Transcriber's note:
Minor typographical errors and inconsistencies have been corrected. Some words had inconsistent hyphenation throughout the book; these have been made consistent.
Ions are shown as Fe+++, instead of using superscripts. There is some inconsistency in the notation used in the original text for chemical formulæ such as Na₂Cr₂O₇(2H₂O). These have been regularized to use the modern mid-dot, for example, Na₂Cr₂O₇ · 2H₂O.
The index entry for Hemlock bark had no page number in the original text, so the correct page number, 34, has been supplied.
On page 152 NaCO₂₃ has been corrected to Na₂CO₃. On page 212, the variable n has been replaced with the correctly subscripted forms n₁ and n₂.

ANIMAL PROTEINS

BY

HUGH GARNER BENNETT, M.Sc. (Leeds)

MEMBER OF THE SOCIETY OF LEATHER TRADES' CHEMISTS; FORMERLY
ASSISTANT LECTURER AND DEMONSTRATOR AT THE LEATHER
INDUSTRIES DEPARTMENT OF THE UNIVERSITY OF LEEDS
AUTHOR OF "THE MANUFACTURE OF LEATHER"

LONDON
BAILLIÈRE, TINDALL AND COX
8 HENRIETTA STREET, COVENT GARDEN
1921

GENERAL PREFACE

The rapid development of Applied Chemistry in recent years has brought about a revolution in all branches of technology. This growth has been accelerated during the war, and the British Empire has now an opportunity of increasing its industrial output by the application of this knowledge to the raw materials available in the different parts of the world. The subject in this series of handbooks will be treated from the chemical rather than the engineering standpoint. The industrial aspect will also be more prominent than that of the laboratory. Each volume will be complete in itself, and will give a general survey of the industry, showing how chemical principles have been applied and have affected manufacture. The influence of new inventions on the development of the industry will be shown, as also the effect of industrial requirements in stimulating invention. Historical notes will be a feature in dealing with the different branches of the subject, but they will be kept within moderate limits. Present tendencies and possible future developments will have attention, and some space will be devoted to a comparison of industrial methods and progress in the chief producing countries. There will be a general bibliography, and also a select bibliography to follow each section. Statistical information will only be introduced in so far as it serves to illustrate the line of argument.

Each book will be divided into sections instead of chapters, and the sections will deal with separate branches of the subject in the manner of a special article or monograph. An attempt will, in fact, be made to get away from the orthodox textbook manner, not only to make the treatment original, but also to appeal to the very large class of readers already possessing good textbooks, of which there are quite sufficient. The books should also be found useful by men of affairs having no special technical knowledge, but who may require from time to time to refer to technical matters in a book of moderate compass, with references to the large standard works for fuller details on special points if required.

To the advanced student the books should be especially valuable. His mind is often crammed with the hard facts and details of his subject which crowd out the power of realizing the industry as a whole. These books are intended to remedy such a state of affairs. While recapitulating the essential basic facts, they will aim at presenting the reality of the living industry. It has long been a drawback of our technical education that the college graduate, on commencing his industrial career, is positively handicapped by his academic knowledge because of his lack of information on current industrial conditions. A book giving a comprehensive survey of the industry can be of very material assistance to the student as an adjunct to his ordinary textbooks, and this is one of the chief objects of the present series. Those actually engaged in the industry who have specialized in rather narrow limits will probably find these books more readable than the larger textbooks when they wish to refresh their memories in regard to branches of the subject with which they are not immediately concerned.

The volume will also serve as a guide to the standard literature of the subject, and prove of value to the consultant, so that, having obtained a comprehensive view of the whole industry, he can go at once to the proper authorities for more elaborate information on special points, and thus save a couple of days spent in hunting through the libraries of scientific societies.

As far as this country is concerned, it is believed that the general scheme of this series of handbooks is unique, and it is confidently hoped that it will supply mental munitions for the coming industrial war. I have been fortunate in securing writers for the different volumes who are specially connected with the several departments of Industrial Chemistry, and trust that the whole series will contribute to the further development of applied chemistry throughout the Empire.

SAMUEL RIDEAL.

AUTHOR'S PREFACE

It has been the author's chief concern that this volume should fulfil its own part in the programme set forth in Dr. Rideal's General Preface.

The leather, glue, and kindred trades have been for many years recognized as chemical industries, but the great development of colloid chemistry in the last few years has given these trades a more definite status as such, and they can now be placed in the category of applied physical chemistry. The time is probably not far distant when some knowledge of pure physical chemistry will be a first essential to students, chemists, chemical engineers, and to all engaged in these industries in supervision, administration, or control. It is hoped that this volume will stimulate the study of these industries from that standpoint.

As the author has previously written upon one of the industries involved herein ("The Manufacture of Leather": Constable & Co.), he has, rather inevitably, found it difficult to avoid altogether his own phraseology. The changes of a decade, however, together with the wider field and newer view-point, have made possible a radical difference of treatment.

The author desires to acknowledge the help he has received from the many books, essays, and researches which are mentioned in the references at the end of each section, especially to Procter's "Principles of Leather Manufacture," and also to thank Dr. Rideal for many useful suggestions. The author would like also to acknowledge here his indebtedness (as well as that of the trade generally) to the work of Dr. J. Gordon Parker, who, through his researches, lectures, and teaching work, has done more than any other man to disseminate a knowledge of practical methods of tanning.

The author's thanks are also due to his brother, Mr. W. Gordon Bennett, M.Sc., A.I.C., M.C., for assistance in proof revision, and to his father, Rev. John Bennett, for some literary criticism.

H. GARNER BENNETT.

Beverly,
June, 1921.

CONTENTS

ANIMAL PROTEINS

INTRODUCTION

Proteins are organic compounds of natural origin, being found in plants and in animals, though much more plentifully in the latter. They are compounds of great complexity of composition, and of very high molecular weight. The constitution of none of them is fully understood, but although there are a great number of different individual proteids, they present typical resemblances and divergences which serve to differentiate them from other groups of organic bodies, and also from one another.

Proteins resemble one another in both proximate and ultimate analysis. They contain the usual elements in organic compounds, but in proportions which do not vary over very wide limits. This range of variation is given approximately below:—

The most characteristic feature of the protein group is the amount of nitrogen usually present. This is generally nearer the higher limit, seldom falling below 15 per cent. This range for the nitrogen content is determined largely by the nature of constituent groups which go to form the proteid molecule. Roughly speaking, proteins consist of chains of amido-acids and acid amides with smaller proportions of aromatic groups, carbohydrate groups and thio compounds attached. In these chains an acid radical may combine with the amido group of another amido acid, the acid group of the latter combining with an amido group of another amido acid, and so on. Hydrogen may be substituted in these chains by alkyl or aromatic groups. There is obviously infinite possibility of variation in constitution for compounds of this character, the general nature of which varies very little. Practically all of the proteins are found in the colloid state, and this makes them very difficult to purify and renders the ultimate analysis in many cases doubtful. It is, for example, often difficult to ascertain their moisture content, for many are easily hydrolyzed with water only, and many part easily with the elements of water, whilst on the other hand many are lyophile colloids and practically cannot be dehydrated or dried. A few, such as gelatin and some albumins, have been crystallized.

The constituent groups have been investigated chiefly by hydrolytic methods. The chains of amido acids are split up during hydrolysis, and individual amido acids may thus be separated. The hydrolysis may be assisted either by acids, alkalies or ferments, but follows a different course according to the nature of the assistant. Under approximately constant conditions of hydrolysis, the products obtained are in approximately constant proportions, and this fact has been utilized by Van Slyke in devising a method of proximate analysis. It is not possible in this volume to enter deeply into the constitution of the different proteids. Reference must be made to works on pure chemistry, especially to those on advanced organic chemistry. It will be interesting, however, to mention some of the amido acids and groups commonly occurring in proteids. These comprise ornithine (1:4 diamido valeric acid), lysine (1:5 diamido-caproic acid), arginine (1 amido, 4 guanidine valeric acid), histidine, glycine (amidoacetic acid), alanine (amido propionic acid), amido-valeric acid (amido-iso-caproic acid), liacine, pyrollidine carboxylic acid, aspartic acid, glutamic acid (amido-glutaric acid), phenyl-alanine, serine (hydroxy-amido propionic acid), purine derivatives (e.g. guanine), indol derivatives (e.g. tryptophane and skatol acetic acid), cystine (a thioserine anhydride), glucosamine, and urea.

There are a few general reactions which are typical of all proteins, and which can usually be traced to definite groupings in the molecule. Amongst these is the biuret reaction: a pink colour obtained by adding a trace of copper sulphate and an excess of caustic soda. This is caused by the biuret, NH(CONH₂)₂ radical or by similar diacidamide groups, e.g. malonamide, oxamide, glycine amide. Another general reaction is with "Millon's reagent," a solution of mercuric nitrate containing nitrous fumes. On warming the proteid with this reagent, a curdy pink precipitate or a red colour is obtained. This reaction is caused by the tyrosine group (p. oxy α amido phenyl-propionic acid). Another general reaction is to boil the protein with 1:2 nitric acid for some days. A yellow flocculent precipitate of "xanthoproteic acid" is obtained, and this dissolves in ammonia and caustic alkalies with a brown or orange-red colour. Another characteristic of proteins is that on dry distillation they yield mixtures of pyridine C₅H₅N, pyrrol C₄H₅N, and their derivatives.

On the subdivision, classification and nomenclature of the proteins much ink has been spilled, and it is impossible in this volume to go into the various systems which have been suggested. It should be noted, however, that some writers habitually use the terms "proteid" or "albuminoid" as synonyms for protein. The classification of proteins adopted in this work is used because it is the most suitable for a volume on industrial chemistry and has the additional merits that it is simple and is already used in several standard works on industrial chemistry. It is based upon the behaviour of the proteins towards water, a matter of obvious moment in manufacturing processes. On this basis proteins may be divided into albumins, keratins and gelatins.

Cold water dissolves the albumins, does not affect the keratins, and only swells the gelatins. The behaviour in hot water confirms and elaborates the classification. When heated in water, the albumins coagulate at temperatures of 70°-75° C., the gelatins (if swollen) dissolve readily, whilst the keratins only dissolve at temperatures above 100° C. Albumins and keratins may be distinguished also from gelatins by adding acetic acid and potassium ferrocyanide to their aqueous solutions. Albumins and keratins give a precipitate, gelatins do not. Another distinguishing reaction is to boil with alcohol, wash with ether, and heat with hydrochloric acid (S.G. 1.2). Albumins give a violet colour, keratins and gelatins do not.

Albumins may be first discussed. They are typified by the casein of milk and by white of egg. Their solutions in water are faintly alkaline, optically active, and lævorotatory. They are coagulated by heat and also by mineral acids, alcohol, and by many poisons. The temperature of coagulation (usually about 72° C.) is affected by mineral salts, the effect being in lyotrope order (see Part V., Section I.). The coagulated albumin behaves in most respects like a keratin. Some of the albumins (globulins) are, strictly speaking, not soluble in cold water, but readily dissolve in weak solutions of salt. The albumins are coagulated from these solutions, as usual, when heated. Into this special class fall myosin (of the muscles), fibrinogen (of the blood) and vitellin (of egg yolk). By a gentle or limited hydrolysis of the albumins with dilute acids in the cold, a group of compounds called albuminates are obtained. They dissolve in either acids or alkalies, and are precipitated by exact neutralization. They may also be "salted" out by adding sodium chloride or magnesium sulphate. They are not coagulated by heat. After further hydrolysis with either acids, alkalies or ferments, very soluble compounds are obtained called albumin peptones or albumoses. These are soluble in alkalies, acids and water, and are readily hydrolyzed further into amido acids and acid amides. They are very similar to the peptones obtained from keratins and gelatins. They are not coagulated by heat.

Keratins are typified by the hair of animals. They soften somewhat in cold water and even more in hot water, but are not dissolved until digested for some time at temperatures exceeding 100° C. With some keratins, however, the cystine group is to some extent easily split off by warm water, and on boiling with water hydrogen sulphide is evolved. The sulphur content of keratins is often greater than the average for proteids. All keratins are dissolved with great readiness by solutions containing sulphydrates and hydrates, e.g. a solution of sodium sulphide. In solutions of the hydrates of the alkali and alkaline earth metals, keratins behave differently. Some dissolve with great ease, some with difficulty, some only on heating and some not even if digested with hot caustic soda. They are dissolved (with hydrolysis) by heating with mineral acids, yielding peptones and eventually amido acids, acid amides, etc. Many keratins have a comparatively low content of nitrogen.

Gelatins are very difficult to distinguish from one another, their behaviour being closely similar to reagents. They are also very readily hydrolyzed even with water, and the products of hydrolysis are even more similar. The gelatins are known together, commercially, under the general name of gelatine. Gelatins of different origin, however, have undoubtedly a different composition, the nitrogen content being variable. If the gelatins are not bleached whilst they are being manufactured into commercial gelatine, they are called "glue." Gelatine is colourless, transparent, devoid of taste and smell. It is usually brittle. Its S.G. is about 1.42, and it melts at 140° C. and decomposes. It is insoluble in organic solvents. When swelling in cold water it may absorb up to 12 times its own weight of water. The swollen product is called a "jelly." Jellies easily melt on heating and a colloidal solution of gelatine is obtained. This "sets" again to a jelly on cooling, even if only 1 per cent. gelatin (or less) be present. The solution is optically active and lævorotatory, but with very variable specific rotation. Some observers have thought that the different gelatins have different specific rotations and may so be distinguished. Gelatins are precipitated from solutions by many reagents, such as alcohol, formalin, quinone, metaphosphoric acid, tannins, and many salt solutions, e.g. those of aluminium, chromium and iron, and of mercuric chloride, zinc sulphate, ammonium sulphate, potassium carbonate, acidified brine. Many of these precipitations have analogies in leather manufacture (see Parts I. to IV.). The gelatin peptones or gelatoses are formed by hydrolysis with acids, alkalies, ferment or even by digestion with hot water only. A more detailed description of the properties of gelatine is given in Part V., Section I. Gelatine is sometimes called "glutin" and "ossein."

Animals are much the most important source of proteins, especially of those which are of importance in industrial chemistry. Proteins occur in nearly every part of all animals, and the "protoplasm" of the living cell is itself a protein. The keratins include the horny tissues of animals: the epidermis proper, the hair, horns, hoofs, nails, claws, the sebaceous and sudoriferous glands and ducts, and also the elastic fibres. The gelatins are obtained from the collagen of the skin fibres, the bones, tendons, ligaments, cartilages, etc. Fish bladders yield a strong gelatin. The albumins are obtained from the ova, blood, lymph, muscles and other internal organs of animals.

The classification of proteins herein adopted fits in well with the scope and purpose of this volume. The keratins are of little importance in chemical industry, but are of immense importance in mechanical industry, e.g. the woollen trade, which is based upon the keratin comprised by sheep wool. The collagen of the hide and skin fibres is of vast importance to chemical industry, and is the basis of the extensive leather trades discussed in Parts I. to IV. The waste pieces of these trades, together with bones, form the raw material of the manufacture of gelatin and glue, as discussed in Part V. The proteids of animals' flesh and blood, milk and eggs form the source of the food proteins discussed in Part VI. The food proteins embrace chiefly albumins, but gelatins and even keratins are involved to some extent.

PART I.—HIDES FOR HEAVY LEATHERS

Section I.—THE RAW MATERIAL OF HEAVY LEATHERS

The term "hide" possesses several shades of meaning. In its widest sense it applies to the external covering of all animals, and is sometimes used derogatively for human skin. In this wide sense, it is almost synonymous with the term "skin." The term "hide," however, has a narrower meaning, in which it applies only to the outer covering of the larger animals, and in this sense is used rather in contrast with the term "skin." Thus we speak of horse hides, cow hides, camel hides, and buffalo hides. It is used in this sense in the title of Part I. of this volume. As such hides are from large animals, the leather which is manufactured therefrom is thick and in large pieces, and is therefore commercially designated as "heavy leather." From the standpoint of chemical industry hides are amongst the most important of animal proteins, and their transformation into leather for boots, shoes, belting, straps, harness, and bags comprises the "heavy leather trade," which is one of the largest and most vital industries of the country. The heavy leather trade predominates over other branches of leather manufacture, not only because of the comparatively large weight and value of the material handled, but also because the resulting products have a more essential utility. There is also a still narrower use of the term "hide," in which it applies only to the domesticated cattle—the ox, heifer, bull and cow—which use arises from the fact that the hides of these are both the largest and most valuable portion of the raw material of the heavy leather industries. In a very narrow sense the term is also sometimes applied only to ox hides, which for most heavy leathers are the ideal raw material.

The Home Supply of hides forms a large important proportion of the total raw material. Its importance, moreover, is rapidly increasing, for the excellence and abundance of the home supply determines the extent to which it is necessary for the industry to purchase its raw material abroad. The position of our national finances makes this an increasingly serious matter, for hides are comparatively a very expensive material.

The quality of our home supply of hides is very valuable, being determined by the conditions of the animal's life, its precise breed, and by other factors such as age and sex. The best hides are usually obtained from animals which have been most exposed to extremes of wind and cold, as such conditions tend naturally to develop a thicker and more compact covering. Broadly speaking, these include the hides from cattle of the northern and hilly districts. The age of the animal when killed is also a dominating factor. Calf skins are very soft, fine grained and compact, the state of rapid growth favouring the existence of much interfibrillar substance. The youngest animals supply suitable raw material for various light leathers (see Part II., Section V., p. [120]), and are also very suitable for chrome work (see Part III., Section III., p. [156]). Bull and cow hides, on the other hand, are from animals whose growth is complete, and show in consequence a lack of interfibrillar substance, coarse fibres and a rough and often wrinkled grain. The resulting leather tends consequently to be spongy, thin, empty and non-waterproof. Intermediate between these extremes are the hides of the ox and heifer, large, yet of good texture, and well supplied with interfibrillar substance. These hides are much the best for sole leather, a firm, smooth-grained and well-filled leather being needed. The term "kip" is often applied to small hides and to hides from large calves. In the trade, however, "kip" is sometimes used also for larger hides, as a verbal enhancement of value; just as a man with a few old fowls is said to keep "chickens." Cow hides tend to be "spready," i.e. to have a large area per unit weight, and are therefore more suitable for dressing leather. Bull hides are thicker in the neck and belly, and thinner in the back, which characteristics reduce their commercial value.

Market hides are sold by weight, and are therefore classified chiefly by their weight, which is marked on near the tail by a system of knife-cuts. The animals are flayed after cutting the hide down the belly and on the inside of the legs.

Of the various breeds, "Shorthorns" yield a large supply of useful hides. The name, however, covers a variety of similar breeds, and the hides therefrom are rather variable in texture and quality. They tend to be greasy owing to high feeding. The "Herefords," obtained from Midland markets, are generally excellent hides for sole and harness leathers. They give a good yield of butt pelt, a stout and smooth shoulder, and are not often greasy. "Devons" yield a good-textured and well-grown hide, but are often badly warbled (see p. [10]). The "Sussex" cross-breeds yield somewhat larger hides. "Suffolk Red Polls," common in East Anglia, yield a good butt, and the cow hides make good dressing leather. "Channel Island" cattle yield very thin hides, but with a fine undamaged grain. Scotch hides possess deservedly the very highest reputation. The climatic conditions favour the production of a hardy race of cattle with thick well-grown hides, yielding a large proportion of butt. These hides are amongst the best obtainable for heavy leather, and particularly for sole leather. "Highlanders," "Aberdeen Angus," "Galloways" are typical breeds, with short neck, legs and straight backs. Cross-breeds are also excellent (e.g. "Scotch Shorthorns"). The natural value of these hides is further enhanced by the usual care in flaying. "Ayrshires" yield good milch cows and consequently yield often a more spready hide. The Welsh breeds for rather similar reasons also yield valuable hides. The Irish "Kerrys" are small but stout, and yield hides suitable for light sole leather. Irish cross-breeds, Shorthorns, have a rather bad reputation, and are often ill flayed.

