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THE PRINCIPLES
OF
LEATHER MANUFACTURE

Frontispiece.

PLATE I.

Section of Calf-skin. (For key, see [Fig. 9].)

THE PRINCIPLES
OF
Leather Manufacture

BY
H. R. PROCTER, F.I.C. F.C.S.

PROFESSOR OF LEATHER INDUSTRIES AT THE YORKSHIRE COLLEGE, LEEDS;
PAST PRESIDENT OF THE INTERNATIONAL ASSOCIATION
OF LEATHER TRADES CHEMISTS

London:
E. & F. N. SPON, Limited, 125 STRAND

New York:
SPON & CHAMBERLAIN, 123 LIBERTY STREET

1903

Dedicated to
PROFESSOR F. L. KNAPP
GEHEIMEN HOFRATH, DR. PHIL. AND DR. ING.
THE PIONEER OF SCIENTIFIC RESEARCH
IN LEATHER MANUFACTURE

PREFACE.

The origin of the present work was an attempt to prepare a second edition of the little Text-Book of Tanning which the Author published in 1885, and which has been long out of print. Though persevered in for years, the work was never brought to completion, partly owing to the constant pressure of other duties, but still more to the rapid advances which have been made in our knowledge of the subject, and in the scientific thought which has been devoted to it. For his share in the initiation of this work, much credit is due to Wilhelm Eitner, Director of the Imperial Royal Research Institute for Leather Industries in Vienna, but the advance he began has been energetically carried forward not only in Vienna, but in the Tanning Schools and Research Institutes of Freiberg, Leeds, London, Liège, Copenhagen, Berlin and elsewhere, and to a less extent in private laboratories.

Under the pressure of this rapid growth, as it was impossible to complete the work as a whole, the Author published an instalment dealing with the purely chemical side of the subject in 1898, under the title of the ‘Leather Industries Laboratory Book’; which has been translated into German, French and Italian, and of which the English edition is rapidly approaching exhaustion.

The present work, which should by right have preceded the Laboratory Book (and which frequently refers to it as “L.I.L.B.”), attempts to deal with the general scientific principles of the industry, without describing in detail its practical methods (though incidentally many practical points are discussed). To complete the subject, a third volume ought to be written, giving working details of the various methods of manufacture; but apart from the difficulty of the subject, and the weariness of “making many books,” the methods of trade are so fluctuating, and dependent on temporary conditions that they have not the same permanent value as the record of scientific advance.

As the present volume is intended to appeal both to the chemist and to the practical tanner, it must to a certain extent fail in both, since many matters are included which are already familiar to the former, and it is to be feared, some, which may prove difficult to the latter. For these and other imperfections the Author claims the indulgence of his Readers.

The Author must here acknowledge his indebtedness to Dr. Tom Guthrie and to Mr. A. B. Searle for assistance in writing several of the chapters; to Dr. A. Turnbull and Mr. F. A. Blockey for much help in reading proofs and preparing the MS. for the press; and to the many gentlemen who have furnished or allowed him to use their blocks and drawings in illustration.

The Yorkshire College,
Leeds.

CONTENTS.

CHAPTER I.

INTRODUCTORY AND HISTORICAL.

Primitive methods of leather manufacture — Use of leather by the ancients — Progress of leather manufacture in England — Methods of production of leather — Vegetable tannages — Combination tannages — Use of aluminium, iron and chromium — Oil- and fat-leathers — Difficulties of scientific treatment PAGE [1]

CHAPTER II.

INTRODUCTORY SKETCH OF LEATHER MANUFACTURE.

The object of tanning — Washing and soaking — Removal of hair by liming — Unhairing by putrefaction — Unhairing and fleshing — Deliming — Bating, puering and drenching — The vegetable tanning process — Currying — Alum, chrome and chamois leathers PAGE [7]

CHAPTER III.

THE LIVING CELL.

The structure of cells — White blood-corpuscles — The yeast-cell — Epidermis cells — The building up of plants PAGE [10]

CHAPTER IV.

PUTREFACTION AND FERMENTATION.

The nature of ferments — Organised and unorganised ferments — Classification of organised ferments — General properties of ferments — The alcoholic fermentation — The action of enzymes or unorganised ferments — The destruction of ferments by heat and antiseptics — The products of fermentation — The fermentations of the tannery — Fermentation in bating and puering — Fermentation in the tanning liquors — Moulds and mildews — Control of fermentation PAGE [15]

CHAPTER V.

ANTISEPTICS AND DISINFECTANTS.

Distinction of antiseptics and disinfectants — Lime — Sulphur dioxide — Manufacture of sulphuric acid — Bisulphites and metabisulphites — Boric acid and borates — Mercuric chloride — Mercuric iodide — Copper sulphate — Zinc salts — Arsenic — Fluorides — Phenol — Use of carbolic acid — Eudermin — Creasote — Creolin — Salicylic acid — Benzoic acid — Cresotinic acid — Anticalcium — “C.T.” bate — Naphthalene sulphonic acid — Naphthols — Hydronaphthol — Oxynaphthoic acid — Carbon disulphide — Formaldehyde — Triformol — Camphor and essential oils PAGE [21]

CHAPTER VI.

THE ORIGIN AND CURING OF HIDES AND SKINS.

Marking of hides — Fellmongering of sheep-skins — The use of salt — Salting of packer hides — Brining — Dry-salting — Indian plaster cures — Analysis of salt-earths — Salt- and iron-stains — Drying of hides and skins — Damage by insects — The warble-fly — Damage by branding — Cockle PAGE [33]

CHAPTER VII.

STRUCTURE AND GROWTH OF SKIN.

Similarity of Mammalian skins — Development of skin — Structure of calf-skin — The epidermis — The structure of hair — The sebaceous glands — The development of hair — The hair-sheath — The hair-muscle — The hyaline layer — The corium — Connective tissue — Fat cells — Striped muscle — Elastic fibres — The unhairing process — The sweating process PAGE [46]

CHAPTER VIII.

THE CHEMICAL CONSTITUENTS OF SKIN.

The keratin tissues — Production of gelatine from connective tissue — Analyses of hide and gelatine — Constitution of gelatine — Analysis and Reactions of gelatine — Decomposition of gelatine — Reactions of gelatine — Chondrin — Coriin — Hide-albumin — “Acid” and “alkali” albumins — Egg-albumin — Vitellin — Casein — Keratins — Elastic fibres — Analytical methods — Kjeldahl process PAGE [56]

CHAPTER IX.

THE PHYSICAL CHEMISTRY OF THE HIDE-FIBRE.

Causes of swelling and contraction — The essentials of the tanning process — The constitution of matter — The nature of molecules — Vapour-pressure — Surface-tension — Solution-pressures — Jellies — Crystals — Osmotic pressure — Electrolytic dissociation — Electrolysis — Reactions of ions — Absorption of water by gelatine — Dehydration by alcohol — Action of acids, alkalies and salts on gelatinous fibre — Physical explanation of swelling — Action of acids on gelatine — Action of alkalies on gelatine — Effect of salt — The pickling process PAGE [73]

CHAPTER X.

WATER AS USED IN THE TANNERY.

Impurities of natural water — Hardness — Soap test — Temporary hardness — Clark’s softening process — Archbutt and Deeley’s softening apparatus — Other appliances — Effect of temporary hardness in tanning and dyeing — Permanent hardness — Boiler scale — Mud — Iron — Alumina — Soda — Copper, lead, etc. — Sulphuric acid — Nitrates and Nitrites — Chlorine — Carbonic acid — Silicic acid — Effect of hardness on plumping — Peaty waters PAGE [93]

CHAPTER XI.

SOAKING AND SOFTENING OF HIDES AND SKINS.

Washing of fresh hides — Danger of putrefaction — Soaking of salted hides and skins — Soaking and softening of dry and dry-salted hides — American wash-wheel — Chemical methods — Difficulty of softening hides dried at high temperature PAGE [108]

CHAPTER XII.

DEPILATION.

Methods of depilation — Sweating process — Liming — Sources of lime — Quicklime — Slaking of lime — Solubility of lime in water — Analysis of lime — “Available” lime — Action of lime on hide — Liming in pits — Suspension limes — Effect of warming limes — Quantity of lime required — The Buffalo method — Action of old limes — Solution of hide substance by limes — Sodium and potassium hydrates — Payne and Pullman’s process — Alkaline carbonates — Alkaline sulphides — Sodium sulphide — Calcium Sulphydrate — Gas-lime — Tank-waste — Lufkin’s liming preparation — Barium sulphydrate — Realgar, or red sulphide of arsenic — “Inoffensive” unhairing solution — Earp’s patent — Unhairing on the beam — Unhairing machines — Vaughn machine — Leidgen machine — Unhairing in stocks and wash-wheel — Jones machine — Fleshing — Vaughn fleshing machine — Rounding PAGE [119]

CHAPTER XIII.

DELIMING, BATING, PUERING AND DRENCHING.

Methods of removing lime and reducing swelling — Use of acids — Lactic, acetic and formic acids — Boral — Sodium bisulphate — Boric (boracic) acid — Borax — “Pulling down” process — Use of ammonium chloride and sulphate — Pickling solutions — Drenching with lactic acid — Metabisulphite of soda — Washing out lime, French process — Nesbitt’s process — Use of carbonic acid — Carbolic acid — Cresotinic acid — Oxynaphthoic acid — “Anticalcium” — “Acrilene bating acid” — “C.T. Bate” — Use of sulphides and polysulphides — Babool pods — Bran-drenching — Bating and puering — Causes of bating effect — Pepsin — Trypsin, or Pancreatin — Wood’s researches — Erodin — Palmer’s experiments — Other artificial bates — Relative effect of dog- and pigeon-dung bates — Analysis of dungs — “Scudding,” or “fine hairing” — Preservation and use of dung PAGE [152]

CHAPTER XIV.

ALUM TANNAGE, OR TAWING.

Nature of leather — Mineral tanning substances — Salts of aluminium — Alums — Aluminium sulphate — Effect of salt in tawing — Basic alumina solutions — Tawing of skins for rugs — Calf-kid manufacture — Glove-kid — Green leather and other combination tannages PAGE [184]

CHAPTER XV.

IRON AND CHROME TANNAGES.

