubility, speed of hydrolysis, and precipitability under definite conditions. But, since a small amount of foreign matter may alter these properties entirely for a given protein and because of the difficulty of separating and purifying proteins, this system of classification is not wholly satisfactory, although it is, perhaps, the best available at the present time. The common names applied to pro- teins, such as keratin, albumin, etc., do not represent individual sub- stances, but groups of closely related proteins whose quantitative separation is very difficult. The most important classes of skin’ proteins, in the order of in- creasing importance to the tanner, are the mucins, albumins, globulins, melanins, keratins, elastins, the unnamed proteins of the grain surface, and the collagens. [Except in the case of fur skins, the first five classes are of importance only because they must be removed from the skin prior to tanning, without injuring the remaining protein matter. In general, the albumins are the only skin proteins soluble in pure water. The globulins are soluble in dilute salt solutions and the mucins and melanins in dilute alkalies. The four remaining classes, which belong to the general group of proteins known as albuminoids, are insoluble in dilute solutions of acids, bases, or salts at room temperature, but all are dissolved and hydrolyzed by boiling solutions of concentrated acids or alkalies. The keratins are dissolved by strongly alkaline solutions be- fore the remaining three classes are seriously attacked and the elastins are easily dissolved by trypsin before any injury is done to the collagen or grain surface. In boiling water, the collagen goes into solution as gelatin, leaving behind a residue of elastin and the proteins of the grain surface. The albumins and globulins are found in the blood and lymph of the skin and also in the fluids of the muscles and nerves. By extracting powdered dog skin with a 10-per cent solution of sodium chloride, under toluene at 37° C., Rosenthal * obtained a quantity of albumins and globulins which, upon coagulating, washing with water, alcohol, and ether, and drying, gave a weight equal to 24 per cent of the total protein of the skin. But a yield of only 4.2 per cent was obtained from calf skin. The albumins are soluble in pure water or in dilute solutions of 2 * Biochemical Studies of Skin. G, J. Rosenthal. J. Am. Leather Chem, Assoc. 11 (1916), 403. 68 THE CHEVISTRYOF LEATHER MANUFACTURE acids, bases, and salts, but are precipitated by the addition of con- centrated mineral acid or by saturating a weakly acid solution with salt. Their solutions coagulate upon boiling, in the presence of a small amount of salt. The globulins generally are insoluble in pure water at the neutral point, but dissolve in dilute neutral salt solutions, from which they can be precipitated by sufficient dilution or by saturating the solution with salt, being most readily soluble in salt solutions of moderate concen- tration. They dissolve freely in dilute solutions of acids and alkalies. Like albumins, their solutions coagulate upon heating. Fibrinogen, an important constituent of the blood, is usually classed as a globulin, but differs from serum globulin in being precipitated from solution by a lesser concentration of neutral salt and of coagulating at a lower temper- ature. It tends to clot upon exposure to air, forming the insoluble © protein fibrin, which action is favored by rise of temperature or agita- tion and is hindered by cooling or the addition of acids, alkalies, or concentrated salt solutions. The clotting action is supposed to be due to the action of an enzyme, thrombin, which is not a normal constituent of blood, but which is formed from the leucocytes and blood plates in the presence of calcium salts. . The mucins are conjugated proteins, of the group known as glyco- proteins, containing both protein and carbohydrate groups in their molecules. They are insoluble in pure water, but, in faintly alkaline solution, give mucilaginous solutions which are precipitated by the addi- tion of acid. It is questionable whether mucins are abundant in the skins of mammals. It has often been assumed that the mucins form the elusive “interfibrillary cementing substance” of the skin, but the existence of a cementing substance in the fibers, other than collagen itself, has not been clearly demonstrated. Rosenthal ® extracted calf skin, previously freed from albumins and globulins, with half-saturated lime water under toluene. Upon render- ing the extract acid with hydrochloric, protein matter was precipitated, which was washed with dilute acid, water, alcohol, and ether, and dried and weighed. The yield of protein, which he called mucoid, equalled about 2.7 per cent of the total protein matter of the skin. The yield from the solid part of the butt was 4.8 per cent against only 1.2 per cent for the loose portions of the belly. Although mucoids are dissolved by dilute alkalies and precipitated by rendering the solu- tion acid, doubt is thrown on Rosenthal’s interpretation of his results by the experiments of Thompson and Atkin,® who showed that hair and wool are partly dissolved by lime liquors and that some of the matter dissolved is precipitated by rendering the solution slightly acid. Since the newly formed epithelial cells are very much more easily at- tacked than hair and wool, much of the material isolated by Rosenthal may actually have been derived from this source. | No very sharp line of distinction can be drawn between the mucins 5 Loc. ctt. : * Note on the Analysis of Lime Liquors. F. C. Thompson and W. R. Atkin. J. Soc. Leather Trades’ Chem. 4 (1920), 15. PevICAE CONSTITUENTS OF SKIN 69 and the mucoids. Hammarsten ‘ differentiates between them as follows: “The true mucins are characterized by the fact that their natural solu- tions, or solutions prepared by the aid of a trace of alkali, are mu- cilaginous, ropy, and give a precipitate with acetic acid which is insoluble in excess of acid or soluble only with great difficulty. The mucoids do not show these physical properties, and have other solu- bilities and precipitation properties.” The melanins are proteins of intense color, usually reddish-brown to black, constituting the pigment of the hair and epithelial cells. They are insoluble in water and dilute acids, as a rule, but dissolve more ‘or less readily in dilute alkalies. They may be extracted with boiling dilute alkali and precipitated by the addition of acid. They contain variable amounts of iron and sulfur in combination. The origin of the melanins is not known with certainty, although it seems probable that they are derived from the blood and lymph. Their development is accelerated by frequent exposure to strong sun- light. Prolonged exposure is followed by a rush of blood to the skin and the production of pigment to protect the tissues against the action of the intense light. This shows itself in the apparent darkening of the color of the skin. The coloring matter of the blood, hemoglobin, belongs to the class of conjugated proteins known as chromoproteins and, like the melanins, also contains iron and sulfur. That the blood and lymph contain substances capable of reacting to produce deeply colored bodies is well appreciated by the tanners. Skins from which the blood and lymph have not been washed are liable to develop stains very difficult to remove, unless special pre- cautions are taken, which will be discussed in Chapter 6 in connection with the preservation of skin to be kept for a considerable period before tanning. The chief constituent of the epidermal system, including the epi- dermis, hair, and epithelial cells of the glands, is the class of proteins known as keratin. The general method of preparing this material for examination is to boil the finely divided sample containing it with water and then to digest the residue with an acid pepsin solution fol- lowed by an alkaline trypsin solution and then to wash it thoroughly with water, alcohol, and finally with ether. Keratin differs chemically from other classes of proteins in yield- ing a comparatively large amount of cystine, upon hydrolysis. In the following table are given the yields of amino acids obtained from keratins from different sources along with those from samples of elastin and collagen, or gelatin. The differences shown by keratins from different sources is interesting, but each sample analyzed probably consisted of a mixture of different keratins more or less contaminated by other proteins. Keratin prepared in the manner described above is naturally very resistant to the action of dilute acids and alkalies, pepsin, trypsin, and boiling water, but it is dissolved by strong alkalies and by water heated 7 Physiological Chemistry. O. Hammarsten. Translation by J. A. Mandel. John Wiley & Sons, New York, 70 THE CHEMISTRY OF.LEATHER MANUFACTURE TABLE T. Per Cent Amino Acid Obtained from Keratin from Collagen Horse Sheep Sheep Goose or Amino Acid Hair® Wool® Horn® Feathers” Elastin™ Gelatin” Glycine) 20. tore ee ny 0.6 0.5 2.6 25.8 o5.50F WARMING 4 ee eee 1.5 4.4 1.6 1.8 6.6 8.7 Waltrie be fates corre ane 0.9 2.8 4.5 0.5 1.0 0.0 POUNCE ince ie oe tere FT ct 15.3 8.0 27-5 7,1 Seriitente yh ar eae 0.6 0.1 ES 0.4 ae 0.4 Aspartic: acid: ~ 4 Maw. 0.3 2.3 2.5 I. sy, 3.4 Glatamie acid: i252)... 3.7 i209 172 eae 08 © 5.8 Cystine On faeces: 8.0 Ae 7.5 a re tng Phenylalanine “2... 0.0 ne 1.9 0.0 3.9 1.4 Tyrosine ne pn i ae 3.2 2.9 3.6 3.6 OCF: 0.01 Fooline’ 3. (aes e's 3.4 4.4 37a 3.5 L.7 9.5 Osyproline! oneness ae < oat ae : 14.1 Plistidigie tere: $5. oe: ge 0.6 car es a ass 0.9 ENV ISIIIG 75 oe oy nee, oa 4.5 ee 20 i 0.3 8.2. LYSING eae Pee oe I.1 a3 0.2 HM Pee 5.9 to 150° C. under pressure. The method of preparation may be criticized on the ground that it does not include young keratin. On the other hand, it may be contended that the proteins of newly formed epithelial cells are not keratins at first, but are later converted into keratins. However, the changes in properties with age are so gradual as to make it almost impossible to draw any sharp line of demarcation. This is a good example of the difficulty of trying to classify proteins strictly according to properties. The cells of the Malpighian layer of the epidermis are readily attacked by trypsin and by solutions of ammonia, but become very much more resistant as they are pushed upward into the corneous layer. In the stratum granulosum of the epidermis, the protoplasm of the epithelial cells has dried up and appears like granules inside of the cells. Walker ** regards these granules as consisting of two sub- stances, keratohyalin and eleidin, presumably stages in the transforma- tion of the protoplasm into the wax and fatty material with which the cells of the corneous layer of the epidermis are loaded. The yellow, elastic fibers interlacing the outer layers of the derma and enveloping the nerves and blood vessels are made up of a class of proteins called elastin. The tendons of the body have been the chief source of elastin used for study, in particular the ligamentum nuche, the tendon at the back of the head of the ox. F. L. Seymour-Jones ™ found that a piece of ligamentum nuche of about 1 square centimeter cross section gave on a testing machine an extension of 150 per cent before breaking, the strain being too small to measure; less than 5 lbs. 8 Abderhalden and Wells. 2. physiol. Chem. 46 (1905), 31. ; * Abderhalden and Voitinovici. Jbid., 52 (1907), 348. 30 Abderhalden and Le Count. IJbid., 48 (1905), 40. 1 Abderhalden. Lehrbuch der physiol. Chem. (1909). 2H. D. Dakin. J, Biol, Chem. 44 (1920), 524. 13 Dermatology. N. Walker. Wm. Wood & Co., New York. * Chemical Constituents of Skin. F. L. Seymour-Jones. J. Ind. Eng. Chem. 14 (1922), 130, Pee VICAL CONSTITUENTS. OF SKIN 7t He also found that the tendon was slowly digested by lime water, although the action may have been due to bacteria. Elastin may be prepared for study by extracting this tendon with dilute sodium chloride solution, washing and then boiling it with water, then with a 1-per cent solution of potassium hydroxide, again with water, and then with acetic acid. The residue is then treated with cold 5-per cent solution of hydrochloric acid for 24 hours, thoroughly washed with water, boiled again with water, and then washed with al- cohol and ether and dried. It then has a yellowish-white appearance. It is not dissolved by boiling water nor by acids and alkalies in the cold, but is easily dissolved by concentrated mineral acids upon heating. The yields of the different amino acids from a sample of elastin are given in Table I. It is, of course, not safe to assume that elastin from skin has exactly the same properties as that from other parts of the body, but the difficulty of isolating some of the skin proteins for study has made it desirable to investigate proteins of the same general classes from parts of the body where they are more easily available, if only to get a suggestion of the properties of the skin proteins. Actually we do find that the elastin of skin behaves much like that from the ligamentum nuche, being resistant to boiling water and to cold solu- tions of acids and alkalies. In glue manufacture, much of the elastin remains in the scutch or residue left after boiling the skin in water. By examining sections of skin under the microscope, after special treatments, we have found that the elastin fibers are not appreciably attacked by dilute solutions of acids and alkalies or by tannery lime liquors, but are easily dissolved by neutral trypsin solutions. ‘These fibers apparently act so as to resist an increase in area of the grain surface of the skin. The proteins of the grain surface are remarkably resistant to most of the ordinary chemical reagents. The thin fibers of this surface are not dissolved by solutions of caustic alkalies sufficiently strong to destroy the collagen fibers, epidermis and hair. In boiling water, they evidently undergo some change in composition, but remain un- dissolved in the form of a thin sheet while the collagen passes into solution as gelatin. They are apparently unaffected by trypsin solu- tions strong enough to dissolve all of the elastin fibers beneath them. But in contact with water having a pH value of about 6, they are easily attacked and liquefied by putrefactive bacteria, although this action can be checked by the addition of a sufficient amount of acid, alkali, or salt. These fibers represent only a very small proportion of the skin by weight, but they are of great importance because they form the grain surface of finished leather, giving it its characteristic appearance. Their position in the grain surface is shown in Fig. 150 of Chapter 106. In tanning and dyeing, they take a color different from that assumed by the collagen fibers, which is noticeable when leather is cut. Any damage to the grain surface reduces the selling value of the leather materially. ‘9m. THE CHEMISTRYCOE eral Er MANUFACTURE Collagen is the most abundant protein of the skin and the one of greatest importance to the tanner, since it is the basis of leather. It constitutes the bulk of the substance of the white fibers of the connective tissues of the derma. Collagen can be prepared for study from fresh skin by removing the other constituents. The adipose tissue is carefully cut away and the skin thoroughly washed. It is then extracted with several changes of 10-per cent sodium chloride solution, in a closed jar set in a tumbling machine, or agitator, in order to remove the soluble protein matter. It is then put back into the same jar with a one-tenth-per cent solution of sodium sulfide containing lime well in excess of saturation and tumbled occasionally for several days, or until the hair is quite loose. The hair and epidermal matters are then removed by scraping the grain surface with a knife blade. The entire grain surface is then cut away, preferably on a splitting machine. The skin is then washed thoroughly to remove most of the lime and is then digested for 5 hours at 40° C. with a solution containing 1 gram of U.S. P. pancreatin, 2.8 grams of monosodium phosphate, and 18 cubic centimeters of molar sodium hydroxide per liter. This removes all of the elastin fibers. The skin is then cut into small pieces and put into a jar of water equipped with a stirring device. Hydrochloric acid is added at such rate as to maintain the solution just faintly acid to methyl orange. When no more acid is required, the pieces are left to wash in running tap water over night. Next day they are soaked in several changes of alcohol to remove the water and then in xylene, after which they are exposed to air until the xylene has evaporated. They are then ground in a mill to a fibrous powder. Collagen thus prepared is known as hide powder. Upon heating with water to 70° C., collagen slowly passes into ~ solution as gelatin. But just what relation gelatin bears to its parent substance collagen is not known with certainty. Hofmeister *® sug- gested that collagen is an anhydride of gelatin and that the change from one to the other is reversible, collagen being regenerated by drying gelatin at 130°C. This heating changes the properties of gelatin so that it swells in water to a lesser extent than before and passes into solution with greater difficulty. In commenting upon Hofmeister’s work, Alexander *® says “It is extremely doubtful if collagen is re- generated under these conditions, the more probable explanation being that, upon driving off the water, the constituent particles of the gelatin approach so close as to form an irreversible gel, thus rendering it insoluble.” C. R. Smith 17 found that gelatin dried at 100° C. and then heated to 128° loses 1.25 per cent in weight. It then swells very slowly and dissolves in water at 35° to 40°, with nearly complete restoration of its jellying power. He concedes that gelatin dried at 128° may 16 Z. physiol. Chem. 2 (1878), 299. 16 Allen’s Commercial Organic Analysis. Vol. 8 (1913), p. 586 17 Mutarotation of Gelatin and Its Significance in Gelation. C. R. Smith. J. Am. Chem. Soc. 41 (1919), 135. CHEMICAL CONSTITUENTS OF SKIN 73 be converted into collagen, but that collagen itself may represent a form of gelatin which is difficult to disperse. Emmett and Giles,’ on the other hand, suggest that the conversion of collagen into gelatin involves an intramolecular rearrangement. Plimmer ¥° says “those proteins which are resistant to the action of trypsin until they have been acted upon by pepsin will have all their units contained in the anhydride ring.” Gelatin is easily hydro- lyzed by either pepsin or trypsin, while it has been generally believed that collagen is hydrolyzed by pepsin, but not by trypsin. This led the author 2° to suggest that Plimmer’s statement corroborated Hof- meister’s view of the anhydride structure of collagen. But Thomas and Seymour-Jones ** have recently demonstrated that collagen is at- tacked by trypsin under the right conditions. The erroneous view that collagen is resistant to tryptic diges- tion unless previously swollen with acid or alkali dates back to a series of studies by Kuthne,?* Ewald and Kuhne,?? and Ewald,?* which were based only upon qualitative observa- tions. Thomas and Seymour-Jones found that trypsin acts most rapidly upon collagen at a pH value of 5.9 and that the action is not appreciably accelerated by soaking the protein previously in solutions of higher ‘or lower pH values such that the protein is not actually hydrolyzed by the acid or alkali. In studying _—- the effects of time and concentra- ; en At : tion of enzyme upon the digestion Fic. 37—Rate of digestion of hide of hide powder by trypsin, they powder by trypsin as a function of adopted the following precedure. time. In each experiment 0.5 gram of hide powder was placed in a centrifuge tube having a capacity of Io cubic centimeters and a conical bottom graduated in units of 0.1 cubic centimeter. In order to bring the hide powder to the optimum pH value, they covered it with 5 cubic centimeters of a phosphate buffer solution having a pH value of 5.9 and a few drops of toluene to check bacterial action. The tube was shaken for 3 hours, then centrifuged for 20 min- utes at 1000 times gravity, and the volume of hide powder read from the 1% J, Biol. Chem. 3 (1907), 33. 39 Loc. ett. , ( ng lone des of Leather Chemistry. J. A. Wilson. J. Am. Leather Chem, Assoc. 12 1917), 108. 21 Hydrolysis of Collagen by Trypsin. A. W. Thomas and F. L. Seymour-Jones, ih ANAS Chem. Soc. (1923); Dissertation, F. L. Seymour-Jones, Columbia University, 1923. 22.W., Kihne. Verhande. Naturhist. Med. Ver., Heidelberg, 1 (1887), 198. 23 A Ewald and W. Kihne. Jbid., 1 (1887), 451. 24 A. Ewald. Z. Biol, 26 (1890), 1. : Fraction of Hide Powder Digested 74 THE CHEMISTRY OF LEATHER MANUFACTURE graduations in the tube. The supernatant liquor was then run away and replaced by 5 cubic centimeters of trypsin solution having a pH value of 5.9 or by the buffer solution where a blank was being run. Toluene was added in every case as a safeguard. The solution was shaken in a ther- mostat at 40° C. for a stated length of time and then centrifuged and the volume of hide powder again read, the loss in volume being taken as a measure of the amount of hide powder dissolved. | The rate of digestion of hide powder by a 0.5-per cent trypsin solution is shown in Fig. 37 as a function of the time. With a solu- tion so concentrated in enzyme, hydrolysis takes place extremely rapidly. It is interesting to note - 6 Bipe hide oonabes also the steady hydrolysis in the se iat 1 blank (without enzyme) at 40° C. time = 30 mine, In Fig. 38 are shown the rates of digestion of fine and coarse hide powders as functions of the concen- tration of enzyme. The fine pow- der consisted of the portion passing through a sieve of 34 meshes to the: inch and the coarse powder of the portion retained by the sieve. A much longer time is required to hydrolyze the coarse powder, as was expected. In Chapter 8 it will be shown that a concentrated solution — of trypsin produces marked hydro- lysis of calf skin only after acting for nearly 40 hours. Here the time required for diffusion of the en-’ 100 200 300 400 500 600 zyme into the skin and complica- miviigrens of Trypsin per Liter tions due to the presence of preteia Fig. 38.—Rates of digestion of fine matter other than collagen play a and coarse hide powders as func- part. In the method described above tions of the concentration of tryp- for preparing collagen for study, the a action of the enzyme does not result in any very serious loss of collagen, but all of the elastin is digested. Collagen is hydrolyzed by concentrated solutions of acids and alkalies in the cold, if sufficient time is allowed. Upon heating the solutions, the hydrolysis proceeds rapidly. In a study of the hydrolysis of gelatin by acids, alkalies, pepsin, and trypsin, Northrop *® found that the course of the early stages of hydrolysis is similar with alkali, trypsin, and pepsin, but quite different with acid. He made a com- parison of the relative velocities of hydrolysis of the various peptide linkings and observed the following important facts. Those linkages which are hydrolyzed by pepsin are also hydrolyzed by trypsin; but trypsin hydrolyzes linkages which are not attacked by pepsin. Of the linkages hydrolyzed by both enzymes, those most rapidly hydrolyzed Fraction of Hide Powder Digested ** Comparative Hydrolysis of Gelatin by Pepsin, Trypsin, Acid, and Alkali. J. H. Northrop. J. General Physiol. 4 (1921), 57. ; CHEMICAL CONSTITUENTS OF SKIN 75 by pepsin are only slowly attacked by trypsin. Those linkages which are most rapidly split by pepsin or trypsin are among the more resistant to acid hydrolysis and least resistant to hydrolysis by alkali. The chemistry of collagen and gelatin forms so large a portion of the chemistry of leather manufacture that further treatment must be reserved for the appropriate chapters. The skin contains a number of non-protein substances in the blood, lymph, and gland secretions. The blood and lymph contain sugars, salts, particularly the phosphates, carbonates, sulfates, and chlorides of sodium and potassium, and fatty matters, including cholesterols and the lecithins, which are phosphorous compounds of fats often existing in loose combination with proteins. Sodium chloride is the chief con- stituent of perspiration, which also contains sulfates, phosphates, and urea, and sometimes sebum. Sebum, the secretion of the sebaceous glands, consists of cholesterols, complex oleins, higher alcohols, and soaps, and is usually found contaminated with epithelial cells, probably those of the sebaceous glands furnishing the sebum. Chapter 4. Ionization of Acids and Bases Commonly Used in the Tannery. | Of vital importance in the use of tannery liquors is the-control of hydrogen-ion and hydroxide-ion concentrations. Irregular variations in these concentrations are almost certain to result in corresponding irregularities in the properties of the leather produced. By juggling the methods of operation until a nearly uniform product was obtained and then rigidly adhering to a developed process, tanners long ago perfected means for keeping hydrogen-ion concentrations reasonably well under control, although without any appreciation as to why cer- tain steps had to be followed. If liquors suddenly became infected with acid-producing ferments, or got beyond the control of the operator from other causes, the result was apt to be disastrous unless the tanner had learned from similar experiences how to correct the trouble. Many of the pioneers who attempted to introduce chemical methods to the industry were handicapped by their inability to compare the activities of acids or bases of different strengths. Too much reliance upon the total concentration of acid, with little or no appreciation of its degree of ionization, has often proved very misleading. It is still not uncommon to find expensive acids being used where cheaper ones would serve the purpose as well or better. Even where an operator had come to appreciate that the determining factor was the hydrogen- ion concentration rather than the total titrable acidity, he was often without the means for determining hydrogen-ion concentrations and there were no easily available figures showing the degrees of ionization of the commoner acids and bases at different concentrations. In order to remedy this situation, Thomas * computed and compiled from the literature a series of tables showing the degrees of ionization of a number of acids and bases commonly used in the tannery; a range from 0.001 to 2 molar is covered. These tables are incorporated in this chapter because it is believed they will make certain portions of the book more readily comprehensible to a greater number of readers and will prove of great value for reference in experimental work on leather manufacture. In making the calculations, Thomas used two modes of procedure. For the weak acids the concentrations of hydrogen ion have been cal- ? Tabulation of Hydrogen and Hydroxyl Ion Concentrations of Some Acids and Bases. A. W. Thomas. J. Am. Leather Chem. Assoc. 15 (1920), 133. 76 Mevi2 alION? OF ACIDS AND BASES , 77 culated from the ionization constants (determined by conductivity measurements) by means of Ostwald’s dilution law, a4 V(I—a) where K is the ionization constant, V the volume in which I gram molecular weight is dissolved, and a the degree of ionization. By rearrangement of the equation, we get —KV + VK?V? 4 4KV Zz K= i ewe But, since the value of K*V? is negligible compared to KV, it can be dropped for the purpose of making the calculations. The following expression, therefore, was used: Per cent ionization = 100\/ KV — 5oKV. For the strong acids, the experimentally determined values for 1ooa at various concentrations were found in the literature. These were plotted against values for logV and a smooth curve was drawn through the points. The desired values were then read from the curve. The hydroxide-ion concentrations of bases were obtained similarly. The figures in the tables may be in error as much as 5 per cent, especially in the cases of the strong acids and bases, but they are the best obtainable at this time. They were obtained from conductivity data and not from measurements by the hydrogen electrode. Acids. Acetic Acid.—Values calculated from the experimentally deter- mined figures of Kendall.? Boric Acid.—Calculated from K = 6.6 X 10719 at 25° C. by Lun- den.* 0.8 molar is saturated solution and since this acid is exceedingly weak, only the concentrations at 0.8, 0.1, 0.01, and 0.co1 molar are given in the table. Butyric Acid.—For concentrations 2 to 0.1 molar, calculated from Poet tO «6at 25° by Ostwald* From 0.1 to 0.001 molar calculated from Ostwald’s experimental values. | Carbonic Acid.—This acid is very weak and its concentration in solution depends upon the pressure of carbon dioxide on the surface of the solution. For this reason no special table was prepared and only two significant concentrations are given here, taken from Kendall.® At 25° the solubility of carbon dioxide in water at 1 atmosphere of pressure of carbon dioxide is 0.0337 mole per liter. The carbonic acid in this solution is 0.33 per cent ionized and hence its concentra- 2 Medd. Vetenskapsakad. Nobelinst, Band 2, No. 38 (1913), 1-27. 37. chim. phys. 5 (1907), 574. *Z. physitk. Chem. 3 (1889), 170. 5J, Am. Chem. Soc. 38 (1916), 1481. “8 THE CHEMISTRY OF LEATHER MANUFACTURE tion of hydrogen ion is 0.coo11 mole per liter, representing a pH value of 3.96. Under ordinary conditions, the partial pressure of carbon dioxide in the air is 0.000353 atmosphere, at which pressure carbon dioxide is soluble to the extent of 0.0oooo11g mole per liter, yielding a hydrogen-ion concentration of 0.000002 mole per liter or a pH value of 5.70. Citric Acid.—For 2 to 0.4 molar, the values of Kendall, Booge and Andrews ® are given. From 0.4 to 0.1 molar, the values are ex- trapolated. From 0.01 to 0.001 molar, the concentrations are calculated from the measurements of Walden.’ Formic Acid.—From 2 to 0.1 molar, the values are calculated from KS A ee ae evel ue Ostwald. From 0.1 to 0.001 molar, they are calculated from Ostwald’s experimental determinations. Gallic Acid.—From I to 0.03 molar, values are calculated from K = 4.0 X Io, as given by Ostwald. From 0.03 to 0.001 molar, values are calculated from Ostwald’s experimental values. Hydrochloric Acid.—The figures for 2 to 0.5 molar are from Jones. Those for 0.5 to o.oor molar are calculated from Kohl- rausch’s ?° experimentally determined values. Lactic Acid.—The figures for 2 to 0.1 molar are based upon the figures of Kendall, Booge and Andrews; ® those for 0.1 to 0.001 molar are calculated from the experimental values of Ostwald.® Nitric Acid.—The 2 to 1 molar values are taken from Jones ; ° those for 0.5 to 0.001 molar are calculated from Kohlrausch’s ?° data. Oxalic Acid.—The only data available are those of Ostwald,® cover- ing the range only from 0.03 to 0.004 molar. This acid is too highly ionized to permit calculations by the dilution law. Phosphoric Acid.—Figures for 2 to 0.1 molar are calculated from the data of Kendall, Booge and Andrews; ® those from 0.1 to 0.001 molar from the experimental data of Noyes and Eastman.™ Salicylic Acid.—Values are based upon the experimental data of Kendall.? 0.0167 molar represents the limit of solubility. Sulfuric Acid.—The figures for 2 to 1 molar are from Jones; ® those for 0.5 to 0.001 molar from the experimental data of Kohlrausch.” Tartaric Acid.—From 2 to 0.04 molar, the figures are calculated from the data of Kendall, Booge and Andrews;® from 0.04 to 0.001 molar, they are calculated from Ostwald’s ® experimental data. Bases. Ammonium Hydroxide.—tThe figures for this weak base are calcu- lated, by means of the dilution law, from K = 1.8 X 10° at 25° C., as given by Noyes, Kato and Sosman.’. Barium Hydroxide—The only available data for this base are ®°J, Am. Chem. Soc. 39 (1917), oe 7Z. physik. Chem. 10 (1892), 5 &Z. phystk. Chem. 3 (1889), Bh ® Carnegie Inst. Publ., No. 60 (1907), 10 Morgan’s Elements ‘of Physical fue hee 4th edition (1908), 519, 11 Carnegte Inst. Publ., No. 63 (1907), 268. 22. physik, Chem. 73 CrgTO), V1 IONIZATION OF ACIDS AND BASES 79 those of Noyes and Eastman," which range from 0.001 to 0.05 molar, upon which the calculations in the table are based. Calcium Hydroxide.—No series of experimental data for this base could be found, but it is so similar to barium hydroxide that prob- ably no great error would arise from the use of the barium hydroxide figures. Potassium Hydroxide.—The 2 molar value is from Jones. Values for I to 0.4 molar and from 0.03 to 0.001 molar are calculated from Kohlrausch’s '° data; those between 0.4 and 0.03 molar are obtained by extrapolation. Sodium Hydroxide-—The 2 molar figure is from Jones;° the others are from Kohlrausch’s !° data. Order of Strengths. Listing the acids in order of increasing strength, or hydrogen-ion activities, we have Boric Carbonic Butyric Acetic Gallic Lactic Formic Citric attaric Salicylic Phosphoric Oxalic Sulfuric Nitric, Hydrochloric Boric is the weakest acid in the list and hydrochloric and nitric are the strongest. The bases in order of decreasing hydroxide-ion activity are Potassium hydroxide Sodium hydroxide sarium hydroxide, Calcium hydroxide Ammonium hydroxide Temperature. __ All of the ‘values given in Tables II to X are for a temperature of 25°C. The temperature coefficient of ionization is small enough to be neglected for most practical purposes. The figures may, therefore, be considered valid for the range of temperature met with in the tannery. 80 THE CHEMISTRY OF LEATHER MANUFACTURE pH Values. The term pH value is now widely used to indicate the value of a, with change of sign, in the expression [H"] = 10+ moles per liter. The use of this term has proved confusing to some because an increas- ing pH value indicates a decreasing hydrogen-ion concentration. But the pH scale has proved of great value for the operator with no knowl- edge of chemistry. He accepts it as a standard scale of acidity and alkalinity, as he does a thermometer for temperature, without caring about its mechanism. He learns, for example, that a given liquor works best at a pH value of 5.5. When the analyst reports to him a value for this liquor of 6.5, he immediately appreciates that the addi- tion of acid is necessary to bring the liquor back to 5.5. The routine worker adopts the pH scale almost as easily as any other system TABLEAH, Hydrochloric Acid " Nitric Acid Moles of acid Per cent Moles Ht Percent Moles H* per liter ionized perliter pHvalue ionized per liter pH value O.00 Pac dt ee 100.0 0.0010 3.00 100.0 0.0010 3.00 OD02 Beta ee eee 100.0 0.0020 2.70 99.5 0.0020 2.70 O004 2 kn fee 100.0 0.0030 252 99.5 0.0030 2.52 OO0A Tins oa 100.0 0.0040 2.40 ° 99.4 0.0040 2.40 DOOGL i tee ee 100.0 0.0050 2.30 99.4 0.0050 2.30 - C.00020 sot esha. 100.0 0.0060 3.22 99.4 0.0060 2.22 OOTP Eas Shtrase 100.0 0.0070 215 99.3 0.0070 2.15 P0080 15s as eee 100.0 0.0080 2.10 99.3 0.0079 2.10 G.000 726 ween cae 99.9 0.0090 2.05 99.3 0.0089 2.05 MOUs cee panes 99.8 0.010 2.00 99.3 0.010 2.00 O02) ccs Bt 98.8 0.020 1.70 99.3 0.020 1.70 O03. .6 0 eee 98.0 0.029 1.54 99.2 0.030 1.52 GA ae ern 97.6 0.039 1.41 08.7 0.039 1.41 O08 aie ete oan 96.8 0.048 142 08.3 0.049 1.31 AR 8 «Pompe ens ioe OR 06.4 0.058 1.24 97.6 0.059 1.23 OTE er fete 95.8 0.067 17 97.3. + 6.068 1.17 CV: wet htae c ee ee 95.6 0.076 Hi2 96.8 0.077 I.II COON eee 95.2 0.086 1.07 06.3 0.087 1.06 re SOR EE Riba 5 Cave 94.8 0.095 1.02 96.0 0.096 1.02 FO.Si 28 ko ae 92.0 0.184 0.74 92.9 0.185 rae ee ate aN ieies ieee 90.1 0.270 0.57 90.7 0.272) 2 aes G.dsdts oe anes 88.7 0.355 0.45 89.4 0.358 0.45 OR eek ee ee 87.5 0.438 0.36 87.9 0.439 0.36 OD eins cies 86.5 0.519 0.28 aie gee Soa OF eee 84.7 0.593 0.23 OST VEet eee Loe 83.3 0.666 0.18 Oud, ene os 81.5 0.734 0.13 EO oo cae 709.6 0.796 0.10 84.8 0.848 0.07 D0 Norte oe ee 69.3 1.386 0.54 73.0 1.478 —0.17 IONIZATION OF ACIDS AND BASES SI of measurement. He soon learns that pH = 7 represents a neutral solution, that values increasing from 7 indicate an increasing alkalinity and values decreasing from 7 an increasing acidity. The investigator in leather chemistry finds it logical to plot variables against —log[H"] rather than against actual hydrogen-ion concentra- tions because of the enormous range covered. For him the adoption of the pH scale has the advantage of eliminating the use of negative values and making his system of record conform to one more desirable for making plant reports, where the use of logarithms, negative values, and conceptions of ionization are often apt to lead to hopeless confusion. The pH values corresponding to the various hydrogen-ion concen- trations have been added to Thomas’ tables in order to increase their usefulness. CASLE LUT. Sulfuric Acid * Phosphoric Acid Moles of acid Per cent Moles H* Percent Moles Ht per liter ionized perliter pHvalue ionized perliter pH value PEQOT ONE Ge 6 a wu iste 07.7 0.0020 2.70 89.0 0.0009 3.05 RIO os es erate were DAF 0.0038 2.42 83.0 0.0017 Pte SNA eS ores oe o's 90.5 0.0054 2.27 ee 0.0023 2.64 TE a eae 88.0 | 0.0070 2.15 73.5 0.0029 2.54 ORT tases «oes s 85.90 0.0086 2.07 70.0 0.0035 2.46 Dis aes 84.2 0.0101 2.00 67.5 0.0041 2.39 ia 0.84 a nl a ee ne 50.9 0.916 0.04 17.7 0.159 0.80 icin aoe 50.7 1.014 — 0.01 17.5 0.175 0.76 CS EN Oe 39.9 1.596 — 0.20 16.1 0.322 0.49 * 100 per cent ionization taken as comple te ionization into H+, Ht, and S04”. fj 100 per cent ionization taken as comple te ionizatidn into H+ and Il,.POy,’. aaa Be Ba Moles of acid per liter TABLE IV. Formic Acid Per cent Moles H* ionized per liter pH value 0.00036 3.44 0.00054 327 0.00066 3.18 0.00080 3.10 0.00090 3.05 0.00100 3.00 0.00109 2.96 0.00118 2.93 0.00126 2.90 0.0013 2.87 0.0019 PAGE 0.0024 2.62 0.0028 2.55 0.0032 2.49 0.0035 2.46 0.0038 2.42 0.0040 2.40 0.0042 2.38 0.0045 Gak 0.0064 2.19 0.0078 2.11 0.0092 2.04 0.0105 1.98 0.0114 1.94 0.0126 1.90 * 0.0136 1.87 0.0144 1.84 0.0150 1.82 0.0206 1.69 Per cent ionized 12.8 9.2 ri) 6.6 5.9 5.4 5.0 4-7 4.4 4.2 3.0 2.4 2.1 1.9 i” 1.55 1.5 1.4 “4 0.9 0.7 0.6 0.57 0.50 0.45 0.42 0.40 0.37 0.30 CHEMISTRY OF LEATHER MANUFACTURE Acetic Acid Moles H* ‘per liter pH value 0.00013 3.89 0.00018 3.74 0.00023 3.64 0.00026 3.58 0.00030 3.52 0.00032 3.40 0.00035 3.46 0.00038 3.42 0.00040 3.40 0.00042 3.38 0.00060 3.22 0.00072 3.14 0.00084 3.08 0.00095 3.02 0.00102 2.99 0.00109 2.96 0.00120 2.92 0.00126 2.90 0.00130 2.89 0.00180 2.74 0.00210 2.68 0.00240 2.62 0.00285 2.55 0.00300 2.52 0.00315 2.50 0.00336 — 2:47 0.00360 2.44 0.00370 2.43 0.00600 2.22 . . PONIZATION OF ACIDS AND BASES 83 TABLE. Vi; Gallic Acid Lactic Acid Moles of acid Per cent Moles Ht Percent Moles H* per liter ionized perliter pHvalue ionized =perliter pH value St a 18.7 0.00019 3.72 30.9 0.00031 3.51 10 Dn 13.4 0.00027 3.57 23.0 0.00046 3.34 I Pa 10.7 0.00032 3.49 18.7 0.00056 3.25 Sa ¢ ae 9.3 0.00037 3.43 16.7 0.00067 3.18 MERE Mears 5 vss 8.4 0.00042 3.38 15.1 0.00076 Baie “Pan Pe ee vane: 0.00046 344 13.9 0.00083 3.08 Ue Seo 7.0 0.00049 S21 12.9 0.00090 3.05 TCS hig as, eae ee 6.7 0.00054 ‘327, 12.2 0.000098 3.01 PRM oe. en 6.2 0.00056 3.25 11.5 0.00104 2.98 ER ems oe oc 5.9 0.00059 2.28 11.0 0.00110 2.06 Tha Clr ae ae 4.1 0.00082 3.09 8.0 0.00160 2.80 Oe een ee is 5 0 oR, 0.00099 3.00 6.6 0.00198 2.70 SOY he oo eee 3.0 0.00120 2.92 5.8 0.00232 2.63 GR eg once als « s 2.70 0.00135 2.87 5.2 0.00260 2.58 CRC. 8 2.50 0.00150 2.82 4.8 0.00288 2.54 RO eric? Sue, 3s 2.30 0.00161 2.79 4.3 0.00301 2.52 USE ORs a 2.20 0.00176 2.75 4.1 0.00328 2.48 TNs ee 2.05 0.00185 278 3.8 0.00342 2.47 ERIE or eis sia 1.98 0.0020 2.70 ou. 0.00370 2.43 es (ee eee 1.40 0.0028 2.55 ag 0.0054 2.27 TE tee pe 1.15 0.0035 2.46 22 0.0066 2.18 Oa eS Sis; 8s 1.00 0.0040 2.40 1.8 0.0072 2.14 Le Sos ee 0.89 0.0045 2.35 1.6 0.0080 2.10 a 0.80 0.0048 2.32 1.5 0.0090 2.05 ee eS has. 0.74 0.0052 2.28 1.4. 0.0098 2.01 Tie Ag hae 0.70 0.0056 2:25 Fr 0.0104 1.98 Wt geo ee 0.68 0.0061 a2 1.2 0.0108 1.97 r= 0.63 0.0063 2.20 tal 0.0110 1.96 EAGT) i 0.8 0.0160 1.80 84 THE CHEMISTRY OF LEATHER MANUFACTURE TABLE VI. Butyric Acip Boric Acip * Moles of acid Percent Moles H* Per cent Moles H* per liter ionized __ per liter pH value ionized per liter pH value O.00Ts ela aan II.4 0.000II 3.06 0.080 0.0000008 6.10 O.002 7 oe oe 8.3 0.00017 3.77 rath: ae O.003 S358 6.8 0.00020 3.70 O.008 ten tee 6.0 0.00024 3.62 ODOStor ea 5.4 0.00027 3.57 O.000 Fi. ce veo 4.9 0.00029 3.54 CUOkst cieeree 4.55 0.00032 3.49 enees ae See OGOOSi0% os sorces 4.3 0.00034 3.47 ‘eit ey P tate ODOG Pesan ures 3.05 0.00036 3.44 Se oe eee oat O.0l 2o2,ce ares 3.8 0.00038 3.42 0.026 0.0000026 5.58 0,022 in oe 27 0.00054 327 lee : pha OWI ee eee aoe 0.00066 3.18 PAG wae eee 1.95 0.00078 i ee O05 chicos weurks re 0.00085 3.07 Lt pe tae sees cae 1.6 0.00096 3.02 COs des eee 1.4 0.00098 3.01 DOG tas eee 1.35 0.00108 2.97 00; eee ee 1.25 0.00113 2.05 es se st atates 1.2 0.00120 2.92 0.008 0.0000080 5.10 O37 vies sae 0.86 0.00172 2.76 ita iS ee ae 0.70 0.00210 2.68 Ue: te ees Pico 0.60 0.00240 2.62 Oe Ie coratgrrre 0.54 0.00270 2.57 OP Oia ee 0.49 0.00294 2.53 ER eer Roe 0.43 0.00301 2.52 one poe ee Oo ee eh ewan 0.41 0.00328 2.48 0.003 0.0000240 4.62 O10). Fok ae soe 0.40 0.00360 2.44 AOE arnt or LOS nae 0.39 0.00390 2.40 2D ee Wee eee 0.27 0.00540 2.27 * roo per cent ionization taken as complete ionization into Ht and H2BO,’. IONIZATION OF ACIDS AND BASES 85 TABLE VIL TARTARIC ACID * Citric Acrip t Moles of acid Percent Moles Ht Per cent Moles H* per liter ionized perliter pHvalue ionized per liter pH value ict) Coe ee ee 65.3 0.0007 15 60.2 0.0006 S22 Oy ea ee 51.0 0.0010 3.00 47.4 0.0009 3.05 2) 7 ae ee 43.0 0.0013 2.89 39.8 0.0012 2.92 Lt) 39.0 0.0016 2.80 36.0 0.0014 2.85 aoe vivid ed + 35.5 0.0018 2.74 33.1 0.0017 277; OOO Meteo ks THE CHEMISTRY OF LEATHER MANUFPACTERe mole per liter, which can be accounted for only on the assumption that more than three-quarters of the water present has ceased to play the role of solvent. The hydration theory assumes that this is brought about by the water combining with the salt. If the rise in hydrogen-ion concentration is due to the removal of water by the added sodium chloride, it should be possible to de- termine the degree of hydration of the salt at any concentration from hydrogen-ion measurements. Assuming this to be so, we should reason as follows: From the above equation, log([H*]/a) = bm. But [H"]/a is the factor by which the acid concentration has been multi- plied by adding m moles per liter of salt. Let w represent the total number of moles of water, free or combined with salt, in 1 liter of solution containing m moles of salt. The moles of free water then equal wa/[H"] and the moles of water combined with one mole of salt equal (w/m) X (1—a/[H’]). Calling this latter value h, we have h = w(1 — 107™) /m. From this, hydration values can be calculated for any concentration of salt. For infinite dilution of salt, the expression becomes greatly simplified, for Limit | HG ch ee But at infinite dilution zw = 55.5 and hence pipes Bote! 9 The calculated number of molecules of water combined with one mole- cule of sodium chloride at infinite dilution would thus be 128 x 0.205 or 20.2, which is in striking agreement with the value 26.5 obtained by Smith ** from a very different type of measurement. Calculations of the degrees of hydration at infinite dilution of the chlorides of potassium, ammonium, and lithium made from the equation h = 128b also agreed fairly well with Smith’s corresponding values. A means is thus afforded to calculate the change of pH value that will be produced by the addition of a neutral chloride to an acid solution. Let I represent the pH value of the acid solution containing no salt, which may be found in the preceding tables. Let F be the pH value after the addition of m moles per liter of salt and H he the number of molecules of water combined with one molecule of salt at infinite dilution. Then F = I—0.0078Hm. The use of this equation does not depend upon the validity of the theory. The measurements of Thomas and Baldwin show that it may be used for the addition of chlorides to sulfuric and hydrochloric acids by substituting the following values for H: 19 A Method for the Calculation of the Hydration of the Ions at Infini iluti McP. Smith. J. Am, Chem, Soc. 37 (1915), 722. oe meniZzellION OF ACIDS AND BASES 93 potassium chloride 15 ammonium chloride 15 sodium chloride 20 lithium chloride 35 barium chloride 50 The effect of adding sulfates cannot, however, be attributed to hydration, since they decrease the hydrogen-i ion concentrations of acid solutions. Their action is probably due to the formation of addition compounds complicated by hydration effects. For the hydrogen-ion concentrations of sulfuric and hydrochloric acid solutions containing neutral sulfates, reference should be made to the original papers of Thomas and Baldwin. ‘For the degrees of ionization of a large number of different salts at various concentrations, the reader is referred to page 35 of the recent book of Kraus.” A skin is subjected to liquors of widely different pH value in passing through the tannery. From a lime liquor having a pH value of 12.5 it may pass into a bate liquor with a pH value of 7.5, then into a pickle liquor of pH —1.5, then into a chrome liquor whose pH value is rising from 3 to 4, and then into a fat liquor at pH = 9. Or, in vegetable tanning, the skin may pass from the bate liquor to a tan liquor whose pH value may be anything from 2.5 to 5.5, de- pending upon the method of operation of the yards. But in spite of the wide variation in pH value to which the skin is subjected in passing through the tannery, the processes are all sensitive to com- paratively small variations in pH value unless each variation is compensated by corresponding changes in the process itself. » 20 The Properties of Electrically Conducting Systems. C., A. Kraus. Chemical Catalog Co., New York. Se baste _ Chapter 5. Physical Chemistry of the Proteins. The physical chemistry of the proteins is one of the foundations upon which leather chemistry is built, but until comparatively recently our knowledge of the chemical reactions of the proteins was hardly sufficient to permit of quantitative treatment. Proteins did not seem to show the stoichiometric relations of orthodox physical chemistry to earlier investigators because they failed to recognize the full number of phases existing in a given system and the necessity for making measurements at definite hydrogen-ion concentrations. ) The way for the quantitative development of the physical chemistry of the proteins was paved by the appearance of Donnan’s theory of membrane equilibria, which was applied by Procter? to the swelling of gelatin and further developed by Procter and Wilson * into a quantita- tive theory of the swelling of protein jellies. In an extensive series of researches, Loeb * has extended this work to include also the osmotic pressure, viscosity, stability, and electrical potential differences of protein systems as well as a general theory of colloidal behavior. This valuable work is now available in book form® and should be consulted as having an important bearing upon leather chemistry. It will be shown in this chapter that proteins conform to the classical laws of physical chemistry and that their reactions are indicated by. well established principles. Donnan’s theory forms the logical start- ing point for this presentation. A good discussion of Donnan’s theory is given in Lewis’ Physical Chemistry ;* we have extended it in this chapter to a consideration of the effects of valency. Donnan’s Theory of Membrane Equilibria. This theory deals with the equilibria resulting from the separation by a membrane of two solutions, one of which contains an ionogen having one ion that cannot diffuse through the membrane, which is permeable to all other ions of the system. As an example, Donnan 1F.G. Donnan. Z, Elektrochem. 17 (1911), 572. Serve of Dilute Hydrochloric Acid and Gelatin. H. R. Procter. J. Chem. Soc. 105 (1914), 313. ( Boe Acid-Gelatin Equilibrium. H. R. Procter and J. A. Wilson. J. Chem. Soc. 109 1916), 307. 4J. General Physiol., 1918-1922. y hei and the Theory of Colloidal Behavior. Jacques Loeb. McGraw-Hill Book Ca.; ew York. - A System of Physical Chemistry, Vol. IT, Thermodynamics, pp. 275-86. Longmans, Green & Co., London, 94 feet a CHEMISTRY OF THE PROTEINS 95 takes an aqueous solution of a salt NaR, such as Congo red, in contact with a membrane which is impermeable to the anion R’ and the non- ionized salt, but will allow Na* or any other ion to pass freely through it. The membrane separates the Congo red solution from an aqueous solution of sodium chloride, which will diffuse from its Solution II into the Solution I of NaR. When equilibrium is established, if a small virtual change is made reversibly at constant temperature and volume, the free energy will remain unchanged; that is, no work will ‘be done. The change here considered is the transfer of dn moles of Na* and Cl’ from II to I. The work, which equals zero, is [Na*] 11 (Sauces NS alt + dn.RT.log (eur == 0, whence ey Cry = [Na |r X< [Cl’]1. (The brackets indicate concentration in moles per liter.) Equilibrium will be established only when the product of the concentra- tions of Na* and Cl’ has the same value on both sides of the membrane. This equation of products, simple though it may appear, is of such fundamental importance in the quantitative development of leather chemistry that any doubt as to its validity should be dispelled at the outset. The derivation of the equation need not involve the use of thermodynamics, since it can readily be visualized. In passing from one phase to the other, the oppositely charged ions must move in pairs, since they would otherwise set up powerful electrostatic forces that would prevent their free diffusion. For this reason a sodium or a chlorine ion striking the membrane alone could not pass through it. But, since the membrane is freely permeable to both Na* and Cl’, when two oppositely charged ions strike the membrane together, there 1s nothing to prevent them from passing through into the solution on the opposite side. The rate of transfer of these ions from one solution to the other depends, therefore, upon the frequency with which they chance to strike the membrane in pairs, which is measured by the product of their concentrations. At equilibrium the rate of transfer of Na* and Cl’ from Solution II to Solution I exactly equals the rate of transfer of these ions from Solution I to Solution II, from which it follows that the product of the concentrations of these ions has the same value in both solutions. It is interesting now to note the effect of complicating the system by the introduction of another salt, such as KBr. Following the same line of reasoning, it will be evident that equilibrium will be established only when the product [K*] X [Br’] has the same value in both solu- tions, and the same is true for the products [K*] & [Cl’] and [Nat] X [Br’]. In fact, with any number of mono-monovalent. iono- gens present in the system, the product of the concentrations of any _pair of diffusible and oppositely charged ions will have the same value in both solutions. Introducing polyvalent ions into the system makes the equation 96 THE CHEMISTRY OF LEATHER MANUFACTURE of products but very little more complicated. When a polyvalent ion strikes the membrane, it will pass through only when an equivalent » number of ions of opposite sign strike the membrane at the same time and pass through with it. The rate of transfer of any dissociated ionogen from one solution to the other is evidently determined by the product of all the ions required to produce the undissociated ionogen. At equilibrium, this product will have the same value in both solutions. It, for example, the system contained the ions Nat and SO”,, then the product [Na*] X [Na*] x [SO”,], or [Nat]? x [SO”,], would have the same value on both sides of the membrane, at equilibrium. The impermeability of the membrane to the anion R’ causes an unequal distribution of ions between the two solutions. In Solution II of the simple system including only the ionogens NaR and NaCl, let x — [Nat esi Gah In Solution I let ye] tee and Zia) ee whereupon [Nat] =y +z. The equation of products may then be written x? = y(y +2)... But here we have the product of equals equated to the product of unequals, from which it is apparent, mathematically, that the sum of the unequals is greater than the sum of the equals, or that 2y + z>2x. The reasoning thus indicates that the concentration of diffusible ions in Solution I, at equilibrium, is greater than in Solution II, and this has been shown to be true in numerous experiments. If we let the excess of diffusible ions of Solution I over Solution II be repre- sented by e, then 2y+z=e2x-+e, or x=y+ a/ ey, which shows us further that + is greater than y or that the concentration of ionized sodium chloride is greater in Solution II than in Solution I. The added sodium chloride does not distribute itself equally through- out both solutions, but, at equilibrium, it is the more concentrated in Solution II. : | | The different distribution of ions in the solutions at equilibrium gives rise, not only to a difference in osmotic pressure, but also to an electrical difference of potential across the membrane. Donnan de- rived the equation for this potential difference by the following thermodynamic reasoning. : In the system just described, let xy be the potential, for positive electricity, of solution I and xyz that for Solution II. Let the ex- PHYSICAL CHEMISTRY OF THE PROTEINS 97 tremely small quantity Fdn of positive electricity be transferred isO- thermally from II to I. In this virtual change of the system from equilibrium, the following work terms must be considered: the change in free electrical energy represented by Fdn (ayy — 1) and the simul- taneous transfer of pdn moles of Na* from II to I and of qdu moles of Cl’ from I to II, where p and q are the respective transport num- bers of the ions, and hence p+q=1. The maximum osmotic work of operation of this transfer of ions is represented by the expression [Na‘]11 [Cl] 1 pdnRT . log (Na‘|r + qdnRT .log in But, since the system is in equilibrium, the electrical virtual work must balance the osmotic virtual work, or Fdn (xt — arr) = pdnRT. log alas +. qdnRT .log Car II [Na']t | Na*|11 Ge bevy ee ev * eo Meigs (Clit sy andp-+q=1. Letting E= ay —a]1, we have rE oS log = volts. This is an equation of fundamental importance in the theory of the mechanism of many reactions involved in leather making. It will now be shown that this equation is still valid when other ions of any valency are added to the system. Consider the general case where an ionogen yielding the ion M** of valency a is added. By applying the above line of reasoning to the potential difference pro- duced by the unequal distribution of the ions of the added ionogen between solutions I and II, we arrive at the equation RT | [M**Jar — —— .log = B= or °8 [MF] where n =a, the valency of M**. But it is evident from the equation of products that [Maya >< [CV ]er = [Mar & [Cl] Arr and that eee ier x [CV \*1 = [Na Jaa [CEP from which it is apparent that [M**]rz _ [Na*]*rr _ x* UR NEM ia et Therefore Rel 2 he Le x E=— .log — = — .log — a Lire y? F y os THE CHEMISTRY OF LEATHER MANUFACTURE At equilibrium, the unequal distribution of the added ionogen between solutions I and II produces exactly the same potential difference as the unequal distribution of sodium chloride. Although the addition of any ionogen must produce a change in the measured potential difference, by disturbing the equilibrium, all ionogens present when equilibrium is again established are producing the same potential difference, regardless of valency. The potential difference can thus be calculated from the determination of the distribution of only one kind of ion between the two solutions. The complexity of systems, such as those just described, is due to the fact that the membrane prevents the diffusion of one kind of ion from one phase to the other. A similar set of conditions is brought about whenever one of a number of ions of a system is prevented from diffusing from one phase to another, which is true for every basic tannery process. When skin protein is brought into equilibrium with various tannery liquors, the diffusion of the protein ions is prevented, not by a membrane, but by their own forces of cohesion. This will be made clear in discussing the swelling of proteins. Swelling of Protein Jellies. When a strip of dry gelatin is soaked in water, it swells by absorb- ing water, increasing in volume from 5 to Io times, depending upon the temperature of the water and the quality of the gelatin. With increas- ing concentration of acid, or alkali, the swelling increases to a maximum and then decreases. The property of swelling in aqueous solutions appears to be common to all proteins under conditions such that they do not pass directly into solution. The swelling caused by acids and alkalies is generally counteracted by the addition of neutral salt or by increasing the concentration of acid or alkali sufficiently. While attempting to arrive at a rational explanation of the molecular mechanism of tanning, Procter was continually confronted by the neces- sity of first explaining the mechanism of swelling and to him belongs the credit of being the first to recognize the almost complete dependence of the science of leather chemistry upon the theory of swelling. In 1897 he started an investigation? of the swelling of gelatin in solutions of acids and salts which has culminated in the Procter-Wilson theory of swelling. Procter’s general method of experimentation was as follows: Sheets of thin, purified bone gelatin were cut into portions containing exactly I gram each of dry gelatin. A portion was put into each of a series of stoppered bottles containing 100 cubic centimeters of hydrochloric acid of definite concentration. A fter 48 hours, which was shown to be sufficient for the attainment of practical equilibrium, the remaining solu- tion was drained off and titrated with standard alkali. The gelatin plates were quickly weighed and the volume of solution absorbed was calculated from the increase in weight of the plates. The swollen 7 Action of Dilute Acids and Salt Solutions upon Gelatin. H, R, Procter. K lloidchem. Bethefte (1911); J, Am, Leather Chem, Assoc. 6 (1911), a rocter. Kolloidchem Meets CAl CHEMISTRY OF THE PROTEINS 99 gelatin was then put back into the bottles and covered with enough dry sodium chloride to saturate the solution which had been absorbed by the gelatin. This caused the gelatin to contract and give up the ab- sorbed solution. After 24 hours, when equilibrium was again estab- lished, the solution expelled by the salt was drained off and titrated to determine the amount of free acid which had been absorbed by the gelatin. A small amount, usually about 1 cubic centimeter, of solution always remained unexpelled by the salt and, although not strictly true, this was assumed to have the same concentration of free acid as the portion expelled, due allowance being made for the increase in volume of solution due to saturating it with salt. The acid still unaccounted for was assumed to be combined with the gelatin base. i A further set of checks was obtained by dissolving the gelatin, dehy- drated by treatment with salt, in warm water and titrating with standard alkali, using both methyl orange and phenolphthalein, the former indi- cating the free acid left in the jelly and the latter the total, including the acid combined with the gelatin base, which was obtained by dif- ference. Experimental values for the volume of solution absorbed by the gelatin, the free acid left in the external solution, the free acid in the jelly, and the acid combined with the gelatin base are shown in Table XI and in Figs. 42 and 43. These were taken from the table on page 317 of Procter’s paper, The Equilibrium of Dilute Hydrochloric Acid and Gelatin. In plotting the results, the concentration of gelatin chloride is taken as the difference between the concentrations of total chloride and free HCl in the jelly. The calculated values given along with the experimental ones will be discussed later in connection with the theory. The Acid-Protein Equilibrium. Procter recognized that gelatin combines with HCI forming a highly ionizable chloride and that the resulting equilibrium is a special case of the membrane equilibria described by Donnan. Instead of tracing the development of the theory of swelling from Procter’s earliest work to its present status, it will simplify matters to present the theory from the deductive reasoning furnished later by Wilson and Wilson.® They set out to prove that the entire equilibria can be determined quantita- tively from the orthodox laws of physical chemistry on the simple assumption that gelatin, or any protein, combines with hydrochloric acid to form a highly ionizable chloride. It seemed that success in this would furnish substantial proof of the correctness of the theory. In order to make the reasoning general, let us consider the hypo- thetical protein G, which is a jelly insoluble in water, is completely permeable to water and all dissolved ionogens considered, is elastic and under all conditions under consideration follows Hooke’s law, and com- 8 J. Chem. Soc. 105 (1914), 313. 