Leather: From the Raw Material to the Finished Product

by K. J. Adcock

This kind of leather is well known to the general public under the name of wash-leather, but it is, perhaps, not so widely known that there is practically no real chamois leather available, since the species of animal bearing this name is almost extinct. Nowadays, "chamois," or "shamoy" is made from the flesh split of sheep skin, and the method whereby the leather is produced is described as the oil tannage. The chamois leather dresser may also do the preliminary work of fellmongering, but more often he receives the pelts, or "fells," from the fellmonger. Although the pelts have been in a lime liquor known as the fellmonger's "gathering limes," the process of liming has to be continued and carefully regulated. Too much liming makes the pelts loose, owing to the development of bacteria. This effect would cause the finished leather to be soft and spongy. On the other hand, under-liming fails to remove sufficient of the cement substance which binds the fibres of the pelts; consequently, the leather produced from these pelts is thin and somewhat gristly. After being limed, the skins are "cobbed" (i.e., the bits of wool or hair left on by the fellmonger are removed) and the pelts are then fleshed by machinery. The next operation, splitting, is very important and requires skilful attention to get good results. The Reeder machine is largely in use, but the band-knife machine is also suitable. The top half of the sheep skin (i.e., the grain side) is utilised for the manufacture of skivers, while the under portion, or flesh side, technically called "lining," serves as the raw material for chamois leather. The best linings are generally sorted out for making into parchment, which, of course, commands a much higher price than chamois. Linings for chamois are then submitted to the operation of either re-splitting or frizing, the object being to remove the loose tissue lying between the grain and flesh. The pelts are re-split in the case of cheap chamois, but frized if intended for choice finish. Frizing is an operation peculiar to the making of chamois and glove leather. It is done with a very sharp knife, similar to the fleshing knife, and on a more upright beam than that used by tanners. The work requires great skill, frizers being among the best paid workers in the trade. Frizing done, the linings are freed from lime by washing them in the drum tumbler, or paddle-vat, through which cold water is allowed to flow continuously for two or three hours. When lamb skins, which are too thin to split, are made into chamois leather, the grain is removed by frizing.

A quick and effective method of deliming is to treat the skins in a weak solution of lactic acid. Some dressers use a drench of pea-flour or bran. The mild acids produced by the fermentation of these materials not only neutralise the lime but also reduce the gristly nature of the skins to a soft, supple condition. The bran infusion is slightly warmed to hasten the process of fermentation, but the temperature must not exceed 100° F. (32° C.). The linings are then rinsed in cold water and sent to the stocking machines, in which they are kneaded until they become quite soft. Either the faller-stocks or the mechanical pushers (Fig. [14]) may be used, the latter being the more modern machine. The operation may require from four to ten hours, the completion being determined by the condition of the skins. "Samming" follows stocking, and for this purpose the linings are hung up in the drying shed until thoroughly dripped, but not dried. In this slightly damp condition they are then prepared for the next process of oiling, which is the most important part of chamois-dressing, as it converts the perishable raw linings into leather. In the United Kingdom, cod oil (generally Newfoundland) is used exclusively, and gives best results. Whale and shark liver, or menhaden oils are often used abroad.

The linings are placed in a tub or vat, a few at a time, and oil is poured over each layer until a sufficient number has been treated to fill the stocking machine. The stocks are run for half-an-hour, or until the oil has penetrated the linings, when they are put back into the vat, where they remain for about an hour. They are then re-stocked, taken into the shed to samm, re-oiled in the vat, and stocked again. These processes are repeated until the skins are thoroughly impregnated with the oil, when they are dried in a warm stove. The skins are not yet converted into leather, which only occurs in oil-dressing, after the oxidation of the oil. This is effected by spontaneous heat, the dry oiled skins being heaped in boxes and covered. Strict attention has to be paid to avoid over-heating the skins, which are turned over and changed at intervals. When the leather ceases to heat, the process is completed. It is then dipped into hot water and mechanically pressed, to remove surplus or uncombined oil, which is collected and sold under the name of "sod oil." The leather is afterwards drummed in warm water and finely cleansed in an alkaline solution, potash, soda, or borax, chiefly the first named, being used for this purpose. They are then rinsed in water, dried, damped, and softened by staking. At this stage, the best skins are sorted out for the glove makers. The others are finished for wash-leather by re-staking, paring with the moon-knife, and smoothing both sides of the skin with a scurfer, or fine pumice-stone. In the warehouse, they are damped, stretched out, piled up, and kept fully extended by placing heavy weights near the edges of each pile of skins.