Iron tannages — Chrome tannages — Chemistry of chromium compounds — Knapp’s method of chrome tannage — Cavallin — Swan — Heinzerling — Hummel’s improvement — Schultz’s method — Theory of the two-bath process — Practical management of the two-bath process — Dennis’s chrome tanning liquor — Procter’s liquors — Theory of basic process — Practical use of basic liquors — Washing and neutralisation — Effect of sulphur on chrome leather — Bluebacking — Fat-liquoring — Dyeing of chrome leather — Glazing and finishing PAGE [198]

CHAPTER XVI.

PRINCIPLES OF THE VEGETABLE TANNING PROCESSES.

Methods of sole-leather tanning — Finishing of sole-leather — Theory of vegetable tannage — Deliming of sole-leather — “Mellowness” of liquors — Penetration of tannage — Drying of sole-leather — Tanning of dressing leathers — Preparation for tannage — Avoidance of “bloom” — Tannage of moroccos and other skins PAGE [220]

CHAPTER XVII.

COMBINATION OF VEGETABLE AND MINERAL TANNAGES.

Early combination tannages — Respective effect of mineral and vegetable tannages — Use of fat-liquor — Action of mineral and vegetable tanning materials on each other — Danish and Swedish glove leathers — Green leathers — Making of fat-liquors — Chrome combinations PAGE [236]

CHAPTER XVIII.

VEGETABLE TANNING MATERIALS.

Distribution of tannin in plants — Structure of barks — Botanical list of important tanning materials PAGE [242]

CHAPTER XIX.

THE CHEMISTRY OF THE TANNINS.

Sources of tannins — General qualities of tannins — Chemical constitution — Catechol- and pyrogallol tannins — Catechins — Tendency of Catechol tannins to darken with light — “Physiological” and “pathological” tannins — Presence of mordant colouring matters PAGE [294]

CHAPTER XX.

THE SAMPLING AND ANALYSIS OF TANNING MATERIALS.

The International Association of Leather Trades Chemists — The American Official Association of Agricultural Chemists — The sampling of material — Preparation of solution for analysis — Extraction of solid materials — Total soluble matter — Evaporations of solutions — The weighing of residues — The determination of non-tannins — The hide-powder filter method — The hide-powder shake method — Determination of moisture — Colour-measurement PAGE [300]

CHAPTER XXI.

THE GRINDING OF TANNING MATERIALS.

Primitive methods of grinding — The bell mill or coffee mill — Disc mills — Disintegrators — Carr’s disintegrator — Carter’s disintegrator — Adjustment of disintegrators — The Williams pulveriser — Myrobalans and Valonia crushers — Sawing mills — Shaving mills — Dyewood cutting machines — Screening of ground materials — Hatching of bark — Disintegrators and fire insurance — Dust from disintegrators — Chain conveyors — Belt conveyors — Vibrating conveyors PAGE [316]

CHAPTER XXII.

THE EXTRACTION OF TANNING MATERIALS, AND THE MAKING OF EXTRACTS.

Leaching — Early forms of leaches — The press-leach system — Handling of liquors — Distributing troughs and valves — Construction of leaches — Influence of temperature — Use of silent boiling jet — Closed extractors — Sprinkling leaches — Manufacture of extracts — Decolorisation of extracts — Soluble extracts — Concentration of extracts — Yaryan evaporator — Multiple effects — The use of extracts in the tannery — Effect of temperature on extraction and colour PAGE [328]

CHAPTER XXIII.

FATS, SOAPS, OILS AND WAXES.

Characteristics of fats and oils — Chemical constitution — Nature and production of soaps — Insoluble soaps — Distillation of fats — Solvents of oils — Drying oils — Saturated fatty acids — Non-drying liquid fatty acids — Less-saturated liquid fatty acids — Castor oil — Tallow — Neatsfoot oil — Wool fat — Holden fat — Distilled wool grease — Distilled stearine — Olive oil — Castor oil — Turkey-red oil — Linseed oil — Boiled oils — Japan for leather — Cottonseed oil — Sesame oil — Cod oil — Shark liver oil — Whale oil — Seal oil — Menhaden oil — Fish oils — Fish tallow — Dégras and Sod oil — Waxes — Sperm oil — Beeswax — Carnauba wax — Japan wax — Volatile or essential oils — Birch oil — Wintergreen oil — Mineral oils and waxes — Vaseline and vaseline oil — Paraffin wax — Ozokerit — Resin oils — Resin PAGE [350]

CHAPTER XXIV.

OIL TANNAGES, AND THE USE OF OILS AND FATS IN CURRYING.

Primitive use of oil in leather manufacture — Chamoising and the production of washleather — Manufacture of Moellon, or Dégras — Sod oil — Formaldehyde leathers — “Crown” and “Helvetia” leathers — Theory of oil leathers — Processes of currying — Theory of the stuffing process — Hand-stuffing — Drum-stuffing — Stuffing of dry leather — “Spueing” and its causes — Fat-liquoring PAGE [378]

CHAPTER XXV.

DYES AND DYEING.

Coal-tar colours — Acid and basic colours — Theories of dyeing — Fixation of colours on leather — Mordants and mordant colours — Curriers’ inks — Glazes and finishes — “Assistants” in dyeing — Bronzing — Fading of colours — Practical methods of leather dyeing — Use of dyewoods — Iron “strikers” — Tannin blacks — Staining — Theory of colour-mixtures — Finishing dyed leathers — Testing of dyes — Injurious effects of metals in dyeing PAGE [394]

CHAPTER XXVI.

EVAPORATION, HEATING AND DRYING.

Theory of evaporation — Boiling point and vapour-pressure — Consumption of heat in evaporation — Heat-units — Mechanical energy of heat — Evaporation by “multiple effect” — Vapour-pressure of atmospheric moisture — Wet and dry bulb thermometers — Heat and air required in leather-drying — Loss of heat by buildings — Quantity of heat given by steam and hot-water pipes — Screw-fans for drying — Centrifugal fans — “Turret” dryer — Downward ventilation — Arrangement of steam-pipes — Hot water pipes PAGE [420]

CHAPTER XXVII.

CONSTRUCTION AND MAINTENANCE OF TANNERIES.

Selection of site — Arrangement of buildings — Fire insurance — Automatic sprinklers — Possibility of extension — Production and distribution of power — Electric motors — Shafts, pulleys and belting — Balancing of machinery — Fire-risk from bark mills — Chain-conveyors — Lubricating oils — Construction of pits — Underground pipes and overhead troughs — Pumps and pumping appliances PAGE [444]

CHAPTER XXVIII.

WASTE PRODUCTS AND THEIR DISPOSAL.

Hair — Fleshings and glue-stuff — Fat — Bate-shavings — Horns — Spent tan — Tan-furnaces — Sewage and other waste liquids — Chemical purification of sewage — Settling tanks — Filter-presses — Bacterial purification of sewage — Tannery waste-liquors PAGE [460]

APPENDIX A.

METHOD OF THE INTERNATIONAL ASSOCIATION OF LEATHER-TRADES CHEMISTS FOR THE ANALYSIS OF TANNING MATERIALS: Corrected to 1901.

Sampling from bulk — Preparation for analysis — Preparation of infusion — Determination of tanning matters and non-tannins — Colour-measurement — Analysis of used liquors PAGE [475]

APPENDIX B.

THE DECIMAL SYSTEM.

Metrical weights and measures — Centigrade thermometer PAGE [481]

APPENDIX C.

METHOD OF ANALYSIS OF TANNING MATERIALS OF THE AMERICAN ASSOCIATION OF OFFICIAL AGRICULTURAL CHEMISTS: Corrected to 1901.

Preparation of sample — Quantity of material — Moisture — Total solids — Soluble solids — Non-tannins — Tannins — Testing of hide-powder — Testing non-tannin filtrate PAGE [482]

APPENDIX D.

LISTS OF COAL-TAR DYES SUITABLE FOR DYEING AND STAINING LEATHER, furnished by Mr. M. C. LAMB.

Colours for staining leather — Colours for dyeing vegetable-tanned leather — Dyeing and finishing chrome-leather — List of colours suitable for chrome-leather PAGE [485]

INDEX PAGE [499]

PRINCIPLES
OF
LEATHER MANUFACTURE.

CHAPTER I.
INTRODUCTORY AND HISTORICAL.

The origin of leather manufacture dates far back in the prehistoric ages, and was probably one of the earliest arts practised by mankind. The relics which have come down to us from palæolithic times, and the experience of the modern explorer, alike tell us that agriculture is a later and a higher stage of development than the life of the hunter; and since, in the colder regions, clothing of some kind must always have been a necessity, we may conclude that it was first furnished by the skins of animals.[1]

[1] See also Gen. iii. 21.

While wet skins putrefy and decay, dry ones are hard and horny; and nothing could be more natural to the hunter than to try to remedy this by rubbing the drying skin with the fat of the animal, of which he must have noticed the softening effect on his own skin. By this means a soft and durable leather may be produced, and this process of rubbing and kneading with greasy and albuminous matters, such as fat, brains, milk, butter and egg-yolks, is in use to this day, alike by the Tartars on Asiatic steppes and the Indians on American prairies; and not only so, but we ourselves still use the same principle in the dressing of our finest furs, and in the manufacture of chamois, and many sorts of lace- and belt-leathers.

Such a process is described in the Iliad (xvii. 389-393) in the account of the struggle over the body of Patroclus:

“As when a man
A huge ox-hide drunken with slippery lard
Gives to be stretched, his servants all around
Disposed, just intervals between, the task
Ply strenuous, and while many straining hard
Extend it equal on all sides, it sweats
The moisture out and drinks the unction in.”

It must also have been early noticed that wood smoke, which in those days was inseparable from the use of fire, had an antiseptic and preservative effect on skins which were dried in it, and smoked leathers are still made in America, both by the Indians and by more civilised leather manufacturers. To this method the Psalmist refers[2] when he says, “I am become like a bottle in the smoke;” and such bottles, made of the entire skin of the goat, are still familiar to travellers in the East.

[2] Ps. cxix. 83.

The use of vegetable tanning materials, though prehistoric, is probably less ancient than the methods I have described, and may possibly have been discovered in early attempts at dyeing; an art which perhaps had its origin even before the use of clothing! The tannins are very widely distributed in the vegetable kingdom, and most barks, and many fruits, are capable of making leather.

The employment of alum and salt in tanning was probably of still later introduction, and must have originated in countries where alum is found as a natural product. The art was lost or unknown in Europe till introduced into Spain by the Moors.