3 ® Colloidal Phenomena and the Adsorption F la. J. A. and W. H. Wilson. Chem, Soc. 40 (1918), 886, phon seerpulas Py an Wilson. J. Am. ST. BONAVENTURE CHEN. ISTRY 100 THE CHEMISTRY OF LETT DRA RY iN UFACTURE bines chemically with the hydrogen ion, but not the anion, of the = HA according to the equation [G] x [H*] = K[GH*]. (1) In other words, the compound GHA is completely ionized into GH* and A’. Now take one millimole of G and immerse it in an aqueous solution of HA. The solution penetrates G, which thereupon combines with some of the hydrogen ions, removing them from solution, and conse- quently the solution within the jelly will have a greater concentration of A’ than of H*, while in the external solution [H*] is necessarily equal to [A’]. The solution thus becomes separated into two phases, that within and that surrounding the jelly, and the ions of one phase must finally reach equilibrium with those of the other phase. At equilibrium, in the external solution, let x= [H*] = [A’] and in the jelly phase let y = [H*] and Params) Ole bs whence [A’] =y+z. It should be remembered that the brackets indicate concentration in moles per liter. It is apparent from Donnan’s line of reasoning, given earlier in the chapter, that the product [H*] * [A’] will have the same value in the external solution as in the jelly phase at equilibrium, or that x? = y(y +2). (2) ‘ As was pointed out above, it is evident from equation (2) that 2y +z> 2x or 2y-+z=2x+e (3) where e is defined as the excess of concentration of diffusible ions of the jelly phase over that of the external solution. Where any two variables are known, all others can be calculated, for from equations (2) and (3) we get the following: x=y+ Vey= Vy? +yz= (2? —e*)/ge. (4) ¥ a= (2 Ae + 4x*)/2=> (2x + e— V/ 4ex + e?)/2 = (z— e)?/4e. (5) z= (x*—y*)/y = Vgex =e +2 Vey. (6) e = (x—y)*/y =z + 2y—2 Vy? yz =— 2x4 Vi4xt 22, Bee PHYSICAL CHEMISTRY OF THE PROTEINS 101 Since [A’] is greater in the jelly than in the surrounding solution, the negative ions of the colloid compound will tend to diffuse outward into the external solution, but this they cannot do without dragging their protein cations with them. On the other hand, the cohesive forces of the elastic jelly will resist this outward pull, the quantitative measure of which is e, and according to Hooke’s law ea ONE (8) where C is a constant corresponding to the bulk modulus of the protein and V is.the increase in volume, in cubic centimeters, of 1 millimole of the protein. Since we have taken 1 millimole of G, beater [GH*] —=1/(V +a) or [G] = 1/(V -+-a) —z (9) where a is the initial volume of 1 millimole of the protein. From (1) and (9) z=y/(V +a)(K+y) (10) and from (6) and (8) z—CV+2WvVCVy. (11) Now from (10) and (11) Oye aj(k + y)(CV +2 VCVy) —y=o (12) where the only variables are V and y. If the molecules or atoms of the protein are not themselves per- meable to all ions considered, the quantity a@ should not be taken as the whole of the initial volume of the jelly, but only as the free space within the original, dry jelly through which ions can pass. For our hypothetical protein, then, we shall consider the limiting case where the value of a is zero. This assumption in the case of gelatin intro- duces errors less than the probable experimental error because of the relatively large values for V over the significant swelling range. [qua- tion (12) thus reduces to V(K + y) (CV +2 VCVy) —y=o. (13) Knowing the values of the constants, K and C, we can plot the entire equilibrium as a function of any one variable. Procter and Wilson *® obtained the value K = 0.00015 for the sample of gelatin used in their experiments by adding successive portions of standard HCl to a dilute solution of the gelatin and noting the corresponding rises in hydrogen-ion concentration. The difference between the con- centration of hydrogen ion that would have been. found upon adding the acid to pure water and that actually found by adding it to the same ~The Acid-Gelatin Equilibrium, Joc. cit. 1o2 THE CHEMISTRY OF LEATHER MANUFACTURE volume of gelatin solution was taken as the amount of acid combined with the gelatin, or as the value of [GH'*] in equation (1). Substitut- ing any two sets of determinations of [GH*] and [H*] in equation (1) and solving the resulting equations simultaneously, the value of “ can be found. C was obtained by substituting experimental values for V Aaa e in Procter's observed results: X= total chloride, e = free HCl, 0.251 O= gelatin chloride . Continuous 0,15 lines represent calculated values, 0,10 Concentrations in Jelly (moles per liter) 0,05 0.10 0.20 0.30 {uH*] in External Solution Fic. 42. —Observed and calculated values for the distribution of HC1 in the system Gelatin-HCl-Water. equation (8). It was found to vary with the temperature and with the quality of the gelatin, but had the value 0.0003 for the sample of gelatin used by Procter and at the temperature of his experiments, 18° C, In order to compare calculated values for V with experimental determinations of the increase in volume of I gram of gelatin, it is necessary to know its equivalent weight. Procter originally regarded gelatin as a diacid base with a molecular weight of 839, but later work by Procter and Wilson showed that it should rather be regarded as acting as a monacid base, with an equivalent weight of 768, in acid solutions not sufficiently concentrated to cause decomposition. 768 & PeystCAlL CHEMISTRY OF THE PROTEINS 103 grams of gelatin combine with a limiting value of 1 mole of hydrochloric acid and the combination resembles that of HCl with a weak monacid base. For this reason we may use the value 768 as the equivalent weight of gelatin. As for the molecular weight of gelatin, no con- vincing figures have yet been produced and it may be questioned whether Continuous line represents calculated values. Circles represent Procter's observed results. Increase in Volume of 1 Gram of Gelatin (c.c.) 0.10 0.20 0.30 {H+] in External Solution Fic. 43.—Observed and calculated values for the degree of swelling of gelatin asa function of the concentration of hydrochloric acid. they would have any real value, if obtained. We look upon a plate of gelatin as a continuous network of chains of amino acids, there being no individual molecules, unless one wishes to look upon the plate of gelatin as one huge molecule. From equation (13) and the values of the constants given above, Wilson and Wilson calculated all of the variables of the equilibrium for gelatin and hydrochloric acid over the range covered by Procter’s 104 THE CHEMISTRY OF LEATHER MANUFACTURE experiments. The important variables are shown in Table XI and in Figs. 42 and 43 along with Procter’s actual determinations. The agreement between calculated and observed values is absolute, within the limits of experimental error. For this reason Procter and Wilson regard their theory as proved, but, if further corroboration is desired, it can be found in the extensive researches of Loeb, some of which will be described later. Jt is worthy of note that no other theory of swelling has yet passed the stage of qualitative speculation. TABLE XI. At Equilibrium Cc. solution absorbed by I g. [ Total chloride} [HCl] V gelatin [HC1] in jelly in jelly Initial in Calcu- Calcu- Ob- Calcu- Ob- Calcu- Ob- [HCI] soln. lated lated served lated served lated served 0.006 0.0011 o35 43.4 44.1 0.0001 0.0005 0.012 0.014 0.008 0.0018 37.5 48.8 48.7 0.0002 0.0004 0.014 0.015 0.010 0.0025 41.7 54.3 59.9 0.0004 0.0004 0.016 0.015 0.010 0.0028 42.7 55.60 58.4 0.0004 0.0004 0.017 0.015 0.010 0.0032 43.2 56.2 53-7 0.0005 0.0005 0.019 0.017 0.015 0.0073 40.8 53.1 57-9 0.002 0.002 0.024 0.020 0.015 0.0077 40.2 52.3 522 0.002 0.002 0.025 0.022 0.015 0.0120 37.5 48.8 51.9 0.005 0.006 0.031 0.027 0.020 0.0122 a4 48.6 4 Oy, 0.005 0.006 0.031 0.027 0.025 0.0170 34.5 44.0 40.4 0.008 0.009 0.036 0.037 0.025 0.0172 34.3 44.7 48.1 0.008 0.009 0.036 0.031 0.050 0.0406 20.7 34.8 36.4 0.026 0.030 0.063 0.061 0.050 0.0420 26.4 34.4 31.1 0.027 0.030 0.005 0.008 aie 0.0576 24.0 212 34.0 0.041 0.043 0.082 0.079 0.075 0.0666 23.0 29.9 27.9 0.049 0.050 0.092 0.095 0.075 0.0680 22.8 29.7 29.1 0.050 0.053 0.0904 0.092 0.100 0.0930 20.7 27.0 23.1 0.072 0.072 0.121 0.126 0.100 0.09044 20.5 26.7 26.4 0.073 * 0.072 0.122 0.121 ae 0.1052 19.8 25.8 29.8 0.083 0.085 0.134 0.128 0.125 0.1180 18.9 24.6 24.4 0.095 0.090 0.148 0.148 0.150 0.1434 17.9 233 24.0 0.118 0.118 0.174 0.173 0.150 0.1435 17.9 23-3 24.2 0.118 0.118 0.174 0.172 0.175 0.1685 17,3 22.2 23.5 0.141 0.138 0.200 0.200 0.200 0.1925 16.3 21-2 20.6 0.164 0.161 0.225 0.229 0.200 0.1940 16.2 alak 227 0.166 0.165 0.227 0.225 0.200 0.1945 16.2 2I.1 22.1 0.167 0.164 0.228 0.226 0.250 0.2450 15.1 10.7 20.2 0.213 0.210 0.279 0.281 0.300 0.2950 14.0 18.2 20.0 0.261 0.260 0.332 0.332 Other proteins which do not dissolve in cold water behave much like gelatin in respect to swelling, although they apparently have different values for the constants, K and C, as well as for equivalent weight. It is interesting to reason from the theory what differences in swelling would result from changes in the values of the constants. Since V = e/C, an increase in the value of C means a corresponding decrease in the degree of swelling. The effect of a change in the value of K, the hydrolysis constant of the protein, is shown in Fig. 44 for a fixed value of C. At K=o, the point of maximum swelling occurs at PHYSICAL CHEMISTRY OF THE PROTEINS 105 x =o and has the value 1/7/C. As K increases in value, the point of maximum swelling decreases in value and occurs at increasing values for +. At K= oo, the point of maximum has the value zero and occurs at 7 = oo According to the theory, all monobasic acids should produce the same degree of swelling of gelatin for any fixed hydrogen-ion concen- The broken line represents the te) (eo) locus of all points of maximum @ (o) swelling for C = 0.0001, J o en) Oo Increase in Volume of 1 Millimole of Protein (c.c.) 0.02 0.04 0.06 (H*] in External Solution Fic. 44.—Family of swelling curves for proteins having the same bulk modulus, but different values for the hydrolysis constant. tration, under constant conditions, provided the gelatin salts formed are ionized to the same extent. It was generally thought that different monobasic acids produce different degrees of swelling, following the order of the well-known Hofmeister series of the ions, until Loeb pointed out that the earlier investigators, through failure to measure the hydrogen-ion concentration, had fallen into the error of attributing to the several acids effects caused merely by differences in hydrogen-ion concentration. He found, at a fixed value for x, that practically the same degree of swelling is produced by all monobasic acids, as well as 106 THE CHEMISTRY OF LEATHER MANUFACTURE such acids as phosphoric and oxalic at concentrations at which they act as monobasic. The calculation of the degree of swelling of proteins in solutions of polybasic acids is not quite so simple as for monobasic acids. Suppose | that G were to combine with the hydrogen ion but not the anion of the polybasic acid H,A. Letting x represent the concentration of the polyvalent anion in the external solution at equilibrium, zg the concen- tration of the anion of the.gelatin salt, and y + zg the total concentration of anion in the jelly, it is evident from the reasoning given above that xt = y*(y +2) and, by inspection of this equation, we see that (a+i1)x< (+ t)y4Z or that (a+1)x+e=(a+I1)y+z. The total concentration of diffusible ions is greater in the jelly than in the external solution by the amount e and swelling in degree directly proportional to e will result. It can readily be seen that as x increases from zero, without limit, e and the degree of swelling increase to a maximum and then decrease, approaching zero, for z has a limiting value since it cannot exceed the total concentration of gelatin. At 4 =0, y=0, ande=o. As ~# increases without limit our equations approach the limiting relations eevee and (a+1)xte=(at+t1)y from which it is evident that +r = y and e =o. The extent of swelling by polybasic acids which combine as such with the protein will be considerably less than that caused by monobasic acids, as Loeb has shown, because fewer anions will be associated with equivalent weights of the protein. For example, for equivalent weights of gelatin sulfate and gelatin chloride, there would be only half as many sulfate ions as chloride ions. For very small values of +, we should therefore expect sulfuric acid to produce only half as much swelling as hydrochloric acid at the same hydrogen-ion concentration and this is actually the case. Repression of Swelling by Salts. The theory accounts quantitatively for the action of neutral salts in repressing the swelling of proteins by acid. In the system described above in which the protein G was immersed in a solution of HA, con- sider the addition of the mono-monovalent salt MN, neither of whose ions combine with G. At equilibrium, let the concentration of M* be represented by u in the external solution and by v in the jelly. It is evident from the general equation of products that the product * (LT H*y EM") ) CAS ae PHYSICAL CHEMISTRY OF THE PROTEINS 107 will have the same value in both phases, or that fete (Yet vy + ¥ + 2) from which Bea (y -- v) 2 2(x- 4). Solving the two preceding equations simultaneously, we get e=—a(xtu)+ V4(x+u)? +2. Now, if the value of x» -+ u increases while remains constant, the value of e, and consequently the swelling, will decrease. The addition of MN to the system increases u and hence must cause a decrease in the degree of swelling, since it increases z only by causing a diminution of the volume of the jelly. It is important to recognize that the repression of swelling by salts does not depend upon any repression of ionization of the protein salt. The salt acts so as to lower the value of e, which is the measure of the force producing swelling. In some cases, the ionization may be repressed to some extent and this would assist in repressing the swell- ing, but in the case of gelatin chloride, the swelling is markedly re- duced long before there is any repression of ionization of gelatin chloride measurable by means of calomel electrodes. The Alkali-Protein Equilibrium. Proteins are amphoteric substances, reacting both as weak acids and as weak bases. In this respect, they retain the properties of the amino acids from which they are formed. Hydrated aminoacetic acid is capable of assuming either a positive or negative charge, or both, by ionizing as acid or base, or both, thus: H+ + ‘OOC.CH,.NH,. HOH = HOOC.CH,.NH,.HOH = HOOC.CH,.NH,.H* + OH’. The ionization constant of a protein as an acid may be represented as follows : LEE eq RO Genet 5 BS AE But foe (On| — K, or [H*)—K,/(0On'] from which [GH] x [OH’] =k[G’], where k = Ky/Ka. But this is essentially the same as equation (1) except for the fact that [H*] is replaced by [OH’]. It is thus apparent that proteins will behave in solutions of increasing concentration of alkali much as they do in solutions of acids so long as they undergo no chemical changes other than that of salt formation. Actually gelatin swells in alkaline solution to a maximum at a concentration of about 0.004 mole of hydroxide ion per liter, above which the swelling diminishes. 108 THE CHEMISTRY OF LEATHER MANUFACTURE In acid solution maximum swelling occurs at a concentration of 0.004 mole of hydrogen ion per liter. The effect of valency is similar in both acid and alkaline solutions. Loeb found that the diacid bases calcium hydroxide and barium hydroxide give points of maximum swelling for gelatin only half as great as the monacid bases. For a given pH value, the amount of swell- TABLE XII. [ISOELECTRIC Pornts or SEVERAL ProrriNs IN TERMS oF —'LoG [H+] or pH Vatrue. — Log [H+] or pH value Reference CASEIN (COW). uubok tei oe ae a eee 4.6 I 4.7 2 4.7 3 Gelatin ad saat? sine asi ape Seek Cee eee ae 4.6 4 4.7 5 Seriim sal bimiwyr sey c.me eae eee ee See 47 6 perim globulin >A oes. 25. st cae ee eee ee 5.4 2 Eee albumen «then)ofic: 22). cee ae 4.8 7 Denatured :serum albumin, 3s... See 5.4 6 Oxybemoriobing ius. 3042 wos eee eee 6.7 9 Carbon monoxide hemoglobin................ 6.8 10 Reduced (hemoglobin 2.5.05 tu. s cee ee 6.8 10 Stroma globulins of blood corpuscles......... 5.0 8,9 Reds blood cellsncas si iene oe eee 4.6 iL Yeast extract proteins, (globulin). ............ 4.6 14 Csliaclin, «Ut uae tetas aaa en ee 9.2 2 Edestin ae cckos ON pet tes ee 5.6 15 Juberin >» (potata) fa, 2s can gue see ee eee approx. 4.0 12 Carrot. protein)... seen ee ee A 4.0 12 Lomato protein’ a; Soy tok ee ee ee % 5.0 12 Nuacleiccacidii, S55 come e cs io eee ae a ie 2.0 13 REFERENCES: 1. Michaelis and Pechstein. Biochem. Z. 47 (1914), 260. 2. Rona and Michaelis. Ibid. 28 (1910), 193. 3. Loeb. J. General Physiol. 2 (1920), 577. 4. Michaelis and Grineff. Biochem. Z. 41 (1912), 373. 5. Loeb. J. General Physiol. 1 (1918), 39. 6. Michaelis and Davidsohn. Biochem. Z. 33 (1911), 456. 7. Sorensen. Compt-rendus trav. lab. Carlsberg, 12 (1915-17). 8. Michaelis and Davidsohn. Biochem. Z. 41 (1912), 102, 9. Michaelis and Takahashi. Jbid. 29 (1910), 439. 10. Michaelis and Bien. Ibid. 67 (1914), 108. 11, Coulter, J. General Physiol. 3 (1921), 309. 12. Cohn, Gross and Johnson. Jbid. 2 (1919), 145. 13. Michaelis and Davidsohn. Ibid. 39 (1912), 496. 14. Fodor. Kolloid. Z. 27 (1920), 58. 15. Michaelis and Mendelssohn. Biochem. Z. 65 (1954), 3; ing is determined by the valency of the ions of opposite sign to that of the protein ions rather than by the specific nature of the ions themselves. In alkaline solution the protein ion is negatively charged, while it is positively charged in acid solution. In a solution, originally alkaline, in which the hydrogen-ion concentration is gradually increased, there must be some point at which the protein becomes electrically neutral ; that is, where it has an equivalent number of positive and nega- tive charges. The hydrogen-ion concentration at which this occurs has meen CHEMISTRY OF THE PROTEINS 10g been called by Hardy! the isoelectric point of the protein. The iso- electric point of gelatin was found by Michaelis and Grineff 12 to lie at a pH value of 4.7 and this value has been repeatedly confirmed by Loeb and others. Thomas and Kelly ** determined the isoelectric point of collagen, or rather hide powder, by means of acid and basic dyes. Portions of hide powder were first wet with solutions of different pH values, then with solutions of basic fuchsin or Martius yellow, and finally washed with solutions having the same pH values as were used to wet the portions initially. The fuchsin left the hide powder deeply stained only at pH values greater than 5 and the Martius yellow only at values below 5, indicating pH = 5 as the isoelectric point of collagen. Porter ** observed that a point of minimum swelling of hide powder occurs at a pH value of 4.8, indicating this as its isoelectric point. Porter also found points of maximum swelling of hide powder at pH values of 2.4 in acid solution and about 12.3 in alkaline solution. Thomas and Kelly compiled a list of isoelectric points of different proteins, taken from the literature, and these are reproduced in Table XII in terms of pH value. Two Forms of Collagen and Gelatin. Quantitative experiments upon alkaline swelling are rendered diffi- cult by the tendency for: the gelatin to pass into solution, which is very much more marked than for acid swollen gelatin. That gelatin and some other proteins undergo a change of form in alkaline solutions is apparent from recent experimental data. Lloyd ** observed a rather significant change occurring in gelatin dissolved in alkaline solution. A comparison between gelatin dissolved in acid solution and gelatin dissolved in alkaline solution was made as follows. Two grams of gelatin were put into a flask containing 200 cubic cen- timeters of tenth-molar hydrochloric acid. After 6 days at 20° C., the gelatin was completely dissolved and 20 cubic centimeters of molar sodium hydroxide were added to the solution, which was then tested and found to be neutral to litmus. 220 cubic centimeters of saturated ammonium sulfate solution were then added and a white, flocculent precipitate formed, which was filtered off. The filtrate was tested and found to be free from protein. The precipitate was insoluble in cold water and was washed several times. It was dissolved in 2 cubic centimeters of hot water and set to a jelly upon cooling. A control experiment made by dissolving 2 grams of gelatin in 220 cubic centi- meters of water with 1.12 grams of sodium chloride behaved in a similar manner. 1 W. B. Hardy. Proc. Roy. Soc. 66 (1900), 110, 12 Biochem. Z. 41 (1912), 373. 18 The Isoelectric Point of Collagen. A. W. Thomas and M. W. Kelly. J. Am. Chem. Soc. 44 (1922), 195. % Swelling of Hide Powder. E.C. Porter. J. Soc. Leather Trades Chem. 5 (1921), 259, and 6 (1922), 83. of . 7°On the Swelling of Gelatin in Hydrochloric Acid and Caustic Soda. D. J. Lloyd. Biochem, J. 14 (1920), 147. 110 THE CHEMISTRY OF LEATHER MANUFACHGRS For comparison, 2 grams of gelatin were put into a flask con- taining 200 cubic centimeters of tenth-molar sodium hydroxide. The gelatin was completely dissolved after 2 days at 20°C. 20 cubic centi- meters of molar hydrochloric acid were then added to the solution, after which it reacted neutral to litmus. 220 cubic centimeters of sat- urated ammonium sulfate solution were added and a white, flocculent precipitate formed, which was filtered off. The filtrate, as in the TABLE SAIL SWELLING OF GELATIN IN PHOSPHATE BUFFER SOLUTION DurRING 4 Days at 7° C. pH value of buffer solution Increase in wt. of Sh we 1 g. dry gelatin Initial - Final Grams 2.90 2.92 13.20 3.50 3.50 9.49 3.96 4.01 7 pie 4.14 4.17 6.91 4.47 4.59 6.68 4.78 4.86 6.20 5.08 5.12 7.02 5.29 5.38 7115 5.57 5.61 ye 5.78 5.80 7.56 6.04 6.08 7.80 6.209 6.29 7.83 6.48 6.49 8.02 6.69 6.70 8.29 6.96 6.904 8.31 7.08 7.10 8.25 7-41 787 8.03 7.68 7.62 7.62 7.97 7.89 8.30 42 8.36 8.59 8.56 8.48 8.60 9.03 8.06 8.78 9.57 9.51 8.91 10.00 9.96 8.98 10.47 10.41 9.24 11.06 10.98 9.55 11.52 11.48 9.95 12.00 F105 10.73 previous experiment, was found to be free from protein. But the precipitate dissolved completely and rapidly in a small volume of cold water and would not set to a jelly even when the volume was reduced to 2 cubic centimeters. Lloyd suggested that gelatin changes from a keto-form to an enol- form in alkaline solution. The gelatin recovered from acid solution and which had the power of setting to a jelly would thus be regarded as the keto-form of gelatin, while that recovered from alkaline solu- tion and which had lost the power of setting to a jelly would be looked upon as the enol-form of gelatin. Miss Lloyd regarded the change in alkaline solution as irreversible, but her experiments do not Peeer AL COBRMISTRY OF THE PROTEINS It show this. Mr. Kern, in the author’s laboratory, added hydrochloric acid to gelatin dissolved in a hot solution of sodium hydroxide until the pH value, as determined by the hydrogen electrode, was reduced to 4.7 and then allowed the solution to cool, whereupon it set to a firm jelly, indicating that the change is reversible. Miss Lloyd’s ex- Increase in Weight of 1 Gram Dry Gelatin (Grams) 4 5 6 7 8 9 10 Selle 12 pH Value of Buffer Solution Fic. 45.—Showing the two points of minimum swelling of gelatin. periment showed merely that it is not readily reversed by the addition only of the quantity of hydrochloric acid equivalent to that of the sodium hydroxide originally employed. In studying the degree of plumping of calf skin as a function of pH value, Wilson and Gallun found two points of minimum, one at 5.1 and the other at 7.6. This work will be described in Chapter 9. Wilson and Kern 7° followed this with a series of experiments upon the swelling of gelatin in buffer solutions and also found two points of minimum, one at 4.7 and the other at 7.7. A description of their work follows. %*The Two Forms of Gelatin and Their Isoelectric Points. J. A. Wilson and E. J. Kern, J. Am. Chem, Soc. 44 (1922), 2633. 112 THE CHEMISTRY OF LEATHER MANUFACTURE A series of buffer solutions was prepared, each member of which had a final concentration of tenth-molar phosphoric acid plus the amount of sodium hydroxide required to give the desired pH value as determined by the hydrogen electrode at 20° C. The pH values ranged from 3 to 12. 200 cubic centimeters of each solution were put into a stoppered bottle and kept in a thermostat refrigerator at 7° C. After the temperature of each solution had reached 7°, a small strip of high grade gelatin of known weight was put into it. All strips were taken as nearly alike as possible and were kept in the solutions at ee for 4 days, after which each strip was quickly blotted off and weighed. The results were carefully rechecked. In Table XIII are given the gain in weight per gram of dry gelatin and the initial’ and final pH values of the buffer solutions. Fig. 45 represents the degree of swelling as a function of the pH value. Wilson and Kern suggested that the two points of minimum rep- resent the isoelectric points of the two forms of gelatin described by Lloyd and this view appears to be substantiated by other data available in the literature. Experiments upon the mutarotation of gelatin led Smith 17 to sug- gest that gelatin exists in two forms: a sol form, having a specific rotation of [a]p =— 141 and being stable at temperatures above 36° 455 and a gel form, with a specific rotation of [a]p = — 313 and stable under 15°, a condition of equilibrium existing between the two forms at intermediate temperatures. The gel form is characterized by its power to set to a jelly, which is lacking in the sol form. Smith cal- culated that a concentration of from 0.6 to 1.0 gram of the gel form per 100 cubic centimeters is required to produce gelation. As the temperature is increased above 15°, the total concentration of gelatin required to produce gelation is increased because of the decreasing proportion of the gel form, which does not exist at all above et Gelatin is the only protein known to show mutarotation, but it gradually loses this property along with its jellying power, when its solutions are kept at temperatures above 70° C. Davis and Oakes #* measured the viscosities of a series of solutions of gelatin at 40° C. at different pH values. Their results are shown jn Fig. 46. A point of minimum occurs at 8, but none at 4.7, the iso- electric point of gelatin as determined by Loeb. They commented upon this as follows: “There may be considerable difficulty in reconciling this minimum viscosity at pH about 8 with the isoelectric point at pH 4.7.” But Davis and Oakes really measured the point of minimum viscosity of the sol form, since their determinations were made at 40° C., whereas Loeb determined the isoelectric point of the gel form. Another case of the apparent disappearance of an isoelectric point when working at a temperature of 40° C. is to be found in the work of Wilson and Daub,!® who experimented upon the bating of calf skin at 7 Mutarotation of Gelatin and Its Significance in Gelation. C. R. Smith. J. Am. Chem. Soc. 41 (1919), 135. * Further Studies of the Physical Characteristics of Gelatin Solutions. C. E. Davis and Er: Oakes. _J. Am, Chem. Soc. 44 (1922), 464. : ae Critical Study of Bating. J. A. Wilson and G. Daub. J, Ind. Eng. Chem. 13 1921), 1137. Pee ioa CHEMISTRY OF THE PROTEINS 113 40° at different pH values. They observed that a point of minimum plumping occurred in the region of pH = 8, but not at pH = 5, the isoelectric point of collagen found by Thomas and Kelly and by Porter. But Wilson and Gallun observed points of minimum plumping of calf skin at both 5 and 8, when working at low temperatures. The recent work of Sheppard, Sweet and Benedict 2° adds further evidence of the existence of critical pH values at both 5 and 8. They obtained a 1.60 1,50 1.40 Absolute Viscosity 1,30: 1,20 ee he, 1,00 DS oad: Sep ipealye cab? pH Value of Gelatin Solution Fic. 46.—Variation of viscosity of I-per cent solution of gelatin at 40° C. with change of pH value. curve for the rigidity of gelatin jelly as a function of pH value exhibit- ing a shoulder at 5 and a flattish maximum between 7 and 9. Apparently the change in gelatin from the gel form to what has been called the sol form takes place both with rise of temperature and with rise of pH value. Since the experiments of Wilson and Kern were performed at 7° C., they were dealing with the gel form of gelatin 20 Elasticity of Purified Gelatin Jellies as a Function of Hydrogen-Ion Concentration, S, E. Sheppard, S. S. Sweet and A. J. Benedict. J. Am, Chem. Soc. 44 (1922), 1857. 114 THE CHEMISTRY OF LEATHER MANUFACTURE in acid solution and actually observed a point of minimum at pH = 4.7, the isoelectric point of the gel form. The appearance of a second point of minimum swelling at pH = 7.7 seems to indicate that between 4.7 and 7.7 the gelatin passes from the gel to the sol form and that the second point of minimum occurs at the isoelectric point of the sol form. It was only by working at temperatures as low as 7° that they were able to prevent the gelatin from passing into solution at the higher pH values. While objection may be raised to the terms gel and sol form as applied to the two forms of gelatin and of collagen, they will serve as well as any until more is known of the transition. Lloyd’s suggestion that the change is a keto-enol tautomerism is. still speculative. Parker Higley,*! at the University of Wisconsin, has recently in- vestigated the absorption spectra of gelatin dispersions of different pH value and plotted a series of curves, at several densities, for the wave length of maximum absorption in the ultra violet as a function of pH value. The curves all show two points of minimum, one at pH = 4.68 and the other at 7.66, coinciding with the points of minimum swelling _ of gelatin. That the two points of minimum have a real existence is thus strikingly confirmed from an unexpected source. The effect upon vegetable tanning of the change of one form of collagen to the other will be discussed in Chapter 13. Electrical Potential Difference between Protein Jelly and Aqueous Solution. It is apparent from the discussion of Donnan’s theory of membrane equilibria that the unequal distribution of ions between a jelly and its surrounding solution must give rise to an electrical difference of po- tential between these two phases whose measure is (RT/F) .log(x/y), where x is the hydrogen-ion concentration of the external solution and y that of the solution within the jelly and this value holds true re- gardless of the valence or number of ions in the system. The potential difference can therefore be calculated from the determinations of pH value in the jelly and in the external solution. Changing from natural to common logarithms and substituting the numerical value for RT/F at 20° C., we get. P.D. = 58log(x/y) = 58(logx — logy) millivolts. But —log y= pH value of the jelly and + log 4 = —pH value of the solution. Hence, BL 20 21, P.D. = 58(pH of jelly minus pH of solution) millivolts. Loeb ** devised a very ingenious method for determining this potential difference directly by means of a pair of calomel cells of equal value 21 Advance note. #2 Cf, Proteins and the Theory of Colloidal Behavior, Pp. 154, PRYSiCAL CHEMISTRY OF THE PROTEINS 115 and a Compton electrometer. A diagram of his apparatus is shown in Fig. 47. The potential difference measured is that of the cell calomel electrode saturated KCl solid jelly external solution saturated roel calomel electrode Everything else being symmetrical, the potential difference measured is that between the jelly and the external solution with which it is supposed to be at equilibrium. In a typical experiment,?* 1 gram of purified gelatin, powdered to a grain size between 30 and 60 mesh, was put into each of a series of solutions of different concentrations of hydrochloric acid or sodium Fic. 47.—Loeb’s apparatus for measuring the potential difference between gela- tin jelly and the surrounding solution. hydroxide. The volume of each solution was 350 cubic centimeters and the temperature 20° C. After 4 hours the volume occupied by each portion of gelatin was measured, the solution filtered off, and the gelatin melted so that the pH values of both jelly and solution could be determined by means of the hydrogen electrode. The gelatin was then allowed to set to a jelly in the receptacle illustrated in Fig. 47 and the potential difference between the jelly and external solution was then measured with a Compton electrometer. The results of such a series are shown in Table XIV along with calculations of the potential differences made from the pH determinations. The cal- culated and observed results are at least of the same sign and order of magnitude, which is a good agreement considering the nature of the experiments and the dilutions of the solutions. It will be shown later that the method is capable of very much better agreement where the complications involved in melting and resetting of the jelly are avoided, as in the measurement of potential difference between a solution of gelatin and a protein-free solution with which it is in equilibrium and from which it is separated by a semi-permeable membrane, especially where the solutions have greater conductivities. 78The Origin of the Electrical Charges of Colloidal Particles and of Living Tissues. Jacques Loeb. J. General Physiol. 4 (1922), 351. 116 THE CHEMISTRY OF LEATHER MANUFACTURE According to the theory, the concentration of free acid in an acid- swollen jelly should be less than that in the external solution and, likewise, the concentration of free alkali in an alkali-swollen jelly should be less than that in the external solution with which it is in equilibrium. TABLE XIV. SUSPENSIONS OF POWDERED GELATIN. After 4 hours at 20° C. pH value of : Vol. of Absorbed External (a) P.D. millivolts Initial normality gelatin solution solution minus Calcu- of solution (mm.) (a) (b) (b) lated Observed O.0OLONGE Cl -- wues ees 28 4.44 235 + 1.09 + 63.0 + 56.0 D.00OSN PE Clas nates 20 4.56 3.55 + 1.01 + 58.6 Ache O.0000N EiGl ewes eae 18 4.79 3.0245 Oe + 51.0 + 26.5 O:000IN SHES. a eae 16 4.85 4.24 + 0.61 + 36.0 + 15.0 Wekett uaGctnet ese coleanas 17 4.89 4.97 — 0.08 — 4.5 — 17.5 O.000IN- Na@H Wor... 18 4.98 5.96 — 0.98 — 57.0 — 59.0 0,.0002N"= NaOH... ...ci5 9 28 5.06 6.24 —1.18 —680 — 61.0 0.0005N NaOH 7.4 50.2.4. 537 5.50 6.46 — 0.96 — 56.0 — 70.0 COOION NaOHus.. 3. ss d0 6.74 7.30 — 0.56 — 33.0 — 66.0 0.0020N NaOH) 2225... .. 47 0.54 10.56 —1.02 —59.0 —46.0 0.0040N NaOH ......... 48 10.15 11.08 — 0.93 — 48.0 — 36.0 This is verified by the figures in Table XIV, which show, for pH values of the external solution less than 4.7, that the hydrogen-ion concentration is greater in the solution than in the jelly, while for pH values of the external solution greater than 4.7, the hydrogen-ion con- centration is less or the hydroxide-ion concentration greater in the solution than in the jelly. Rhythmic Swelling of Protein Jellies. Sheppard and Elliott 24 made a study of the causes of the reticula- tion of the surfaces of photographic negatives that has a bearing upon a similar kind of trouble sometimes occurring in the vegetable tanning of skins. During the fixing or washing of a negative, the wet gelatin layer sometimes becomes more or less finely wrinkled or corrugated, the network of puckers forming a pattern extending either over the whole of the negative or only over part of it. They found that this reticulation can be produced by the combined action of a swelling agent and a tanning agent. Fig. 48 represents a print from a negative treated to produce reticu- lation by Mr. Daub in the author’s laboratories. The plate was flashed, — developed, fixed with sodium thiosulfate, washed, and then immersed in a solution of wattle bark extract containing 5 grams of tannin and 0.2 mole of acetic acid per liter; the temperature was kept at 24 The Reticulation of Gelatine. S. E. Sheppard and F. A. Elliott. J. Ind. Eng. Chem. 10 (1918), 727. ri PHYSICAL CHEMISTRY OF THE PROTEINS 117 28°C. After several minutes the gelatin surface began to pucker at isolated points and this action gradually spread over the entire sur- face, producing series of ridges of swollen gelatin with valleys of hardened and contracted gelatin in between. Following this action, the silver particles migrated from the hardening portions into the swelling ridges, giving the negative the mosaic-like appearance shown in the print. Often the puckering became well pronounced before the Fig. 48.—Reticulation Produced on Photographic Negative. migration of silver particles was noticeable. Sheppard and Elhiott liken the effect to the production of Liesegang rings. The acid tends to cause a swelling of the gelatin while the tannin tends to cause a hardening and contracting action. But the acid diffuses relatively very rapidly whereas the diffusion of the tannin is greatly retarded both by its high molecular weight and by its tendency to combine with the gelatin, forming a compound less permeable and having a much lower power of swelling than the original gelatin. The action becomes greatly accelerated as the temperature is raised towards the melting point of the gelatin jelly. When the action is prolonged 118 THE CHEMISTRY OF LEATHER MANUFACTURE at higher temperatures, provided the jelly does not dissolve, a second and much coarser series of puckers begins to form, tending to mask the finer pattern. In the coarser pattern, the peaks of the ridges may be from one to several millimeters apart. The reticulation of the surface of skin in tanning is a very serious matter as the pattern formed is permanent and materially reduces the selling value of the leather. The pattern formed on skins is usually of the coarser variety and would hardly pass as an artistic sample of embossing, which the photographic negative might do, because of the fineness of the pattern and distribution of silver particles. The reticu- lation of skin may attend the injudicious use of acid in attempting to plump the leather during tanning, or it may occur where acid-producing ferments get the upper hand in a yard where fresh liquors are normally used. The corrective is to prevent the swelling action, either by neutralization of the acid or by the addition of salt. Structure of Gelatin Solutions and Jellies. Procter’s *° investigations of the behavior of gelatin jellies led him to regard them as having a structure consisting of a network of molecules cohering to each other, but leaving interstices large enough to permit the passage of water and simple molecules and ions. The long chains of amino acids making up the protein molecules are peculiarly fitted to produce such a structure through combination of the acid and basic terminals of these chains. A hot solution of gelatin may be looked upon as a true solution consisting of individual gelatin molecules, or at least of comparatively small polymerized groups, but the molecules orientate themselves, as the solution cools, so as to leave a minimum of free energy, the most active acid groups tending to unite with the most active basic groups until a continuous network is formed throughout the system. A block of jelly might thus be looked upon as an enormous, single molecule. Such a view is not radical in the light of modern theories of crystal structure. According to the Procter-Wilson theory of swelling, when a block of gelatin jelly is immersed in a solution of hydrochloric acid, the solution passes into the jelly, filling up the interstices. Of the ionized gelatin chloride, which then forms, the chloride ions remain in the solution in the interstices while their corresponding gelatin cations form part of the network and are not in solution in the same sense as the anions. In tending to diffuse into the outer solution, the anions exert a pull upon the cations forming part of the network, causing an increase in volume of the jelly proportional to the pull exerted, so long as the elastic limit is not exceeded. That gelatin jellies are truly elastic and follow Hooke’s law may be taken as proved chemically by the agreement between calculated and observed results shown in Table XI. More recently Sheppard and Sweet 28 proved by measure- 7° The Structure of Organic Jellies. H. R. Procter. Proc, Seventh International Con- gress of Applied Chemistry, London, 1909. *° The Elastic Properties of Gelatin Jellies. S, E. Sheppard and S. S. Sweet. J. Am. Chem. Soc. 43 (1921), 539. } PreAl: CHEMISTRY OF THE PROTEINS 119 ments of rigidity that gelatin jellies follow Hooke’s law nearly up to the breaking point. Loeb’s work on the viscosity of gelatin solutions, to be discussed presently, indicates that the initial step in gelation is the combination of individual molecules to form large aggregates, possibly in a manner similar to the growth of crystals. Bogue *’ pictures this process as the formation of catenary threads by the union of the individual molecules end to end. The manner in which fibrous curds of soap are formed led McBain ?® to a similar view regarding the structure of soap jellies and solutions. He attributes the elasticity of gels to the formation of an exceedingly fine filamentous structure. Innumerable molecules placed lengthwise and held together by forces of residual valence are assumed to make up these fine threads, which may be microns or millimeters in length. Considering the nature and variety of the amino acids composing the gelatin molecule, as shown in Table I of Chapter 3, we should hardly expect the polymerization of gelatin to take place along a single line, but in every direction and probably with cross chains grow- ing to support chains increasing in length in other directions. The increasing viscosity of gelatin solutions with time, upon cooling, would thus be attributed to the increasing size of the particles; the formation of a rigid jelly to the final union of the large particles, forming a structure continuous throughout the entire system. There is an abundance of evidence to support Procter’s view of the structure of jellies and Loeb’s view that gelatin solutions, after standing for a time at temperatures below 35° C., always contain par- ticles of jelly consisting of aggregates of gelatin molecules. A number of supporting lines of evidence are given in a review of the literature by Thompson.?® Graham showed long ago that the velocity with which crystalloids diffuse through gelatin jellies is only very little less than the velocity through pure water. This slight reduction in velocity is in no way comparable with the apparently great physical difference in state be- tween the jelly and water. Although the viscosity of a gelatin jelly is too great to be measured by the methods usually applied to liquids, simple molecules move through it as though in a medium of viscosity nearly that of water. The network theory explains this by assuming that the diffusing substance actually is moving through the pure water or aqueous solution in the interstices of the network. Any slight diminution in velocity can be accounted for by the small portion of any cross section of the jelly occupied by the gelatin network. The same holds true for gelatin solutions, the diffusing substance being able to pass through the particles of jelly in suspension almost as rapidly as through the solution surrounding the particles. ts ga and Constitution of Glues and Gelatines. R. H. Bogue. Chem. Met. Eng. 23 (1920), 61. 78 Colloid Chemistry of Soap. J. W. McBain. Brit. Assoc. Advancement Sci. Third Report on Colloid Chemistry (1920), 2. ( pies of Gelatin Solutions. F. C. Thompson. J. Soc. Leather Trades Chem. 3 T1919), 209. 120 THE CHEMISTRY OF LEATHER MANUFACTURE Thompson shows from the work of Dumanski *° that the con- ductivity of a solution of potassium chloride in gelatin jelly is no less than in pure water when a correction is made for the small volume actually occupied by the gelatin network, whereas, if the apparent viscosity had any effect, the conductivity should be reduced by the gelatin to\a minute fraction of its value in pure water. | The vapor pressure of even a 20-per cent gelatin jelly is prac- tically the same as that of water, indicating the presence of pure water in accordance with the network theory. By placing a strain upon gelatin jelly in one direction, double re- fraction is produced, a property always associated with a definite structure and with anisotropy. Even dilute solutions of gelatin show double refraction on compression or when passed between two cylinders rotating in opposite directions. With increasing strain, the effect is increased up to a point corresponding to an elastic limit. This in- dication of structure even in gelatin solutions corroborates the views of Loeb and of Bogue. The fact that the viscosity of gelatin solutions is lowered by simply agitating the solution is another piece of evidence in favor of the existence of a structure in gelatin solutions and still further evidence is furnished by Loeb’s work on the viscosity of gelatin solutions and Bogue’s measurements of plasticity, to be described later. Relation of the Osmotic Pressure and Viscosity of Gelatin Solu- tions to the Swelling of Gelatin Jellies. In an extensive series of experiments, Loeb has shown that the variations in osmotic pressure and viscosity of gelatin solutions with change of pH value or of concentration of salt, parallel the corre- sponding variations in the degree of swelling of gelatin jellies, which is what would be expected on the basis of the theory of protein-salt formation described above. This parallelism is shown by the curves in Figs. 49 to 54. In each determination *! of the two series of experiments performed to get the curves shown in Fig. 49, 1 gram of powdered gelatin was put for 1 hour at 20° C. into 100 cubic centimeters of acid solu- tion of definite strength. The volume of the gelatin was measured, after settling, in a graduated cylinder and the pH value of the jelly was determined after melting. The volume is plotted against the pH value of the jelly and not that of the external solution, which was always lower, as explained in the discussion of the theory of swelling. The curves in Fig. 50 were obtained by rapidly heating to 45° C. solutions of 0.8-per cent gelatin containing different amounts of acid, maintaining this temperature for 1 minute, cooling rapidly to 24°, and immediately determining the viscosity at 24°. The viscosity is plotted against the pH value of the gelatin solution.®2 °° Z. physik, Chem. 50 (1907), 553. *1Ion Series and the Physical Chemistry of the Proteins; II. Jacques Loeb. J. General Physiol. 3 (1920), 247. Ton Series and the Physical Properties of Proteins; I. Jacques Loeb. J. General Physiol, 3 (1920), 85. om pee sICAL CHEMISTRY OF THE PROTEINS I2I In the experiments whose results are shown in Fig. 51, collodion bags, cast in the form of Erlenmeyer flasks having a volume of 50 cubic centimeters, were filled with I-per cent gelatin solutions con- taining different amounts of acid. Each bag was closed with a rubber stopper fitted with a glass tube serving as a manometer and put into 4 he Temp. 20°. fs 60 ae 40 pr HeS04 Oe » Ba 20 ® o © 1 gram of powdered gelatin Pe 0.8 gram gelatin dissolved in LOO-c. c,. solution Relative Viscosit (water = 1) 4007 4 gram gelatin dissolved in LOO) G, uC. solution HoS04 Temp. 24°. Osmotic Pressure (m. m. of water) re) ro) oO 2 3 4A pH Value of Gelatin Solution Variables as Functions of pH Value. Fic. 49.—Volume of powdered gelatin. Fic. 50.—Viscosity of gelatin solution. Fic. 51—Osmotic pressure of gelatin solution. a beaker containing dilute acid solution of the same kind as was used in making up the gelatin solution. When osmotic equilibrium was established, the level of solution in the manometer was recorded and plotted against the pH value of the gelatin solution.** The measurements were made at 24°. The explanation of the parallelism between the curves for swell- ing, viscosity, and osmotic pressure as functions of pH value is that 88 Donnan Equilibrium and the Physical Properties of Proteins; II, Osmotic Pressure. Jacques Loeb. J. General Physiol. 3 (1921), 691. 122 THE CHEMISTRY OF LEATHER MANUFACTURE the variation in each case is due to the same fundamental cause, namely, the establishment of a Donnan equilibrium. In the viscosity measure- ments, the solutions contain aggregates of gelatin molecules capable of swelling with change of pH value and, since the viscosity must in- crease with the increasing volume occupied by the gelatin, we should 3 25 oo a 0.5 gram of powdered gelatin pa ie 8 uA Temp.<20°, on 16 pH value 3.0. fo do od 10 hb i 0.5 gram of powdered gelatin o 2.5 suspended in 100 c. c. of on solution =" 2.0 | ake : Temp. 20°. PP pH value 3.0. re BE 1.5 rs i?) cr 400 o~ 1 gram of gelatin dissolved 26 in 100 c. c. of solution @ ® 300 ope Temp. 24°, u pH value 3.5. 2 200 at s > #8 e a #100 0,02 0.04 0.06 Moles NaCl or NaNOg per Liter Variables as Functions of Concentration of Added Salt. Fic. 52.—Volume of powdered gelatin. Fic. 53.—Viscosity of gelatin suspension. ' Fic. 54.—Osmotic pressure of gelatin solution. expect the viscosity to rise and fall with the degree of swelling of the gelatin particles. In the experiments on osmotic pressure, we have an application of the Donnan equilibrium which is considerably simpler than that involved in the swelling of jellies, although of a similar kind. In the swelling and osmotic pressure experiments, we note that the points of maximum given by sulfuric acid are only half as great as those given by hydrochloric acid, which is in harmony with the theory, since the divalent sulfate ion has no greater diffusion pressure than PHYSICAL CHEMISTRY OF THE PROTEINS 123 the monovalent chloride ion and is only half as numerous for equivalent concentrations of gelatin salt. In Figs. 52, 53, and 54 are given curves showing the depressing effect of increasing concentration of neutral salt upon the volume of powdered gelatin,** the viscosity of a suspension of powdered gelatin,** and the osmotic pressure of a solution of gelatin.*® Again we find a parallelism in the results that would be expected from the theory. Osmotic Pressure and Membrane Potentials. A discussion of the mechanism of the osmotic pressures exerted by protein solutions may serve to make the theory of swelling, which is the more important in leather chemistry, a little clearer. The collodion bags used in Loeb’s experiments were permeable to water and simple acids, bases, and salts, but not to dissolved proteins. Let us consider a solution of gelatin chloride and hydrochloric acid contained in a collodion bag which is brought into contact with pure water. Hydrochloric acid diffuses out through the membrane until equilibrium is established between the external solution and the gelatin solution inside the bag. The outside solution contains only hydrochloric acid, but the inside solution contains both hydrochloric acid and_ gelatin chloride. At equilibrium, in the outside solution, let x = [H*] = [CV] and in the inside solution let y=([H*] | z = [gelatin ion] whence PCr y+ z. It is apparent from the reasoning given early in this chapter that at equilibrium | x? = y(y + 2) and that Oy a2 > 2X: The greater concentration of diffusible ions of the inside solution, 2y-+z2, must give rise to an osmotic pressure proportional to the quantity ¢ in the expression e= 2y + z— 2x. This assumes that the gelatin exerts no osmotic pressure of its own, which may not be strictly true. A correction would have to be made by adding to e an amount corresponding to the osmotic pressure of the gelatin. But Loeb ** has shown that any such correction that may be necessary is less than the probable experimental error of measurement. * Donnan Equilibrium and the Physical Properties of Proteins; III, Viscosity. Jacques Loeb. J. General Physiology 3 (1921), 827. 35 Donnan Equilibrium and the Physical Properties of Proteins; I, Membrane Potentials. Jacques Loeb. J. General Physiol. 3 (1921), 667. eo The DD Ses of the Influence of Acid on the Osmotic Pressure of Protein Solu- tions. Jacques Loeb. J. Am, Chem. Soc, 44 (1922), 1930. 124 THE CHEMISTRY OF LEATHER MANUFACTURE When +, y, and zg are determined in the solutions, the osmotic pres- sure can be calculated. At 24°C. the osmotic pressure, in terms of millimeters pressure of a column of water, equals 2.5¢ X 10°. For casein chloride, Loeb found that the observed osmotic pressure ap- proximated the value 250000e as closely as the determinations could be made. Because of the unequal distribution of ions between the inside and outside solutions, there must be an electrical difference of potential set up between the two solutions whose measure at 20° C., as in the case of the jellies, is given by the formula P.D. = 58(pH inside minus pH outside) millivolts. In determining the potential difference between the inside and out- side solutions, Loeb used an apparatus similar to that shown in Fig. 47. The collodion bag containing the inside solution was hung in the. beaker filled with the external solution. The manometer tube of the collodion bag was replaced by a funnel. ‘The capillary tube of the right hand calomel cell was dipped into the funnel so as to make contact with the inside solution. ‘Lhe potential difference of the system was then measured by means of a Compton electrometer. S TABLE XV. GELATIN SOLUTIONS AT 24° C. pH value of Osmotic Inside Outside (a) P.D (millivolts ) Moles pressure solution solution minus Calcu- Ob- NaNO; per liter (mm. ) (a) (b) Ch lated = served INOUE Tt or eee 435 3.58 3.05 0.53 31.2 31 | O.0002d1 tera sine ee 405 3.56 3.08 0.48 28.3 28 ) CL0004BS On se ki cael any 3.51 3.10 0.41 24.0 24 : G.00007 5. oe suse Siew 335 3.46 3.11 0.35 20.7 22 i G.OCLQS oo ecgis none ae 280 3.41 3.14 0.27 16.0 16 ' COD 30 cas ki oats eee 215 3.30 ait7 0.19 11.2 12 ODO070 cae. orca aa ec 134 3.32 3.20 0.12 7.0 7 GORE s 2. Oe aoe 85 3.20 3:22 0.07 4.1 4 OO3T2 cance ces ae 63 3.25 3.24 0.01 0.6 O Further quantitative proof of the correctness of the theory is fur- nished by the data in Table XV, showing the depressing effect of increasing concentration of neutral salt upon the osmotic pressure and potential difference of a system in which an acid solution of gelatin is separated from a gelatin-free solution by means of a collodion membrane.*’ The osmotic pressure curve is plotted in Fig. 54. When equilibrium was established, the pH values of both inside and outside solutions were determined and the potential differences were determined in the manner described above. The potential differences were also calculated from the pH determinations, the factor 58.8 being used for 24°. The agreement between calculated and observed results is as nearly perfect as could be hoped for. ** The Colloidal Behavior of Proteins. Jacques Loeb. J. General Physiol. 3 (1921), 557.. 7 PHYSICAL CHEMISTRY OF THE PROTEINS 125 With increasing concentration of salt, the pH values of the inside and outside solutions approach each other. According to the theory, the distribution of any ion between the two solutions is similarly affected by the addition of salt; i.c., the logarithms of its concentration in the inside and outside solutions, respectively, approach each other, bring- ing about a lessening of the difference in total concentration of dif- fusible ions between the two solutions. It is this effect rather than any supposed repression of ionization of the protein salt that is responsible for the reduction in the swelling of jellies and the osmotic pressure, viscosity, and potential difference of protein systems. Changes in Viscosity of Gelatin Solutions with Time. When hot solutions of gelatin are allowed to cool, their viscosities increase with time until they finally set to rigid jellies. Loeb attributes this to the formation of aggregates of gelatin molecules, the viscosity increasing with the average size of the gelatin particles. The curves in Figs. 55 and 56 show that this increase in viscosity with time fo) 3 . é » & b pH value = 2.7. b> sc - } n oO ° 2 a > ot as Temp. 20°. 10 20 30 40 = 50 10 20 30 40 £50 Time in Minutes Time in Minutes Fic. 55.—Increase in viscosity with Fic. 56.—Change in viscosity with time of 2-per cent solutions of time of 2-per cent solutions of gelatin sulfate of different pH gelatin chloride at different tem- values. ; peratures. is materially influenced both by the pH value and temperature of the gelatin solution.** The effect of pH value was determined by rapidly heating 2-per cent gelatin solutions containing different amounts of sulfuric acid to 45° C., cooling rapidly to 20°, and then maintaining % The Reciprocal Relation_between the Osmotic Pressure and the Viscosity of Gelatin Solutions. . Jacques Loeb. J. General Physiol. 4 (1921), 97. ; 120 THE CHEMISTRY OF LEATHER MANUFACTURE this temperature while viscosity measurements were made at intervals of 5 or 10 minutes. An increasing concentration of acid tends to pre- vent the formation of aggregates; the viscosity increases most rapidly at the isoelectric point. The effect of temperature was determined by rapidly heating 2-per cent gelatin chloride solutions having a pH value of 2.7, to 45°"G5 cooling rapidly to the temperature at which the viscosity measurements were to be made, and maintaining this temperature while determina- ° Q am Nrero ° &% D OSS S 160 140 120- 100 ; 604! 1 ‘ / H i 404!) pay Viscosity (angular deflection) ee) oO £06 wire #30 j y 2041 Hi / M,/, da 4 LO: 20 30° 40a Revolutions per Minute Fic. 57.—Viscosity-plasticity curves for a 20-per cent gelatin solution. tions were made at intervals of 5 or 10 minutes. The remarkable point to be observed is that the viscosity increases with time at temperatures below 35° C., but decreases with time at higher temperatures. Bogue *® measured the viscosities of gelatin solutions at different temperatures by means of a Macmichael torsional viscosimeter. At each temperature he made measurements for a number of different speeds of rotation of the cup. A set of these is shown in Pigess%, °° The Sol-Gel Equilibrium in Protein Systems. R. H. Bogue. J. Am. Chem. Soc. 44 (1922), 1313. RiRaE age ok ape: nie See i ff y EE: +4 PewiCAL GOEPMISTRY OF THE PROTEINS Te The continuous lines cover the range of actual observation and the dotted portions represent the curves extrapolated to zero speed of rotation. For all temperatures above 34° C. the extrapolated curves pass through the origin, indicating truly viscous flow. But for lower temperatures, the curves do not pass through the origin; they indicate a finite deflection for an infinitesimal speed of rotation, showing that here we have an example of plastic flow. The gelatin solutions at ‘lower temperature actually possess a measurable degree of rigidity. This is further evidence in support of Smith’s view that at temperatures above 35° gelatin in solution exists in a form having no power of gelation. As the temperature is lowered, some of this sol form changes into a gel form which has the power of gelation. As the temperature is lowered, the proportion of gel form to sol form increases until at 15° and lower temperatures all of the gelatin exists in the gel form. The structure of the aggregates of molecules of the gel form is such as to impart to the solution the rigidity observed by Bogue. When an acid solution of gelatin contained in a collodion bag at 20° C. is brought into equilibrium with a pure aqueous solution of the acid, the solution actually is separated into 3 phases. The gelatin solution within the bag has a hydrogen-ion concentration less than that of the external solution, but greater than that of the solution absorbed by the aggregates of gelatin molecules suspended in the gelatin solu- _tion. Loeb *” has shown that, with increasing proportion of aggregates to dissolved gelatin, the variation of pH value produces an increasing effect upon viscosity, but a decreasing effect upon osmotic pressure measurements, as would be expected. Theory of Salting Out and the Stability of Colloidal Dispersions. Protein solutions and other so-called emulsoid colloids differ from the suspensoid colloids, such as colloidal gold, in requiring relatively very high concentrations of salt to precipitate their sols. It is gen- erally admitted that the stability of colloidal dispersions is increased by the electrical charge usually associated with the particles. However, but little quantitative work has been done on the actual determination of this charge. Powis *°.measured the potential difference at the oil-water boundary of an emulsion of cylinder oil and found that the emulsion was stable only when the absolute value of the potential difference exceeded 30 ~ millivolts. When it was reduced to any value lying between plus or minus 30 millivolts, coagulation took place, but at a rate which was independent of the voltage. It was pointed out by the author *4 in 1916 that Donnan’s theory of membrane potentials is applicable to suspensoids as well as to pro- tein jellies. A gold sol may be taken as a typical example. When gold is dispersed in water, the presence of chloride, bromide, iodide, or 40 The Relation between the Stability of an Oil Emulsion and the Potential Difference at the Oil-Water Surface eae and the Coagulation of Colloidal Suspensions. F, Powis. Z. physik. Chem. 89 (1914) = pease of Colloids. j. a "Wilson. J, Am, Chem, Soc. 38 (1916), 1982. 