All the varieties of the home supply are subject to various defects, which influence seriously their commercial value. One of these defects is warble holes or marks, caused by the Ox Warble fly (Hypoderma bovis). This is a two-winged fly about half an inch long. The larva of this fly, the "Warble maggot," lives and thrives in the skin of cattle, and causes a sore and swelling. The life-history of this insect is still in dispute, but it is generally thought that the eggs are laid in the hair on the animal's back, and the young larva eats its way through the hide until just below the dermis, and there feeds until mature. It then creeps out of this "warble hole," falls to the ground, pupates for a month, after which the imago or perfect insect emerges from the chrysalis. Hides which have been thus infected have, in consequence, often quite a number of holes through the most valuable part of the hide, thereby rendering it unsuitable for many kinds of leather. Even old "warbles" which have more or less healed up are a weakness, and warbled hides and leather fetch a decidedly lower price than undamaged. Another of these defects is bad flaying. Clearly the hide should be as little cut as possible, but many of our market hides are abominably gashed and often cut right through. This, of course, often reduces seriously the commercial value of the hide. Careless treatment after flaying also results in another common defect, viz. taint. As the term implies, the hide is partly putrefied, sometimes only in patches, but sometimes also so extensively as to render the hide quite rotten and quite incapable of being made into leather at all. Hides are of course putrescible, and dirt, blood, dung and warm weather encourage rapid putrefaction. As market hides are usually uncured, this defect is constantly appearing, and is a cause of considerable loss. Other defects are due to injuries to the animal before it is killed, e.g. brands, scratches due to hedges and barbed wire, old scabs, goad and tar marks. All these reduce the value of the hide.

All the defects in hides involve a very serious loss to the community, and the time is rapidly approaching when their continuance is insufferable. The loss is not usually very considerable to any individual, though very large in the aggregate. The hide is a minor part of the beast's value, and a somewhat damaged hide does not involve a very serious loss to the farmer. Some with typical stupidity regard a few warbles as "the sign of a healthy beast." These defects involve practically no loss to the hide merchant, tanner or currier, as each pays less for damaged material. The loss falls upon the community, and the time is ripe for the community to insist upon the elimination of these defects. The national resources will be for some years strained to their uttermost, and preventable damage must be considered intolerable. The principal defects in hides are preventable, and ought to be prevented. The warble fly could, by a united effort, be rendered before long practically extinct, a task which is facilitated by the fact that it is not migrative. Bad flaying and careless treatment of hides resulting in putrefaction are still more easily remedied. The communal slaughter-house is long overdue from the standpoint of public health, and would, under conditions of cleanliness and skilled workmanship and oversight, also solve the problem of ill-flayed and tainted hides.

The question of the raw material is of first importance to the leather trades. There was, before the commencement of the European War, a steadily increasing scarcity of hides, causing a constant increase in their price. This was due partly to the fact that cattle were increasing at a less rate than the population, partly to the growth of civilization, and more extensive use of leather in proportion to the world's population, and partly to the constant discovery of new uses for leather, e.g. for motor cars, aeronautics, etc. The question of raw material was under these conditions serious enough. The terrific slaughter, necessary at the same time to provide the belligerents with food and the army with leather, is bound to result in a serious crisis for the leather industries; and in conjunction with the country's financial condition, will make it absolutely necessary that all care should be taken with the raw material of one of our most important industries. The farmer who pays no heed to the warble fly, the man who gashes the hide in flaying and who allows the hide to putrefy, are equally criminal with the man who throws bread crusts into the dustbin.

It is impossible to foresee, as yet, anything in the nature of a satisfactory solution to the problem of raw material, especially in respect to heavy leather production, for the food question will rank first in the popular mind, and the earlier slaughter enjoined for the more economical production of meat will scarcely tend to increase the proportion of heavy hides.

The Foreign Supply of hides is also of great importance and value. In the case of imported hides precautions to prevent putrefaction are essential, and some method of "curing" is always used.

Salting the hides is one of the most satisfactory methods for temporary preservation. The action of salt is hygroscopic, and mildly antiseptic. Moisture is withdrawn from the hides, which are then under conditions no longer favouring the growth of bacteria. Well-salted hides will keep for years, especially if quite clean. A light salting is also useful for a short preservation, and is becoming common in hide markets and tanneries during the summer and autumn months. Salting is a method used extensively in the United States. The "packer hides" of the stockyards are carefully and systematically salted with about 25 per cent. of salt and stored in cool cellars. The hides are so piled up in heaps, that brine easily drains away. The great disadvantage of salting is the so-called "salt stains." These stains have been ascribed to the iron in the salt, to the iron in the blood, to calcium sulphate in the salt, and also to chromogenic bacteria, whose development is favoured by salting. The relative importance of these factors is not yet satisfactorily determined, but cleanliness and pure salt tend to eliminate the trouble.

Drying the hides is a less satisfactory cure. The principle is similar, viz. removal of moisture. Dried hides are, however, much drier than salted, and are quite hard and horny, hence the name "flint hides." The hides also lose much weight, a considerable advantage in reducing freight. Tropical hides are often flint-dry, and where preservatives are expensive or unprocurable, it is often the only practicable method of cure. Nevertheless, the method has many serious disadvantages, and is difficult to execute. If dried too slowly the hides putrefy partially; if too quickly they dry on the outside, and the interior is left to putrefy. The fact that hides are of uneven thickness, and the climate often hot, increases the difficulty, and often results in partial destruction of the fibrous structure of the hide. When dried, moreover, the hides are still subject to the attacks of insect larvæ, for the prevention of which the usual sprinkling of naphthalene or arsenic is only an imperfect remedy. This method of cure is also a nuisance to the tanner, who has to employ labour, pits and time in attempting to restore the hides to their original condition, and often loses up to ten per cent. of the goods in so doing. Dried hides are also subject to the presence of anthrax.

Dry Salting the hides is an excellent method of curing. As the name implies, it combines methods of drying and salting which are used alternatively. The method is used extensively in South America. A modified form of it is also used for preserving the "E.I. kips," which are cured, however, not with common salt, but with earth containing up to 70 per cent. of sodium sulphate. Dry-salted hides are largely free from the defects of dried hides, but of course are more trouble to the tanner in the process of soaking (see Section II., p. [16]) than the wet-salted goods.

Freezing the hides is now a commercial process. On the whole the process is satisfactory, but the expansion of water after freezing may tend to damage the hide fibres.

Sterilizing the hides has been frequently suggested, but no method has yet been advocated which does not interfere either with the tanning processes or with the quality of the finished leather.

Hides from the European Continent, usually wet salted and well flayed, exhibit much the same variable quality as the home supply, those from highland districts tending to be thick, yet even, well grown, tight textured and smooth grained, whilst those from lowland regions are less satisfactory. Thus hides from the Swiss Alps and Scandinavia have ranked high, whilst the spready Dutch cows are typical of a lowland hide. In the hides which once came from Germany the same features appear. Bavarian highland hides had an excellent reputation, whilst those from Berlin, Cologne, etc., tended to be long in shank and not well grown. French hides are often ill flayed, and Spanish and Portuguese are often subject to scratches. Italian hides have a very good name, being small but stout in butt.

The American supply is important. South America yields an excellent class of hide, salted or dry-salted. They are from an excellent breed of animals, slaughtered and flayed with every care, and efficiently cured. A most serious defect in this class of hide is the "brand," which is both deep and large and in the most valuable part of the hide. One side, however, is usually unbranded, so that each hide yields one good "bend." These hides, e.g. "Frigorifics," have recently been much more extensively tanned in Britain because of the shortage in the home supply of market hides caused by the European War. South America also yields good horse hides. North American hides are usually wet-salted (e.g. packer hides). They are usually good. Central America yields mostly dried hides exhibiting usual defects.

The Asiatic supply comprises the frozen China hides, which are clean but small, with flaying of uncertain quality. There are the buffalo hides from Asia and East Europe, which are suitable for cheap and sole and strap leather, and also the dry-salted "E.I. kips," obtained from a small breed of Indian cattle, and extensively made into upper leather. The Asiatic humped cattle also provide a limited supply. The African supply is of increasing importance. The tropical parts yield dried hides of uncertain quality, but the more temperate parts of South Africa yield a growing supply of good quality.

REFERENCES.
"The Manufacture of Leather" (Bennett), pp. 27-37.
"Principles of Leather Manufacture" (Procter), pp. 33-56.
"The Ox Warble or Bot Fly" (E. Ormerod).
"The Making of Leather" (Procter), pp. 2-22.

Section II.—THE PREPARATION OF PELT

Before hides are tanned it is necessary for them to pass through a series of preparatory processes. The object of these processes is to obtain from the hide the true hide substance in a pure and suitable condition. Each class of leather has its own appropriate processes, the adjustment of which largely determines the quality of the finished article. So prominent is the influence of these preparatory methods that the paradox "good leather is made before tanning" is in trade circles almost a platitude. These processes, sometimes lumped together under the general name of "Wetwork," comprise soaking, liming, beam house work and deliming. These will be discussed in turn.

The term applied to the hide after these processes, but before tannage, is "pelt."

Soaking has for its object the cleansing and softening of the hides, chiefly by means of water. It aims at the removal of dirt, blood, dung, and curing materials by washing. The process is usually simple, and is much the same for all classes of leather. The ideal to be aimed at is to restore the hide to its condition when it left the animal's back. Cleanliness in leather manufacture is as essential at the commencement as anywhere, for the hide is in its most putrescible state. The soluble proteids (blood, lymph, part of dung, etc.) which always adhere to hides encourage the rapid growth of putrefactive bacteria, and cannot be washed away too soon. Dung is often difficult to remove, being caked on the butt end amongst the hair. Soaking only softens it, and mechanical removal is usually necessary. If such substances are not removed, they go forward with the goods into the lime liquors, causing stains, loss of hide substance, and counteracting plumping.

The detailed method and time of soaking are determined mainly by the nature of the cure. One of the purposes of the soak liquors is to dissolve the salt used in curing hides and to rehydrate the hide and make it again soft and pliable. As a 10-per-cent. salt solution exerts a solvent effect on hide substance, it is necessary soon to change the first soak liquor of salted goods.

Market hides, which are uncured, require the least soaking, the cleansing effect being most needed. The hides are inserted into pits ("water dykes") of water for a few hours, and the water changed once or twice. The soaking should not be prolonged as the hides are so putrescible, and where it is customary to leave the goods in a soak liquor overnight, it is advantageous to add a little slaked lime to the water before inserting the goods. This not only softens hard water, but is mildly antiseptic and plumping, and forms a suitable introduction to the liming proper. Each pit contains a "pack" of 30-50 hides, according to its capacity, which varies in different tanneries from 1000 to 2000 gallons. Tainted goods, which are indicated by a characteristic white colour on the flesh side and by loose hair, need a preliminary washing either in a "drum," "tumbler" or in a "paddle." This ensures a rapid change of liquor and the removal of most of the putrefactive agencies. Bad cases may need the application of antiseptics, such as immersion in 0.1 per cent. carbolic acid; but if possible these should be avoided, as they lengthen the time required for liming. After drumming or paddling, tainted goods should be placed directly into a lime liquor.

Salted hides need very similar treatment to uncured hides, but the soaking is longer, because of the dehydration caused by salting. Hence they receive also a greater number of changes of water, three or four usually, but often more. As much loose salt as possible should be shaken from the hides before insertion into any liquor. The employment of drum or paddle before pit soaking is extremely useful to effect the rapid removal of superficial salt, and is also useful after pit soaking to remove the last traces.

Dried and dry-salted goods need a soaking still more prolonged, up to one week if water alone be used. With the assistance of caustic soda, however, the process can be shortened to about two days. The first soak liquor should consist of a 0.1 per cent. solution of caustic soda, and after the goods have been inserted twenty-four hours, they will be materially improved by a few hours' drumming or paddling. Another caustic soda soak will complete the process. Sodium sulphide crystals may replace caustic soda, but about three times the weight will be needed. Carbonate of soda and caustic lime also are a convenient commercial substitute for caustic soda. For 10 lbs. caustic soda, use 36 lbs. carbonate and 7 lbs. lime. Extra lime should be added in all cases when the water is hard. Acid liquors will also soften dried and dry-salted goods, but such processes do not fit in so well with the subsequent liming. The use of putrid soaks and stocks may be now considered out of date.

Liming follows soaking, and consists essentially in immersing the hides for 7-10 days in milk of lime. The chief object in view is to loosen the hair and prepare for its mechanical removal. Liming takes place in pits, the tops of which are level with the limeyard floor. The lime is slaked completely and mixed well with water in the pit, being particularly well plunged just before the insertion of a pack of goods. Saturated limewater is only a 0.13-per-cent. solution. The goods are occasionally "handled" i.e. hauled out of the pit and reinserted after plunging ("hauling" and "setting"). This is necessary to keep the liquor saturated with lime. The hides are inserted one by one, each being "poked down" to ensure its contact with the liquor. The goods are invariably immersed first in a previously used lime liquor. Most tanneries now carry this out in a systematic way, so as to ensure regularity in the process. As the goods are large and heavy it is less laborious to carry out the whole process in one pit. In this "one-pit system" the goods are inserted for (say) four days in an old used lime liquor, with occasional handling; this liquor is then run to the drain and a new liquor made up in the same pit, into which the goods are inserted for (say) five days. They are then hauled and sent to the unhairers. Each pack thus gets two liquors, old and new.

A better method is the "three-pit system." In this case each pack receives three liquors and has (say) three days in each, first an "old lime," then a "medium lime," and finally a "new lime." This system ensures a greater regularity of treatment, and is deservedly the most popular method for liming hides for sole leather. After being used once as a "new lime," a liquor then becomes a "medium lime," and after being thus used becomes the "old lime" which receives the green hides from the soaks. The system involves the goods being shifted twice to another pit, which is more laborious than reinsertion into the old pit, but if the limeyard be arranged in "sets" or "rounds" of three pits, the shift is usually only to the adjacent pit. One special advantage of this system is that the top hides in one pit become the bottom hides in the next pit, and vice versâ. Rounds of more than three pits are sometimes used.

Many factories have now adopted systems in which there is no handling at all. The hides are suspended in lime liquors which are agitated by mechanical contrivances (e.g. Tilston-Melbourne process), or by jets of compressed air (e.g. Forsare process). The goods are soaked and limed "mellow to fresh" by changing the liquors by means of pumps, air ejectors, etc. Thus the hides need no labour from first being inserted until drawn for depilation.

In liming, the whole of the epidermis as well as the hair is loosened, and is subsequently removed in depilation. The corium or true hide substance becomes much more swollen by imbibation of water, and when taken out of the new lime is "plumped" to very firm jelly. This plumping is a matter of prime importance to the tanner. The coarser fibres are thereby split up into the finer constituent fibrils, which fact assists very materially in obtaining a quick and complete tannage, good weight, and a firm leather. During the liming, the natural grease of the hide is saponified or emulsified, which prepares for its removal in scudding. Liming is thus a complex process: the hair is loosened, the hide is plumped, and the grease is "killed." All these results may be hastened by the use of other alkalies in addition, and most heavy leather yards assist the liming by adding also sodium sulphide or caustic soda or both. Sodium sulphide is a powerful depilatant, and will alone unhair hides easily in strong solutions even in a few hours. As in solution it forms caustic soda by hydrolysis, it possesses also the powerful plumping and saponifying powers characteristic of the latter. The addition of arsenic sulphide (As₂S₂) (realgar) to the lime when slaking causes the presence of calcium sulphydrate in the lime liquors thus made. This is also a powerful depilatant, but not much used for heavy leather.

The function of the lime in depilating is complex and has occasioned much discussion. Its main purpose, however, is that of a partial antiseptic. When hides putrefy, one of the first results is that the hair is loosened. In America depilation by "sweating" is carried out commercially by such a mild putrefaction, the lime liquor permits a similar fermentation at a slower rate, and all tannery lime liquors are swarming with putrefactive bacteria. Liming is thus a safer method than sweating, which may be easily carried too far. Various workers have isolated specific organisms—Wood a bacillus, Schmitz-Dumont a streptococcus—but it seems highly probable that the limeyard bacteria are just the common organisms of putrefaction sorted out or selected by the exact nature of the liquor and the method of working the limes. Many putrefactive bacteria are very adaptable and could easily accommodate themselves in this way. It is known that the exact nature of the culture medium has a great influence on the rate of development of such organisms, and which particular species thrive and obtain predominance in any limeyard will depend upon the amount and nature of the dissolved organic matter available as food, and upon the exact alkalinity and the concentration of other apparently inert substances, such as common salt and sodium, calcium and arsenic salts. Hence no two lime liquors operate alike, and approximate regularity is only assured by systematic method. In handling and shifting, the organisms are subjected to further selection, and the most adaptable survive. It is probable that different species may act symbiotically. The depilating organisms of lime liquors are probably mostly anærobes, but some may be anærobic by adaptation. It is probable that ærobic ferments commence the depilation, but this will be done before the goods are put into work, or at any rate before they reach the limes. More strictly, it is the enzymes secreted by bacteria which are directly responsible for the hydrolytic work; these enzymes are chiefly proteolytic (proteid splitting), but the lipolytic (fat splitting) enzymes have also a place.

The lime, however, not only limits and selects the course of the putrefaction, but also affords more positive assistance. Lime plays its own hydrolytic part and assists the depilation by purely chemical action. Lime will unhair without the assistance of bacteria, but its action is slow and forms a minor part of the operation in the average limeyard. This action is due chiefly to its progressive formation of calcium sulphydrate from the cystine group of the softer keratins. Lime also plays an essential part in assisting the putrefactive fermentation. It softens the keratins and thus assists the bacterial attack, it hydrolyzes other proteids and provides the bacteria with food in solution, the calcium ion increases the proteolytic action of certain enzymes, and finally the apparently inert excess of undissolved lime has an accelerating effect on the bacterial activity.

In the average limeyard these various functions are inextricably mixed up, and it is impossible to assign any definite proportion of the total depilatory effect to any of the factors at work. Lime alone will unhair, bacteria alone will unhair, and sulphides will also unhair without lime or bacteria, but in the limeyard all three agencies are at work. Putrefactive fermentation, however, obtains a good start. Ærobic fermentation commences with the slaughter of the animal, and the anærobic organisms soon commence their part, and are at work in the hide house and soaks. On entering the limes, the purely chemical hydrolytic action of lime is added to that of the bacterial enzymes as well as the action of lime as bacterial assistant, and the three continue to operate side by side. Each gives rise to the formation of calcium sulphydrate, whose own special solvent effect is superadded. If sulphydrates be deliberately added to the liquors there is yet another factor assisting. Speaking broadly, the bacterial enzymes have their maximum activity in the old limes, and the chemical action of sulphydrate formed from the keratin cystine is also at a maximum in these liquors. The chemical action of added sulphide, and the simple hydrolytic action of calcium hydrate have their maximum activity in the new limes. Most observers would agree that in practice the bacteria shoulder the greater part of the work.

From the limeyard is taken about the only waste bye-products of the tannery, viz. the residues from the soak and lime pits. These consist mainly of lime and chalk, with some hair and dung, and possibly a little sulphide. The sludge possesses some value as a manure, especially if from the soak pits on account of the greater nitrogen content. (Part VI., Section I.)

The Beam House Work consists in the mechanical removal of those parts of the hide not wanted for leather manufacture. Unhairing removes the hair and the epidermis made loose in liming. The hides are placed over a sloping "beam" with a convex surface, and the hair scraped off with a blunt concave and double-handled knife. The hides are then thrown into a pit of water. The hair is carefully collected, washed well with water, preferably centrifuged, and then dried out by a current of warm air. It forms a valuable bye-product. White hair is usually kept separate and fetches a higher price. Fleshing is the next process. The hides are again placed over a beam, with the flesh side (i.e. the side nearest the flesh) uppermost. Skilled workmen then cut off, with a sharp convex knife, the fat, flesh and connective tissue left in flaying. Rounding is usually the next process. The unhaired and fleshed hide is spread out flat and cut up into butt, shoulder and a pair of bellies. These parts have different commercial values, and may afterwards be tanned by different methods for very different purposes—for dressing leather, and sometimes even for sole leather. Scudding is the last piece of beam work. The fleshed hides (whether rounded or not) are washed, or at least rinsed, with water, and again placed on the beam grain side up. They are then scraped with a rather sharp concave knife, to remove "scud," which consists of hair roots and sheaths, lime soaps, fat, pigment and other dirt. Short hair is shaved off by a very sharp hand knife.