Leather manufacture reached considerable perfection in ancient Egypt. A granite carving, probably at least 4000 years old, is preserved in the Berlin Museum, in which leather-dressers are represented. One is taking a tiger-skin from a tub or pit, a second is employed at another tub, while a third is working a skin upon a table. Embossed and gilt leather straps have been found on a mummy of the ninth century B.C., and an Egyptian boat-cover of embossed goat leather, as well as shoes of dyed and painted morocco, are still in comparatively good preservation. The art is of very early date in China, and was well understood by the Greeks and Romans. In the Grosvenor Museum at Chester is the sole of a Roman caliga, studded with bronze nails, which is yet pretty flexible. After the fall of the Roman empire many arts were lost to Europe, and it was not until the Moorish invasion of Spain that the art of dyeing and finishing the finer kinds of leather was reintroduced.

England was very backward in this manufacture up to the end of the last century, owing to the fossilising influence of much paternal legislation, and of certain excise-duties, which were only repealed in 1830. Since this time the art has made rapid strides, especially in the use of labour-saving machinery, and England may at the present moment be considered fairly abreast of any other country as a whole; though in some special manufactures we are surpassed by the Continent and by America. In making comparisons of this kind, it must, however, be remembered that, especially in sole-leather tannage, the most rapid progress has been made during the last few years in those countries which were more backward, and that therefore our superiority is much less pronounced than formerly, and in a few years will probably cease to exist unless marked improvements are introduced in the methods of production.

In the sketch of the development of leather manufacture which has just been given, it has been implied that its object is to convert the putrescible animal skin into a material which is permanent, and not readily subject to decay, while retaining sufficient softness or flexibility for the purposes for which it is intended. As these range from boot-soles to kid-gloves, there are wide divergences, not only in the processes employed, but also in the materials used and in the principles of their application.

The most important method of producing leather is by the use of vegetable tanning materials, and this is perhaps the only one which is really entitled to be called “tanning,” though the distinction is not very strictly adhered to. It includes the whole range—from sole leather, through strap, harness and dressing leather, to calf and goat skins, and the various sumach tannages which yield morocco and its imitations. All of these products but the first and the last undergo, after tanning, the further processes of “currying,” of which the most important operation consists in “stuffing” with oily and fatty matters, both to increase the flexibility and to confer a certain amount of resistance to water. Sumach-tanned skins are not strictly “curried” but usually receive a certain amount of oil in the process of “finishing.”

Next in importance to the vegetable tannages are the “tawed” leathers produced by the agency of alum and salt, including the “white leathers” for belt laces and aprons, and calf- and glove-kid. A connecting link between tanning and tawing is found in the “green leather,” “Dongola,” and “combination” tannages, in which alum and salt are employed in conjunction with vegetable tanning materials, and especially with gambier.

Salts of several of the metals, and particularly those of aluminium, iron, and chromium, have the power of converting skin into leather; and processes in which salts of chromium are used have recently attained very considerable commercial importance.

In the production of calf- and glove-kid, in addition to alum and salt, albuminous and fatty matters, such as egg-yolk, olive oil and the gluten of flour, play a considerable part, and are thus linked both to the primitive methods in use by the Indians and Kalmucks, and to those by which “crown” and “Helvetia” leather, and many other forms of belt- and lace-leathers are now produced by treatment with fats and albumens.

From these again the step is a short one to the “chamois” and “buff” leathers, and the German “fettgar” leathers, in which oils and fats only are used; and these are probably again related chemically to leather produced by the aid of formaldehyde and other aldehydes.

In an attempt to view all these complex processes from the scientific standpoint, the reader should constantly realise that the present methods of leather manufacture are the results of tens of centuries of experience, and of innumerable forgotten failures, and must not therefore expect that they can be easily superseded. Science must follow before it can lead, and its first duty is to try to understand the reasons and principles of our present practice, for we can only build the new on the foundation of what has been already learned. Another fact, which is scarcely understood by the practical man in his demands on science, is that in leather manufacture every question which is raised seems to rest on the most recondite problems of chemistry and physics; the chemistry of some of the most complex of organic compounds, and the physics of solution, of osmose, and of the structure of colloid bodies—problems which are yet far from completely conquered by the highest science of the day.

It may seem bold to attempt the scientific treatment of such a subject at all; and, indeed, it must be admitted that our knowledge is still far from adequate for its complete accomplishment, but enough has been done to lay a foundation for future work, and this can at least be summarised and arranged in an available form. The subject falls naturally into two sections, in the first of which the processes of manufacture would only be described in general terms, and with sufficient fulness to enable the reader to understand the scientific considerations on which they are based, and the methods of investigation which can be applied to them; while in the second an effort should be made to give working details of the various processes sufficient to enable those with a general knowledge of the trade to experiment successfully in its various branches. It was at first intended that these two sections should be published in one book as a second edition to the Author’s ‘Text-book of Tanning,’ but owing to the long delay in its publication, it was decided to publish the first section under the present title ‘Principles of Leather Manufacture,’ leaving the latter section ‘Processes of Leather Manufacture’ to a later, and I fear, somewhat uncertain date; while the strictly chemical portion has already appeared in the ‘Leather Industries Laboratory Book,’ frequently referred to in the following pages under the abbreviation “L.I.L.B.” Where quantities and details are given, they must not be taken as recipes to be blindly followed; or even, in every case, as the best known methods; but rather as mere guides to experiment, which must be modified to suit varying conditions and requirements. It is the special virtue of the scientific, as opposed to the merely traditional way of looking at such questions, that knowing the cause and effect of each part of the process, it can so adjust them as to get over difficulties, and to suit novel conditions. It is needless to add that many methods are jealously preserved as trade secrets, and full details are frequently unattainable.

After what has just been said, it may be well to emphasise the great importance of practical knowledge and experience to the leather manufacturer. Even in trades which have reached the highest scientific development, such, for instance, as the manufacture of the coal-tar colours, the small experiments of the laboratory are not transformed into manufacturing operations without experience and sometimes even failure; and this must still more often be the case in a trade like that of leather-making, where our knowledge of the actual changes involved is still so incomplete. On the other hand, the cost of experiments on a manufacturing scale is usually so heavy that the least scientific must admit the advantage of learning all which the laboratory can teach before venturing on anything more; while even our present imperfect knowledge of the chemical changes involved will often warn us off hopeless experiments, and give us hints of the directions in which success may be attained. A knowledge of chemistry will probably prove at least as important to the future of our trade as that of mechanics has been in the past.

CHAPTER II.
INTRODUCTORY SKETCH OF LEATHER MANUFACTURE.

The object of tanning has been stated to be the rendering of animal skin imputrescible and pliable, but as we now rarely require leather with the hair on, preliminary processes are needed to remove it, and to fit the skin for tanning, and the nature of these processes has great influence on the subsequent character of the leather produced.

The first step is usually a washing of the skin to remove blood and dirt; while, where it has been salted or dried, a more thorough soaking is needed to remove the salt, and to restore the skin to its original soft and permeable condition.

The hair is then loosened by softening and partial solution of the epidermis structures (see [p. 47]) in which it is rooted. This is most generally accomplished by soaking for some days in milk of lime, which is occasionally assisted by the addition of caustic alkalies or of sulphides. When the latter are used in concentrated solution, the hair itself, as well as the epidermis tissues, is softened and destroyed in the course of a few hours. The lime not only serves to loosen the hair, but swells and splits up the fibre-bundles of which the hide tissue is composed, and so fits it to receive the tannage (cp. [p. 125]).

For some purposes a regulated putrefactive process is substituted for the liming; the hides or skins being hung in a moist and warm chamber (see [p. 119]), when the soft mucous layer which forms the inner part of the epidermis is disintegrated, partly by direct putrefaction, partly by the action of the ammonia evolved, so that the hair can be scraped off. In this case the hide-fibre is not swollen, and the necessary swelling has to be obtained by subsequent processes.

In whatever way the hair has been loosened, it is scraped off with a blunt and somewhat curved two-handled knife on a sloping rounded “beam” of wood or metal; this operation being termed “unhairing” (see [p. 144]).

This is generally followed by “fleshing,” which is performed on the same beam with a somewhat similar knife, which, however, is two-edged and sharp. In this operation, portions of flesh, and the fat and loose tissue which underlie the true skin (see [p. 147]) are removed by scraping and cutting. Machines for fleshing are also largely in use for certain purposes (see [p. 148]).

For sole leather, the hide, after some washing in soft water to cleanse from lime, is then ready for the actual tanning process; but for the softer leathers more thorough treatment is needed to remove the lime, and to still further soften the skin by solution and removal of a portion of the cementing substance of the fibres.

This treatment is generally of a fermentive or putrefactive nature, and the most common form is that known as “bating,” which consists in steeping in a fermenting infusion of pigeon- or hen-dung. The theory of its action is not yet thoroughly understood, but the effect is largely due to the unorganised hydrolysing ferments produced by the bacteria present; while at the same time the lime is neutralised and removed by the weak organic acids and salts of ammonia which are produced; and the fibre which had been plump and swollen with lime, becomes extremely relaxed and flaccid.

In the lightest leathers, such as kid- and lamb-skins for gloves, and goat and sheep for moroccos and the like, dog-dung is substituted for that of fowls, and the process is then called “puering” (see [p. 170]).

These processes are often followed by “drenching,” which sometimes indeed takes their place, the skins being soaked in a fermenting bran infusion. In this, the small quantities of acetic and lactic acid formed by fermentation are the active agents, neutralising and dissolving the lime, and cleansing and slightly plumping the pelt (see [p. 166]).

The tanning process which follows consists in soaking the pelt in infusions of various vegetable products containing bodies of the class known as “tannins,” which have the power of combining with skin-fibre and converting it into leather.

If at first strong infusions were used, they would act too violently on the surface of the skin, hardening and contracting it so that the subsequent tannage of the interior would be impeded, and the “grain” or outer surface would be “drawn” and wrinkled. This is avoided by the use at first of very weak infusions which have already been used on goods in a more advanced stage. In the later part of the process much stronger solutions are employed, and the hides are frequently “dusted” in them with ground tanning material.

In the case of sole leather, these processes may require from two to twelve months for completion; after which the leather is dried, smoothed, and compressed by mechanical means, and is then ready for use.

Dressing-leathers, ranging from calf-skins to harness-hides, receive a much shorter tannage, and the subsequent treatment with fats and oils, which, together with mechanical manipulations, constitute “currying.” The thin film of grease distributed over the surface of the fibres renders them supple, and to some extent waterproof.