128 THE CHEMISTRY OF LEATHER MANUFACTURE hydroxide ion in concentrations ranging from 0.00005 to 0.005 normal has a marked stabilizing effect on the sol produced and the particles are negatively charged. The effect seems to be due to the ability of these ions to form stable compounds with the gold. Fluoride, nitrate, sulfate, and chlorate ions decrease the stability of gold sols, which is significant in view of the fact that they do not form stable compounds with gold.*? In Fig. 58 let A and B represent two gold particles stabilized by potassium chloride. In combining with the gold, the chloride ions have imparted their negative charges to the particles. But the potassium ions are still left in solution, although their field of motion is restricted to the thin film of solution wetting the particles because they must continue to balance the negative charges on the particles. The volume P'tG. 58.—Particles of stable gold sol showing enveloping films of aqueous solution. of the film of aqueous solution enveloping a particle will be measured by the surface area of the particle and the average distance that the potassium ions are able to travel from the surface. Let us now consider the case where an amount of potassium chloride — is present in the sol too small to cause precipitation. The enveloping film will contain potassium ions balancing the charges on the par- ticles as well as ionized potassium chloride. The surrounding solution will have potassium and chloride ions only in equal numbers. In the surrounding solution let x = [K*] = [CV] in the enveloping film let y= [Cl] and z= [K*] balanced by charges on the particles, whence y +z represents the total concentration of potassium ion. As was shown in the discussion of Donnan’s theory, the product [K*] [Cl’] must have the same value both in the enveloping film and in the surrounding solution at equilibrium. Hence x? = y(y +z). “ The Electrical Synthesis of Colloids. H. T, Beans and H. E. Eastlack. J. Am. Chem, S06. 37 (1915), 2667. PHYSICAL CHEMISTRY OF THE PROTEINS 129 The surface layer of solution will have a greater concentration of ions than the surrounding solution by the amount 2y + z— 2x. This un- equal distribution of ions will give rise to a difference of potential between the enveloping film and the surrounding solution whose measure is RE Scout 2x E = —— log — = —=— log ———————__— FP ay im —z+ V74x*+2 But now, if we increase + without limit while g remains constant, E must decrease, approaching zero as a limit, since limit {ieee a log ee xX — © B —= «= OA. V Ax? It is thus evident that the difference of potential between the envelop- ing film and the surrounding solution will be a maximium when there is no free potassium chloride present and will decrease, approaching _ zero, as the concentration of potassium chloride is increased without limit. The particles shown in Fig. 58 are prevented from coalescing be- cause there is a sufficiently high potential difference of the same sign Fic. 59.—Coagulation of gold sol initiated by reduction of potential difference between enveloping films and the surrounding solution, by the addition of potassium chloride. between the surrounding solution and each enveloping film. The electrostatic repulsion is determined by this potential difference rather than by the absolute electrical charge on the particles because the surface film completely envelops the particles and endows them with its own properties. | When enough potassium chloride has been added to lower the potential difference to a point where it is no longer able to overcome the attractive forces between the particles and the surface tension of the enveloping film, the particles move toward each other and the enveloping films of two or more particles blend into one, as shown in Fig. 59. It is at this point that the actual charges themselves come into play and probably determine the nature of the precipitate. 130 fo CHEMIST eee ea MANUFACTURE We have now only to substitute for the solid particle with its enveloping film the molecular network with aqueous solution filling up the interstices to make this theory of salting out apply to gelatin and similar proteins. By referring back to Loeb’s data in Table XV, it will be noted that the potential difference between an acid solution of gelatin and a gelatin- free solution with which it was in equilibrium was reduced to less than one millivolt by the addition of 0.031 mole per liter of sodium nitrate. If we may assume a similar lowering of potential difference between highly dispersed gelatin particles and the dispersion medium by the addition of this quantity of salt, it would follow that coagula- tion as a function of this difference of potential is not independent of the properties of the disperse phase. A gelatin solution shows no tendency to precipitate in the presence of 0.03 mole of sodium nitrate, but Powis found that his emulsion of cylinder oil ceased to be stable when the potential difference was reduced to 30 millivolts. Half- saturating a gelatin solution near the neutral point with ammonium sulfate will cause its precipitation, but we have as yet no data in- dicating the extent to which the potential difference is lowered before the precipitation begins. Thomas * has called attention to the fact that the stability of col- loidal dispersions may be determined more, in some cases, by the at- traction between the dispersed phase and dispersion medium than by the difference of potential at the interface. The low degree of attrac- tion between oil and water was probably responsible for the coagula- tion of Powis’ emulsion at 30 millivolts. Apparently a potential difference of less than one millivolt is sufficient to prevent the precipita- tion of certain protein solutions because of the attraction existing be- tween the protein and water. The attraction between sugar and water appears to be so great that no potential difference at all is required to keep it‘in solution. Loeb’s work, taken in conjunction with investigations in the author’s laboratories, indicates that the lowering of the potential difference of protein systems is not brought about by repression of ionization of the protein salts, as has often been supposed, but rather by the mecha- nism of the Donnan equilibrium just described. In gelatin systems in which the potential difference has been lowered to a very small value, we find no repression of ionization of gelatin chloride measurable by means of calomel electrodes. Moreover, there is no need to postulate such repression in order to account quantitatively for the observed results. | An application of this theory of salting out to soap solutions furnishes a needed addition to McBain’s** theory of soap solu- tions, in .which it would be well also to look upon the micelle as an aggregate of monovalent ions rather than as a complex polyval- ent 10n. aiwecee gelation Theory of (Collada! Diveersion, 6 a ** Colloidal Electrolytes. Soap Solutions and Their Constitution. J. W. McBain and C. S. Salmon. J. Am, Chem, Soc. 42 (1920), 426. ‘ a a a a PHYSICAL CHEMISTRY OF THE PROTEINS 131 Adsorption. Ever since Gibbs showed that the concentration of the solute must be greater at the surface than in-the bulk of solution where the solute lowers the surface tension of the’ solution, there has been a tendency to look upon this proof as an explanation of the fact that substances of great specific surface reduce the concentration of solute in many different kinds of solution with which they are brought into contact, The error in this tendency lies in the fact that Gibbs’ work applies only to the lowering of the surface tension by a substance actually in solution. Since, in many cases, it has not been found possible to determine the actual concentration of solute in the layer of solution immediately in contact with the surface of the material causing a> decrease in concentration of solute in the bulk of solution, any con- _ clusions as to the causes of such decrease have been open to question. In the case of gelatin, however, it has been found possible to measure concentrations in the absorbed solution and this has thrown considerable light on the phenomenon known as adsorption. Adsorption is a term now widely used to indicate the removal of solute from solution by a material in contact with the solution. An empirical formula was proposed by Freundlich #® which agrees ap- proximately with some observed results over limited ranges, provided the two constants required in the formula can be selected to suit the findings. The formula may be represented as follows: Wossiax”, where w is the amount of solute removed from solution by unit quantity of the adsorbing material, x is the final concentration of solute, and a and b are constants selected to suit the occasion. Freundlich mentions that b may vary from o.1 to 0.5, but a very much more. ' The very nature of the equation makes it capable of fitting a great variety of data, especially since the constants may be selected as de- sired, but it doesn’t explain anything. Referring back to Table XI, we find that the total quantity of chloride in the gelatin jelly at equilibrium, represented by V(y ++ z), can be represented as a function of the hydrogen-ion concentration by the use of Freundlich’s formula. Letting V(y + z) = 7.33x°-#?, we can plot a curve for the total quantity of hydrochloric acid, combined and uncombined, which has been ab- sorbed by the jelly that agrees fairly closely with both the calculated and observed results given in Table XI, although not quite so well as do the calculated and observed results with each other. Plotting logV(y +z) of the above equation against logx, we get a straight line, but the observed results never give a perfectly straight line, but vary in the same directions as do the calculated results of Table XI. The curve for the concentration of gelatin chloride shown in Fig. 42 also can be represented approximately by Freundlich’s formula by letting z—o0.10x™*. The formula is a convenient means of represent- * Kapillarchemie. H. Freundlich, Leipsic, 1909. 132 THE CHEMISTRY OF LEATHER MANUFACTURES ing a’reaction approximately over a limited range, which it is able to do merely because many variables give curves that are nearly parabolic in shape. | | Adsorption, so far as it pertains to gelatin jellies, is a manifesta- tion of chemical combination complicated by the separation of the solution into two phases. We see no reason for looking upon adsorp- tion by other materials in any different light. In the case of suspensoids, we are dealing with two phases of the solution apparently analogous to those of gelatin systems, the film of solution enveloping the particles corresponding to the solution absorbed by the jelly. For a more elaborate treatment of certain phases of modern theories of the physical chemistry of the proteins, the reader is referred to Loeb’s “Proteins and the Theory of Colloidal Behavior” ** and to Bogue’s “Chemistry and Technology of Gelatin and Glue.” *® -». 46 McGraw-Hill Book Co., New York, 1922. Chapter 6. Preservation and Disinfection of Skin. Practically every country in the world supplies hides and skins for leather manufacture. The skins from large, fully grown animals are usually called hides, those from half grown animals of the larger variety kips, while those from small or very young animals, or those intended for furs, are ¢alled skins. For example, as the calf grows into a cow, its skin remains a skin until it reaches a weight of about I5 pounds in the wet state, when it becomes a kip, while it becomes a hide at about 30 pounds. These figures are necessarily arbitrary, but serve to indicate the general scheme of classifying skins accord- ing to size. A bull hide may weigh more than 100 pounds. A sheep skin always remains a skin because it never assumes great size. The skin of the full grown East Indian buffalo is called a kip because it is smaller than the ordinary cow hide. For convenience, the term skin is used in its general sense throughout this book to include hides and kips, except when referring to specific cases. The fact that animals are generally raised and slaughtered for food rather than for purposes of leather manufacture makes the tanner’s chief raw material a by-product of the packing industry. For this reason a decreasing consumption of leather has but little influence upon the continued supply of skins, although it does tend to lower their market value. On the other hand, a brisk demand for leather generally does not in itself stimulate the raising and slaughtering of cattle, but rather has the effect of increasing the vigilance against damage to the existing supply of skins by putrefaction, careless handling, or the ravages of insects. Raw skins are highly putrescible and, since a considerable period of time usually elapses between the slaughter and the first tannery operation, it 1s necessary to subject them to some method of preservation as soon as possible after flaying. Salting. The commonest method of preserving skins, where they do not have to be transported very long distances and where salt is reasonably cheap and plentiful, is salting or curing, as it is sometimes called. The skins are laid out flat, flesh side up, and covered with salt in amount equal to about one quarter of their weight. Often they are placed in piles so arranged that the sides are higher than the center, which keeps the brine from flowing away, but this is undesirable unless 133 134 THE CHEMISTRY OF LEATHER MANUFACTURE the skins have previously been washed free from blood. Sometimes they are soaked in a concentrated solution of salt first and then covered with dry salt. The object is to get the salt to diffuse completely through the substance of the skins, which may require only a few days for light skins or weeks for heavy hides. Each skin is then folded up, hair side out, and in this condition sent to the market. Where the blood and lymph have been removed from the skins immediately after flaying and enough pure salt has been used to give a nearly saturated solution in the skins, putrefaction is reduced to an almost negligible degree and the skins may be kept for a long time with comparative safety. Common salt is most widely used, but sodium sulfate and other neutral salts are also effective and actually used in some places. Salt Stains. A defect commonly found in salted skins is the appearance of peculiar stains, usually either rusty brown or greenish blue in color, which are sometimes very difficult to remove and only become in- tensified and darkened through contact with sulfide-lime liquors or vegetable tan liquors, substantially lowering the market value of the leather. Because they are a source of loss and annoyance to the tanner, efforts have been made, from time to time, to determine their cause and methods for preventing them. Some stains disappear when the un- haired skins are pickled with a solution of sulfuric acid and salt, but others are resistant even to this process as ordinarily conducted. These stains received the name salt stains from the general belief that they were caused by the salt used in curing. At any rate, it was ap- preciated that their frequency of occurrence was influenced by the composition of the salt and the method of its application. The percentage of stained skins was especially high in those parts of Europe where edible salt is taxed and the salt used for curing must be denatured. The use of commercial aluminum salts, particularly those containing iron, was looked upon with suspicion and the scientific men of the industry began to seek other denaturing materials that would tend to prevent rather than to cause stains. One important school of thought regarded bacterial action as being largely responsible for the formation of the stains and sought de- naturing materials capable of checking bacterial growth. Paessler * found that the percentage of stains appearing on skins could be greatly reduced by curing with salt denatured with 3 per cent of its weight of anhydrous sodium carbonate. His discovery was put into general use and had the important effect of considerably decreasing the percentage of stained skins. : | Schmidt ? showed that bacterial action could be effectively checked by using salt previously sprinkled with a I2-per cent solution of zinc chloride and this method has been used to some extent to prevent salt 1 Salting of Hides and Skins. J. Paessler. Ledertech, Rundschau (1912), 137. , ? Depreciation of Skins in Process. C. E. Schmidt. Shoe & Leather Rep., March » I9II. ; PRESERVATION AND DISINFECTION OF SKIN — 135 stains. But, after making a series of comparative tests, Paessler * claimed that zinc chloride was no more effective than sodium carbonate in preventing salt stains. Romana and Baldracco * suspected the blood and lymph as the source of the stains and tried washing the skins very thoroughly after flaying and before adding the salt. On skins thoroughly washed they found no stains at all. They also found that the stains could be prevented by adding to the salt used in curing 1 per cent of its weight of sodium fluoride. - Kitner ° suggested that many stains are caused by delaying the salt- ing operation until bacterial action has already considerably advanced. He advised a more thorough elimination of water by heavily salting the skins, draining off as much brine as possible, and then resalting. The brine drained off carries with it proteins which are very susceptible to putrefaction. Yocum ® observed that salt stains occurred much more frequently in summer than in winter and were most abundant where the skins had had greatest contact with the air or had been kept for the longest period in the salted condition. Tests for iron were obtained on pieces of filter paper previously moistened with acetic acid and placed on the stains. Where stains still appeared on the finished leather, he obtained a test for iron in the stained, but not in the unstained parts. But iron was often found in the ash of fresh skins which showed no stains when tanned at once without salting. This seemed to in- dicate that the staining was due to a change in the condition of the iron present which enabled it to combine with the skin. He was able to produce stains on skins by treating them with hemoglobin and sug- gested that the hemoglobin of the blood might have been the source of the staining material. Becker 7 made extended studies of yellow, orange, and red stains on skins and isolated from them bacteria which, in pure cultures, were able to produce the corresponding stains. He also found that adding salt, up to 10 per cent of the weight of the skin, favored the action of these bacteria, while greater amounts retarded it. He warned against the use of an insufficient quantity of salt in curing, storing the skins in a warm, damp atmosphere, and of allowing dirt and filth to re- main on the skins. As a means of preventing these stains, he recom- mended dipping the skins in a 0.25-per cent solution of mustard oil, followed by the application of plenty of clean salt denatured with sodium carbonate. Not being able to reproduce the blue stains by bacterial action alone, he admitted that these might be due to chemical changes other than those involving bacteria. , The great stress placed upon the role played by bacteria in the formation of salt stains adds interest to the work of Abt,§ ® who main- 8 Soda as a Denaturant for Hide Salt. J. Paessler. Ledertech. Rundschau (1921), 169. 4 Salting of Hides and Avoidance of So-Called Salt Stains. C. Romana and G, Bald- racco. Collegium (1912), 533. 5 Theory of Salt Stains. W. Eitner. Gerber (1913), serially. ®Salt Stains. J. H. Yocum. J. Am. Leather Chem, Assoc. 8 (1913), 22. 7 Salt Stains. H. Becker. Collegium (1912), 408. 8 Origin of Salt Stains. G. Abt. Collegium (1912), 388 _.® Microscopical Examination of Skin and Leather Applied to the Study of Salt Stains. Ibid. (1914), 130. 136 THE CHEMISTRY OF LEATHER MANUFACTURE tained that most of the salt stains he had examined in France were not caused by bacterial action. Particularly bad cases of staining were traced to the presence of crystals of calcium sulfate in the salt used for curing. ‘The stains themselves always contained considerable quan- tities of calcium phosphate as well as iron. ‘The stained regions always: gave more intense qualitative tests for iron than the unstained regions, but analysis showed the same actual quantity of iron in both. He pictured the stain formation as follows: Calcium sulfate present in the salt used for curing is precipitated as phosphate through contact with ammonium phosphate derived from the nucleic acids of the skin. The ammonium sulfate thus liberated then reacts with insoluble ferrous carbonate, naturally occurring in the skin, forming the soluble ferrous sulfate, which forms a stain by combining with the skin protein. Abt attempted to follow the progress of the staining under the microscope and found that the cell nuclei disappear as the staining increases. ‘The connective tissues gradually disintegrate, but he could find no bacteria between the altered fibers, nor did the disintegration resemble the type of decomposition producd by bacteria. He thought the iron probably originated either in the chromatin of the cell nuclei or from the blood. A second type of stain contained no calcium phos- phate, but the epithelial cells were strongly pigmented. These stains he regarded as due to the fixation of the pigment by mineral matter in such a way as to prevent its decomposition by the lime liquors later on. Abt also recommended adding sodium carbonate to salt to be used for curing because it precipitates the calcium salts present and also exerts an antiseptic and dehydrating action. Although Abt contended that most of the stains which he had ex- amined were not caused by bacterial action, he admitted that bacteria might play an important part in the formation of other types of stains. In fact, he *° isolated an organism from one stain capable of producing a brown color on gelatin in the presence of traces of calcium phosphate and iron. At least three different explanations have been offered to account for the effectiveness of sodium carbonate in preventing salt stains. Abt attributed it to the precipitation of calcium salts which might be present in the salt used for curing. Paessler and others looked upon it as due to the production of an alkalinity unfavorable to the action of the bacteria thought to be responsible for the stains. Moeller,?+ - however, suggested that the staining is a tanning action, due to such agents as the melanins or to iron and sulfur bacteria, but that this tanning action cannot proceed in alkaline solution. It is, of course, obvious that the sodium carbonate has the important effect of preventing iron salts from passing into solution, in which condition they would be free to combine with the skin forming the stains. Summing up the work of various investigators, it would appear that salt stains are of several kinds and inay be produced directly by bacteria, such as Becker’s chromogenic organisms, or by soluble iron 70 Role Played by Bacteria in Production of Salt Stains. Collegium (1091 204. 1 Origin of Salt Stains. W. Moeller. Collegium (1917), seriall Core, fresh kVATION AND DISINFECTION OF SKIN 137 salts. These iron salts may be introduced in the salt used for curing or may be formed from the insoluble iron salts already present in the skin, either by chemical action, as described by Abt, or through the intervention of bacteria.- The blood and lymph of skins furnish, an excellent medium for bacterial growth and contain compounds of both iron and phosphates. The following simple rules represent the best means known to the author for preventing these undesirable stains and, it would seem, ought to be quite effective, if carefully observed at the point of slaugh- ter. Immediately after flaying, the skins should be washed very thoroughly in running water to remove as much blood, lymph, and other soluble matter as possible and then salted uniformly in all parts with plenty of clean salt, free from iron and containing about 4 per cent of its weight of anhydrous sodium carbonate. During the time required for the salt to diffuse completely through the skins, they should be kept in a cool place and the brine formed should be allowed to drain away, carrying with it any soluble proteins not previously washed out. Salt equal in amount to at least one quarter of the weight of the skins should be used. Proper curing of skins is necessary, not only: to prevent the formation of stains, but also to prevent putrefaction that would otherwise impair the yield and substance of the leather. Drying. In tropical countries, like Java and India, from which skins are often transported very long distances, the simplest and most economical method of preserving skins is to dry them. ‘This is true for all regions where salt and antiseptics are scarce. Moreover, drying reduces the weight of the skin by about 70 per cent. In the absence of moisture, putrefactive bacteria are practically without action on the skin proteins, although the drying does not always kill the bacteria. When this method of preserving skins is intelligently controlled, very little damage to the skin results. In hot climates, care must be exercised to prevent excessive heating of parts of the skin which are still wet or the protein matter may decompose. Sometimes skins are dried so rapidly that the outer layers feel quite dry, while the interior is still moist enough to permit putrefaction. Skins packed and shipped in this condition are liable to considerable damage. Defects of this kind usually cannot be detected until the tanner attempts to soak the skins back, when they may actually disintegrate or the grain and flesh layers may tend to separate, due to the hydrolysis of the protein matter in the interior. If the drying has been unduly prolonged at high temperatures, the tanner may have considerable difficulty in soaking the skins back to their normal water content. The skin tissues continue to live for some time after the death of the animal and, in the living condition, are not readily subject to putrefaction. It is therefore desirable to dry skins as soon as possible after flaying. ‘They should first be cleansed thoroughly by washing away all the blood and lymph and then suspended freely in a current 138 THE CHEMISTRY OF LEATHER MANUFACTURE of cool air until dry. Where conditions are such that drying cannot be effected sufficiently rapidly to prevent putrefaction, as in damp climates, it is customary to treat the skins first with some antiseptic, such as naphthalene, which acts also to protect the skins against the attacks of insects during drying. The advantages of drying, as a means of preserving skins, are simplicity and speed of operation, independence of a supply of pre- servative material, and low transportation costs for the skins. The disadvantages are the difficulty of wetting the skins back later to their normal water content, the almost impossibility of detecting damage to the skin proteins until they are wet back, and the fact that dried skins may carry disease-producing bacteria or their spores in a form likely to spread infection. Salting and Drying. Sometimes the methods of salting and drying are combined to ad- vantage. ‘The skins are first salted in the usual manner, the brine is allowed to drain away, and they are then allowed to dry slowly. The salt has the effect of hindering putrefaction during the drying. This method is extensively used in some parts of India, but the salt used is a native earth which, according to Procter,!? consists chiefly of sodium sulfate mixed with sand containing insoluble compounds of iron and aluminum. This material is made into a very thin paste, which is brushed onto the flesh side of the skins. Next day more of the paste is rubbed onto the flesh side of the outstretched skin and rubbed into it with a porous brick. After 3 or 4 saltings, the skins are dried under cover and are ready for export. The iron present in the salt sometimes causes a staining of the skins when they are kept for a long time in a moist atmosphere. Pickling. Skins may be preserved by pickling in a solution of sulfuric or hydrochloric acid and sodium chloride. A solution made about N/20 as to acid and 2N as to salt is efficient. This method is not in general use for fresh skins because of the complications involved in attempt- ing to bring them into an alkaline condition later on for unhairing. But for sheep skins, already dewooled, it is a widely used method and convenient, because the skins are then ready for chrome tanning with- out further treatment. The value of this method for preserving sheep skins is increased by the fact that wool is often more valuable than the skin. The skins are frequently purchased by wool pullers, who remove the wool by methods to be described in Chapter 8, and then lime, bate, and pickle them, in which condition they are stored or resold to tanners. ‘This method of preservation permits the immediate use of the wool without destroying the skin or forcing it directly into the tanning process. #2 Principles of Leather Manufacture, 2nd edition. H. R. Procter. D. Van Nostrand Co., New York. * PRESERVATION AND DISINFECTION OF SKIN _ 139 In pickling, the skins are usually thrown into a vat, equipped with a paddle wheel for keeping the liquor and skins well stirred and con- taining a strong solution of salt with a definite excess of sulfuric acid, which is controlled by analysis. The skins are left in the pickle liquor until equilibrium has been practically reached, which is determined by noting when there is little further decrease in concentration of acid with time. This may require anywhere from 4 to 24 hours, depending upon the thickness and condition of the skins and upon the equilibrium concentration of acid selected. Equilibrium is reached more quickly when more concentrated solutions of acid are used, but, if too strong a solution is used, it may be necessary to remove some of the acid prior to tanning by washing the skins in a concentrated neutral salt solution. After pickling, the skins are allowed to drain and are then stored in a damp condition until the tanner is ready to put them into process. Disinfection. Infectious diseases among cattle are common in many countries, particularly in Asia. For this reason some kind of disinfection of skins to be transported from infected areas is necessary in order to prevent the spread of disease germs. Much attention has been paid to preventing the spread of rinderpest,. foot-and-mouth disease, and. the much dreaded anthrax, which occasionally proves fatal to human beings infected with it. Various governments have issued rules to be fol- lowed in disinfecting skins from regions known to be infected. The greatest precautions have been directed against the spread of anthrax because of the danger to human life, but any treatment effective against this disease may be considered effective against the others as well. Anthrax is the disease caused by the spore-bearing bacillus anthracis. The bacillus possesses a short rod-like form and is easily destroyed. According to Seymour-Jones,’? drying alone will kill the rod bacillus. The spore, on the other hand, is very resistant to methods of disinfec- tion that do not cause some injury to the skins, and it is this that makes the problem of disinfecting skins a difficult one. Anthrax spores have been found in dried skins and in blood clots on hair and wool, but seldom, if ever, in wet salted skins. Practical methods of disinfection are limited because so many disin- fectants are injurious to the skin and reduce its value for leather making. Consequently only a few workable methods have been devised. Of these, the best known is that of Seymour-Jones,'* who recommends its employment at the point of export rather than of import because of the danger of spreading the disease during transit. It consists in soaking the dried skins for from I to 3 days in a I-per cent solution of formic acid containing 0.02 per cent of mercuric chloride. They are then soaked for an hour in a saturated solution of common salt, drained, and baled for shipment. 18 Anthrax Prophylaxis in the Leather Industry. Alfred Seymour-Jones. J. Am. Leather Chem, Assoc. 17 (1922), 55. a 144 Formic-Mercury Anthrax Sterilization Method. Alfred Seymour-Jones, London (1910). 140 THE CHEMISTRY OF LEATHER MANUFACTURE Procter and Seymour-Jones?* studied the rate of absorption of formic acid and mercuric chloride during the soaking operation at a . number of different concentrations, using 1 liter of solution per 100 grams of dried skin. The concentration of acid in the solution always fell slowly during a period of 20 hours, but that of the salt at first increased and then dropped, finally approaching a limiting concentra- tion. The initial increase in concentration of mercuric chloride was. found to be the result of a greater initial rate of absorption or pene- Moles Mercuric Chloride per 100,000 Liters, Concentration of Li quor 2 4 6 8 LOG ete 14 16 ee 20 Hours of Contact of Skin and Liquor Fic. 60—Change in composition of solution with time in the Formic-Mercury © Process for sterilizing skins. tration of water and acid than of the salt. The results of one of their experiments are shown in Fig. 60. a The absorption of water caused by the acid renders the skin almost as soft as in the fresh state and the subsequent immersion in saturated sodium chloride solution brings it into a condition resembling that of salted skins. Seymour-Jones points out that skins in this condition are not only properly disinfected, but that they present less of a gamble to the tanner because they show any defects in the skin that would not be visible when the skin is in the dried state. aS Schattenfroh ‘® proposed a method of disinfection involving the © Seymour-Jones Anthrax Sterilization Method. H. R. Procter and Arnold Seymour- Jones. Leather Trades Review through J. Am. Leather Chem. Assoc. 6 (1911), 85. 1® A Harmless Method for the Disinfection of Skins against Anthrax. A. Schattenfroh. Collegium (1911), 248. eee pee tON AND DISINFECTION OF SKIN 14! soaking of infected skins in a solution containing 10 per cent of sodium chloride and 2 per cent of hydrochloric acid at 40° C. for 3 days. Much debate has waged over the relative merits of the Seymour-Jones and Schattenfroh methods. Tilley,’ after experimenting with both methods, concluded that the Seymour-Jones process is effective, but only provided the concentration of mercuric chloride is as high as 0.04 per cent and the skins are not subjected within a week to treatment with sodium sulfide or other substance that would neutralize the disin- fectant. It should, therefore, be effective where the disinfection is carried out at a foreign port before shipping. Seymour-Jones,'* in reply, pointed out that neutralization of the disinfectant by sodium sulfide would take place only in the unhairing process, whereas, under conditions existing during this process, the sodium sulfide itself is a | perfect sterilizer of anthrax spores. This would seem to eliminate any possible danger of anthrax infection from skin or leather that had passed through the usual lime and sulfide method of unhairing. Tilley found the Schattenfroh method effective when the hides were allowed to remain in the acid-salt solution for 48 hours or longer. Schnurer and Sevcik,?® however, applied the Schattenfroh process to very heavy hides and obtained 4 positive tests of infection out of 11 made after the hides had been in a solution containing 2 per cent of hydrochloric acid and Io per cent of sodium chloride for 72 hours. They attributed the more favorable results obtained by Schattenfroh to the fact that he experimented with very thin skins. Using the Sey- mour-Jones process on very heavy hides, they found it. necessary, in order to get complete sterilization in 24 hours, to increase the concen- tration of mercuric chloride to 0.2 per cent, but hides so treated were found by Eitner not to have suffered for tanning purposes. They also found it necessary to degrease heavy sheep skins before applying the Seymour-Jones process, as otherwise a ten-fold dose of mercuric chloride was required. Seymour-Jones objected to the Schattenfroh method on the ground that it is workable only under laboratory conditions and that its factors of time, temperature, and general manipulation are not suited to prac- tical operations. Ponder,?? investigating methods of disinfection for the Leathersellers Company of London, and Abt,*4 of the Pasteur Institute, Paris, working for a syndicate of French tanners, both re- ported in favor of the Seymour-Jones process. Apparently neither process does any injury to the skins that can be detected in the finished leather, according to the findings of numerous investigators. Abt, however, has pointed out that hides would contain no anthrax spores, if they were dried in the sun immediately after flaying, and this view is supported by Seymour-Jones. 17 Bacteriological Study of Methods for the Disinfection of Hides Infected with Anthrax Spores. F. W. Tilley. J. Am. Leather Chem, Assoc. 11 (1916), 131. 18 The Formic-Mercury Process for Sterilizing and Curing Dried Hides. Alfred Sey- mour-Jones. J. Am. Leather Chem, Assoc. 12 (1917), 68. 1® Anthrax Disinfection of Hides. J. Schnurer and F. Sevcik. Tierdrztliches Zentralbl. through J. Am. Leather Chem. Assoc. 8 (1913), 174. 20 A report to Worshipful Company of Leathersellers, 1911. C. Ponder. 21 Disinfection of Anthrax Infected Hides and Skins, Pasteur Institute, 1913. G. Abt. Chapter 7. Soaking and Fleshing. As received at the tannery, skins contain much material unsuitable for leather manufacture and which would introduce serious complica- tions, if not removed as early in the process as possible. For this rea- son every effort is made to remove each undesirable constituent as soon as it can be done efficiently. The preparation of skin for tanning is carried out in a department of the tannery known as the beamhouse and includes, not only the removal of the undesirable parts, but also the regulation of the degree of swelling of the skin proteins. Ears, cheeks, hoofs, and tails are trimmed from skins still pos- sessing them and the flesh, or adipose tissue, is removed by working the skin in a fleshing machine, which forces the flesh side of the skin against a revolving roller set with sharp blades, which cut away the adipose layer. The trimmings and fleshings make up the tannery by-. product known as glue stock and are disposed of for manufacture into glue and gelatin. On the hair side of the skin, the epidermis is made up of a network of membranes, forming the walls of the epithelial cells, impermeable to the soluble proteins of the skin as well as to other material having large molecules or consisting of aggregates of molecules, while on the flesh side the adipose tissue consists of layers of fat cells bound to- gether by extensive series of semi-permeable membranes. It will, therefore, be readily appreciated why the adipose tissue must be removed before the skin can be thoroughly cleansed and freed from soluble protein matter. ; The collagen fibers of the skin are joined together at the lower boundary of the derma in such manner as to give increased strength to the skin. In fleshing, it is important to remove all of the adipose tissue without cutting into the derma, which would weaken its structure as well as lower the leather yield. But reference to Fig. 7 will show: that this is not difficult where the skin is in its normal state. The lower boundary of the derma is sharply defined and the adipose tissue is not joined securely to it at all points. But where the skin has under- gone partial or complete drying, satisfactory fleshing becomes a more difficult operation. During the ordinary methods of drying, protein jellies suffer a change of shape, as well as of size, depending upon their initial shape, the resistance offered to shrinkage in any direction, the rate of drying, and many other factors. This was prettily illustrated by Sheppard 142 SOAKING AND FLESHING 143 and Elliott! with blocks of gelatin jellies. The photographs shown in Figs. 61 to 64 were kindly furnished by Dr. S. E. Sheppard of the Eastman Kodak Co. Fig. 62 shows four stages in the drying of a cube of 20-per cent gelatin jelly which was freely suspended in the air. No. 1 represents the original block of jelly, Nos. 2 and 3 inter- mediate stages in the drying, and No. 4 the dried block. At first the drying naturally proceeds most rapidly at the corners, or trihedral angles, and the faces of the cube become curved outward, as shown in No. 2, giving convex surfaces under tension. This is rapidly followed by the drying and hardening of the edges, forming a rigid framework, so that the bulk of the jelly now behaves as though suspended inside of a rigid wire frame. The faces now gradually recede and the edges become somewhat incurved until a sort of inner cube is formed with connected flanges reinforcing it, any cross-section through this having an J-beam structure, as though the drying proceeded in a manner developing the greatest resistance to stress. The flange-like edges appear to form sections of hyperboloids with a common focus at the center of the cube. Fig. 61 shows three stages in the drying of a sphere of gelatin jelly. Even here the drying is not uniform, but the surface becomes puckered and wrinkled. The dried forms of two cylinders of gelatin jelly are shown in Fig. 64 and their end views in Fig. 63. One base of the first and both bases of the second cylinder were allowed to adhere to rigid sur- faces during the drying. The shrinkage in area of these bases being prevented, the reduction in volume had to be compensated by greater shrinkage in other directions. In the drying of a thin coat of gelatin jelly on a glass plate, the shrinkage takes place almost entirely in the direction perpendicular to the plane of the glass surface. Upon soaking dried blocks of gelatin in water, the swelling pro- ceeds in the direction counter to that followed during drying and the blocks tend to assume the shapes and sizes they possessed before drying. During the drying of skin, the distortions of shape suffered by the insoluble protein constituents are further complicated by the tendency for the fibers to adhere to each other. Before a skin can be fleshed satisfactorily, it is necessary to soak it in water long enough so that all of the insoluble protein constituents may swell to their normal sizes and shapes. When the skin is not uniformly swollen, the boundary between the derma and adipose tissue cannot be made to lie in a single plane. The fleshing machine would then cut the skin so as to leave the flesh side apparently smooth, but in so doing would either leave a considerable amount of adipose tissue on the skin to interfere with the proper cleansing of the skin or else injure the skin by cutting into the derma. The flesh side would look smooth enough upon coming from the machine, but would be ragged and irregular in thickness after the skin had been soaked further or swollen in the liquors used later. F. L. Seymour-Jones says that in Europe it is customary not to flesh 1The Drying and Swelling of Gelatin. S. E. Sheppard and F. A. Elliott. J. Am. Chem, Soc. 44 (1922), 373. Fig. 61.—Three Stages in the Drying of a Sphere of Gelatin Jelly. = =e i ee ee ae Fig. 64.—Two Cylinders of Gelatin Jelly Dried with One and Two Faces, Respectively, Adhering to Rigid Surfaces. 144 SOAKING AND FLESHING 148 skins until after at least a preliminary liming. In America, tanners of goat skins usually flesh them after liming. Heavy, dried hides not only require a more drastic treatment than light, fresh skins, but are also better able to stand it without injury to the resulting leather. In order to get better and more uniform results, the tanner sorts the skins he receives according to weight and general condition. A suitable number of skins, all as nearly alike as possible, are assembled into a unit lot and kept together throughout the process. The treatment is then determined by the average size and condition of the skins as well as by the kind of leather desired. Very large hides are often cut into two sides along the line of the back bone, for convenience in handling. Where the skins come to the tannery in a perfectly fresh condition, the soaking and fleshing operations are extremely simple. After the skins have been trimmed, the adhering blood and dirt are removed by tumbling the skins for half an hour or more in an open drum through which water is flowing. They are then fleshed, after which they are soaked in several changes of clean, cold water containing salt or a small quantity of alkali, the object of which is to free them from soluble protein matter that would otherwise coritaminate the liquors used to loosen the hair and epidermis. The purpose of the salt, or alkali, is to render the globulins soluble so that they may be removed along with the albumins. For dried, or partially dried, skins it 1s necessary to soak the skins both before and after the fleshing operation. The first soaking is pri- marily for the purpose of swelling the insoluble proteins back to their normal sizes and shapes so that the fleshing operation may be carried out efficiently. The second soaking is for the purpose of freeing the skin from soluble protein matter. The time required for the first soaking depends upon the extent to which the skins have been dried. Completely dried skins absorb cold water extremely slowly. Since skins, as received at the tannery, are almost invariably contaminated with proteolytic bacteria, the use of warm water in soaking is somewhat risky, unless the process is very carefully watched. It is usually preferable to hasten the swelling of dried skins by adding small quantities of acid or alkali to the soak waters. Because of the attention centered on the Seymour-Jones process of disinfecting skins, described in the preceding chapter, formic acid has often been used as a swelling agent, although other acids can be used equally as well by applying a simple system of chemical control. Alka- _ lies, however, are more suitable where the skins are subsequently to be treated with alkaline liquors to loosen the hair. Sodium sulfide is most commonly employed to swell dried skins because it requires less careful control than the use of more caustic materials, such as sodium hydroxide. In soaking, a gallon of water is usually used per pound of wet skin or for one-fifth of a pound of completely dried skin. Making the initial concentration of alkali about 0.02 normal is usually enough to initiate the swelling without causing damage either to the skin cr the hair. The 146 THE CHEMISTRY OF LEATHER MANUFACT Oe solution after using is then only very faintly alkaline, the greater por- tion of the alkali having combined with the protein matter. The alkaline liquor is used only for the first soaking after which the skins are moved into fresh water each day until swollen to normal. Sometimes the absorption of water and softening of the skins is assisted by tumbling them in revolving drums with water between suc- cessive soakings. This is usually done with heavy, dried hides or sides. As a rule, salted skins can be fleshed after soaking for only one day, or less. After fleshing, it has been the custom to soak the skins in successive changes of water until practically all of the salt has been removed. ‘The salt diffuses out from the skin much more rapidly than the soluble protein matter, so that continuing the soaking until all of the salt has been removed is not unduly prolonging the process where it is desirable to free the skin as far as possible from soluble protein matter. This custom, however, has created a widespread, but erroneous, impression that it is dangerous to carry salt into the lime liquors. On the contrary, salt assists in the unhairing and plumping of skins by the ordinary lime liquor. Its action in this respect appears to be due to the fact that it increases the hydroxide-ion concentration of alkaline solutions in general.’ 7 : Defects in finished leather are often traceable to the soaking opera- tion. Although bacterial action is the chief source of danger, the skin may suffer from other causes. The tissues of the body do not neces- sarily die with the animal, but may continue to live for an indefinite period, if sufficiently well supplied with nourishment. For this reason it is conceivable that the sudden chilling of a fresh skin may exert an effect upon the muscles and glands of the thermostat layer. If, for example, the erector pili muscles were suddenly contracted and para- lyzed by chilling, the result would be a permanent roughening of the surface of the skin. There have been cases where an unusual roughness of the grain surface of leather seemed to result from the sudden im- mersion of the warm skins, before tanning, in water near the freezing point. But the danger from proteolytic bacteria makes the use of warm water undesirable for soaking. Cold water should be used, but the operations should be so conducted that the temperature of the skins falls eradually. : How long the various parts of the skin continue to live and function after the animal has been flayed remains to be determined. We do know, however, that the skin undergoes changes of one sort or another practically from the moment of flaying. McLaughlin? noted that the rate of swelling of hide in saturated lime water decreases during the first two or three hours following the flaying of a freshly killed animal. A strip of hide put into lime water containing undissolved lime in excess 30 minutes after flaying swelled about 30 per cent more in 120 ? The Hydrogen- and: Hydroxyl-Ion Activities of Solutions of Hydrochloric Acid, Sodium and Potassium Hydroxides in the Presence of Neutral Salts. H. S. Harned. J. Am. Chem. Soc. 37 (1915), 2460. 3 Post-Mortem Changes in Hide. G. D. McLaughlin. J. Am. Leather Chem. Assoc. 16 (1921), 435. ; ; hee SOAKING AND FLESHING 147 hours than a corresponding strip put into the lime water 210 minutes after the flaying. This is, of course, not surprising in view of the fact that many changes are known to occur in skin, after the death of the animal, all of which would tend to retard the swelling in lime water. The coagu- lation of the blood, during which fibrinogen is converted into fibrin, would tend to retard the penetration of lime into the skin and the partial drying of some of the tissues would act in a similar manner. Decomposition of some of the protein constituents would yield simpler bodies capable of forming salts of calcium, which would serve to repress the swelling of the proteins by calcium hydroxide. It is pos- sible also that some of the proteins capable of swelling are gradually broken down into simpler bodies not having the power to swell. Where the preservation of a skin has been done carefully and intelligently, these changes appear not to have any detrimental effect upon the leather produced. The author has tested this by comparing the tannage of skins properly preserved and kept for months before tanning with the tannage of skins put into process within an hour of the death of the animals; no appreciable differences could be detected by chemical, physical, or microscopical examinations of the final leathers. But where there is carelessness in handling, the skins may suffer irrep- arable damage before the soaking operation has been completed. The commonest source of danger in soaking is bacterial action. Although the inner surface of the skin on the living animal may be free from bacteria, it acquires them from the atmosphere very rapidly from the instant of flaying and acts as an ideal medium for the repro- duction of bacteria. By the time the skin reaches the soak vats, it is usually contaminated with countless millions of bacteria. Many species of these bacteria are known to secrete enzymes, which may prove as harmful as the bacteria themselves. The chief practical object to be gained from a study of the bacteria common to tannery soak waters is to find means of destroying them, or at least of preventing them from doing any damage to the skins. An extensive series of investigations of the bacteria and enzymes present in tannery liquors has been made by Wood.* Andreasch * isolated a number of species of bacteria from tannery soak liquors of which he identified the following: Bacillus fluorescens liquefaciens (Fliigge). . megaterium (de Bary). . subtilis. . Mesentericus vulgatus. mesentericus fuscus. . mycoides (Fltgge). . liquidus (Frankland). . gasoformans (Eisenberg). SoleclusMecleclesles * Properties and Action of Enzymes in Relation to Leather Manufacture, J. T. Wood, J. Ind. Eng. Chem. 13 (1921), 1135. ; ° Der Gerber, 1895-6; J. Soc. Chem, Ind., 1896-7. 148 THE CHEMISTRY. OF LEATHER MANUPAGIR White bacillus (Maschek). Proteus vulgaris. Proteus mirabilis. : B. butyricus (Hueppe). White streptococcus (Maschek). Worm shaped streptococcus (Maschek). Grey coccus (Maschek). Fig. 65.—Typical Plate Culture on Gelatin of Soak Water Used for Softening Dried Sheep Skins. All these may be classed as putrefactive organisms that secrete a variety of enzymes, many of which act energetically on hide substance. Fig. 65, taken from Wood’s paper, shows a typical plate culture on gelatin of a soak water used for softening dried sheep skins, in which no chemicals were used. The development of the colonies had to be stopped by the application of formaline vapor before many of the species had time to develop; otherwise the whole plate would have been liquefied. SOAKING AND FLESHING 149 Rideal and Orchard ® examined the action of B. fluorescens lique- faciens on gelatin to which had been added Io per cent of Pasteur’s solution to serve as nutrient medium. The gelatin was completely liquefied in three and one-half days. It was shown that the liquefac- tion of the gelatin was due to an enzyme secreted by the bacteria. The liquefied gelatin was alkaline and had a slight odor suggesting putre- faction, but contained no hydrogen sulfide. A notable feature was the small amount of ammonia and volatile bases produced; only 0.2 gram of ammonia per 100 cubic centimeters was produced even after 16 days’ incubation. In bacterial action of a certain type, one of the first effects to be noticed is the loosening of the hair, a condition known to the trade as hair-slippiness. Either the bacteria, or the enzymes which they secrete, act upon the soft epithelial cells of the Malpighian layer of the epider- mis, liquefying them and thus effecting a separation of the whole of the epidermis and hair from the rest of the skin. This action alone is not harmful, but the bacteria develop rapidly and soon begin to attack the fibers in the grain surface and the skin is permanently injured. ‘This effect shows itself in the finished leather in the form of dull spots, or what is known as pitted grain. In some cases the bacteria attack the heavier collagen fibers without injuring the fibers of the grain surface. When the bacteria attack the proteins of the thermostat layer, they weaken the connection between the fibers of the grain surface and those of the reticular layer; in the finished leather the grain surface then tends to peel off and its looseness of connection with the main body of the skin gives it the appearance known as pipy grain. : Chemists not familiar with the chemical composition of fresh skin sometimes fall into the error of assuming that the presence of nitroge- nous matter in a used soak liquor indicates that the collagen fibers have been attacked. One of the objects of soaking skins is to remove the soluble proteins so that they will not be carried forward to contaminate the liquors used to loosen the hair. Bacteria may become lodged just under the grain surface of the skin and resist the action of the various liquors through which the skin passes. They then become the source of many most annoying troubles. They may produce dull spots or stains or hydrolyze the fats used later to soften the leather. Hydrolyzed and oxidized fats are the _ common sources of spews appearing on the surface of finished leather. In the use of what is known as the putrid soak, bacteria are put to work by being made to assist in the softening of dried hides. But this method is not only an obnoxious one, but one so difficult to control that some damage very often accompanies the softening action. The method is seldom used in modern countries, but in some parts of India dried skins are softened by soaking them in putrid pools of liquor containing all kinds of tannery refuse. In most tanneries, no attempt is made to utilize the bacteria of the soak waters. On the contrary all practical means available are used ® Analyst, Oct., 1897. 10 THE CHEMISTRY OF LEATHER MANUFACTURE to prevent bacterial action in the soaking operation. In a study of the effect of hydrogen-ion concentration upon the activities of putrefactive bacteria, the author has found that they are most active between the pH values 5.5 and 6.0. This probably explains the value of using alkaline soak waters; the liquefaction of skin by bacteria at a pH value of 5.5 is usually greatly retarded or even completely checked by raising the pH value to 12. A similar effect is observed by lowering the pH value to about 3 by the addition of acid. Procter’ has pointed out the advantages of using sulfurous acid in the soak waters. It assists in the absorption of water by the skin and at the same time prevents bacterial action. He found that no putrefaction takes place, even if the skins are later retained for a con- siderable time in water, and the acid has little or no solvent effect on the collagen fibers, whose strength is well preserved. Alkalies are about equally effective as acids both in the softening of dried skins and in checking bacterial action and are generally pre- ferred because they assist rather than retard the action of the lime liquors in loosening the hair. 3 Aside from the use of acids and alkalies, the chief precaution taken against bacterial action in the soaks is the use of plenty of clean, cold water. If the temperature of the water is not allowed to rise above 1o° C. and plenty of clean water is used, the skins are not likely to ' suffer any serious damage from the soaking operation itself. ‘Principles of Leather Manufacture, Second Edition (1922), 16r. Ghapter so: Unhairing and Scudding. After the skins have been trimmed, cleansed, freed from adipose tissue and soluble matter, and have again become soft through absorp- tion of their normal water content, they are ready for the series of operations involved in the removal of the epidermal system. It will be recalled from Chapter 2 that this system includes the epidermis, hair, and the sebaceous and sudoriferous glands and differs from the true skin under it in origin, structure, method of growth, and chemical com- position. The several parts of the epidermal system differ markedly in their resistance to chemical reagents and it is rather fortunate for the tanner that the part most readily digested is the portion of the Malpighian layer resting on the grain surface. When the epithelial cells of this layer are destroyed, the rest of the epidermis and the hair become completely separated from the true skin and can easily be removed mechanically. Sweating. What is probably the oldest method known for unhairing skins received the name sweating from the nature of the process in its more highly developed state. It consists of little more than the putrefaction of the cells of the Malpighian layer. Since it is only necessary to allow a fresh skin to remain for a day or two in a warm, damp place to cause a loosening of the hair, the method was probably discovered very early in the history of the human race. It is not improbable that the acci- dental discovery of this action first revealed to the ancients the advan- tages of unhaired skins for certain purposes. ‘Because of the danger of serious damage to the skins in the sweat chambers, unless the process was very carefully watched and controlled, it ceased to be popular for the best grades of skins after safer methods of unhairing were devised. It is still in use in some tanneries for the lower grades of skins, such as the cheaper classes of dried hides and sheep skins where the wool is valued more highly than the skin. The skins are generally hung from beams in a closed room in which the air is kept warm. and humid. The temperature, humidity, and ventilation must be carefully controlled. During the process a con- siderable quantity of ammonia is evolved and this assists in the unhair- ing action. Just as soon as the hair slips easily, the skins are removed from the sweat chamber and dumped into saturated lime water. The lime water serves to retard further bacterial action and to cause the skins 151 Fig. 66.—Vertical Section of Sheep Skin. (After 42 hours in sweat chamber.) Location: butt. Eyepiece: none. Thickness of section: 20 wu. Objective: 16-mm. Stains: Van Heurck’s logwood, Wratten filter: H-blue green. Daub’s bismarck brown. Magnification: 45 diameters. 