Gloving Leather

Progress in the art of making leather for gloves has been rapid during the last few years; but further important developments are expected, particularly in the process of tanning. Practically the only method that has been used for many decades for converting kid, lamb, and sheep skins into gloving leather is that known technically as "tawing," which consists of treating skins with alum, salt, egg-yolk, flour, and a vegetable oil. These substances change skins into extremely supple and "stretchy" leather, but when this is made into gloves it is far from ideal in wear, because it fails to keep the hands warm in cold, wet weather, it is easily soiled and cannot be cleaned without great expense, and it is not very strong in texture. Combination tannages have lately been produced, however, which remove the defects of alumed leather. By means of a light chrome tannage after tawing, the leather is strengthened and made more resistant to water, and can be cleaned with a damp rag or sponge. By tanning skins with the formaldehyde process, or with Neradol, the artificial tannin, for making into suède leather, the finished article is not only washable but also resists the action of alkalies and soap. The adaptation of combined tannages in the manufacture of gloving leathers has only lately been developed, and further improvements will doubtless be effected before long.

Lamb, kid, goat, and sheep skins constitute the raw material for gloving leathers, although deer and antelope skins are also used to a small extent. Real kid skins are the best wearing dress gloves, but the great majority of so-called "kid" gloves are made of lamb skins. The raw kid and lamb skins are chiefly of European, Arabian, and Indian origin. Sheep skins from the Cape provide the raw material for a large number of men's gloves, and leather of very good quality can be produced from the best grades. Most of the skins are preserved by drying, or by salting and drying, although some kid skins are wet-salted and packed in barrels for export. It would save leather-dressers a great deal of trouble if they could always get wet-salted skins; but the object of drying them is to reduce the weight and lower the cost of freight. Soaking is done by methods already described (p. 65). Loose flesh and pieces of fat are cut off in order to facilitate the action of the depilitant, of which the best for glove leather is undoubtedly a paste of lime and red arsenic. Seven or eight parts of lime to one of arsenic is a satisfactory proportion, the quantities to be mixed depending on the number of skins to be treated, as a fresh mixture should be made for each lot, or "pack" as the tanner terms it. The lime should be well broken up, or, better still, pure powdered lime should be used and the red arsenic well mixed with it; a little water is then added to slake the lime gradually, and the mixture is stirred to promote chemical reaction. The compound is further diluted with water until it has the right consistency and the colour has changed. The reaction generates great heat, and the "paint" should, therefore, not be used at once. The flesh side is mopped with the paint and the skins are folded flesh to flesh. After a few hours, or as soon as the hair or wool is loosened, the skins are dehaired or dewoolled. The hair or wool is not allowed to come into contact with the depilitant, otherwise it would be damaged. In large yards, the white hair is separated from the coloured, as it is worth nearly twice as much. Wool is sorted into different qualities, of which the number may vary from four to eight, or even nine, according to the class of skins treated. The pelts are then thoroughly washed and placed in lime liquors, where they remain for one or two weeks, being hauled and set in the usual manner. Fleshing and piecing or trimming are the next operations, and then follows the very important process of puering, which, in the case of glove leather, must be thoroughly done so as to reduce the pelts to a very soft and flaccid condition. Success in the making of glove leather depends largely on the "puering" process.