The lighter fancy leathers, such as morocco, are dyed, and undergo many complex processes to fit them for their required purposes and improve their appearance.

Many skins such as calf, glove, and glacé kid, are not tanned, but “tawed” by a solution of alum and salt, which is often supplemented with mixtures of flour and egg-yolk to fill and soften the leather.

Salts of chromium are also employed in place of alum and salt, and produce an equally soft, but more permanent and enduring leather.

Lastly, wash-leather, or so-called “chamois,” and buff-leather are produced by fulling the prepared pelt with fish or whale oil, which converts the skin into leather by subsequent oxidation, during which aldehydes are evolved.

CHAPTER III.
THE LIVING CELL.

The larger part of the materials employed in leather manufacture are organic in their origin, and the skin itself is an organised structure, while the life-processes of putrefaction and fermentation play a large part in the tannery. Some knowledge, therefore, of biological structures and processes is necessary to a full understanding of much which follows, and a few words are not out of place with regard to the foundations of life itself.

The bricks of which all living structures are built are the living “cells” and their products, and these first elements differ little, if at all, whether the life is animal or vegetable, the distinction being produced rather by the way in which they are put together, than by differences in the cells themselves. This is so much the case that it is often difficult to decide in which of the two classes to place the simplest organisms, since most of these forms are capable of active movement, and their modes of nutrition and reproduction are common to both kingdoms.

In its simplest form, the cell, whether animal or vegetable, is strictly speaking not a cell at all, but consists merely of a minute mass of living jelly or protoplasm. Such is the amœba found in water and damp soil, such are the lymph-cells and white blood-corpuscles of our bodies, and such also some stages at least of the lowest forms of fungi, like the Æthalium septicum which is sometimes found on old tan-heaps as a crawling mass of yellow slime. If a drop of saliva be examined with the microscope under a cover-glass, with one-sixth objective and small opening of diaphragm,[3] a few scattered semi-transparent objects will be found, of the apparent size of a lentil or small pea, and of rounded form. These are lymph-corpuscles ([Fig. 1]). Their contents are full of small granules, and if they be observed quickly, or if the slide be kept at about the warmth of the body, it will be noticed that these are in constant streaming motion. If the warmth can be kept constant, which is difficult without special apparatus, and the cells can be observed from time to time, it may be seen that they lose their circular form, and put out protuberances (pseudopodia, “false feet”) one of which will gradually increase in bulk, till it absorbs the whole cell, which thus crawls about. It will now readily be understood how these cells wander through all the tissues of the body, passing through the smallest pores like the fairy who put her finger through a keyhole, and grew on the other side till she was all through! This independent vitality, in a warm and suitable nutrient liquid, may continue for more than a week, and, in the case of amœba, quite indefinitely.

[3] For details of microscopic manipulation in this and the following chapter see L.I.L.B., p. 234 et seq.

Fig. 1.—Lymph-corpuscle of frog, showing gradual change of form. (Ranvier.)

It is possible that by close attention, a rounded or elongated body, somewhat like an oil-globule, may be seen within the cell, though it is generally more obvious when the latter has been killed and stained with a weak solution of iodine. This is the nucleus, and within it is a still smaller speck called the nucleolus, which bears an important, and as yet little understood, part in the life-history of the cell. After a period, it undergoes certain somewhat complicated changes, and divides into two, the nucleus elongates, and also divides, each half carrying with it a portion of the living protoplasmic jelly, and thus forming two complete and independent cells. This is the life-history, not only of the lymph-cell, but with more or less modification, of every living cell or tissue.

Fig. 2.—Yeast-cells, much magnified.

These cells, like all living things, feed on the nutriment which surrounds them, and even enclose small particles of solid food, which are gradually dissolved and disappear. In this way the white blood-corpuscles are said to feed upon and destroy the still smaller organisms which gain access to the blood, and which might otherwise cause disease. The matter which cells consume is not, of course, destroyed, but simply converted into other forms, some of which are useless, or even poisonous to the cells, and which, like the secretions of higher animals, are discharged into the surrounding fluids; while others are retained, and contribute to the growth of the cell. Thus most vegetable cells secrete cellulose, or plant-tissue, which forms a wall enclosing the protoplasm, and so justifies the name of cell. If to warm water and a little sugar we add enough yeast to render it slightly milky, and examine it like the saliva, we shall have before us typical vegetable cells of the simplest form ([Fig. 2]). There is the same granular protoplasm, and there is the nucleus, though it cannot be seen without special preparation, the rounded spaces which look like one, being simply filled with transparent fluid, and called vacuoles. There is, however, no motion, as in the case of amœba, for the cells are enclosed in a tough skin of cellulose, which will be evident if they are crushed by putting some folds of blotting paper on the cover-glass, and pressing it with the handle of a needle or a rounded glass rod, when the protoplasm will be forced out and the skin remain like a burst bladder. This will be more obvious if the cells are previously stained with iodine or magenta, which will stain the protoplasm, but not the membrane. It is easy to observe the multiplication of the yeast-cells, which is somewhat different to that of the corpuscles. Instead of enlarging as a whole, and dividing into two equal cells, a small bud appears on the side of the parent-cell, and enlarges till it becomes itself a parent-cell with buds of its own. These do not break away at once, and hence chains and groups of attached cells are formed which are easily noticed in growing yeast if a microscope be employed. The principal nutriment of yeast is grape-sugar or glucose; and much more of this is consumed than is needed to produce the cellulose wall and the substance of new cells; just as in the animal, sugar, starch and fat are consumed to give heat and energy. In the yeast, this extra sugar is split up into carbon dioxide, which escapes as gas, and to which yeast owes its power of raising bread; and into alcohol, which in too large proportion is poisonous to the yeast itself.

Fig. 3.—Epithelium-cells. Ranvier.
p, pressure-marks; g, granular protoplasm.

In examining the saliva for lymph-cells, it is probable that some much larger objects may have been noticed of irregular polygonal outline and with a well-marked nucleus. These are cells from the lining epithelium of the mouth, and only differ from those of the epidermis of skin in their form and size ([Fig. 3]). Note the markings caused by the pressure of overlapping cells. In these cells the wall is formed of keratin or horny tissue, which takes the place of the cellulose of the yeast.

Fig. 4.—Penicillium glaucum, a common green mould.

Other simple forms of cell are those of Saccharomyces mycoderma or torula which forms a skin on the surface of old liquors, and which much resembles a small yeast; and of the various ferments which are found in liquors, bates and drenches, which will be more fully described in the chapter following.

Many of these, such as the acetic and lactic ferments, which, like all other bacteria, multiply by division, do not separate, but remain connected in chains or chaplets, like a string of beads. From these, the step is not a long one to the hyphæ or stems of the higher moulds, which are too frequently found on leather which has been slowly dried, and which consist simply of tubular cells which elongate and divide by the formation of septa or cross-partitions, and thus build up a complicated plant-structure ([Fig. 4]). As we proceed higher in the scale of plant and animal life, the forms and products of the cells become more varied, and instead of one single cell, fulfilling all the functions of the plant or animal, each class of cell has its own peculiar duties and properties, while all work together for the maintenance of the complex structure of which they form a part.

CHAPTER IV.
PUTREFACTION AND FERMENTATION.

The chemical changes produced by the unicellular plants, such as yeasts and bacteria, to which allusion has been made in the last chapter, are known as fermentation and putrefaction, and are of such importance to the tanner, both for good and evil, that the subject must be treated in some detail. No scientific distinction exists between fermentation and putrefaction, though it is customary to restrict the latter term to those decompositions of nitrogenous animal matter which yield products of disagreeable smell and taste.

The organisms which are the cause of both fermentation and putrefaction are known by the general term of “ferments.” This term has also been extended in recent years so as to include the so-called “unorganised ferments” (enzymes, zymases) which are active products secreted by the “organised ferments” or living organisms.

These latter are again divided into three classes:—

• 1. Moulds.
• 2. Yeasts (Saccharomycetes).
• 3. Bacteria.

The members of one class are distinguished from those of another by their form, and, more especially, by the substances they produce during their life-history. All three classes are now considered to be fungi.

All ferments possess the following three properties:—

1. They are nitrogenous bodies.

2. They are unstable, i.e. they are destroyed by heat, chemicals, etc.

3. A relatively small quantity of the ferment is capable of producing great changes in the substances upon which it acts, especially if the products of the change can be removed as they are formed.

The general character of fermentation will be best understood by a closer study of the yeast cell, which has already been described ([p. 12]), and its life-history briefly sketched. It has been shown that it is a growing plant of a very simple type, belonging to the fungi. These are devoid of the green colouring matter which enables the higher plants to utilise the energy of sunlight to assimilate the carbonic acid of the atmosphere, exhaling its oxygen, and employing its carbon for the building up of tissue; and they must therefore, like animals, have their nutriment ready formed, and capable of supplying energy by its oxidation. For yeast, as has been stated, the appropriate nourishment is glucose, or “grape-sugar.” This is broken down, in the main, into the simpler compounds, alcohol and carbonic acid, while a small portion is utilised for the building up of the cell and the formation of secondary products. The main reaction is represented by the following equation:

Yeast cannot directly ferment ordinary cane-sugar (C₁₂H₂₂O₁₁), but secretes a substance called invertase, which so acts on the sugar as to break it up, with absorption of one molecule of water, into two molecules of fermentable glucose (dextrose and levulose) which serve as nourishment for the yeast.[4] This invertase is the type of the series of bodies which are known as “unorganised ferments,” enzymes, or zymases, differing from the organised ferments in being simply chemical products without life or power of reproduction, but capable of breaking up an unlimited quantity of the bodies on which they act, without themselves suffering change. The way in which this is done is not clearly understood, but some parallel may be found to it in the action of sulphuric acid on alcohol, of which it will convert an unlimited quantity into ether, without itself suffering any permanent change. The action of enzymes is limited to breaking down complex bodies into simpler forms, often with absorption of water, as in the case of sugar, while some of the products of living ferments are often complex, a part of their nutriment being broken down into simple products such as carbonic acid, marsh gas and ammonia, to supply the necessary energy to elaborate the remainder.

[4] Compare O’Sullivan and Thompson, Jour. Chem. Soc., 1890, p. 834; 1891, p. 46.