152 Fig. 67.—Vertical Section of Thermostat Layer of Sheep Skin. (After 42 hours in sweat chamber.) Location: butt. Eyepiece: 5X. Thickness of section: 20 Uw. Objective: 16-mm. Stains: Van MHeurck’s logwood, Wratten filter: H-blue green. Daub’s bismarck brown. Magnification: 135 diameters, 153 * 154 THE CHEMISTRY OF LEATHER MANUFACTURE to swell somewhat by absorption of water; the skins upon coming from the sweat chamber are in a very flaccid and slimy condition. Wilson and Daub? recently made a study of the sweating process under the microscope. Pieces of fresh sheep skin were kept in a closed receptacle having an atmosphere saturated with water vapor at 38° C. At frequent intervals strips of skin were removed for sectioning and examining under the microscope. At the end of 42 hours, the wool could be rubbed off with ease and the skin had apparently suffered no damage. The odor of ammonia in the receptacle after the first day was very pronounced. | The first sign of action visible under the microscope was the sepa- ration of the cells of the Malpighian layer from one another and from the surface of the derma. This action gradually spread to the outer- most layers of cells of the sebaceous and sudoriferous glands. On the second day the action had proceeded so far that the epidermis, glands’ and wool were completely separated from the derma and many of the epithelial cells had completely disintegrated. A section of the skin after being in the sweat chamber for 42 hours is shown in Fig. 66. The upper portion of the section is shown in Fig. 67 at a much higher magnification. It will be noted that the corneous layer is still intact, but the Mal- pighian layer has almost completely disintegrated, the linings of the hair follicles are broken up, and the glands have all been loosened and separated from the derma. Fig. 66 should be compared with Fig. 28, which represents a section from the same skin fixed in Erlicki’s fluid within an hour after the death of the animal. In practice, the systematic cleaning of the sweat chambers is neces- sary in order to prevent the increase of undesirable organisms that may be carried in from time to time. Hampshire? investigated the cause of a pitting, or liquefaction in spots, of the grain and flesh surfaces of sheep skins, a damage known to the trade as run pelts. He found that the pitting was caused by several species of wormlike organisms belong- ing to the family Nemathelminthes and growing to a length of about one millimeter. Apparently they are killed by simple drying. They were found in great numbers in the sweat chambers, but not on skins which had not yet entered the chambers. In laboratory experiments, they produced a pitting of the skin in the presence of a small amount of ammonia, such as is always present in the sweat chambers. It was found that uniform slipping of the wool could be produced by incubat- ing the skin in a clean vessel which excluded all organisms other than those present on the incoming skin, and skin treated in this way was free from pitting. It would seem that the danger of run pelts can be completely avoided by making certain of the cleanliness of the sweat chamber before the skins enter. Upon coming from the sweat chamber, the skins are usually put *The Mechanism of Unhairing. J. A. Wilson and Guido Daub. Presented before th Leather Division at the 64th meeting of the American Chemical Society. Publication ee photomicrographs reserved for this book. : * Causes of Run Pelts in the Sweating Process. P., Hampshire. J. Soc. Leather Trades Chem. 5 (1921), 20. UNHAIRING AND SCUDDING 155 into saturated lime water and left there for a few hours or over night. Although this treatment is not essential and is sometimes omitted, it has the advantage of decreasing the danger of damage to the skins through putrefaction. The next step is the actual removal of the hair and epidermis. In modern practice, this is accomplished by means of an unhairing machine in which the skin is backed by a rubber slab and blunt knife blades pass over the hair side, under low pressure, rubbing Grams CaO Left per 100 Grams Dry Skin ~ 1 2 3 4 5 6 ry Hours of Washing Fic. 68.—Removal of lime from unhaired skin by washing. off the hair and epidermis. Often the blades are set in rollers which rotate as they pass over the skin. The skin is then placed over a beam and scudded. The beam, from which the beamhouse derived its name, is a convex wooden slab sloping upward from the floor, at an angle of about 30°, to a point about three feet higher, which gives it a length of about six feet. The beamster, leaning over the beam, pushes a specially designed, two-handled knife over the skin downward and to left and right, forcing the remnants of the glands, lime soaps, dirt, and any remaining hairs out of the hair follicles and pores.. This operation is known as scudding. Goat skins can be scudded satisfactorily by machine after the bating 150 THE CHEMISTRY OF LEATHER MANUFACTURE operation, but the author knows of no machine that can replace a good beamster for scudding calf skins after liming. Scudding can usually be done better by hand than by machine because the hair follicles slope in many different directions. If the knife stroke is made in the direction of the hair, from root to tip, the dirt in the follicles is easily squeezed out, whereas there is a tendency for-it to be trapped by a stroke in the opposite’ direction. There is a sufficient degree of transparency to a limed skin to enable the beamster to see the dirt and pigment in the follicles and he directs his knife first one way and then another until the skin appears clean. He is also on the lookout for fine hairs not removed by the machine. The bulb of a new hair is as deeply seated as that of an old one, but there may not be enough of the new hair protruding above the surface of the skin to be gripped by the knives of the unhairing machine. After the scudding operation, the skins are washed thoroughly to remove as much lime as possible. This washing is of considerable im- portance because any great excess of lime carried forward interferes with the later processes. It is customary to wash the skins in a revolv- ing drum through which fresh water is continually passing. Wood ® followed the removal of lime during washing and showed that little is to be gained by continuing the washing for more than two hours. The tendency, however, is to wash the skins for a shorter time than this and to take care of the residual lime by other means. Fig. 68 shows the extent of lime removal with time during a typical washing operation. The lime left in the skins appears to approach a limiting value, due to the lime which has carbonated as well as that in chemical combination with the skin. Liming. The commonest method in use today for effecting the separation of the epidermal system from the true skin is also one of ancient origin and is known as liming from the fact that saturated lime water is used. Formerly a lime liquor was prepared simply by filling a vat with water and adding calcium hydroxide greatly in excess of saturation. The skins, after soaking, were put into this liquor and allowed to remain there until the hair and epidermis had become so loosened that they could be rubbed off with very little pressure. Often the skins were removed each day and fresh lime added in order to hasten the action. But with a fresh lime liquor it usually required weeks for the skins to get into a state where the hair would slip easily. It was discovered that less time was required for each succeeding lot of skins passing through a given liquor. The longer a liquor was used the more it became charged with ammonia, other protein decomposition products, bacteria and enzymes, all of which assisted in loosening the hair, The older liquors, however, attacked the collagen fibers to a greater extent and also produced less swelling of the skin proteins than fresh liquors. * The Puering, Bating and Drenching of Skins, J, T, . London (1912). 8 J Wood. E. & F. N, Spon, ae ee UNHAIRING AND SCUDDING 157 As more was learned of the action of lime liquors, it became cus- -tomary to employ a series of liquors for each lot of skins. The skins were put first into the oldest liquor in order to start the loosening of the hair. Each day they were moved into a fresher liquor and finally into one quite fresh. This system is still in use in some tanneries, but the modern tendency is toward quicker methods. When lime alone was used in making lime liquors, it usually re- quired from one to three weeks to cause the hair to slip easily, during which time a considerable amount of collagen became hydrolyzed, espe- cially in old liquors or in liquors not kept completely saturated with lime at all times. Bacteria are very sensitive to changes in pH value and many proteolytic bacteria present in lime liquors which are compara- tively inactive at a pH value of 12.5, that of an ordinary lime liquor, become very active as the pH value falls to lower values. In order to guard against the danger of incomplete saturation of the liquors with lime, mechanical agitators have been devised, one of the simplest being a paddle wheel set in the vat. By keeping the undissolved lime continually stirred up, the solution is kept almost at the saturation point. With increasing demand for speed of operation and conservation of the skin collagen, sharpening agents have come into wide use, the prin- cipal ones being arsenic sulfide, sodium sulfide, and sodium hydroxide. The judicious use of these materials, in conjunction with lime, has reduced the time required to unhair skins from weeks to as many days. More attention was paid also to temperature. In some of the old tan- neries not equipped to heat the liquors, a much longer time had to be allowed for unhairing in winter than in summer. It 1s now customary to maintain a uniform temperature of from 20° to 25° C. the year round. Arsenic disulfide was one of the first sharpening agents to be em- ployed. It was mixed with the lime before slaking in the proportion of about one part of sulfide to twenty-five parts of lime and from this mixture a liquor was made of such concentration that the hair would not be damaged, but would slip easily in two or three days. Sodium sulfide is now used more commonly than arsenic, being cheaper and somewhat more effective in loosening the hair. It is used at about 0.01 molar concentration in a solution kept saturated with lime. The action of a lime liquor sharpened with sodium sulfide upon a calf skin is illustrated in Fig. 69. A fresh calf skin was put into a solution containing 0.7 gram of Na,S per liter and calcium hydroxide well in excess of saturation. The liquor was agitated frequently and kept at a temperature of 25°C. Strips of the skin were examined at intervals as in the study of the sweating process. The skin from the sweating process was in a soft, flaccid condition, while that from the lime liquor was plump and rubbery, but the fate of the epithelial cells of the Malpighian layer was the same in both cases. Sections of speci- mens taken at intervals showed these cells slowly disintegrating and leaving the corneous layer, hairs, and glands separated from the derma. Fig. 69 shows a section taken after the skin had been in the lime liquor for 48 hours. Part of the upper region of the section is shown at Fig. 69.—Vertical Section of Calf Skin. (After 48 hours in lime liquor.) Eyepiece: none. Thickness of section: 40 u. Objective: 32-mm. Stains: Weigert’s resorcin-fuchsin Wratten filters: B-green; E-orange. Location: butt. and picro-red. Magnification: 25 diameters, 158 i te he Fig. 70.—Vertical Section of Thermostat Layer of Calf Skin. (After 48 hours in lime liquor. ) Eyepiece: 5X. Objective: 16-mm. Wratten filters: B-green; E-orange, Magnification: 135 diameters, Location: butt. Thickness of section: 40 wu. Stains: Weigert’s resorcin-fuchsin and picro-red. 159 160° THE CHEMISTRY AOR ea roeh MANUFACTURE higher magnification in Fig. 70. The section is from the same skin as that shown in Fig. 18, which represents the fresh skin as it existed in life. The lime has completely destroyed the Malpighian layer of the epidermis and the corneous layer appears as a nearly continuous line somewhat separated from the true skin. The epithelial cells of the hair follicles have been completely broken up leaving the hair, with adhering patches of corneous layer, free to be swept out by the action of the unhairing machine. The sudoriferous glands have disintegrated, leav- ing empty spaces, and the sebaceous glands may be seen lodged in pockets opening into the hair follicles. The erector pili muscles are still intact and can be seen runing upward to the left from the region of the hair bulbs. In the thermostat layer, as well as in the deepest layer of the skin, the elastin fibers appear as fine, black threads. These fibers do not appear prominently in Fig. 18 because this section was stained with the object of showing greater detail in other parts. Although the hair loosening operation can be effected easily in a single liquor acting for two or three days, some tanners still prefer to use a series of liquors, claiming that they get a result better adapted for the particular kinds of leather they desire to make. They lessen the extra amount of labor involved in handling the skins by a system of reeling from vat to vat. The skins are all hooked or tied together, the head of one to the tail of another, and the whole lot is passed over a reel from one vat to another, the last skin in being the first to come out. The skins are put first into the oldest liquor and then reeled into a fresher liquor each day until ready to be unhaired. Plumping and Falling. When animal skin is immersed in dilute solutions of acid or alkali, the protein matter ‘swells by absorbing some of the solution, but the effect to a casual observer is not so much one of swelling as of increased resiliency of the skin, due to its fibrous structure. ‘The collagen fibers, in swelling, tend to fill up the interstices between them and the full increase in volume of the protein matter is not evident from the appear- ance of the skin. .A skin in which the fibers are not swollen may con-_ tain practically as much water as one whose fibers are swollen, as in lime water, but the bulk of the water in the first skin is held only loosely between the fibers and may be squeezed out by the application of slight pressure, whereas that in the second is present within the substance of the fibers, like the water absorbed by a solid block of gelatin jelly, and cannot be removed, except by the application of enormous forces. During the swelling of the protein matter, the tanner observes in the skin an increasing resistance to compression, to which he has given the name plumping, the term falling indicating the reverse action. Wood, Sand and Law ‘ devised an apparatus for determining when *The Quantitative Determination of the Falling of Skin in the Puering or Bating Process. J. T. Wood, Hi. J, Si Sand ‘and: D. J. Law, J, Soc. Chem. Ind. 31 (1912), 210 and 32 (1913), 398. UNHAIRING AND SCUDDING 101 a skin had become completely fallen during the bating process which consisted of a sensitive thickness gauge in which the pressure exerted upon I square centimeter of skin could be varied by means of weights. The point of complete falling of a skin was taken as that at which no recovery in thickness of the skin took place upon removing the weights. The apparatus was also used to measure the apparent modulus of elas- ticity of the skin and this was considered to be a measure of the degree of plumping. This method suggested to Wilson and Gallun® another which is more suitable for certain purposes. Their apparatus consisted of a Randall and Stickney thickness gauge ** with a flat, metal base upon which a small piece of skin could be placed, and a plunger, having a circular base I square centimeter in area, capable of pressing on the surface of the skin under constant pressure. The apparent thickness of the skin, as shown on the dial of the instrument, being determined by the position of the plunger, decreased with time as the plunger caused an increasing degree of compression. For this reason and in order to get comparative readings, all gauge readings were taken a fixed length of time after dropping the plunger onto the skin. In order to measure the degree of plumping of skin in a given liquor under fixed conditions, they first measured the resistance to compression of a small piece of skin under standard conditions. This same piece of skin was then subjected to the conditions of the test and its resistance to compression measured again. In each case the gauge reading was taken as a measure of the resistance to compression. The ratio of the final to the initial gauge reading is a measure of the degree of plumping of the skin. Their measurements of the degree of plumping of calf skin as a func- tion of pH value are given in Chapter 9. If a skin in the alkaline state is plumped or swollen excessively, it suffers permanent distortion and the value of the final leather is lowered. Some knowledge of the degree of plumping of skin in liquors used for unhairing is therefore much to be desired. Atkin ® was able to reason from the work of Procter, Wilson, and Loeb, which was discussed in Chapter 5 in connection with the swelling of protein jellies, that arsenic disulfide is preferable to sodium sulfide for certain kinds of skin where fineness of grain surface is of para- mount importance in the finished leather. Loeb showed that diacid bases produce a maximum swelling of gelatin jelly only half as great as that produced by monacid bases. Atkin confirmed this for the swelling of hide powder and showed that the weak base ammonium hydroxide produces as much swelling as sodium hydroxide at the same pH values. When arsenic disulfide is slaked with lime and used in a fresh liquor, the solute consists only of calcium hydroxide, calcium sulfhydrate, and calcium sulfarsenite. But when sodium sulfide is used as the sharpening agent for a lime liquor, sodium hydroxide and -sodium 5 Direct Determination of the Plumping Power of Tan Liquors. J. A. Wilson and A. F, Gallun, Jr. Ind. Eng, Chem. 15 (1923), 376. @ Made by Randall and Stickney, Waltham, Mass. *Notes on the Chemistry of Liine Liquors Used in the Tannery, W, R, Atkin. 7 J. Ind. Eng. Chem. 14 (1922), 412, 162 THE CHEMISTRY OF LEATHER MANUFACTURE sulfhydrate are present. It would therefore be expected that the use of sodium sulfide would result in a greater plumping of the skin than the use of arsenic sulfide, which gives a liquor containing only divalent cations. In actual practice, when arsenic sulfide is used to sharpen lime liquors for the unhairing of goat skins in the manufacture of glazed kid leather, the final leather has a smoother and silkier grain surface than when sodium sulfide is used in the lime liquors. It might be inferred from this that it is preferable to use arsenic sulfide for all kinds of skin where smoothness of grain is desired, but this is not necessarily so. All skins are not equally sensitive to injury through plumping. What may prove to be excessive plumping for goat skins may not have any deleterious effect at all on a calf skin and one type of calf skin might be more resistant to permanent distortion than another. The greater speed of action and lower cost of sodium sulfide makes its use preferable in all cases where it does no harm to the skins. It sometimes happens that a skin can be unhaired less readily the more it is plumped. This seems to be due to the overlapping scales of the hair, which open upward as shown in Fig. 6. When the skin is put into a liquor in which it swells considerably, the hair becomes tightly pinched by the skin and at the same time the scales become distended, their ends wedging themselves into the sides of the follicles in such manner as to resist any attempt to pull the hair out. If the fine hairs are not removed from a skin while it is still in the alkaline condition, but are allowed to remain in place until after the tanning operation, they again become firmly fixed in place, apparently because of the distention of the hair scales and the permanent plumping of the skin produced by the tannage. Fresh vs. Mellow Lime Liquors. A much used lime liquor, charged with decomposition products of the skin, bacteria and enzymes, is usually referred to as mellow. Where unsharpened lime liquors are used, a mellow liquor causes a much more rapid loosening of the hair and much less plumping of the skin than a fresh liquor. This difference is not due to any difference in hydroxide- ion concentration for Wood and Law? have shown that a mellow lime liquor has a pH value practically the same as that of pure saturated lime water. They found also that the pH value is but little affected by the addition of small quantities of sodium sulfide and this has been con- firmed in the author’s laboratories. The decrease in plumping power of a lime liquor with use may be ascribed to the calcium salts formed, which tend to repress the swelling of proteins by calcium hydroxide. But the increasing power to loosen the hair must be attributed to the protein decomposition products, bacteria, enzymes, or the lesser swell- ing of the skin at the same pH value, or possibly to a combination of all four factors. | Wood and Law regard the growth of bacteria in lime liquors as the principal factor in the production of mellowness. They examined 7 Light Leather Liming Control. J. T. Wood and D. J. Law. Collegium (1912), 121. UNHAIRING AND SCUDDING 163 an old lime liquor in which skins had been worked for 3 to 4 weeks and obtained a count of 50,000 bacteria per cubic centimeter of a type capable of developing in ordinary nutrient gelatin containing ammonia. They identified Micrococcus flavus liquefaciens and B. prodigiosus, both of which are known to produce proteolytic enzymes. The bac- teria found on the roots of wool from the sweating process were found to be capable of growing in a liquid as alkaline as 0.05 normal. These appear to be similar to the bacteria commonly present in mellow lime liquors and Wood considers it highly probable that the unhair- ing action both in the sweat chamber and in mellow lime liquors is due to the same bacteria, not necessarily belonging to a single species. Stiasny * also showed that bacteria play an important role in old lime liquors. An untreated mellow lime liquor caused a loosening of the hair of calf skin in 24 hours, but in a test where chloroform was added to the same liquor to check bacterial action the liquor was not able to cause any loosening of the hair in 3 days. A portion of the untreated liquor was freed from ammonia by heating to 60° C. and passing carbon dioxide-free air through it for 4 hours. It then showed an unhairing power as great as before, but a lesser solvent action on the hide substance, indicating that the unhairing action is due to bac- terial action rather than to the ammonia ordinarily present in mellow liquors. Since sterile lime water appears to have but little unhairing action on skins, it was long thought that bacteria were necessary for this action, where no sharpening agent was employed. But Schlichte ® found that skin previously sterilized by the Seymour-Jones process, with mercuric chloride and formic acid, could be unhaired easily after two weeks of contact with saturated lime water under sterile conditions. Wood and Law," however, pointed out that the action may have been influenced by the previous swelling of the skin in the sterilizing solu- tion. This is intelligible from the viewpoint of Stiasny,‘ who regards proteins as peptones held together relatively loosely by means of secondary valency forces. The peptones are considered to be built up of peptides held together by forces of primary valence. He as- -sumes that the swelling of a protein jelly causes a diminution in the forces holding the peptones together. On this basis, the swollen protein, or one in which the bonds between the peptones had been weakened through previous swelling, would be attacked by hydrolyzing agents much more readily than the unswollen protein. In support of this view, he finds that collagen is attacked by trypsin very much more rapidly when swollen by potassium thiocyanate or iodide solutions and that the action then goes only to the peptone stage. It was suggested by the author * that barium and calcium hydroxides ® The Nature of the Liming Process. E. Stiasny. Gerber (1906); English translation, J. Soc. Leather Trades Chem. 3 (1919), 129. ®A Study of the Changes in Skins during Their Conversion into Leather, A. A. Schlichte. J. Am. Leather Chem. Assoc. 10 (1915), 526 and 585. 7 Note on the Action of Lime in the Unhairing Process. J. T. Wood and D, J. Law. Peer, Chen. Ind. 35 (1916), 58s. ™ Some Modern Problems in Leather Chemistry. E. Stiasny. Science 57 (1923), 483. ( Alia of Leather Chemistry. J. A. Wilson. J. Am. Leather Chem. Assoc. 12 1917), 108, 164 THE CHEMISTRY OF LEATHER MANUFACIGRe hydrolyze proteins to a lesser extent than the hydroxides of sodium or ammonium because of the higher valency of the cations. The swell- ing of proteins in alkaline solution is due to the pull of the cations of the protein salt, which tend to diffuse from the region of high con- centration of ions in the jelly to the region of lower concentration in the surrounding solution. If this pull is sufficiently great, we might reasonably expect a breaking up of the units making up the protein jelly. A sodium or ammonium ion exerts its entire pull upon a single unit, whereas the pull of a divalent cation is divided between two units, making the tendency towards decomposing the protein only half as great. This valency effect, however, is not the only one playing a part in sterile unhairing liquors because the mere replacement of half of the hydroxide ions of lime water by sulfhydrate ions is sufficient to cause a very marked increase in the rate of unhairing. Wood and Law suggested that Schlichte’s observation of the unhairing power of sterile lime water is further complicated by the formation of sulfur compounds by the action of lime on the easily dissolved sulfur of the hair. Such compounds are capable of loosening the hair. Unhairing by Means of Other Alkalies. Pure solutions of sodium hydroxide and sodium sulfide quickly destroy the hair and epidermis when sufficiently concentrated. A 2-per cent solution of Na,S at 25°C. will dissolve the hair and epi- dermis from the surface of a calf skin in about 2 hours, during which time only a comparatively small amount of collagen is destroyed. This treatment has been applied with considerable success to heavy hides, especially those which had previously been dried, and was a great help in speeding up the production of army leathers during the war. The hides were put into the sulfide solution, which was agitated by means of a paddle wheel. After several hours the hides were transferred to a solution of sodium bicarbonate or calcium chloride in order to stop the caustic action of the sodium sulfide. They were then washed and were ready for bating or tanning. The hair was completely dis- solved from the surface of the hides in the sulfide liquor, but the action was so rapid that they had to be removed before the sulfide had diffused into them to the depth of the hair bulbs. As a result, the hair bulbs were usually left in the hides intact, as could be shown by examining sections under the microscope, but this apparently did not lower the value of the leather in any way. With this method of unhairing, it was found economical to use the same liquor for a number of consecutive lots of skins, adding just enough fresh sodium sulfide each time to maintain the necessary con- centration. The liquors soon became heavily charged with protein decomposition products which are soluble in alkaline solution, but are precipitated by rendering the solution faintly acid. Kadish and UNHAIRING AND SCUDDING 165 Kadish ** made use of this fact in a scheme for recovering this nitrog- enous matter as fertilizer. The waste liquors were run into a mixing chamber where they were reacted upon by sulfuric, sulfurous, or other acid. The precipitated nitrogenous matter was separated from the mother liquor and the hydrogen sulfide was recovered separately in such manner as to make the entire operation continuous. Using sodium hydroxide instead of the sulfide, a similar unhairing action is obtained, but the skin becomes much more swollen and plumped. For the finer grades of light skins, where a smooth grain surface is required, neither sodium hydroxide nor sulfide solutions can be used alone because of the rough grain resulting from the excessive plumping. It is not an uncommon practice in dewooling sheep skins to paint them on the flesh side with a paste made of a mixture of lime and sodium sulfide. The skins are then folded, wool side out, and left until the sulfide has diffused into the skins as far as the hair bulbs. When these are destroyed, the wool can be pulled or brushed out. As a rule, the skins are thrown over a beam and the wool is worked off by a beamster. The skins are then limed, washed, bated, and pickled, in which condition they may be kept until required for tanning. Sometimes the paste is made from lime and arsenic sulfide. Solutions of ammonia in twice-molar concentration have a very marked unhairing action on fresh skins. The author found that fresh calf skins could be unhaired quite satisfactorily after only two hours’ immersion in such a solution. The skin swells but very little and the grain surface is left remarkably smooth and silky. If the skin is left in the solution longer than is necessary, however, there 1s danger of it suffering damage because of the powerful action of the ammonia on the collagen fibers. Since the unhairing’ powers of ammonia have long been known, it has often been wondered why its use has not become widespread. In an investigation, the author found that it could not be relied upon for unhairing the ordinary run of skins in commerce because its action is influenced by the previous treatment of the skin. On some skins, the ammonia would loosen the hair only in patches. In one experiment, a piece of fresh calf skin was cut into two pieces. One was put directly into twice-molar ammonia solution and the hair was loosened quite satisfactorily in two hours. The other was soaked in molar acetic acid for an hour, washed, neutralized with ammonia, and then put into the twice-normal ammonia solution. But there was no appreciable loosening of the hair after several hours. Stiasny “* studied the effect of adding different salts upon the un- hairing action of ammonia. He used a series of liquors each consisting of half-normal ammonia and 0.07 normal chloride of sodium, calcium, barium, or zinc. One liquor contained ammonia alone. A piece of fresh calf skin was put into each. After 2 days the piece in am- VY, H. Kadish, U. S. patent 1,269,189 (1918); V. H. and H. L. Kadish, U. S. patent 1,298,960 (1919). 144The Nature of the Liming Process, loc. cit. 106 THE CHEMISTRY OF LEATHER MANUFACTURE monia alone had increased in weight 65.5 per cent, the one in the solution containing sodium chloride 45.8 per cent, in calcium chloride 14.9 per cent, in barium chloride 19.8 per cent, and in the solution containing zinc chloride 31.4 per cent. The hair was loosened in the solution of ammonia alone and in the one containing sodium chloride, but not in the others. Atkin’ has pointed out that the dif- ference in repression: of swelling by the different salts may be at- tributed to the valency of the cation. It is, of course, evident that the difference in unhairing action may be explained in the same way. Stiasny, however, looked upon the difference in action as due to the formation of complexes between the ammonia and the divalent cations, giving salts of the type Ca(NH,),Ch. Unhairing by Means of Acids. In 1916, Mr. J. T. Wood sent the author a piece of calf skin which had been sterilized by the Seymour-Jones process. The formic acid had caused a loosening of the hair, which Mr. Wood says was marked in 8 days. Thuau*® and Nihoul !’ had previously shown that sulfurous acid will cause a loosening of the hair of skins, if used in solutions that will prevent the swelling of the skin, as in the presence of salt. Marriott’® found that salted hide could be unhaired by immersion in 0.25-per cent acetic acid solution for Q days. In no case was the hair loosening by means of acid as satisfactory as can be obtained in alkaline solution. The acid seems to attack only the deepest layer of the epithelial cells of the Malpighian layer, leav- ing most of the epidermis intact, to be removed with the hair. It seems doubtful that acid will ever replace alkaline solutions for unhairing. Unhairing by Means of Pancreatin. In 1913, Rohm ** described a process for unhairing and bating skins in one operation, involving the use of an alkaline solution of pan- creatin. Since then pancreatin has often been listed as an unhairing agent. In 1920, Hollander *? described Réhm’s process as having a number of advantages over the old system of liming and claimed that it depends entirely upon enzyme action for unhairing. According to his description, the skins ‘are first soaked for 1 day in dilute sodium ‘hydroxide solution and then transferred to a dilute solution of sodium bicarbonate to which the enzyme is added after the swelling due to the alkali has been counteracted. Twenty-four hours later the hair is completely loosened and can be rubbed off. Wilson and Gallun ?! investigated this method with the object of © Notes on the Chemistry of Lime Liquors Used in the Tannery, Joc. cit. 16 Unhairing with Sulfurous Acid. U. J. Thuau. Collegium (1908), 362. % Unhairing with Sulfurous Acid. E. Nihoul. Bourse aux Cuirs de Lidge (1908), 8. 18 Acid Unhairing. R, H. Marriott. J. Soc. Leather. Trades Chem. 5 (1921), 2. 12 A New System of Liming. O. Réhm. Collegium (1913), 374; J. Am. Leather Chem, Assoc. 8 (1913), 408. : 7° Unhairing Hides and Skins by Enzyme Action. C. S. Hollander, J. Am. Leather Chem. Assoc, 15 (1920), 477. » Pancreatin as an Unhairing Agent. J. A. Wilson and A. F, Gallun, Jr. Ind. Eng. Chem. 15 (1923), 267. _UNHAIRING AND SCUDDING 167 determining the specific rdle played by the enzyme. They made a pre- liminary examination by soaking pieces of thoroughly cleansed calf skin in 0.05 molar sodium hydroxide solution for 1 day, replacing the solution next day by 0.1 molar sodium bicarbonate solution, and 5 hours later transferring the pieces to a solution made by diluting 18 cubic centimeters of molar sodium hydroxide, 2.8 grams of monosodium phosphate, and 1 gram of U.S.P. pancreatin to 1 liter. The pH value of the solution was found to be 7.52 at 25° C., lying well within the range of optimum activity of this enzyme. Two experiments were run at a temperature of 25° C., but in one the solutions were left ex- posed to air, as would be the case in practice, while in the other they were covered with a layer of toluene to check bacterial action. After the pieces had been in the enzyme solutions for 24 hours, the hair of the pieces from the solutions exposed to air could be rubbed off with the greatest ease, leaving the erain surface clean and white, but that of the pieces from the solutions under toluene remained firmly fixed. This seemed to indicate that the unhairing action obtained at 25° was not due to the enzyme, but probably to proteolytic bacteria or their products. Because of the doubt thus cast upon the role played by pancreatin sn this method of unhairing, Wilson and Gallun carried the investiga- tion further, paying particular attention to the action of pancreatin at 4o° C., the temperature of its maximum activity. The studies were made upon pieces of fresh calf skin, about 5 x3 inches, which had been thoroughly soaked and cleansed. Each experiment was carried out both at 25° and at 4o°C. The action of the enzyme solution upon the skin in each test was compared with the action of a blank identical with the enzyme solution except for the fact that it con- tained no enzyme. This solution was prepared by diluting 18 cubic centimeters of molar sodium hydroxide solution and 2.8 grams of monosodium phosphate to 1 liter and all enzyme solutions were made by adding to it 1 gram of pancreatin per liter. The pH values did not vary more than o.1 from the value 7.6 in any case. The enzyme solutions and blanks as well as solutions used for the pretreatment of the skin were all covered with a layer of toluene to check bacterial ‘action. The results were checked on separate occasions with pieces of skin from different sources. The effect of pancreatin upon skin not previously soaked in sodium hydroxide solution, or any other swelling agent, was studied first. After 24 hours of contact of skin and solution, little action was notice- able either at 25° or 40°, but after 48 hours the collagen fibers of the skin in the enzyme solution at 40° began to dissolve very rapidly, the action proceeding from the flesh side, but there was no indication © of the hair becoming loosened. On the other hand, the skin in the blank at 40° and those at 25° in both blank and enzyme solution still remained but little affected. It was evident that pancreatin has a more powerful solvent action upon the collagen fibers than upon the epidermis of a skin not previously swollen with acid or alkali. The time factor involved in the destruction of the collagen fibers is in- 168 THE CHEMISTRY OF LEATHER MANUFACTURE teresting. The action seemed to indicate that the fibers were coated with some material more resistant to tryptic digestion than the col- lagen beneath it. Possibly this supposed covering may be found to bear some relation to what Seymour-Jones 7? has called the fiber “sarcolemma.”’ | In the next series of experiments, the pieces of skin were kept for 24 hours in 0.05 molar sodium hydroxide solution at 25° and 40° C., respectively. The solutions were then replaced by 0.1 molar sodium bicarbonate solutions of corresponding temperatures, and 5 hours later by the enzyme and blank solutions, in which the skins remained for 24 hours. The unhairing action in the enzyme solution at 40° was completely satisfactory, indicating that, at this temperature, pan- creatin may be considered an unhairing agent for calf skin previously swollen in dilute sodium hydroxide solution. A very slight unhairing action was noticeable in the blank at 40°, evidently due to the previous treatment with alkali. No unhairing action could be detected in the blank or enzyme solution at 25°. The preceding series of experiments was then repeated exactly, except that 0.05 molar hydrochloric acid solution was substituted for the alkali as the swelling agent. At 25° there was no visible un- hairing action either in the blank or enzyme solution. In the hydro- chloric acid solutions in the bath at 40°, the pieces of skin began to jelly; there was no further change in the piece transferred to the blank at 40°, but the piece put into the enzyme solution at 40° was quickly destroyed, the collagen passing into solution, leaving the epidermis and hair floating in the liquor. The opposite effects of acid and alkali upon the skin at 40° is interesting. 0.05 molar sodium hydroxide solution hydrolyzes the epidermis more rapidly than the collagen fibers, whereas 0.05 molar hydrochloric acid hydrolyzes collagen much more rapidly than it does the epidermis. The experiment was repeated except for the fact that the pre- treatment with hydrochloric acid was done at 25° and the digestion with pancreatin at 40°. After the skin had been in the pancreatin solution for 24 hours, the hair was completely loosened, showing that the effectiveness of pancreatin as an unhairing agent depends upon the previous swelling of the skin, but regardless of whether the swell- ing is caused by acid or alkali. The fact that pretreatment with sodium hydroxide in the experiment with alkalies was done at 40° did not seriously influence the result for, when another piece of skin was soaked in 0.05 molar sodium hydroxide solution at 25° for a day and then in the pancreatin solution at 40°, the unhairing action was entirely satisfactory. Experiments dealing with the action of pancreatin upon skins previously treated with ammonia were carried out exactly like those of the sodium hydroxide series, except for the replacement of the. 0.05 molar sodium hydroxide solution by 0.50 molar ammonium. hydroxide solution. The hair was loosened to some extent by the ( Bye ey of the Skin. Alfred Seymour-Jones. J. Soc. Ecotier Trades Chem. 2 1916), 203. d : a ee tls ok UNHAIRING AND SCUDDING 169 pretreatment with ammonia, more at 40° than at 25°. After the pieces had been in the blank and enzyme solutions for 24 hours, they all showed some unhairing action, but in no case was it entirely satisfac- tory. The degree of action might be given a very rough rating by calling that in the enzyme solution at 40° 75 per cent, that in the blank at 40° 50 per cent, and that in both blank and enzyme solutions at 25° 25 per cent. Evidently the pretreatment of skin with ammonia, which is itself an unhairing agent, does not assist the unhairing action of pancreatin nearly so much as pretreatment with materials whose action is primarily to swell the skin. Combined Bating and Unhairing by Means of Pancreatin. Wilson and Gallun extended their investigation to an examination of the effect of the pancreatin upon the elastin fibers of the skin, the work of Wilson and Daub having indicated previously that the fundamental action of bating is the removal of elastin fibers from the skin. The work of Wilson and Daub will be described in the next chapter. Pieces of skin were taken from the various experiments after the pancreatin had acted upon them. These were imbedded, sectioned, stained, and mounted for examination, as described in Chapter 2. When the pancreatin method of unhairing is used in practice, the liquors are left exposed to air. The experiments of Wilson and Gallun show that the hair loosening can then be effected at a temperature of 25°C., but that the action is apparently not due to enzyme, but rather to bacteria, since it is checked by covering the solutions with toluene. But, if pancreatin is not the active agent, we should expect the action not to be accompanied by elastin removal. Fig. 71 cor- roborates this view; where the hair loosening was effected by a pan- creatin solution at 25°, exposed to air, the epidermis is disintegrated and the hair loosened, but the elastin fibers remain undissolved and show in the upper half of the picture as fine, black threads running nearly horizontally. In the unhairing experiments where the skin from the enzyme solutions at 40° C. had not previously been swollen with acid or alkali, microscopic examination showed that all of the elastin had been dis- solved away from the flesh side of the skin in 24 hours, but none from the region just under the epidermis. The hard corneous layer of the epidermis had apparently acted as a membrane impermeable to the enzyme. In the ordinary methods of unhairing, such as liming, the unhairing agent acts upon the cells of the Malpighian layer, which lie between the corneous layer and the derma. The impermeability of the corneous layer to the enzyme explains why the pancreatin did not attack the Malpighian layer and loosen the hair. In acid or alkaline solutions, the corneous layer swells considerably and is thereby ren- dered more permeable. It is also attacked by the enzyme, when in the swollen condition, as shown by the fact that no corneous layer could be found in the sections examined. Fig. 71.—Vertical Section of Thermostat Layer of Calf Skin. (After 1 day in 0.1-per cent pancreatin solution at 25° C.) Location: butt. Eyepiece: 5X. Thickness of section: 30 pw. Objective: 8-mm. Stains: Van MHeurck’s logwood, Wratten filter: H-blue green. Daub’s bismarck brown. Magnification: 170 diameters, 170 Fig. 72.—Vertical Section of Thermostat Layer of Calf Skin. (After 1 day in 0.1I-per cent pancreatin solution at 4o° C.) Location: butt. Eyepiece: 5X. Thickness of section: 30 wp. Objective: 8-mm. Stains: Van MHeurck’s logwood, Wratten filter: H-blue green. Daub’s bismarck brown. Magnification: 170 diameters. 171 172 THE CHEMISTRY OF LEATHER MANUFACTURE Fig. 72 shows a section of calf skin which had been soaked in sodium hydroxide solution previous to digestion with pancreatin at 40° C., under toluene. Not only is the epidermis destroyed and the hair loosened, but the skin is completely bated, as shown by the absence of elastin fibers. An interesting attempt to unhair skins by means of enzymes naturally occurring in the skin is that of H. C. Ross.** A 1-per cent solution of ammonium hydroxide is used to inactivate the foreign enzymes, while the thrombase found in the skins is activated by the addition of calcium lactate or polysulfide. It is mentioned that the thrombase may be assisted by the addition of trypsin or other pro- teolytic enzymes which will work in an alkaline medium. The unhair- ing is effected without destroying the epidermis, so that large sections thereof can be removed with the hair attached. Subsequent bating is unnecessary. In preparing dressing leathers, the solutions are heated, while for sole leathers cold liquids are employed, these allowing plumpifg to take place to a greater extent. How nearly the actual mechanism of this method of unhairing is suggested by the descrip- tion of the patent is open to question, but it would be interesting — to see a study made of it along lines similar to those of the experiments of Wilson and Gallun. Skins prepared for unhairing and scudding by means of pancreatin solutions are unhaired on a machine, scudded on the beam, and then washed, after which they are ready for tanning without further treat- ment. Skins from lime liquors are unhaired, scudded, washed and then either bated, delimed, drenched, or pickled before tanning. Some tanners put the skins directly into old vegetable tan liquors with- out giving them one of these treatments, but the tan liquor then becomes a deliming agent and has little value other than that of removing lime. Apparently anything that will hydrolyze the newly formed cells of the epidermis without injuring the rest of the skin is a satisfactory unhairing agent. Lime owes its popularity to the safety attending its use. Its limited solubility makes it possible to maintain a con- stant hydroxide-ion concentration at about 0.03 mole per liter simply by using an excess. This concentration is high enough to retard putre- faction considerably and yet not great enough to injure the skin itself, since the solute is a diacid base. It is entirely possible, however, that the popularity of lime will wane when some of the newer methods of unhairing reach a higher stage of development. *8 British Pat. 169,730, March 25, 1920. Chemical Abstracts 16 (1922), 853. Chapter 9. Bating. Perhaps the most curious of all the processes involved in making leather is that of bating. Little is known of its origin because it WaSea secret process, but: it is at least some centuries old. After the skins are taken from the lime liquors, unhaired, scudded, and washed, they still contain lime in the form of carbonate and in com- bination with the skin proteins. At this stage they are plump and rubbery and tanners have experienced many difficulties due to putting the stock directly into certain types of vegetable tan liquors when it was in this condition. The object of bating is to prepare the un- haired skins for tanning and originally consisted in keeping. them in a warm infusion of the dung of dogs or fowls until all plumpness had disappeared and the skins had become so soft as to retain the impression of thumb and finger when pinched and sufficiently porous to permit the passage of air under pressure. When hen or pigeon manure was used, the process was called bating, and when dog dung was used, it was called puering, but the term bating is now applied to the process generally, regardless of the materials used. The difference in terminology naturally disappeared with the advent of artificial bating materials. A common method for treating light skins was to put them into a vat filled with a liquor containing about Io0 grams of dog dung per liter, kept at a temperature of 40° C. by means of steam. A paddle wheel kept the liquor and skins in motion. During the action, the skins gradually lost the plumpness acquired in the lime liquors and became soft and raggy. The completion of the process was de- termined by the attainment of a certain degree of flaccidity, which the workmen could judge only after long experience. Hen or pigeon manure was sometimes used for light skins, but was more commonly applied to heavy hides because it penetrates more rapidly than dog dung, due apparently to the fact that it contains also the urinary products, especially urea. For many years this remained one of the mysterious processes of the tannery. It gave some tanners an improved product, which they could get in no other way known to them. But during the past thirty years there has been a persistent effort to determine the es- sential reactions of bating so that it might be carried out more reliably and with less offensive materials, or that it might be done away with 173 174 THE CHEMISTRY OF LEATHER MANUFACTURE entirely by treating the skins differently at other stages. For ex- ample, it had been suggested that the only important function of the bate is the removal of the insoluble lime compounds from the skin before tanning. But this was contested by those who believed that merely removing the lime was not sufficient. They regarded bating as a process necessary for the removal of certain undesirable protein constituents of the skin. In order to settle this question, investigators have made extensive studies of dungs, and of the skins and liquors, both before and after the process. - The greatest pioneer work in this field has been carried out by J. T. Wood, whose investigations, coupled with practical developments by O. Rohm and others, have led to the almost complete replacement of the obnoxious dungs by pancreatic enzymes. In his book, Wood ? says: ‘When learning the trade as an apprentice every fault in the leather was attributed to this part of the work, and the troubles and miseries of the ‘puer shop’ first caused me to take up the study of puering. I was determined to know the causes underlying the process. Puering is not only a filthy and disgusting operation, but is prejudicial to health, and in the nature of it is attended by more worry and trouble than all the rest of the processes in leather making put together.” Wood found the mineral matter of dungs to consist chiefly of the sulfates, chlorides, carbonates, and phosphates of sodium, potas- sium, ammonium, and calcium, and some silica. The most important organic constituents seemed to be the bacteria, enzymes, cellulose ma- terials, and fats. He found both peptic and tryptic enzymes, a rennin, an amylolytic enzyme, and a lipase. Since the bate liquor is usually faintly alkaline, it seemed likely that trypsin was active in the process and it was later shown that this enzyme does produce some of the effects of dung upon the skin. Wood also isolated from dog dung a species of B. coli which was found to yield an enzyme capable of acting upon the skin like trypsin. Artificial bates are now to be found upon the market which con- tain pancreatin, ammonium chloride, and supposedly inert fillers and these have largely supplanted the dung bates formerly used. But ma- terials other than those containing tryptic enzymes have also appeared on the market, as bates, to revive the old question as to the fundamental object to be attained by bating. These materials apparently give sat- isfactory results for some kinds of leather, even though some of them consist merely of carbohydrates, which yield organic acids by fermentation. The dung bates evidently had several different func- tions, but apparently all manufacturers of artificial bating materials did not concentrate their attentions upon the same functions. Numbers of preparations of quite different properties are sold as bating ma- terials and this has served to aggravate the confusion as to what constitutes a bating material. The several purposes served by these materials will be considered separately. , es Puering, Bating and Drenching of Skins. J. T. Wood. E. & F.N. Spon, London 1912). BATING 178 Falling. The one property which all of the various types of bating materials have in common is that of reducing the degree of swelling of the protein constituents of the limed skin, which action is known to the trade as falling. Indeed it would have been practically impossible for any artificial preparation to pass as a bate that did not have this property, because the degree of flaccidity of the skin was the accepted measure of the nearness to completion of the bating process. It will be apparent from the discussion of the swelling of protein jellies given in Chapter 5 that the degree of falling of a skin must be a function of hydrogen-ion concentration and also of the concentration of neutral salts. Wilson and Gallun 2 measured the degree of plumping of calf skin as a function of pH value by means of their method, which is described in Chapter 8. Pieces of unhaired skin, each about 2 centimeters square, were cut from the butt of a calf skin so as to insure the greatest degree of uniformity of structure. These were freed from lime by washing in a 1I2-per cent solution of sodium chloride containing a small amount of hydrochloric acid, and then neutralized in cold, sat- urated sodium bicarbonate solution. They were then washed and bated by keeping at 40° C. for 24 hours in a solution containing 0.1 gram of U.S.P. pancreatin, 2.8 grams of monosodium phosphate, and 18 cubic centimeters of molar sodium hydroxide solution per liter, giving a pH value of 7.7. Microscopic examination showed that this procedure removed all of the elastin fibers. The pieces were then washed in cold, running tap water, having a pH value of 8, for 24 hours. They were then kept in distilled water in the refrigerator at 7° C. until used for the tests. The condition in which the skin existed in this state was taken as a standard, as it was found to be easily reproducible. A series of 24 large reservoirs of test solutions was prepared, each having a final concentration of tenth-molar phosphoric acid plus the amount of sodium hydroxide required to give the desired pH value as determined by the hydrogen electrode. A range of pH values from 4 to II was covered. In each test a piece of skin in standard condition was placed in the Randall and Stickney thickness gauge described in Chapter 8. The gauge reading in every case was taken exactly five minutes after dropping the plunger onto the piece of skin. This was called the initial gauge reading. The skin was then shaken with water to bring it back to its natural shape and then put into 200 cubic centimeters of standard buffer solution of the desired pH value and kept in a thermostat refrigerator at 7° C. so as to reduce to a minimum any tendency towards putrefaction. After 24 hours, each solution was replaced by fresh buffer solution. After 4 days more, there being 2The Points of Minimum Plumping of Calf Skin. J. A. Wilson and A. F, Gallun, Jr, Ind. Eng. Chem. 15 (1923), 71. 170. THE CHEMISTRY OF LEATHER MANUFAC practically no change taking place in the pH values of the solutions, it was assumed that equilibrium was established and the pieces were removed and their thicknesses measured again. ‘The results are given in Table XVI. The ratio of the final to the initial gauge reading is a measure of the degree of plumping of the skin and this is plotted as a function of the pH value in Fig. 73. TABLE XVI. UNHAIRED CALF SKIN IN CONTACT WITH BUFFER SOLUTIONS OF DIFFERENT pH VALUuEs. Gauge readings in mm. (average of pH value of solution duplicates ) at 20° C. Initial Final Ratio * Initial Final 1.421 2.729 1.92 3.96 3.97 1.205 1.885 1.56 4.14 4.17 1.269 1.431 pa ike 4.47 4.49 1.439 1.290 0.90 4.78 et aS 1.489 1.305 0.88 5.08 5.07 1.299 1.161 0.89 5.29 . 5-25 1.347 1.239 0.92 5.57 5-57 1.388 1.306 0.904 5.78 5.72 212 1.263 1.04 6.04 6.08 1.226 Bee ke 1.04 6.20 6.29 1.391 1.478 1.06 6.48 6.42 1.248 1.343 1.08 6.69 6.68 1.435 1.514 1.06 6.96 6.88 1.292 1.362 1.05 7.08 7.00 1.379 1.415 1.03 7.41 7.41 1.413 1.385 0.98 7.68 7.62 1.393 1.407 1.01 - 7.97 7.89 1.515 1.520 1.00 8.42 8.44 1.428 1.427 1.00 8.56 8.50 1.253 1.343 1.07 9.03 9.13 1.258 1377 1.09 9.59 9.64 1.219 1.388 1.14 10.00 9.98 1.240 1.621 13 10.47 10.51 1.289 2.206 1.71 11.06 11.08 * This ratio is a measure of the degree of plumping of the skin, The significance of these two points of minimum plumping has been discussed in Chapter 5. By comparing Fig. 73, with Fig. 45, it will be seen that the plumping of calf skin varies in much the same way as the swelling of gelatin with change of pH value. Apparently collagen undergoes a change of form, possibly an internal rearrange- ment, in passing from an acid to an alkaline solution and the two points of minimum represent the isoelectric points of the two forms. The degree of plumping at any point between 4.5 and g.O is rela- tively so small that the skin would pass as completely bated, if judged solely by its fallen condition. Wood, who was probably the first to apply the hydrogen electrode to tannery liquors, observed that the pH value of fresh dung bate liquors varied from about 4.7 to 5.4, whereas the bating of a pack of skins raised it to points lying BATING 177 between 6.4 and 8.4. In a lime liquor, which has a pH value of about 12.5, the skin is very plump and rubbery. But when it is brought into equilibrium with a liquor having a pH value lying between 4.5 and 9.0, it becomes fallen and flaccid. The author has observed that when putrefaction starts in pro- tein solutions the pH value of the solution generally tends to shift into the region 5.5 to 6.0, regardless of what it may have been in- Degree of Plumping of Skin (final/initial gauge reading) 4 5 6 if 8 c ane) ta Ba ee pH Value of Buffer Solution Fic. 73.