Read Ebook: Leather: From the Raw Material to the Finished Product by Adcock K J

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Even the apparently trivial matter of removing the hide from the cheeks and face is economically important, for, unless those parts are removed so as to get the maximum surface, they are only fit to be cut off and thrown in a pit with other pieces and roundings which are made into glue.

Imperfect preservation is another serious form of damage to hides and one that cannot be easily remedied in some of the hot climates. Hides and skins may be simply dried, salted and dried, wet-salted, treated with arsenic solution and dried, brined or pickled with acid and salt; of these methods the last-named is the most effective, but is not practicable, or, at least, has not yet been applied practically to the cure of hides and calf skins. Salt is not available, or is too costly in most tropical countries; hence, Chinese, Indian, Mexican, Colombian, and Arabian hides and skins are generally exported in a dry condition. Even when the hides are dried under the best conditions in a cool and shady place, they are of less value than a fresh or a wet-salted hide, owing to a certain loss of gelatinous matter in softening them before they can be placed in the lime liquors. But it not infrequently happens that hides are dried by exposure to the hot sun, or perhaps in a strong current of air. In either case, the hide is much reduced in value and may be irreparably ruined. The effect of submitting hides to these conditions is that the exterior surface becomes rapidly dry and, naturally, contracted, so that the air or heat cannot reach the interior, which retains moisture. This moist inner layer may be quite thin, but it contains sufficient nutrient to develop putrefactive organisms, so that when the hide is soaked in water it practically falls to pieces. The effect of hot sun or heat of any kind is, of course, disastrous to raw hides and skins, and there have been not a few claims on shipping companies as the result of storing hides near the boilers of ships.

Salt is almost invariably used for curing both hides and calf skins, but though it is a good preservative it has one or two minor defects. It contains too much water, and is liable to contain traces of iron which is inimical to both raw hides and leather in process of manufacture. Common salt is also liable to cause stains which cannot be removed in later processes, and which are even accentuated in the tan liquors. One trade chemist attributes these stains to the presence of calcium sulphate or gypsum in the salt, which is converted into calcium phosphate by the action of the phosphoric acid in the nuclei of the hide on the sulphate of calcium. Another well-known technical chemist is certain that stains are produced by the growth of bacteria, and to prove his assertion prepared in gelatine several cultures from salt-stained skins. Practical men generally attribute the stains to the presence of blood on the hides or skins at the time of curing, and the majority of the stains are probably due to this cause, although the presence of calcium sulphate as an impurity of the salt would undoubtedly contribute to this defect. Blood contains a percentage of iron, and, with other extraneous matters, should be washed from the hides before salting them.

Fortunately, chemists have lately paid attention to the advantages of the use of pure salt in various industries, with the result that at least two chemically pure products are now available. The use of these salts should be general for the cure of hides and skins, as they are quite dry, and, therefore, easily spread. As a curing agent, they are much more effective and lasting than common salt.

The use of glauber salts is recommended by the International Commission for the Preservation, Cure, and Disinfection of Hides and Skins instead of ordinary salt, where the latter is unobtainable. The preparation of a sterilised salt, however, renders its export a practical proposition to almost any part of the world.

Although the loss due to bad curing and flaying is very great, it is quite small compared with the damage caused by natural defects.

The ravages of disease cause a great wastage of hides and skins, as animals infected with anthrax are immediately destroyed and cremated in all civilised countries, while, in Great Britain, foot-and-mouth disease is kept in check by the same drastic method. In many other countries, the infected cattle are isolated, treated with an antiseptic hoof-and-mouth wash and generally cured, as it is a mild fever which soon runs its course, although it is very contagious. The germ of foot-and-mouth disease has not yet been discovered, for the most powerful microscope fails to reveal its presence, but cattle readily show the complaint, as their hoofs and mouths become covered with swollen lesions.

Another kind of fever, known as "tick," was prevalent in the southern part of the United States, but this disease was eventually eliminated by systematically "dipping" the cattle three or four times a year. The cattle "dip" used effectually prevented the ravages of the fly which caused the disease. A similar method has of late years been adopted in South Africa, with the result that cattle-raising in that country is developing rapidly.

A peculiar natural defect is found in many South American goat skins, especially the Brazilian, which are often badly scratched by the animals rubbing themselves against cactus plants. Although, perhaps, more of an artificial than a natural defect, the scratches caused by cattle rubbing their hides against barbed wire constitute a serious, but easily avoidable, loss. Such a barbarous system of fencing ought never to be used.