Very many different unorganised ferments are known to exist, as they are not only produced by yeasts and bacteria, but are formed by the cells of higher plants and animals; thus the digestive principles, pepsin, trypsin, ptyalin, are of this character—ptyalin, like diastase, converting starch into sugar; and such bodies fulfil many functions both in animal and vegetable economy. In fermentation, as in disease, it is often difficult to distinguish what is due to the direct action of bacteria, and what to the unorganised ferments which they produce, and the question is further complicated by the fact that in most natural fermentations more than one ferment-organism is present. Sometimes the action of the unorganised ferments may be distinguished by the fact that the addition of chloroform has little effect on their activity while it paralyses that of the living organism. By exposure to high temperature both are destroyed, the bacteria, yeasts and moulds being killed and the unorganised ferments coagulated like white of egg, and so rendered inoperative. Many antiseptics also destroy the activity of both organisms and enzymes; but others, like chloroform, have no action on the latter. In some cases, as in that of invertase, the actual zymase can be precipitated by alcohol from its aqueous solution, filtered off, and restored to activity by transference into water. Since both classes of ferments are destroyed by high temperatures, all fermentation-processes are completely and permanently arrested by exposure to sufficient heat, and subsequent preservation in vessels so closed that no new ferment-germs can gain access. A familiar instance is that of tinned meats. All fully developed bacteria are destroyed by a very short exposure to a boiling temperature, and most by 60° to 70° C., but many species produce spores which are extremely difficult to destroy. The thermophilic bacteria discovered by Globig and further investigated by Rabinowitsch,[5] thrive at a temperature of 60° C. About eight species are known, and they take part in the heating of hay and similar fermentations where high temperatures are involved, and are therefore presumably present in spent tan.

[5] Centr. Blatt für Bakt., II. Abth. vol. i. p. 585.

For absolute sterilisation it is therefore necessary either to boil under pressure so as to raise the temperature to, say 110° C., or to heat repeatedly for a short time to temperatures of 80°-100° C. at successive intervals of 24 hours, in order to allow the spores to develop. This process is frequently performed for bacteriological observation in flasks or test-tubes merely stopped with a plug of sterilised cotton-wool, which has been found to efficiently filter the germs from the air which enters through it (see L.I.L.B., p. 270).

The ferment-organisms cannot thrive and multiply unless they have proper nourishment and conditions of growth, the amount of moisture and the temperature being two of the most important of the latter. Use is made of this in the preservation of many articles of food, etc., since by ensuring that at least one of the conditions necessary for growth shall be absent, these substances are prevented from decomposing. For instance, hides are preserved by drying them; the absence of sufficient moisture hindering the growth of any organisms in them so long as they are dry, but as soon as they become somewhat damp, putrefaction commences at once.

The waste products of organisms are often poisonous to themselves, and for this reason fermentations frequently come to an end before the whole of the substance is fermented. Thus neither beer nor vinegar can be obtained of more than a certain strength by direct fermentation, the alcohol or acetic acid checking the growth of their respective ferments. A solution of glucose “set” with the lactic ferment of sour milk will only produce lactic acid to the extent of about half a per cent.; but if chalk be added, the lactic acid will be neutralised as produced, and the fermentation will go on till the whole of the glucose is converted into insoluble calcium lactate.[6] When this is accomplished the lactic ferment dies from want of nutriment, and its place is taken by another organism, of which some germs are sure to be present, which ferments the calcium lactate into calcium butyrate. If the nourishment fails, or the conditions become less favourable for one ferment than for some other which exists even in small quantity in a liquid, the former is quickly overgrown and killed, and the latter takes its place. Thus the ordinary ferment of the bran drench will die out rapidly unless constantly transferred to fresh bran infusions.

[6] For the practical preparation of lactic acid, the solution may contain 7¹⁄₂-11 per cent. of glucose, and some nitrogenous nourishment. The solution should be slightly acid. See Journ. Soc. Ch. Ind., 1897, p. 516.

Many of the products of bacteria (like those of some of the higher plants) are intensely poisonous both to animals and man. Many of the severe symptoms of disease are caused by these poisons produced in the body. Thus the tetanus-bacteria produce a poison similar in its effects to strychnine, and quite as virulent. Not only are such poisons produced by disease-bacteria in the body, but frequently also in the earlier stages of putrefactive fermentation. The latter are known as ptomaines, and when present in cheese and preserved foods are liable to cause poisoning. Such putrefactions are often unaccompanied by any disagreeable odour or flavour.

The fermentations which are most important in the tannery are, firstly, the ordinary putrefaction which attacks hides as well as other animal matter, and which is usually a complicated process carried on by many sorts of bacteria and other micro-organisms. This may be regarded as generally injurious to the tanner; but it is utilised in the “sweating” process for depilation and in the “staling” of sheepskins, in both of which advantage is taken of the fact that the soft mucous layer of the epidermis, which contains the hair-roots, putrefies more rapidly than the fibrous structure of the hide itself. In soaking also, use is made of the power of putrefactive ferments to dissolve the cementing substance of the hide, though in this case with doubtful advantage to the tanner. In the liming process putrefaction makes itself felt when the limes are allowed to become stale and charged with animal matter, softening the hide and finally rendering the leather loose, empty and inclined to “pipe.” Here the effect is in many cases useful if not carried too far.

In bating and puering, the action is almost entirely due to the enzymes and other products of bacterial activity, the original chemical constituents of the dung being apparently of minor importance. Naturally the liquid is adapted to the growth of many other organisms beside those acting most advantageously on the hide, and injury in the bates from wrong forms of putrefaction is very common, if indeed it is not always present in greater or less degree.

In drenching, the effect is, at first, entirely due to the weak acids produced by bacterial fermentation of the bran, but becomes complicated in its later stages by putrefactive and other fermentations which may be desirable or otherwise.

In the tanning liquors, fermentation is not so marked, but is of great importance owing to the production of acids by bacterial action from the sugars present in the material. The acids themselves are apt to be fermented and destroyed, principally by the oxidising action of Saccharomyces mycoderma and the higher moulds (see [p. 14]), which also act destructively on the tannins.

The effect of these acids on the hides is to swell them and to neutralise any lime they may contain. They also give to the liquors a characteristic sour taste, as a consequence of which, liquors containing acetic and lactic acids are usually known in the tannery as “sour liquors.”

It is doubtful whether the action of fungi is completely stayed even by the drying process. The heating of leather in the sheds is due to bacteria and the higher moulds, and Eitner considers their growth one of the causes of the “spueing” or “gumming” of curried leathers.

From what has been said, it is obvious that, with regard to fermentations, a double problem is presented to the leather manufacturer, since he desires to utilise those which make for his advantage, while controlling or destroying those which are injurious. The first step to a solution of these problems is a more complete knowledge of the organisms which serve or injure us, that we may, as it were, discriminate friends and enemies. We may then approach the question in two ways. Taking the drenching process as an example, we may on the one hand introduce a “pure cultivation” of the right ferment into a sterilised bran infusion, and so induce only the one fermentation which we require; or, on the other hand, as different ferments are affected in varying degrees by antiseptics, we may perhaps choose such as permit the growth of the organism we want, while killing or discouraging the rest. We may also arrange the nutriment, temperature, degree of acidity and other conditions, so as to favour one organism rather than another. All three methods have been applied in brewing with good results.

CHAPTER V.
ANTISEPTICS AND DISINFECTANTS.

“Antiseptics” are often defined as substances which check putrefaction without necessarily destroying bacteria and their spores, while “disinfectants” are poisonous to ferment-organisms, and actually destroy them; great differences exist in the extent of their sterilising power, and the whole distinction is one rather of degree than of kind, and has little practical value. Thus common salt is incapable of killing most bacteria, even in concentrated solution, though it holds putrefaction in check both by withdrawing water from the hide and by directly preventing the multiplication of bacteria. If the salt be washed out of the hide, putrefaction is at once resumed by the organisms present. Hides, on the other hand, which have once been sterilised by powerful disinfectants, such as phenol (“carbolic acid”) or mercuric chloride, do not again putrefy till the organisms which are killed are replaced by fresh ones from outside. The action of sodium sulphate, and many other salts, is similar to common salt in this respect, while a large proportion of the aromatic compounds are permanently disinfectant, though their efficiency varies with the species of bacteria involved.

Biernacki and others have shown that some disinfectants when extremely diluted actually stimulate alcoholic fermentation, and probably the growth of other ferments, e.g. mercuric chloride 1 in 300,000, salicylic acid 1 in 6000, and boric acid 1 in 8000, and in many cases organisms become habituated to antiseptics in doses which would at first have proved fatal.

The number of antiseptics available is now so great that it is impossible to give a detailed account of all, but the following are among those which are best known and have been practically employed.

Lime possesses some antiseptic properties, and is largely used in the preservation of fleshings before they are sent off to the glue factory. They are most conveniently stored in a large vat filled with a strong milk of lime. Dilute solutions of caustic alkalies have an effect similar to that of lime.

Common salt, sodium chloride, NaCl, acts to a certain extent by its solubility and a dehydrating effect on animal tissues common to chlorides, which removes water from hides and other materials which it is used to preserve. Probably the latter characteristic has a good deal to do with its effect in checking the development of bacteria, since many species thrive quite well in weak salt solutions, and some even in brine, and the dehydrating effect of the salt enables it to harden many animal tissues if used in sufficient quantity, the water they contain running away in the form of brine.

Ordinary rock salt frequently contains ferric chloride, and this, either originally present in the salt, or in some cases derived from the action of the latter upon the iron contained in the blood, is the cause of what is known as “salt-stains.” These show but little during the liming of the hides, unless sulphides are used, when stains appear of a greenish black, from the formation of sulphide of iron; when, however, the hides come into the tanning liquors, black or blue stains are produced by the action of the tannin, which are partially removed by the acids of the liquors during the tanning process, but generally show to some extent in the finished hide. There is another species of salt-stain, not apparently due to iron, but to the colouring matter produced by some fungoid or bacterial growth, which it is practically impossible to remove, and which is stated to be sometimes caused by the use of old salt with which hides have been previously salted. Iron stains are most readily recognised by the use of a solution of potassium ferrocyanide or thiocyanate slightly acidified by hydrochloric acid. If this be applied to the leather, the stains will be changed from a blackish to a blue, if the former, or a red colour if the latter salt has been used. A more absolutely conclusive proof is to lay a piece of filter paper soaked in dilute hydrochloric acid upon the stain, and then to test for iron upon the paper with ferrocyanide or thiocyanate. The freedom of the paper itself from iron must be ascertained before use. Iron-stains produced in the salted state are more difficult to discharge than those which are caused later in the tanning process, since iron salts have distinct tanning power, and attach themselves firmly to the untanned fibre. On the Continent, where common salt is heavily taxed, alum, carbolic acid, naphthalene and other materials are frequently added to it to “denaturise,” or render it incapable of being used as food, and these additions are often the cause of trouble to the tanner.