—Showing the two points of minimum plumping of calf skin. itially. The putrid dung bates would, therefore, tend to reduce the pH value of the limed skin from 12.5 to a value approaching 6. But the bate liquor contains phosphates, which act as buffers, and the full drop in pH value is prevented. The phosphate is thus a safeguard against putrefaction of the skin, which would be quickly damaged if the pH value were allowed to drop to the range of maximum rate of putrefaction, 178 THE CHEMISTRY OF LEATHER MANUFACTURE Many so-called bating materials probably serve chiefly to reduce the pH value of limed skins to the region of minimum plumping. The value of this fallen condition is readily apparent for skins which are to be tanned in vegetable tan liquors. ‘Tannins diffuse only very slowly through swollen skin, but when the skin is in a fallen con- dition, the tarinins are enabled to diffuse rapidly into the spaces be- tween the fibers, greatly hastening complete penetration. There is a fallacy in the assumption that plump leather can be produced only by putting skin into the tan liquors in a plump condition. The solidity of the resulting leather is determined more by the reaction of the liquor itself than by the degree of plumping of the skin when first put into the liquor. | The manufacture of materials capable of bringing limed skin into the condition of minimum plumping is obviously a simple matter. It is only necessary to incorporate a buffer material with one which will tend to lower the pH value of the limed skin to a final value of about 8. Among the materials used for this purpose are boric acid, ammonium chloride, weak organic acids and materials yielding acids by fermentation, and acid sodium phosphate. The author observed five successive lots of skins pass through an artificial bate liquor con- taining sodium phosphate, which was entirely uncontrolled, and 0.5 was the greatest deviation in pH value from the normal value of 8.0 during the entire period of operation. Whére it is desired only to bring the skins into a fallen condition, the process can be carried out very effectively using only sodium phosphate and the occasional addition of hydrochloric acid to maintain a pH value of about 8. Regulation of Hydrogen-Ion Concentration. Although the degree of plumping of a skin is a function of the hydrogen-ion concentration, the action of a bate liquor in lowering the pH value of limed skin has an importance independent of the question of plumping. Nearly 80 per cent of the bated weight of a skin is due to water, or rather bate liquor. Even though the skin may be washed, the water will assume a pH value depending upon the substances held in combination with the skin. This adhering solution will therefore have an effect upon the tan liquor into which the skins are put. If the pH value of this adhering solution is very variable, difficulty will be experienced in vegetable tanning because the rate of tanning, the rate of diffusion of the tan. liquor into the skin, the color value of the tan liquor, and its tendency to oxidize are all functions of the pH value. Keeping constant the pH value of the solution adhering to the skins entering the tan liquors is a factor of great importance and one which made the old dung bates almost a necessity to the tanner who had no other way of controlling the pH value. The actual pH value, within limits, was probably of less importance than keeping it constant at some arbitrary value, which could be met by establishing conditions in the tan yard to correspond. BATING 179 Deliming. Many persons have looked upon bating chiefly as a process for removing the combined lime from the skins. In using a dung bate, Wood found from 3 to 6 per cent of lime, calculated as calcium oxide on the dry skin, before bating and only from 0.5 to 0.9 per cent after bating and all of this appeared to be present as neutral salt. Artificial bates, however, do not all have the property of removing calcium from the skin. Upon investigating the operation of a bate liquor containing phosphates and ammonium chloride and having a pH value of 8.4, the author found no diminution of the calcium con- tent of the skin during bating, although the skins had become com- pletely fallen and practically all of the lime had been converted into neutral or insoluble salts. Apparently insoluble calcium phosphate had formed in the skin, where it remained. In cases like this, the process can hardly be called efficient as a means of deliming. Where nearly complete removal of calcium compounds is essential for the best opera- tion of later processes, it is much better to employ a properly controlled acid liquor, such as those to be described in the next chapter. Bacterial Action. Bacteria play an important role in the action of dung bates, being instrumental in the removal of lime from the skin as well as in lower- ing the pH value to the region of minimum plumping. Some of the bacteria, or their products, also attack portions of the skin itself, as shown by the appearance of nitrogenous matter in solution. In Fig, 74 is shown a typical plate culture on gelatin? of a dung bate liquor in actual use. Becker * isolated 54 varieties of bacteria from dog dung and studied the actions of many of them upon skin. He found one, which he called B. erodiens, capable of producing a falling action of limed skin similar to that of the dung bate itself. An artificial bacterial bate was developed independently by Wood in England and by G. Popp and H. Becker in Germany, but they later joined forces and _ per- fected the artificial bate known as erodin, which consists of a nutrient material to which a pure culture of B. erodiens is added before using. This material has been used on a commercial scale and found to be a satisfactory substitute for dung for some kinds of leather. Since B. erodiens does not secrete tryptic enzymes, Wood has sug- gested adding to it bacteria obtained from the roots of wool in the sweating process which secrete a mild form of proteolytic ferment. The susceptibility of erodin liquors to become contaminated by for- eign bacteria presents an obstacle to any very widespread increase in their use. In using erodin, Wood has observed that the fresh liquor * Cf. The Properties and Action of Enzymes in Relation to Leather Manufacture. J. T, Wood. J. Ind. Eng. Chem. 13 (71621 )Sorr3s. * Bacteriological Reactions in the Leather Industry. H. Becker. Z, 6fent, Chem, to (1904), 447. 130 THE CHEMISTRY OF LEATHER MANUFACTURE usually has a pH value of about 6.6 and this increases to about 7.3 during the bating operation. Cruess and Wilson ® isolated 10 varieties of bacteria from pigeon dung and found that the falling of limed skins could be brought about by pure cultures in dilute skim milk. If the bating operation were unduly prolonged, the skin proteins became hydrolyzed, but they found Fig. 74.—Typical Plate Culture on Gelatin of Puer Liquor. that danger from this source could be greatly minimized. by using a liquor containing 0.5 per cent of glucose. They pointed out that the glucose was decomposed into acids which checked bacterial action and assisted in the removal of lime from the skin. The prevailing opinion is that bating is not produced directly by the bacteria, but rather by the products which they secrete. Of these, the enzymes are regarded as the most important because the reduc- tion in pH value of the skin, with consequent falling, can be brought about by simple chemical means not generally regarded as constituting the process of bating. | 5A Bacterial Study of the Bating Process. W. Cruess and F. H. Wilson. J. Am. Leather Chem, Assoc. 8 (1913), 180. : BATING 181 Enzyme Action and Elastin Removal. Wood * separated the enzymes from dog dung by precipitation from solution with alcohol and showed that the enzymes, in conjunction with ammonium compounds, were capable of bating skins. In view of the fact that the bate liquor was alkaline, it seemed pretty certain that trypsin must be the principal enzyme acting. Wood and Law’ later showed that there were at least five different enzymes present in dog dung, as follows: A peptic enzyme resembling stomach pepsin. A tryptic enzyme resembling pancreatic trypsin. A rennin (coagulating enzyme). An amylolytic enzyme. A. lipase. baie aa he Where a skin contains an abundance of fat cells, the lipase probably exerts an important function in hydrolyzing and emulsifying the fats. In 1908 Rohm ® patented the use of the enzymes of the pancreatic juice and ammonium salts as a bating material. This mixture now known as oropon has come into wide use and has largely supplanted the dung bates formerly used. Recently there has been a concerted effort to determine just what part is played by pancreatin in the bating process. As a measure of the elastin content of skin, Rosenthal ® used the per cent of nitrogenous matter that could be rendered soluble by tryptic digestion. By this method he found that bating with oropon reduced the elastin content of calf skin from 10.36 to 0.31 per cent, calculated on the dry basis. The author’s later investigations of the bating process by means of the microscope, however, indicate that Rosenthal’s method of deter- mining the elastin content of skin is unreliable. Apparently a large portion of the matter included as elastin was derived from the other protein constituents of the skin or their hydrolytic products. Upon examining a dung bate liquor used to bate sheep grains, Wood found that nitrogenous matter had been dissolved equivalent to only one per cent of the total protein matter of the skins. As nearly as can be judged from microscopic observations, this represents approximately the percentage of elastin present in the skin. Seymour-Jones *° also suggested that the function of bating is the removal of the elastin fibers of the skin. In collaboration with J. T. Wood, Seymour-Jones carried out an interesting experiment on the bating of sheep skin. The “flywing” grain of a sheep skin was split from the main body of the skin, called simply flesh for convenience, 6 Notes on the Constitution and Mode of Action of the Dung Bate. J. T. Wood. J. Soc. Chem. Ind. 17 (1898), tort. 7 Enzymes Concerned in the Puering or Bating Process. J. T. Wood and D. J. Law. J. Soc. Chem. Ind. 31 (1912), 1105. ®U. S. Pat. 886,411, May 5, 1908. ® Biochemical Studies of Skin. G. J. Rosenthal. J. Am. Leather Chem. Assoc. 11 (1916), 3. 10The Physiology of the Skin. Alfred Seymour-Jones. J. Soc. Leather Trades Chem. 4 (1920), 60. ~ Fig. 75.—Vertical Section of Calf Skin. (After liming and unhairing, before bating.) Location: butt. Eyepiece: none. Thickness of section: 40 u. Objective: 32-mm. Stains: Weigert’s resorcin-fuchsin Wratten filters: B-green; E-orange. and picro-red. Magnification: 25 diameters, 182 Fig. 76.—Vertical Section of Calf Skin. (After bating, before tanning.) Location: butt. Eyepiece: none. Thickness of section: 40 u. Objective: 32-mm. Stains: Weigert’s resorcin-fuchsin Wratten filters: B-green; E-orange. and picro-red. Magnification: 25 diameters. 183 1834 THE .CHEMISTRY OF LEATHER MANUFACTURE and both grain and flesh were cut into halves along the backbone. One grain and one flesh were bated with pancreol, a pancreatin preparation similar to oropon, while the other halves were delimed with acetic acid, but not bated. All four pieces were then tanned with sumac. There was comparatively little difference between the bated and unbated flesh halves, but the grain samples were very different from each other. The bated grain was soft and even, with the hair-holes clean and clear, but in the unbated grain the hair-holes appeared to be glued up and the surface had a rough, contracted appearance. He con- cluded that elastin present in the region of the grain membrane must be digested. before tanning in order to produce a satisfactory grain surface, but that the bating of the skin under the grain is not only unnecessary, but often undesirable. The difference which Seymour-Jones found between the two grains was probably not due entirely to the bating process, since one was treated with acetic acid while the other was not. This means that the unbated grain would be subjected to the action of tan liquor at a lower pH value than the bated grain. But as the pH value of a fresh tan liquor is lowered, there is an increasing tendency for it to produce in the grain layer of a skin the rhythmic swelling described in Chapter 5. This shows itself first in a roughening of the grain, similar to that described by Seymour-Jones, and with further drop in pH value the corrugation of the surface appears. The roughening of the grain which had not been bated may have been aggravated by the © presence of the elastin fibers, but the chief cause was probably the lower pH value. Wilson and Daub *: ?? undertook to settle definitely the question of the removal of elastin in the bating process by means of the microscope. They prepared sections of calf skin taken both before and after bating with a solution of pancreatin and found that the process removes all of the elastin fibers, if sufficiently prolonged. Fig. 75 shows a sec- tion of calf skin taken after liming, unhairing, scudding, and wash- ing, but before bating. The elastin fibers show as a thick, black band just under the grain surface; the magnification here is not sufficiently great to show each individual fiber. Another layer of elastin fibers — appears at the flesh boundary. The main body of the skin contains no elastin fibers excepting those surrounding blood vessels, nerves, and muscles. Fig. 76 shows an adjoining section of the same skin taken after bating for 24 hours in 0.oI-per cent pancreatin solution at 40° C., having a pH value of 7.5. The author has recently received a letter from Mr. L. Krall of Geneva, Switzerland, claiming priority in discovering, by means of the microscope, that the chief function of bating is the removal of elastin fibers from the skin. His experiments, performed at the University of Geneva from 1914 to 1916, proved that the elastin fibers of skin can be entirely removed by digestion in an infusion of dog dung at 40°C. His photomicrographs show that the action of dung is 11 The ‘Mechanism of Bating. J. A. Wilson. J. Ind. Eng. Chem. 12 (1920), 1087. 2A Critical Study of Bating. J. A. Wilson and Guido Daub, Ibid., 13 (1921), 1137. BATING 185 practically identical with that found by Wilson and Daub for pan- creatin, thus furnishing further evidence of the soundness of Wood’s conclusion that pancreatin is the active constituent of dung in bating. Krall’s important paper ‘* was unfortunately buried in a private bulletin. After examining hundreds of sections of skin, taken before and after bating, at high magnifications and with the employment of a great variety of stains, Wilson and Daub came to the conclusion that the removal of elastin is the primary function of bating and that the other actions associated with dung bates can all be produced by the simple chemical control of the processes other than bating. The fall- ing of the skin, however, always accompanies the removal of elastin because the range of pH values over which pancreatin acts upon elastin is such as to reduce the plumping of limed skin to the point accepted as a measure of the completion of the bating process. In studying the progress of bating, Wilson and Daub observed cross sections of skin taken before and after bating and estimated the per cent of elastin removed by the treatment. For this purpose, the sec- tions were prepared and stained as described in Chapter 2. The enzyme which they employed was a commercial sample of U.S.P. pancreatin which showed by analysis: water, 6.3 per cent; ash, 6.8 per cent; nitrogen, 11.0 per cent; chlorine, 1.7 per cent; phosphates, as phosphorous pentoxide, 3.5 per cent; sulfate, none. By the method for determining tryptic activity described by Sherman and Neun,’* 10 milligrams of the sample acting upon I gram of casein in 100 cubic centimeters of solution for 1 hour at 40° C. and at pH value of 7.33 digested 51 milligrams of nitrogen, As a matter of caution, it should be pointed out that this does not give a correct measure of the activity of the enzyme so far as its power to digest elastin is concerned. The author suggests that the elastin-digesting power of a bating material be determined solely by the amount of elastin which a given sample can digest from skin under rigidly defined conditions. The activity of the sample on casein or gelatin may be entirely misleading as regards its value as a bating material. For each series of experiments, Wilson and Daub cut a piece of limed and unhaired calf skin into strips about 2x0.5 inches. There is a small, but appreciable, difference in time required for complete removal of elastin from skins of different thickness and for this reason care was exercised in selecting all strips for any one series from the same part of the same skin, so as to have them all as nearly identical as possible. Each strip was put into 500 cubic centimeters of liquor, a volume large enough to prevent the skin from seriously altering the concentration of the liquor. The liquors were all put into dark brown bottles to shield them from the light and were kept in a large Freas thermostat for the stated lengths of time at 40° + 0.01° C., the optimum temperature for most enzyme actions.*° 18 Ferments in the Tannery. L. Krall. Societe Anonyme, anc. B. Siegfried. Zofingue, Switzerland. Private bulletin, June, 1918. 14H. C. Sherman and D. E. Neun. J. Am. Chem, Soc. 38 (1916), 2199. < The Chemistry of Enzyme Actions. K. G. Falk. The Chemical Catalog Co., New ork, 186 THE CHEMISTRY OF LEATHER MANUFACTURE Every liquor contained 0.02 mole per liter of added phosphoric acid to act as a buffer, in addition to the enzyme, and the potassium hydroxide required to give the desired hydrogen-ion concentration. The pH value of each liquor was determined both before and after the digestion period by means of Hildebrand electrodes and a Leeds and Northrup potentiometer, excepting where it was proved by previous test that the results obtained by the Clark and Lubs series of in- dicators were sufficiently accurate. Except for the more strongly acid and alkaline solutions, the change in pH value during digestion was practically negligible. Estimates of the per cent of elastin removed were made on the basis of removal from the grain layer only. In some cases all of the elastin was removed from the grain layer before half of it was removed from the flesh layer. Since the shaving operation removes practically all of the flesh elastin, its removal in bating is of little importance, As a rule, a preliminary series covering a very wide range was run, followed by a second series covering only the active range of the enzyme. A third series was usually run as a check. Effect of Hydrogen-Ion Concentration. It is well known that the hydrogen-ion concentration is an impor- tant factor in determining the rate of digestion by enzymes. Using 0.1 gram of pancreatin per liter and digesting for 24 hours, complete removal of elastin from the skin was obtained only between the pH values 7.5 and 8.5. ay S =P “a EGS 3 = B38 bce m a eS ws Oey eis BV ofa no O.2 Fee ous 25 28 3 aoexes fe See a ois Oe & 5 One a airs pot oO as Sas ~ og wae 9 Sa Eon = nat Ae) Vo oe po 66s OG 35 ed O°m a ye | #2» ¢ 3 04 aa gs 00 eee 2 og ae eS ise. ba ane &10.E Sule Han 95 I 196 THE CHEMISTRY OF LEATHER MANUFACTURE while Fig. 82 is from a young heifer calf skin. It will be noted that the older skin has relatively fewer elastin fibers, although they extend into the skin to a greater absolute depth. This greater depth necessitates leaving the hide in the bate liquor for a longer time, so that the enzyme may diffuse to the most deeply seated fibers, but, on the other hand, there is less reason for removing the elastin fibers from the heavier skin, because they are relatively fewer. : Fig. 83 shows the elastin fibers of a sheep skin before bating and Fig. 84 those of a hog skin. The elastin fibers of the hog skin are very sparsely. scattered; the heavy band of elastin fibers passing obliquely upward to the right, across the center, is apparently there for the purpose of protecting the erector pili muscle, which it surrounds. Figs. 81, 82, 83, and 84 should be compared with Figs. 11, 18, 28, and 30, respectively, of Chapter 2, which show sections taken from the same skins when fresh. Effect of Elastin Removal on the Final Leather. Wilson and Daub attempted to determine the practical value of bating by comparing bated and unbated skins. A limed calf skin was cut into halves along the line of the backbone, the elastin was com- pletely removed from one half by means of pancreatin, while the other half was simply treated with dilute ammonium chloride solu- tion having a pH value of 8, in order to reduce its degree of plumping to that corresponding to what is accepted as the bated state. Both halves were then thoroughly washed. It was recognized that an exact comparison of the two halves during tanning could not be made, if the pH values of the absorbed solutions were very different. Every effort was made to have the pieces identical, excepting for elastin content. The most noticeable difference was observed during the early stage of vegetable tanning. The surface layers of the skin naturally tan ‘ more rapidly than the fibers in the interior and there is a tendency for the grain surface to expand temporarily to a greater extent than the rest of the skin. The elastin fibers in the unbated half evidently tended to prevent this expansion and the result of the tension pro- duced was a slightly harsh feel, although the grain appeared tight and smooth to the eye. The grain of the completely bated half, however, actually expanded, giving the skin temporarily a wrinkled appearance, although the grain felt very soft and silky. When both halves had become completely tanned, this difference had almost dis- appeared. In the finished leather, the only difference in appearance was a slightly lighter color in the bated half. Photomicrographs of exactly corresponding points on the grain surface of the two halves are shown in Fig. 85. The difference in appearance of the grain sur- face in the two cases is practically negligible. In carrying out prac- tical tests of this kind, tanners usually fail to appreciate the im- portance of having the test pieces in equilibrium with solutions of BATING 197 the same pH value and often attribute to differences in bating dif- ferences in the properties of the leather actually caused by differences in pH value. | While bated and unbated finished leathers appear much alike to the eye, there are perceptible physical differences, such as one might expect to find in view of the fact that the elastin fibers have been removed from under the grain of the bated leather. The desirability of completely, or even partially, removing elastin from skin depends upon the use to which the leather is to be put. Bated leathers are Not Bated Fig. 85.—Grain Surfaces of Tanned Calf Skin. Eyepiece: none. Wratten filter: K2-yellow. Objective: 48-mm. Magnification: 7 diameters. usually a little softer than unbated leathers, but this is desirable for some leathers and undesirable for others. Wood ** believes that. it is not necessary, or even desirable, to remove all of the elastin in bating, but that it is sufficient for the elastin fibers to be broken up or weakened, in order that the desired suppleness may be obtained. Digestion of Collagen during Bating. Although Thomas and Seymour-Jones have shown that pancreatin hydrolyzes collagen, the work of Wilson and Daub indicates that no serious loss of collagen occurs where the pH value is kept within the limits 7.5 to 8.0 and the action is stopped just as soon as all of the elastin fibers have been dissolved. Unduly prolonging the bating operation is sure to result in a very considerable hydrolysis of collagen, with corresponding decreases in the yield and firmness of the leather. Often an apparently heavy loss of collagen during 24 The Properties and Action of Enzymes in Relation to Leather Manufacture. Loc. cit. 198 THE CHEMISTRY OF LEATHER MANUFACTURE bating may be attributed to a previous breaking down of the collagen by excessive liming, putrefaction, or contact with liquors containing much ammonia. Manufacturers of glove leather sometimes make use of these agencies in order to get a very soft leather. They leave the skins in the lime liquor until a considerable amount of hydrolysis of collagen has taken place and then subject the skins to a prolonged bating. Much valuable collagen is thus lost, but the skins are thereby rendered more suitable for a specific purpose. Chapter 10. Drenching and Pickling. In the final preparation of the skin for tanning, the pH value of the solution absorbed by the skin and with which the skin is in equilibrium must be adjusted to suit the particular method of tanning to be employed. During liming, this solution has a pH value of about 12.5; during bating, a pH value of about 7.5. Before skins can be tanned properly by any of the common methods of tanning, the pH value of this solution must be lowered considerably below the value 7-5. During vegetable tanning, the pH value of the liquor is usually less than 5 and in chrome tanning less than 4. By using tan liquors containing the proper excess of acid, the adjustment of pH value may be made in the tan liquor itself. But this is often a very difficult matter where the process is not under rigid chemical control. Drenching. For certain classes of leather, it is customary to subject the bated skins, before tanning, to a process known as drenching. Sometimes the bating process is omitted, as entirely unnecessary, and the skins are drenched directly after the washing following the unhairing process. The drench liquor is prepared by mixing 5 to 10 grams of bran per liter of water at 30° to 35° C. and allowing the mixture to ferment, with the formation of organic acids. The skins are put into this liquor contained in a vat equipped with a paddle wheel which keeps the liquor well stirred. In some tanneries, the fermentation is carried out in special tanks and only the clear, decanted, acid solution used on the skins. The acid: dissolves any lime remaining in the skin and brings the skin into a more suitable condition for tanning. The particles of bran also exert a sort of cleansing action upon the skin, tending to absorb dirt and greases. The treatment is usually continued for several hours, but the completion of the process is determined by skilled workmen, who have learned to judge by the feel and appear- ance of the skin just when it is ready for the particular tanning process to be employed. | During the process, there is a considerable evolution of gas, which tends to cause the skins to float to the surface. In a drench in actual use, Wood * found that the gases had the following composition: 7 The Properties and Action of Enzymes in Relation to Leather Manufacture. J, T. Wood. J. Ind. Eng. Chem. 13 (1921); 1135. 199 200 THE CHEMISTRY OF LEATHER MANUFACTURE Carbon dioxide 25.2 per cent Hydrogen sulfide trace Oxygen 2.5 Hydrogen 40.7 Nitrogen 26.0 The acids produced per liter were Formic 0.0306 gram Acetic 0.2042 Butyric 0.0134 Lactic 0.7907 Only an insignificant quantity of other materials were formed during drenching, trimethylamine being the chief. It was found that the starch of the bran is converted into glucoses and dextrin by the action of an amylolytic enzyme, cerealin, discovered by Mege Mouries.? It resembles the diastase of translocation de- scribed by Brown and Morris? in their work on the germination of grass seeds. It transforms starch into dextrin and glucose, whereas ordinary malt diastase transforms starch into dextrin and maltose. The action of cerealin is much slower than that of diastase. The sugars are then fermented by bacteria (Bacillus furfuris) with the formation of the organic acids listed above. The principal acid pro- duced is lactic; the acetic acid is produced directly from the glucoses without any preliminary alcoholic fermentation by yeasts. In the hands of experienced operators, the drenching process sel- dom gives much trouble, but it is not quite foolproof. If the acidity of the liquor increases rapidly and the skins are not removed in time, they become excessively swollen and may even be destroyed by hydro- lysis, especially if the liquor is very warm. How much enzymes play a part in this hydrolysis is not yet known. Apparently danger from this source can be prevented by adding salt to the liquor to repress ‘the swelling of the skin just as soon as it becomes very noticeable. In his review of the damage to skins that may be caused by im- proper control of the drenching operation, Wood * points out that the discovery of the effectiveness of salt in preventing the destruction of skin in an acid liquor that would otherwise cause excessive swelling represents the origin of the modern pickling process. Sometimes the fermentation may not proceed in the usual manner and the liquor, instead of becoming acid, turns slightly alkaline, fre- quently becoming bluish black, due to the presence of chromogenic bacteria. Under these conditions the skin is rapidly attacked by proteolytic organisms, but may be saved if transferred in time to a solution of acid and salt. When the fermentation is accompanied by a very rapid evolution * Compt. rend. 37 (1853), 351; 38 (1854), 505; 43 (1856), 1122; 48 (1859), 431; 50 es 4067, Chem, Soc. 57 (1890), 458 4 Bacon Bating and Drenching of Skins, p. 237. DRENCHING AND PICKLING 201 of gas, the skins may be damaged by the formation of gases inside of the skin which burst out through the grain surface, leaving small holes. A damage very similar in appearance may be caused by pro- teolytic bacteria developing on the grain surface, each colony forming a small hole. This usually results from operating the drench at too high a temperature. A high temperature, especially in the presence of an excess of acid over that normally present, may result in a con- siderable amount of hydrolysis of collagen and the leather will feel rather spongy and empty. When bacteria attack the grain during drenching, the surface of the finished leather may show dull patches, as though it were etched. In one instance, Eitner® found that this was caused by Bacillus megaterium, which formed a slimy film over the grain surface, which was attacked by a proteolytic enzyme secreted by the bacillus. Wood and Wilcox ® showed that if the acids ordinarily found in the drench are used in pure solution in the proportions in which they occur in the drench, the action upon the skin is the same, except for being more rapid. With the appreciation of the fact that the active constituent of the drench is the acid formed, tanners began to sub- stitute pure solutions of organic acids, such as lactic and acetic. These could be used with safety, simply by adding the acid at such rate as to keep the solution just neutral to methyl orange. Hydrochloric acid, being cheaper, is often used, although it makes the control more deli- cate. In this way practically all of the lime can be removed from the skins and the skins then combine with a sufficient amount of the acid so that they do not reduce the acidity of the ordinary vegetable tan liquor into which they may be put. But even when pure solutions of acid were employed to drench skins, no fixed rule could be made for all tanneries. If the vegetable tan liquors contained a considerable amount of salt and other soluble nontannins, the drench could be operated at a lower pH value with safety. Where fresh liquors of tanning materials containing a relatively small proportion of nontannin were used, there was danger of the skins being damaged by the rhythmic swelling described in Chapter 5, whenever the pH value of the drench fell below some fixed value, which depended upon the composition of the tan liquor employed. This trouble can be avoided by the addition of salt to the tan liquor, but the remedy may be almost as undesirable as the disease, since many tan liquors are precipitated by the addition of salt. In gen- eral, the purer the first tan liquor into which the skins are put after drenching, the more delicate must the control of the drenching operation be. It sometimes happens that the tan liquors employed contain easily fermentable sugars, which are continually being converted into organic acids. In such cases, the use of a drench prior to tanning may be undesirable and even the bating operation may be unnecessary, where 5 Gerber (1898), 204. 6 Further Contribution on the Nature of Bran Fermentation. J. T. Wood and W. H. Wilcox. J. Soc. Chem, Ind, 12 (1893), 422. 202 THE CHEMISTRY OF LEATHER MANUFACTURE the removal of elastin is not important. The tan liquor itself actually becomes a drench and the lime salts formed serve to prevent rhythmic swelling. Where the skins have been drenched prior to putting into the tan liquor, the acid present may prove excessive and the skins will be spoiled. One tanner may employ a non-acid tan liquor preceded by a drench, another may use acid tan liquors and do away with the drenching operation, and yet both may produce the same kind of finished leather. But one would not dare to adopt only a part of the other’s methods, which might prove disastrous; he must adopt all or none. This will serve to explain why it is not possible to outline quantitatively a rigid system of bating, drenching, deliming, or any other process, so that it may be used in any tannery. All fundamental operations in any one tannery are interdependent and a change, even one for the better, in one operation might necessitate a corresponding change in nearly every other operation. | Pickling. The pickling operation differs from drenching chiefly in the fact that salt is used in conjunction with the acid. Formerly it was the cus- tomary practice to soak the limed or bated skins in a vat containing dilute sulfuric acid until they became somewhat swollen and then to transfer them to a saturated solution of sodium chloride, which repressed the swelling. Now it is more common to use the acid and salt in solution together, the preliminary swelling having been found unnecessary and sometimes undesirable. A satisfactory pickle liquor for most purposes consists merely of a molar solution of sodium chloride to which sulfuric acid is added in the desired amounts. Pickle liquors are used for a number of different purposes, the . chief of which are the preparation of skin for chrome tanning and the preservation of unhaired skins so that they may be kept for an indefinite period before tanning. In preparing skins for chrome tanning, the concentration of acid most desirable to use depends upon the degree of basicity of the chrome liquor employed. The more concentrated the acid in the pickle liquor, the more quickly does the system tend to reach a condition approxi- mating equilibrium. Furthermore, the more concentrated the acid solution absorbed by the skin, the more quickly will the chromium salts penetrate into the interior of the skin during the tannage. On the other hand, if the concentration of acid is too great, the rate of fixation of chromium by the skin will be reduced to an undesirable degree, unless the excess of acid is neutralized by the addition of sodium bicarbonate, borax, or other agent, during the tannage. Pickling has the advantage over drenching that it is extremely easy to control chemically. If the concentration of salt is not allowed to fall below half-molar, the pickle liquor can be controlled by simple titrations, using methyl orange as indicator. Regardless of the variable amounts of lime which the skins may contain before pickling, they DRENCHING AND PICKLING 203 can all be brought into a uniform condition simply by so regulating the concentration of acid that all skins finally reach equilibrium with solutions of the same concentration. When used in this way, the pickling process becomes a stabilizer of inestimable value in chrome tanning. When the equilibriumi concentration of acid is maintained at 0.05 normal or greater, the pickling of light skins requires only a few hours, but for weaker solutions and for heavy hides, the stock must remain in the liquor over night. In acid solutions greater than 0.01 normal, there is practically no danger of the skins being attacked by bacteria. The salt present is sufficient to prevent undue swelling at any pH value so that the process may be considered entirely safe, if only ordinary care is used. For preserving skins, after bating, it is sufficient to bring them into equilibrium with a solution containing I mole of sodium chloride and 0.01 mole of sulfuric acid per liter. The liquors may be used for several consecutive lots of skin as the calcium sulfate formed is soluble in acid solution. The skins are usually pickled in vats equipped with paddle wheels, which keep the skins and. liquor in motion, greatly hastening the attainment of equilibrium. After equilibrium has been established, the skins are withdrawn from the liquor and thrown over wooden horses to drain. They may then be kept in a damp condition for many months. It is often desired to tan such skins later in vegetable tan liquors of such composition that they would be precipitated by the salt and acid present in the skins. In such cases, the skins are first depickled by soaking in paddle vats containing a solution of half-molar sodium chloride to which borax is added at such rate as to keep the solution neutral to methyl orange. When equilibrium has been established, the skins are transferred to a wash wheel and the salt washed out by means of running water. They are then ready for tanning. Depickling is unnecessary in the case of chrome tanning. In the control of pickle liquors, it must not be assumed that the _ decrease in concentration of acid is caused only by its neutralization ' by lime. Two other factors contribute to the decrease. The bated skins usually contain about 80 per cent by weight of water, only 20 per cent representing collagen. Part of the decrease is caused by the dilution by this water. The author has found that 1 gram of col- lagen combines with approximately 0.00133 gram equivalent of acid. By making allowance for the decrease in concentration of acid caused by dilution and by combination with the collagen, the amount consumed in neutralizing lime can be roughly approximated. Chapter II. Vegetable Tanning Materials. It has been known since prehistoric times that raw skin is colored and rendered imputrescible by contact with aqueous solutions of ma- terials obtained from many forms of plant life. The active principle, - which is widely distributed throughout the vegetable kingdom, is a class of complex organic compounds known as tannin. By vegetable tanning is meant the combination of tannin with the protein matter of skin to form leather. Among the materials which have assumed commercial importance as a source of tannin for leather manufacture are barks, woods, leaves, twigs, fruits, pods, and roots. Tanning extracts obtained from differ- ent sources show very different properties, which is due in a large measure to the foreign matter extracted with the tannin. Classification. Many attempts have been made to classify tanning materials ac- cording to their behavior in tanning practice, but this varies so widely with the nature and proportions of foreign matters extracted with the tannin that attempts at classification on this basis have not yet re- sulted in any scheme of great practical value. The properties of a tanning extract depend more, in many cases, upon the method of extraction or the conditions under which it is used in the tannery than upon its source in nature. By suitably controlling the conditions of tanning, it has been found possible to get practically the same result from tanning materials otherwise exhibiting markedly different properties. | The tannins themselves, however, seem to fall chemically into two general classes, which have been named pyrogallol and catechol from the fact that tanning materials usually yield the one or the other of these two substances upon dry distillation. Upon fusion with sodium hydroxide, the pyrogallol tannins yield sodium gallate while the catechol tannins yield sodium protocatechuate. The pyrogallol tannins con- tain about 52 per cent of carbon as against about 60 per cent in the case of the catechol tannins. The two classes exhibit a number of different properties by which they may be differentiated. All tannins seem to possess in common the property of precipitat- ing gelatin from solution and this is used as a test to indicate the - presence of tannin in solution. ‘The reagent is made by dissolving 10 204 VEGETABLE TANNING MATERIALS 205 x grams of gelatin and 100 grams of sodium chloride in 1 liter of water. One drop of the gelatin-salt reagent is added to 5 cubic centimeters of the solution suspected of containing tannin. Under ordinary conditions, a precipitate is formed if more than a trace of tannin is present. The sensitivity of this test and the conditions under which it may fail to operate will be discussed in Chapter 12. When ferric salts are added to tannin solutions, a deep blue color is formed in the presence of pyrogallol tannins and a deep green in the presence of catechol tannins. All tannins are precipitated by lead acetate, but if the solution is first made approximately normal to acetic acid, the pyrogallol tannins only are precipitated by the addition of lead acetate, the catechol tannins remaining in solution. On the other hand, the catechol tannins are precipitated by the addition of an excess of bromine water, while the pyrogallol tannins remain in solution. A common method for differentiating between pyrogallol tannins and those of the catechol group is to add 10 cubic centimeters of 40-per cent formaldehyde solution and 5 cubic centimeters of concentrated hydrochloric acid to 50 cubic centimeters of the tannin solution and to boil the mixture for half an hour in a flask fitted with a reflux condenser. Catechol tannins are completely precipitated by this treat- ment. The solution is cooled and filtered. To 10 cubic centimeters of the filtrate are added 5 grams of sodium acetate crystals and 1 cubic centimeter of a I-per cent iron alum solution. A strong bluish violet coloration will appear if pyrogallol tannins are present, but none if the original solution contained only catechol tannins. The separation of the tannins into these two groups and the exten- sive studies made of the reactions of the many different kinds of tanning materials have furnished the basis for a scheme of qualitative recognition of vegetable tanning materials which is sometimes of value in detecting adulteration in commercial tanning extracts. One of the best of these qualitative schemes is that of Procter.* When liquors containing pyrogallol tannins undergo fermentation in the tan yard, they usually deposit finely divided ellagic acid, which appears as sludge in the bottom of the vat or as bloom on the surface of the leather. Catechol tannins, on the other hand, yield a difficultly soluble material called reds, or phlobaphenes. Sources of Tanning Materials. Only the more important raw materials will be mentioned here; for more comprehensive lists, the reader is referred to the standard work of Dekker 2 and to the books of Procter* and Harvey.* Among the barks used most widely as a source of tannin are those of the several varieties of oak. Oak bark is one of the few materials furnishing both pyrogallol and catechol tannins, although the latter predominate. Tan- 1 Leather Chemists’ Pocket-Book. H. R. Procter. E. & F. N. Spon, London (1912). 2 Tanning Materials. J. Dekker. Verlang von Gebrtider Borntraeger, Berlin (1913). 8 Principles of Leather Manufacture. H. R. Procter. D. Van Nostrand Co., New York (1922). Tanning Materials. A. Harvey. Crosby Lockwood & Son, Londen (1921). 206 THE CHEMISTRY OF LEATHER MANUFACTURE ning extracts obtained from oak bark have long been favorites for the production of leather where firmness and solidity are desired. Hem- lock bark is used extensively in the United States for the manufacture of heavy leathers. Extracts made from the barks of the larch, spruce, and fir are used to a very considerable extent both in America and in Europe. The barks of the mimosa, or wattle, the mallet, and the several species of mangrove, which are grown in Australia and South Africa, are very rich in tannin. Babool bark is commonly used in India and willow and birch in Russia. The leather known as Russia calf was originally tanned with birch bark, to which it owed its characteristic odor. As a general rule, the tannins of the barks belong to the catechol group. Among the woods, that of the quebracho, grown in South America, is probably richest in tannin, ‘The tannins of chestnut and oak woods find application in the manufacture of sole leather for blending with other materials. Quebracho tannin is of the catechol type, while that of chestnut and oak woods is of the pyrogallol type. The extract obtained from the cutch wood of India is widely used as a mordant jn the dyeing of leather, Recently an extract of the wood of the osage Orange tree has appeared on the market both as a natural dyestuff and as a tanning agent. The most important extracts obtained from leaves and twigs are those of the gambier of India and the sumac of Sicily. The stoner belongs to the catechol and the latter to the pyrogallol group. Gambier is one of the mildest tanning materials known, a property which it apparently owes to the large amount of nontannins present in the extract. It is used as a mordant and, in mixtures with other materials, in the manufacture of light leathers. Sumac is commonly used to tan the grain splits of sheep skins for hat bands, etc., and as a mordant. It is rather easily decomposed by boiling water. A variety of unripe nuts and pods form a much used source of tannin, usually of the pyrogallol type. These often contain easily fermentable sugars and, by their use in tanning, it is often possible to do away with the acid drench to which skins are sometimes subjected prior to tanning. The light colors obtained when using materials like these, which yield acids by fermentation, may be explained, in the light of recent Investigations, by the fact that the color of a tan liquor as well as that of the leather it produces becomes lighter the lower the PH value. The pods of the algarobilla and divi-divi, grown exten- sively in Central and South America, and the dried, unripe nuts of the myrobalan tree of India are used jn mixtures with other materials that do not yield acids so readily. In the preparation of some mixtures, valonia, from the acorn cups of the Turkish oak, is favored. Another easily fermentable tanning extract is obtained from the babool pods of India, which contain both pyrogallol and catechol tannins. Among the roots used as a source of tanning extracts, those of the palmetto, grown in the United States, and the canaigre, grown in Mexico and Australia, are perhaps the most common. The latter is rich in catechol tannin and has a tendency to ferment rather easily. VEGETABLE TANNING MATERIALS 207 Where there is no chemical control of the tan liquors, the selection of tanning materials must be governed by the nature of the operations preceding and following the tanning process, as well as by price and availability of the materials. While quebracho extract, for example, is an excellent tanning material, its pure solutions are hardly suitable for receiving consecutive lots of raw skin containing much lime. Their naturally low acidity would be quickly neutralized and the tannin would then be precipitated by the lime, or oxidized, and cease to tan properly. But this danger would be greatly lessened by the use of a mixture of tanning extracts containing acid-producing materials, like those in divi- divi or myrobalans. Leaching. It is still common to find tanneries equipped to extract the tannin from the raw materials grown in neighboring districts, although the manufacture of tanning extracts has now become a separate industry, which has proved useful in making a greater variety of materials avail- able to the individual tannery. One of the oldest systems for leaching raw materials, and the one most commonly used in tanneries, is known as the open vat method. The bark, or other material, is broken into small pieces and then shredded in a bark mill. The leaching tanks are usually arranged in batteries of about eight and are fitted with perforated false bottoms on which the bark is placed. The bottom of each tank is fitted with a pipe through which liquor may be drawn off or pumped from one tank to another. When fresh bark is put into a given tank, liquor is run onto it which has been used to leach the bark in all of the seven other tanks. This strong liquor is finally drawn off and pumped into a storage tank. The bark is then leached with liquor which has passed through only six other tanks. The eighth leaching of this bark is made with fresh water, after which the bark is dumped and discarded. Fresh water is used to leach only the most nearly exhausted bark. As the liquor becomes stronger in tannin, it is run onto fresher bark, and finally onto the previously unleached bark. As soon as each tank is dumped, it is again filled with fresh bark and becomes the head vat in the cycle, which is continuous. The object of this system of leaching is to get final liquors as concentrated as possible. In the tannery, the liquor in the storage tank is used as needed, but in the extract plant it is necessary to evaporate off most of the water so as to make its subsequent transportation practical. The extraction of the raw material is often facilitated by the use of mechanical devices. Sometimes the leaching tanks are equipped with mechanical stirrers or with pipes for bubbling air up through the liquor. In another system, the tanks are replaced by revolving drums, used on the same principle as the open vats, the liquor being pumped from one drum to another. In still another system, the bark, or other material, is forced through a trough in one direction, by means of a screw conveyor, while water flows over the bark in the 208 THE CHEMISTRY OF LEATHER MANUPACl Ure opposite direction. At the point of entry of the fresh water, the bark is practically exhausted and is dumped onto a pile from which it is subsequently moved to the furnaces for fuel, or is disposed of in some other way. At the point of entry of the bark, the liquor is richest in tannin and is conducted to the storage tank. In many extract plants, autoclaves are employed in order to leach the bark under pressure, which increases the yield obtained. The liquor is pumped from one autoclave to another, just as in the open vat system. In a system only recently devised, the raw materials are leached in autoclaves under a vacuum. ‘The advantage claimed for this is that the liquor may be kept boiling at a very low temperature, giving increased yields, but not at the expense of the quality of the ex- tract. The relative merits of the pressure and vacuum systems will probably be brought out more clearly when they have been more thoroughly investigated. Effect of Temperature. The rate at which tannin can be extracted from the raw material increases with the temperature of the water used, but so also does the rate at which the dissolved matter decomposes. The variation of the ratio of these two rates with temperature determines the optimum temperature that it is desirable to employ and this is different for different materials. It is customary to extract the fresh material at a low temperature and to increase the temperature of extraction until the material is practically exhausted. In using the open vat system for ordinary barks, it is a good plan to have the fresh water at the boiling point and to allow its temperature to fall slowly to about 60° C. as it passes over fresher bark. The temperature of the liquors can be controlled by having suitable heating coils placed in the tanks just under the false bottoms. : Effect of Hardness and Alkalinity of the Water. When a very hard, alkaline water is used in leaching, the tannin yield is very low and the extract is dark in color and of poor quality. This has been the subject of numerous investigations, from which the general conclusion has been drawn that the use of a soft water in leaching is imperative. But the recent work of Wilson and Kern seems to indicate that the question of hardness of the water used is of less importance than the pH value of water and liquor. Effect of pH Value on the Color of Tan Liquors. Wilson and Kern ° made a special study of the effect of pH value on the color of gambier and quebracho liquors. Two tan liquors were prepared, one from gambier and the other from quebracho extract. To 5 The Color Value of a Tan Liquor as a Function of the Hydrogen-Ion C i J. A. Wilson and E. J, Kern. J. Ind, Eng, Chem. 13 (1921), 1025. : oT eee VEGETABLE TANNING MATERIALS 209 each was added sufficient phosphoric acid to bring the pH value to 2. 5 as determined by the hydrogen electrode. The phosphoric acid was added to act as a buffer in preventing large changes in pH value upon long standing. To equal portions of each, sodium hydroxide was added to give series of tan liquors ranging in pH value from 3.0 to 12.0 and all having a tannin content of 1 per cent, as deter- mined by the Wilson-Kern method, to be described in the next chapter. The gambier series varied in color from light straw at 3.0 to a very deep red at 12.0. The quebracho series was similar in color excepting that the liquors of lower pH value had a touch of violet. Either series suggested a standard series of colors such as is used in the indicator method of determining hydrogen-ion concentration, except for the fact that a light precipitate formed in all liquors having a pH value of 4.0 or less. The difference in color was evidently a true indicator effect, for any member of one series could be made to match any other member simply by bringing it to the same pH value. All members of either series appeared practically identical when brought to a pH value of 3.0. This complete reversibility of color change, however, was not found when liquors at higher pH values were allowed to stand long exposed to air. Effect of pH Value on the Oxi- _ dation of Tan Liquors. Two complete series of each ex- tract were poured into test tubes; the tubes of one series of each were tightly stoppered, while the others were left open to the air. Next day the liquors in the stop- pered tubes showed practically no change, but the others had become darker in color, the more so the higher the pH value. When the liquors in a series not exposed to air were all brought to a pH value of 3.0, they all assumed practically the same color. But when those of a series that had been exposed to GAMBIER QUEBRACHO Cubic Centimeters of Precipitate from 100 cc, Tan Liquor After Bringing pH Value to 3.0 air were all brought to 3.0, they 4.5.69 "By S09 33 did not assume the same color, but PH ence tien were darker the higher the pH : ; _ Frc. 86—Showing how tendency of a value during the period of reoe tan liquor to form a precipitate when sacle to air; furthermore a pre- brought to a pH value of 3 varies cipitate settled out from those with its pH value during a period of whose pH values had been in the exposure to air. vicinity of 9. ; This precipitate formation is very curious. A complete series of each extract was allowed to stand exposed to air in shallow dishes for 3 days; the liquors were then made up to original volume and poured 210 THE CHEMISTRY OF LEATHER MANUFACTURE into 100-cubic centimeter graduate cylinders. Each was brought to a pH value of 3.0 by the addition of hydrochloric acid and allowed to stand over night. Next day the volume of precipitate from 100 cubic centimeters of original liquor was read from each cylinder. The results are shown in Fig. &6. Keeping a solution of either extract exposed to air while its pH value is 9 causes it to yield an enormous precipitate when its pH value is subsequently brought to 3.0. But keeping it exposed to air when its pH value is greater than 10 apparently prevents its precipita- tion when brought to 3; all such liquors remained brilliantly clear. The addition of a great excess of acid, however, caused all liquors to precipitate, while any precipitate could be completely redissolved by th addition of sufficient alkali. | Another interesting fact is that the liquors exposed to air when their pH values lay between 8 and 9 gave much trouble with the hydrogen electrode. After bubbling hydrogen through them for only a few minutes, the voltage would fall rapidly towards zero. Even when brought to a pH value of 3.0, the liquors still gave this trouble, making it necessary to check the results by means of indicators. No such trouble was encountered with liquors exposed to air at pH values below 7 or above 10. Apparently pH —9 is a critical point in the oxidation of tan liquors. The curves in Fig. 86 show that this effect of oxidation is ap- preciable at all pH values from 6 to about 10. Most hard waters have pH values lying within this range and many of them have pH values higher than 8. : Effect of pH Value on the Precipitation of Tan Liquors. Wilson and Kern ® also studied the effect of pH value on the pre- cipitation of quebracho liquors. Four series of solutions of solid quebracho extract were prepared according to the official method of the American Leather Chemists Association,’ except for the additions of sulfuric acid, hydrochloric acid, sodium hydroxide, and calcium hydroxide, respectively, to the four series to produce approximately the desired pH value before making each solution up to the re- quired volume. The pH values were finally determined at 20° C. by means of the hydrogen electrode and the solutions were analyzed according to the official method. The effect of the added acid of ne upon the per cent of insoluble matter found is shown in ig. 87. The solution receiving no addition of acid or alkali had a pH value of 4.60. As the pH value was lowered from this, by the addition of either sulfuric or hydrochloric acid, there was an increase in the per cent of insoluble matter found, sulfuric acid proving the more effective in causing precipitation. With increasing pH value, there * Effect of Hydrogen-Ion Concentration upon the Analysis of Vegetable Tanning Ma- terials. J. A. Wilson and E. J. Kern. J. Ind. Eng. Chem. 14 (1922), 1128. ; 7J. Am, Leather Chem. Assoc. 16 (1921), 113. VEGETABLE TANNING MATERIALS 211 was first a decrease in the amount of insoluble matter and the un- filtered solution gradually became more nearly transparent. In the case of the liquors containing sodium hydroxide, this continued with- out a break, the liquor having a pH value of 11.35 being quite trans- parent. But at the neutral point, an abrupt change occurred in the solu- tions containing calcium hydroxide; with further rise in pH value, the tannin was precipitated in increasing amounts. If these data may be applied quantitatively to raw tanning ma- terials in general, it is evident that the precipitation of tannin by lime may be prevented by keeping the pH value of water and liquor, dur- ing extraction, under 7. But to avoid appreciable oxidation effects, the material should not be ex- tracted at pH values greater than 5, which may be accepted tentatively Added Acid Added Alkali - Original Extract) Insoluble Matter (Pero ent 5 2 4 as the optimum pH value for leach- pH Value of ‘Tan tiqnees ing, since, with decreasing values rie 98 pe ; 8 ' f Fic. 87.—Effect of pH value on per ere 1S an increasing amount o cent of insoluble matter in solution material precipitated. Where only of quebracho extract. hard water is available for leaching, it would seem the part of wisdom to add to it, before using, a sufficient quantity of acid to lower its pH value to 5. Clarifying, Decolorizing and Drying. In the manufacture of tanning extracts for sale, it is desirable that the extract should be clear, have a good color, and be dried to a degree sufficient to make handling and shipment easy. Clarifica- tion, which consists merely of the removal of finely divided matter in suspension, is effected by settling and decantation, by filter-pressing, or by centrifuging. Where the extract manufacturer has carried the extraction of the raw material nearly to the limit, the extract is apt to have a dark color, which is-not desirable. This seems to be due to the extraction of foreign matters at the high temperatures used, or, in some cases to oxidation. A common method of clarifying and decolorizing some ex- tracts is bymmeans of blood albumin. The tan liquor is treated with a solution of blood albumin and then heated to a temperature of 70° C., at which the albumin coagulates and carries down with it the suspended matters, some of the deeply colored bodies, and some tannin. The clear liquor is decanted off and the sludge is filter-pressed to re- 212 THE CHEMISTRY OF LEATHER MANUFACTURE cover the adhering tan liquor. Although some tannin is lost in this way, the color of the extract is greatly improved. A number of other methods of decolorizing involve the treatment of the tan liquor with chemicals.. Sulfur dioxide and sodium bisul- fite are often used. Some brightening of the color would naturally be expected from the lowering of the pH value of the liquor by sul- fur dioxide, but the total effect seems to be more complex than this, | since some of the suspended and difficultly soluble matters are thereby rendered soluble. Apparently the reducing action of sulfur dioxide plays: aapart... 4 There are naturally numerous methods in use for drying extracts. Since high temperatures and contact with the air during drying are undesirable, much of the drying is done in specially constructed vacuum dryers. As these have been greatly improved, from time to time, it has become possible to dry extracts to greater extents without causing them to suffer any damage. Formerly it was customary to reduce the water content of most extracts only to from 50 to 60 per cent, but now it is not uncommon to find extracts on the market having a water content as low as Io per cent. Chapter 12. The ‘Tannins. Some idea of the volume of literature which has appeared dealing with the composition of tanning materials may be gained from the bibliography, compiled by Dean,’ of the more important papers pub- lished prior to 1910, which lists 273 papers. It is remarkable that the greatest work on the organic chemistry of the tannins was accom- plished by the same man who did most to elucidate the complex struc- ture of the proteins, Emil Fischer. Among the numerous papers by Fischer and his coworkers, telling of their work which led to the discovery of the composition of tannin, may be mentioned one en- titled “Synthesis of Depsides, Lichen-Substances and Tannins,” 2 which is something in the nature of a review. The tannin studied by Fischer was that obtained from nutgalls, the so-called gallotannic acid and purest form of pyrogallol tannin. As early as 1852, Strecker * concluded that tannin was a compound of glucose and gallic acid. He was supported by the works of van Tieghem,* who found glucose among the hydrolytic products of tannin, and Pottevin,? who effected the hydrolysis with the enzyme of Aspergillus niger. But the variation in proportion of glucose found weakened the view, which gave way to that of Schiff,* who regarded tannin as digallic acid: Be ee OOF! Ga SCOOS. S ee oO HO COOH. Although Schiff’s formula for tannin was widely accepted, it was shown very definitely that digallic acid is not tannin. The formula showed no asymmetric carbon atom in the molecule to account for the optical activity of the natural tannin and it could not account for the high molecular weights observed. By observing the electrical conductivity, hight absorption, and behavior towards arsenic acid, Walden? showed that Schiff’s digallic acid is very different from natural tannin. » Fischer and Freudenberg * first set out to determine whether the 'On the Composition of Taaning Materials; Bibliography 1828-19009. A. L. Dean. le Am. Leather Chem. Assoc. 6 (1911), 172. ; * Emil Fischer, Ber. 46 (1913), 3253. J. Am. Chem. Soc. 36 (1914), 1170, SA, Strecker. Ann. 81 (1852), 248; 90 (1854), 328. *P. van Tieghem. Annal, d. Sciences naturelles V. Serie Botanique (1867), 210. SH. Pottevin. Compt. rend. 132 (1901), 704. Pet ecu. er,-4 (1871), 232; 967; 12. (1879), 33. TP. Walden. Ber. 30 (1897), 3151; 31 (1898), 3167. 