The most effective way of getting rid of the pest is to destroy the larvae, either by cutting them out and crushing them under foot, or by piercing them with a hot needle. No satisfactory dressing has yet been found, but Prof. Carpenter states that sulphur dioxide is effectual, if a good method of applying it can be devised.

While sheep skins are immune from the attacks of the warble fly, they are often damaged by the blow-fly, lice, keds, and ticks; by scab caused by the action of a mite or acarus; and by "cockle," which causes a wrinkled grain. The origin of cockle is not definitely known, but it is a seasonal defect which begins to show on a large number of skins in December and does not disappear until the sheep are shorn in the following spring.

TANNING MATERIALS

Tanning materials are derived from the vegetable, mineral, and animal kingdoms.

The vegetable materials used are woods, barks, shrubs, leaves, and fruits, either in their natural state or in the form of extracts. The majority of the minerals have a more or less tanning effect on animal fibres, but the principal are basic chrome salts, formaldehyde, alum and salt. Titanium, iron, cerium and potassium salts also convert skins into leather, but are not yet used commercially.

The animal matters that will convert skins into leather consist of oxidised oils , fats, and brains .

Each of these classes of tanning materials has characteristic effects which render them easily distinguishable. The use of combinations of vegetable and mineral tannins has lately increased, and it is possible that the blending of the two classes of materials may produce an ideal tannage for certain classes of leather. In fact, this result is already claimed for a chemically combined tanning material which, according to the American patent, is prepared by the following method: 125 lb. of solid quebracho extract is dissolved in the same weight of hot water and allowed to cool; 16 lb. of commercial caustic soda dissolved in two or three times its weight of water is added, and the mixture agitated about half an hour; 150 lb. of chromium sulphate is then added. In this way an insoluble tannate of chrome is produced, but, on boiling and agitating, it changes to a greenish brown colour and forms a sulphotannate of chrome. The combination of the alum tannage and gambier has been used successfully for years past. Another combination which has given good practical results is the tannage with alum and chrome salts in the manufacture of glove leather.

The vegetable materials containing tannin should be arranged botanically, but the following classification is simpler for practical purposes.

NATURAL TANNING MATERIALS

TANNING EXTRACTS

Of these materials, only about twenty are of much importance commercially, the principal being oak, chestnut, quebracho, hemlock, valonia, gambier, myrobalans, mimosa or wattle, sumach, mangrove, divi-divi, spruce, larch, and babool.

OAK BARK is still an important material, but is rarely used alone. The bark from English oaks contains from 8-14 per cent. of tannin as estimated by the impregnation of a standardised hide powder in a given quantity of the tanning material in solution. Owing to its weakness in tannin compared with other materials, oak bark tans very slowly. Used for sole leather, it would not produce the essential quality of firmness and solidity, and it is now customary to use a stronger tanning material, such as valonia, or valonia extract, or gambier in the latter stages of the process. This is the nearest approach to the pure oak bark tannage of former days, and, if carefully regulated, is a great improvement on the old method.

If dressing hides and calf skins required for boot upper leather are bark-tanned, the tannage is often completed in a sumach liquor, the object in this case being to lighten the colour so that the leather can be dyed evenly.

In England, oak bark is harvested in April and May, when the sap rises in the tree. Rings are cut round the tree soon after it is felled and the bark is peeled from the tree with a special tool which is forced between the bark and the wood. It is peeled in narrow strips about 3 ft. in length, and on delivery to the tannery is stacked in huge ricks. If harvested in a good, dry condition, the bark is said to improve with age, although analytical tests have shown that there is always a certain loss of tannin. An old rick is much darker in colour than a new one, owing to exposure to the air. Coppice bark from young trees is preferred by tanners, as it is free from ross and generally contains more tannin than the rough bark.

In view of the modern demand for materials in extract form, English oak bark would almost certainly be preferred in the form of a concentrated liquid, if the supply of the raw material was plentiful within a limited area. During the last few years it has not met with a ready sale, owing to the large supply of other materials, but it would doubtless regain some of its former popularity if it were prepared in the form of an extract containing about 25 per cent. of tannin. The only oak bark extract on the market is the American chestnut oak .

OAK WOOD is very largely used for the manufacture of tanning extract, especially in Hungary and Canada. The extract contains from 24-28 per cent. of tannin, and is extensively used in the tannage of heavy leathers, as it strengthens the liquors and hastens the process, while keeping the quality of the leather at a high standard.