Sodium sulphate, Na₂SO₄, has little if any disinfectant power in dilute solution, but if used in the calcined form (anhydrous sodium sulphate) as proposed by Eitner[7] as a substitute for common salt in preserving hides, it withdraws water from the hide and crystallises with 10 Aq (about 56 per cent.). This does not run away like brine, but remains in the hide, which retains its weight, and remains plump and swells well in the limes and liquors, which chlorides have a great tendency to prevent; 10-15 per cent. on the weight of the hide is sufficient, while salt must be used in nearly double this quantity. Care must be taken that the sulphate used is free from bisulphate, NaHSO₄, which has a powerful swelling effect upon the hide-fibre, like sulphuric acid. The neutral sulphate does not redden methyl orange or litmus. Pickled skivers may be in part preserved by the sodium sulphate formed by the action of sulphuric acid upon the salt employed in the pickling bath (see [p. 90]).

[7] Gerber, 1880, p. 185.

The stronger mineral acids have considerable antiseptic power, and are of course especially fatal to such ferments as thrive best in alkaline solutions. The use of sulphuric acid in pickling skivers has already been alluded to, and a very dilute solution applied without salt to raw hides prevents putrefaction, though the principal object in using it is to plump the hides and produce a fictitious weight and substance which disappear on tanning. Such hides of course have a powerful acid reaction to litmus. Sulphuric acid in small quantities has been used with advantage in soaking E.I. kips. A very small excess of hydrochloric acid will sterilise putrid effluents, and no doubt nitric or sulphuric acid would have the same effect. The powerful effect of mineral acids on animal fibre, and their solvent action on cements and iron, preclude however, their general use as antiseptics.

More important is the use of sulphurous acid and sulphur dioxide, which, from their mild acidity and great antiseptic powers, are capable of a variety of useful applications. Considerable doubt has been raised as to the germicide power of sulphur dioxide, and it is certain that the dry gas is less effective on dry objects than when applied in solution, or to moist materials, as is almost invariably the case in the tannery. It may possibly be more efficient in its action on some moulds and putrefaction-ferments than on the pathogenic bacteria which have been most frequently used to test the power of disinfectants; but in practice it is found extremely useful in the brewery and in gelatine manufacture, and there is no reason that it should be less so in the tannery.

The gas is most conveniently produced by burning sulphur, which produces double its weight of sulphur dioxide. If used for “stoving” drying rooms and other places infested with moulds, care must be taken to avoid risk of fire. A shallow cast-iron pot set on bricks or sand is generally the most suitable vessel, and the sulphur may be ignited by a piece of red-hot iron or a rag which has been previously dipped in melted sulphur. It is corrosive to metalwork, and bleaches many colours, but does not produce any marked injurious effect on leather, though the sulphuric acid formed by oxidation may, if not removed, ultimately make it tender.

For many purposes a solution of the gas is required, and this is most easily made by burning the sulphur in a small metal or firebrick stove from which the fumes are sucked through a “scrubber,” which, on a small scale, is conveniently made of large glazed sanitary pipes, packed with coke or broken earthenware, over which water is allowed to trickle. The lowest pipe has an opening for a branch pipe, which is connected with the stove and rests on three bricks in a tub, which collects the acid solution and forms a water-seal to prevent the escape of gas. Above the inlet for the gases is fixed a wooden grating on which the coke rests. The scrubber may be 10-15 feet in height and connected at the top with a chimney or steam ejector to produce the draught. The arrangement is illustrated in [Fig. 5]. Another method is to burn the sulphur in a closed cylinder and to force the products through water with an air-compressor or steam-jet injector.

In place of using a scrubber, the fumes may be blown by a steam ejector direct into a tank. This is a very good arrangement for washing and bleaching hair, etc., but where large quantities of solution are required is inferior to the scrubber. Ejectors of hard lead or regulus metal should be used, and are less acted on by the dry gases than by the very dilute moist exhaust from the scrubber (see [p. 335]).

Fig. 5—Sulphurous acid apparatus.

Bisulphites have also strong antiseptic properties. “Bisulphite of soda” (hydric sodic sulphite) solution may be made by supplying the scrubber with solution of soda-ash or washing soda; bisulphite of lime, by using milk of lime or packing the scrubber with chalk or limestone (free from much iron) in place of the coke. In either case a much stronger solution is obtained than with water alone.

Boakes’ “metabisulphite of soda”[8] is a very convenient source of sulphurous acid when the latter is wanted in small quantities. It is an anhydrosulphite, Na₂O.2(SO₂), and contains 67·4 per cent. of its weight of SO₂. One molecule of the salt (= 190) requires one molecule of H₂SO₄ (= 98) to set free the whole of the sulphurous acid. For many purposes the sulphate of soda formed may be neglected and the acidified solution used direct.

[8] Patented by Boakes, Ltd., Stratford, London, E.

For analysis of sulphites and sulphurous acid solution, see L.I.L.B., pp. 16 and 37.

Boric acid, borax and other borates are not very powerful disinfectants. They have no injurious action upon the skin, but to be effective require to be employed in pretty strong solutions, say 1 per cent., and their comparatively high cost unfits them for general use as antiseptics in the tannery, though boric (boracic) acid is very useful as a drenching and deliming agent (see [pp. 156], [229], and L.I.L.B., p. 37).

Mercuric chloride, corrosive sublimate, HgCl₂, is an extremely powerful antiseptic, preventing the growth of some species of bacteria in solutions so dilute as 1 in 300,000 (Koch). 1 in 14,000 is disinfectant (Miquel), but its power varies very much upon different organisms (Jörgensen states that 1 in 400 is required to kill Penicillium glaucum), and it is unsuited for most purposes in leather manufacture, both from its extremely poisonous character, and because it is rendered inactive by various substances present in the materials used.

Mercuric iodide dissolved in iodide of potassium solution was patented by Messrs. Collin and Benoist as an antiseptic in tanning, but it is ineffective for the same reasons as mercuric chloride; although under favourable circumstances it is even more powerful than the latter.

Copper sulphate, zinc chloride and sulphate, and many other metallic salts are powerful antiseptics, but have only a limited application in leather industries, and do not usually actually sterilise. Arsenic (arsenious acid), which has been used in curing hides, is an excellent insecticide, but not particularly effective as an antiseptic; and sulphide of arsenic (realgar) when used in limes (see [p. 139]) seems to have but little antiseptic effect. Arsenious acid is easily soluble in alkaline solutions.

Fluorides have been suggested as antiseptics in the tannery, but do not seem of much practical value.

The most important antiseptics at present are those derived from coal tar, and belonging to the aromatic series. Of these, the phenols (carbolic acid, cresol, etc.) are the most used.

Pure phenol, “pure crystallised carbolic acid,” is hydroxybenzene C₆H₅(OH), but the crude forms which are generally employed contain cresols and higher members of the series in which one or more of the atoms of hydrogen are substituted by CH₃ groups. These are oily bodies scarcely soluble in water, and even pure phenol is only soluble in cold water to the extent of some 7 per cent. Crude carbolic acid should not be employed in the tannery, since the insoluble oily particles stain the hide, and render it unsusceptible of tanning. Suitable carbolic acid should be of a pale yellow colour when fresh (though it will darken on exposure to air and light), and it should be wholly soluble in a sufficient quantity of water. Its specific gravity should be 1·050 to 1·065. For methods of chemical examination, see L.I.L.B., p. 40. A saturated solution of carbolic acid sterilises hide completely against most putrefactive organisms, but has a sort of tanning effect, adhering obstinately to the fibre so that it cannot be removed by washing; and hides which have been cured with it cannot be unhaired by sweating, though they may be limed in the usual manner, if somewhat more slowly. Care should be taken in mixing with water or liquor, as undissolved drops will produce the same effects as those of the crude acid. Hides are occasionally stained, as has just been described, by salt which has been denaturised with common sorts of carbolic acid. Eitner recommends the use of a solution of carbolic acid in an equal weight of crude glycerine, which readily dissolves in water, and seems to prevent any injurious effect on the hide.

An aqueous solution containing 1 per cent. of carbolic acid is sufficient for mere sterilising of hides, but if it be desired to preserve them for a long period, stronger solutions (up to 4 per cent.) may be employed.[9]

[9] Gerber, 1889, p. 98.

Quantities so small as 1 part per 1000 control the fermentation of liquors, and prevent the formation of moulds on the surface, economising tannin, and preserving vegetable acids already present, but at the same time lessening their production by fermentation, and therefore sometimes leading to difficulties in the early stages of tanning. Carbolic acid is not, strictly speaking, an acid, but rather of the nature of an alcohol, although it forms weak combinations with bases. It is a powerful narcotic poison, and if dropped on the skin in a concentrated form it produces severe burns; these are best treated with oil, while in cases of poisoning, oil and chalk must be administered internally, but if the quantity of carbolic acid taken has been large, are not likely to be effective. From its cheapness and efficiency, carbolic acid is likely to be increasingly used, although for special uses some of the newer antiseptics have great advantages.

Eudermin is a tar-oil manufactured by Speyer and Grund, of Frankfort-on-Main, which is intended as an antiseptic addition to stuffing greases to prevent mould and spueing. It is recommended for the purpose by Eitner[10] and can be used in proportions such as 10 per cent. of the grease. Creasotes and cresols can be dissolved in oils and stuffing greases, and act as antiseptics, though less powerfully than in aqueous solution. Rosin oils and turpentine have also antiseptic properties.

[10] Gerber, 1893, p. 41.

Creasote, “heavy coal oil,” or “dead oil,” is a complex mixture of hydrocarbons, phenols and cresols, obtained by distillation of coal tar, heavier than water, and almost insoluble in it. It is largely used as a preservative for timber. Carbolineum is an oil of this class, boiling at over 300° C., and intended for application to wood. One or more coats are applied to the dry wood at a temperature of 80° C. The workman’s hands must be protected by gloves, as the hot creasote raises painful blisters. Eitner[11] recommends its use for preserving pits, posts and other woodwork in tanneries. Wood-creasote is a somewhat similar product obtained from wood-tar.

[11] Gerber, 1889, p. 183.