The beam work demands a certain amount of skill from the workmen, especially from the flesher, whose sharp knife may prove very wasteful in incompetent hands. Hand labour was slowly but surely being replaced by machinery before the war, and war-time conditions have greatly accelerated the rate of transition. Beam house machinery is rapidly becoming universal. The machines are cumbrous and expensive in cost and in power, but machine work is quicker, less laborious, and needs much fewer workmen. Many types of machine have been suggested, but the most useful are those in which the hides pass over rollers and are simultaneously acted upon by a rapidly revolving cylindrical knife with spiral blades, one half being a left-handed and the other a right-handed spiral, so that the hide is scraped outwards as well as in the direction of motion. The part of the hide being acted upon rests on a pneumatic roller. By changing the type of spiral knife cylinder the machine will unhair, flesh or scud.

Deliming is a general name covering a number of similar operations whose primary object is the neutralization and removal of the caustic lime and soda in the plumped pelt, or at any rate on the surface of the hide. This is a preparation for the tan liquors. All the tannins and many associated substances darken rapidly with oxidation when in alkaline solution, so that to place the fully limed hide in a tan liquor would give a dark-coloured leather. A short insertion in a bath of weak acid would secure the elimination of surface lime and the disappearance of this difficulty, but there are other purposes in deliming. The more completely lime is removed the more the plumped pelt "falls" into a soft, pliable, unswollen and relaxed condition, and this change assists very materially in the production of a soft dressing leather, suitable for boot uppers, bags, etc. For such leathers, therefore, the deliming must be much more complete than for sole leather, in which the object is to obtain a firm and plump leather.

In the case of the softer dressing leathers, experience indicates the advisability of allowing some further bacterial action on the interfibrillar substance in order to produce the requisite pliability and softness. This is secured by "bating" the hides. This process consists in immersing the goods into a cold fermenting infusion of hen or pigeon dung. The infusion is made in a special tub or pit with warm water and allowed to stand for a day or two until the fermentation has commenced, and then run into the bating pit through a coarse filter such as sacking. The hides are immersed for some days, but are handled frequently to ensure an even effect. The bate is always slightly alkaline. The caustic alkalinity increases rapidly at first owing to the diffusion of caustic lime, then at a slower rate, afterwards slowly declining. This is explained by the production of organic acids, and their salts with weak bases from the dung infusion by the action of bacteria. The total alkalinity of the bate liquor increases rapidly at first owing to the diffusion of lime and its liberation of organic bases, then very slowly, but towards the end of the operation the total alkalinity increases very rapidly indeed, owing probably to the commencement of a violent anærobic fermentation which produces ammonia and other organic bases, and which heralds the approach of a putrefactive action, which if allowed to continue for even a short time will ruin the hides. Bating is consequently a risky process, and needs experienced oversight. For goods which need only a mild bating, there is the alternative of giving a longer liming in older limes. This of course involves more bacterial hydrolysis, and perhaps does it in a safer, more economical and certainly in a less offensive manner. Bating is often followed by a further deliming by acids. Boric, lactic, acetic, formic and butyric acids are all used, and with care even hydrochloric and sulphuric acids may be employed. Innumerable "artificial" bates have been put on the market, but most are merely weak acids, acid salts or salts of strong acids with weak bases. An American "bacterial bate" consists of a lactic fermentation of glucose in the presence of glue.

Closely similar to bating is "puering," investigated by Wood (see p. [94]).

Drenching is another fermentive deliming process. In this the goods are inserted into an infusion of bran. This is made by scalding the bran with hot water, and allowing it to stand until it is about 70°-90° F. The infusion is then "inoculated" with a few gallons of old drench liquor, and the goods are immersed. This fermentation has been examined carefully by J. T. Wood. First the enzyme cerealin converts bran starch into glucose, which is then fermented by the drench bacteria with the production of lactic acid, some acetic acid and small amounts of formic and butyric acids. The butyric fermentation is liable to become too violent. These acids, as they are formed, neutralize the lime in the hides and plump the pelt slightly (see pp. [107-109]).

Various gases (carbon dioxide, hydrogen, nitrogen, methane and sulphuretted hydrogen) are involved, and the proportion produced in the pelt itself has a peculiar opening effect on the hide fibres. The activity of the drench can be decreased by dilution and by using a less starchy bran, and can be increased by adding pea meal or rye meal. Drenching usually follows bating. Scudding sometimes follows deliming.

The theory of the volume and elasticity changes of pelt during preparation will be better understood after considering the behaviour of gelatine gels (pp. [200-219]). The determining factors are the nett charge of hydroxyl ions on the disperse phase, resulting from ionic adsorptions, and the lyotrope influence of dissolved substances on the continuous phase.

In softening dried hides the swelling may be due to either influence, but the latter tends to loss of hide substance and the production of soft leather.

In liming, the nett adsorption of hydroxyl ions is the principal factor, but the lyotrope influence of the alkali cations and of the impurities is important. Plump pelts are those in which the contained water is in a relatively greater average state of compression. Few substances can assist plumping, but many can hinder it. In plumping all lyotrope influence is objectionable, and "sharp" (pure) alkali solutions are required. Mellow limes reduce elasticity and plumpness by lyotrope influence.

In bating and puering the essential change is that before the process the swelling is due chiefly to adsorption of hydroxyl ions, whereas afterwards it is due chiefly to a composite lyotrope influence.

REFERENCES.
"Principles of Leather Manufacture," Procter, pp. 108-184.
"The Manufacture of Leather," Bennett, pp. 49-113.
"Lyotrope Influence and Adsorption in the Theory of Wetwork,"
Bennett, J.S.L.T.C., 1920, pp. 75-86.
"Analytical Examination of Bating," Bennett, Leather Trades Review, 1911, p. 972, and 1912, p. 28.
"The Bating, Puering and Drenching of Skins," by J. T. Wood.

SECTION III.—VEGETABLE TANNAGE

All tannages have for their object the conversion of the readily putrescible hide tissue of the corium (the pelt) into an imputrescible, insoluble and permanent material called "leather" which, possessing considerable strength and pliability, is capable of application to a variety of useful purposes. The conditions necessary for this transformation have been clearly stated by Procter. [1] For the production of leather from pelt "it is not only necessary to dry the fibres in a separate and non-adherent condition, but so to coat them or alter their chemical character that they are no longer capable of being swelled or rendered sticky by water." Whatever substance will secure this permanent dehydration of the hide fibres in a separate condition is called a "tanning material." The change from pelt to leather is known as "tannage," the process is termed "tanning," and those who undertake it are "tanners."

[1] "Principles of Leather Manufacture," p. 184.

In "vegetable tannage" the tanning materials are of vegetable origin, and contain a group of organic compounds called "tannins" which are extracted by the infusion of these materials with water. Pelt, when immersed in these infusions, is converted into leather, rather slowly; but a gelatin solution gives an immediate precipitate of "amorphous leather," even if the tannin infusion be exceedingly dilute. The tannins are aromatic compounds of phenolic character, and contain carbon, hydrogen and oxygen only, but our knowledge of their chemical constitution is exceedingly small owing to their instability and colloid nature, which make impossible their preparation in a pure state. They are all, however, derived from either catechol or pyrogallol, and yield these substances if carefully heated to about 200° C. The tannins are soluble in water, alcohol, acetone, ethyl acetate and acetic acid, but insoluble in benzene, chloroform, carbon disulphide, petroleum ether, dilute sulphuric acid and pure ethyl ether. The aqueous infusions of the tannins are in reality colloidal solutions; i.e. heterogeneous systems of two phases. The systems are lyophile, or, more particularly, hydrophile, i.e. there is an affinity between the two phases. As usual with lyophile systems the two phases may be considered as both liquid, and an aqueous infusion of tannin forms an emulsoid sol, which therefore is subject to the phenomenon of adsorption. The tannins are all precipitated by solutions of basic lead acetate and copper acetate, and many of them with varying completeness by solutions of many other metallic salts and hydroxides, of basic dyestuffs and of alkaloids. They give dark colorations with ferric salts.

The tannins are widely distributed in plant-life, but only in a limited number of cases do the plants contain sufficient tannin to render them of commercial importance. Tannin is found in all parts of plants, but usually in greatest amount in the bark or fruit. The tannins are classified into "pyrogallol tans" and "catechol tans," according to the parent phenol. This classification is confirmed by their chemical, analytical and practical behaviour, and the vegetable tanning materials may be classified into the same two groups, for, although even the same plant contains both pyrogallol and catechol tans, it is usual to find in any one part of the plant that one group is predominant.

Pyrogallol tans, which are oftenest obtained from fruit or leaves, contain usually about 52 per cent. of carbon. Used alone they produce a rather soft and porous leather. Associated with them—in many cases probably as decomposition products—are certain other substances of well-known properties and constitution. These substances are not only typical of the group, but also form the most valuable clue to the chemical constitution of the group and the key to their chemical behaviour. One of these substances is gallic acid (3:4:5 trihydroxy-benzoic acid C₆H₂(OH)₃COOH), which possesses properties very similar to the tannins, but does not precipitate gelatin and will not itself make leather. Another of these substances is ellagic acid C₁₄H₆O₈, a double lactone of a hexa-hydroxy-diphenyldicarboxylic acid. This is deposited as an insoluble yellow powder from infusions of many pyrogallol tans, by boiling with dilute acids only, allowing them to stand for a few days. In practice the deposit is found as mud at the bottom of the tan pits, and also upon the leather, to which it strongly adheres. It is technically known as "bloom." It is insoluble in acids and cold alcohol, but soluble in alkalies. It is a feeble dye-stuff. The pyrogallol tans yield very different amounts of bloom. Other associated substances are the sugars. In practice these sugars ferment to lactic, acetic, and other acids which cause "sour" liquors. Such liquors plump the hides and tend to give firm, thick leather. These acids also probably cause increase of adsorption of tannin by the hide and therefore assist in giving "good weight." Solutions of pyrogallol tans all give a blue-black colour with a dilute solution of ferric alum. If a solution of sodium arsenate be added to an infusion of pyrogallol tan diluted until no longer distinctly coloured, and the mixture allowed to stand for about two hours, a green colour develops at the surface of the liquid. The reaction is due to gallic acid or a similar grouping, and is, in the author's experience, the most satisfactory qualitative test for the group. Another test is to mix equal volumes of a 0.4 per cent. infusion of tan and a 10 per cent. solution of sodium bisulphite; a few drops of 10 per cent. potassium chromate are added, and either a transient blood-red colour or a more permanent deep purple is obtained. The former colour is due to gallic acid. If a tannin infusion be largely diluted with hard water and a little iodine solution added, the pyrogallol tans yield either a purple-red or a dark blue colour, the former being a reaction of gallic acid. Pyrogallol tans yield no precipitate with bromine water. They yield a yellow or brown colour when one drop of infusion is added to concentrated sulphuric acid.

Myrabolans is one of the most important of the pyrogallol tanning materials. It is a name given to the dried fruit of Terminalia chebula and other species of Indian trees. The nuts resemble an elongated walnut. They are dried and exported from many parts of India to all parts of the world, but largely to this country. The varieties of commerce are named according to origin and quality: thus we have "J1's," i.e. Jubbelpore, No. 1 quality, "R1's" (Rajpore, No. 1), "B1's" (Bhimley, No. 1), etc. The little difference in tannin strength (about 32 per cent.) in these varieties is usually compensated by corresponding differences in price. The quality of myrabolans cannot be safely judged by appearance. Much bloom is deposited by myrabolans liquors, especially by "J's." Myrabolans are amongst the most sugary of tanning materials, containing up to 5½ per cent. It is therefore one of the best materials for giving a plump leather. Broadly speaking, those varieties which yield most sugar yield least bloom, and vice versâ. Myrabolans tannin has a small affinity for hide substance and penetrates the hide very slowly. It gives a "mellow" tannage, but a bright, good colour, which characteristics are imparted to the leather when the material is blended with other materials containing dark or astringent tannins. When used alone it yields a rather spongy leather, and it is not considered a good weight-giving material, though its acid-producing powers are very helpful to other more astringent tannins.

Valonia has been the other staple tanning material of the heavy leather trade. It is the acorn cup of oaks common in Asia Minor and Greece, chiefly the Turkish oak (Quercus ægilops). The fruit is gathered when ripe and dried in layers of about one foot deep until the acorn drops out, Smyrna is the great export centre. Greek valonia is obtained from many parts of the Archipelago and mainland. It is gathered in a more immature condition and includes the acorn. It is considered slightly inferior in strength and colour to the Smyrna valonia. The exterior of the acorn cup is covered with rather scaly protuberances known as "beard," which contains usually about 40 per cent. of tannin. The cup alone contains usually about 25 per cent. tannin, and the whole about 30 per cent. The valonia tannin has been thought to contain two chemical individuals, only one of which produces bloom. Parker and Leach [2] found that the tannin of the cup produces more bloom than that of the beard, and that Smyrna valonia yields more bloom than Greek. The more bloom is deposited, the less acid will be produced. Under all conditions the yield of bloom is large, and its deposition in and on the leather assists materially in giving the weight and water-resisting powers associated with sole leather which has been largely tanned with valonia. The valonia tannins have only a moderate affinity for hide, which, like myrabolans, they penetrate very slowly. When used alone the leather is less yellow than that from myrabolans, and is also duller. After most of its bloom has been deposited valonia makes a very suitable tannage for dressing leather, and in conjunction with gambier has been largely thus used. Since the outbreak of war the Turkish product has, of course, not been available for importation.

[2] J.S.S.I., 1903, 1184.

Sumach[3] is the other pyrogallol tan of commercial importance. It consists of the leaves and small twigs of the Sicilian sumach (Rhus coriaria) cultivated in Italy extensively for export. The leaves are hand picked, dried and often ground to powder. It contains 26-28 per cent. of a tannin which yields little or no bloom, but much gallic acid. It is an unstable tannin, and its infusion rapidly ferments. Sumach is a very valuable tanning material, and when used alone gives an exceedingly durable leather of excellent light colour. It gives a soft mellow tannage, and is therefore most suitable for light leather tanning, and is extensively used for this purpose. It is used, nevertheless, in large quantities by the heavy leather tanners for finishing purposes, for it contains some organic reducing agent which exerts a powerful bleaching action on other tannages, and which assists to brighten as well as lighten the rather dull appearance of leathers largely tanned with valonia. It is rather an expensive tannin, but most manufacturers find that its results are worth its cost.

[3] Also spelt Sumac and Shumac, and always pronounced like the latter.

Other pyrogallol tans are also used to a limited extent. Algarobilla and divi-divi are the fruit pods of several species of American cæsalpina. They are strong in tan (45 per cent.) and yield a light-coloured and bright leather, but are unstable tans, yielding much bloom. Babla is a small pod yielding a mellow tannage and much gallic acid. Celavinia is another pod containing no colouring matter and giving an almost white leather. The tannin is closely similar to that of oak galls. These last were once extensively used for tanning in Austria. Willow bark is used for tanning in Russia and Denmark. Valuable pyrogallol tannins are obtained from oak wood and chestnut wood, but the woods are not used in tanning as the percentage of tan is so small.

Catechol tans, often obtained from barks, contain usually about 60 per cent. of carbon. They are seldom used alone, for they usually have little or no sugar associated, and hence their liquors do not either "sour" or "plump." They can be used alone if artificially acidified, but without acidifying or blending would give a rather flat leather, though possibly firm. They yield no bloom or gallic acid, but have associated with this other characteristic substances. Of these the catechins are the most typical, and have been considered as the parent substances of the catechol tans.

The catechins are white crystalline substances, apparently isomers with the general formula C₁₅H₁₄O₆. They have different melting-points, and varying amounts of water of crystallization, but are otherwise exceedingly similar in properties. They are sparingly soluble in cold water, but freely in hot, and in alcohol and ether. They are precipitated by lead acetate, mercuric chloride and albumin, but not by gelatin, tartar emetic or alkaloids. In gambier liquors they are especially strong, and sometimes crystallize on the side of the pits, being thus known as "whites." The phlobaphenes or "reds" are also typical of catechol tans from which grow catechins; they can be formed by boiling with dilute mineral acids. They are considered to be anhydrides of the catechol tans. They are difficultly soluble in cold water, but freely in hot, and in cold alcohol and dilute alkalies. They are true tannins and alone are capable of making a red leather, but in practice are often found as mud in the tan liquors owing to their limited solubility. They naturally influence the colour of leather made with catechol tans, which is usually distinctly redder than the leather made from pyrogallol tans. Infusions of catechol (cp. catechin) give a green-black colour with iron alum. The sodium arsenate test gives a red colour due to catechin. The chromate and iodine tests mentioned for pyrogallol tans give negative results with the catechol tans, but bromine water gives a precipitate, and sulphuric acid a crimson colour.

Mimosa bark is one of the most important catechol tans. It is usually obtained in this country from Natal ("Natal bark"); but the tree (Sydney green wattle, Acacia mollissima) is a native of Australia. It is being cultivated now extensively in South Africa, and forms a most valuable portion of the Empire's stock of tanning material. Its more extensive use has been long recommended by the author[4], but its gradually increasing employment in British tanneries has been greatly accelerated by the war, which has prevented its delivery in Germany and has cut off Turkish valonia from Britain. It yields about 30 per cent. of a stable and excellent tannin, and will produce a firm, durable leather, with a colour much less red than that obtained from many other catechol tans. It is an astringent tan, and if carelessly used yields a harsh or even "drawn" grain. Most of the tannin is easily extracted, yielding a clear infusion which penetrates fairly quickly and gives good weight. It contains less than 1 per cent. of sugar, which unfortunately rapidly ferments to carbonic acid, so that it is not a good plumping material. It makes in all respects an excellent blend with myrabolans. Like all catechol tans, the resulting leather darkens on exposure to sunlight.

[4] J.S.C.I., 1908, 1193.

Oak bark, from Quercus robur, is the ancient tanning material of Britain, and is still used to a limited extent. It contains about 13 per cent. of tannin and is mainly a catechol tan, but also contains a pyrogallol derivative. It yields catechin, and gives a red colour with the sodium arsenate test, but also will yield some bloom and gallic acid, and gives a blue-black with ferric salts. The tannin itself is exceedingly similar to that of mimosa bark, but the material contains about 2½ per cent. of sugar, which makes it possible to employ oak bark alone for making sole leather. It is noted for yielding a sound, durable leather of good typical tan colour. Its tannin combines well with hide and penetrates quickly. The fatal disadvantage of oak bark is its weakness in tannin strength compared with other materials. This results in heavy freight and heavy cost per unit tannin, bulky storage, expensive handling in the factory, comparatively large bulk of spent tan, after relatively greater trouble in extracting, and the impossibility of making the strong liquors so necessary in these days to produce good weight in a short time. No satisfactory extract has yet been made from it.

Pine bark, from Pinas abies, is one of the staple materials of the Continent. It contains up to 14 per cent. of a catechol tan, and, unlike most of this group, contains a high proportion of sugar and will give good results alone. Hemlock bark has been the staple tanning material of North America. It is obtained from the hemlock, or Pinus canadensis. It contains up to 11 per cent. of tan and much phlobaphene, and yields a characteristic red leather of good quality, but which rapidly darkens with sunlight. It contains some sugar, but is usually employed in conjunction with sulphuric acid or with sugary materials. Mallet bark yields another catechol tan similar to that of mimosa, but somewhat less astringent and more yellow in colour. Quebracho wood and mangrove bark have been used, but are now made into extracts (pp. [38] and [41]).

Leaching.—Whatever class of leather is being made, and whatever blend of tanning materials is being employed, the tannins must be efficiently extracted by water in order to make the tanning liquors. This process is called "leaching." The tanning materials, after being ground, crushed or shredded, are placed in large pits arranged in "rounds," "sets," or "batteries" of 6, 8 or 10 units, through which water is percolated systematically, so as to secure a continuous extraction. Water itself is added to only one of the pits of material. The liquor produced is passed on to the next pit, and then to the next, and is continually gathering strength. After passing thus through the series, the liquor becomes the source of the strong extracted tan liquors which are used in the tannery proper. With this system the stronger leach liquors are being acted upon by fresh material, and the nearly "spent" material is being acted on by the weakest liquors, and finally by water, thus ensuring a complete extraction. In the press leach system, which is now practically universal, the bottom of one pit communicates with the top of the next, and the liquor presses round by gravity flow caused by a few inches "fall." Liquor is thus constantly percolating downward through the material in each pit. The "head leach" and "tail leach" are always adjacent in a double row of pits, and when the material in the latter is quite spent, it is "cast," and the pit is filled with fresh material. The liquor is then pressed round into this pit by adding water to the tail leach. Hot water is used to secure better diffusion. At least two such sets of leaches ("taps" and "spenders") are necessary to spend the material of the average tannery and to obtain liquors of the necessary strength.