VALONIA is the acorn cups of an oak tree which grows abundantly in Asia Minor and the Greek Archipelago. No other part but the acorn cups is exported. The harvest in Asia Minor takes place in August, when the fruit ripens and the cups can be easily beaten from the trees. They are left to dry on the ground and are then sent to stores in seaport towns, and principally to Smyrna. The drying is still further completed in spacious warehouses, where the cups are spread out and turned over until fermentation ceases. During this process the acorns shrink and are rejected. The cups should be perfectly dried and very hard before export. The Turkish valonia contains from 30 to 35 per cent. of tannin, and is of much better quality than the Greek, which is usually harvested before it is ripe, and, therefore, contains the acorn. As the acorn has practically no tannin value, the Greek valonia contains only 25 to 28 per cent. tannin. There are several grades of valonia, the best going to Russia, Austria, and Italy. English tanners seem to prefer the lower qualities at present, probably because the price is much less than that of the best grade.

The beard of the valonia cup is much richer in tannin than the shell, and, as several of the spines become detached during the storage of the material, there is always a certain quantity of beard on offer. This may contain up to 42 per cent. of tannin, but its price is usually the same as that of ordinary valonia. Of late years, very large quantities of valonia have been made into extract at two works in Smyrna. The production of extract will no doubt increase, with a corresponding reduction in the export of the raw material. The great advantages of the extract over the natural cup are its superior strength of tannin , easier solubility, uniformity of quality, lower cost per unit of tan, and guaranteed purity.

Valonia is well adapted for the tannage of sole leather in conjunction with oak bark, for it deposits a heavy bloom , imparts weight and solidity, and increases the resistance of the leather to moisture.

THE CHESTNUT TREE probably provides the next tanning material of importance. This must not be confused with the chestnut oak, an American tree which also yields a very useful tannin. The chestnut is indigenous to the South of France and Italy, where the forests have been considerably reduced in size to meet the great demand for this popular tanning material. The greater part of the denuded forests have not been replanted with the chestnut, as the land has been put under cultivation whenever possible. A further depletion has been caused by the ravages of an insect, which turns the interior of the wood quite black and renders it unfit for tanning purposes. It will, naturally, be several years before the supply is exhausted, even if no reafforestation is undertaken. As it is the most important tanning material grown in France, and the chestnuts are used as a food, steps may be taken to cultivate the trees on areas unsuitable for agriculture. Liquid chestnut extract contains from 30 to 32 per cent. of tannin, and, when decolorised, gives a light brown colour to the leather. It is rarely, if ever, used alone, but generally in conjunction with quebracho, valonia or myrobalan extracts.

THE CHESTNUT OAK TREE is indigenous to America and the wood yields a very good tanning extract, containing up to 30 per cent. of tannin. This material is the principal tanning agent used in America, where the tannages are roughly classified in three sections: oak, hemlock, and the union . American tanners also use other materials to a smaller extent, chiefly for blending with the principal tannins. Quebracho and spruce extracts are specially favoured.

QUEBRACHO COLORADO is a tree indigenous to South America, the best wood for tanning purposes being found in the Gran Chaco district in the north of Argentina, and in Uruguay. The wood contains from 17 to 22 per cent. of tannin, and is so hard and heavy that it sinks in water. In fact, its name is derived from two Portuguese words meaning "axe-breaker."

After felling the trees, they are cut up into logs about 4 ft. in length and either exported in this state to Hamburg, Havre, and Liverpool, or sent to the numerous factories in close proximity to the forests to be made into extract, in which an enormous trade has of late years been developed. Very little of the natural material is now used, as, even after cutting the wood into chips, the tannin is extracted only with great difficulty, whereas the extract can be treated with sulphites, alkalies, or neradol to render it easily soluble, besides which the concentration of the material raises the percentage of its tannin to 65 or even 70 per cent. Owing, perhaps, to faulty preparation, this tanning extract was not well received at first, but it is now among the principal tannins and increases in importance every year.

MYROBALANS is the unripe fruit of an Indian tree and contains from 35 to 40 per cent. of tannin which gives a light colour to leather. This material is useful both for light and heavy leathers, but is generally used in admixture with other tannins. It deposits much bloom and is largely used for brightening the dark colour produced by other tannins. A large quantity of this material is now made into extracts, which are more convenient to handle and more uniform in strength of tannin. Natural myrobalans have the appearance of shrivelled nutmegs, except that they are yellowish in colour; they are very hard and require a special milling machine to reduce them to powder. The quality of myrobalan nuts varies in different districts, the best being Bhimlies and Jubbalpores.