8 E. Fischer and K. Freudenberg. Ber. 45 (1912), 919. 213 214 THE CHEMISTRY OF LEATHER MANUFACTURE glucose found by Strecker was really a constituent or only a chance impurity of tannin. They started with the purest technical tannin available. Assuming that the tannin molecule had no carboxyl group, they proceeded to separate it from acid impurities by rendering its solution slightly alkaline and extracting it with ethyl acetate, a method discovered independently and published previously by Paniker and Stiasny.° As they had anticipated, the tannin dissolved in the ethyl acetate, leaving the sodium gallate in the aqueous solution. They accepted this as proof that the tannin possessed no free carboxyl group. Applying this method of purification to different kinds of commercial tannin, they obtained products that were practically identical. After hydrolyzing the purified tannin with sulfuric acid, they found between 7 and 8 per cent of glucose. In the purest sample of tannin examined, they found one molecule of glucose combined with ten molecules of gallic acid. No phenolcarboxylic acid other than gallic could be found in tannin, even when the hydrolysis was effected by means of alkali. With excess of alkali and exclusion of air, large yields of alkali salt of gallic acid were obtained in relatively pure condition. It appeared to Fischer that the surest way to prove his assump- tions regarding the structure of tannin was to synthesize it. He started out with the idea that tannin contains no carboxyl and that, conse- quently, the gallic acid must all be bound as an ester, a condition that would be fulfilled by regarding tannin as an ester-like combination of one molecule of glucose with five molecules of digallic acid, after the manner of pentacetyl glucose. The investigations of Fischer and his collaborators are so extensive as to require treatment in a separate volume and the reader is referred to the recent book by Freudenberg,’® who is continuing Fischer’s work on the tannins. Fischer ‘* succeeded in preparing penta-m-digalloyl- B-glucose, which was proved to be an isomer of the tannin from Chinese nutgalls. The formula for the so-called gallotannic acid may thus be written NE ce eee NG hE ‘ ie emie der Nattirlichen Gerbstoffe. K. Freudenberg. J, Springer, B li 1 E, Fischer and M. Bergmann. Ber, 51 (1918), 1760. 8 Je Spree (1920). THE TANNINS 215 Freudenberg has suggested a classification of the tannins more dis- tinctive than the catechol-pyrogallol system mentioned in Chapter 11. He would divide them into two main classes, the first consisting of hydrolyzable tannins in which the benzene nuclei are united to larger complexes through oxygen atoms, and the second of condensed tannins, in which the nuclei are held together through carbon linkages. Where both kinds of compounds are present, as in ellagic acid, the classifica- tion is decided by the genetic connection with other tannins. The first group embraces three classes: (1) depsides, esters of phenolycarboxylic acids with each other or with other oxyacids; (2) the tannin class, or esters of phenolcarboxylic acids with polyatomic alco- hols and sugars; and (3) glucosides. The most important criterion of the first group is hydrolysis to simple components by enzymes, particularly tannase or emulsin. Freudenberg and Vollbrecht?? have recently discussed the isolation and determination of the activity of tannase, which is secreted by Aspergillus niger. The second group of tannins are not decomposed to simple com- ponents by enzymes. They are generally, but not always, precipitable by bromine and condense to amorphous tannins, or reds, of high molecular weight, when treated with oxidizing agents or with strong acids. They are divided into two classes according to whether or not phloroglucin is present. With the exception of some simpler ketones, oxybenzophenones and oxyphenylstyrylketones, the catechins belong to the phloroglucin class, which include the tannins of quebracho and probably also those of oak bark. That the reds, or phlobaphenes, precipitated by acid from solu- tions of quebracho and gambier extracts are oxidation products is indicated by the curves in Fig. 86, which show that the quantity of precipitate obtained is greatly increased by previous oxidation. The actual composition of the phlobaphenes is not yet known. The ellagic acid, or bloom, formed in solutions of tanning extracts of the pyrogallol group, is of very much simpler composition than the phlobaphenes. The formula aN sa wk Jo.co-h z OH for ellagic acid, suggested by Graeb,'* is the most satisfactory thus far proposed. Practical Definition of Tannin. The great classical work on the structure of the tannins is still too far from complete to enable one to apply organic chemistry to practical tanning, excepting, perhaps, in the study of the reactions 12 Z. physiol. Chem, 116 (1921), 277. 1% Chem, Ztg. (1903), 129. 216 THE CHEMISTRY OF LEATHER MANUFACTURE of particular groups present in the tannin molecules. The struc- tures of the tannins of the catechol group are still entirely unknown. As in the case of the proteins, it has been found necessary to deal with the general properties of the tannins from the standpoint of physical rather than organic chemistry. All tannins seem to have the property of precipitating gelatin from solution and of combining with the protein matter of hide. fibers, forming a compound resistant to washing. Any natural vegetable material having this property in aqueous solution has generally been accepted as tannin, and this has been made the basis for the various methods of determining tannin now in use. The portion of soluble matter which neither combines with collagen to form a compound resistant to washing nor precipitates gelatin from solution is known as nontannin, 5 The Gelatin-Salt Test for Tannin. In testing a solution for the presence of tannin, it is customary to add to it one drop of a solution made. by dissolving 10 grams of gelatin and 100 grams of sodium chloride in a liter of water, a precipi- tate or turbidity indicating the presence of tannin. This reaction has been the subject of numerous investigations for more than a century. Its sensitivity as a means of detecting tannin in solution has recently been studied by Thomas and Frieden.1* They found that the added gelatin is completely precipitated when the ratio of gelatin to tannin does not exceed 0.5; a great excess of gelatin prevents precipitation. Thomas and Frieden studied the precipitation of tannin by gelatin at different pH values and concentrations of salt. Using a gelatin solution containing no salt, they obtained a maximum precipitation of gallotannic acid, in pure solution, at a pH value of 4.4; at pH values below 4 or above 5, the solutions became opalescent, but no precipitate formed. The effect of adding sodium chloride was to widen the range of pH value over which a precipitate was obtainable: it apparently had no effect upon the sensitivity of the test between the pH values 4 and 5. Using various commercial tanning extracts, they found that the optimum range for precipitation of tannin by gelatin varied from 3-5 to 4.5, quebracho, wattle, and hemlock precipitating most readily at pH values slightly above 4.0 and gambier, oak, and larch at values slightly below 4.0. The limits of dilution at which tannin could be detected by means of the gelatin-salt reagent were found to depend upon the proximity of the solution to the optimum pH value for precipitation, which is different for each kind of extract, but apparently always lies between 3-5 and 4.5. At the optimum pH value, gambier, the least sensitive to the test, could be detected at a concentration of 1 part of tannin to I10,000 parts of water. Wattle, the other extreme, could be de- tected at a dilution of 1 to 200,000. When the commercial extracts ** The Gelatin-Tannin Reaction. A. W. Thomas and A. Frieden. Ind. Eng. Chem (1923); (advance copy). ‘ : THE TANNINS 217 were simply diluted with distilled water, no attention being paid to the final pH values, the sensitivity of the tests was greatly de- creased. ‘The least sensitive was then hemlock at 1 part in 6,500 and the most sensitive was gambier at I part in 30,000. They also found that the age of the gelatin-salt reagent has no effect on the sensitivity of the test, provided bacterial action is prevented by means of toluene. The Determination of Tannin. Although a general discussion of analytic methods is outside the scope of this book, the question of determining tannin demands some attention here because of its importance in leather chemistry and the fact that the methods in common use do not determine the actual tannin content of tanning materials, but include as tannin a variable fraction of nontannin, which, in the extreme case of gambier, is twice as great as the tannin content itself. For more than a century leather chemists have struggled with the question of determining tannin and numerous methods have been proposed. Of these, the only one which has really survived is that known as the hide powder method. But even this is used in different parts of the world with different modifications. For a review of the various methods proposed up to 1908, the reader is referred to Procter’s book.*® It will serve our purpose here to give an outline of the official method of the American Leather Chemists Association, which is similar in principle, although not in all details, to those employed in various parts of Europe. A. L. C. A. Method '* “American Standard” hide powder is specially prepared by giving it a light tannage with chrome alum, washing it practically free from soluble matter, and squeezing it until it contains not less than 71 nor more than 74 per cent of water. The solution of tanning material for analysis must contain not less than 0.375 nor more than 0.425 gram of tannin per 100 cubic centimeters, as found by this method. To 200 cubic centimeters of this solution is added such an amount of the wet hide powder as contains not less than 12.2 nor more than 12.8 grams of dry hide powder and the whole is shaken for 10 minutes. The detannized solution is separated from the powder by squeezing through linen and is then filtered through paper, after the addition of kaolin, the solution being returned to the paper until the filtrate is quite clear. The amount of residue from an aliquot portion of this filtrate, after correcting for the water introduced by the hide powder, is taken as a measure of the nontannin in the original material. The difference between the total soluble matter and the nontannin is called #® Leather Industries Laboratory Book. H, R. Procter. E. & F.N. Spon, London (1908). 46 For further details, see J. Am. Leather Chem. Assoc. 16 Ci9Z2T), Tres: 218 THE CHEMISTRY OF LEATHER MANUFACTURE tannin. ‘The other determinations of the method need not concern us here. 3 It will be apparent from the discussion of the equilibria of pro- tein systems in Chapter 5 that the method involves two false as- sumptions: one that the hide powder combines only with tannin; the other that the solution absorbed by the collagen jelly has the same concentration as that in the surrounding solution. It may be men- tioned that the former assumption introduces errors vastly greater than the latter. As long ago as 1903, Procter and Blockey 1” showed TABLE XVII. RESULTS OF TREATMENT OF PuRE GALLIC Acrtp Sotutions By A. L. C. A. Metuop. (Using 47 grams of wet hide powder (73 per cent water) to 200 c.c. of solution.) Gallic Acid Nontannin Tannin Grams per liter Per cent Per cent 8.88 54.0 46.0 4.44 47.1 52.9 2.22 43.8 56.2 EG 40.4 59.6 TABLE XVUL EFFECT OF ALTERING ProporTION oF Hinze Powper upon AMountT oF GALLIC AcIp REMOVED FROM A 0.888-PER CENT SOLUTION. (Using principle of A. L. C. A. method.) Wet Hide Powder (73 per cent water) Nontannin Tannin Grams per 200 C.c. Per cent Per cent 5 91.8 8.2 10 86.0 14.0 25 69.6 30.4 50 52.1 47.9 75 43.7 56.3 that hide powder removes from solution considerable amounts of such nontannins as gallic acid, quinol, and catechol. Wilson and Kern 18 showed this even more strikingly by subjecting pure solutions of gallic acid to the A.L.C.A. method of tannin analysis. By varying the concentration or the proportion of hide powder, practically any results desired could be obtained. Tables XVII and XVIII show that the A.L.C.A. value for tannin decreases with increasing concen- tration of the solution and increases with the proportion of hide powder. Using a solution of I gram of gallic acid per liter, the method indicates a tannin content for the sample of about 60 per cent, even though it contains none at all. _™ Absorption of Non-Tanning Substances by Hide Powder and Its Influence on the Seana: of Tannin. H. R. Procter and F. A, Blockey. J. Soc. Chem. Ind. 22 (1903), 2 7®The Nontannin Enigma. J. A. Wilson and E. J. Kern. J. Am. Leather Chem. Assoc. 13 (1918), 429. Pa hei an NSS 219 Wilson-Kern Method. With the object of avoiding the palpable errors of the A.L.C.A. method, Wilson and Kern !® set out to devise a method that would determine exactly what is called for in the practical definition of tannin, namely, that portion of the soluble matter of vegetable tanning materials which will precipitate gelatin from solution and which will form compounds with hide fiber which are resistant to washing. The principle of their method is to shake a convenient amount of the tannin solution with a known quantity of purified hide powder until all tannin has been removed from solution, as determined by the gelatin-salt test. The tanned powder is then washed free from soluble matter including the nontannin removed from solution by the hide powder, which is responsible for the large errors in the A.L.C.A. method. It is then carefully dried and analyzed for tannin as in the regular procedure for vegetable-tanned leathers, and from this figure the per cent of tannin in the original material may readily be calculated. In order to show the workability of this method, Wilson and Kern selected 8 typical tanning materials showing great differences in prop- erties, especially in so-called astringency.. The solid quebracho ex- tract and the four liquid extracts of oak bark, larch bark, chestnut wood, and osage orange are typical samples of the best of these ma- terials on the American market. The gambier is the ordinary pasty product from the East Indies; the sumac, consisting of ground leaves and small twigs, is from Palermo; and the hemlock bark came from the forests of Wisconsin. The extracts were simply dissolved in hot water, cooled slowly, and made up to the mark. The bark and sumac were finely ground and leached by percolation, only the extracted por- tions being used after making up to a definite volume. In each teser ie grams of hide powder (of known hide substance content) were put into a wide-mouthed, rubber-stoppered, half-pint bottle, the tanning ma- terial dissolved in 200 cubic centimeters of solution was added, and the whole was shaken in a rotating box for 6 hours. The amount of material that could be used was limited by the amount of tannin that the hide powder was capable of taking up in 6 hours. On the other hand it was desirable not to use too little, since the less the amount of tannin fixed per unit of hide sub- stance,. the less the accuracy of the method, since the tannin was determined by difference. Whenever the liquor, after the 6-hour shak- ing, gave a turbidity or precipitate with the gelatin-salt reagent, the test was repeated with less material. The tanned powder was washed by shaking with 200 cubic centi- meters of water for 30 minutes, squeezing through linen, and re- peating the washing operation until the wash water showed no color and gave no test with ferric chloride solution. Nontannins like gallic acid give a dark coloration upon the addition of ferric chloride solu- tion. Except for the osage orange and chestnut wood extracts, which % The True Tanning Value of Vegetable Tanning Materials. J, A. Wil ; Kern. J. Ind. Eng. Chem, 12 (1920), 465. J Met. B20, THE CHEMISTRY OF LEATHER MANUFACTURE rACarA 4 Sv ZV gz LV Judd Jog jel1oqe yy Ul UIUURL [elioyeyl S°9S grr £46 Bee, v'Z6 oS°Z ZS 60°Z St 0) zVvg o°S6 zoel SEZ 9o°Z S611 G2 it £641 QS°Z1 LIZ SS°9 £28 OS"O1 O' IST QeSI 6'°9v 2Z'$ £28 S¢ OI QILI CA 69h £z'9 9°28 OV'II QIeI £291 I'Zv1 9z'6 O°161 £611 6°£gz O11 Qe PZo1 Q'9f or ZI Q'9£ 6£°Z1 sueIyy eile 60°S I1‘9 LL 29 €s°6 62'S e338 che 06°9 Ze'R SQ'Il oS'h 90°8 QS'II go's z1'0 ISI gE°Z L£v'6 OI! ff°9 OI'e1 Coe £6°18 ge"gZ zo'bZ ¥Z0g VS QZ g452 $9'9Z 06°22 1G'SZ 00°Ig gf 62 gL bl £S°62 6£°6Z VS'9Z ace ate So SZ ZOVL te Qz'0 oo mee cz'o cz’0 Z1°0 Zo0 itome) gto Iz’0 61°0 coo ozo of’o €1°o vo vz'0 z1'0 9z'0 Qz'0 vz'o of'o ceo cf’o sueiny (s0Ualayip (zoo XN) Wey pesn punoy Aq) uluueyz souerqsqus A Bee urluue |, QoUeSGNS spiPFT JO ‘8 OOI Jog JapMOg oply{ pouuey, Jo siskjeuy os8ejus0I9g T10 Qz0 glo Z1'0 60°0 £1'0 zI‘O £z'0 (oh xe) OI'O £10 Ilo g0°0 60°0 60'0 vee) 60'0 Zoo coo £1'o zZI'0 £0'0 g0°0 VIO ysV “XIX HTAVL go'el / Ape | 30°71 ara €Q7ZI ZQ‘ZI SLEi Qzz1 OL II Sorat Z7QTI Cv'zi zSgI SgeI OS ‘ZI eer 6I'II PS‘zI 9L7Z1 £201 v6°6 IQ‘f1 a (gah fe! QS‘II Joye £92 Sree 3'°gr py Ae S"z9 eee) SLE o's ¢°Z9 o'Sz ocy 5°L9 o'Sc o'sv 40) oSZ OOO! o'oSI Clr Sst 8'8I Joy tod suIeIs [elIoyey Sie SU Pee Aan ee ages mre era) ee eee JoIqurer) ec eote ec eee eee eee ee sane JoIquier) C9 Ege Se Pins 8 Oe 8:70) a18-79 (816 ce JOIquier) ° eoaeeees ee eer oe ces eeoee eee IsUueIO, adesCO IsUeICQ) aseS_O asUueIO asesoO eee e eee deuUINS eer eeeee deUINS eevee aoe oeuns poomM ynuysoyy poo, yuysey7y pooM ynuysoy’) -st55* yaeg yore] sees: yea yoreq Nines. Slee errr ans yieg YoieT vseeeees yp ¥eO tseseees yea ¥eO seeeeee reg ¥eO eee eeree ee eee eee ereeeeeee es ere ere ee ee eee eee ee ee ee ore ee ylegq 3oopwoyy yiegq yoorwopy yleg yoojwopy “se oyseiqon¢g oeee oyseiqon(¢) oyseiqong¢ [elioye yy THE TANNINS 221 are unusual in several respects, not more than 12 washings were re- quired to free the powders from nontannin, which shows that the line of demarcation between tannin and nontannin is fairly sharp for the commoner materials. The wash water continued to extract coloring matter from the powders tanned with osage orange until after the fiftieth washing, while as many as 25 washings were required to free the powders tanned with chestnut wood from soluble matter producing a dark color with ferric chloride. All wash water was tested with the gelatin-salt reagent, but in every case the test was negative. The washed powders were dried at room temperature for 24 hours or longer and then analyzed for water, ash, fat and hide substance. The per cent of hide substance was taken as the per cent of nitrogen multiplied by 5.62. The difference between 100 and the sum Oietie percentages of water, ash, fat and hide substance was taken as the per cent of tannin in the tanned powder. The parts of tannin per 100 parts of hide substance divided by the parts of tanning material used per part of hide substance gave the per cent of tannin in the original material. The results for the 8 materials examined are given in triplicate in Table XIX. Comparison of A. L. C. A. and Wilson-Kern Methods. The two methods just described give very different results. A careful comparison is therefore desirable, especially since it will assist in giving a better understanding of the vegetable tanning process and of what is ordinarily called tannin. It should be remembered that the great majority of tannin values quoted in the literature were obtained either by the A.L.C.A. method or by some method based upon similar principles. The Wilson-Kern method is still too new to have found general acceptance. TABLE XX. Percentage Analysis of Material Per- Wilson- centage A. L. C. A. Method Kern Error in Insoluble Soluble Matter Method A.L.C.A Material Water Matter Nontannin Tannin Tannin Method CHeBIACHO 55... 17.87 ma6 6.96 68.01 47.41 43 Hemlock Bark.... . 8.90 74:33 6.71 10.00 6.17 63 De a Sie rn 52.66 3.68 19.46 24.20 12.88 88 Panenusatk... shows how the determination of tannin, by both the A.L.C.A. and the Wilson-Kern methods, is affected by change of pH value. The latter method gives a practically constant value over the wide range 3.6 to 7.3. Where the falling off in per cent of tannin occurs at pH values higher than 7, indicated by the broken line, the results should not be con- sidered as found by this method because in each case the residual solu- tion gave a test for tannin, by the gelatin-salt test, whereas the method specifies that the determination is to be discarded whenever such a test is *4 Influence of Degree of Acidity on the Tannin Content of Solutions. F. C. Thompson, K. Seshachalam, and K. Hassan. J. Soc. Leather Trades Chem. 5 (1921), 380. °° Effect of Hydrogen-Ion Concentration upon the Analysis of Vegetable Tanning Ma- terials. J. A. Wilson and E. J. Kern. J. Ind. Eng. Chem. 14 (1922), 1128. 2 4 6 8 10 pH Value of Tan Liquor THE TANNINS 23 obtained. The values obtained were included in the curve in order to show the effect of pH valtte on the rate of tanning. 7 Modified Wilson-Kern Method. Im order to meet the demand for a simpler method, Wilson and Kern 2° modified their method as follows: Standard hide powder is further purified by washing with water to free it from soluble matter, then dehydrating with alcohol, then soaking in two changes of xylene, and then drying. The tan liquor is filtered, as in the A.L.C.A. method, and only the soluble portion used, 100 cubic centimeters being shaken with 2 grams of purified hide powder for 6 hours. The tanned powder is allowed to wash over night in a specially designed percolator and is then dried and weighed. The increase in weight of the dry powder represents the weight of tannin in the roo cubic centimeters of solution used. Wilson and Kern compared the modified and original procedures of their method and found that they give practically identical results for all ordinary extracts. For further details, the original papers should be consulted. Potential Difference of Tannin Solutions. In Chapter 5 it was pointed out that the stability of a colloidal dispersion is determined less by the absolute value of the electrical charge on the particles than by the electrical difference of potential between the film of solution wetting the particles and the bulk of the surrounding solution. In the Procter-Wilson theory of tanning, to be discussed in Chapter 13, the astringency of a tan liquor in prac- tice is assumed to be a function of the potential difference between the solution immediately in contact with the tannin particles and the bulk of the tan liquor as well as of the potential difference between the tan liquor and the collagen jelly. Grasser *’ studied the electro- chemistry of tannin solutions, but obtained confusing results of rather doubtful value, which may be due to his failure to control or measure the hydrogen-ion concentrations of the liquors. Thomas and Foster *° were more successful. Using the U-tube elec- trophoresis method described by Burton,” they succeeded in measur- ing the potential differences of tannin solutions under different con- ditions. Table XXVIII shows a series of values obtained for tan liquors made from 8 typical tanning materials. It is interesting | to find gambier, the mildest tanning material, with the lowest potential difference and quebracho, the most astringent, with the highest poten- tial difference. The order of decreasing conductivity of these solutions 26"Tne Determination of Tannin. J. A. Wilson and E. J. Kern. J. Ind. Eng. Chem, 13 (1921), 772. 5 : : 27 Electrochemistry of Tannins. G. Grasser. Collegium (1920), 17, 49, 277, 332. t 28 The Colloid Content of Vegetable Tanning Extracts. A. W. Thomas and S. B. Foster. J. Ind. Eng? Chem, 14 (1922), 191. 28 Physical Properties of Colloidal Solutions. E. F. Burton. Longmans, Green & Co., London (1916). 232 THE CHEMISTRY OF LEATHER MANUFACTURE was sumac, gambier, oak bark, larch bark, hemlock bark, chestnut wood, osage orange, quebracho. It is evident that the potential differ- ence is not a simple function of the conductivity, but is influenced by the kind as well as the amount of electrolyte present. TABLE XXVIII. PoTENTIAL DIFFERENCES OF TANNINS FROM DIFFERENT SOURCES. Grams Potential Total Soluble Difference Extract Matter per liter volts Gambler s(Cubeyts ccs. wash ee se oe ee 18.7 — 0.005 OakeDatiasce cere calcaneus ee 17.0 — 0.009 Ciresiniutewood-thrsh ies ono 17.8 — 0.009 Hemlockmoat Kit, o. tek cee 16.7 — 0.010 SUMACT. + ictal buss «lia pee see 19.6 — 0.014 BALCH SHAT Rawat sa he ak ale ae 19.5 — 0.018 (Jsape ornige 7 tet ae ee 13:7 — 0.018 (?) Onebracho7= 2. oo vee ee eee 11.0 — 0.028 If the absolute value of the electrical charge on the particles re- mains constant, according to the theory given in Chapter 5, the potential difference at the surface should decrease with increasing concentra- tion of electrolyte, or increase with decreasing concentration. Thomas and Foster found that the potential difference of solutions of quebracho extract actually does increase with decreasing concentration, as shown in Table XXIX. The addition of acid decreases the value of the potential difference by lowering the absolute value of the electrical charge, which holds true for negatively charged dispersions in general. This is shown in Table XXX. TABLE? ix PoTENTIAL DIFFERENCES OF SOLUTIONS OF QuEBRACHO EXTRACT, Concentration Potential Grams Dry Solids Difference per liter volts 32 — 0.024 16 — 0.028 8 — 0.029 4 — 0.030 TABLE XXX. EFFECT OF ADDITION OF ACID. (16 grams of solid quebracho extract per liter.) 0.1 N HCl added per liter Potential Difference cubic centimeters volts 9) — 0.024 10 — 0.014 15 — 0.010 20 approx. 0 The effect of dialyzing a tan liquor is to lower the concentration of electrolyte, which we should expect to increase the potential dif- THE TANNINS 233 ference. The values in Table XXXI show that this actually occurs, although part of the increase may be attributed to dilution. TABLE XXAXT EFFECT OF DIALYSIS. Potential Grams Extract Hours Final Difference Extract in 250 cc. Dialyzed Volume cc. volts OE a Ge 4 60 415 — 0.033 PUSAUEUOEATIO“ iy oic.accescceces 4 24 370 — 0.024 Oo ee 4 24 460 — 0.026 Oa SS a ll 8.2 24 390 — 0.029 BIeGMOCIeDaTK ....2........ oa 24 ove — 0.024 Isoelectric Points of the Tannins. Thomas and Foster 2° later extended their investigations in an at- tempt to determine the isoelectric points of tannins from different sources. The various tanning extracts were dissolved in a citrate buffer mixture having a pH value of 2.0 and the solutions were finally adjusted to the desired pH values by means of the hydrogen electrode. The buffer was apparently necessary to eliminate, or delay, the sec- ondary actions, such as diffusion of the boundaries and change of re- action of the extracts due to electrolysis, which behavior had nullified previous experiments. Between the pH values 2.5 and 2.0, the direction of migration of the tannin particles changed from anodic to cathodic in solutions of the extracts of oak bark, hemlock bark, wattle bark, sumac, and gambier. In the case of quebracho, there seemed to be no movement in the U-tube at the pH values 3.0 or 2.5, but at 2.0 the movement seemed to be slightly cathodic. Quebracho was precipitated by the buffer and only the clear, supernatant liquor could be used, which may account for the inability to obtain more definite results. Until they are located more definitely, the isoelectric points of the tannins may be accepted as lying between the pH values 2.0 and 2.5, at least those of hemlock, oak, and wattle barks, sumac, and gambier. Precipitation of Tan Liquors. In the hope of throwing some light upon the colloidal nature of the tannins, Thomas and Foster studied the action of various electrolytes upon a great variety of tan liquors. Aqueous solutions of different tanning extracts were made up so that 100 cubic centimeters of solu- tion contained 4 grams of solid matter. The solutions were made at 85° C., cooled to 25°, and then adjusted to final volume. The stock solution was then centrifuged for 5 minutes at 1000 times gravity in 80 The Electrical Charge of Vegetable Tannin Particles. A. W. Thomas and S. B. Foster. Ind. Eng. Chem. (1923); (advance copy). 234 THE CHEMISTRY OF LEATHER MANUFACTURE order to throw down coarse suspended matter. Portions of 25 cubic centimeters were put into 100-cubic centimeter, graduated oil tubes. Then 25 cubic centimeters of the electrolyte were added, the solutions were allowed to stand for 15 to 30 minutes for precipitation to start, and were then centrifuged for 5 minutes at 1000 times gravity, The volumes of the precipitates were recorded and plotted against the concentrations of electrolyte employed. The results may be most conveniently studied by grouping them wader the names of the various electrolytes used. Each available ex- tract was not tested with all electrolytes because, in some cases, pre- liminary experiments indicated that further work would be fruitless. Monovalent Cations. Potassium chloride. Concentrations of potassium chloride from 0.02 to 4 molar gave only negligible amounts of precipitate with gambier and quebracho. Oak bark gave a gradually increasing salting out effect. Oak Bark 8) Larch Bark Hemlock Bark | Chestnut Oak Bark 4 Quebracho Vol. of Precipitate (c.c.) Vol. Precipitate (c.c.) MM MM M M M 234 6M MM MM MM M oM 10050 2510 4 2 7 10050 2010 495% Concentration of Acid Concentration of Sulfuric Acid Fic. 92.—Precipitation of Tannins by Hie 03.—Precipitation of Tannins by Hydrochloric and Phosphoric Acids. Sulfuric Acid. © Since gambier and quebracho represent extreme types of tanning ex- tracts, no further tests were made with this salt. It must be borne in mind that the solutions to which the neutral salts were added were made simply by dissolving the extracts in distilled water and had pH values in the vicinity of 4.5. Hydrochloric acid. Concentrations from 0.01 to 6 molar were used. Gambier and quebracho gave large amounts of precipitate only at high concentrations of acid and, since this was not a simple colloid precipitation, no further experiments were attempted. A salting out effect was obtained with oak bark. (See Fig. 92.) Sulfuric acid. Quebracho, hemlock bark, oak bark, and larch bark gave progressively increasing amounts of precipitate with increasing concentration of acid, as shown in Fig. 93. No precipitate was ob- tained with sumac until molar concentration was reached, when gummy THE TANNINS 238 masses were thrown down, similar to those obtained with aluminum sulfate. At 4 molar concentration, a flocculent precipitate was formed. Phosphoric acid. Gambier began to give an appreciable precipitate only at 4 to 7 molar concentration. With sumac a gummy mass was thrown out at 2 molar, as was observed upon the addition of sulfuric acid and aluminum sulfate, and at 4 to 7 molar a flocculent precipi- tate formed which left the supernatant solution almost colorless. Quebracho was progressively salted out. (See Fig. 92.) Acetic acid. Experiments with quebracho, sumac, gambier, and oak bark were run with concentrations of acid from 0.005 to 4 molar. There was no appreciable precipitation in any case. At the higher concentrations the suspended matter began to dissolve. Formic acid. Concentrations from 0.005 to 12.5 molar were used. Sumac, chestnut oak bark, larch bark, gambier, and hemlock bark oi » tae) ke (lactic) 4 ae uw UM UM UM C2 34M 6077 .20)-10 Mi an “ uebracho (formic) » Vol. of Precipitate (c.c.) «a Vol, Precipitate (c.c.) ow eo ee 300 100 BO 25 10 6 @ Concentration of Acid Concentration of Barium Chloride Fic. 94.—Precipitation of Tannins by Fic. 95.—Precipitation of Tannins Formic and Lactic Acids. by Barium Chloride. gave no precipitation up to 4 molar, at which concentration the sus- pended matter began to dissolve. Quebracho and quercitron bark were precipitated, but the precipitate redissolved at from 2 to 4 molar. (See Fig. 94.) . Lactic acid. Concentrations from 0.005 to 2 molar were employed. The effects of this acid were similar in kind, but not in degree, to those with formic acid. (See Fig. 94.) The precipitates with que- bracho and quercitron redissolved at lower concentrations of lactic than of formic acid. Since lactic is the weaker acid and since this redissolving was not found with hydrochloric or sulfuric acids, the effect must be due to chemical properties other than those of the hydrogen ion. This is an important point to consider in the chemical control of tan liquors. Divalent Cations. Barium chloride. On account of the limit of solubility, this salt was employed up to only 0.5 molar. The salting out effect is shown in Fig. 95. 236 THE CHEMISTRY OF LEATHER MANUFACTURE Calcium chloride. Concentrations up to 2 molar were used. As with barium chloride, increasing amounts of precipitate were obtained with the different tanning materials used, as shown in Fig. 96. At the same concentration of these salts the different extracts gave in some Fw 8 FF HN HD 3 oo ? “e, / Gambier ™--..\ MM M MM MM M Volume of Precipitate (c.c,) Vol. of Precipitate (c.c,) MMMM MM M M 2 3M 20010050 2010 4 B T 800 40020010060 2010 4 Concentration of Calcium Chloride Concentration of Aluminum Sulfate Fic. 96.—Precipitation of Tannins F'1c. 97.—Precipitation of Tannins by by Calcium Chloride. Aluminum Sulfate. cases less, and in others more, precipitate, showing the presence of substances reacting with barium and calcium ions to form compounds of different solubilities. Trivalent Cation. Aluminum sulfate. In the pre- cipitation of negatively charged colloidal dispersions, aluminum sulfate is not only a powerful pre- cipitant, but it also gives the “ir- regular series” or “tolerance zone” which is typical of the action of weak base cation-strong acid anion salts, as shown by Buxton and Teague,** and by Freundlich and Schucht.*? The concentrations of aluminum sulfate used ranged from 0.00125 to 0.5 molar. The “irregular series” effect was obtained with gambier, sumac, oak bark, and quercitron bark. Precipitation generally set in at 0.00125 molar concentration, rose rapidly to a maximum, dropped off into a “tolerance zone,” and then started upward again, as shown in Fig. 97. Those which gave no “irregular series,” at least up to 0.5 molar concentration of the salt, were osage orange, quebracho, camel cutch, chestnut wood, chestnut oak bark, hemlock bark, and larch bark, shown %Z. physik. Chem. §7 (1907), 76. 82 ITbid., 85 (1913), 641. Vol, Precipitate (c.c,.) F wo YQ fF TD DH \“Hemlock Bark & 2 3) = Gg M MMM MM OM OM 800400200100 50 20 10 4 Concentration of Aluminum Sulfate Fic. 98.—Precipitation of Tannins by Aluminum Sulfate. THE TANNINS 237 Hydrochloric Acid Sulfuric Acid Formic Acid Volume of Precipitate (c.c.) ai 2 3 4 5 6 7 pH Value of Tan Liquor Fic. 99.—Precipitation of Tannins of Quebracho Extract as a Function of pH Value. in Fig. 98. Precipitation started at 0.00125 molar and increased grad- ually to about 0.1 molar, where there was an abrupt upward trend similar to a salting out effect. These extracts are not so sensitive to precipitation by dilute solutions of aluminum sulfate as those shown Hydrochloric Acid —o—_—_o—__—__6— Sulfuric Acid ———_@—__—___@__"__ Volume of Precipitave (c.c.) af 2 3 4 5 6 7 pH Value of Tan Liquor Fic. 100.—Precipitation of Tannins of Gambier Extract as a Function of pH Value. 238 JHE CHEMISTRY ON LEATHER MANUFACTURE in Fig. 97. Bengal cutch seemed to be in a separate category, since It was unaffected by the addition of aluminum sulfate. Hydrogen-Ion Concentration. The effect of hydrogen-ion concentration upon the precipitation of solutions of quebracho, gambier, larch bark, and oak bark by sulfuric, hydrochloric, and formic acids is shown in Figs. 99, 100, IoI, and to2. Solutions of sumac, hemlock bark, and wattle bark were not precipitated by these acids with increasing acidity to pH =1. It is evident that the volume of precipitate formed is not a function of Hydrochloric Acid —O-—_—_—_- ee w es \) Sulfuric Acid —___9____ —_q—___ n e oi Formic Acid Volume of Precipitate (c.c.) hw = bo ro) roy ro) Oo e oO pH Value of Tan Liquor Fic. 101.—Precipitation of Tannins of Larch Bark Extract as a Function of pH Value. hydrogen-ion concentration alone, since the three acids give curves of different shapes. Wherever a precipitate formed, the amount invariably increased with increasing hydrogen-ion concentration where hydrochloric and sul- furic acids were used. But an increasing concentration of formic acid dissolved the precipitate, or the suspended matter in cases where no precipitate had previously formed. The precipitates obtained with hydrochloric acid were found to be soluble in strong alcohol and in 9 molar lactic acid. On shaking up with water, these precipitates dispersed, but gradually settled out more or less completely in 24 hours. In the case of oak bark and quebracho, it was found that approximately two-thirds of the original solid matter present had been precipitated at pH = 1. THE TANNINS | 239 When the pH value was increased by the addition of sodium hydroxide, there was increasing solution, clear liquids being obtained in every case at pH = 8. The effect of adding calcium hydroxide, how- ever, is very different, as will be recalled from Fig. 87 of Chapter 11. Hydrochloric Acid —o—__9————_6— Sulfurie Acid Formic Acid Volume of Precipitate (c.c.) 1 Z 3 4 s 6 7 pH Value of Tan Liquor Fic. 102.—Precipitation of Tannins of Oak Bark Extract as a Function of pH Value. At pH values above 7.2 increasing amounts of precipitate are obtained with increasing pH value. The conduct of the extracts examined by Thomas and Foster shows that they contain.a large amount of colloidal matter of a type of disper- sion with properties between those of the intermediate and hydrophilic dispersions. From the colloidal point of view, vegetable tanning ma- terials furnish an almost unexplored field; the work outlined in this chapter cannot be considered as more than a good start. Chapter 13. Vegetable Tanning. Raw skin is readily putrescible in the wet state. Upon drying, the collagen fibers become glued together and the skin becomes very stiff. Although the dried skin will not putrefy, it again becomes putrescible as soon as it comes into contact with water. Thousands of years ago the discovery was made that the properties of skin substance change completely when the wet skin is brought into contact with the aqueous extract of those forms of plant life which have since come to be classed as vegetable tanning materials. The action which brings about this change of properties is known as vegetable tanning and the com- pound of skin protein and tannin as leather. Under normal con- ditions, the fibers of leather do not glue together upon drying and they are not putrescible even in the wet state. The practice of tanning is greatly complicated by the necessity for endowing the leather with many delicate properties, according to the use to which it is to be put, all of which are markedly affected by slight differences in manipulation. The effect produced by any single change in the tanning process depends upon the nature of every one of the numerous operations preceding and following that in which the change has been made. In the manufacture of one type of leather, a skin may be subjected to scores of different operations and a slight change in any one of these may necessitate changes in nearly all of the others in order to preserve the specific properties desired in the finished leather. It is this fact that renders most practical treatises of leather manufacture of so little value to the tanner. Were he to try to adopt an operation described in the literature which was better in itself than the one he was using, he might find that the change would spoil his leather because of its failure to harmonize with all of the other opera- tions peculiar to his particular process. There are, however, certain broad principles of tanning which are followed generally. ‘Two conditions may be accepted as essential to successful tanning: the first that the natural physical structure of the skin shall be changed but very little; the second that the degree of tannage shall be as nearly uniform as possible throughout the skin. The second condition, in a large measure, is essential to the first. The physical means widely adopted to preserve the natural struc-_ ture of the skins during tanning is to suspend them freely from sticks with the heads hanging downward in the tan liquors, care being taken to see that the unhaired skin is free from creases or wrinkles, which 240 VEGETABLE TANNING 241 would be permanently fixed by the tannage. Usually the lower end of each skin is tacked onto a stick and the skin is then spread out carefully so that it hangs in its natural condition when immersed in the tan liquor. The supporting stick rests upon a rectangular frame floating in the liquor. The skins are not subjected to any violent mechanical agitation until the grain surface has been “set” by the tannage and the tannins have penetrated into the skin for a considerable distance. If skins from the beamhouse were put directly into strong tan liquors of such reaction that the rate of combination of tannin with the skin protein was abnormally great compared to the rate of dif- - fusion of tannin into the interior of the skin, the tendency for the outer layers to assume an area different from that of the skin as a whole would cause a distortion of the skin that would be permanent. In such a case, the liquor is called very astringent. The fact is often overlooked that the reaction of the solution previously absorbed by the skin may be as important in bringing about this condition as the reaction of the tan liquor itself. In fact a given tan liquor may appear very astringent to a pickled skin and yet very mild to a skin taken directly from the bate liquor. The distortion may show itself as coarse wrinkles, as the finer reticulation illustrated in Fig. 48 of Chapter 5, or merely as a rough grain surface. Since these distortions greatly lower the value of the leather, every effort is made to avoid them. The practical means adopted by the tanners to eliminate this danger is to hang the skins from the beamhouse first in a tan liquor which has been used to tan a great many lots of skins previously and in which the ratio of nontannin to tannin is very great. Each day the skins are then moved into stronger and fresher liquors until completely tanned. The effect of an increasing ratio of nontannin to tannin in the tan liquor is to increase the ratio of the rate of diffusion of the tannin into the skin to the rate of combination of the tannin with the skin protein. This has the obvious effect of making the rate of combination more uniform throughout the skin, and consequently lessening the tendency towards distortion. The ideal process would be the one in which combination was entirely prevented until the tannin was uniformly distributed throughout the skin and then allowed to proceed uniformly by a suitable change of reaction of the liquor. An- other safeguard which tanners have been forced to adopt, without understanding its mechanism, is so to regulate the reactions of both the tan liquors and the solution absorbed by the skin proteins just prior to tanning that the tanned and untanned portions of the skin protein do not tend to assume greatly different specific volumes. The progress of the diffusion of the tan liquor into the skin is determined by cutting off a strip and observing the color of the freshly exposed portfon. The raw portion is white and the tanned layers usually a deep brown. When the tannin has penetrated almost to the middle of the skin, it is customary to take the skins off from the sticks and pile them into vats known as handlers or layers. The name handler is used when the skins are handled from vat to vat at fre- 242 THE CHEMISTRY OF LEATHER MANUFACTURE quent intervals until completely tanned. The name layer is used for the vats in which heavy hides are laid away for long periods, during which the tannin diffuses very slowly into the interior. Although hardly more than a week is consumed in the diffusion of the tan liquor into a light skin, months are required in some processes of sole leather manufacture. : Where great solidity is required, as in sole leather, it is not sufficient merely to convert all of the collagen into leather. The volume of the collagen fibers increases as more tannin combines with them. After the hides have become completely colored throughout, it is customary to treat them with very strong tan liquors with the object of getting as much tannin fixed as possible, and mechanical agitation of one kind or another is often employed. Usually the weight of sole leather is further increased by the incorporation of glucose and magnesium sulfate in the leather. The Structures of Tanned Skins. In the manufacture of leather for definite purposes, the choice of the kind of skin is of the greatest importance. By varying the nature of the tanning process, the properties of the leather can be varied, but not sufficiently to make one kind of skin suit all purposes. Advan- tage is taken of the variety of skins furnished by nature in order to simplify the tanning process itself. Fig. 103 shows a vertical section taken from the butt of a steer hide tanned for sole leather. The natural solidity of this hide 1s so great that a heavy degree of tannage would not have been necessary in order to produce a leather suitable for shoe soles. This particular leather was heavily tanned with oak bark extract, but was not loaded — with glucose and magnesium sulfate. A section of vegetable tanned calf skin is shown in Fig. 143 of Chapter 14, where it was put for direct comparison with chrome tanned calf made from part of the same skin. It is interesting to compare its structure after tanning with that of calf skin in the fresh state, shown in Fig. 18 of Chapter 2. The leather is typical of the finest grade of finished shoe upper leather. Fig. 104 shows a section of vegetable tanned sheep skin just as it came from the tan liquors. Note the great contrast which it pre- sents to the leather made from steer hide or calf skin. The holes and empty spaces left by the wool and glands give the leather a spongi- ness that makes it unsuitable for many purposes. The upper layer is often split from the rest of the skin and used in bookbinding, for hat bands and for the linings of expensive shoes instead of cloth. Sheep skin leather is sometimes used as a substitute for kid leather in the manufacture of gloves, where its softness is an asstt. The raw skin is shown in Fig. 28. | Fig. 105 is a section of vegetable tanned leather from the butt, or shell, of a horse hide. This is finished leather ready for use in the manufacture of the uppers of heavy, waterproof shoes. The raw VEGETABLE TANNING 243 hide, at much lower magnification, is shown in Fig. 31. It will be noted that the leather has been split into layers through the portion known as the glassy layer, only the upper layer being used. The compactness of the fibers in the bottom third of the leather makes it waterproof and almost airtight. Leather from this part of the horse hide is known as Cordovan. The section should be compared with Fig. 145 of Chapter 14, which shows a section from the same butt which has been chrome tanned; the contrast is. striking. The peculiarity of the horse hide is that this compact fibrous structure is found only in the butt, the rest of the hide being very loose in texture. Fig. 106 shows a section taken from the same piece of leather as that shown in Fig. 105, but from a point further up the back beyond the boundary of the glassy layer. Its softness and -. sponginess has found for it a use in the manufacture of heavy gloves. The section shown in Fig. 107 is that of a vegetable tanned hog skin. A section of the fresh skin is shown in Fig. 30. When the flesh side of the leather was shaved to make it smooth, the bottom of the pocket of the hair follicle was cut away, leaving the hole running right through the leather, as shown in the figure. This is typical of hog leathers; wherever there were bristles in the original skin, holes pierce the final leather. The roughness of the grain surface of the leather gives it a place in the manufacture of saddles, football covers, purses, etc. | ) Fig. 108 shows a vertical section of salmon leather taken directly from the vegetable tan liquors. It should be compared with the section of fresh skin shown in Fig. 36. The gap in the upper portion is the follicle once occupied by a scale. The structure of the leather makes it suitable for belt lacings. The raspy feel of certain kinds of shark leather is explained by the section shown in Fig. 109. Shark leather has recently been tried for shoe uppers, in which case the hooks are removed prior to tanning. The fibrous structure resembles that of other fishes. Fig. 110 shows a vertical section of vegetable tanned alligator skin. This type of leather finds an outlet in the manufacture of bags and cases. Fig. 111 shows a section of leather made from the skin of a horned toad.. Although these skins are very small, they make very pretty doilies and fancy purses. In both the alligator and toad skins, the fibrous structure resembles that of the fishes. A section of leather made from camel skin is shown in Fig. 112. It is remarkable for its compact structure, which would make it suit- able for belting leather or for light soles. Figs. 113 and 114 show portions of the section of a vegetable tanned walrus hide and Figs. Ir5 and 116 show sections of the tanned hide of a hippopotamus.* The most remarkable thing about these hides is their great size. The actual thickness of the walrus leather was 24 millimeters and that of the hippopotamus leather 30 millimeters. In order to show the entire 1The hippopotamus, walrus, and camel leathers were very kindly furnished by Professor Douglas McCandlish of the University of Leeds, England, , : oLessol ra v 4 Co Ge gee ge ge} cae, ae “e Pe i) Fig. 103.—Vertical Section of Steer Hide Leather. (Sole leather.) Eyepiece: none. Objective: 48-mm. Wratten filter: H-blue green. Magnification: 15 diameters, Location: butt. Thickness of section: 40 pw. Stain: none. Tannage: vegetable. 244 Fig. 104.—Vertical Section of Unfinished Sheep Leather. Location: butt. Thickness of section: 30 u. Stain: none. Tannage: vegetable. 245 Eyepiece: none. Objective: 16-mm. Wratten filter: H-blue green. Magnification: 46 diameters. Fig. 105.—Vertical Section of Horse Leather. (Cordovan—from shell.) Location: butt. Eyepiece: none. Thickness of section: 20 u. Objective: 16-mm. Stain: Daub’s bismarck brown. Wratten filter: H-blue green. Tannage: vegetable. Magnification: 70-diameters. 246 Fig. 106.—Vertical Section (From spongy part of back near shell. ) Thickness of section: 20 u. Objective: 16-mm. Stain: Daub’s bismarck brown. Wratten filter: H-blue green. Tannage: vegetable. Magnification: 70 diameters. of Horse Leather. Location: back. Eyepiece: none. 247 Fig. 107.—Vertical Location: butt. Thickness of section: 30 u. Stain: none. Tannage: vegetable. 248 Section of Hog Leather. Eyepiece: none. Objective: 16-mm. Wratten filter: K3-yellow. Magnification: 46 diameters. Fig. 108.—Vertical Section of Unfinished Salmon Leather. Location: side. Thickness of section: 20 u. Stain: none. Tannage: vegetable. Eyepiece: 5X. Objective: 16-mm. Wratten filter: B-green. Magnification: 110 diameters. 249 Fig. 109.—Vertical Section of Shark Leather. Location: (?). Thickness of section: 50 uw. Stain: none. Tannage: vegetable. Eyepiece: 5X. Objective: 16-mm. Wratten filter: B-green. Magnification: 75 diameters. 250 Fig. 110.—Vertical Section of Alligator Leather. Location: back. Thickness of section: 50 uw. Stain: none. Tannage: vegetable. Eyepiece: none. Objective: 16-mm. Wratten filter: G-yellow. Magnification: 45 diameters. 251 Fig. 111.—Vertical Section Location: butt. Thickness of section: 20 u. Stain: none. Tannage: vegetable. 252 of Horned-Toad Leather. Eyepiece: 5X. Objective: 8-mm. Wratten filter: H-blue green Magnification: 220 diameters. Fig. 112.—Vertical Section of Camel Leather. Location: butt(?). Eyepiece: none. Thickness of section: 30 wu. Objective: 32-mm. Stain: none. Wratten filter: B-green. Tannage: vegetable. Magnification: 30 diameters. 293 Portions of Vertical Section of Walrus Leather. Fic. 113.—Region of Grain Surface. fic, 114.—Region 22 Millimeters Below Grain Surface. Location: butt(?). Eyepiece: 5X. Thickness of section: 40 p. Objective: 16-mm. Stain: none. Wratten filter: G-yellow. Tannage: vegetable. Magnification: 68 diameters, 254 Portions of Vertical Section of Hippopotamus Leather. Fic. 115.—Region of Grain Surface. Fic. 116.—Region 28 Millimeters Below Grain Surface. Location: butt(?). Eyepiece: 5X. Thickness of section: 40 wn. Objective: 16-mm. Stain: none. Wratten filter: G-yellow. Tannage : vegetable. Magnification: 68 diameters, 255 ie 256 THE CHEMISTRY OF LEATHER MANUFACTURE thickness of the leather at 68 diameters, a picture about seven feet high would be required. The walrus must be very sensitive to touch, if we may judge from the highly developed papillae which protrude everywhere from the grain surface. In neither of these leathers were the roots of the hair removed and the fat cells surrounding the hair bulbs of the walrus were still-intact as though the unhairing liquors had not pene- trated that deeply. Except for the huge collagen fibers in the reticular layer, and the great size of the hide, the walrus hide resembles that of the common hog. It is interesting to compare the fibers of these leathers with those of the smaller skins, but the differences in magnification must be taken into consideration. It has often been supposed that the tanning action consists of a coating of the skin fibers with tannin, but observations of sections under the microscope indicate that this is not the case. The outer surfaces of the skin act as filters, permitting only the soluble matter to pass into the interior, where it subsequently diffuses into the sub- stance of the fibers, which assume a uniform color throughout when tanning is finally complete. In finished leather, contrary to what seems to be the general belief, we find no coating of the surfaces of the fibers nor any material precipitated in the spaces between them. Rate of Diffusion of Tan Liquor into Gelatin Jelly. The great length of time required to tan heavy leathers is due to the very slow rate of. diffusion of the tannin into the interior of the hides. Because of the difficulty of measuring the extent of pene- tration of tan liquors into raw hides, studies of the rate of diffusion are usually made with tubes of gelatin jelly. Hoppenstedt ? noted that different tanning extracts diffused into gelatin jelly at different rates, the order of increasing rate of diffusion being mangrove bark, quebracho, hemlock bark, algarobilla, valonia, oak bark, myrobalans, chestnut wood, gambier, divi-divi, sumac. Later Thomas ** showed that the rate of diffusion of tanning ex- tracts into gelatin jelly increases with the ratio of nontannin to tannin in the extract. For typical samples, he found the rate of diffusion increasing in the order quebracho, hemlock bark, larch bark, oak bark, chestnut wood, gambier, sumac, agreeing with the results obtained by Hoppenstedt. This is also the order for decreasing astringency of these materials, as ordinarily used. The same order is roughly borne out in experiments dealing with the rate of diffusion into cow hide. The action of nontannins in increasing the rate of diffusion of tannins into skin may be explained as follows: Tannins and cer- tain nontannins form compounds with collagen, but the collagen-tannin compound is very stable, while the collagen-nontannin compounds are * Diffusion of Tannins through Gelatin Jelly. A. W. Hoppenstedt. J. Am. Leather Chem, Assoc. 6 (1911), 343. *a Order of Diffusion of Tanning Extracts through Gelatin Jelly. A, W. Thomas. J. Am. Leather Chem, Assoc. 15 (1920), 593. MEGEITABLE TANNING 257 considerably dissociated. The nontannins, having a much smaller molecular weight than the tannins, diffuse more rapidly into the skin. When the slowly moving tannin reaches a point where it would combine with collagen, it cannot do so because the point is already occupied by nontannin. Tannin that would otherwise have combined with collagen near the surface of the skin is thus enabled to pro- ceed into the interior and the measured rate of penetration is thereby increased. ‘This action is more marked the greater the concentration of nontannin capable of combining with collagen. The collagen-tannin compound being much the more stable, tannin replaces nontannin as fast as the collagen-nontannin compound hydrolyzes. According to the Procter-Wilson theory of tanning, to be dis- cussed presently, the rate of tanning, and also of the combination of collagen with certain nontannins, can be decreased either by increasing the electrolyte concentration or by lowering the positive electrical charge which collagen possesses in acid solution, which can be accom- plished by decreasing the acidity. We should therefore expect the constituents of a tan liquor, both tannin and nontannin, to penetrate skin more rapidly as the acidity of the tan liquor is decreased to the isoelectric point of collagen. Thomas prepared a 5-per cent dispersion of gelatin in hot water containing 0.1 per cent ferric chloride and poured it into a series of test tubes to three-quarters of their capacity. When the dispersions had set to jelly, equal volumes of solutions of different extracts were poured on top of the jellies, which were then placed in an ice box. All of the extract solutions were made to contain 1 per cent of dry solid matter. Tannin and some nontannins react with ferric chloric giving very deep green or blue colors, which served to indicate the extent of the penetration. In 96 hours the gambier had penetrated 18.0 millimeters as against only 4.8 millimeters by the quebracho. It was, of course, the extent of penetration by certain nontannins that was measured, as these diffuse more rapidly than the tannin. Wilson and Kern * treated a large volume of a dispersion of gelatin in dilute ferric chloride solution with tartaric acid until its pH value was reduced to 2.5, as determined by the hydrogen electrode. Equal portions were then treated with sodium hydroxide to give the desired pH values, which ranged from 2.5 to 11.0. Dilutions were such that the final dispersions contained 5 per cent of gelatin and 0.1 per cent of ferric chloride, as in the experiments of Thomas. Solutions of gambier and quebracho extracts were treated with tartaric acid to give a pH value of 2.5. Equal portions were then treated with sodium hydroxide to give series of pH values the same as in the series of jellies. Each final liquor contained 1 gram of solid matter of the original extract per 100 cubic centimeters. aeaye The gelatin dispersions were poured into test tubes and allowed to set. Onto each was poured a given volume of tan liquor having the same pH value as the jelly. Both the quebracho and gambier series * Effect of Change of Acidity upon the Rate of Diffusion of Tan Liquor into Gelati Jelly. J. A. Wilson and E. J. Kern. J. Ind. Eng. Chem. 14 (1922), 45, * a 2588 THE CHEMISTRY OF LEATHER MANUFACTURE were run in duplicate. They were kept in the ice box and examined at intervals for 96 hours. The extent of the diffusion of the tan liquors into the jellies is shown in Fig. 117, the measurements being taken after 96 hours. In each case the duplicate series were practically identical. Gambier, which has a high ratio of nontannin to tannin, be- gins to penetrate at a pH value of 3.0 and reaches its maximum rate at pH = 6.0. Quebracho, on the other hand, scarcely shows any penetration until pH = 4.7, the isoelectric point of gelatin, is reached. At pH values greater than 9, however, the quebracho liquor penetrates at the greater rate, possibly because of its higher tannin content. Studies were also made of the effect of change of pH value upon Sq hape ae the rate of diffusion of tan liquors — 7 8 9 10 into cow hide. With increasing pH Value pH values up to about 8, there is Fic. 117.—Rate of Diffusion of Tan 4 distinct increase im rate Of dit- Liquor into Gelatin Jelly as a Func- fusion, but because of the flaccid tion of pH Value. nature of hide at pH = 8 it is dif- ficult to make accurate measure- ments of the rate of diffusion. At pH values below 3 and above 11 the hide swells considerably and becomes rubbery and distorted. Extent of Diffusion in Millimeters Rate of Tanning as a Function of Time and Concentration of Tan Liquor. An extremely important series of investigations of the nature of the vegetable tanning process has recently been begun by Thomas and Kelly, which promises to throw much light on the mechanism of this very complex process. Their first studies*® were devoted to the ef- fects of time and concentration. In their preliminary experiments, por- tions of purified hide powder were shaken with definite quantities of un- filtered solutions of tanning extracts for stated lengths of time, washed free from soluble matter, and then analyzed for the purpose of deter- mining the amount of tannin combined with a unit of hide substance. © In the more concentrated liquors, however, an error was introduced by the occlusion of insoluble matter by the hide powder, which was included as combined tannin because it was not removed later by washing. Time and Concentration Factors in the Combination of Tannin with Hide Substance. A. W. Thomas and M. W. Kelly. J. Ind. Eng, Chem. 14 (1922), 202. 