The Manufacture of Extracts.—In addition to the use of the natural tanning materials described above, modern leather manufacturers employ also a variety of "tanning extracts," i.e. vegetable tanning materials in which the tannin has been already extracted, and which are supplied in form of a solid or concentrated liquid. Such extracts only need to be dissolved in warm water in order to make a tan liquor, and the cost and trouble of leaching is avoided. They are a great convenience as making strong liquors of definite strength. Many vegetable tanning materials are too weak in tan for the tanner to leach, and indeed to justify the cost of importation have been made available by manufacturing an extract at the source of the material. With such weak materials the extract manufacturer has had to secure a much more complete extraction than in ordinary leaching, and to concentrate his infusions by means of steam-heated vacuum pans. With such experience he has naturally begun to make extracts also from the stronger materials, such as myrabolans and mimosa bark, and it is now possible to have a tannery without any leaches at all. Tanners also have begun to realize the advantages not only of more rapid and complete extraction, but also of doing the work for themselves, and extract factories are beginning to appear as an adjunct to the larger tanneries. The more complete extraction of tan also involves a greater extraction of unwanted colouring matters, hence decolorization is a feature of extract manufacture.

Chestnut Extract is from the wood of the Spanish chestnut (Castanea vesca), which contains 3-6 per cent. of a valuable pyrogallol tan very similar to that of valonia. Its weight-giving and water-resisting powers are as good as valonia, and its penetrating power is even better, so that it forms an exceedingly suitable material for the modern short tannage, and also for drum tannages. The extract is manufactured extensively in France. The wood is stripped of bark and usually piled for some months to dry and to allow the resins to become insoluble. Some factories, however, use the green wood direct. There are two methods of extraction, viz. in open vats and in closed vats under pressure. The two methods yield extracts which differ in composition and properties. In either case the vats have a capacity of up to 3000 gallons, and hold up to 6½ tons of wood. They are arranged in series, as in leaching, and the liquor passes in succession through all the vats over wood less and less spent. The temperature is highest in the vat containing the fresh water and nearly spent wood. In open vats of wood or copper the temperature is near boiling-point, whilst in the closed autoclaves (copper or bronze) the pressure reaches about two atmospheres and the temperature about 130° C. (266° F.). The series may contain 5, 7, 9 or even 12 vats, and the liquor obtained has a strength of 3° to 4½° Beaumé (22° to 33° Bkr.).

After extraction the liquor is allowed to stand, and much insoluble matter settles out—resins, wood, fibre, etc. The clarified and settled liquor is then passed through a cooler up to about 55° C., and then run into the decolorizing plant, a deep vat fitted with a copper steam coil and mechanical stirrer attached to power. The best decolorizer is bullock's blood, which is run into the vat and well mixed. The temperature is next raised to about 70° C., causing the blood albumin to coagulate. It carries down with it a little tannin, but much colouring matter. After standing a few hours the settled liquor is run off direct to the evaporator. A multiple-effect evaporator is usually employed, and the concentrated liquor, which has a strength of about 25° Beaumé, is run into suitable oak casks. The extracts contain 27-32 per cent. tannin. An extract made with open vats has about 7 per cent. soluble non-tanning matters, whilst a "pressure extract" may contain up to 12 per cent. of these "non-tans." Pressure extracts obtain also a better yield of tannin, which more than compensates for the slightly lower price. Open extraction yields, however, the purer product and an extract with better penetrating powers, and is consequently the more suitable for drum tannages. Chestnut extract is extensively used by the heavy leather tanners.

Oakwood Extract is manufactured from the wood of the common oak (Quercus robur). The centre of the industry has been the oak forest of Slavonia. The wood contains 2-4 per cent. of a tannin very similar to that of chestnut wood, but somewhat more astringent.

The manufacture is also similar to that of chestnut extract, but decolorization is often omitted, and greater care has to be taken and in other ways to keep the colour within limits. One of these is to strip the wood more completely of bark. Another is to operate at as low a temperature as possible, about 110° C. The extraction is made in large circular vats about 14 feet high and holding about two tons material. A battery is composed of about eight vats or extractors. Open extraction is used, and the liquor is passed forward after 2-3 hours' boiling, so that the material is spent in about 24 hours. A liquor of about 5° Be (36 Bkr.) is obtained, and the strength of the material reduced from 4 to ¼ per cent. of tannin. Getting rid of insoluble matter is a difficulty, and is attained by settling, by rapidly cooling, and then passing through a filter press of wood. For evaporation a double-effect vacuum pan is preferred, which operates first at about 113° F., and afterwards at 140° F. with a higher vacuum. The liquor is concentrated from 5° to 25° Beaumé (s.g. 1.036 and 1.210 respectively).

The extract has a much higher colour than chestnut, and is not used now as much as some years ago. As the principal supply was German, it has been unavailable.

Quebracho Extract is made from the wood of the South American tree Loxopteryngium Lorenzii, which contains about 20 per cent. of a typical catechol tan. It is associated with a little catechin, much phlobaphene, but practically no sugar. The tannin is very astringent, penetrates quickly and gives a firm red leather which darkens on exposure to light. It is not noted for weight-giving powers. The wood itself, as chips or shavings, has been used in British tanneries, to a limited extent, but the great bulk of the material is made into extract chiefly in South America. The crude "extract," made by evaporating aqueous infusions of the wood, is largely exported for refinement in Europe. It is also refined on the spot to a large extent and converted into solid extract containing 60 per cent. of tannin.

The great difficulty with quebracho has been the disposal of the phlobaphenes, and a great variety of quebracho extracts are now available which deal with this problem in different ways. In some the more soluble reds are simply left in the extract under the idea that they are really tannins and may be of some use in some part of the tanning process; in others they have been removed by settling and filtration at appropriate temperatures and concentrations; in most, however, they have been solubilized by treatment with alkalies, in the presence of reducing agents, notably by heating with sodium bisulphite in closed vats. The base combines with the phlobaphenes, which are made completely soluble and available for tanning. Sulphurous acid is evolved, and its reducing powers assist materially in retaining and promoting a good colour in the product. Such "sulphited extracts" are now extensively manufactured in this country from the imported "crude" extract, and sold as liquid extracts containing 30, 35 or 40 per cent. of tan according to the requirements of the buyer; "mixed extracts" which are solubilized quebracho blended with about 15 per cent. of myrabolans, are also used.

By solubilizing quebracho with excess of bisulphite an extract is obtained which possesses considerable bleaching powers, and such extracts are also extensively manufactured for the "vatting" or bleaching of heavy leather after tannage. The excess of sulphurous acid not only bleaches the leather, but also swells it up and thus permits a further absorption of strong tan liquor, which is conducive to good weight. These bleaching extracts are usually of 36-38 per cent. strength in tan.

Gambier is an extract of the leaves and twigs of the eastern shrub Nauclea gambier. It is a catechol tan of peculiarly mellow quality and great practical value. It contains much catechin, but little phlobaphene, and yields a beautifully soft leather, but without weight. It is an exceedingly suitable material for the early stages of tanning, and is much liked for tanning leathers that have to be curried, and is widely used in the manufacture of upper leather. It is, however, an exceedingly expensive tannin, and the extract is made in a very crude way by Chinese and Malays without much supervision. Hence its strength in tan and general quality is extremely variable. The plant is cultivated for the purpose of extract manufacture, and prunings are taken in the plant's third year. They are bruised and boiled with water in the open. The infusion is strained, concentrated, and poured into cooling vessels in which it sets to a paste. Two varieties of gambier are well known, "cube gambier" and "block gambier." In the latter the extract remains as a paste containing 25 to 40 per cent. of tannin. It is sold in oblong blocks of 1 or 2 cwt., either wrapped in cocoanut matting or in wooden boxes. Cube gambier is made by running the concentrated syrup into trays 2 inches deep and drying in the sun. When partly dry, it is cut up into 1½-in. cubes and dried further on cocoanut matting. The rough "cubes" as imported contain 40-50 per cent. of tannin.

Myrabolans Extract is now largely manufactured in this country. A liquid extract of 25, 30 or even 35 per cent. strength is made for home consumption, and a solid extract for export. The light colour, high strength and easy extraction of the natural material have all facilitated the task of the manufacturer.

The material is extracted in open vats or stills of copper, which take one ton or more of nuts. A battery of 4, 6 or 8 of such stills is usually employed, and the temperature is kept well below boiling-point except in the vats containing the nearly spent material. The liquors move forward quickly, and the material is quite spent in 24 hours. The material when cast contains less than ½ per cent. of tannin. The liquor obtained is 40°-50° Bkr. (6-7½ per cent. tan), and after settling is concentrated at 40°-50° F. in a single effect vacuum pan, which though more costly in steam is quicker than the multiple effects, and gives the low temperature required. For solid extract the more concentrated liquor is run direct into tarred bags, in which it soon solidifies.

Hemlock Extract is manufactured from the North American pines and imported into this country to some extent. It gives a very red colour.

Mangrove Extract is made from the bark of Rhizophona Mangle and other species of mangrove which grow freely in the tropical swamps of West Africa, Borneo, etc. Much solid and liquid extract has been made from this material, but is not very popular on account of its harsh tannage and dark red colour.

Pine Bark Extract (Larch extract) is made in Sweden from the Norway spruce (Pinus abies). It is slightly sulphited and gives a good colour. It is a liquid extract of about 30 per cent. strength, and is sometimes used as a chestnut substitute. It should not be confused with the so-called "spruce" or "pine wood" extract, which is a paper trade bye-product and contains ligneous matters rather than tannin.

American Chestnut Extract, made from the chestnut oak, is either a liquid or a solid extract in powder form. It gives a wretched brown-black colour, which is quite unsuited to the usual British needs.

THEORY OF VEGETABLE TANNAGE.

Vegetable tannage is a phenomenon of colloid chemistry. The old arguments as to whether tanning was a chemical or a physical process have been rendered obsolete by the advent of a new set of explanations, which, though shedding light on many obscure points, have enormously increased the complexity of the problem. In vegetable tannage an emulsoid gel (pelt) is immersed in a complex emulsoid sol (tan liquor), which immersion results, not in simple reaction or change, but in a series of changes.

One of these changes is adsorption. Pelt is a gel which possesses a great development of surface. It not only exhibits like gelatine the phenomenon of imbibition and dehydration to a very marked extent, but also possesses a very fine fibrous structure due to its organic origin; thus pelt possesses an enormous specific surface, further intensified by the preparation processes previously discussed, which split up the hide fibres into smaller bundles and into much finer constituent fibrils. Tannins, on the other hand, are hydrophile colloids which in water form emulsoid sols, and which may thus be expected to exhibit the phenomenon of adsorption. A tan liquor usually contains several tannins in addition to other closely similar substances, also in colloidal solution, and is therefore a sol of considerable complexity. The immersion of pelt into a tan liquor results in an adsorption, which consists essentially in an inequality of concentration in the sol, the greater concentration being at the interface. This inequality between the surface concentration and the volume concentration of the sol, is due primarily to considerations of surface tension and surface energy, and exists before the immersion of the pelt. The surface layer having excess over the volume concentration, any considerable extension of surface in a fixed volume of sol must produce a very considerable decrease in the volume concentration. This is what occurs when pelt is immersed in a tan liquor, the immersion being the considerable extension of surface. It should be especially remembered that the inequality of concentration is in the sol, on the liquid side of the interface. In adsorption, the substance adsorbed, i.e. the excess at the surface, is too frequently regarded as bound to the solid immersed. This is because the excess is in the layer which wets the solid and remains wetting it when the solid is removed. Thus the immersion of pelt produces primarily only a change in the distribution of the tannins in the liquor. It follows from this that the adsorption is an equilibrium, and that if the sol be diluted, the equilibrium will become the same as it would have been by immersing the pelt directly into the dilute solution. Thus, if pelt be first immersed in one tan liquor and then into a weaker one it will yield tan to the latter solution.

The chief object in heavy leather tanning is to obtain the maximum possible adsorption in the minimum possible time, or in other words, to obtain good weight quickly. The amount adsorbed is proportional to the actual extension of surface, i.e. the adsorption is a function of the specific surface of the adsorbent. Hence, to obtain good weight it is necessary to develop in the pelt its maximum possible specific surface. This is one of the objects of "plumping," which splits up the fibres. It is attained also by the solution of interfibrillar substance in limes and bates.

The amount adsorbed is also a function of the volume concentration in the sol after equilibrium is reached. Hence the better weights are obtained with stronger liquors.

The adsorption law is

y/m = ac^1/n

where y is weight adsorbed by the weight m of adsorbent, and c the volume concentration after adsorption; a and n are numeral constants. Hence weight is determined by the strength of the liquor which the goods finally leave. The commencement of tannage is necessarily in weak infusions, in order to secure the maximum diffusion into interior of the fibres before they become heavily coated on the exterior. As the equilibrium is being established in such liquors the volume concentration diminishes, and thus makes it less likely that good weight will be attained; hence it is necessary in practice to move the goods constantly into fresh liquors of gradually increasing strength, and so maintain the rate of adsorption and save time. A further consequence of the adsorption isotherm is that as y varies as c^1/n and n is > 1, y is increased appreciably only by a relatively large increase in c. Hence, though stronger liquors give better weight, there is a limit beyond which any further gain in weight is not justified by the enormous increase in the concentration necessary to attain it. Such great increase in c is impracticable not only on the ground of expense, but also on account of the great viscosity of the sol.

The amount of adsorption depends also upon the exact nature of the sol. It has been previously pointed out that the tannins differ largely in their penetrating and weight-giving powers. Some are readily adsorbable and are deposited in great concentration at the surface of the fibre, but for good weight it is necessary to use also the less adsorbable and more diffusible tans, which penetrate the fibre itself. Hence it is necessary for good weight to use a blend of materials, and so supply many grades of liability to adsorption. It is particularly advantageous to blend judiciously the two main types of material, the pyrogallol and catechol tans. It is also necessary for good weight to present to the pelt the more diffusible and less adsorbable tannins first, in order to secure the maximum diffusion into the interior of the fibre before the exterior of the fibre is heavily coated with the heavily adsorbable and astringent tans. The least adsorbable materials are therefore used in the early stages of tanning, and the most adsorbable materials at the end of the tanning process. Thus gambier is added to the early liquors (suspenders), solubilized quebracho to the later liquors (handlers), and mimosa bark extract to the final liquors (layers). There is also another excellent way of ensuring this progressive astringency of the liquors; this consists in leaching the required blend of materials together (or mixing them in the case of extracts) and presenting the mixed infusion to the nearly tanned goods, which adsorb chiefly the more astringent tannins. The liquor is then used for goods at a less advanced stage of tanning, which again take the most adsorbable constituents. This is repeated until the stage is reached when the fresh pelt is inserted into the nearly exhausted liquor, which naturally contains only the least adsorbable substances. This system is almost universal, and in practice is known as "working the liquors down the yard." It has the additional advantage of being a systematic method of economically exhausting ("spending") the tan liquors. When free acid is present in the tan liquors, it tends to distend the fibres composing the pelt by a strong and rapid adsorption. Thus distended or plumped the fibres present a still greater surface for adsorptive operation, but the distension naturally leaves less space between the fibres for the diffusion of the sol. Hence acid or "sour" tan liquors give in the long run more weight, but tan more slowly. Pelt tanned whilst thus plumped forms naturally a thicker and less pliable leather. This occurs in tanning sole leather, to a less extent with heavy dressing leather, and to a very small extent in the case of softer dressing leathers.

In addition to adsorption, there is another phenomenon of colloid chemistry in operation, viz. the mutual precipitation of the sols in the liquid by the gels in the hide. In most sols the disperse phase is electrically charged. The sol therefore possesses electric conductivity, and migration occurs in the electric field to the cathode or anode according to the nature of the charge. Oppositely charged sols precipitate one another, the precipitate containing both colloids. The maximum precipitation occurs when the + charge of one sol exactly equals and neutralizes the-charge of the other. There is thus an electrical equivalence; an amount of sol which is equivalent to a given amount of the other. This is not a chemical equivalence, however, and the precipitate is not a chemical compound in spite of its fairly constant composition. The composition of the precipitate, indeed, is not quite constant, for the optimum precipitation may not correspond exactly with the electrical equivalence, being influenced by the number of particles required, their size (dispersity), the rate of mixture, and the relative concentrations of the sols. This mutual precipitation is exhibited by emulsoids as well as suspensoids, but the charge (+ or-) on an emulsoid is in many instances largely an accidental matter, being determined by the medium in which it happens to be, its normal condition being electrical neutrality. Gelatin and pelt are such emulsoids, and a positively charged gelatin sol has been observed to precipitate a negatively charged gelatin sol. It is thought, however, that gelatin is primarily a positive sol. Pelt (whether delimed or not) is rapidly acidified by the quickly penetrating and strongly adsorbed organic acids of the old tan liquors and becomes positively charged before the tannins are adsorbed. The positive charge increases with the acidity of the liquor. Other emulsoids are not electrically neutral, but are electrically charged and exhibit considerable conductivity. Into this class fall the tannins, and in tanning it is thought that there is a mutual precipitation of the negative tannin sol with the positive hide gel, the precipitation of the negative sol being favoured by the acid condition of the liquor. The effect of increasing acidity soon falls off, however, as a saturation limit is soon reached. This mutual precipitation of colloids in tanning is in reality but an extension of the adsorption theory, which explains the predominant effect of H+, and OH-on the electric charge by stating that these ions are more readily adsorbed than other ions, and that as OH-is more readily adsorbed than H+ most sols are negative to water.

In addition to the adsorption phenomena described, there are in vegetable tannage secondary changes which are slow and "irreversible." These changes are obscure and are difficult to investigate. Oxidation, dehydration and polymerization have all been suggested, but there is little direct evidence. Certain it is, however, that time renders the tannage more permanent. It perhaps should be pointed out that in the very strongest tan liquors the viscosity of the tannin sol is so great that adhesion would be a better term than adsorption. There is no abrupt division between the two phenomena.

In the theory of vegetable tannage there is another factor the importance of which has been strongly emphasized by the author, viz., lyotrope influence. This has been most conveniently discussed in connection with gelatin gels (pp. [200-219]), but its effect on hide gels is analogous. It has also an effect upon the diffusion and gelation of the tannin and non-tannin sols (cp. pp. [129] and [174]).

Mechanical Operations.—In the tanyard the liquors are almost invariably divided up into sections, called "rounds" or "sets," in which the mechanical operations are different in aim and method. In the first pits entered by the goods there is rapid adsorption in spite of the low concentration and small astringency, and the great aim is to obtain evenness of action and a good level colour. It is also necessary to maintain the rate of adsorption. All the aims are attained by frequently moving the goods. Heavy leather is suspended vertically in the pits of tan liquor and handled up and down as well as forward from pit to pit. Such pits are termed "suspenders." In the earliest suspenders it is indeed advantageous to have the goods in constant motion. This is done by suspending on wooden frames which are rocked gently by mechanical power; such pits are termed "rockers." For dressing leather in which firmness and smooth grain are not so essential, the goods may be paddled in the first liquors. This is occasionally done with stronger liquors for the express purpose of working up the "grain" pattern. The goods after passing through the suspenders are usually passed to "handler" rounds, in which they are moved less frequently. In these pits the goods are laid horizontally one above the other. One advantage of handlers is that the goods flatten thoroughly and straighten one another by their own weight; another is that more goods can be placed in one pit than in suspenders. They are not so convenient to work, however, as suspenders, and the goods do not feed so rapidly. Hence the tendency is now to tan more in suspension, and to economize labour by an extension of the rockers. The handling of the goods is also saved by pumping the liquors and by working rounds of suspenders or rockers like the press leach system, with the difference that the stronger liquor is pumped in to the head pit, and the liquor passes upwards through the goods.

Finally the goods are placed in "layers" or "layaways," in which they remain undisturbed for a decidedly longer time. These pits contain the strongest liquors of the yard, and their principal function is to complete the tannage and give weight and firmness by the adsorption of bloom, reds, etc., in the interior of the hide. The goods are placed in horizontally, and are dusted in between with fresh tanning material which maintains the local strength of the liquor and keeps the goods somewhat apart. Drum tanning attains a more rapid penetration of the pelt by giving constant motion in stronger infusions. It is of course liable to result in an under-tannage of the interior of the fibre. After the goods have been "struck through" in the ordinary way, however, drumming in extract is increasingly used as a substitute for much labour in handling, and also to save the time spent in the early layers.