SUMACH is a valuable tanning material, and is used for a large proportion of the light and fancy leathers. It is a small bush plant which grows in Italy, Spain, Southern France, America, and Algeria, but of the numerous varieties the Sicilian is by far the most important. Sumach is one of the few materials cultivated on an extensive scale; most tanning materials are derived from natural sources and, chiefly owing to the length of time before trees reach maturity, it would not be a paying proposition to cultivate them. The sumach shrubs are propagated from small cuttings and the leaves can be picked at the end of the first year, but it is better to allow the shrubs to become more firmly established before stripping them. The leaves are dried and sometimes exported whole, especially for the use of the silk manufacturers in Lyons; but they are more often ground to a fine powder. All sumachs should be ventilated to remove foreign matters and all traces of iron, which would cause dark bluish stains on the leather. "Ventilation" is effected by passing currents of air, preferably with a fan, through a narrow room, when the pure sumach is sent forward, while the heavier particles of dirt and small pieces of wood remain behind. Sometimes the process is repeated, and the best brands of sumach are generally described as "pure, extra ventilated." As far as possible, the male plants are cultivated in Sicily, where the best sumach is grown. Female sumach , grown in parts of Italy, is weaker in tannin than the male, but is rarely sold separately. The serious amount of adulteration formerly practised by the admixture of inferior plants, and particularly of lentisco led to the Italian Government taking strong action a few years ago, and it is now possible, for a very small sum, to have any consignment inspected and analysed by the Government. Lentisco is now sold separately and is used for common work.

Sumach has been successfully introduced into Australia, but its development is retarded owing to difficulties of labour, which render competition with the European product almost impossible. An inferior sumach is grown in America, chiefly in the State of Virginia. It contains from 15 to 20 per cent. of tannin, and produces a darker coloured leather than Sicilian, the best qualities of which contain 27 to 30 per cent. of tannin.

A useful test for finding out if a sumach has been adulterated is to treat a small quantity with strong nitric acid, which destroys the structure of the leaves. The mass is washed and neutralised with an alkali, when the appearance of the midrib and veins of the leaves of the common adulterants are easily recognised.

GAMBIER or TERRA JAPONICA is a crude extract of a shrub indigenous to the Malay Peninsula. Nearly the whole of the production is shipped from Singapore. The leaves and twigs are boiled in an iron vessel, and when the mass has become syrupy it is strained through a rough sieve into a shallow tub, where it is cooled. The liquor is stirred while cooling and rapidly thickens. Before it sets, it is cut into 1 in. cubes and thoroughly dried. Good qualities contain from 50-65 per cent. of tannin. An inferior product, called "block gambier," is made by allowing the syrupy mass to set in large blocks weighing about 2 cwt. each. These are packed in coarse matting. The strength of tannin varies from 30 to 40 per cent. Gambier is a good tanning material, but its use has been declining for some years past owing to its being frequently adulterated with sago and other farinaceous plants. However, a pure gambier extract, manufactured on the latest scientific principles, has been placed on the market, and there will undoubtedly be a revival of the use of this valuable tannin. This pure gambier is prepared at Asahan, in Sumatra, and is guaranteed to contain a minimum of 38 per cent. of tannin.

Gambier can be used to advantage in keeping up the strength of bark liquors in the tannage of sole leather and hastening the process, while it may be used alone for the tannage of boot upper leather and dressing hides. It produces an exceptionally mellow and plump leather. It is preferable, however, to complete a gambier tannage with a little oak wood or quebracho extract, in order to fix the tannin principle of gambier, which, perhaps on account of its viscosity, does not readily combine with the fibres of the skin.

The bark contains from 20 to 45 per cent. of tannin. The cultivation of wattle in Australia seems to have declined, owing to the high cost of labour and inability to compete with the mimosa bark imported from South Africa, where it is cultivated on a very large scale and where labour conditions are more favourable for the growers, as is clearly shown by the fact that the wattle growers in Australia successfully petitioned the Government a short time ago to place a duty of ?1 10s. per ton on the imported bark.

The introduction of the industry into South Africa was quite an interesting adventure. A Mr. Vanderplank brought the seeds from Australia to England about seventy years ago, and afterwards took them to South Africa, where, in recognition of certain services a few months after his arrival, he was granted a farm by the Dutch Government. He then planted the seeds of the black wattle, which grew so well that it was only a question of developing the industry. It was some years before any African bark was exported, and only ?11 worth was shipped in 1886. In 1911, the exports had risen to ?288,000.