5 The Concentration Factor in the Fixation of Tannins by Hide Substance. Ibid. (1923); (advance copy). VEGETABLE TANNING 209 In their most recent work, Thomas and Kelly adopted a method practically identical with the modified Wilson-Kern method of tannin analysis described in Chapter 12, except for the fact that no attempt was made to detannize the various solutions completely. All tan liquors were centrifuged and filtered and only the clear filtrates used in the experiments. The use of filtered liquors with hide powder gave results which were more uniform and which probably represent actual tanning conditions more. closely, since the surfaces of the whole skin act as filters, permitting only the soluble matter to come into contact with the great bulk of the skin protein. WATTLE BARK pH = 4.1 Grams Tannin Fixed by 100 Grams Hide Substance Grams Tannin Fixed by 100 Grams Hide Substance 20 40 60 80 100 120 140 20 40 60 80 100 120 140 Grams Solid Matter per Liter ; Grams Solid Matter per Liter Fic. 118—Rate of Tanning as a Fic. 119.—Rate of Tanning as a Function of the Concentration of Function of the Concentration of Tan Liquor. Time, 24 hours. Tan Liquor. Time, 24 hours. Portions of purified hide powder equal to 2 grams of anhydrous substance were shaken with I00 cubic centimeters of tan liquor of the desired concentration and for fixed intervals of time. The powder was then washed until the. wash water no longer gave a dark color upon the addition of a drop of ferric chloride solution. It was found that the ferric chloride test is capable of detecting I part in 75,000 of either gallic acid or pyrogallol. The powders, freed from soluble matter, were dried in a current of warm air and then completely dried in the oven. The increase in weight of the absolutely dry ma- terial was taken as the amount of tannin fixed by 2 grams of hide powder. Figs. 118 and 119 show how the rate of tanning varies with in- creasing concentration of solutions of quebracho, hemlock bark, larch 260 THE CHEMISTRY OF LEATHER MANUFAC bark, gambier, oak bark, and wattle bark extracts. The mild action of gambier, as contrasted with the astringency of quebracho, is graphically shown by the steep rise of the quebracho curve compared Grams Tannin Combined with 100 Grams Hide Substance GAMBIER. GAMBIER (39 grams per liter) (76 grams per liter) 24 hours . e4 hours AK BA (43 grams per liter) (88 grams per liter) 24 hours 24 hive WATTLE BARK WATTLE BARK (37 grams per liter) (61 grams per liter) (Concentrations are given in terms of dry solid matter) 2 weeks 24 hours £1 4 6 8 ee ee 2 4 6 82 0a pH Value of Tan Liquor Fic. 120.—Rate of Tanning as a Function of pH Value. with that of the gambier series. It is remarkable that all extracts give curves of similar shape and having points of maximum at the relatively low concentrations ordinarily used in practice. Thomas and VEGETABLE TANNING 261 Kelly showed definitely that the rise and fall in the curves cannot be attributed to variations in hydrogen-ion concentration, but is due to the increasing concentration of the other constitutents of the tan liquors. QUEBRACHO 3 QUEBRACHO (18.3 grams per liter) ; 18.5 g. per liter e 24 hours 6 hours HEMLOCK BARK HEMLOCK BARK (24 grams per liter) (80 grams per liter) 21.5 weeks $$ *&—_*— 2 weeks —G—__9-—_._-o— J day 24 hours LARCH BARK LARCH BARK (49 grams per liter) (90 grams per liter) Grams Tannin Combined with 100 Grams Hide Substance 24 hours 24 hours foeee 6 8. 10° 12 2a. An 6) eee LOa Le pH Value of Tan Liquor Fic. 121.—Rate of Tanning as a Function of pH Value. One explanation given for the appearance of points of maximum in the curves is that the rate of combination of tannin and hide sub- stance increases so rapidly, with increasing concentration of tan liquor, 262 THE CHEMISTRY OF LEATHER MANUFACTURE that it soon reaches a point where the surfaces of the hide fibers quickly become so heavily tanned that they are rendered less permeable to the tannin remaining in solution. The interior of the fibers are thus prevented from tanning so rapidly, which accounts for the smaller amount of tannin fixed by the hide powder in the stronger solu- tions. Another explanation is furnished by the work of Thomas and Foster,° who observed that the electrical difference of potential at the surface of tannin particles decreases with increasing concentration of tan liquor. This would lessen the attraction between the tannin particles and the protein jelly and thus cause a decrease in the rate of combination. This seems the more probable explanation because a greater rate of diffusion of tan liquor into skin is obtained in practice by using more concentrated solutions. The curves represent the resultant of two effects: the increasing concentration of tannin tends to cause an increase in the rate of tanning and the increasing concentration of nontannin tends to cause a decrease in the rate of tanning. The point of maximum represents the point at which the effect of the increasing concentration of nontannin becomes greater than that of the tannin. These curves are in agreement with the findings of a number of investigators that highly concentrated tan liquors are very much less astringent than those of moderate concentrations. In practice, the degrees of astringency of tan liquors seem to follow curves similar to those in the figures. The use of concentrated liquors in tanning has been suggested by Seymour-Jones* and by Enna,® but the idea seems not to have been widely adopted, probably because it introduces complications in the later processes not easily overcome without some loss in quality of the finished leather. Rate of Tanning as a Function of pH Value. Thomas and Kelly® next turned their attention to the effect of the pH value of tan liquors upon the fixation of tannin by hide substance. The procedure adopted was the same as in the studies of the effect of concentration. In each case the pH value of the tan liquor, as determined by the hydrogen electrode, was adjusted to the desired value by the addition of sodium hydroxide or hydrochloric acid, Figs. 120 and 121 show the effect of change of pH value on the rate of the tanning of hide powder by solutions of quebracho, gambier, oak bark, wattle bark, hemlock bark, and larch bark ex. tracts. The curves contain a mine of information that requires careful study. The most elaborate set of curves is that for hemlock bark exiract, 8 Ind. Eng Chose eed eae Tanning Extracts. A. W. Thomas and S. B. Foster, . erates. Tanning of Sole Leather. Alfred Seymour-Jones. J. Soc. Leather Trades Chem. ® Rapid Tannage. Fini Enna. Ibid., 1 (1917), 36. ®The Hydrogen-Ion and Time Factors in the Fixation of Tannins by Hide Substance. A. W. Thomas and M. W. Kelly. Ind. Eng. Che ‘ ; Di i i Kelly, CokumbisiUaree ress g tem. (1923); Dissertation, Miss Margaret W. VEGETABLE TANNING =. 263 which may be discussed as typical. In the concentration experiments, a tan liquor containing 24 grams of solid matter per liter gave a much greater rate of tanning than one containing 80 grams per liter, but the curves in Fig. 121 show that this is dependent upon the pH value; at pH = 5, the more dilute solution tans at the greater rate, while at 2 and at 8, the more concentrated solution tans at the greater rate. In tanning for 24 hours, there is a steep rise in all curves to the left of pH = 5, which is exactly what one would expect, knowing that the positive electrical charge on collagen increases as the pH value falls from the isoelectric point and that the tannins are nega- tively charged at pH values higher than 2. In some cases a falling off in rate of tanning as the pH value drops below 2 is noticeable, but it must be remembered that the great tendency for collagen to swell and to hydrolyze at high acidities makes it difficult to get reliable data at pH values as low as 2. The most curious parts of the curves are those bétween the pH values 5 and 8. Since tannin particles are negatively charged in this region, the question that naturally arises is the possibility that the collagen may become increasingly positive with rise of pH value from 5 to about 8. This might seem an absurd view were it not for the two points of minimum plumping of calf skin found by Wilson and Gallun and shown in Fig. 73 of Chapter 9. Here it was suggested that collagen undergoes a change of form, possibly an internal re- arrangement, in passing from an acid to an alkaline solution and that the two points of minimum, at pH = 5.0 andsat pli t="7.7, 1epresent the isoelectric points of the two forms. We may refer to collagen stable in acid solution as form A and collagen stable in alkaline solu- tion as form B. As the pH value is increased from 5.0 to 7.7, if the conversion of form A into form B proceeds at a greater rate than the formation of negatively charged ions of form A, then we should ex- pect the net charge on the collagen structure to become increasingly positive, which would result in an increased rate of tanning. The question was raised in discussion as to whether any fixation of tannin actually took place at pH values below 2 and above S2ein all of the experiments described, the powders were washed with dis- stilled water immediately after being taken from the tan liquor. Dis- tilled water usually has a pH value of about 5.8, due to dissolved carbonic acid, and this would tend to make the pH value of the solution absorbed by the collagen jelly approach the value 5.8 before it was all washed out and the observed fixation of tanning might have occurred during the washing rather than during the shaking with tan liquor. Thomas and Kelly showed, however, that fixation actually does take place at pH values below 2 and above 8. They prepared a solution of wattle bark extract containing 40 grams of solid matter per liter and hydrochloric acid to bring the pH value to 0.87. Four portions of hide powder were tanned with this solution for 24 hours in the prescribed manner and then two were 264 THE CHEMISTRY OF LEATHER MANUFACTURE washed with distilled water and two with a hydrochloric acid solu- tion having a pH value of 0.87 until no more tannin could be ex- tracted. The latter two were then washed free from hydrochloric acid with distilled water. The two powders washed with the acid solution were found to contain an average of 0.739 gram tannin com- bined with the original 2 grams of hide powder against 0.987 gram tan- nin for the powders washed with distilled water. This shows that, al- though washing with distilled water causes an increase in combined tannin found, there is actually a fixation of tanning taking place at pHi'0.87; Tanned Powders Washed with Distilled Water Tanned Powders Washed with A Buffer Solution of Same pH Value as Tan Liquor to is*] Oo oO Grams Tannin Fixed by 100 Grams Hide in 24 Hours h oO a 2 3 4 5 6 7 8 ¢ 10 pH Value of Tan Liquor Fic. 122.—Showing the Rate of Tanning as a Function of pH Value and also the Effect of Washing the Tanned Powders with Buffer Solutions having the Same pH Value as the Tan Liquor Used in Tanning. They then prepared two series of solutions of hemlock bark ex- tract, containing 24 grams of solid matter per liter and having pH values ranging from 1 to 10, as determined by the hydrogen electrode. Portions of hide powder were tanned in each series for 24 hours in the prescribed manner and then the powders of one series were washed with distilled water, while those of the other were washed free from soluble tannin with solutions having the same pH values as the liquors in which the powders were tanned. For pH values of 4 or less, the solutions used for washing contained only hydro- chloric acid; for pH values from 5 to 9, they were made from M/rs sodium phosphate adjusted to the desired pH value with HCl or N aOH ; for pH = 10, a solution of sodium hydroxide was used. The final washing was done with distilled water. The results are shown in a SS an VEGETABLE TANNING 265 Fig. 122. The two curves are not identical, but show plainly that tannin combines with hide substance at all pH values from I to Io. Where a buffer solution was used to wash the hide powder tanned at pH = 5, a greater fixation of tannin occurred. Salts at low con- centration have the property of increasing the fixation of tannin at pH = 5, as will be shown later. Stability of the Collagen-Tannin Compound. at Different pH Values. While studying the action of solutions of acid and alkali upon leather previously freed from water soluble matter, Wilson and Kern* found that tannin was extracted by dilute solutions of alkali, but not of acid. In an attempt to locate the pH value at which the collagen- tannin compound begins to hydrolyze, they performed the following experiment. A large amount of purified hide powder was tanned with quebracho extract at a pH value of 4.6, washed free from all soluble matter with distilled water, and then dried. Seven large reservoirs of buffer solutions were prepared by making up solutions of tenth- molar phosphoric acid with sodium hydroxide to produce the pH values 5, 6, 7, 8, 9, 10, and 11, respectively. Eight-gram portions of the tanned powder were put into Wilson-Kern extractors ™ and extracted with 4 liters of buffer solution, taking just 6 hours for all of the solution to percolate through the tanned powder. Each portion was extracted with a solution of different pH value. The extracted powders were washed free from buffer solution with distilled water and were then dried and analyzed for comparison with the original powder. All extracts were brought to a pH value of 4 and then tested for tannin with the gelatin-salt reagent. The buffer solutions extracted only negligible amounts of nitrogen from the powders. The results are shown in Table XXXII. TABLE XXXII. ANALYSES OF TANNED Hine Powper BErorE AND AFTER WASHING WITH SoLUTIONS OF DIFFERENT pH VALUES. Before After washing with solution of pH= washing 5 6 vk 8 9) 10 II mR ey aa se nw ees fe 0.2 0.4 0.5 0.4 0.6 0.3 0.5 0.4 Hide substance (N x 5.62).. 84.2 83.9 83.9 83.9 84.2 847 84.9 85.3 Tannin (by difference)..... 15.6 15.7 15.6 15.7 15.2 15.0 14,60) Gi43 Per cent of total tannin ex- oo Lo rer none none none 2.6 3.8 6.4 8.3 Test for tannin in extract...... neg. neg. meg. pos. pos. pos. pos. Leather tanned at pH = 4.6 is apparently resistant to hydrolysis by solutions having pH values up to some point between 7 and 8, but is at least partially hydrolyzed, and with increasing speed, as the 10 Stability of the Hide-Tannin Compound at Different pH Values. J. A. Wilson and “i J. Kern. Presented before the Leather Division of the American Chemical Society, Sept. 1922, * “11 For description, see J. Ind. Eng. Chem. 13 (1921), 772. 2066 THE CHEMISTRY OF LEATHER MANUFACTURE pH value is increased above 8. This adds some weight to the sug- gestion that 7.7 represents the isoelectric point of one form of collagen. But, taken in conjunction with the finding of Thomas and Kelly that collagen and tannin form stable compounds at pH values greater than 8, it also supports their view that the collagen-tannin compound formed in alkaline solution is different from that formed in acid solution, which will be made clearer when their later experiments are described. Effect of Neutral Salts upon the Rate of Tanning. The effect of the concentration of sodium chloride or sulfate upon the rate of tanning of hide powder by solutions of gambier and hem- GAMBIER (sodium chloride) GAMBIER (sodium sulfate) ~so1,0-M salt eo 0 2.0-M salt HEMLOCK BARK HEMLOCK BARK (sodium sulfate) (sodium chloride) ‘© 0.5-M salt no salt ——o1.0-M salt “9 2.0-M salt Grams Tannin Fixed by 100 Grams Hide Substance Grams Tannin Fixed by 100 Grams Hide Substance go. -8--—--9 1,5-M salt 2 5 8 pH Value of Tan Liquor Before pH Value of Tan Liquor Before Adding Sodium Chloride Adding Sodium Sulfate Fic, 123.—Effect of Sodium Chloride Fic. 124—Effect of Sodium Sulfate and pH Value upon the Rate of and pH Value upon the Rate of Tanning. Time, 24 hours. Tanning. Time, 24 hours. lock extracts, at different pH values, has recently been studied by Thomas and Kelly.'’* In each test, 100 cubic centimeters of tan liquor, a weighed amount of salt, and the equivalent of 2 grams of water-free hide powder were put into a bottle and shaken, in a rotating box, for 24 hours. The contents were then transferred to a Wilson-Kern extractor, filtered, and washed until the washings gave no coloration with ferric chloride solution. The tanned powders were then dried in a vacuum at 100° C. and weighed, the increase in weight of the dry powder being taken as tannin fixed. 2 The Influence of Neutral Salts upon the Fixation of Tannins by Hide Substance. A. W. Thomas and M, W. Kelly. Ind. Eng. Chem. (1923); (advance copy). VEGETABLE TANNING 267 In order to guard against including as fixed tannin any matters rendered insoluble by the added salt, blanks were run leaving out the hide powder and corrections were made where necessary. The insoluble matter of the extracts was first removed by centri- fuging strong solutions, which were then diluted to contain 40 grams of solid matter of the tanning extract per liter, after adjusting the pH value to 2, 5, or 8, by addition of hydrochloric acid or sodium hydroxide. The effect of sodium chloride is shown in Fig. 123 and that of sodium sulfate in Fig. 124. At pH =2, both salts retard tanning to a very considerable extent, although sodium sulfate is always much more effective in this respect than sodium chloride. In each case the extent of the retardation is greater the higher the concentration of salt. At pH =8, the action of the salts is similar, but less pro- nounced. At pH = 5, the action is still less pronounced and is even reversed by concentrations of sodium chloride less than twice molar, which seem to cause an increase in rate of fixation of tannin. The marked reduction in the rate of tanning at pH = 2 exerted by the salts is probably due primarily to the reduction of the electrical differences of potential between the collagen jelly and the liquor on the one hand and between the liquor and the surface film surrounding the tannin particles on the other. The potential difference between collagen jelly and liquor is probably at its maximum value in the vicinity of pH = 2 and, consequently, the depressing action of salt should be greatest at this point. According to the Procter-Wilson theory of tanning, a diminution in this potential difference must result in a decrease in rate of tanning. The greater effect of sodium sulfate may be attributed to the divalent sulfate ion, as explained in Chapter 5. With the decreasing potential difference between the liquor and the surface film of solution in contact with the tannin particles, there would be an increasing tendency for the tannin particles to form aggregates and finally to precipitate out, further decreasing the rate of combination of tannin with collagen. The effect of hydration of the added salt is to remove water from the role of solvent, as explained in Chapter 4, and this would cause a virtual increase in concentration of tannin. Thomas and Kelly point out that opposed to this, within certain limits, would be the tendency for the salt to cause an aggregation of the particles of tannin. These opposing actions may explain the behavior of sodium chloride at pH=s5. At this point the potential difference between the collagen jelly and the liquor would be near its minimum value and hence would be but little affected by the salt. The effects of hydration and of aggregation would therefore be much more pronounced at this point, and Thomas and Kelly suggest that the increase in rate of tanning by molar and half-molar sodium chloride may be due to the hydration effect and the decrease in rate of tanning by the stronger sodium chloride solution and the sodium sulfate solutions to the aggregation factor. They are continuing their studies of the action of salts upon the vegetable tanning process. 268 THE CHEMISTRY OF LEATHER MANUFACTURE Degree of Plumping of Skin as a Function of Concentration of Acid and Salt in Tan Liquors. Tanners of heavy leathers usually attach much importance to the degree to which the skin is swollen, or plumped, during the tanning operation. It is generally assumed that greater yields of leather are obtained when the skin is tanned in a highly plumped condition. If | the plumping by means of acid is carried to excess, however, the skins will be ruined. The first sign of danger in this direction is a wrinkling and reticulation of the grain surface of the skin. A rapid tanning of the surfaces of the skin follows, rendering them almost impermeable to the tannin remaining in solution, and the fibers in the interior remain taw and swell considerably, assuming a glassy appearance. If left long in this condition, especially in warm liquors, the collagen hydrolyzes and the skin is damaged beyond hope of recovery. TABLE XXXIII. DEGREE OF PLUMPING OF CALF SKIN Propucep BY TAN Liguor CONTAINING 25 GRAMS OF OAK BARK ExtTrRAcT PER LITER AND Lactic ACID AND SopIUM CHLORIDE AS SHOWN IN THE TABLE. Moles per Liter Gauge Readings in MM. Final Lactic Sodium (average of triplicates ) pH Value acid chloride Initial Final Ratio *.. “at 26°U None None 1.346 2.150 1.60 4.63 0.0025 bi 1.411 2.343 1.66 3.904 0.0050 ‘ 1.383 2.699 205 3.74 0.010 m 1.433 3.842 2.68 3.47 0.025 a 1.470 4.504 3.10 3.05 0.050 “a 1.360 4.497 3.31 2.81 0.100 - 1.434 5.100 3.56 ee By 0.05 1.456 4.522 3.11 2.49 . 0.10 1.458 3.918 ~ 2.69 2.47 0.25 1.461 3.483 2.38 2.43 2 0.50 1.420 2.182 1.54 2.37 * This is a measure of the degree of plumping. Wilson and Gallun** studied the-effect of acids and salts upon the plumping of calf skin in tan liquors, using their method, which is described in Chapter 8. The effect of lactic acid and of sodium chloride upon the degree of plumping of calf skin in a solution of oak bark extract is shown in Table XXXIII and in Figs. 125 and 126. For this experiment a piece was selected from the butt of a calf skin, after liming, unhairing, and washing, of as nearly uniform thick- ness as possible and cut into squares having a side of about 2 centi- meters. These were delimed by washing with several changes of 0.o1-molar hydrochloric acid containing 10 per cent of sodium chloride, then kept over night in a saturated solution of sodium bicarbonate containing 10 per cent of sodium chloride, washed thoroughly, and 18 Direct Determination of the Plumping Power of Tan Liquors. J. A. Wilson and A. F, Gallun, Jr. Ind. Eng. Chem. 15 (1923), 376. VEGETABLE TANNING 269 finally bated for 5 hours at 40° C. ina solution of 1 gram per liter of pancreatin, having a pH value of 7.6. The pieces were then washed for 24 hours in running tap water and were kept under distilled water in a refrigerator at 7° C. until used. The resistance to compression of each piece of skin was measured by means of a Randall & Stickney thickness gauge with a flat, metal base, upon which the piece of skin was placed, and a plunger, having a circular base I square centimeter in area, capable of pressing on the surface of the skin under constant pressure. The gauge reading was taken, in every case, exactly two minutes after dropping the plunger onto the skin. 3.54 OAK BARK (lactic Acid) OAK BARK (lactic acid) Degree of Plumping (final/initial gauge reading) Degree of Plumping (final/initial gauge reading) ~ a eepeoeOr o.0 4,0 4.5 O10 0,8 (016 Ok 0.5 pH Value of Tan Liquor Moles Sodium Chloride per Liter Fic. 125.—Effect of pH Value of Tan Fc. 126.—Effect of Sodium Chloride Liquor upon Degree of Plumping upon Degree of Plumping of Calf of ‘Calf Skin. Skin in Tan Liquor Acidified with Lactic Acid. 7 Eleven tan liquors were prepared as indicated in Table XXXII. The gauge readings of pieces of the standard skin were taken and they were then shaken with water to bring them back to their normal shape, after being compressed in the gauge. They were then put into the tan liquors and allowed to remain there for 24 hours at 20°C. The final eauge readings were then taken. In each case 3 pieces of skin were put into 100 cubic centimeters of tan liquor and the agreement be- tween the triplicate determinations was satisfactory. The degree of plumping caused by the liquor is measured by the ratio of the final to the initial gauge reading. The actions of the acid and the salt are not exactly the same as they would be in pure water, but are complicated by the tanning action of the liquor, which decreases the swelling power of the skin. The general tendency of the acid, nevertheless, is to swell the skin and the action of the salt to counteract this swelling. McLaughlin and Porter ** made a rather interesting study of the change in weight of limed steer hide during immersion in tan liquors 4 On the Swelling and Falling of White Hide in Vegetable Tan Liquors, G. D. McLaughlin and. R. E. Porter, J. Am. Leather Chem. Assoc. 15 (1920), 557. 270 THE CHEMISTRY OF LEATHER MANUFACTURE of various compositions. Unfortunately the pH values of the liquors were not determined and, in many cases, it is not clear how much of the change observed is due to variation of hydrogen-ion concentration. Rapid Tannages. Numerous accounts appear in the literature of attempts to hasten the tanning process, especially for heavy leathers. But few of these have yet developed to a point where the mechanism of the process is well defined. In many cases, it appears likely that the added ac- celerator acts only indirectly by bringing about a more favorable reaction of the tan liquor itself. One process for hastening tanning that seems, on the face of it, to merit further investigation is that described by Cross, Greenwood, and Lamb.’* In the course of investigations on the hemi-celluloses of seed endosperms, the authors studied their compounds with tannin, which may be made to form apparently homogeneous jellies. From previous experience in the dyeing of silk, the authors conceived the idea of controlling the astringency of the tannin by using it in the form of a compound with the hemi-cellulose. They found that the use of “gum tragasol” in conjunction with the tannin solution caused a very rapid penetration of tannin into the skin. Complete penetra- tion of very thick hides was obtained in two or three days, although the reduced rate of combination between collagen and tannin required the keeping of the hides in the liquor for a somewhat longer time than this. A similar type of process is that proposed by Turnbull and Car- michael *° in which the tanning materials are dissolved in a jelly formed of a starch solution. Another process intended to hasten the tanning of heavy hides is that of C. W. Nance?” and known as the vacuum process. The hides are put into a tank, which is then evacuated to a pressure of 0.5 lb. per square inch. The temperature is then gradually raised to the point at which water boils at this pressure. The tan liquor is introduced and the temperature allowed to fall slowly to permit the hide to absorb tan liquor to replace the water lost by boiling. By a proper regulation of temperature and pressure as well as concentra- tion of tan liquor, it is claimed that an enormous reduction in time of tanning can be effected. ; Attempts have been made at various times to hasten the tanning process by the application of an electric current. The hides are placed between. carbon electrodes and the current turned on; in moving towards the anode, the tannins are thus made to penetrate the hide. Ridea] and Evans 18 pointed out that to get good results the conductivity of and ta ae Bos Colorin uthlauegs: © F: Crm G. Vs Greenwood re e pate Serer i ss Poe rete es - British Pat. 110,470, Feb, 24, 3O17. 3 7 Some Experiments on the Theory of Electro-Tanning, <. Ri : pe ee ches 2 sattisraeoe y nning. E. K. Rideal and U. R. Evans. VEGETABLE TANNING 271 the liquor must be very low and that the cathodes should be made of carbon and the anodes of copper. Williams *® found that a direct current causes a rapid destruction of pure gallotannic acid, which did not take place when an alternating current was used. Following the presentation of Rideal and Evans’ paper, J. G. Parker said that he had experimented with electrical tanning and doubted that it had any advantages over systems not involving the use of the electric current. At any rate, it has not been adopted very widely as yet. Much attention has been paid recently to the effect of adding organic compounds containing sulfonic groups to vegetable tan liquors upon the rate of penetration of the tan liquor into the hide. Among the materials commonly used may be mentioned the lignosulfonic acids obtained from the so-called sulfite cellulose, a by-product in the manu- facture of paper from wood pulp, and also the synthetic products known as syntans, discovered by Stiasny, which will be discussed in Chapter 15. These materials act much like certain nontannins naturally occurring in vegetable tanning materials in lessening the astringency of the liquors and hastening the penetration of tannin into the skin. Apparently they have lower molecular weights than the tannins, which enable them to diffuse into the skin more rapidly. Since they actually combine with the collagen, they retard the combination of the true tannins with collagen, which thus permits tannin to diffuse into the interior that would otherwise have combined with collagen at the outer surface. This makes them valuable materials to use in the early stages of tanning. Whether or not the sulfonic groups which they possess are harmful for some kinds of vegetable tanned leathers has been the subject of debate, but has not yet been clearly settled. The acid character of these sulfonic groups gives the liquors a very low pH value, which in turn causes a lightening of the color of both the liquors and the leather. In some cases there seem to be combina- tions between the tannins and the sulfonic compounds, resulting in com- pounds less easily precipitable than the original tannins. The synthetic materials seem also to cause a reduction of some of the more highly oxidized tannins, which may explain in part the lesser tendency of certain mixtures to precipitate upon the addition of acid. Theory of Tanning. Until it became possible to treat the chemistry of the proteins in a quantitative manner, there was little hope of developing a quantita- tive theory of tanning. Numerous attempts to determine the relative combining weights of gelatin and tannin led only to variable and often apparently contradictory results because of the failure to appreciate the existence of uncontrolled variable factors. A review of the older Set on theories of tanning would be of little more than historical value. The modern theories of tanning are following the general trend of % Inquiry into Electrical Tannage. O. J, Williams, Collegium (1913), 76. aye THE CHEMISTRY OF LEATHER MANUFACTURE development of the chemistry of the proteins. One school of thought treats the theory of tanning from the viewpoint of the physical chem- istry of the proteins and the other from that of organic chemistry. Procter-Wilson Theory. The line of investigation of the physical chemistry of the proteins started by Procter led naturally to the conception of the mechanism of tanning formulated by Procter and Wilson.2® The work leading to the formulation of this theory is given in detail in Chapter 5 and need not be repeated here. When in equilibrium with a tan liquor having a pH value lying in the range 2 to 5, collagen may be looked upon as constituting an aggregate of complex cations balanced by much simpler anions held in the solution immediately in contact with the collagen structure by the same forces that hold all oppositely charged ions together. We may assume that the collagen composing a hide fiber has a structure corresponding to that of gelatin when set to a jelly. The theory may be pictured very simply by considering a piece of skin in contact with a solution containing only tannin and the acid HA. When equilibrium is established between the collagen and the acid, in the tan liquor let x = [H*] = [A’] and in the jelly phase of the collagen let Vea and z—= [CH*] (ie., concentration of collagen cation) whence (Aaa yg | The equilibrium conditions are exactly analogous to those described for gelatin, from which it is apparent that there will be an electrical difference of potential between the jelly phase and the external solution expressible quantitatively by : rope as log — RY oe oe VA - — x inne 2x Each tannin particle is negatively charged and, consequently, must have associated with it an equivalent number of cations held in the solution immediately in contact with the particle, which we may call the surface film for convenience, although it makes no difference to the theory whether the tannin particle is solid, like a gold particle, or a jelly particle capable of absorbing solution. Let the concentration of these cations be represented by z, and the concentration of the anion A’ in the surface film by y,; the total concentration of cation then equals y, + 2,. The electrical difference of potential between the surface film and bulk of solution then equals ee of Vegetable Tanning. H. R. Procter and Je As Bisco? J, Chem. Soc, 109 VEGETABLE TANNING 273 eat 2 ae eal & 2% E, = —— log — os —— log ee? peeia OV dae fey F yi F It is evident that E and FE, are of opposite sign. According to the Procter-Wilson theory, the first important action in the mechanism of tanning results from the tendency for EF and FE, to neutralize each other. The initial rate of tanning will, therefore, be measured by the sum of the absolute values of the potential differences, or 7 a! ll ES aN Bo (24+ V4 +27) (Ha -+ 4x? F217) In this expression, z is measured by the absolute value of the electrical charge on the collagen and 2, that on the tannin particles, while x represents the hydrogen-ion concentration of the tan liquor. For a fixed value of x, an increase in value of either zg or g, evidently causes an increase in the rate of tanning. _ Now, if we introduce a salt, say sodium chloride, and let its con- centration in the tan liquor at equilibrium be represented by u = [Na*] = [Cl’], from the reasoning in Chapter 5, we see that the initial rate of tanning is now determined by the expression RT tog Ceemen ste 40X52 F eee 40x 1-0)? 2") [—z, + V4(x + u)? +2,7] It is apparent that an increase in u, provided it does not increase z or 4, will cause a decrease in rate of tanning. This explains, in part, the retarding effect of salts upon the rate of tanning. If u is increased without limit, the value of the above expression becomes zero. When the surface film surrounding the tannin particle has joined the solution constituting the jelly phase of the collagen and thus neu- tralized the potential difference which each had against the external solu- tion, the actual charges on the collagen and tannin are free to neutralize , each other, as in the combination of any two oppositely charged ions which tend to form a slightly dissociated salt. Like the physical chemistry of the proteins, outlined in Chapter 5, this theory is capable of almost indefinite extension by mathematical treatment. Since the quantitative testing of the theory has only just been begun, such extensions may well be left for some future time. It is worthy of note, however, that the theory has proved a valuable guide in the development of tanning processes and no facts observed in tanning practice have yet been shown to be out of harmony with it. It is interesting to speculate on the probable combining ratio of collagen and pentadigalloyl glucose. Taking the author’s value of 750 as the equivalent weight of collagen and assuming that each digalloyl 274. THE CHEMISTRY OF LEATHER MANUFACTURE radical is capable of combining with collagen, we arrive at a combining ratio of 340 parts of tannin to 750 parts of collagen, or 45.3 per 100 parts of collagen. It may be only a coincidence, but this ratio repre- sents the minimum possible for vegetable tanned leather to pass as fully tanned, at least in the author’s experience. On the other hand, when skins are allowed to remain in the tan liquors for months, the ratio approaches the value of 90 parts of fixed tannin per 100 of col- lagen, but the author has never known it to pass this value in practice. Of course it is appreciated that different tannins may have different molecular weights, which would cause some deviation in the ratio to be expected. Any supposition as to the combining proportions of col- lagen and tannin is admittedly highly speculative in view of our meagre knowledge of the mechanism of tanning, but where so little is known, such speculations are valuable in forming a nucleus from which to build. The Procter-Wilson theory does not concern itself with the con- stitutions of the collagen cation and tannin anion, nor does it deal with possible combinations of collagen and tannin where these have electrical charges of the same sign, a condition which rarely, if ever, occurs in tanning practice. Thomas and Kelly 74 have recently started an investigation to de- termine the nature of the combination of collagen and tannin at different pH values. Trunkel ?* had previously shown that the water- insoluble compound of gelatin and tannin can be resolved into its components by digesting with ethyl alcohol, provided the digestion is carried out before the precipitate has dried. After drying, the gelatin- tannin compound is unaffected by alcoholic digestion. Thomas and Kelly studied the effect of alcohol upon collagen tanned at different pH values. Tan liquors were prepared having pH values of 1, 3, 5, 7, and 9. In the study of hemlock bark extract, portions of hide powder con- taining I gram of dry protein were shaken for 24 hours, at room temperature, with 50 cubic centimeters of tan liquor containing 2.7 grams of solid matter of the hemlock extract. The tanned powders were then filtered and washed in Wilson-Kern extractors until the wash water gave no color upon addition of ferric chloride. The wet powders were then transferred to Thorn extractors and extracted with g5-per cent alcohol. In this type of extractor, the material is ex- tracted by the hot vapors as well as by the condensed solvent. At intervals the alcoholic extracts were transferred to beakers, evaporated to dryness, dried for 4 hours im vacuo at 100° C. and weighed. After apparently complete extraction, the tanned powders also were dried im vacuo and weighed, the loss of tannin due to the alcohol extraction being calculated by comparison with a control series not treated with alcohol. 21 The Difference in Kind or Degree of Tannin Fixation as a Function of the Hydrogen- Ion agape, ec A. W. Thomas and M. W. Kelly. Ind. Eng. Chem. (1923); (advance copy). ot ‘#2 Gelatine and Tannin. H. Trunkel. Biochem, Z, 26 (1930), 458, Se VEGETABLE TANNING 2475 Table XXXIV shows the results obtained from weighing the residues from the alcoholic extracts and Table XXXV those obtained from the dry weights of the extracted leathers. Apparently alcohol de- composes most easily those leathers which were tanned at pH values lying between 3 and 5, the region in which tanning is usually done in practice. It is also apparent that leathers tanned at pH values greater than 5 are much more resistant to decomposition than those tanned at values less than 5. Table XXXV also shows the effect of extracting previously dried leathers with alcohol; drying evidently brings about a more permanent fixation of the tannin. TABLE XXXIV. ExtTRACTION BY ALCOHOL OF FIXED TANNIN FROM LEATHERS TANNED WITH Hemiock Bark Extract AT DIFFERENT pH VALuges. Ficures OBTAINED BY WEIGHING THE Dry RESIDUES FROM THE ALCOHOLIC EXTRACTS. Per cent of Total Fixed Tannin Removed Tanned at pH by Extraction for Value of 1 hour 45 hours gt hours les nts tina cs so 6.1 19.3 23.3 On a 77 24.1 28.9 MT eee ko ss oak x 7.8 17.1 22.0 an Ree an 3.1 6.9 9.8 Cl oo a 4.2 6.3 8.4 TABLE XXXV. (Same Experiment as Described in Table XXXIV. But the figures in this table were obtained by weighing the dry leather after the alcohol extraction.) Per cent of Total Fixed Tannin Removed Tanned at pH by 91 Hours’ Extraction of the Value of Wet leather Dried leather Moy a lk oc 0% 16.6 0.0 Se chee o's where ois'.5 > ae 23.3 2.8 ct Se ian, Pollo ey ae oa 25.1 4.4 Oo) Aa eee ae 4.6 0.6 a ee eee 0.6 0.0 A curious finding is that the figures in Table XXXV show a smaller loss of tannin than those in Table XXXIV. Some light is thrown upon this difference by a series of experiments with gambier. These were similar to the hemlock series except for the fact that the 50- cubic centimeter portions of tan liquor contained 2 grams of dry gambier solids. Table XXXVI shows the results obtained by weigh- ing the dry leathers after extraction with alcohol. The leather tanned at a pH value of 9 actually shows a gain in weight upon extraction with alcohol. Thomas and Kelly suggest the hypothesis that this gain may be due to an oxidation of the alcohol to aldehyde followed by an aldehyde tannage. Tan liquors absorb oxygen readily at a pH value of 9 and darken in color as oxidation proceeds. The author has found | 276 THE CHEMISTRY OF LEATHER MANUFACTURE that oxidized tannins present in leather cause an oxidation of un- saturated oils used in fatliquoring leather, as determined by the ratio of oxidized to unoxidized fatty acids subsequently extracted from the leather. It is therefore not unreasonable to suppose that tannins which have been oxidized at a pH value of 9 may be able to bring about an oxidation of the alcohol molecule. TABLE XXXVI. EXTRACTION By ALCOHOL OF FIXED TANNIN FROM LEATHERS TANNED WITH GAMBIER EXTRACT AT DIFFERENT pH VALUES. FIGURES OBTAINED BY WEIGHING Dry LEATHERS AFTER THE ALCOHOL EXTRACTION. Per cent of Total Fixed Tannin Removed by Ex- Tanned at pH traction for 91 Hours Value of of the Wet Leathers DV sileiel sueavejeteca hue’ elise s a ierets 17.5 Pept, ih MR Ie tee gs 26.3 ee Pe Dae ee ee? ee 19.5 ec Oe Te 0.7 ine See ere oe eee (Gain of 13.8 per cent!) The most important finding in this work is that the kind of fixation of tannin by collagen at pH values lower than 5 is different from that at pH values greater than 5. The simple theory of Procter and Wilson does not take into consideration the complex organic re- actions which apparently occur in tanning with liquors having a pH value greater than 5, nor the changes in the collagen-tannin compound which take place upon drying and aging. Oxidation Theory. Of the various theories of tanning treating the subject from the standpoint of organic chemistry, the oxidation theory supported by Meunier, Fahrion, and others is the only one meriting serious con- sideration. Meunier ** and his co-workers found that skin could be converted into leather by bringing*it into contact with a solution of benzoquinone. The color of the skin changed successively to light rose, to violet, and to brown. A leather of remarkable resistance to boiling water was obtained. An observation of great theoretical sig- nificance was that a portion of the quinone was reduced to quinol during the tanning action. Meunier concluded that part of the quinone had been reduced by the oxidation of the collagen and that only the oxidized collagen entered into combination with the remaining quinone. He likened the action to that of quinone upon aromatic amines: C,HsNH, + 2C,H,O2 = CsH,;N(C,H,O,) + C,H,(OH). ; 2C.H;NH, + 3C,.H,O2 = (CsH,N),C,H,O, + 2C,H,(OH). _ % Modern Theories of the Various Methods of Tanning. L. Meunier. Chimie & indus. trie 1 (1918), 71, 272; English translation, J. Am. Leather Chem. Assoc, 13 (1918), 530, VEGETABLE TANNING 277 Assuming the existence of primary amino groups in the collagen mole- cule, the compounds formed might be represented by the formulas: O R—N—O h—N= >C.H, | > CoH, O R—N—O Fahrion 24 suggested the following reactions: 2RNH, + O=R—NH—NH—R-+H,.O R—NH R—NH—O In vegetable tanning materials, Meunier assumes that quinones are formed by oxidation and that it is these which react with collagen to form leather. Powarnin”® objected to the assumption of the formation of quinones by oxidation and suggested that they are formed by a. tautomeric change, thus CH CH ES aN fd Cee OT HC CH—O Peal = [ae*| | he, CC, OH HC CH—O ee Wee CH Cliss enol form keto form The enol form was supposed to be stable chiefly in alkaline solution and the keto form in acid solution. According to Powarnin’s view, only the keto form has tanning properties, the action being represented as follows: tannin protein leather The organic chemistry of vegetable tanning has not yet passed the stage of speculative hypotheses. 24W. Fahrion. Mon. sci. (1911), 3613; (1914), 112. 26 Active Carbonyl and Tannage with Organic Substances. G. Powarnin. (1914), 634. Collegium Chapter 14. Chrome Tanning. Although the tanning of skins by means of chromium salts is only of comparatively very recent origin, a large proportion of the world’s supply of light leathers is now tanned by this process. In 1858 Knapp? described a process for tanning skins with salts of aluminum, iron, and chromium, but chrome tanning did not come into prominence commer- cially until after the appearance of the patents of Augustus Schultz, of New York, in 1884. In Schultz’s process, the skins, after bating or deliming, were tumbled in a solution of potassium bichromate and hydrochloric acid until the chromate had completely penetrated the skins, after which they were allowed to drain. They were then tum- bled in a solution of sodium thiosulfate acidified with hydrochloric acid, which reduced the chromate to chromic salt, in which condition it combines with the skin protein, yielding a very stable leather. Schultz’s system of tanning is known as the two-bath process. In 1893 Martin Dennis patented a system for tanning skins directly in a solution of basic chromium chloride along the lines suggested by Knapp in 1858. This one-bath process was naturally to be preferred to the two-bath process and soon gained precedence over it, except in the manufacture of glazed kid leathers, where the two-bath process seemed to yield a leather having more desirable properties. No explanation has yet been forthcoming as to why this should be so, although the author believes that the same results can be obtained by the one-bath process, if the reaction of the liquor is made to equal that of the liquor present in the skins during the second bath of the two-bath process. In the ordinary two-bath process, the acidification of the sodium thio-. sulfate causes a deposition of sulfur in the leather, which affects its properties, but a similar result can be obtained by the use of sodium thiosulfate in the one-bath process. On the other hand, the precipita- tion of sulfur in the two-bath process can be avoided by using sodium bisulfite as the reducing agent. Basic chromium sulfate was found to be superior to the chloride for one-bath tanning, as well as cheaper, and is now almost universally used. | Commercial one-bath chrome liquors are usually made from chrome alum or from sodium bichromate. A popular method is that devised 1 Nature and Essential Character of the Tanning Process and of Leather. F.2K - Ener Buchhandlung (1858); English translation, J. Am, Leather Chem, Anoeiee 1921), 658. 278 ee a a CHROME TANNING 279 by Procter? in which acidified sodium bichromate solution is reduced to chromic salt by the addition of a solution of glucose. A great variety of reducing agents have since been suggested or patented for the purpose. A very convenient method, described independently by Balderston * and Procter,* consists in passing sulfur dioxide gas into a solution of sodium bichromate until the reduction is complete. The equation usually given for the reaction is as follows: Na,Cr,O; + BOO. +- H,O — Na.5O, + 2CrOHSO,, but this merely indicates the relative basicity of the final liquor, the end product probably being much more complex than this. In modern practice, it is customary to pickle the skins from the beamhouse before chrome tanning, as described in Chapter 10. This has the advantage of bringing all skins into a uniform condition. After pickling, the skins are put either into a drum or a paddle vat and tumbled or paddled with chrome liquor until completely tanned, which condition is determined by placing a cutting in boiling water. Any untanned portions are converted into gelatin and this causes the piece of skin to shrivel up and curl. When the skin is completely tanned, it is apparently entirely unaffected by boiling water. The rate of penetration of the chromium salts into the skin, the rate of tanning, and the properties of the resulting leather are markedly influenced by changes in concentration of chromium salt, neutral salt, and hydrogen ion. Conditions are made even more complex by the important effects of time, temperature, and degree of hydrolysis of the chromium salts. All of the variables must be adjusted to suit each other as well as the condition of the skin as it enters the tan liquor and the processes to which it is to be subjected after tanning. Probably no two tanneries operate exactly alike and very few would dare to deviate far from the practice found to give good results under their particular conditions of operation. . Chromium Collagenate. It now seems fairly well established that acid and basic radicals form definite salts with proteins. There is nothing novel about the assumption that chromium, or other metallic radical, can form with collagen a series of salts that might be called chromium collagenates. Moreover, such an assumption is a convenient one, even though it may be shown later that the compound formed is much more complex than would be indicated by the term chromium collagenate. If the author’s ® value of 750 be accepted for the equivalent weight of collagen, then the smallest amount of chromic oxide required to 2H. R. Procter. Leather Trades Rev., Jan. 12, 1897. -§L, Balderston. Shoe & Leather Rep., Oct. 18, 1917. 4H. R. Procter. J. Roy. Soc. Arts. 66 (1918), 747. ( oa teas of Leather Chemistry. J. A. Wilson. J, Am. Leather Chem, Assoc, 12 1917), 108. 280 THE CHEMISTRY OF LEATHER MANUFACTURE convert I00 grams of collagen into the chromium salt would be (152x 100)/(6x 750), or 3.38 grams. Lamb and Harvey ® found that chrome leather showing less than 2.8 to 3.0 per cent of chromic oxide, based on the dry leather, was invariably undertanned. Based upon actual collagen, the figure would be about 3.4. That the figure 3-38 has some significance will be made more apparent later. This value assumes that all of the three bonds of the chromium ion have entered into combination with the protein and we should expect such a compound to be extremely stable, which chrome leather undoubtedly is. The most exhaustive studies yet made of the combination of collagen and chromium at measured concentrations of hydrogen ion, chromic oxide, and neutral salt are those of Thomas and his collaborators, which will be given in some detail. Hydrolysis of Chromium Salts. Being a salt of a strong acid and a weak base, chromic sulfate hydrolyzes to a very considerable extent in aqueous solution, yielding free sulfuric acid and a series of basic chromic sulfates. Thomas and COMMERCIAL CHROME DIQUOR) Baldwin ® = fol ae change in immediately degree of hydrolysis of chromic salts, under various conditions, by measuring changes in hydrogen-ion concentration. Studies were made of solutions of C.P. chromic sulfate and chromic chloride and also of a typical commercial chrome liquor, which showed by analysis: Cr.Onz, 14.3 per cent; Fe.O3, 1.9 per cent; immediately Al,Os, 0.2 per cent; SOs, 23.5 per cent; Cl, 0.2 per cent; and = ba= sicity corresponding to the formula Cr(OH)1.2(SOu)o.9, the sulfate present in excess of that called for by this formula being present as so- CHROMIC SULFATE PH Value of Chrome Liquor rw) es) a ° LO. 8720 “320 M46 dium sulphate. Grams Cro0z per Liter Effect of dilution: Fig. 127 Fic. 127.—Effect of Dilution upon the shows the pH values of both chro- Hydrolysis of Chrome Liquors. mic sulfate and the commercial chrome liquor at different concen- trations. Strong solutions of each were diluted to increasing extents and the hydrogen-ion measurements were made immediately and also after the diluted solution had stood for 7 or 9 days. With both mate- ®° Estimation of Chromic Oxide cs Chrome Tanned Leather. M. C. Lamb and A, Harvey. Collegium (London Edition) (1916), 201. ; The Acidity of Chrome Liquors. A. W. Thomas and M. E. Baldwin. J. Am. Leather Chem. Assoc. 13 (1918), 192. ‘ ® Contrasting Effects of Chlorides and Sulfates on the Hydrogen-Ion Concentration of Acid Solutions. J, Am. Chem. Soc. 41 (1919), 1981. CHROME TANNING 281 rials, the effect of dilution is naturally to raise the pH value, but, upon standing, the pH value of the commercial liquor continues to rise, while that of the chromic sulfate falls. ‘Effect of added acid or alkali: Fig. 128 shows the effect of adding sulfuric acid or sodium hydroxide to solutions of the commercial chrome liquor and of pure chromic sulfate. In each case a given amount of concentrated chrome liquor was mixed with a definite volume of stand- ard sulfuric acid or sodium hydroxide and the mixture was diluted to 50 cubic centimeters. The hy- drogen-ion concentration was deter- COMMERCIAL mined immediately after mixing 4,0 ane and diluting and also after definite | . ; oe 3.5 intervals of time. The addition of acid causes a corresponding increase 3.0 in hydrogen-ion concentration, but _ this causes a repression of hydroly- 2.5 sis of the chromium salt. The curves show that changes in degree of hydrolysis require a considerable 8 & 4 length of time; after the addition of B 4,5 acid, the hydrogen-ion concentra- 4 eer: tion continues to fall for many days, « a SULFATE approaching, but never reaching the § 3,5 ot value it had before the addition of & the acid. & 3,0 ‘ The lowering of the hydrogen- & ae ion concentration by the addition of 7*® a alkali causes an increase in the de- 2,0 Shlemaetietats gree of hydrolysis of the chromium - a eee salt and the hydrogen-ion concen- 1B EL tration continues to rise towards the = | ~ _ | after 30 de; value it had before adding the a alkali. The long time required for such systems to reach equilibrium, after a disturbance, increases the difficulty of investigations of the BG «Add ed’ 0. 2M 1p 20¢00x ua0H chemistry of chrome tanning. og. c ae ee eee eee ° rams Chromic Oxide Effect of neutral salts: In 5, 128 Rffect of Added Acid or Chapter 4 it was pointed out that Aitatin unde tain Ledeen ae the hydrogen-ion concentration of Chrome Liquors. acid solutions is increased by the addition of neutral chlorides and decreased by the addition of neutral sulfates. The effect of increasing concentration of different salts in solutions of sulfuric and hydrochloric acids was shown in Figs. 39 and 40. In Fig. 129 are shown the changes in hydrogen-ion concentration occurring when various salts are added to solutions of the commercial chrome liquor and of pure chromic sulfate. The curves are strikingly similar to those in Figs. 39 and 40. The time effect is shown in the case of sodium chloride; the other measurements were made 30 days. 30 20 10 10. 20 30 acid alkali 282 THE CHEMISTRY OF LEATHER MANUFACTURE after adding the salt in order to give time for the re-establishment of equilibrium. In order to avoid the complications obtained by mixing chlorides with chromic sulfate, Thomas and Baldwin also studied the effect of neutral chlorides upon a solution of pure chromic chloride. Fig. 130 shows the effect of adding increasing amounts of various neutral salts Ammonium Sulfate (after 30 days) Sodium Sulfate Magnesium Sulfate PH Value of Chrome Liquor Moles Salt per Liter Fic. 129.—Effect of Neutral Salts on the Hydrogen-Ion Concentration of Chrome Liquors Containing 13.86 Grams of Chromic Oxide per Liter. to a solution of the green modification of chromium chloride; measure- ments of hydrogen-ion concentration were made immediately after adding the salt and diluting to definite concentration and also after the solutions had stood for 50 days. Where no salt was added, there was a rise in pH value upon standing. The complex nature of the time effect after adding salt to a chrome liquor is shown in Fig. 131. Commercial chrome liquor and sodium chloride were mixed and diluted so that the final concentration of sodium chloride was twice molar and of chromic oxide 13.86 grams per liter. Determinations of hydrogen-ion concentration were made every CHROME TANNING 283 10 minutes for 4 hours and then at longer intervals for 3 days. It is evident that the action causing the increase in hydrogen-ion concentra- CHROMIC CHLORIDE SOLUTION pH Value of Chrome Liquor 1 2 3 4 Moles Salt per Liter Fic. 130.—Effect of Neutral Chlorides on the Hydrogen-Ion Concentration of a Solution of Chromic Chloride Containing 13.77 Grams of Chromic Oxide per parter, tion is subject to a time factor as well as the hydrolysis of the chromium salt. Following the observation by W. Klaber that chrome liquors can be made more basic, without causing precipitation, if salt be added to 3,0 COMMERCIAL CHROME LIQUOR (13.86 grams Crj0, per liter) pH Value of Chrome Liquor 4 8 ee 16 20 24 360 Time in Hours Fig. 131.—Change in Hydrogen-Ion Concentration of Chrome Liquor with Time after the Addition of 2 Moles of Sodium Chloride per Liter. them previously Wilson and Kern® noted that the resistance of a chrome liquor to precipitation by alkali is increased by the addition of *The Action of Neutral Salts upon Chrome Liquors. J. A, Wilson and E, J. Kern J. Am, Leather Chem. Assoc. 12 (1917), 445. 284 THE CHEMISTRY OF LEATHER MANUFACTURE all neutral salts‘and that sulfates are even more effective than chlorides. ‘he amount of alkali required to start precipitation in a chrome liquor was determined by titrating 10 cubic centimeters of filtered liquor with o0.IN sodium hydroxide until the first permanent turbidity appeared, as seen by looking through the liquor. tor a given chrome liquor, 3.7 cubic centimeters of standard alkali were required. To another portion of 10 cubic centimeters was added 0.04 gram molecule of sodium chloride; in this case 6.8 cubic centimeters of the standard alkali were required to start precipitation. Kepeating the experiment, taking in each case 10 cubic centimeters of the chrome liquor and 0.02 gram molecule of added salt, the following amounts of standard alkali were required to start precipitation in the presence of the neutral salt indi- cated: none, 3.7; KBr, 3.9; KCl, 40; KNO,, 4.2; NH Cl ais Nace 5-4; MgCh, 0.2; MgsO,, 10.5; NazsO,, 11.4; and (NH,)25O,, 11.6. cit least some of the effect ot the chlorides may be attributed to their action in increasing the hydrogen-ion concentration of the liquor. ‘lhe sulfates, however, decrease the hydrogen-ion concentration, but, since they increase the stability of the chrome liquor, it seems likely that they form addition compounds with the chromium salt less easily precipi- tated than the simpler salt. Diffusion of Chromium Salts into Protein Jellies. In vegetable tanning, the rate of diffusion of tannin into the fibers of the skin increases with increasing pH value. In chrome tanning, the reverse is true. An increasing pti value causes the molecules of chromium salt to form aggregates ot increasing size, greatly reducing the rate at which they dittuse into the skin. When neutral skin substance is brought into contact with a chrome liquor, both the free acid present in the liquor and the basic chromic sait begin to dittuse into it. But the greater rate of diffusion of the acid causes the liquor to become more basic. Procter and Law +¥ studied the relative rates of dittusion of the free acid and chromium salt of a chrome liquor into gelatin jelly by allowing a faintly alkaline solution of gelatin and phenolphthaiein to set in a Nessler tube and pouring the chrome liquor on top of the jelly. As the acid diffuses into the jelly, it discharges the color of the phenolphthalein, while the extent ot diffusion of the chromium salt:can be followed by its color the combination of both acid and chromium with the gelatin has a retarding eftect upon the rate of diffusion. ) this differential diffusion may not occur where the common practice of pickling skins prior to tanning is used. When the skins contain a great excess of acid, the chromium salt diffuses into them very rapidly but the rate of combination of chromium and collagen is corresponding! agers and : becomes necessary to neutralize some of the acid Rees the skins can become completely tanned meated by the chromium cai » oven note ee ea 70H. R. Procter and D. J. Law. J. Soc. Chem. Ind, 28 (1909), 297. CHROME TANNING 285 The Time Factor in Chrome Tanning. The progress of the chrome tanning of hide powder with time has been studied by Thomas, Baldwin, and Kelly.11 12 They first examined the action of the commercial chrome liquor described above. The chrome liquor was diluted to contain 17 grams of chromic oxide per liter. Two hundred-cubic centimeter portions were poured upon 5-gram portions of hide powder in glass-stoppered bottles. These mixtures were kept at room temperature (about: 20-00: ); agitated frequently, and Gltered off at definite intervals,—1, 2, 4, 6, 8, 12, 24, 48, 72, and 96 hours. They were filtered by suction on a dry paper in a Buchner funnel, the Gltrate was set aside for analysis, and the tanned powder was washed with 500 cubic centimeters of water in order to remove chromium salts ro) ° ° ° @ COMMERCIAL CHROME LIQUOR er Liter ed ro) ro) ro) J Pp ™ 0.0005 Moles Hydrogen-Ion 10 20 30 40 50 60 70 80 90 Time in Hours Fic. 132,—Change in Hydrogen-Ion Concentration of Chrome Liquors with Time. not chemically combined. The washed powder was partially dried at 4o° C. and then completely at TOO, es The filtered liquors were analyzed for hydrogen ion, acidity, and chromic oxide and the tanned powders for sulfate, chromic oxide, ash, and hide substance (nitrogen x 5.62). Measurements of hydrogen-ion concentration were made immedi- ately upon filtration of the liquors and, in order to exclude the possibility of attributing natural hydrolytic changes to absorption by hide sub- stance, parallel measurements were made upon a portion of the chrome liquor having no contact with hide powder. The parallel sets of meas- urements are shown in Fig. 132. In Fig. 133 are shown the amounts of chromic oxide and of sulfate combined with 1 gram of skin protein at different intervals of time. These were obtained from the analyses of the washed powders after tanning. The broken lines represent calculations made from analyses of the chrome liquors on the assumption that the concentration is uni- 11 The Time Factor in the Adsorption of the Constituents of Chrome Liquor by Hide Substance. A. W. Thomas, M. E. ‘Baldwin, and M. W. Kelly. J. Am, Leather Chem. Assoc. 