The heavier cresols are so little soluble in water as to be valueless in their ordinary form as antiseptics, but several preparations are made under the names of “Creolin,” “Jeye’s fluid,” “Lysol,” “Izal,” “Soluble phenyl,” etc., in which they are treated with additions of soap or alkalies, which cause them to emulsify or dissolve in water, generally as milky liquids. These are powerful germicides and have the advantage over phenol of being non-poisonous. 0·1 to 0·5 per cent. solution of creolin will sterilise hides after bating so that no putrefaction takes place in the liquors. Mr. J. T. Wood specially recommends creolin for the general purposes of the tannery, disinfecting pits and tubs, and for checking the action of puers and drenches on goods which have gone a little too far, by throwing them into a 0·2 per cent. solution.

Salicylic acid, orthohydroxybenzoic acid, C₆H₄OH(COOH), is now artificially prepared from phenol. It is much less poisonous than the latter and has no smell, which makes it valuable for certain purposes, but is too dear for most technical applications. Many bacteria appear to become gradually habituated to its action, and the same is true of phenol to a less degree.

Salicylic acid is closely related to protocatechuic and gallic acids, and, like these, gives a blackish colour with iron salts. It is freely soluble in hot water, but very sparingly in cold. The addition of 1-2¹⁄₂ parts of sodium phosphate, sulphate, or potassium nitrate to each part of salicylic acid greatly increases its solubility. It seems much more powerful in preventing the development of bacteria than carbolic acid; a solution of 1 part of salicylic acid in 666 of water is said to be equal in this respect to 1 part of carbolic in 200.

Benzoic acid, C₆H₅COOH, though not much employed, except in medicine, is a still more powerful disinfectant, and has the advantage of being non-poisonous to human beings.

“Cresotinic acid,” which is derived from the cresols as salicylic acid is derived from phenol, is more soluble than salicylic acid. It is not very poisonous, and a powerful disinfectant. In a crude form it has been introduced by Hauff, of Feuerbach, for bating or removing lime from hides. This it does very well, though without the softening action of a true bate. It has a tendency to produce a pinkish stain, and in some degree a sort of tanning of the fibre. Its price, moreover, is rather high for extensive technical use. (See also [p. 162].)

“Anticalcium” is a more recent preparation introduced as a bate by the same firm.[12] It is a solution of mixed sulphonic acids derived from cresols, and has considerable disinfectant powers. It removes lime very effectively, but from its acid character somewhat swells the skin. It is used very successfully as a drench for thin skins ([p. 163]).

[12] Gerber, 1895, p. 133.

“C.T.” (coal-tar) bate is a grey crystalline pasty mass, with a tarry smell, and is chemically very similar to anticalcium if not identical with it.

Naphthalene sulphonic acid has strong antiseptic properties. Its use in bating has been patented by Burns and Cross. (See [p. 163].)

Naphthols, C₁₀H₇(OH).—These bodies, which have the same relation to naphthalene as the phenols to benzene, are powerful antiseptics; and naphthalene itself appears to have antiseptic power, and is occasionally used for denaturising salt. There are two naphthols, varying in the position of the OH group in the molecule, and denominated α and β, of which α naphthol is the more powerful antiseptic and the less poisonous, though β, being cheaper, is the common commercial article. It is said that quantities so small as 0·1-0·4 grams of α naphthol per liter are sufficient to prevent the development of microbes, while of β naphthol about ten times that quantity is required.

Naphthols are not very expensive, but their value is diminished by the fact that they are insoluble in water. They are soluble in alkaline solutions, but their compounds with bases are of much lower antiseptic value, and the same is true of their alcoholic solutions; when an alcoholic solution is added to water the naphthol is precipitated, but if an addition of soap or camphor be made to the alcoholic solution, the naphthol remains in a very finely divided condition, if not dissolved.

Adopting Eitner’s suggestion with regard to oxynaphthoic acid (see [below]), hides may no doubt be sterilised by treatment first with an alkaline naphthol solution, and then with a very dilute acid to set the naphthol free.

“Hydronaphthol,” β tetra-hydro-naphthol, C₁₀H₁₂O, is obtained by the reduction of β naphthol by sodium (Rideal). It seems to be an excellent disinfectant.

Oxynaphthoic acid, α hydroxynaphthoic acid, C₁₀H₆(OH)COOH, which bears the same relation to naphthol as salicylic acid does to phenol, is cheaper than salicylic acid, and said to be a more powerful antiseptic. Its salts have no antiseptic power. In its commercial form it is a reddish crystalline powder, inodorous, but with a burning taste, and its dust causes violent sneezing. It is scarcely soluble in water, and is said to undergo some change on keeping which lessens its germicide power; it is readily soluble in alcohol, and the solution produces a milky fluid on mixture with water. Such a solution containing 15 grams of the acid in 4 liters of water, will sterilise a hide. Eitner recommends[13] that it should be dissolved in dilute soda solution, and the hides, after soaking in it, passed through water slightly acidified with hydrochloric acid, as has been suggested in the case of naphthol; the method is also applicable to creosotinic acid, the hides being permanently sterilised so that they cannot be unhaired by sweating, though they will lime in the usual manner.

[13] Gerber, 1888, p. 101; 1889, pp. 99 et seq. See also [p. 163].

Carbon disulphide.—Moret has suggested an aqueous solution of this compound as an antiseptic, and it seems to have considerable sterilising powers, but from its inflammability, poisonous character, and unpleasant smell, it is not likely to come largely into use.

Formaldehyde, COH₂, has recently been introduced as an antiseptic in aqueous solution containing 40 per cent. of formaldehyde together with a little formic acid, under the names of “formalin,” “formol,” etc. It seems to have great disinfectant powers, and may possibly be valuable in various processes of leather manufacture as it becomes cheaper, but has a curious hardening tanning effect on hide fibre and gelatinous matters, so that in very dilute solution it will produce leather.[14] The vapour of formaldehyde, or of its condensation-product paraform, may be employed to harden microscopic preparations. 1 part of formaldehyde, and consequently 2¹⁄₂ parts of “formalin” in 12,000 parts of water, is said to sterilise, and this proportion would form a good disinfectant solution. Even in considerably larger proportion than the above, it does not appear to be poisonous, and thus possesses the bactericidal power of sublimate without the latter’s poisonous properties. Formaldehyde has another advantage over most, if not all other antiseptics, in that it may be used as well in the gaseous as in the liquid state, and on that account it is largely employed in the disinfection of rooms or of articles which would be spoiled if they were to be wetted, as the gaseous formaldehyde, though thoroughly disinfecting them, will not injure the colours of materials of the most delicate fabrics.

[14] Gerber, 1897, p. 67; ibid., 1899, pp. 101, 205, 218.

On account of its capability of rendering gelatinous matters hard and insoluble in water, formaldehyde requires to be employed with great care, but 0·2-0·3 per cent. may be successfully used in admixture with egg-albumen in the preparation of “seasoning” in the finishing of morocco leather. It is also used commercially to produce different varieties of white leather for soldiers’ accoutrements and similar purposes ([p. 380]).

Triformol (tri-oxymethylene, “paraform”) is a product of the polymerisation of formaldehyde, and is prepared by evaporating a solution of the latter to dryness on the water-bath. It is said to be more powerful than formalin in its antiseptic properties, but has not entered very largely into use as a disinfectant, though considerable use is made of it to “fix” bacteria in gelatin for bacteriological purposes.

Camphor and essential oils, as well as oil of turpentine, have considerable antiseptic powers, and the cheaper essential oils such as those of winter-green, black birch, sassafras and aniseed are frequently employed, especially in America, in preserving pastes, finishes and seasonings, and at the same time covering offensive odours. The odour of essential oils becomes much more powerful as they are diluted, and very small quantities suffice for the purposes mentioned. Birch-tar oil, such as is used to give the scent to Russian leather ([p. 372]), has considerable antiseptic effect.

CHAPTER VI.
THE ORIGIN AND CURING OF HIDES AND SKINS.

A considerable proportion of the hides and skins used in leather manufacture are those of animals killed by the butcher for food, and these are frequently employed by the tanner without any preliminary curing. Domestic hides and skins are now generally sold by auction in weekly markets in the principal towns, after sorting and classification in weight and quality.[15] This is in many respects an improvement on the old method of purchase direct from the butcher, but it often leads to delay in delivery, and in hot weather hides suffer from putrefaction. In most cases, the damage is not sufficient seriously to affect the durability of the leather, but the delicate membrane of the “grain” is injured, and the hide or skin unfitted for coloured leather, or any purpose where small damages to appearance are important. Butchers are adverse to the use of salt, because it withdraws water from the hide in the form of brine, and so causes it to lose weight; but much injury would be saved by a light salting, and all hides or skins on which the hair is “slipping” should be regarded as damaged for fine leather manufacture.

[15] The weight of English market-hides as credited to the butcher is usually marked on the edge of the butt near the tail, by cuts with a knife, the mode of numeration being sufficiently explained by [Fig. 6], in which cuts crossing the horizontal line each represent 20 lb., that above it 10 lb., while less amounts are expressed in Roman figures.

On the Continent weights are usually given in pounds of half a kilogramme (50 kilos = 110 lb. English). In Paris the marking is on the tail, and is also shown on [Fig. 6].

Fig. 6.—Method of marking weight on hides; 97 lb.

Sheep-skins are not usually bought direct by the tanner, but by the fellmonger, who removes the wool; and as this is usually of much greater value than the skin, the latter is frequently handled very carelessly, and its quality sacrificed for the sake of real or fancied improvement to the wool. In very many cases the skin is “sweated” or “staled” by hanging in a warm and moist chamber, heavily charged with ammonia derived from the putrefaction of the skin, until the wool is sufficiently loosened to be “pulled.” If this treatment is conducted with extreme care the skin may escape serious injury, but in most cases the grain is weakened, and the foundation is laid of damage, which makes itself felt throughout the tanning process. For the purposes of the tanner, a much better way is to lime the skins by painting with thick limewash on the flesh-side, and after folding the skins down the back, flesh-side in, to prevent as much as possible the access of the lime to the wool, to place them in a pit, and cover them with water, till the wool is loosened by the penetration of the lime through the skin. A still more satisfactory method, and one which is in general use in the American stockyards, and to some extent also in Europe, is to wash the skins in water to free them from blood and dirt, and then, laying them in a wet condition, flesh side up, to paint them with a solution containing about 25 per cent. of sulphide of sodium, thickened with lime. The skins, as they are painted, are doubled down the back, flesh-side in, and laid on a floor, overlapping each other like tiles on a roof, for some hours, or overnight, till the wool is sufficiently loosened to pull, after which the pelts are limed and treated in the ordinary way. As a general rule the English fellmonger keeps his skins in lime till they are sold to the tanner, and as in small yards some time is taken to accumulate a parcel, the earlier skins may suffer great injury from overliming. Even sweet fresh limes dissolve the cementing substance of the fibre, and increase the naturally loose texture of the sheep-skin, but the injury is much more considerable when old and stale limes, charged with ammonia and bacterial products, are employed, as is frequently the case. In the American stockyards the skins are generally limed only for the necessary time to act upon the grease, and to swell and differentiate the fibres, and are then at once puered, drenched and preserved by “pickling.” For details of “pickling” see [p. 89]. It is very probable that the Pullman process of liming ([p. 137]) would answer well for fellmongered skins, as goods will keep for a considerable length of time uninjured after treatment with calcium chloride.