DIVI-DIVI is the dried pods of a Central American tree. It has also been successfully cultivated in India. The pods are rich in tannin, containing anything from 40-50 per cent., but its value is discounted by its liability to fermentation, which, however, may be checked to some extent by the use of antiseptics, such as carbolic acid, formaldehyde, or by the addition of synthetic tannin, neradol. If this tendency to fermentation and oxidation of the colouring matter could be checked completely, divi-divi would be a valuable material, as it makes a firm leather of good colour. When dried, the pods curl up in the shape of the letters S and C. The tannin is found in the husks of the pod. The seeds, which contain no tannin, are so hard that it has not yet been found profitable to extract the oil from them. Very similar tanning materials to divi-divi are cascalote, indigenous to Mexico, and algarobilla which grows in Chili. Cascalote is chiefly used by Mexican tanners and is rarely exported. Algarobilla is not available in large quantities, otherwise it would be largely used in Great Britain, as it does not ferment so readily as divi-divi, and is even richer in tannin.

HEMLOCK is an important tanning material, both the bark and the wood being extensively used in America. The wood is now generally converted into extracts in factories built near the principal forests. Of late years, this extract has been imported into the United Kingdom in fairly large quantities, in order to produce a cheap red sole leather to compete with the American hemlock-tanned leather. It contains only about 25 per cent. of tannin, but its value is increased by its contents of insoluble non-tannins, which give weight and solidity to the leather. Hemlock really gives a strong, durable leather, but in America the practice of using artificial weighing materials, such as glucose and Epsom salts, with a reduced quantity of tanning material, has considerably lowered the value of this leather.

In addition to the materials described, there are several of minor importance which can only be briefly mentioned.

CANAIGRE is the tuberous root of a dock plant indigenous to Mexico and the Southern States of America. It is fairly rich in tannin and yields a moderately firm leather. It contains too large a proportion of starch, however, and cannot be described as a really satisfactory tannin. Moreover, it is not harvested economically and the only way to make a satisfactory tannin of it is to convert it into extract and remove the starchy matters near the source of supply, if anyone dare take the risk of establishing a factory in Mexico.

BABLAH or BABOOL grows in India, Egypt, and the Sudan. The bark of the babool tree is one of the principal tanning materials used in India for hides, calf, and sheep skins, which are sent in large quantities to Great Britain in a rough-tanned state and dressed there. It contains 15-20 per cent. of tannin, which readily oxidises in the leather in contact with light, turning into a bright pink colour. It also seems to weaken the fibres of animal tissues, and, for that reason, babool-tanned leather was condemned by the Society of Arts Commission on Bookbinding.

The pods contain from 20-30 per cent. of tannin and give a mellow and plump leather similar to that produced by gambier. The bleaching of the material is troublesome, and it is probable that its use would be increased if the tannin were prepared in the form of extract.

Commercial tannic acid, used for medicinal purposes, is prepared from galls or excrescences on oak trees growing in Asia Minor. These galls are caused by an insect puncturing the small branches and producing abnormal growth in the perforated parts. The acid is gallotannic, which, if used for making leather, would produce a soft, spongy, and nearly white leather. This tanning material is used a little by Near Eastern tanners, but the result is unsatisfactory.

CLASSIFICATION OF TANNING MATERIALS

Tanning materials are divided into two main classes: Pyrogallol, Catechol. The pyrogallol tannins give a bluish-black colour, and the catechol tannins a greenish-black, with iron salts. Bromine water does not precipitate pyrogallols, but causes a precipitate with catechols. Pyrogallols yield ellagic acid , which improves the waterproof qualities of leather. On the other hand, catechols contain a large proportion of insoluble reds, or phlobaphenes, which are deposited between the fibres and thus give solidity to the leather. Pyrogallol tannins give a light coloured, soft leather, and can be used alone satisfactorily; but heavy leathers, such as sole and belting, need a blend of both kinds of tannin. The pyrogallol tannins include sumach, chestnut, myrobalans, divi-divi, oakwood, algarobilla, chestnut oak, willow, and galls. The catechols include quebracho, gambier, hemlock, mimosa or wattle, mangrove, larch, birch, canaigre, and cutch. Oak bark and valonia contain some of the properties of both classes of tannins.

There are sub-divisions of these two classes, arranged according to chemical tests, by which one tanning material can be distinguished from another.

TANNING EXTRACTS

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