15 (1920), 147. 2 The Time Factor in the Adsorption of Chromic Sulfate by Hide Substance. A. W. Thomas and M. W. Kelly. Ibid., 15 (1920) 487. 286 THE CHEMISTRY OF LEATHER MANUFACTURE form throughout all of the solution present in the system during tanning, We know from Chapter 5 that this assumption is not true, that the solution absorbed by the collagen jelly is less concentrated than the outer solution. This explains why these curves show lower values for combined sulfate and chromic oxide. Thomas and Kelly were well aware of this fact and presented the calculations made from the analysis of the solutions for the purpose of demonstrating the fallacies in this from analysis of leathers _from analysis of liquors Crg0z Combined with 1 Gram Hide Substance ”_— — Milligrams SOz Combined Megs, with 1 Gram Hide Substance 10 20 30 40 50 60 70 80 90 Time in Hours Fic. 133.—Progress of Combination of CrOs and SO; with Hide Substance during 4 Days’ Contact with Chrome Liquor containing 17 Grams Cr.O; per Liter. method, which is commonly used for measuring the extent of “adsorp- tion” of substances from solution by skin and other materials. The discrepancy is increased in the case of the sulfate determination by the fact that some combined sulfate is removed by washing, whereas the chromium-collagen compound is very stable. | They next .studied the action of a solution of pure chromic sulfate, but, using the same procedure, got only erratic results. They found it necessary, when using the pure salt, to soak the hide powder in water before adding the chrome liquor. The fact that the pure salt gave a solution very much more acid than the commercial salt may have had something to do with this. Five-gram portions of hide powder were CHROME TANNING 287 5 ha 0,020 F<} Ma mw ® 0,015 | 10,0104 ~~ ae @ 0,005 a Z = 10 20 30 40 50 60 70 64 Time in Hours days Fic. 134.—Change in Hydrogen-Ion Concentration of Chrome Liquors with Time. placed in each of a series of glass-stoppered bottles, 50 cubic centimeters of water were added to each, and the powders were allowed to soak over night. Then 150 cubic centimeters of chromic sulfate solution were r= + 2 150 La 3+ 125 Seah Ree eR ee — a3 oad ve 421007) // CHROMIC SULFATE SOLUTION 83 751 wen «75 from analysis of leathers eaey OO eid. 2° from analysis of liquors _ = @ 140 32 8&8 120 a CHROMIC SULFATE SOLUTION £0 Pa ee Se teres SE aes 100 2 Wee es cate Aine wail, / ere 80 / ff at / from analysis of leathers 2) { Bihar em, ipeee d 8 from analysis of liquors Mo) ae emeetn epee Te aa Ae ph ey bald ee eee oe ee 20 23 Ca! z 5 10 15 20 25 30 35 64 Time in Days Fic. 135.—Progress of Combination of Cr.O; and SO; with Hide Substance during 64 Days’ Contact with Chromic Sulfate Solution containing 16.4 Grams Cr,Os per Liter. ; 288 THE CHEMISTRY OF LEATHER MANUFACTURE added, making 200 cubic centimeters of liquor with a concentration of 16.4 grams of chromic oxide per liter. The rest of the experiment was performed just as in the case of the commercial chrome liquor, except for the fact that a time period of 64 days was covered. The variation in hydrogen-ion concentration in the control liquor and in the filtrates from the tanning tests is shown in Fig. 134. The trend of the curves is different from that of those in Fig. 132. The amounts of chromic oxide and of sulfate combined with 1 gram of hide substance are shown in Fig. 135. When the calculations were. made from the analyses of both leathers and liquors, the same sort of differences were noted as with the commercial chrome liquor. The only reliable figures are those obtained from the analyses of the leathers, shown by the continuous lines. The amount of chromic oxide combined with 100 grams of hide substance approaches a limiting value of 13.8 grams. Because this is almost exactly 4 times the author’s value of 3.38 grams, calculated to be the smallest amount of chromic oxide required to convert 100 grams of collagen into the chromium salt, Thomas and Kelly referred to it as tetrachrome leather. A comparison of Figs. 133 and 135 will show that the rate of tanning is very much less in the chromic sulfate solution, in which the hydrogen- ion concentration is about 20 times as great as in the commercial liquor. It will also be noted that the amount of chromic oxide combined with I gram of skin protein is greater for the commercial liquor after 4 days than the limiting value in the case of the pure chromic sulfate and has not reached a limiting value in 4 days. The significance of this will be made more apparent presently. The Concentration Factor in Chrome Tanning. The effect of the concentration of a chrome liquor upon the fixation of chromium by skin protein has been studied by Baldwin ** and by Thomas and Kelly.***° A solution of the commercial chrome liquor, described above, was made having a concentration of 202 grams of chromic oxide per liter and this was used at various dilutions to study the effect of concentration. A 200-cubic centimeter portion of each dilution was poured into a bottle containing hide powder equivalent to 5 grams of water-free hide powder. Another portion of each solution was set aside and at the expiration of 48 hours the hydrogen-ion con- centration was determined. The bottles were shaken at intervals for 48 hours and the contents were then filtered off by suction. Analyses were made of the liquors and of the tanned powders after washing and drying, the methods employed being the same as in the studies of the time factor. , % The Effect of the Concentration of a Chrome Liquor upon Adsorption by Hide Sub- stance. M. E. Baldwin. J. Am. Leather Chem. Assoc. 14 (1919), 433. %* The Effect of Concentration of Chrome Liquor upon the Adsorption of Its Constitu- ents by Hide Substance. A. W. Thomas and M. W. Kelly. J. Ind. Eng. Chem. 13 (1921), 31. * Equilibria between Tetrachrome Collagen and Chrome Liquors; the Formation of Octachrome Collagen. Ibid., 14 (1922), 621. CHROME TANNING 289 Fig. 136 shows the variation in hydrogen-ion concentration in the filtrates and also in the control liquors which had no contact with hide powder. Fig. 137 shows the effect of concentration upon the amount of chromic oxide fixed by 1 gram of skin protein in 48 hours. Where the calculation was made from the analyses of the liquors, a ridiculous result was obtained, as expected. In this calculation it is assumed that the decrease in concentration of the liquor represents the amount of solute combined with the hide powder, but the concentration of the stronger liquors is actually increased by the introduction of hide powder, 0,006 (COMMERCIAL CHROME LIQUOR 0,005 0,004 0,003 Moles Hydrogen Ion per Liter 25 50 Ot we eLOO. sel 25 Grams Chromic Oxide per Liter Fic. 136.—Change in Hydrogen-Ion Concentration of Chrome Liquors with Increasing Concentration. due to the absorption of a greater proportion of water to chromium salt than existed in the solution before the introduction of the hide powder. It is interesting to note that the method of calculation giving these ridiculous results is the same in principle as that of the official method of tannin analysis of the American Leather Chemists Associa- tion, described in Chapter 12. The determination of combined chro- mium by analysis of the washed leathers corresponds to the Wilson-Kern method of tannin analysis, also described in Chapter 12. The reason for the point of maximum at a concentration of 15 grams of chromic oxide per liter is not entirely clear, although a number of causes may be assigned to the falling off in rate of combination at higher concentrations, among which may be mentioned the increasing hydrogen-ion concentration, as shown in Fig. 136, the increasing salt concentration, and the probability of the formation of addition com-: pounds, It is interesting to compare this curve with those for the rate 2900 THE CHEMISTRY OF LEATHER MANUFACIORS of vegetable tanning as a function of concentration, shown in Chap- ter 13. The experiments just described were repeated exactly, except for the fact that the hide powders were kept in the chrome liquors for 8.5 months. The results are shown in Fig. 138. Curiously enough a point of maximum occurs having a value of 26.6 grams of chromic oxide per 100 grams of skin protein, which is approximately 8 times the author’s calculated minimum of 3.38. Moreover a point of inflection occurs in the curve at a point giving just half of the maximum value. On the (from analysis of leathers) LIQUOR Milligrams Cro0q Combined with 1 Gram Hide Substance 50 100 150 200 Grams Chromic Oxide per Liter Fic. 137.—Effect of Concentration of Chrome Liquor upon Combination of Chromium with Hide Substance. assumption that they had obtained octachrome collagen, Thomas and Kelly calculated the combining weight of collagen as 94, a value of the same order of magnitude as those of the amino acids making up the protein molecule. This degree of combination of collagen and chro- mium is the highest ever reported in the literature. The fact that only a tetrachrome collagen was obtained after 64 days of contact with chromic sulfate solution, whereas an octachrome collagen was obtained with the commercial chrome liquor may possibly be explained by the differences in hydrogen-ion concentration of the two series of liquors. On this assumption, only half of the total number of carboxyl groups are capable of attaching chromium bonds at the higher acidities. The possibility is thus suggested that a series of col- lagen salts from monochrome to octachrome might be obtained by tanning hide powders for a sufficient length of time at different pH CHROME TANNING 291 values. Experiments to settle this point will be made as soon as the opportunity affords. The reversibility of the action leading to the formation of tetra- chrome collagen was studied by Thomas and Kelly. Since the chrome Tetrachrome Collagen "4 Milligrams Cro0, or 50g Combined with 1 Gram Hide Substance 50 100 150 200 Grams Chromic Oxide per Liter Fic. 138.—Effect of Concentration of Chrome Liquor upon Combination of Chromium and Sulfate with Hide Substance. tanned hide powder does not lose any measurable amount of chromium upon several hours’ washing, they decided to allow tetrachrome collagen to remain in contact with solutions of varying chromium content for several months. (after 8.5 months) Cro0g Milligrams Cro0g or 50g Combined with 1 Gram Hide Substance 50 100 150 200 Grams Chromic Oxide per Liter Fic. 139.—Effect of Concentration of Chrome Liquor upon Combination of .Chromium and Sulfate with Tetrachrome Collagen. Portions of tetrachrome collagen containing just 5 grams of hide substance were placed in a series of 12 bottles and covered with 200- cubic centimeter portions of chrome liquor of various concentrations. The bottles were kept sealed to prevent evaporation and were shaken once a week. At the end of 8.5 months the contents were filtered and 292 THE CHEMISTRY OF LEATHER MANUFACTURE < the powders washed free from soluble matter, dried, and analyzed. The results are shown in Fig. 139. They show that a hydrolysis of the chrome collagen and collagen sulfate compounds takes place in water and in very dilute chrome liquor, which was also shown by an increase in hydrogen-ion concentration. With further increase in concentration of the chrome liquor, there is a steady addition of Cr,O; and SOs, approaching the condition of octachrome collagen. But with further increase in concentration the COMMERCIAL CHROME LIQUOR @ oO (17 grams Crp0, per liter) on oO ~ oO oO oO Hide Substance in 24 Hours nw nes oO oO (pickled calf skin) Milligrams Crp0z Combined with 1 Gram 1 2 3 Moles of Added Salt per Liter Fic. 140.—Retardation of Chrome Tanning by Neutral Salts at Various Concentrations. curve does not fall below the tetrachrome value. This shows that the curve in Fig. 138 does not represent the equilibrium condition of a reversible reaction. On the contrary, it suggests that the fall in the curves is due to the increasing hydrogen-ion concentration, which inhibits the combination of collagen and chromium. Effect of Neutral Salts upon Chrome Tanning. In studying the effect of neutral salts upon the chrome tanning of calf skin, Wilson and Gallun 1° selected sodium sulfate and the chlorides of ammonium, sodium, lithium, and magnesium because of their differ- ent degrees of hydration in aqueous solution. The commercial chrome 7° The Retardation of Chrome Tanning by Neutral Salts. J. A. Wilson and E. A. Gallun. J. Am. Leather Chem, Assocz 15 "C1g20), 273. ; CHROME TANNING 203 Moles of Salt per Liter CREB EAU Ess {Tf p) Kind of Salt Lithium Chloride \) / | Magnesium Chloride ( Fig. 141.—Strips of Chrome Leather, All Originally of Equal Area, Taken After Tanning for 24 Hours and Then Boiling in Water for 5 Minutes and Drying. Compare with Curves in Fig. 140. 294 THE CHEMISTRY OF LEATHER MANUFACTURE mixture which they used showed by analysis: Cr2Os3, 24.2 per cent; Fe,Os;, 0.6 per cent; Al,Os, 2.7 per cent; SOs, 39.5 per cent; Cl, 0.4 per cent; and basicity corresponding to the formula Cr(OH)1.4(SO.)o.s. Solutions of this chromium preparation were mixed with solutions of the various neutral salts so as to give liquors containing 17 grams of chromic oxide per liter and definite quantities of salt. In each test a strip of pickled calf skin, 16 square inches in area, was covered with 200 cubic centimeters of chrome liquor, in a bottle, and shaken at intervals for 24 hours. The pieces of skin were then washed by shaking with successive changes of water until the wash water gave only a very faint test for chloride or sulfate. Strips of equal area were cut from each piece and immersed in boiling water for 5 minutes, in order to determine the nearness to complete tannage. The remaining portions were cut into small pieces, dried and analyzed. The effect of increasing concentration of the chlorides of ammonium, sodium, lithium, and magnesium upon the amount of chromic oxide combined with a unit of hide substance in 24 hours is shown in Fig. 140. In Fig. 141 are shown the strips of skins after being subjected to the action of boiling water for 5 minutes. The appearance of these strips parallels the curves, in a rough sort of way; that is, they gen- erally show the greatest shrinkage in area where the amount of com- bined chromic oxide is smallest. The first strip in each series proved to be fully tanned and showed no shrinkage in area. In another series of tests, chrome liquor having a concentration of 10 grams of chromic oxide per liter was used and the effect of sodium sulfate was studied in addition to the effect of the chlorides noted above. The results for the chlorides were practically the same as in the first experiment, except for the observation of a point of minimum in the sodium chloride curve at 2 moles per liter of salt and a slight rise to 3 moles. In the case of the sodium sulfate, there was a steady drop in fixation of chromic oxide from 10.09 grams per 100 of hide substance when no added salt was present to 3.57 when the solution was saturated with the salt. Wilson and Gallun attributed the action of the chlorides, in part, to their hydration. The removal of .water from the réle of solvent would have the practical effect of increasing the concentration of all of the constituents of the chrome liquor, expressed in terms of the free solvent. Fig. 137 shows that increasing the concentration of the chrome liquor from 17 grams of chromic oxide per liter does retard the rate of combination of collagen and chromium. That the chlorides actually concentrate the solute in the free solvent is also indicated by the fact that they increase the hydrogen-ion concentration as measured by the hydrogen electrode. But at half-molar concentration the effect. only of magnesium chloride is in its expected relative position; being most highly hydrated it should produce the greatest effect, which it does. At 3-molar concentration all 4 chlorides are in an order inverse to that expected unless it may be assumed that at very high concentrations of chrome liquor a further increase in concentration causes an increased rate of tanning. CHROME TANNING 295 The action of sulfates is in a category different from that of the chlorides, as shown by the fact that they decrease the hydrogen-ion concentration of acid solutions and of chrome liquors. Wilson and Gallun suggested that the retarding action of sodium sulfate was prob- ably due to the formation of addition compounds between the added salt and the chromium compounds which tan less readily than the original chromium compounds, but that the action was further com- plicated by the hydration of the sodium sulfate. a H 5 m 120 @ s g 110 16. ° §.5 28 Cr,0, per liter aaerone S 1004 ws 1 a < AGS See A Se aeceaaaed DOA’ Naso A oe Dae eee Nall © a ‘\ Bots oneers e 80 ay ne vent eer as is] SS ces 2 Nees. Sp ea, q Ate Nacl Bt, 70 i =) 60 » : Na,s a,90 4 Sols =. Te) nl “go sibs 5: 5 os oO we on 8 pepe we epee’ 12) 20 .0S 9D.. GSo.n Vv og. BO. On Sa Hen 326 FINISHING AND MISCELLANEOUS OPERATIONS 327 face which has been cut away from the leather below it at a thickness of less than 15 microns. (The exact measurement of thickness could not be made because this was the first cutting of the paraffin block at 15 microns that included any leather at all and was taken so as not to lose the upper surface.) The section was stained for 2 minutes in a I-per cent solution of indigo carmine, The specimen was taken from a skin about to be fatliquored. The average distance between the fibers of the surface is about 2 microns. If size of particle alone counted, it would merely be nec- essary to prepare the dispersion so as to get particles having a diameter less than this. The large empty spaces in the figure are the openings of the empty hair follicles. In the ordinary course of fatliquoring, these probably become filled with oil as soon as the emulsion breaks. Fatty Acid Spews. Sometimes when finished leathers are chilled, the surface becomes coated with a white crystalline deposit resembling a thin film of snow. ‘This deposit, known to the trade as spew, usually consists of saturated fatty acids having a high melting point. For this reason, many tanners try to avoid the use of oils containing much stearin or free stearic or palmitic acids. Where sulfonated oils have been used, the deposit may contain sulfonated fatty:acids, which are more difficult to remove from the surface of the leather than pure stearic or palmitic acid. Although the spew does no harm to the leather, it ig undesirable because it detracts from its appearance. Its removal, however, is a comparatively simple matter and may be effected by wiping the surface with a cloth wet with naphtha or with a soap solution. Fahrion® found that the splitting of a fat, with the liberation of saturated fatty acids, is not the only cause of spew of this kind. Glycerides may appear on the surface of leather which have a lower melting point than those left in the leather. But when the fatty acids are liberated from the spew, they are found to have a higher melting point and a greater tendency to crystallize than the fatty acids liberated from the glycerides remaining in the leather. A glyceride mixture is likely to cause spewing if another mixture can be separated from it by fractionation which has a lower melting point, but a higher con- tent of saturated fatty acids. He points out that the less the tendency of a fat to crystallize, the more suitable it is for purposes of fatliquoring. When fatliquors containing saturated fatty acids are used, the danger of spewing can be greatly lessened by incorporating in the fat- liquor materials like mineral oils or sulfonated castor oil. These remain liquid at ordinary temperatures and are solvents for the fatty acids and glycerides which form ordinary spews. When a piece of finished leather spews, it is often noted that the greatest deposits of fatty acids occur where the leather is thinnest. This is due simply to the fact that leathers fatliquored in the ordinary way have a higher total fat content in the thin parts than in the heavier 6 Properties of Fat in Leather. W. Fahrion. Gerber 43 (1917), 123. 328 THE CHEMISTRY OF LEATHER MANUFPACTGRe regions. In fact the fat content of the various parts may be taken as inversely proportional to the thickness. This is easily explained by the fact that the oil globules are deposited only on the surfaces of the leather during fatliquoring. Equal areas of leather thus receive the same amount of oil. If the butt is twice as thick as the shanks, it will receive only half as much oil per unit volume. For the same reason, if a very thick skin is fatliquored along with a very thin one, the thick one will not get enough oil, while the thin one will get more than its share. A type of spew less common, but more difficult to remove, than that made up of saturated fatty acids is the resinous spew caused by the oxidation of unsaturated fatty acids in the leather. If a vegetable tanned leather contains much soluble tanning material, the tendency for the free tannin to absorb oxygen from the air increases upon fat- liquoring due to the resulting increase in pH value. The oxidized tarinin seems to give up its oxygen readily to the unsaturated fatty acids of cod and similar oils. The higher the pH value of the solu- tion in the leather and the greater its content of free tannin and oxidizable fatty acids, the greater will be the danger of the formation of resinous spews, consisting of oxidized fatty acids. Most tanners appreciate that it is undesirable to allow a large amount of soluble neutral salts to remain in the finished leather and, since such salts are easily washed out during the process, there is no good reason why they should not be removed before finishing. Never- theless leathers are occasionally found on the market showing a de- posit of salt crystals on the surface, resembling spew. It is, of course, an easy matter to differentiate between such salt deposits and ordinary fatty spews. When leather is kept in a cool, damp place for a long time, it is apt to be subject to the growth of molds. Sometimes these show the light green color of the common mold penicillium glaucum, but often they appear like a white, powdery deposit, resembling the spew sometimes occurring on leathers fatliquored with sulfonated neatsfoot oil. Coloring. Leather is dyed either before or after the fatliquoring process, depending upon the kind of leather produced and the nature of the other operations. Natural dyestuffs are still employed for coloring some kinds of leather, but have been largely supplanted by artificial products. In coloring vegetable tanned leather, basic dyestuffs are usually preferred because they combine readily with tannin and give more intense shades than the acid dyestuffs. When vegetable tanned leathers are to be colored with basic dye- stuffs, they must first be washed so that no free tannin will diffuse into the dye bath, where it would be precipitated by the dye and tend to cause spottiness and discoloration of the leather. The leather is sometimes given a short rinsing in sodium carbonate solution in FINISHING AND MISCELLANEOUS OPERATIONS 329 order to free the grain surface from precipitated tannin and to strip off any excess of fixed tannin, the object being to get clearer and more uniform coloring. The leather is then drummed in a solution of a salt of titanium or antimony, such as titanium potassium oxalate or antimony potassium tartrate, which serves the double purpose of acting as a mordant and of precipitating any remaining free tannin that might otherwise diffuse into the dye bath. The leather is then given a thorough washing and is then drummed in a solution of the dyestuff. Since basic dyestuffs are precipitated by hard waters, where these must be used, they should first be acidified with acetic acid, which will prevent precipitation. An objection to the use of acid dyes is the fact that strong acids are necessary to develop the color. The sulfuric acid often used exerts a detrimental effect upon vegetable tanned leathers, if not neu- tralized after coloring. Where later neutralization is objectionable, it is much better to use formic acid to develop the color since this acid appears to have no harmful effects whatever, if not used in ex- cess. It has often been stated that acid dyes are faster to light than basic ones, but this is not uniformly true for leather. Chrome leather may be dyed in a manner similar to that of veg- etable tanned leather if it is first given a light surface retanning with a vegetable tanning material, such as gambier or sumac. The leather, after tanning and neutralizing, is drummed for a short time in a dilute tan liquor, washed, and then mordanted with a titanium or antimony salt, washed again and finally colored, after which it is fatliquored. Where the fatliquoring may cause a bleeding of the color, the dyeing must follow the fatliquoring. By the use of acid dyestuffs, chrome leather may be dyed directly, without the necessity for a vegetable retanning. In this case, how- ever, either the fatliquoring must be done before the coloring or else the color must be fixed in some way before fatliquoring. One method of accomplishing this is to divide the coloring operation into two parts, the leather being dyed first with an acid dye and then with a basic dye. Since the two kinds of dyestuffs coprecipitate each other from solution, forming insoluble lakes, they show little tendency to bleed into the fatliquor. . Direct colors may be used on either chrome or vegetable leathers. Alizarine and developed dyes find a use in the coloring of chrome leather. Sulfide dyes are frequently used where it is necessary to get a color very resistant to washing. For lists of the individual dyes and details of their application to leather, the reader should consult the various practical handbooks on leather dyeing. The shade of colored leathers may be darkened, or “saddened,” by drumming them in solutions of salts of iron, copper, or other heavy metals. In the production of black leathers, it is common to drum the leather first in a slightly alkaline solution of logwood extract and then in a solution of ferrous sulfate to develop the black. This is often topped by a second dyeing with some artificial black dyestuff. 330 THE CHEMISTRY OF LEATHER MANUFACTURE After the leather has been dyed, it is customary to rinse it in cold water, smooth it out by slicking, lightly oil the surface, and then dry it. The chemistry of leather dyeing has not yet overtaken the art, although the process is primarily a chemical one. The effect obtained from a given color bath is a function of pH value, concentration, temperature, etc., but the literature contains no record of investiga- tions of leather dyeing under rigidly controlled conditions of pH value, etc., like those on tanning described in Chapters 11 to 14. One nae infer that similar principles are involved in both tanning and yeing. Finishing. For descriptions of the numerous mechanical operations and machines used in making leather, including splitting, shaving, slicking, samming, drying, staking, rolling, brushing, boarding, plating, glazing, and embossing, reference should be made to books treating the subject from a more obviously practical viewpoint, like those of Procter,’ Lamb,® and Rogers.® When the leather has been dried, after coloring, it is subjected to various mechanical operations in order to give it the desired physical properties. Ordinary shoe upper leather is coated with a size or finish in order to make it water repellent and more pleasing to the eye. These finishes usually have as a base an aqueous dispersion of gelatin, casein, blood albumin, egg albumin, gum tragacanth, or Irish moss, and are often mixed with colors or pigments. Another popular finishing material consists of a mineral pigment ground in a solution of shellac and borax in water; this is sprayed onto the grain surface of the leather by means of an atomizer. | Patent leathers are coated with a varnish made from boiled linseed oil, driers, and pigments. The leather is dried in a hot oven after the varnish has been applied. The surface is then rubbed smooth and a second coat applied, after which it is again dried in the oven. This may be repeated several times to get the desired effect. The surface of the leather is finally exposed to the sun or to ultra-violet rays for several hours. Suede or ooze calf leathers are made from the skins of slinks, chrome tanned, colored, and finished on the flesh side. The so-called buck leathers are made from chrome tanned cow hide, split to give a sufficiently thin layer, including the grain. The grain is buffed on an emery wheel and then dusted with a dry pigment. The names of - many of the commoner leathers indicate the method of finishing rather than the animal furnishing the skin. ee of Leather Manufacture. H. R. Procter. D. Van Nostrand, New York ® Leather Dressing. M. C. Lamb. Leather Trades Publishing Co., London (1909). ® Practical Tanning. A. Rogers. Henry Carey Baird & Co., New York (1922) a ; AUTHOR INDEX Abderhalden, E., 66, 70 Abt, G., 135-7, 141 Alexander, J., 72 Alsop, W. K., 228 Andreasch, F., 147 Andrews, J. C., 78 Apostolo, C., 316 Atkin, W. R., 68, 161, 166, 305-6 Balderston, L., 279 Baldracco, G., 135 Baldwin, M. E., 80, 91, 92-3, 280-3, 285-8 Bassett, 305-6 Bast, T. H., 8, 18 Beans, H. T., 128 Beck, A. J., 324 Becker, H., 135-6, 179 Benedict, A. J., 113 Bergmann, M., 214 Bien, Z., 108 Blockey, F. A., 218 Blockey, J. R., 306 Bogue, R. H., 119-20, 126, 132 Bonnet, F., 307 Booge, J. E., 78 Bowker, R. C., 324 Brown, 200 Burton, D., 306 Burton, E. F., 231 Buxton, B. H., 236 Carmichael, T. B., 270 Churchill, J. B., 324 Conn. EK. J., 108 Corran, J. W., 296 Coulter, C. B., 108 Cross, C. F., 270 Cruess, W., 180 Dakin, H. D., 70 Daub, G., 8, 112, 116, 154, 169, 184-91, 196-7 Davidsohn, H., 108, 189 Davis;-G.. E., 112-3 Dean, A. L., 213 Dekker, J., 205 Dennis, Martin, 278 Donnan, F. G., 127-30 Dumanski, A., 120 94-9, I14, I2I-3, Eastlack, H. E., 128 Eastman, G. W., 78-9 Eitner, W., 135, 141, 201 Elliott, F. A., 116-7, 143-4 Emmett, A. D., 73 Enna, Fini, 262, 319 Evans, U. R., 270-1 Ewald, A., 73 Fahrion, W., 276-7, 317, 327 Fales, H. A., 89 Falk, K. G., 185, 189 Fischer, Emil, 66, 213-4 Fodor, A., 66, 108 Foster, S. B., 231-9, 262, 295-6 Freudenberg, K., 213-5 Freundlich, H., 131, 236 Frieden, A., 216-7 Gage, S. H., 16 Gallun, Albert F., Jr., 35, 111, 113, 161, 166-72, 175-7, 188, 263, 268-9, 324 Gallun, Arthur H., 8, 14 Gallun, Edwin A., 292-4 Garelli, F., 316 Gibbs, J. Willard, 131 Gies, W. J., 73 Graeb, 215 Graham, Thomas, I19, 315 Grasser, G., 231, 319-20 Greenwood, C. V., 270 Grineff, W., 108-9 Gross, J., 108 Hammarsten, O., 69 Hampshire, P., 154 Hardy, 3W.~ 8,,;. 109 Harned, H. S., 80, 146 Platt woken) 324 Harvey, A., 205, 280 Hassan, K., 230 Hey, A. M., 319 Higley, Parker, 114 Hill, J. B. 321 Hofmeister, F., 72-3, 105 Hollander, C. S., 166 Hoppenstedt, A. W., 256 How Tek, 3i4 Hough woe alesis Hull, M., 189 332 Jackson, D, D., 314 Jettmar, J.°413 Johnson, O. C., 108 Jones, H. C., 78-9 Kadish, H. L., 164-5 Kadish, eVsitl, 4 1O4=n Kato, Y., 78 Kelly, Margaret W., 8, 109, 113, 258-67, 274-6, 285-92 : Kendall, J., 77-8 Rem sh, J. 265-6, 283-4 Klaber, W., 283 Knapp, F., 278 Kohlrausch, F. W. G., 78-9 Krall, L., 184-5 Kraus, Cl AY 3 Kihne, W., 73 Lamb, M. C., 270, 280, 330 Law, D. J., 160, 162-4, 181, 284 Le Count, 70 Lees A, B36 Lewis, W. C. McC., 94, 296 Lloyd, Dorothy J., 109, III, 114 Loeb, Jacques, 8, 94, 104-6, 108-9, 112, 114-6, 119-27, 130-2, I61 Loeb, M. H.,:8 [oie =} ahs ei ko Lumiére, A., 311 Lumiere, L., 311 Lundén, H., 77 McBain, J. W., 1109, 130 McCandlish, D., 8, 243 McLaughlin, G. D.,, 146, 269 Mandel, J. A., 69 Marriott, R. H., 166 Mendelssohn, A., 108 Merryman, G. W., 321 Meunier, L., 276-7, 316-9 Michaelis, L., 108, 189, 206 Moeller, W., 136, 324 Morris, 200 Mouries, M., 200 Nance, C. W., 270 _ Nelson, J. M., 89 Y Neun obi 189 \ Nihoul, E., 166 \Northrop, J. H., 74, 187 \Noyes, A: A., 78-9 Oakes, Eee Vase Orchard, 149 Ostwald, W., 77-8 Pae§sler, J., 134-6 Paniker, R., 214 Parker, J. G., 271 III-4, 208-11, 218-31, 257, AUTHOR ANDEX. Payne, M., 318 Pechstein, H., 108 Plimmer, R. H. A., 65, 73 Poma, G., 89, 90 Ponder, -C3-r4a Popp, G., 179 Porter, Hs (tog Porter, R. E., 269 Pottevin, H., 213 Powarnin, G., 277 Powis,. F., 127,° 330 Procter, H. R, 8, 94, 98-104, 118-9, 138, 140, 150, 161, 205, 217-8, 220, 272-6, 279, 284, 297, 313, 330 Pullman, E., 318 Pullman, J., 318 Ricevuto, A., 306 Richards, T. W., 307 Rideal, E. K., 149, 270-1 Rogers, A., 330 Rohm, O., 166, 174, 181, 314 Romana, "Cp igs : Rona, P., 108, 206 Rosenthal, G. J., 67-8, 181 Ross, \H. Cree Salmon, C. S., 130 Sand, Ho J Ss 9166 Schattenfroh, A., 140-1 Schiffss Hoare Schlichte, A. A., 163-4 Schmidt, C. E., 134 Schnurer, J., 141 Schucht, H., 236 Schultz, A., 278 Schultz, G. W., 226 Seshachalam, K:, 230 seveik, Eouiat seyewetz, A. 311, 316-7 Seymour-Jones, Alfred, 16, 35, 38, 139, 141, 163, 166, 168, 181, 184, 262 Seymour-Jones, Arnold, 140 Seymour-Jones, Frank L., 8, 70, 73-4, 143, 188, 197, 305-8 Sheppard, S. E., 8, 113, 116-8, 142-4 Sherman, H. C., 185, 189 Siewert, 308 Smith, \C, RiGya stro Smith, G. McP., 92 Sommerhoff, E. O., 316 Sorensen, S/) Pein te sosman, R. B., 78 Stiasny, E., 163, 165, 214, 271, 319 Strecker, A., 213-4 Sweet, 9: 53 119 erie Takahashi, D., 108 Teague, O., 236 Thomas, Arthur W., 8, 14, 73, 74, 76-93, 109, I13, 130, 188, I9I, 197, 216, AU LORAIN DEX, 231-9, 256-67, 274-6, 280-3, 285-92, 295-6, 324. Hac, 68, Thompson, 305-6 suuat, WU; J., 166, 315 Tilley, F.:W., 141 Trunkel, H., 274 Turnbull, A., 270 Van Tassel, E. D., Jr., 324 Van Tieghem, P., 213 Voitinovici, A., 70 Vollbrecht, E., 215 II9Q-20, 230, 333 Walden, P., 78, 213 Walker, N., 70 Wells, 70 Whitmore, L. M., 324 Whitney, 308 Wilcox, W. H., 201 Williams, O. J., 271 Wilson, F. H., 180 Wilson, Wynnaretta H., 99-106 Wood, J. T., 147-8, 156, 160, 162-4, 166, 174, 176, 179-81, 197, 199-201, 308 Yocum, J: H. 135 SUBJECT INDEX Acetic acid, effect upon vegetable tan liquors, | 235 ionization of, 77, 82 Acids formed in drenching, 200 Acids, order of strength, 79 Acid unhairing, 166 Adipose tissue, 23, 33 vertical section of, 32 Adsorption, 131-2 Aging of leather, 228-30, 274-6 Alanine, 65, 70 Albino rat leather, grain surface of, of Albumin fixative, 18 Albumins, 67-8 isoelectric points of, 108 Albuminoids, 67 Albumose, action of 189 Aldehyde tanning, 318-9 Aigarobilla, 206, 256 Alligator leather, 243 vertical section of, 251 Aluminum salts, precipitation of vegetable tan liquor by, 236 tanning with, 311-3 Amino acids, 65-6, 70 Ammonium chloride, effect upon bating, I90-I chrome tanning, 292-4 pH value of acid solutions, 89-90 pH value of chrome liquors, 283 precipitation of chrome liquor by alkali, 284 Ammonium hydroxide, ionization of, 78, 88 unhairing with, 165 Ammonium sulfate, effect upon pH value of acid solutions, 89-90 pH value of chrome liquors, 282 precipitation of chrome liquor by alkali, 284 Amorphous tannins, 215 Amylolytic enzymes, 174, 181, 200 Inthrax, disinfection against, 139-41 Attimony salts in mordanting leather, trypsin upon, : 329 Arginine, 66, 70 Arstnic disulfide, unhairing with, 157 \ Arteries, 33, 38 vertical section of, 32 Aspartic acid, 65, 70 Aspergillus niger, 213-5 Astringency, 224, 231, 241, 260, 262 Autoclave, leaching in, 208 Babool bark, 206 Bacillus anthracis, 139 butyricus, 148 coli, 174 erodiens, 179 fluorescens liquefaciens, 147, 149 furfuris, 200 gasoformans, 147 liquidus, 147 megeterium, 147, 201 mesentericus fuscus, 147 mesentericus vulgatus, 147 mycoides, 147 prodigiosus, 163 subtilis, 147 Bacterial action in bating, 179-80 drenching, 200-1 liming, 162-3 pancreatin unhairing liquors, 167 production of salt stains, 134-7 soaking, 146-50 sweating, I51-6 Baldness, 31 Barium chloride, effect upon pH value of acid solutions, 89-90 pH value of chrome liquors, 283 precipitation of vegetable tan liquor, 235 Beers hydroxide, ionization of, 78, Bases, order of strength, 79 Bating, 173-98 as a. hydrogen-ion concentration control, 178 bacterial action in, 179-80 effect of ammonium chloride, 190-1 pancreatin, 189-90 pH value, 185-9 time, 188-90 to delime skins, 179 to digest collagen, 197-8 to reduce plumping, 175 to remove elastin, 181-97 \ 334 \ \ \ YUSIEC LINDE X Bating, with bacteria, 179-80 with dung, 173, 184 with pancreatin, 181-97 Beam, 155 Beamhouse, 142 Beamster, 155 Bear leather, grain surface of, 37 Birch bark, 206 Bismarck brown stain, 19 Bismuth tanning, 316 Blackheads, 31 Bleaching leather, 322 Blood, 75 decolorizing tanning extracts with, 211-2 Bloom, 205, 215 Boiling test in chrome tanning, 293-4 Boric acid, ionization of, 77, 84 Bran in drenching, 199 Bromine tanning, 316-7 Buck leather, 330 Bulb, hair, 28, 30-64 Butyric acid, ionization of, 77, 84 Calcium chloride, precipitation of vege- table tan liquor by, 236 Calcium hydroxide, ionization of, 79 precipitation of vegetable tan liquors, 210-1 unhairing with, 156-60 Calcium sulfarsenite, unhairing with, 157 Calf leather, grain surface of, 36, 197 horizontal section through grain sur- face, 326 vertical sections of, 2098, 299, 303 (see also under leather) Calf skin, 48-53 vertical sections of (before tanning), fresh, 49, 52, 53 from lime liquor, 158, 159 unhaired in pancreatin solutions, 170, I71 before bating, 182 after bating, 183 Camel leather, 243 grain surface of, 37 vertical section of, 253 Canaigre, 206 Carbol-xylene, 17 Carbonic acid, ionization of, 77 Casein, hydrolysis by trypsin, 189 isoelectric point of, 108 Caseinic acid, 66 Catechins, 215 Catechol tannins, 204 Cells, epithelial, 23 335 Cells, fat; 33)'35, 51 growth of, 25 migratory, 35 Cerealin, 200 Cerium tanning, 316 Chamois leather, 317 Chemical constituents of skin, 65-75 Chestnut wood, 206, 219-39, 256 Chinese nutgalls, 214 Chlorine tanning, 316-7 Cholesterol, 75 Chrome leather (see leather) Chrome liquors, dialysis of, 307 diffusion into protein jellies, 284 effect of added acid or alkali, 281 dilution, 280 , neutral salts, 281-3 salts of hydroxyacids, 297 electrophoresis of, 305-8 hydrolysis of, 280-4 manufacture of, 278-9 pH value of, 93; 280-97 precipitation of, 283-4 tanning with, 278-308 ultrafiltration of, 307 Chrome retan leather, 309 vertical section of, 310 Chrome tanning, 278-308 boiling test in, 203-4 effect of concentration, 288-92 neutral salts, 292-6 pH value, 285-97 salts of hydroxyacids, 297 time, 285-8 theory of, 305-8 Chromium collagenate, 279-80 Chromium salts (see chrome liquors) Chromogenic bacteria, 135-6 Citric acid, ionization of, 78, 85 Clarifying tanning extracts, 211-2 Clearing agent, I7 Cod fish skin, 64 vertical section of, 62 Collagen, 35, 72-3 combining weight, 273, 279-80 compounds with chromium, 279-80, 305 tannin, 273-4 fibers, 33, 35, 39-64 Poa by acids or alkalis, 168, 169 enzymes, 73-4, 168, 197-8 hot water, 72 isoelectric point of, 100, 175-7 relation to gelatin, 72-3 two forms of, 109, 176 Colloidal dispersions, tanning with, 316 theory of stability of, 127-30 336 Coloring leather, 328-30 Color value of tan liquor, effect of oxidation, 209-10 pH value, 208-9 Combining weight of collagen, 273, 279-80 gelatin, 102-3, 311 Conductivity of gelatin dispersions, 120 Connective tissue, 23, 33, 35, 38-64 Co-operation between university and industry, 12-4 Cordovan leather, 58, 243, 304 grain surface of, 36 vertical sections of, 246, 301 Corneous layer of epidermis, 26-7, 39- 64, 70, I51-4, 157-9, 169 Cow hide, 39-48 horizontal sections of, 43, 44, 45, 46, 47 vertical sections of, 40, 41, 192 Cow leather, grain surface of, 36 vertical section of, 310 Curliness of hair, cause of, 29, 51 Cutch wood, 206 Cystine, 65, 70 Dandruff, 25 Decolorizing tanning extracts, 211-2 Dehydrating specimens of skin, 17 Deliming, 179, 199-203 Depsides, 215 Derma, 23, 33-64 Dialysis of chrome liquors, 307 vegetable tan liquors, 233 Diastase of translocation, 200 Diffusion into protein jellies of chrome liquor, 284 vegetable tan liquor, 256-8 Digallic acid, 213 Disinfection of skin, 139-41 Divi-divi, 206, 256 Double refraction in gelatin disper- sions, I20 Drenching, 199-202 Dried forms of gelatin jellies, 144 Drying of gelatin jellies, 143 skins, 137-8 tanning extracts, 212 Dung, composition of, 174 use of in bating, 173 Dyestuffs, use of in coloring leather, 328-30 Ectoderm, 25. Edestin, isoelectric point of, 108 Egg albumin, isoelectric point of, 108 Elasticity of gelatin jellies, 118 SUBJECT INDEX Elastin, 70-1 hydrolysis by pancreatin, 181-97 Elastin fibers, 35, 42, 182-3, 192-5 removal in bating, 181-97 effect upon leather, 196-7 Electrical tanning, 270-1 Electrophoresis of chrome liquors, 305-8 vegetable tan liquors, 231-3 Eleidin, 27, 29, 70 Ellagic acid, 205, 215 Emulsifying agents, 324 Emulsin, 215 Enzymes, action in bating, 181-91 unhairing, 166-72 activation of, IQI amylolytic, 174, 181, 200 cerealin, 200 emulsin, 215 inactivation of, 187-91 lipase, 174, I81 list of in dung, 181 pancreatin, 73-5, 166-72, 181-91 pepsin, 73-5, 181 rennin, 174, I81 tannase, 215 thrombase, 68, 172 trypsin (see pancreatin) | Epidermal system, 25, 27-33, 39-64, 60 Epidermis, 23, 25, 30-64 horizontal section of, 43 vertical section of, 26 Epithelial cells, 23 tissue, 23 Erector pili muscle, 31, 39-60 Erlicki fixer, 17 Erodin, 179 Falling of skin, 160, 175 Fascia, superficial, 23, 33 Fat cells, 33, 35, 39-58 Fatliquoring, 323-5 Fatty acid spews, 327-8 Ferric chloride test for nontannin, 259 Fertilizer from sulfide unhairing liquors, 165 Fiber sarcolemma, 35, 168 Fibrin, hydrolysis by trypsin, 189 Fibrinogen, 68 Finishing leather, 330 Fir bark, 206 Fish skins, 62-4 Fixative, albumin, 18 Fixing specimens, 16-7 Flesh, 23, 33 Fleshing, 33, 142-50 Follicle, hair, 29, 39-64 Formic acid, absorption by skin, 140 SUBJECT INDEX Formic acid, ionization of, 78, 82 precipitation of vegetable tan liquor by, 235, 237-9 use of in coloring leather, 329 disinfecting skins, 139-41 Freckles, 27 Fuchsin stain, 19 Gallic acid, effect upon tannin analysis, ionization of, 78, 83 Gallotannic acid, 213-4 Gambier, 206, 215-7, 219-39, 256-66, 276 Gases formed in drenching, 200 Gelatin, © combining weight of, 102-3, 31! hydrolysis of, 74-5 isoelectric point of, 108 mutarotation of, I12 reaction with aluminum salts, 311 chromium salts, 308-11 tannin, 308 relation to collagen, 72-3 two forms of, 109-14 Gelatin dispersions, absorption spectra of, 114 conductivity of, 120 double refraction of, 120 osmotic pressure of, 120-5 plasticity of, 126-7 structure of, 118-20 vapor pressure of, 120 viscosity of, 112-3, 120-3, 125-7 Gelatin jellies, diffusion of chrome liquor into, 284 diffusion of vegetable tan liquor into, 256-8 dried forms of, 144 drying of, 143 elasticity of, 118 reticulation of, 116-8 structure of, 118-20 swelling of, 98-123 Gelatin-salt test for tannin, 216-7 Glands, 27, 31, 33; 39-60 Glassy layer of horse hide, 58 Gliadin, isoelectric point of, 10 Globulins, 67-8 isoelectric points of, 108 Glucose, use in reduction of sodium dichromate, 279 Glucosides, 215 Glue stock, 142 Glutamic acid, 66, 70 Glycine, 65, 70 Goat leather, 304 vertical section of, 302 Goat skin, 56 Gold sol, theory of stability of, 127-30 Goose flesh, 31 218-24 337 Grain membrane, 38 Grain surface, 38 horizontal section of from calf leather, 326 loose, 149 photomicrographs of from leathers, albino rat, 37 bear, 37 calf, 36, 197 camel, 37 cow, 36 guinea-pig, 37 hog, 30 horned toad, 37 horse, 36 kid, 36 salmon, 37 _sheep, 36 pipy, 149 pitted, 39, 149, 154 proteins of, 71 rough, 146 structure of, 326 Guinea-pig leather, 60 grain surface of, 37 Guinea-pig skin, 60 vertical section of, 61 Hair, 27-31, 39-64 bulb, 28, 39-64 vertical section of, 28 curliness, cause of, 29, 5! follicle, 29, 39-64 papilla, 29, 48 pigment, 31 root, 29 scales, 29 photomicrograph of, 30 shaft, 29 Halibut skin, 64 vertical section of, 62 Handlers, in vegetable tanning, 241 Hematoxylin-eosin stain, 18 Hemi-celluloses, use of in tanning, 270 Hemlock bark, 206, 216, 219-39, 256, 259-66, 274-5 | Hemoglobin, isoelectric point of, 108 Hide powder, digestion by trypsin, 73-4 preparation of, 72 Hippopotamus leather, 243 vertical section of, 255 Histidine, 66, 70 Histology of skin, 15-64 Hog leather, 243 grain surface of, 36 vertical section of, 248 Hog skin, 57 vertical sections of, 57, 195 Hooke’s law, 101, I! 338 Horned toad leather, 243 grain surface of, 37 vertical section of, 252 Horse hide, 58-60 vertical section of, 59 Horse leather, 242-3, 304 grain surface of, 36 vertical sections of, 246, 247, 301 Human skin, 21-39 vertical sections from back, 22 heel, 24, 26 scalp, 21 Hydration of ions, 91-3, 267, 292-6 Hydrochloric acid, effect of neutral salts on pH value, ionization of, 78, 80 precipitation of vegetable tan liquor by, 234, 237-9 _ Hydrogen-ion concentration, effect of neutral salts upon, 89-93, 146, 281-4 effect upon aldehyde tanning, 319 bating, 185-9 chrome tanning, 285-97 color of vegetable tan liquors, 208-9 determination of tannin, 230-1 diffusion into protein jellies of chrome liquor, 284 vegetable tan liquor, 257-8 fatliquoring, 325 gelatin-salt test for tannin, 216-7 hydrolysis of collagen by pancre- atin, 73-4, 168, 197-8 elastin by pancreatin, 185-9 leaching of vegetable tanning ma- terials, 208-9 osmotic pressure of protein sys- tems, I2I-4 oxidation of vegetable tan liquors, 209-10 plumping of skin, 175-7 precipitation of vegetable tan liquors, 210-1, 237-9 putrefaction, 150, 177 stability of collagen-tannin com- pound, 265-6 swelling of protein jellies, 98-114, 175-7 vegetable tanning, 262-7 viscosity of gelatin dispersions, 112-3, 120-3, 125-7 values for different solutions (see pH values) Hydrolysis of chromium salts, 280-3 collagen, 70-4, 168, 189, 197-8 elastin, 70, 181-97 SUBJECT INDEX Hydrolysis of keratin, 69-70, 151-4, 157-60, 164-72 leather, 265-6, 274-6 ~ Hydroxyacids, salts of, effect upon chrome tanning, 297 Imbedding specimens of skin, 17 Inactivation of enzymes, 187-91 Interfibrillary cementing substance, 68 Spee of acids and bases (tables), O- effect of neutral salts, 89-93 temperature, 79 order of strengths, 79 tables showing pH values, 80-8 Iron tanning, 313-4 Isoelectric points of collagen, 109, 175-7 gelatin, 108, 109-14 other proteins, 108 tannins, 233 Isoleucine, 65 Keratin, 29, 60 hydrolysis of, 69-70, I5I-4, 157-60, 164-72 Keratohyalin, 70 Keto-enol tautomerism in proteins, 110, 114 tannins, 277 Kid leather, 56 grain surface of, 36 Kid skin, 56 vertical section of, 55 Kips, 133 Lactic acid, effect upon plumping of skin in vegetable tan liquor, 268-9 ionization of, 78, 83 precipitation of vegetable tan liquor by, 235 Lamb fur, 313 vertical section of, 312 Larch bark, 206, 216, 219-39, 256, 259, 261-4 Layers, in vegetable tanning, 241-2 Leaching of vegetable tanning mate- rials, 207-9 Leather, action of sulfuric acid on, 323 aging of, 228-30, 274-6 alum, 311-3 ancient, II bleaching, 322 chamois, 317 chrome, 278-308 comparison with vegetable leather, 298-304 chrome retan, 309-10 coloring, 328-30 SUBJECT INDEX Leather, decomposition by hot alcohol, 274-6 fatliquoring, 323-5 finishing, 330 grain surface of (see grain surface) horizontal section of, 326 hydrolysis of, 265-6, 274-6 loading, 242 mordanting, 329 oxidation of fat in, 328 resistance to washing, 225 spew on, 327-8 tensile strength of, 324 vegetable, 240-77 comparison with chrome leather, 298-304 vertical sections of, alligator, vegetable, 251 calf, chrome, 299 vegetable, 208 calf slink, chrome, 303 camel, vegetable, 253 cow, chrome retan, 310 goat, chrome, 302 hippopotamus, vegetable, 25 hog, vegetable, 248 ; horned toad, vegetable, 252 horse, chrome, 301 vegetable, 246, 247 lamb slink, alum, 312 salmon, vegetable, 249 shark, vegetable, 250 sheep, vegetable, 245 steer, vegetable, 244 walrus, vegetable, 254 Lecithins, 75 Leucine, 65, 70 Ligamentum nuchae, 70-1 Lignosulfonic acids, 320 Limed skin, washing of, 155 Lime liquors, bacterial action in, 162-3, 166 pH value of, 93, 157, 162, 199 unhairing with, 156-60 Lipase, 174 Lithium chloride, effect upon chrome tanning, 292-4 pH value of acid solutions, 89-90 chrome liquors, 283 Logwood, use in coloring leather, 329 stains for skin sections, 18-9 Lymph, 75 ducts, 35 Lysine, 66, 70 Magnesium chloride, effect upon chrome tanning, 292-4 pH value of acid solutions, 89-90 precipitation of chrome liquor by alkali, 284 339 Magnesium sulfate, effect upon pH value of acid solutions, 89-90 chrome liquor, 282 precipitation of chrome liquor by alkali, 284 Mallet bark, 206 Malpighian layer of epidermis, 25-6, 70, I51-72 destruction by acids, 166 alkalis, 156-65 bacteria, 151-6, 162-3, 166 enzymes, 166-72 _ Mangrove bark, 206, 256 Melanins, 27, 67, 69 Mellow lime liquors, 162 Membrane equilibria, 94-132 potentials, 96-8, 123-5 Mercuric chloride, absorption by skin, 140 use of in disinfecting skins, 139-41 Mesoderm, 25 Micrococcus flavus liquefaciens, 163 Microtomy, 18 Migratory cells, 35 photomicrograph of, 34 Mimosa bark (see wattle bark) Moellon degras, 317 Mordanting leather, 320 Mounting sections, 19-20 Mucins, 67-9 Mucoids, 69 Muscles, 23, 31, 39-60 Muscular tissue, 23 Mutarotation of gelatin, 112 Myrobalans, 206, 256 Nemathelminthes, 154 Nerves, 31, 33, 38 vertical section of, 32 Nervous tissue, 23 Neutral salt effect, 89-93, 146, 281-4 Nitric acid, ionization of, 78, 80 Nucleic acid, isoelectric point of, 108 Nucleus of a cell, 25, 46 Nutgalls, 214 Oak bark, 206, 215-6, 219-39, 256, 259-69 Oak wood, 206 Oil tanning, 317-8 Ooze leather, 304, 330 vertical section of, 303 Open-vat leaching, 207 Oropon, I81 Osage orange wood, 206, 219-39 Osmotic pressure of protein systems, 120-5 effect of pH value, 121-4 neutral salts, 122-4 Oxalic acid, ionization of, 78, 86 Oxidation of _ fat in leather, 328 vegetable tan liquors, 209-10 340 -Oxybenzophenones, 215 Oxyhemoglobin, isoelectric point of, 10 Oxyphenylstyrylketones, 215 Oxyproline, 66, 70 Palmetto, 206 Pancreatin, action upon albumose, 189 casein, 189 collagen, 73-4, 168, 197-8 elastin, 181-97 fibrin, 189 gelatin, 74-5 hide powder, 73-4 skin, 166-72, 181-98: bating with, 169-72, 181-97 unhairing with, 166-72 Pancreol, 184 Papilla, 25, 38, 42, 58, 256 hair, 29 Pars papillaris, 26 Patent leather, 330 Penicilium glaucum, 328 Pentacetyl glucose, 214 Pentadigalloylglucose, 214 Pepsin, 73-5, 181 Peptones, 65 Persian lamb fur, 313 vertical section of, 312 Phenylalanine, 65, 70 Phlobaphenes, 205, 215 Phloroglucin tannins, 215 Phosphoric acid, as a buffer in bating, 184-6 ionization of, 78, 81 precipitation of vegetable tan liquor by, 235 Photomicrography, 20 Physical chemistry of the proteins, 94- 132 pH value, acids (tables), 80-6 aldehyde tan liquors, 319 bases (tables), 87-8 bate liquors, 93, 176-9, 184-91, 197, 199 bleaching solutions, 322 chrome liquors, 93, 280-97 definition of, 80 effect of (see hydrogen-ion concen- tration ) fatliquors, 93, 325 lime liquors, 93, 157, 162, 199 pancreatin unhairing liquors, 167 pickle liquors, 93, 203 syntan solutions, 320 vegetable tan liquors, 93, 199, 208-11, 237-9, 257-8, 260-76 Pickling, 138-9, 202 SUBJECT INDEX Picro-indigo-carmine stain, 19 Picro-red stain, 19 Pigment, 27, 31, 69 Pilo-motor nerves, 31 Pipy grain, 149 Pitted grain, 39, 149, 154 Plasticity of gelatin dispersions, 126-7 Plate cultures of puer liquor, 180 soak water, 148 Plumping of skin, 160 effect of lactic acid, 268-9 pH value, 175-7 sodium chloride, 268-9 in vegetable tan liquor, 268-9 measurement of degree of, 161 reduction in bating, 175 two points of minimum, 175-7 Polypeptides, 65-7 Pores, 27 Potassium bromide, effect upon pre- cipitation of chrome liquor by alkali, 284 Potassium chloride, effect upon pH value of acid solutions, 89-90 chrome liquors, 283 precipitation of chrome liquor by alkali, 284 vegetable tan liquor, 234 Foe hydroxide, ionization of, 79, Potassium nitrate, effect upon precipi- tation of chrome liquor by alkali, 284 | Potential difference of colloidal dispersions, 127-30 protein systems, 114-6 vegetable tan liquors, 231-3 Precipitation of chrome liquors, 283-4 fatliquors, 325 sols, theory of, 127-30 vegetable tan liquors, 209-11, 233-9 Preservation of skin, 133-9 Prickles, 25 Proline, 66, 70 Protein-acid equilibria, 98, et sequ. Protein-alkali equilibria, 107, et sequ. Protein jellies, diffusion of chrome liquor into, 284 vegetable tan liquor into, 256-8 drying of, 143 elasticity of, 118 rhythmic swelling of, 116-8, 201, 241 structure of, 118-20 swelling of, 98-123, 160, 175-7, 268-9 Proteins, classification of, 67 hydrolytic products of, 65-6, 70 isoelectric points of (table), 108 physical chemistry of, 94-132 SUBJECT Proteins, synthesis of, 66 Proteoses, 65 Proteus mirabilis, 148 vulgaris, 148 Protoplasm, 25, 70 Puering (see bating) Puer liquor, plate culture of, 180 Putrefaction, effect of pH value, 150, 177 Putrid soak, 149 Pyrogallol tannins, 204 Quebracho wood, 206, 215-6, 219-39, 256-65 Quinol, 276-7, 319 Quinones, 276-7 tanning with, 318-9 Rapid tanning, 270-1 Rat leather, grain surface of, 37 Reds, 205, 215 Rennin, 174 Resistance of leather to washing, 225 Resorcin-fuchsin stain, 19 Reticular layer of skin, 39-64 vertical section of, 34 Reticulation of photographic negative, Ti7 Rhythmic swelling of protein jellies, 116-8, 201, 241 Rochelle salt, effect upon chrome tan- ning, 297 Root, hair, 29 Run pelts, 154 Salicylic acid, ionization of, 78, 86 Salmon leather, 243 grain surface of, 37 vertical section of, 249 Salmon skin, 64 vertical section of, 62, 63 Salting out, theory of, 127-30 Salting skins, 133-8 Salt stains, 134-7 Scales, hair, 29 Scalp, vertical section of, 21 Scudding, 155 Sebaceous ducts, 44 glands, 27, 31, 39-60 Sebum, 75 Sectioning, 18 Serine, 65, 70 Serum albumins and globulins, isoelec- tric points of, 108 Shaft, hair, 29 Shark leather, 243 vertical section of, 250 Sharpening agents in liming, 157 INDEX Sheep leather, 242 grain surface of, 36 vertical section of, 245 Sheep skin, 51-6 vertical sections of, fresh, 54 from sweat chamber, 152, 153 before bating, 194 Shell of horse hide, 58 Sides, 145 Silicic acid tanning, 315 Skin, chemical constituents of, 65-75 dehydrating, 17 disinfection of, 139-41 drying of, 137-8 falling of, 160-2, 175-7 fixing, 16-7 histology of, 15-64 imbedding, 17 plumping of, 160-2, 175-7, 268-9 preservation of, 133-9 salting, 133-8 sampling, 16 sectioning, 18 sections and descriptions of skins from alligator, 64 calf, 48-53, 158-9, 170-1, 193 camel, 64 cod fish, 62, 64 cow, 39-48, 192 goat, 56 guinea-pig, 60-1 halibut, 62, 64 hippopotamus, 64 hog, 57, 195 horned toad, 64 horse, 58-60 human, 21-39 kid, 55, 56 salmon, 62-4 shark, 64 sheep, 54, 152-3, 194 steer, 39 walrus, 64 Soaking, 142-50 Soak, putrid, 149 Soak water, plate culture of, 148 Sodium carbonate, as a preventive of salt stains, 134-7 Sodium chloride, effect upon chrome tanning, 292-6 pH value of acid solutions, 89-93 chrome liquors, 282-3 plumping of skin in vegetable tan liquor, 268-9 precipitation of chrome liquor by alkali, 284 vegetable tanning, 266-7 Sodium dichromate, 278-9 341 342 Sodium fluoride, as a salt stains, 135 Sodium hydroxide, as an unhairing agent, 164 effect upon hydrolysis of chrome liquor, 281 ionization of, 79, 87 Sodium sulfate, effect upon chrome tanning, 294-6 pH value of acid solutions, 89-90 chrome liquors, 282 precipitation of chrome liquor by alkali, 284 vegetable tanning, 266-7 Sole leather, 242 vertical section of, 244 Spew on leather, 327-8 Spruce bark, 206 Spruce extract, 321 Stability of chrome liquors, 283-4 collagen-tannin compound at differ- ent pH values, 265-6 colloidal dispersions, 127-30 fatliquors, 325 vegetable tan liquors, 209-11, 233-9 Staining sections, 18-9 Stearamid, 324 Steer leather, 242 vertical section of, 244 Sticks, in vegetable tanning, 241 Stratum corneum, 26-7, 39-64, 70, 151-4, 157-9, 169 granulosum, 26-7, 70 lucidum, 26-7 mucosum, 25-7, 70, I51-72 Stripping chrome leather, 297 vegetable leather, 265-6, 322 Stuffing, 323-5 Sucrose, effect upon chrome tanning, 295-6 Sudoriferous glands, 27, 31, 33, 39-60 Suede leather, 303-4, 330 Sulfide unhairing liquors, from, 165 Sulfite cellulose, 271, 321 Sulfonated oils, 323 Sulfuric acid, action upon leather, 323 combination with skin during chrome tanning, 285-92 effect of neutral salts upon the pH value of solutions of, ‘90-1 effect upon the hydrolysis of chrome liquor, 281 ionization of, 78, 81 precipitation of vegetable tan liquor by, 234, 237-9 é Sulfurous acid, use of in soaking, 150 Sulfur tanning, 316 Sumac, 206, 219-39, 256 preventive of fertilizer SUBJECT INDEX Superficial fascia, 23, 33 Sweat ducts, 27 glands, 27, 31, 33, 39-60 Sweating process of unhairing, 151-6 Swelling of protein jellies, 98-123, 160, 175-7; 268-9 effect- of neutral salts, 106-7, 122, 200, 202, 268-9 pH value, 98-114, 175-7 polyvalent ions, 106, 108, 161 family of curves of, 105 rhythmic swelling, 116-8, 201, 241 two points of minimum, 109-14, 175-7 Syntans, 271, 319-20 Synthesis of proteins, 66 tannins, 214 Tan liquors (see vegetable tan liquors) Tannase, 215 Tanning, evolution of art of, 11-2 Tanning extracts, manufacture of, 207- 12 Tanning with aldehydes, 318-9 aluminum salts, 311-3 bismuth salts, 316 bromine, 316-7 cerium salts, 316 chlorine, 316-7 chromium salts, 278-308 colloidal dispersions, 316 iron salts, 313-4 oils, 317-8 quinones, 318-9 silicic acid, 315 sulfite cellulose, 321 sulfur, 316 syntans, 319-20 vegetable tanning materials, 240-77 Tannins, 204, 213-39 amorphous, 215 catechol, 204, 215 definition of, 215-6 determination of, 217-31 electrical charge on, 231-3 formation from nontannin, 226 gelatin-salt test for, 216-7 isoelectric points of, 233 mordanting chrome leather with, 329 oxidation of, 209-10, 215 phloroglucin, 215 precipitation of, 209-11, 233-9 purification of, 214 | . pyrogallol, 204, 215 qualitative tests for, 205, 216-7 synthesis of, 214 Tartaric acid, ionization of, 78, 85 Tensile strength, effect of grease upon, 324 Su DIE Iielly EX Thermostat layer of skin, 31, 39-64 horizontal sections of, 44, 45, 46, 47 vertical sections of, 41, 153, 159, 170, 171, 192, 193, 194, 195 Thrombase, 68, 172 Thumb print, 38 Titanium salts in mordanting leather, 320 Trimming, 142 Trypsin (see pancreatin) Tryptophane, 66 Tuberin, isoelectric point of, 108 Tyrosine, 65, 70 Ultrafiltration of chrome liquors, 307 Unhairing with acids, 166 ammonium hydroxide, 165 arsenic disulfide, 157 bacteria, 151-6, 162-3, 166 calcium hydroxide, 156-60 calcium sulfarsenite, 157 enzymes, 166-72 sodium hydroxide, 164 sodium sulfide, 157, 164 sweating, 151-6 University, co-operation with industry, 12-4 Vacuum tanning, 270 Valine, 65, 70 Valonia, 206, 256 Vapor pressure of gelatin dispersions, 120 Vegetable leather (see leather) Vegetable tan liquors, color value of, 208-10 dialysis of, 233 diffusion into protein jellies, 256-8 electrophoresis of, 231-3 oxidation of, 209-10 343 Vegetable tan liquors, plumping of skin in, 268-9 potential difference of, 231-3 precipitation of, 209-II, 233-9 Vegetable tanning, 240-77 effect of concentration, 258-62 neutral salts, 266-7 nontannin, 257 pH value, 262-5 time, 258-62 rate increased by electricity, 270-1 hemi-celluloses, 270 nontannin, 241, 271 sulfite cellulose, 271 syntans, 271 vacuum, 270 theory of, 271-7 Vegetable tanning materials, 204-12 classification, 204-5 leaching of, 207 effect of various electrolytes, 210- at sources, 205-7 Veins, 33, 38 vertical section of, 32 Viscosity of gelatin dispersions, 120-3, 125-7 112-3, Walrus leather, 243, 256 vertical section of, 254 Washing of limed skin, effect of time of, 155 Wattle bark, 206, 216, 225-39, 259-64 Willow bark, 206 Wool, mune eae of segment of, Wool aaliee 138 Zinc chloride, as a preventive of salt stains, 134 att ‘gic