REFERENCES.
Procter, "Principles of Leather Manufacture," pp. 220-350.
Bennett, "Manufacture of Leather," pp. 113-179.
Bennett, "Celavinia and Babla," L.T.R., 1914, 122.
Dumesny and Noyer, "Manufacture of Tanning Extracts."
Theory:—
Meunier and Seyewetz, Collegium, 1908, 195.
Stiasny, Collegium, 1908, 117-159, 289, 294, 337.
Procter and Wilson, Collegium (London), 1917, 3.
Wilson, Collegium (London), 1917, 97, 100, 105.
Moeller, Collegium (London), 1917, 13, 38, 46, 103; and J.S.L.T.C., 1917, 22, 56, 92.
Bennett, J.S.L.T.C., 1917, 130-133, 169-182; 1918, 40; 1920, 75-86; S.L.R., 1916, March.

SECTION IV.—FINISHING PROCESSES

After the tannage is complete, leather is hung up to dry. In the case of heavy leather this drying must be very carefully carried out in order to obtain a product of satisfactory appearance and saleable qualities. Associated with the drying are many mechanical operations (scouring and rolling) which assist very materially in imparting the desired qualities. After tanning, however, the quality of the final product is most strongly influenced by the amount of grease added in finishing. Some grease is always used in finishing, partly because even sole leather requires some measure of pliability and partly because a coating of oil over the leather during drying prevents the loose tannin from being drawn to the surface of the leather by capillarity, thereby causing dark and uneven patches and a "cracky" grain. The added grease is also a contribution to the "weight" of the finished article—a primary consideration for heavy leather, which is usually sold by weight. The finishing processes, indeed, tend to be dominated by this consideration, and become a series of efforts to retain as much tannin and add as much grease as are consistent with the requirements of the class of leather being manufactured. Sole leather does not contain more than about 2 per cent. grease, or its firmness is impaired. Belting leather, in which considerable pliability is needed, may contain about 9 per cent., whilst harness leather, which must be exceedingly tough and durable, may contain up to 13 per cent. of fatty matters. Upper leathers, which need to be soft and pliable as well as waterproof and durable, are very heavily "stuffed" and often contain up to 30-40 per cent. of grease. Sole leather is thus rather distinct from the rest, which are called "curried," "stuffed," or "dressed" leathers. The actual drying out before, after and between the various mechanical operations, each have an appropriate degree of wetness. In this country the drying is usually under the prevailing atmospheric conditions and is known as "weather drying." The goods are suspended by hooks or strings or by laying over poles in special sheds fitted with louvre boards by which the rate of drying can be roughly controlled. Weather drying is cheap, but exceedingly slow, and in unfavourable weather is very unreliable. The goods, moreover, need constant attention to obtain an even result. Steam pipes are usually laid along the shed floors, and are used in winter and damp weather to accelerate the drying, and also in the final shed stove to remove the last traces of moisture. Wet weather, however, will not stand a high temperature, and steam drying is better avoided when possible. Air-dried leather still contains about 14 per cent. of moisture. Many systems of shed ventilation have been suggested to hasten the drying and to secure a better control of the process. In one system a screw fan is fitted at one end of a shed (without louvre boards) and sucks air through the goods from an inlet at the other end. The air can be heated by a steam coil near the inlet. In another system a centrifugal fan blows air through an arrangement of pipes which distributes it to the drying sheds, and discharges it close to the floor by various branch pipes. The outlets are near the roof. A system of dampers permits hot air, warm air and the used wet air to be blended in the desired proportions. In America turret drying has been used. The sheds are vertically above one another and have latticed floors. Heated air is admitted at the bottom and rises through the goods up the building just as in a chimney. For many of the finishing operations it is important to obtain the leather in a uniformly half dry or "sammed" condition. This may be done by careful drying, and wetting back the parts that have become too dry with tepid water or weak sumac liquor, and then leaving the goods "in pile" until of uniform humidity. It may also be done by "wetting back" leather which has been completely dried out. There are also "samming machines" which by means of rollers squeeze out the excess liquor. Sole leather is dried out and finished immediately after tanning, but dressing leather is often "rough dried" out of tan liquors and wet back for finishing when required. Dressing leather is often treated in different factories; tanners selling it as rough leather and "curriers" finishing it.

Scouring is one of the first operations in finishing leather. The grain side is wet and worked with brushes and stones until the bloom and loose tannin are removed. This process aims at producing a good even colour and level surface, but is liable to cause a loss of weight. Dressing leather is often scoured on both grain and flesh, and weak soap or borax solutions are used to assist the process. In this operation hand labour has been now quite superseded by machine work. A great variety of machines have been devised. The mechanical working of leather takes place in various parts of finishing. These operations, known as "striking," "setting," "pinning," "jacking," may be carried out often by the same machine as used for scouring, but with a change of tool. The object of these operations is to get rid of wrinkles and creases, to produce softness, pliability and area, and to remove superfluous moisture, grease, dirt. The tools are of steel, brass, slate or vulcanite. Scouring is often effected by putting the goods into rotating drums together with extract and sumach. The bloom is removed by friction, the colour is improved by the sumach, whilst the extract keeps up the weight.

In finishing sole leather firmness is enhanced by "rolling." A brass roller passes to and fro over the goods with the exertion of considerable pressure. The operation is carried out by machinery.

Shaving is an important operation in the case of many dressing leathers. Its object is to produce a uniform thickness of the leather and an even surface on the flesh side. The sammed goods are laid over suitable beams and shaved with special sharp knives which possess a turned edge. This hand process, which demanded considerable skill, is fast becoming extinct, and machine shaving is already almost universal on account of its greater speed. The machines consist essentially of two rollers, one of which is smooth, whilst the other is a spiral knife-blade cylinder (cp. Section II., p. [23]). The sammed goods are held in the hands and placed over the smooth roller, which is raised to the cutting roller by a foot treadle. A number of similar operations ("flatting," "whitening," "buffing") are carried out by a suitable change of tool. In all these operations good samming is important.

Splitting is another important operation on tanned leather. In this process the leather is cut parallel to its grain surface, thus yielding two pieces with the same area as the original, the "grain" and the "flesh split." It is essentially a machine operation, and is carried out by presenting the carefully sammed leather to a sharp knife-edge, towards which it must be constantly pressed. The "band-knife" machine is the most popular arrangement. The knife is an endless belt, which continually revolves round two pulley wheels of equal size. In between these the knife is horizontal, and is then used for splitting. The sammed leather is pushed towards the blade by two feed rollers, and the grain passes above the knife on to a small platform, whilst the flesh or "split" passes below and falls to the ground. Emery grinders and thick felt cleaners in the lower part of the machine keep the knife in good condition. The adjustment of the machine is delicate and requires considerable experience. With care splits may be obtained down to 1/16" thick, and sometimes as many as 6 or 7 splits are obtained from one hide.

Oiling is still usually done by hand, and cod oil is still preferred for many classes of goods. Of recent years there has been a great extension of the use of sulphonated oils, which have the valuable property of forming an emulsion with water or tan liquor. With these materials it is easier to ensure the goods being completely covered with oil. The penetration of the oil into the leather is also quicker and more complete. These oils have often the disadvantage of leaving solid fats on the exterior of the leather, which gives it an ugly smeared appearance.

Stuffing the dressing leathers is carried out in a variety of ways and with a variety of materials. The old process of hand stuffing employs a mixture of tallow and cod oil called "dubbin." This is made by melting the ingredients together and allowing them to cool with constant stirring to a nearly homogeneous salve. The dubbin is brushed thickly on to the flesh side of the sammed leather, which is then hung up to dry. As the moisture dries out the oils and soft fats penetrate the leather and leave the more solid fats on the outside. The proportions of tallow and oil are varied with the time of year and with the method of drying, for if the dubbin be too soft it will run off the leather, and if too hard will not penetrate it so well.

Drum stuffing is a more modern development in which a higher temperature is employed, about 140° F. The drum is heated up by steam or by hot air, and the sammed goods are then inserted and drummed for a few minutes until they are warmed. The drum is fitted with a heated funnel containing the melted grease, which is run in through the hollow axle. After a half to three-quarters of an hour's drumming the grease is completely absorbed by the leather. The drumming is continued for a while until the goods have cooled. Whilst still warm they are "set out" to remove creases and superfluous grease. Drum stuffing is not only quicker than hand stuffing, but also makes it possible to use the hard fats, and so make a leather which carries more grease without appearing greasy. Thus in drum stuffing, paraffin wax and wool fat are used, and their penetration assisted by small proportions of cod oil or dégras. If the leather be too wet the grease is not absorbed, whilst if it be drier than usual the leather will take more grease, but the resulting colour is not so good. There is also another method of stuffing which originates from the Continent. It is known as "burning in" and involves the use of still higher temperatures (195° to 212° F.). Wet leather will, of course, not stand this temperature, so that it is first necessary to make the leather absolutely dry. This is effected by drying in stoves at temperatures up to 110°-115° F. There are two ways in which the grease is applied. In one method the melted grease is poured by a ladle on to the flesh side and brushed over until evenly distributed. A second application of grease is made to the thicker parts. The hides are then put into warm water (120° F.) for about a quarter of an hour, and then drummed for half an hour. In the other method the goods are completely immersed in the melted fats for a few minutes in a steam-jacketer tank at a temperature of 195° F. After softening in water at 120° F. the goods are drummed. "Burning in" is used for the heavier dressing leathers such as belting and harness. It does not give good colour, but permits the employment of still more hard fats.

REFERENCES.
Procter, "Principles of Leather Manufacture," pp. 223, 378.
Bennett, "Manufacture of Leather," pp. 251-312.
Bennett, "Principles of Leather Stuffing," Leather Trades Review, 1911, 186.

SECTION V.—SOLE LEATHER

Leather for the soles of boots and shoes is a matter of essential interest to all, and forms one of the best appreciated applications of animal proteids to useful purposes. Methods for its manufacture are as numerous as the factories producing it, hence all that can be done is to describe broadly the general method which is typical of our time, to classify the many varieties into types, and to indicate the recent changes and present tendencies.

Sole leather is mainly manufactured from butt pelt, and the great aim is to produce a firm, thick, waterproof and smooth grained leather which will bend without cracking. It must have a light tan colour to be saleable, and contain as much weight as possible to be profitable.

The modern mixed tannage of "sole butts" or "scoured bends" generally utilizes ox-hides of the Scotch and English markets, though salted Continentals and South Americans are also employed. After the usual soaking a short and sharp liming is given. The special aim in liming sole hides is to obtain the maximum plumping effect with the minimum loss of hide substance. Both these achievements are necessary to obtain good weight. The limes should be kept as clean as possible, which is best obtained by putting clean hides into work. This reduces bacterial activity and loss of hide substance. The "shortness" of the process is attained by the use of sodium sulphide (from 2 to 16 ozs. per hide of sulphide crystals), by which depilation may be accomplished easily in about nine days. The amount of sulphide should be increased somewhat in the short-hair season and in cold weather. Some factories take up to about 12 days using less sulphide, whilst others will lime in about a week by using the larger quantities. The amount of lime used varies enormously, and is invariably in great excess of the actual requirements. "Probably 2-3 per cent. on the green weight of the hides is all that can be really utilized, the remainder being wasted." [5] This amounts to about 2½ lbs. lime per hide, but in practice it is more frequent to find 7, 8, 9 or even 10 lbs. per hide being used. The excess is innocuous, owing to the limited solubility of lime. Some excess is desirable, to replace in the liquor the lime adsorbed by the goods in plumping, to assist bacterial activity (p. [21]), and also because in sharp lime liquors the undissolved portions do not remain so long in suspension. The use of sulphide and other alkalies does not "make it possible" to reduce the amount of lime used, it merely renders the excess more superfluous. The use of sulphide not only shortens the process, but also sharpens it, on account of the caustic soda produced by hydrolysis. Usually for sole leather, however, it hardly sharpens it sufficiently, and it is very common to add also caustic soda (or carbonate of soda) to the limes. About 2 ozs. caustic soda (or its equivalent in carbonate) is used per hide. The hides are limed generally by the three-pit system, giving about three days in each pit. They should be handled each day in the first pit (old lime) and once in the other pits.

[5] Procter, "Principles of Leather Manufacture," p. 129.

Unhairing and fleshing by hand labour is still common, in order to avoid great pressure on the plumped hide. Scudding should be very light, and in some yards is entirely omitted. Only the lime on the surface of the hide should be removed by deliming, and this immediately prior to the insertion of the butts into the tan liquor. This is to ensure good colour and yet keep the butts plump. Boric acid is the best for this purpose, using 10-15 lbs. per 100 butts. The goods are inserted (and preferably rocked) in a dilute solution for a few hours only. About the same quantity of commercial lactic acid may be substituted for the boracic. This deliming can also be accomplished by adding the acid to the worst suspender in the tanyard.

To obtain firmness and plumping it is necessary that the early liquors in tanning should be more acid than for other leathers. With old methods of tanning one could trust to the natural sourness of the liquors to complete the deliming and replump the goods with acid. In such cases any deliming was also unnecessary. In the modern yard, however, we get "sweet" liquors coming down the yard, partly on account of the greater proportion of extract used and partly because the liquors themselves are not so old. Hence it is now practically always necessary to acidify artificially the tan liquors. This may be done by adding a few gallons of lactic, acetic, formic, or butyric acid to the handlers and suspenders, especially in the winter and spring. It is now increasingly common to place sole butts in a special acid bath after they have been in tan liquor for about a week. This bath is often made from sulphuric acid, and may be 1 or 2 or even 4 per cent. in strength.

The actual tanning of sole butts lasts three to four months, and just prior to the war the tannage consisted often of about one-third myrabs, one-third valonia, and one-third extract. The myrabs and valonia were leached together, and the extract added to the best leach to make layer liquors of the required strength. Some mimosa bark was generally used also, and now it is extensively employed to replace the valonia. The most widely preferred extract is chestnut, but quebracho, myrabs extract and mixtures have also a prominent place, and mimosa bark extract an increasing importance. It is recognized that this tannage is if anything too mellow, and that if only a smooth grain and plump butt can be ensured in the first weeks of tanning, it is much better for sole leather to employ the most astringent tans possible and the sharpest liquors (i.e. liquors with a small relative proportion of soluble non-tannin matters). Hence there is the tendency in sole-leather tanning to employ fresh clear liquors for the butts and use up the more mellow liquors on the "offal" (shoulders and bellies).

Four types of sole butt tannage will now be described, all of which illustrate the methods employed in a modern mixed tannage.

1. The first type consists in a four-months tannage, in which the liquors are worked down the yard.

The butts pass first through the suspenders (20°-40° Bkr.) in about a week, and are rocked in the first liquors. They next enter the handlers (40°-55°) rounds of eight pits, six floaters and two dusters. Myrabs, or a mixture with algarobilla is used as dusting material. The goods remain in this set for two weeks, and should then be struck through. The suspender handlers (55°-65°) are next entered, in which they remain up to three weeks in suspension, being shifted forward on alternate days. The goods now enter the layers, of which four are given: first 70° for one week; second 75° for two weeks; third 80° for three weeks; and fourth 90° for a month. The goods thus take sixteen weeks to tan, of which ten weeks (62½ per cent.) are in layers.

The system of working the liquors is expensive, and is only possible if the butt liquors can be spent out by the offal. The best or fourth layer, 90°, is made from the best leach liquor, 65°, and extract (chestnut with some oakwood or mimosa bark). After use it becomes the second layer, 75°. The third layer, 80°, is also made from fresh leach liquor and extract (chestnut with some myrabs or mixed extract). After being used thus it is used for the first layer, 70°. The used first and second layers are mixed together and used partly to form the belly layers, and partly to make a sharp liquor for the handlers (55°-40°) by diluting with 40° leach liquor and adding quebracho extract. The old handler liquor is run to the suspenders (40°-20°), and finally used for colouring off the offal in drum or paddle 18°. The suspender handlers (65°-55°) are made from fresh leach liquor and chestnut extract. They are afterwards used to make shoulder layers. The course of the liquors is shown on p. [59].

It will be seen that fresh leach liquor and fresh material are used to each set except the suspenders, which must have some mellowness to ensure plumping and smooth grain. Layer liquors are used twice only, and then (when only five weeks old) pass to the handlers. These are further sharpened by fresh leach liquor and fresh extract and dry materials. The forward handlers are fresh liquors with fresh extract. This tannage is fairly typical of high-class sole leather, in which the liquors are worked down the yard, but worked towards the offal, which thus receives liquors with relatively greater proportions of mellow tans and soluble non-tans.

2. The second type consists in a tannage of about four months, in which the liquors are not worked down the butt yard. In this method also there is an attempt to save much of the labour in handling, first by shortening the time in the handlers by one week (as compared with the above), and second by fusing the two progressive handler sets into two sets of equal strength, through which the goods pass more slowly and with less disturbance.

The goods go through the suspenders (10°-25°) in about a week, rocking in the early liquors, and then into large rounds of handlers (30°-45°) for one month. The handlers consist of floaters and several dusters, in which the butts are laid away with 1-3 cwt. myrabs. The goods next enter the layers, of the same strength as in Type 1, and in which they remain the same time. The total tannage is thus 15 weeks, of which 10 weeks (nearly 67 per cent.) are in layers.

The best or fourth layer is made up from leach liquor and extract, and is then used successively as a third, second and first layer, and then passes to the offal layers. The handler liquor is made entirely from fresh leach liquor and quebracho extract, and is a sharp liquor of greater strength than its Bkr. strength would indicate. The old handler liquor is run to the butt suspenders. The course is represented thus:—

3. The third type consists of a short three-month's tannage in which the liquors are worked straight down the yard. To compensate for the short time it is necessary to have stronger layer liquors in which the goods spend a still greater proportion of their total time. The stronger liquors involve a greater proportion of extract, particularly of quebracho, which fact causes the whole of the liquors to be sharper than their Bkr. strength indicates, and justifies them being worked straight down the yard.

The goods go through suspenders (20°-40°) as usual one week, and then pass into suspender-handlers (40°-60°) for two weeks, and thence to the layers. In the first two of these (65° and 70°) they are actually in suspension, a week in each liquor. They are then dusted down for ten to eleven days, first in 85° and then in a 95° liquor, and finally for a month in a liquor of 110°. The total tannage is thus twelve weeks, of which nine weeks (75 per cent.) are in layers. There is considerably less handling than in Type 2, and it is more convenient, the goods being in suspension.

4. The fourth type is also a three-month's tannage. In this it is attempted to obtain even greater weight with still less labour. The layer liquors are kept much stronger by the more extensive use of extract, and this makes it impracticable as well as too costly to run these liquors down the yard. They are therefore repeatedly strengthened with extract and used again.

The goods go through suspenders (20°-40°) as usual one week, and then through a round of suspender-handlers (40°-55°) consisting of fresh sharp liquor from the leaches together with quebracho extract. They are in this set two weeks, and then are laid away. They receive three layers: first, 105° for 2 weeks; second, 110° for three weeks; and finally, 120° for a month. Of the twelve weeks, therefore, nine weeks (75 per cent.) are spent in layers. In this method the goods are immersed in 3 per cent. sulphuric acid after passing through the suspenders.

There is possible, of course, a tremendous number of variants of the above types. The number of handler rounds is determined by the number of butts being dealt with. With a large number it is more easily possible to arrange for them to be in progressive strength as in Type 1. There are also many systems of working the layers, of which the most notable is to make the second or third layer from fresh leach liquor and extract, and strengthen it with extract for the succeeding layers. It is then used as a first layer and worked down the yard.

The bellies and shoulders often go through separate sets of liquors, but it is common to put them through suspenders, and even handlers together. They receive, of course, a distinctly shorter tannage, and are often drummed with extract before laying away or after the first layer. By way of illustration, the course of the offal and their liquors may be given in the case of Type 1. The shoulders and bellies are coloured off in a paddle or drum with old butt suspender liquor, which is then quite exhausted. They then pass through suspenders (18°-40°) together in 4-5 days, and go through a handler round (40°-55°) for 3 weeks, including one duster. The bellies are removed after 2 weeks, and given three layers (60°, 70°, 80°) of a week each. They receive, therefore, nearly 6 weeks in all. The shoulders also have three layers (60°, 65° and 80°) of 2, 3 and 4 weeks respectively.