In most of the English tanneries a decoction of dog manure is used, at a temperature not exceeding 90° F., but on the Continent the artificial puer, oropon, is preferred. It is much safer to use and more uniform in its action than excrement, which develops bacteria rapidly in contact with gelatinous pelts, and could ultimately destroy them entirely. After puering them, the pelts are well washed and submitted to the process of drenching, which consists in putting the skins into a warm infusion of bran or pea-flour and leaving them covered until the following morning. The slightly acid fermentation causes the pelts to rise to the top of the vat. They are pushed into the liquor again with a pole and stirred round. This is repeated three or four times to prevent damage to the grain. The process is often done in the paddle-vat (Fig. [25]), in which the bran liquor is circulated for several hours before the pelts are allowed to remain quiescent. Drenching thoroughly purges the pelts of the last traces of lime, and puts them in suitable condition for being made into leather. The pelts are then rinsed in tepid water and "scudded" on the grain with a slate or vulcanite tool, shaped somewhat like a dehairing knife. The scud removed consists of dirt, dissolved lime salts, short hairs, and pigment. Machines are rapidly replacing manual labour for this operation.

The alum tannage, known technically as "tawing," is largely used for kid and lamb gloves. The tawing mixture is composed of alum, salt, egg-yolk, and wheaten flour. The proportions used vary considerably in different tanneries, but the following is a typical recipe: 4 lb. alum, 2 lb. salt, 1 lb. salted egg-yolk, or the yolks of twenty fresh eggs, and 5 lb. flour for 100 lb. of pelts. The flour is made into a paste, the egg yolk is diluted in warm water and mixed with the flour, the salt and alum are dissolved and added, and the mixture thoroughly stirred. A suitable quantity of water (about 2 gals. per 100 lb. pelts) is then placed in the drum tumbler, the tawing mixture is added, and the drum revolved for a few minutes before putting the pelts in. The process is completed in two or three hours in the case of thin skins. It is a good plan, however, to leave them at rest in the drum for a day, after which they are piled up overnight to allow further combination of the tawing materials with the fibres of the pelts. The leather is then dried out completely, damped in clean sawdust, or by sprinkling with water, levelled by shaving if necessary, staked over an upright knife fixed in a wooden stand or by machine, and dried in a hot stove. In this condition, or in the "crust," as dressers term it, the leather is allowed to remain several weeks to "age," a most essential process for the production of soft, and supple glove leather.

Dressing and dyeing are begun as soon as the leather is satisfactorily aged. The skins are uniformly soaked in warm water, dyed, and re-dressed with egg yolk ("re-egged"), to which a small quantity of olive oil, or a sulphonated oil, is added. Some dressers prefer to give the second tawing mixture before dyeing, but the advantage of dressing the leather after dyeing is that the colour is securely fixed. In "re-egging," many dressers use a similar mixture to the first dressing. The dyeing process is of great importance, since the colour must be fast. The leather is dyed either in the drum or on a convex table. In the former case, the leather is naturally coloured both sides, while, in the latter, it is stained with a brush on the grain side only. Staining is the more difficult method. Kid glove leather may be dyed with aniline colours, or, as more generally practised, with natural dye-woods as a base and aniline dye for top-colouring. The great advantages of the latter method are economy in dye-stuffs and increased depth of colour. The skins are first prepared for dyeing by brushing with, or drumming them in, an alkaline solution. Stale urine was largely used for this process, but ammonical salts are now generally preferred, if only for sanitary reasons. The skins are then drummed or paddled in, or brushed with, dye-wood liquids which have been carefully strained. A large selection is available, including fustic, cuba wood, saffron, peachwood, logwood, sappan wood, cutch, Persian berries, gambier or terra japonica, and golden tan bark.

Light and medium brown can be obtained from these dye-woods without the aid of aniline colours; but for dark shades, and to increase the brilliancy of other colours, a top dye or coal-tar dye is often given.