Where hides or skins cannot be used at once in the fresh state, there is probably no better method of preserving them than the use of salt. Although salt is not fatal to bacteria, it so slows bacterial growth, partly by its direct antiseptic effect on many organisms, and partly by withdrawing water from the skin, that well-salted skins can be kept in good condition for almost an unlimited time. Where it is only required to preserve goods for a week or two, a moderate sprinkling on the flesh side is efficient, but if they are to be preserved for any length of time, more thorough treatment is necessary. It is said that however carefully hides are salted they deteriorate if kept in this condition above twelve months.

The method of salting employed in the Chicago stockyards for “packer” hides may be taken as a good type of a thorough salting. The hides are first trimmed from useless “switches,” and any large portions of adhering fat are removed. The curing takes place in large and cool cellars, with concrete floors. The detail is well given in the following extract from the ‘Shoe and Leather Reporter’:—

“Great care is taken to make the sides of a pack higher than the middle, so that the brine which is made by the juices of the hide coming in contact with the salt will be retained. The brine can only escape by percolation and hence the fibre of the hides is thoroughly cured. The floor of a hide cellar is usually of concrete, and a pack is from 15 to 20 feet long and as wide as the space between the posts which support the floor above. The sides of a pack are built first to a height of from 4 to 6 inches; the cross layers are then put on, generally three on each side, two being inside and one having the butts drawn out to the edge. In a pack 20 feet long, the side layers will contain about 25 medium-sized hides each, and a cross-layer 12 or 14. To begin a pack a truck-load of hides is run along to the front of the place selected, one spreader grasps the butt and his partner the head of a hide, and together they carry it to what is to be the rear of the bed. The hide is then dropped, so that the folded back is parallel to and from 15 to 20 inches from the inside line of the posts, the head a trifle closer than the butt. The front man takes the dewlap and front shank in his left hand, and extends his right along the belly of the hide as far as is necessary to raise the edge, the rear man holding the flank with one hand and the hind shank with the other. They keep their legs well out of the way of the salt thrower, who with a single throw covers the whole hide, being particular that enough salt strikes against the edges held by the men to make a pronounced ridge when they are lapped down. A little salt is thrown on the hair surface and the butt folded over about a foot. The folded edge is then drawn out even with the outer line of the pack. More hides are placed the same way until the corner is high enough. After this, each hide is put further forward to make a level surface from rear to front, the heads at the front corner being folded back as the butts were at the starting place. The other side is built the same way, and then the cross layers are put on alternately until the pack is level, when sides are again built as before. In putting on the first hides of the cross layers, they are thrown over the edge, to lap back again when the salt is thrown on; the layer is then continued on to the front. The spreader who holds the butt does the guiding in every case. He drops the butt down at exactly the proper place, takes the upper flank and shank in each hand, sets one foot on the lower shank to keep it firm, and throws the one in his hands from him with considerable force. The man at the head watches his partner, keeps the folded hide taut, and drops it at the same time as the latter. He takes the fore-shank at the knee in the one hand and the upper head-piece in the other, and setting his foot on the lower side, throws the upper side forward simultaneously with the rear man. Two expert spreaders, accustomed to working together, spread a hide at a single throw, but some little straightening has to be done by hand before the hide is ready for the salt. A gang composed of two spreaders, one salt thrower and a salt trucker put down forty hides an hour. When gangs are doubled, two men do all the spreading; the other two place the hides where they can be got at conveniently. A double gang put down eighty hides an hour. The salt trucker brings the salt to the pack in box-trucks open at one end to permit the entrance of a shovel. The salt thrower keeps the edges and corners of the pack full of salt. He must see that every part of the flesh-side is well covered. Each hide takes two scoop shovelfuls of ground rock or coarse white salt, mixed with an equal quantity of old or second salt. The salt thrower throws the shovel forward and to one side and back again with a peculiar swinging jerk, causing the salt to fall regularly over the entire surface of the hide. The ease and rapidity with which a gang operates depends greatly upon the efficiency of the salt thrower. When the pack gets too high to be comfortable for the men, it is brought to a dead level and covered over with clean salt. It then presents a very neat and workmanlike appearance. Spreaders and salt throwers receive 20 cents an hour, and truckers get 17¹⁄₂ cents. When the temperature is kept at an even average, two weeks is ample time to cure the hides.

“In ‘taking up,’ two men strip the hides from the pack. As they were put down from the rear to the front, they are taken up in the reverse direction. No matter how much loose salt is lying on the top, the man knows exactly where to place his hand on a shank; as the hides are moved forward, the loose salt is thrown off toward the front. One man takes away the salt as it accumulates and trucks it to the salt bins, where it is mixed with new, to be used again. A ‘horse’ made of a network of scantling about 3¹⁄₂ feet wide by 6 feet long, and standing 2¹⁄₂ feet from the floor, is placed in front of the pack, on this the hides, flesh side down, are shaken to remove the salt that is clinging to them. This process requires four men, one at each corner. The hide is brought down heavily on the horse twice, and then spread on the floor flesh side up for examination by the inspectors, of which there are two, one representing the house and the other the buyer of the hides. They sweep off any salt that may be left, and examine for cuts, sores, brands, manure and grubs. They also see that the hide is properly weighed and classified. If the contract calls for a special trim it is now done. Two men then roll the hide, beginning by lapping over the shanks, head and neck. Then the sides are folded over and lapped again, leaving the roll 15 to 18 inches wide. The ends are thrown inward, slightly overlapping each other; a final fold is then given, and the hide is ready to be tied. Rope the size of clothesline is used for tying, and is cut into lengths of about seven feet. It takes three men to tie for a gang such as we have described. After tying, the neat bundles are weighed and loaded on the cars for shipment. A small tare is allowed the buyer. Ordinary workmen in hide cellars get 17¹⁄₂ cents an hour, and inspectors 25 cents an hour.”

About 25 per cent. of salt on the green weight of the hide is required for thorough curing. Rock salt merely crushed is frequently employed, but this is very liable to contain iron in the form of oxide and chloride, which causes the peculiar marbled markings known as “salt-stains.” It is therefore much better to use a white crystallised salt, though it is possible even in this case that stains may arise from the iron present in the blood. Some salt-stains appear also to be due to the action of pigment-bacteria, and not to contain iron. A reddening of the flesh side is often noticed in hides which have been kept in salt long or under unsatisfactory conditions, and is very frequent in wet-salted South American hides. Such hides are said never to produce so firm a leather as those which are sound.

Hides are not unfrequently cured by steeping in salt brine, instead of strewing with dry salt. This method is principally resorted to in order to give fictitious weight. Brined hides do not plump well in tanning, the leather is not so good in quality as from those salted with dry salt, and the cure is much less efficient.

Many hides are not only salted but also dried in order to preserve them. Not much detail has been published with regard to the methods used, which no doubt vary much in different places, but probably in some cases the hides are salted in pile and in others by brining, and then hung up to dry. The principal object of this drying is to economise weight and cost of transport, but it makes the hides much more difficult to wash and soften for tanning, and probably the crystallisation of the salt has a weakening effect on the fibre. Hides cured in this way are styled “dry salted.”

A large number of the hides of the small native cattle of India are imported into this country in a dry-salted condition. The following particulars of their cure are taken from a paper by the Author and Mr. W. Towse.[16]

[16] Journ. Soc. Ch. Ind., 1895, p. 1025.

Dry-salted, or, as they are commonly called “plaster cures,” such as those of Dacca and Mehapore, are thickly coated with a white material, which in the first instance is merely the insoluble portion of a saline earth used in the cure; though in many cases it is applied in larger quantities than necessary, with the simple object of giving weight. The salting is thus described by Mr. W. G. Evans, who some years since had considerable experience as a tanner at Cawnpore:—

“The salt used by the natives is a salt-earth; and is so called by them. It is found extensively in the districts of Cawnpore, Agra, Delhi, Lucknow, Patna, etc., and has no doubt something to do with the localisation of the hide-curing and kindred industries in these places. The mode of procedure used is pretty much as follows:—the salt-earth is mixed into a very thin paste, and this is lightly brushed on to the flesh side one day, and the hide allowed to remain over night under cover. Next day, for best hides, the same solution is again spread on the flesh side of the outstretched hide and rubbed into it with a porous brick, and then for legitimate salting, the hide is allowed to dry under cover. If for export, the saltings may be three or four, and the hides are treated out in the open, subject to the intense heat of the sun; which accounts for the number of hides which go back in the soaks in England and elsewhere.”

“We had a clause in our agreement with hide-factors, that any hides which did not come down to natural suppleness in two days in clean water were to be returned. Of arsenic curing I know nothing, and it is not so much in vogue as formerly. There is quite a trade in Cawnpore, Lucknow, Allahabad, etc., in treating old and inferior hides with new for export, and great efforts are made by native holders to get their stocks down before the rains commence, as they say, and rightly I think, that hides are not worth so much after the rains by 30 per cent. The peculiar latent moisture of the rains affects them very detrimentally.”

Under certain circumstances this mode of cure gives rise to extensive iron-staining of the skins, and analyses of the material scraped off Dacca and Mehapore kips were undertaken with a view to elucidating the causes of this injury. The following are the results of the analyses referred to, which were made upon the residue after the rather considerable quantity of fibrous organic matter, which had been scraped off with the cure, had been destroyed by ignition, together no doubt with traces of ammoniacal salts:—

The soluble salts of the Dacca cure were also analysed separately with the following result:—