The course of the liquors is shown thus:—

The tanned butts are piled for 2-3 days, sometimes rinsed to remove dusting material, and then scoured either by machine or by drumming with sumac and extract. This removes bloom, but causes some loss of weight. "Vatting" or "bleaching" now follows, in which it is attempted not only to bleach the colour of the leather, but also to impart as much weight as possible. The vat liquor is made several degrees stronger than the last layer by means of quebracho bleaching extract and good coloured chestnut or myrabs extract. The liquor is kept warm by a steam coil, at about 100° F., but not much more without risk. The goods remain in the bleach liquor 2-3 days and are then horsed or suspended to drain. Sumach is sometimes used in the vats. A new vat liquor must be made up after some weeks' use. The goods are sometimes rinsed in weak sumac liquor before vatting to get good penetration, and sometimes after to ensure good colour.

The butts are next oiled and hung up in a dark shed and allowed to dry slowly and evenly to an "india-rubbery" consistency and rather slimy feel. They are then "struck out" by machine, wiped, re-oiled and again hung up to dry, preferably with sulphonated oil. After a short drying to a suitable and even condition they are "rolled on," and, possibly after further drying, "rolled off" with greater pressure, and then dried for a day or two with the help of a little steam. Finally they are machine brushed and sent to the warehouse, where they are weighed and classified.

The offal is often drum oiled. It needs more striking and is more difficult to obtain in suitable condition for striking, rolling. It is treated similarly to butts, but often also goes for dressing leather, and may be split. It is of some interest to compare the above processes with that once very popular manufacture of "bloomed butts" in the West of England from South American salted hides. These receive a liming from 12-14 days, using 12-16 lbs. of lime per hide. They receive then a tannage of about 9 months, comprising 3 weeks in suspenders (20°-40°)—very sour and mellow liquors—4 weeks in handlers (40°-55°), 4 weeks in dusters (60°), 4 weeks in round made from hemlock extract (60°), and 20 weeks in six layers (60°-90°) in which they were dusted heavily with valonia. Oakwood extract was used for the layers, which took 57 per cent. of the total time. The butts were scoured in a much-dried condition, so that only the loose and surface bloom was removed. No bleaching was given in the modern sense.

In the old oak-bark tannage of sole leather up to 12 months were taken for tanning, two-thirds to four-fifths of which time the goods were in layers. The strongest liquor rarely exceeded 50° even where valonia and gambier were also used, and rather more than 30° if not.

It will be understood from the above that the tendency for many years has been to shorten the time and the labour required for tanning. Drum tanning is obviously the next stage in shortening the time. In one such process the butts are put through suspenders (25°-40°) for 2 weeks, drummed for 12 hours in an 80° extract liquor, and finally in a neat extract 200° for 36 hours. Drum tanned sole leather, however, is not as yet of good quality; the grain is not smooth, and the heavy weight finish (striking and rolling) needed to counteract this tendency is liable to cause poor "substance." The leather, too, readily wets and goes out of shape. Possibly some drumming may be adopted to save time in the early layers, but the most serious rival to the 3 months' tannage is the waterproof chrome sole leather (Part III., Section V., p. [173]).

REFERENCES.
Parker, J.S.C.I., 1902, 839.
Procter, "Principles of Leather Manufacture," p. 220.
Bennett, "Manufacture of Leather," pp. 179, 259.
Bennett, J.S.C.I., 1909, 1193.

SECTION VI.—BELTING LEATHER

The manufacture of belting leather is well illustrated by the tanning and finishing of "strap butts." In general, the tannage presents many points of great similarity with the tannage of sole leather; indeed, the resemblance is so close that in some factories there is little difference observed, and the currying and finishing operations are relied on to produce the desired difference in final results. Nevertheless, there is considerable difference in the type and ideal of the two leathers, which may be expressed in trade parlance as a greater "mellowness" for the belting leather, and in the best methods of manufacture this fact is in evidence throughout the whole process of manufacture.

In liming, there need be little difference between sole and belting hides, and a sharp treatment of 9-10 days, by the three-pit system, with a day or two extra in the coldest weather, would meet ordinary needs. For the conservation of hide substance and for the saving of time a shorter liming is sometimes given, in which more sulphide is employed than is usual for sole leather. Even the very short processes of liming, 1 to 3 days, which involve the use of strong solutions of sodium sulphide, have been successfully employed for belting leather. The tendency to harsh grain with such processes is not so serious a defect with belting as with sole leather, and can be minimized by careful deliming. American and Continental factories tend to favour the use of those quick processes which employ warm water in addition to sulphide. The hides after a short liming in sulphide limes are immersed in warm water, which greatly accelerates both the chemical and bacterial actions. For example, after about 3 days' liming, in which both old and new limes are used as usual, the hides may be thrown into water from 100°-105° F., and will be ready for depilation in 7 or 8 hours.

Even a stronger liming may be given, especially if the soaking is unusually prolonged. Such processes undoubtedly save hide substance, and the pelt is obtained more free from lime, but they have the disadvantage that the natural grease of the hide is only imperfectly "killed" (i.e. saponified or emulsified), and may interfere with the normal course of the tannage. The plumping is also apt to be insufficient. On the other hand, liming processes are also used in which a mellower liming or a longer liming is preferred in order to produce the desired degree of softness and pliability in the finished leather. Belting must not be too soft, of course, and it will be clear that the required difference from sole leather can be produced either in liming or tanning or partly in both. These considerations also decide whether bating is to be omitted or not. A hard astringent tannage in sour liquors after a sharp liming might make bating essential, but in these days it is usual to avoid it and produce the effect in other ways. A light bating of a few hours is sometimes given, but it is more unusual to delime the grain thoroughly with boric acid, using up to 20 lbs. per 100 butts. Crackiness is a fatal defect in strap butts, so that a sound grain must always be obtained. Generally speaking, therefore, strap butts receive more washing in water, and rather more deliming than sole leather, even when they are not bated. It is also usual to scud much more thoroughly, and to round a larger proportion of butt, especially in length.

The tannage is usually carried out with a blend which includes a much greater proportion of the fruit tans, and correspondingly less of extract.

Distinctly more myrabs are used than in sole leather tannages, in the dry material, and amongst the extracts chestnut is preferred to quebracho, and myrabs to mimosa bark, though all these may be used in some degree. In the past the most favoured extract has been undoubtedly gambier, which gives a tannage which is easily curried and imparts the required mellowness to the uncurried leather. The great expense of this material, however, together with the advent of drum stuffing and shorter tannages in stronger liquors, have tended to cause a considerable reduction in the proportion used for strap butts, and to limit its employment to the earlier stages of tanning.

The same tendencies for reducing the time taken to tan, employing stronger liquors, and securing economy of labour in handling, have been evidenced in the tannage of strap butts as in sole butts. It is nevertheless true that, broadly speaking, strap butts receive rather more handling and rather weaker liquors than sole butts. A greater amount of mechanical assistance is also employed with early stages (paddling, drumming, rocking). This is less objectionable for curried leather than for sole butts. The handling is more usually in suspension. The liquors are usually worked straight down the yard as a greater mellowness is needed in the early liquors than for sole butts. The offal is given a separate tannage and often used for different purposes, e.g. the shoulders for welting and the bellies for fancy goods. Plumping with sulphuric acid is generally considered inadmissible for strap butts. It has been shown that leather containing sulphuric acid tends to perish after the lapse of a number of years. Sole leather will be worn up before this effect is observed, but belting is an article which is intended to last much longer, and the use of sulphuric acid is consequently inadvisable. Plumping must be obtained, to a considerable extent, but must be achieved by the organic acids (lactic, acetic, formic and butyric acids). A few gallons of such acids are consequently added to the handlers, especially in the winter and spring. Less may be used in the autumn, when the layer liquors which fermented in the summer months have worked down to the suspenders. A mixture of these acids is usually better than any one alone, for they not only differ very considerably in price, but also have different powers of neutralizing lime and plumping the goods. Lactic acid (M.W. 90), Acetic acid (M.W. 60), and formic acid (M.W. 46) are each monobasic acids; consequently 3 lbs. formic will neutralize as much lime as 4 lbs. acetic or 6 lbs. lactic. Their plumping powers are somewhat influenced by the anion. In determining what quantities to take, the commercial strength of the acids must also be considered. Formic is often 80-90 per cent. pure, acetic 60-80 per cent., and lactic 40-60, but may be as low as 25 per cent. The blend must be adjusted accordingly. As strap butts do not need the firmness of sole leather, less of these acids may be used than for sole butts.

The exact nature of the tannage and the strength of the liquors is largely influenced by commercial considerations. If the manufacturer is both tanner and currier, he need not go to such great expense in strong liquors and in time in layers, for he can obtain some of this weight in currying. If, however, the tanner sells the butts rough dried, he must naturally aim at obtaining greater weight in tanning.

The actual details of the tanning processes are as usual very varied, but may be classified according to type, just as in the case of sole butts.

Illustrations will now be given.

Type 1, which may be compared with Type 1 for sole butts, is a tannage of about 5 months. The goods pass through suspenders (8°-30°) in 2½ weeks, and then pass to the handlers (30°-50°), in which they remain a month; they are then put into suspension again and pass through the suspender handlers (40°-55°), which takes 2½ weeks. In this round much gambier is added, and the goods are frequently handled. Four layers are usually given, viz. first layer 55°, one week; second layer 60°, two weeks; third layer 65°, four weeks; and fourth layer 75°, four weeks. The tannage is thus 20 weeks, of which 11 weeks (55 per cent.) are in layers. Extra layers may be given to heavier goods, using stronger liquors made up with extract. All liquors work straight down the yard.

The tannage consists of 35 per cent. myrabs, 35 per cent. valonia, 10 per cent. Natal bark, and 20 per cent. extract, chiefly gambier, though some chestnut and quebracho are used.

Type 2 represents the modern tendency to use stronger liquors and a shorter time. The strap butts pass through the suspenders (22°-50°) in 1½ weeks, during about a third of which time they are rocked. They next pass through two sets of suspender-handlers (50°-67° and 67°-80°), which takes a month, and thence to the layers. Three layers are given (85°, 90° and 100°), in which the goods remain one, three and four weeks respectively. The tannage is thus 13½ weeks, of which 8 weeks (nearly 60 per cent.) are in layers. The liquors work down the yard. Longer time may be given to heavier goods. The tannage consists of 40 per cent. myrabs, 35 per cent. valonia or Natal bark, and 25 per cent. extract, chiefly chestnut, though some gambier may be added to the suspenders.

However tanned, strap butts are first dried out rough over poles. This assists in making the tannage permanent, on account of secondary changes discussed in Section III., p. [46]. They are next wet back for currying by soaking in water or sumach liquor for a few hours and piling to become soft and even. The first operation is "skiving," which is a light shaving on the flesh side, carried out by a sharp slicker with a turned edge. The butts are next scoured thoroughly by machine on both flesh and grain, and sumached in a vat for several hours at 100° F., after which they are slicked out and hung up in a cool shed to samm for stuffing. Hand stuffing is often still preferred, with tallow and cod oil. The butts are next set out, and it is important that this should be thoroughly done. Machines are now generally used, and the goods are often reset after further drying. After drying out completely they are given a light coating of tallow and laid away till wanted for cutting up into straps, which is now done by machinery.

A Continental method for making belting leather is to give 6 weeks in a suspender set (70°-24°) of twelve pits arranged on the press system, running two fresh liquors a week, and to give them two layers (24° and 28°) of 6 and 8 weeks. The material is chiefly pine bark, but some oak bark, valonia, myrabs and quebracho are also used. The goods are stuffed by "burning in," molten fat being poured on the flesh side.

REFERENCE.
Bennett, "Manufacture of Leather," pp 194, 295.

SECTION VII.—HARNESS LEATHER

When discussing the question of oak bark (Section III.), reasons were advanced for its decreased use and popularity. These were quickly appreciated in the sole leather trade, but the obsolescence of oak bark in the dressing-leather section was much more prolonged, partly because there was less pressing need to obtain good weight in the actual tanning, and partly because in some branches of dressing leather, such as belting and harness, a leather was required of great durability and toughness, for which qualities oak bark tannage had a deservedly high reputation. Hence harness leather manufacture affords a good illustration of the transition between the methods of the late nineteenth and those of the twentieth century. With the use of oak bark lingered the old methods of liming, bating and tanning in weak liquors for a long time with plenty of gambier. Hence in this section it will be necessary to observe a gradual transition of method, both in wet work and tanning. It should be pointed out that this transition has not been and is not going on in all factories at the same rate. Many factories remain in which the old methods are still preferred at some stages of the manufacture, and some remain in which many of the changes indicated below have not taken place at all. The leather trade has always been considered conservative in its methods, but it should be realized that much of the prejudice in favour of old methods is due to the public, and that after all tanners and curriers, like other business men, have to suit their customers. The march of industry is not like a regiment in line; it is rather more like nature, a survival of the most adaptable.

Hides for harness leather are limed in various ways, of which the following are types.

1. A rather mellow liming of 10-15 days (longer than for sole leather), in which nothing but lime is used, and a certain amount of old liquor used in making up the new limes. The liming was carried out by the one-pit system, but the goods and liquors were kept clean by a good soaking process. Hence the loss of hide substances was not very great; goods so treated were bated before tanning.

2. A shorter liming than the above by the three-pit system. This saved time (taking 9-10 days), saved hide substance, and ensured greater regularity of treatment. The limes were about as mellow, but a little sulphide (2-4 ozs. per hide) was used to assist the depilation, especially during the short-hair season. These goods were also bated.

3. A distinctly longer liming, 15-16 days, in mellower limes. This differed from Type 1 also in the respect that greater regularity was ensured by the three-pit system; a foot or two of old liquor was used in making up the new lime. More hide substance was lost than in either of the above processes, but this was deliberate, the object being to dispense with bating, which is always light for harness hides. Thus a longer and mellower but systematic liming was used as a substitute for shorter liming and bating. No sulphide was used in this process.

4. A short liming of 6-7 days, using up to 12 ozs. of sulphide per hide. The object here is to save time and hide substance. The three-pit system is preferred. Bating again becomes necessary, but the pigeon-dung bate is replaced by artificial bates, less objectionable, quicker, and more scientific in management.

5. A still shorter process of about five days, using still more sulphide (about 16-20 ozs. per hide), together with some calcium chloride to reduce harshness. In such a method there is a tendency to revert to the one-pit system, which involves rather less labour. The three-pit system shows to a great advantage in the longer processes of liming when the process is reduced to five days; there is little difference between the two, for a one-pit system is a two-liquor method. Hence again an artificial bate is used.

The various methods of liming, together with analogous variations in tannage, have resulted in great variety in bating. Sometimes up to three days' bating has been given at 70° F., but more often the goods are merely immersed overnight, and then delimed with boric acid, but with sulphide processes it is an advantage to use some of the commercial bates of the ammonium chloride type, and finish off with boric acid. Scudding is always more thorough than for sole or belting, the hides are rounded into long butts which include most of the shoulder "harness backs." The goods are sometimes bate shaved.

A few tannages will now be outlined, in order of historic type.

Type 1 may be taken to represent the so-called "high-class" process in which oak bark myrabs and valonia are the staple materials. A good deal of gambier is also used, and a little myrabs and chestnut extract are helpful in attaining the desired strength of liquor. The "backs" go first through suspenders (8°-30°), which takes up to three weeks, and then in to handlers (30°-40°) for four weeks, consisting of rounds of clear liquor. They next go through a duster round, in which they are put for a week with oak bark and myrabs into a liquor of 45°. Four layers are given (50°, 55°, 60° and 65°), in which the goods remain for two, three, four and five weeks respectively, oak bark being the chief dusting material. The tannage is thus for twenty weeks. Light backs receive less time in the layers (only 11 weeks). If the tanner is also the currier, the fourth layers are omitted. He then saves five weeks and gets the weight in the stuffing.

Type 2 is a tannage in which oak bark and valonia are replaced by myrabs, mimosa bark and chestnut extract. It is therefore considerably cheaper and probably no less durable. Expense is also curtailed in handling. The harness backs go through suspenders (16°-30°) in two weeks, handlers (30°-45°) in four weeks, and then receive four layers of the same strength as in Type 1, but only one, two, three and four weeks respectively. The last layer is omitted for light harness, and an extra layer of 75° is given if the tanner is not the currier also. Thus the usual tannage is 16-20 weeks, of which 10-14 weeks (63-73 per cent.) are in layers.

Type 3 is a tannage which may consist of myrabs (55 per cent.), valonia or mimosa bark 25 per cent., and extract (26 per cent.). The extract is chiefly quebracho, though some chestnut may be used. More valonia and less myrabs may be used if desired (and when possible), and myrabs extract will then replace quebracho and chestnut. The goods are coloured off in drums or paddles, and then pass through two sets of suspenders handlers (20°-55° and 55°-75°). They are handled up and down very frequently in the first set and rapidly pass into stronger liquors. The backs then receive three floaters at 80°, in each of which they remain one week. The tannage is completed by three layers: first, 85° for one week; second, 90° for one week; third, 95° for two weeks. The tannage is thus 11 weeks, of which 7 weeks involve little labour. If the tanner is not the currier, still stronger liquors may be used.

In all these tannages little or no acid is used for plumping, as the natural acids of the liquors are sufficient to ensure what is necessary in this direction for this class of leather. A little organic acid or even boric acid may be used in the earliest liquors for deliming purposes, when necessary. After tanning the goods are dried out and sorted in the rough state. Harness is a somewhat broad term, and there is scope for considerable variety in classification. The hides are sometimes not rounded until after tanning. The finished article may be any grade between heavy harness for artillery and leather for ordinary bridles.

In currying heavy black harness, the backs are soaked and sammed for shaving. Lighter goods may be machine shaved, but the heaviest are shaved lightly by hand over the beam or merely "skived" with the shaving slickers. The neck needs most attention, and it is often advisable to stone by machine and split. The scouring should be thorough, on flesh and grain. This is done by machine, and not only cleans the goods from bloom, dirt and superfluous tan, but also assists in setting out. Sumaching may be for several days, merely overnight or even only for a few hours, being stoned after wetting back to temper. Hand-stuffed goods get a coat of cod oil first, and during the drying are often well set out. Drum-stuffed goods are well set out by machine, and after some drying, stoned and reset by hand. It is now usual to buff the grain, i.e. remove the coarser parts by light shaving. This prevents cracking in the finished article. The goods are blacked with logwood, iron and ammonia, thinly dubbined again, again well set out and tallowed. Setting out, indeed, may be done at any convenient opportunity. The superfluous grease is removed by slicking, scraping, brushing with a stiff brush, and finally with a soft brush.

For brown harness the goods are more carefully selected, more thoroughly scoured and sumached, and bleached frequently with oxalic acid. They are hand stuffed, stained twice, and after the usual setting out, glassing and brushing, are finally rubbed with flannel.

For bridle leather the goods are carefully shaved but are not stuffed, being merely oiled with cod oil on flesh and grain. They are dried out before scouring, and then sized, set out, stained and resized. The goods are heavily glassed during the finishing.

REFERENCE.
Bennett, "Manufacture of Leather," pp. 195, 297.

Section VIII.—UPPER LEATHERS

The manufacture of leather for the uppers of boots and shoes embraces a bewildering variety of goods, suitable for anything between a baby's shoe and a man's shooting boot. Almost all degrees of lightness, softness, and waterproofness are in demand. A great variety of finish is also involved, determined by the ingenuity of the currier and the ever-changing fancy of the public. Even greater is the variety of methods by which all these results are obtained by methods which superficially seem quite different; the desired qualities being imparted in one case largely by the tannage and in another case almost entirely by the currying. Under such circumstances the selection of types becomes a problem.

The variety, moreover, commences from the earliest stages, the selection of the raw material. Upper leather may be made from light calfskins, heavy calfskins, kips (home and foreign), light dressing hides and heavy dressing hides, which last may replace any of the former after splitting to the required substance. In this section it will be necessary to take kips as typical of the rest, and to use it in a rather broad sense, including heavy calf and light dressing hides.

Speaking quite generally, kips for upper leather receive usually a long and mellow liming, a thorough bating and a sweet and very mellow tannage in weak liquors. In currying they are well scoured and set out, heavily stuffed and stained black, being sometimes finished on the grain and sometimes on the flesh. These outstanding features of upper-leather methods will be further illustrated by a brief account of the tanning of kips (light hide and heavy calf), and outlining the best known types of finish for butt, shoulder and belly.

The goods receive usually a long and mellow liming of 14-16 days, using only lime as a rule. In some factories lime liquors are used repeatedly for successive packs to an almost indefinite extent. Dissolved hide substance, ammonia, mud and dust, and bacteria accumulate for months and sometimes for years. It is obvious that in such liquors "putrefaction" is a more correct term than "liming" for the depilation. Such methods have been used even in recent years, but there has now been a tendency for some time to make the liming more methodical. Such old limes make a leather which is empty, loose, and dull grained, but the defects are minimized by the system of stuffing heavily and finishing the flesh, and hence the ancient lime remained with surprising tenacity. Even so late as 1903 we find that Procter with characteristic caution could write, "Probably no lime ought to be allowed to go for more than three months at the outside limit without at least a partial change of liquor." It is within the writer's experience to find an upper leather factory with limes which had never been emptied for over three years. In other factories, however, there has been a revulsion of feeling with regard to such processes, and it has been found advantageous to adopt a more scientific routine, in which the lime pits are cleaned out at regular intervals. There is little doubt that a mellow liming is desirable, but this can be secured by blending some old lime liquors with fresh lime liquor in a systematic manner. Similar considerations apply to the question of working the various packs through the limes. It is clear that with a mellow liming a one-pit system is quite possibly satisfactory, but the revulsion of feeling against a lack of method produced a method of liming more elaborate than usual, and it is now not uncommon to find kips limed in a "round" of 6-8 pits, the goods passing through each pit. They remain in one pit about two days, and are shifted forward. In the green or old limes the goods are handled up and down. The old limes are, of course, mellower than the new and exert the desired softening effect. The working is quite analogous to that of a round of handlers. Unhairing is sometimes assisted by the use of arsenic sulphide. E.I. kips need a thorough soaking before any liming; several days are usually needed. The old methods involving putrid soaks and stocks may be considered out of date, and it is usual to soften back in caustic soda or sulphide soaks with some assistance by drumming. A little sulphide is sometimes added to the older limes to continue the treatment.

The goods are next thoroughly bated and delimed. The hen or pigeon dung bate is still usual, and probably gives the best results, though closer approximations have been made of recent years on artificial lines. Some bating with solution of hide substance seems necessary for these goods. The lighter goods are often drenched also to complete the deliming, using 6 per cent. bran on the weight of pelt. The heavier goods are more often treated with boric acid after bating, which not only delimes completely and gives a soft relaxed felt, but also acts as antiseptic and stops the action of the bate, a matter of some importance (see Section II.). Lactic acid may substitute boric, in which case about 2 per cent. on the pelt weight of 50 per cent. acid may be required. It is important to avoid a strong solution and local excess, hence lactic acid must be added gradually so that the liquor is never stronger than 0.2 per cent. Drumming and paddling is an advantage in deliming.

The tannage is light in most cases, partly because some of the finished goods are sold by area, but partly also because even if sold by weight, the weight is obtained quicker and more easily by stuffing, which course is also often preferable to obtain the desired mellow feel, waterproofness and durability. Hence it is seldom that strong liquors are employed. The tannage is also mellow, on account of the softness and pliability required; no acids are consequently employed, and no material which is liable to yield sour liquors. Gambier is easily the first favourite amongst the tanning materials, whilst oak bark comes second. It should be observed, however, that a hypothetical tannage of equal weights of cube gambier and oak bark is in reality a tannage by four-fifths gambier and one-fifth oak bark, on account of the relatively greater strength of the former. This observation is so apposite with respect to some tannages that it is nearly correct to say that the tannage is gambier and the oak bark an excuse for having leaches through which the gambier liquors may be run occasionally to clear and to sharpen slightly. No serious theoretical objection to such a method is possible if the liquors are weak and the system of working the liquors is scientific and the process carefully regulated. Upper-leather tannages, however, have scarcely merited scientific praise. It is often a case, not of poor methods, but of no method at all. The same lack of system, principle, and regularity observed with regard to the limeyard has been equally obvious in the tanyard, when perhaps the need was even greater. Even a mellow tannage has varying degrees of mellowness possible to it; there still remains the question of the soluble non-tans. However, method in the upper-leather tanyard has often been conspicuously absent. There has been many a factory where any one tan liquor was as good as any other in the yard. In the writer's experience are two such cases: in one the liquors were all 25° Bkr., in the other they were all 0° Bkr. In such cases, handling the goods from pit to pit is somewhat futile, and handling forward from set to set still more so. Hence it is possible to find dressing leather tanned by putting it slowly through one round of handlers, adding a few buckets of gambier where it apparently is necessary. It is, from one point of view, surprising to see what serviceable and excellent-looking upper leather can be manufactured by such happy-go-lucky processes. It is, however, also possible to see how this may occur. Gambier is a stable tan, and no souring and little decomposition take place in gambier liquors. It is also extremely mild and non-astringent, and is always used in weak liquors. The hides, moreover, are completely delimed, and there is little danger of bad or uneven colour. Tanning under these conditions is at its easiest; it is almost more difficult to spoil the goods than make them right. Under such conditions tanning deteriorated rather than improved in method. When neglecting it made little difference to the finished leather, it was neglected.

This state of affairs, however, was embarrassing whenever a tanner wished to try any other tanning material. The expense of gambier and oak bark made valonia and mimosa bark into obviously desirable alternatives and substitutes. Methods which would tan with gambier, however, would not work with Natal bark or valonia, and many a tanner has had to revise his method of tanning from end to end. The use of myrabs also raised the problem of souring, and it has become evident that "working the liquors down the yard" is as desirable a method for dressing leather as after all other tannages. It will be clear from the above that types of upper-leather tannages are less typical than for other leathers, but nevertheless the more progressive manufacturers have for some years now been working on sounder lines, economically and scientifically. In such cases it is now usual to pass the goods through at least two sets of handlers, and through liquors of gradually increasing strength. Occasionally dusters or layers are given, especially for the heavier goods. The tannage is nearly always commenced now by paddling the goods in the oldest liquor. This paddling may be anything from half an hour up to twenty-four hours. It is sometimes desired to work up a "grain," and the old liquor is then often sharpened by the addition of fresh gambier or leach liquor.

The same tendency to save labour in handling is to be observed in upper leather tannages as in sole and other dressing leather factories. There is also a tendency to obtain rather more weight in tanning by using stronger liquors, and in the heavier goods to shorten somewhat the time taken. The following methods may be taken to illustrate modern processes, in order of evolution. They all last about seven weeks.

Type 1.—In this process the kips are first paddled in an old liquor (3°), and passed to the first handlers (3°-30°) for three weeks. After working through this set they pass through the second handlers (20°-30°), in which they are not handled quite so frequently. They are in this set also three weeks. Heavy goods may then receive a floater (30°) for another week.

Type 2.—In this process the goods are paddled, and then enter a large handler round (8°-30°), through which they pass in five weeks. The goods are handled frequently in the early stages. The tannage is completed by one layer of two weeks (30°). The layer is made by the ancient method of putting the goods and dust alternately into an empty pit, and then filling up with liquor from the best leach. Oak bark, valonia and myrabs are used as dust, though sumach and gambler have been used.

Type 3.—In this process an attempt is made to save handling and obtain more complete tannage. The goods are paddled for three to five hours in a rather sharp liquor of 10°, and are then handled well for a week in the first handlers (5°-20°). The goods then go through the second handlers (20°-45°) in six weeks, and heavy goods may then receive an extra floater (45°) for one week.

In type 1 the leaching material is two-thirds oak bark and one-third valonia; in type 2 it is half oak bark and half mimosa bark; in type 3 it is one-third oak bark, one-third valonia or Natal bark, and one-third myrabolans. In all cases the strongest handler is obtained from the leaches, and made up to the required with strong infusion of gambier. When the liquor has passed through the forward handlers, it is returned to the leaches to clear and sharpen, and then run to the green handlers. After passing through this round it is returned to the paddle, from which it passes to the drain. The rest of the paddle liquor may be from the forward handlers. It is often customary to obtain the best liquor from the second leach, and allow the best leach to stand for a few days. This allows the bloom to deposit in the leaches. The system secures the result desired, but the deposition of bloom involves a loss of tannin, which waste makes the system expensive.

Heavier dressing hides are tanned by methods similar to the above, but with floaters, dusters and occasionally layers added after they have passed through two sets of handlers. Thus they may have first handlers (8°-18°) two weeks; second handlers (40°-45°) for six weeks, making twelve weeks in all. Lighter goods may receive two rounds, being two weeks in each.

After tanning, the kips are rounded usually into butts, shoulders and bellies, to which different finishes are given. The currying may be illustrated by selecting types, but it must be borne in mind that there is much elasticity in this matter. Thus kips may be made into waxed butts, satin shoulders and lining bellies, but also may be cut down the back in "sides," both of which are finished limings.

Waxed kip butts are a type of many similar upper leathers (waxed shoe butts, waxed calf, waxed splits, etc.). The finish is on the flesh side. The kip butts are soaked carefully, and shaved by machine. They are then drummed in sumach for an hour or two, slicked out and sammed for stuffing. The sumaching is also the scouring unless the goods be too heavily bloomed. The samming is often done by machine. Drum stuffing follows, wool fat and stearin being staple greases, with varying amounts of degras and cod oil, and of tallow and cod oil. A little paraffin wax and resin are also used sometimes. The goods are well slicked out and dried. They may be now dubbined and laid away to mellow for whitening, which consists of a careful shaving of the flesh by a turned-edge slicker or by machine. The grain is stoned, set out and "starched," and the butts grained by boarding the flesh. In the waxing, one of two courses may be adopted. The butts may be blacked with lampblack and oil, "bottom sized" with glue, soap and logwood, and then "top sized" with glue, dubbin, beeswax and turpentine; or they may be given a "soap-blacking" of soap and logwood and lampblack, applied by machine, and sized once only.

Dressing hide butts may also be given a grain finish, such as the "memel butts" for heavy uppers. The butts are soaked, shaved or split, sumached in drum, and preferably thoroughly scoured on flesh and grain. They are then sammed and heavily stuffed in the drum. The grain is buffed and stained black with logwood, ammonia and iron solution (curriers' ink). The butts are then dried, set out, thinly sized and slowly dried.

When dry on the face they are printed or embossed by machine to give the characteristic memel pattern and dried out completely. They are then grained four ways. The grain is finished by a coating of linseed oil containing resin, and the flesh is whitened, French chalked and glassed.

Shoulders for "satin" receive a currying which strongly resembles the "waxed" finishes, but the smooth finish is on the grain side. The grain is buffed, and blacked, dubbined, set and reset, with intermediate drying, and is sized and finished by compositions similar to those used for waxed leathers. The flesh is whitened. Satin hide and satin calf are dressed similarly.

Shoulders may also be finished for "levant." After soaking, splitting, and shaving to substance, they are drum-sumached, machine sammed, and oiled up to dry. They are stained with logwood on the grain, and at once printed with the typical "levant grain," blacked and dried out. They are then softened by machine, seasoned with logwood and albumen, glazed, grained and oiled lightly with mineral oil. It will be observed that stuffing is omitted.

Bellies may be dressed for linings. After soaking and splitting to the required substance, they are bleached in a weak and warm solution of oxalic acid, and drum-sumached at 110° F. After slicking well out they are hand-stuffed on the grain with dubbin and water, or merely oiled, and hung up to samm. They are then set-out flesh and grain. If the grain be coarse, it is buffed and reset. After drying out the flesh is fluffed and the grain dusted with French chalk.

In this section may be conveniently discussed the manufacture of legging leather. Whilst in many respects a typical dressing leather there are some rather important differences from the average upper leather. Broadly speaking, the differences are that legging leather needs a smooth grain, greater firmness and more thorough tannage on account of the absence of stuffing.

The liming and bating are somewhat similar to dressing leather, though a shorter liming with sulphides and a milder bating would be in order. The tannage is mellow, but not so much as is usual for upper leather. Thus gambier is used, but more valonia and myrabs are employed, and the liquors may be strengthened with chestnut and quebracho extracts. The hides are rounded before tanning into long butts or backs, and the tannage is commenced in suspenders (18°-40°), which are kept acid by the addition of lactic or acetic acid, in order to obtain the required firmness; the goods are three weeks in these liquors. The backs next go through rounds of dusters (40°-50°), in which they are put down with oak bark and Natal bark. They are six weeks in this section, and then pass to the layers. Three layers are given, first 50° for one week; second 55° for two weeks; and third 60° for two weeks. The tannage thus takes fourteen weeks.

In finishing, the goods are soaked and split, and then scoured flesh and grain. They are heavily sumached, slicked out thoroughly, oiled up with linseed oil and dried out. They are then next damped back, stoned and flatted. After further wetting and tempering they are dressed with Irish moss and tallow on the flesh, and with gum tragacanth on the grain. They are glassed whilst drying out, and then stained twice and glassed again. They are again brushed, seasoned and glassed by machine.

REFERENCE.
Bennett, "Manufacture of Leather," pp. 197-201 and 301-308.

SECTION IX.—BAG LEATHER

Hides to be tanned for bag leather receive a treatment which is little different in fundamental principle from that of dressing hides for upper leather, except that the tannage is usually shorter. Hides for bags and portmanteaux represent a type of dressing leather in which the outstanding features are that the goods are split but not rounded. The splitting is done at all stages, according to the requirements of the tanner. Some tanners split "green," i.e. split the pelt itself. The advantage of this is that the fleshes may then be treated in quite a different way, e.g. pickled or given a much cheaper tannage. Other manufacturers split after tanning, the advantage being that there is much less material to handle. The general opinion, however, favours a middle course in which the hides are split after being in the tan liquors for a short time. The advantage of this course is that the hides are easiest to split under these conditions—a great consideration—being coloured through with tan, just a little plumped, but not hard. A smoother flesh is obtained together with more even substance. Here again, however, are differences; some tanners prefer to split after two days, others after two weeks in tan. Much depends upon the nature of the tan and the strength of the liquors.

For this class of work, flat, spready and evenly grown cowhides are obviously the most suitable material, and are invariably used. It is important, however, that the grain be good, and free from scratches and similar defects. The tannage must be sweet and mellow, i.e. contain no acid and little astringent tan. Hence myrabolans and gambier have always been the favourite tanning materials. A soft and mellow tannage is the more important, inasmuch as the leather is not heavily stuffed with grease in finishing. These types of method for tanning split hides will now be outlined, and the nature of the currying then indicated.

Type 1.—In this a long mellow liming of 15-16 days is given, much like that described for harness leather in Section III., p. [72], Type 3. Only lime is used, but the liquors are not allowed to get dirty. The three-pit system is much the best. The hides are trimmed at the rounding tables, and then bated in hen or pigeon dung for three days at 75°-85° F. The deliming is commenced by washing in tepid water before bating, and is completed by a bath of boric acid, using up to 30 lbs. acid per 100 hides as necessary. In this and other processes for split hides it is essential to obtain all the lime out, but to do no plumping with acid. Lactic acid may also be used, but it is not so convenient to hit the neutral point with it.

The tannage consists of oak bark and myrabs together with gambier. These may be partly replaced by Natal bark, valonia, and quebracho respectively. It is sometimes desired to have a smooth finish, but sometimes to work up a "grain." In the latter case the hides are first put through colouring pits containing fresh leach liquor. In these they are constantly handled for a few hours. A little experience indicates which leach liquor will serve the purpose. The hides then go through the "green handlers" (8°-20°) in two weeks. The liquor is the old forward handler liquor made up with gambier. The hides may be sammed and split up at this stage, but the heavier goods may be tanned further. These heavies and the grains of the split hides now go through the "forward handlers" (20°-40°) for four weeks, and the heaviest goods given two layers (40°) of two weeks each, and making ten in all.

Type 2.—In this a shorter liming of 8-9 days is given with the help of sulphide. No dung bate is used, but the goods are washed with water and bated with ammonium chloride and boric acid. The tannage is chiefly of myrabs, but some valonia or Natal bark may be used together with chestnut extract and some quebracho. Gambier is used in the early liquors. The goods are coloured off in drum or paddle and tanned in several sets of handlers, viz. green handlers (15°-35°) three or four days; second handlers (35°-60°) two weeks; forward handlers (60°-80°) 1½ weeks; and floaters (80°-90°) for three weeks. The tannage is thus 6½ weeks in all. The arrangement of pits is a matter of local convenience, and the number of sets of equal strength is determined by the number of hides being tanned. The hides are split green or after passing through the green set. After tanning they are oiled with cod oil and dried out.

Type 3 is illustrated by American methods. The goods are tacked on laths or racks with copper nails in order to ensure smooth grain. They are then suspended in tan liquors. The tannage is largely with gambier and in weak liquors, which also help to give smooth grain. The tendency is to employ handler rounds involving a rather large number of pits, and to work these on the press system. Handling is also saved by plumping the liquors instead of shifting the goods forward, and by rocking the suspenders instead of handling up and down. The hides are split after about a month, and the heavier grains laid away in hemlock liquors.

Type 4.—This is a rapid process throughout. The hides are limed in 6-7 days with the help of sulphide, and "bated" by washing in warm water and then in cold to which hydrochloric acid is gradually added, finishing off again in tepid water. The hides are now coloured off in paddles, put through a small handler round (11°-20°) for half a week, and then split. The grains are drum tanned in a mixture of chestnut and quebracho extract, over a period of about three days in which the liquor is strengthened gradually from 30° to 50°. The fleshes are drum tanned with the old grain liquors after strengthening with quebracho.

The split hide grains for bag work, after tanning, are drummed in sumach, rinsed, drained, and oiled up to dry out, with some setting out. After wetting back they are shaved if necessary, hand scoured, and heavily sumached again to get a light even colour. The goods are slicked out, oiled up to samm, reset and dried out. They are next stained, sammed, printed by machine, dubbined or tallowed, "grained" (see Part II., Section I., p. [97]), brushed and rubbed with flannel.

REFERENCE.
Bennett, "Manufacture of Leather," pp. 202, 308.

SECTION X.—PICKING BAND BUTTS

It is the paradox of vegetable tannage that the less the pelt is tanned the stronger is the leather produced. The manufacture of butts for picking bands affords a good illustration. What is required is a leather of maximum toughness, pliability and durability. Any factor reducing the tensile strength of the leather is fatal. Hence, compared with most other tannages, picking band butts are under-tanned. To ensure the desired softness and pliability, moreover, it is necessary to have a mellow liming, rather heavy bating, and a soft mellow tannage in sweet and weak liquors. The required durability and the necessity for weak liquors both point to oak bark as the most suitable tanning material, assisted by some gambier in the early stages.

A good quality hide is chosen, and given a long and mellow liming of about 15-16 days. The one-pit system may be used, and the hides are put into an old lime for about five days with frequent handling and then placed in a new lime which is made up in a pit containing about a foot depth of the old liquor. After about twelve days another ⅓ cwt. of lime may be added.

After unhairing and fleshing the goods are bated in pigeon dung for four days at a temperature of about 78° F., handling twice on the first and last days. The bating is stopped and the deliming completed by paddling with boric acid (15 lbs. per 100 butts).

The tannage is commenced by paddling in a spent handler liquor (4°) to which a little gambier has been added. The butts then go through the first handlers (5°-15°), which are rounds of ten pits in which the goods are handled every day in the first week, and alternate days in the second week, and are shifted forward twice a week in the next pit. The goods are therefore in this set for five weeks. Gambier is added to these liquors as needed. The butts next pass to duster rounds of four pits, in which they are dusted down in a liquor of 20° for four weeks with 1-2 cwt. of oak bark. The liquor is obtained from the leaches, and afterwards run alternately to the leaches and to the first handlers. As many as six layers are now given of 20°-25° strength, in which the butts are dusted down with 2-3 cwt. oak bark for three weeks. The layer liquors are received from and returned to the leaches, which are made from the "fishings" from the layers. The tannage lasts, therefore, 27 weeks, of which 18 weeks (two-thirds) are in layers.

Shorter tannages are now often given, using stronger liquors, much as in ordinary dressing leather.

The tanned butts are rough dried, and then wet in for shaving. They are thoroughly scoured, flesh and grain. They are next drummed for three-quarters of an hour in sumach, struck out and hung up to samm. Hand stuffing is best, to avoid any tendering owing to high temperature, but drum stuffing is also used. After setting out and stoning on the grain they are stuffed with warm cod oil and laid away in grease for several weeks, re-oiling occasionally. They may be stained before stuffing.

REFERENCE.
Bennett, "Manufacture of Leather," pp. 203, 310.

PART II.—SKINS FOR LIGHT LEATHERS

SECTION I.—PRINCIPLES AND GENERAL METHODS OF LIGHT LEATHER MANUFACTURE

The term "skin," like the term "hide," in its widest sense applies to the natural covering for the body of any animal, but is generally used with a narrower meaning in which it applies only to the covering of the smaller animals. Thus we speak of sheep skins, goat skins, seal skins, pig skins, deer skins, and porpoise skins. It is in this sense that it will be used in this volume. The treatment of such skins to fit them for useful purposes comprises the light leather trade. Whilst this branch of the leather industry is certainly utilitarian, the artistic element is a great deal more prominent in it than in the heavy leather branch. Thus the light leathers are often dyed and artistically finished, and their final purposes (such as fancy goods, upholstery, bookbinding, slippers, etc.) have rather more of the element of luxury than of essential utility. The total weight and value of the skins prepared, and of the materials used in their preparation, are naturally considerably smaller than those of the heavy leather trade. In the latter, moreover, one has to consider the purpose in view from the very commencement of manufacture and vary the process accordingly, but in light leather manufacture one aims rather, in the factory, at a type of leather such as morocco leather, and only after manufacture is it fitted to such purposes as may be particularly suited to the actual result. These results depend very largely upon the "grain pattern" which is natural to the skin of any one species of animals. Hence in Part II. of this volume it has been found most convenient to deal with the different classes of skins in different sections. Just as the hides of ox and heifer were much the most numerous and important of hides, so also naturally are sheepskins the most prominent section of the raw material of the light leather trade. This is the more true because the skin is valued for its wool as well as for its pelt; indeed, the wool is often considered of primary importance, and receives first consideration in fellmongering. Unfortunately for the light leather trade, sheepskins, though most numerous, do not give the best class of light leather, the quality being easily surpassed in strength, beauty and durability by the leather from goat or seal skins.

In the wet work for the preparation of skins for tannage much the same general principles and methods are embodied as in the case of hides, but with appropriate modifications. As soft leathers are chiefly wanted, a mellow liming is quite the usual requirement for all skins. It is also usual to have a long liming, for some skins (like those of sheep and seal) have much natural fat which needs the saponifying influence of lime and lipolytic action of the enzymes of the lime liquors; whilst other skins (like those of goat and calf) are very close textured and need the plumping action of the lime and a certain solution of interfibrillar substance. In consequence of the long mellow liming, sulphides are not usually necessary, and indeed sodium sulphide is not usually desirable, on account of its tendency to make the grain harsh. It is used, however, for unwoolling sheepskins, in such a manner that the grain is not touched. Similarly caustic soda is seldom required, and the yield of pelt by weight is usually a small consideration. Systems of liming show some variety. The one-pit system is very common, and is less objectionable for a long mellow liming, but rounds of several pits are also used, and in some cases even more than one round. This is obviously conducive to regularity of treatment, and as the work involved in shifting the goods is much less laborious than in the case of heavy ox hides, it would seem a preferable alternative. The depilation of sheepskins involves very special methods of treatment (sweating and painting) on account of the importance and value of the wool, the quality and value of which would be impaired by putting the skins through ordinary lime liquors. The pelts, however, are limed after unwoolling.

In deliming light leathers the process of puering is widely used (see p. [25]). This consists in immersing the skins after depilation in a warm fermenting infusion of dog-dung. In principle this disgusting process presents a close analogy with bating, and indeed the two terms are both used somewhat loosely, but there are nevertheless several points in which the two processes are radically different. The dog-dung puer is a process carried out at a higher temperature than the fowl-dung bate; it is also a much quicker process, and the infusion employed is generally more concentrated. Whilst the fowl-dung bate is always slightly alkaline to phenolphthalein the dog-dung puer is always acid to this indicator, and the course of the puering may be conveniently followed by testing the pelts with it. The mechanism of the two processes is also probably somewhat different. The mechanism of the dog-dung puer has been largely made clear by the researches of Wood and others, and been found due partly to a deliming action by the amine salts of weak organic acids and partly to the action of enzymes from a bacillus of the coli class, which received the name of B. erodiens, and which effects a solvent action on the interfibrillar substance. As we have noted (Part I., Section II., p. [24]), the fowl-dung bate involves two fermentations, in each of which (ærobic and anærobic) several species of bacteria are probably active. Wood found the bacteria of the bate to be chiefly cocci, and ascribed part of the difference in mechanism by the nature of the media, which in the bate includes also the urinary products. In the dog-dung puer, also, a lipolytic action is probably an essential part of the total effect. The puer gives a much more complete deliming and a much softer and more relaxed pelt than the bate, it is therefore particularly suited to the needs of light leather manufacture. The puering action has been imitated fairly successfully by artificial methods. "Erodin" (Wood, Popp and Becker) involves the use of B. erodiens and a suitable culture medium including organic deliming salts: "Oropon," "Pancreol" and others involve the use of ammonium chloride and trypsin, together with some inert matter.

Light-leather goods are usually drenched after puering. They are also often split green after the wet work. Sheepskins thus yield "skivers" (the grain split), whilst the flesh split is often given an oil tannage (see Part IV., Section III.). The greasy nature of sheep and seal skins necessitates the processes of "degreasing." In the case of sealskins this is done largely before liming, but with sheepskins either after being struck through with tan, or after tannage is complete. Sheepskins are often preserved in the pelt by pickling with sulphuric acid and salt, which process forms a temporary leather. The fibres of the pelt are dried in a separate condition, but the adsorption is easily reversible and the pelts may be "depickled" by weak alkalies and afterwards given an ordinary vegetable tannage.

In the vegetable tannage of skins for light leathers, the same theoretical considerations have force as in the heavy-leather section, but the former has its own rather special requirements and aims. Generally speaking, a softer and more flexible leather is required, but these qualities must not be imparted by stuffing with grease as in the currying of dressing leather, because a bright and grease-free result is usually required. Hence it is important that a sweet mellow tannage be given. The durability of the leather is also a primary consideration for goods intended for bookbinding, upholstery, etc., and the tannage must be arranged to impart this quality and avoid anything tending to cause the perishing of the fibre. Thus oak bark is a popular tanning material, and sulphuric acid very definitely avoided. The tannage must be fast, and take the dyestuffs well, and for the production of light shades of colour in dyeing must be a light-coloured tannage. All these qualities are imparted by sumach, which also fits in excellently with the other general requirements, such as softness, brightness and durability. Hence sumach is the principal light-leather tanning material, but the tendency is to employ other materials—oak bark, myrabs, and chestnut extract—to do much of the intermediate tanning, so that the expensive and useful sumach may be used for setting the colour and grain at the commencement, and for brightening, bleaching and mordanting the leather at the end of the tanning process. Weight is generally no consideration, but area is often a definite aim, partly because some goods are sold by area and partly because the striking out, setting out and similar operations improve the quality of the leather by giving evenness of finish. Leather well struck out, moreover, is less liable to go out of shape. As the grain pattern is so important in the finished leather, appropriate care must be taken during tannage. If a smooth or a fine grain finish is wanted, for example, the goods must not be allowed to get wrinkled, creased, doubled or unduly bent to and fro during the tanning. For such goods, suspension, careful handling and even the "bag tannage" may be desirable, whilst for coarser and larger grains paddles or drums may be more extensively used.

Amongst the finishing processes dyeing holds an important position. The nature of the process has many points of similarity with that of tanning. The great specific surface of pelt is probably more enhanced than otherwise during tannage, at any rate with light leathers, owing to the isolation of fibres, and consequently leather is as liable as pelt to exhibit adsorption. The dyestuffs, on the other hand, are substances very easily adsorbed. Some (like eosin and methylene blue) are crystalloids, some (like fuchsin and methyl violet) are semi-colloids, whilst others (like Congo red and night blue) are undoubted colloids forming sols (usually emulsoid) with water as dispersion medium. The crystalloids and semi-colloids may also be obtained in colloidal solution, sometimes being so changed on the mere addition of salts to the solution. In addition, the pelt has been mordanted with tannin. If, however, leather has been kept long in the rough-tanned or "crust" state, this may not be so effective, owing probably to the secondary changes in tanning (Part I., Section III., p. [46]), but such leathers are usually "retanned" or prepared for dyeing by sumaching (which process also incidentally bleaches). The tannin mordant assists materially in the fixation of the dyes. In the case of basic dyestuffs, lakes also are formed, i.e. there is a mutual precipitation of oppositely charged colloids (+dye,-tannin). The dyeing of leather is thus a case of colloid reactions even more complicated than that of tanning.

Another finishing operation typical of the light leathers is "graining" or "boarding." In this the skins after dyeing and drying are worked by a board which is covered by cork, rubber, perforated tin or other material, and so grips or "bites" the leather. The object of "graining" is to work up the grain pattern by pushing or pulling a fold on the skin with the board. The nature of the grain varies with the thickness and the hardness of the skin, with the amount of pressure applied, with the nature of the board, with the direction of the boarding and with the total number of directions boarded. There is thus infinite scope for variety of finish, and hence arise bold grain, fine grain, hard grain, straight grain, cross grain, long grain, etc. The operation requires considerable skill and experience. In the case of skins with little natural grain (such as sheepskin) embossing and printing machines impress the desired pattern.

In seasoning, a dressing is applied containing essentially albumins and emulsified fats, e.g. egg albumin and milk. Colouring matters are also often added to intensify or modify the shade. After seasoning the goods are usually "glazed" by a machine which rubs the seasoned grain with considerable pressure, by a glass or hardwood tool, and so produces a high gloss, for which the seasoning is very largely a preparation. Light leathers are very lightly oiled with linseed or mineral oil.

REFERENCES.
Procter, "Principles of Leather Manufacture," pp. 220, 394.
Bennett, "Manufacture of Leather," pp. 36-41, 55, 85-90, 92-112, 312, 332.
Wood, "Puering, Bating and Drenching of Skins."
Lamb, "Leather Dyeing and Finishing."

SECTION II.—GOATSKINS

Goatskins are amongst the most valued raw material for the manufacture of light leather. The leather obtained from them is of the very finest quality in respect to durability and adaptability to the principal purposes in view. The texture of the fibres in goatskin is exceedingly compact and very strong, whilst the grain exhibits naturally a characteristic pattern which renders it most suitable for a grained finish. Hence for purposes like upholstery, bookbinding, slippers, it forms almost an ideal material. The tanning and finishing of goatskins into "morocco leather" may indeed be taken as a quite typical example of light leather manufacture.

The skins are obtained from all quarters of the globe where goats exist, and the excellent quality of the leather produced has created a demand which is greater than the supply. This is due not only to the demand for morocco leather, but also to the popularity of the goatskin chrome upper leathers such as "glacé kid" (see Part III., Section IV.). The large American trade in the latter has produced the saying that wherever there is a goat there is an American waiting for it to die! The European supply of skins is somewhat limited. They are obtained from the Balkans and Bavaria, in which case they are small, fine-grained and plump skins. The Swiss goatskins are larger, and have also a fine grain; they are well grown and well flayed. Scandinavian skins have a poor reputation, being very flat. The African supply is important; Abyssinian skins are exceedingly compact and tough, and are very suitable for "bold grain" finishes. The Cape skins are particularly large, strong and thick, but their quality is often impaired by the cure, the skins being flint-dry, and, like hides so cured, prone to unsoundness. Large quantities of goatskins also come from the East. Many of these are imported in a tanned state (E.I. Goat). These skins are tanned with turwar bark, which contains a catechol tannin. They are also heavily oiled with sesame oil, and need degreasing. The tannage is also stripped as far as practicable, and the skins retanned with sumach before finishing. They make good morocco leathers for many purposes, but the primary catechol tannage renders them ineligible for finishing under the specifications of the Committee of the Society of Arts. The skins have a Persian or Indian origin. India also supplies a large number of raw dried goatskins which are small and of variable quality. These, however, are more extensively used for chrome uppers.

Goatskins are imported in either a salted or a dried condition. The great aim of soaking is to obtain the skins in a thoroughly soft condition. Hence the soaking is prolonged, and some mechanical treatment is desirable in addition to various steepings in water. To be certain of softness it is desirable to avoid the use of alkalies in the soak waters, for although they cause hydration of the fibres by imbibition, they also have a plumping effect which is not wanted at this stage. Salted goatskins are first immersed in water and left until the following day. This dissolves the salt. They are then stretched and given a fresh soak liquor of water only to soften further, clean, and remove the rest of the salt. This second water lasts only a few hours, and the goods are then drummed well in running water. This not only cleans quickly, but has an excellent softening effect. They are again returned to a soak liquor, then softened mechanically by working them over a beam. This treatment must be repeated, drumming again if necessary, until the skins are perfectly relaxed and thoroughly softened. If the treatment be very prolonged it becomes advisable to use antiseptics in the soak waters after the first drumming. Solubilized (or emulsified) cresols of the "Jeyes fluid" type are the most suitable antiseptics, but too much must not be used or the sterilization affects the liming, in which bacterial action is needed. Flint-dry skins are left longer in the first soak, which should be of water only. They are then given a fresh soak liquor containing 0.2 per cent. of sodium sulphide. Sometimes a 1.0 per cent. solution of borax is used instead; it softens excellently, is antiseptic, and avoids the plumping effect, but is rather expensive. The goods are next drummed well, and resoaked and worked as for salted skins. In either case the soaking takes about a week.

The liming of goatskins presents some points of contrast with the methods used for other skins. These differences are due to the exceedingly tight and compact nature of the skin fibres. This compactness of texture makes it quite necessary to dissolve the interfibrillar substance to a greater extent than usual, and also to plump the fibres and split them into the constituent fibrils. These effects are essential to obtain a rapid and complete tannage and a soft leather. Too much bacterial action should be avoided, however, or the brightness and soundness of the grain may be impaired, which would be a fatal defect in such a leather. Hence the liming is long rather than mellow, and sharp limes rather similar to those required for sole leather are often used. Another result of the tight texture of goatskin is that depilation is not easily effected. This feature is rather intensified by the deepness of the hair-root. Hence it is usual to employ sulphides to assist the depilation. In one method two rounds of five pits are used. The skins are given about two days in each pit, so that the liming lasts approximately three weeks. In the first round, which consists of rather mellow limes, arsenic sulphide is used to assist depilation. Up to 6 per cent. on the weight of lime is added during slaking. This is a comparatively large amount of arsenic sulphide, and the depilation is considerably hastened; the skins indeed are unhaired after passing through this round, i.e. after about 10 days' liming. In the next round the object is plumping, and caustic soda (or carbonate) is added to the lime liquors in quantities comparable to those suggested for sole leather (Part I., Section V., pp. [55, 56]). In this round the goods stay also for about 10 days. An alternative to the above process is to hasten the earlier part of the liming by employing sodium sulphide instead of realgar. More sulphydrate may be obtained in solution in this way, and the unhairing may be in about half the time. The sulphide of soda also commences the plumping action which follows in the next round, but this alternative has the disadvantage that the skins are unhaired whilst the pelt is swollen with sulphide, which renders the grain both harsh and tender and consequently more liable to damage by the unhairer's knife.

Deliming is by puering and drenching, and is often associated with a further mechanical working of the goods. The skins are inserted into a puer liquor at 85° F. and thoroughly pulled down. The caustic alkalies should be completely neutralized. A slight cut into a thick part at the butt end should develop no pink colour with phenolphthalein. The skins should be thoroughly relaxed, and the swelling so much eliminated that they are quite soft, weak and "fallen." The resilience and elasticity of the plumped skins should have quite disappeared, and the impressions of hand or thumb should be readily retained by the pelt. The grain should appear white and possess a soft and silky feel. In this condition they are again worked over the beam to soften further if possible. They are then rinsed and again worked over the beam. Drenching follows with 10 per cent. of bran on the pelt weight, the operation commencing at 85° to 95° F., and lasting till next morning. The skins are next scudded thoroughly to remove all dirt, but carefully so as not to damage the grain.

In tanning, sumach and oak bark are the staple materials. Sumach gives a much lighter colour, and hence it is used alone for goods that are to be dyed the lighter shades, but oak bark is a "faster" tannage and more preferable for dyeing in those cases where blacks and very dark shades are wanted. For ordinary purposes a blend is usually employed. A feature of oak bark, also, is that it tends to make a firmer leather, so that the proportion used must be adjusted with this fact in mind as well as the question of colour. For firmer moroccos the skins may pass through a handler round of oak-bark liquors (10°-20°) in which a certain amount of sumach is added to the liquors. The sumach is leached and assists both in tanning and bleaching as the liquor works through the round. The old liquor is run to a paddle, and the tannage is commenced by paddling the drenched skins in this liquor. It is advantageous both for the tannage and for the efficient "spending" of the sumach if this liquor be slightly warmed. In the early pit liquors the goods are very frequently handled. There is, however, the usual tendency of the times to save labour in this direction, and hence it is common to have several paddles with liquors of gradually increasing strength, followed by a shorter round of handlers in which the handling is more infrequent. Instead of paddles latticed drums may be inserted into pits containing liquors. These, however, are not quite so convenient. In some tanneries, especially where sumach only is employed, the tannage is in paddles throughout. A new liquor is made up with fresh sumach and is used repeatedly until exhausted. A three-paddle system sometimes obtains, in which case the operation closely resembles the three-pit system of liming (Part I., Section II., p. [19]), and the skins pass through an "old" liquor, a "medium" liquor and a "fresh" liquor. The goods need not be paddled the whole day through, and indeed in the later stages this is undesirable. The packs remain several days in each liquor and take up to 14 days to tan. Two to three bags of sumach are needed for about 20 dozen goatskins. This method of tanning is efficient and convenient for bold-grain finishes, on account of the constant tumbling and bending of the skins which tends to work up a grain. For very soft leathers and fine-grain finishes, however, the "bag-tannage" or "bottle tannage" is favoured. In this method the pelt is stitched up by machine to form a bag, grain outwards, leaving a "neck" in the hind shank. The bag is nearly filled with a fairly strong infusion of sumach, inflated with air and tied up at the neck. The bags are then placed into a vat of warm sumach liquor, in which they just float. The bags are pushed down and the liquor stirred up, so that the goods are in constant motion. After a few hours they are piled on a rack, and the tan liquor of the interior is caused to diffuse through the skins by the pressure due to the weight of the pile. The bags are refilled with fresh and stronger sumach liquor and the process is repeated. The skins are thus lightly but effectively tanned in about 24 hours, and the leather has very fine grain and soft feel. However tanned the skins are dried out after tanning, and sorted in the "crust" according to size and colour. The larger skins are preferred for upholstery and the smaller for fancy goods and bookbinding.

To illustrate the course of finishing operations, the case of hard-grain morocco for bookbinding may be given as typical. The goods are wet back with warm water and drummed for 1-2 hours in warm sumac to prepare for dyeing. They are then struck out by machine, sammed and shaved. Dyeing follows, with acid colours, in a drum. The goods are run first in a little water and the dyestuff added very gradually through a hollow axle. The acid required (preferably formic) is added later to develop the full shade. Warm solutions are used, and the dye bath is practically exhausted. The goods are next placed in cold water to wash off superfluous liquor and free the skins from acid. They are then horsed to drain, struck out and hung up to samm. They are seasoned with milk and water and piled to temper. They are "tooth rolled" in the glazing machine two ways: right-hand shank to left fore shank and vice versâ, and piled again. After wetting back again they are "wet grained" by hand with a cork board in four directions: belly to belly, shank to shank, and across as before, and finally from neck to butt. They are immediately hung up in a warm shed to dry, and to fix the grain. They are then softened by "breaking down" with a rubber board, top seasoned, piled to temper and dry, brushed lightly, piled again, brushed more heavily, and dried out. They are finally softened by graining in three directions: shank to shank and across, and neck to butt. They are then brushed again. If these skins are wanted for upholstery they are shaved after dyeing, and nailed on boards to samm. They are also dried out in a cooler shed or "stove," to ensure softness.

REFERENCE.
Bennett, "Manufacture of Leather," pp. 39, 55, 89, 111, 204, 344, 396.

SECTION III.—SEALSKINS