Leather: From the Raw Material to the Finished Product

by K. J. Adcock

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.

The natural dyes are further developed with "strikers," which mainly consist of metallic salts. Iron, copper, and zinc sulphates, nitrate and acetate of iron, bichromate of potash, and titanium salts (titanium lactate, titanium potassium oxalate, and tanno-titanium oxalate) are the most important. The lactate, sold commercially under the name of "corichrome," is especially suitable, as, unlike the mineral acid salts, especially the sulphates, it has no destructive effect on the fibres of the leather.

Dye-woods are now concentrated in the form of a paste, or dry extract, the latter being the more reliable. They are also very convenient to use and dissolve, while mixtures are, of course, easily prepared. A good tan shade on Cape sheep can be obtained by mordanting the leather with a solution of 1 lb. of bichromate of potash for every 100 lb. of leather, drumming it in 8-9 lb. of pure gambier, and then with a mixture of cuba wood extract, 1 lb.; fustic extract, 3/4 lb.; Brazil wood extract, 1/2 lb.; and logwood extract, 1/2 oz. After drumming the leather in this dye liquor for about an hour, the colour is developed with corichrome. If a darker shade be required, the leather can be treated with a suitable basic dye. After dyeing the leather, some dressers only fat-liquor it with egg-yolk and a small quantity of olive oil, while others prefer to re-dress it with a similar mixture to that used for tawing, namely, alum, salt, egg-yolk, and flour; but, where titanium salts are used, the latter method is not essential, because titanium has tanning properties. When dry, the leather is ready for finishing, but it is advisable to keep it in store for a few days before packing it in damp sawdust or sprinkling it with water to prepare it for the operation of staking. Anything more unlike leather would be difficult to imagine at this stage, but, after stretching the skin in the staking machine, or by drawing it over the upright stake, the dry, stiff, and shrivelled leather is reduced to a very supple condition. The flesh side of the leather is then pared with the moon-knife, or in the shaving machine, to equalise the thickness. In some works, a special tool which pares the leather on a flat table is preferred; this particular operation is called "doling." The flesh side is finished by fluffing it on the emery or carborundum wheel (Fig. [33]). Finally, the grain is brushed and polished with the glass sleeker, or ironed.

Chamois leather has been largely used for gloves of late years, but this leather has the defect, in common with suède leathers, of getting soiled much more quickly than grain leathers, such as kid, lamb, or Cape sheep. Nevertheless, suède and chamois gloves are likely to remain fashionable to a more or less extent. The manufacture of chamois leather is described on page [144]. Sun-bleached skins are the best for dyeing, especially if delicate shades are wanted. The frontispiece shows a field covered with skins bleaching in the sun. Chemically bleached leather is likely to become discoloured after dyeing. Defective skins are often dyed with pigments (dust colours), and this system is also applied to skins which have to be dyed such delicate shades as cannot be produced by wood or aniline colours. Although it gives attractive results to the eye, and certainly covers up any defects of the grain, this method of dyeing is not altogether satisfactory, as the leather remains unpleasantly dusty in wear for quite a long time.

The dyeing of chamois with wood-dyes or coal-tar colours is by no means easy, but this method gives the best results when successful. The grease must first be removed from the leather with a solution of 5 lb. of borax or 3-1/2 lb. of soda for every 100 lb. of leather. If the leather is still greasy on the surface, a further quantity of soda or borax is given, after which the leather is well washed in warm water, sumached, rinsed to remove the particles of sumach, and mordanted with titanium salts. The dyeing is then done with anilines or wood-dyes, or a combination of both, and this is followed by fat-liquoring with egg-yolk and a sulphonated oil. The finishing operations are staking and fluffing.

To get a good, fast black on chamois and suède leathers is one of the difficult processes in the leather trade, although it is easier to get a good black on alumed or chromed leather than on vegetable-tanned. Alumed leather is washed in a solution of borax or carbonate of ammonia to remove uncombined dressing in order to prepare it for dyeing. Chrome-tanned suède leather does not need this preparation. The leather is first mordanted with dye-wood extract, of which a suitable mixture is logwood and fustic, or logwood and quercitron, in the proportion of 4 lb. and 2 lb. to every 100 lb. of leather. After drumming the leather in this solution for about an hour, a weak solution of copperas (ferrous sulphate) and bluestone (copper sulphate) is added, and the milling is continued for twenty minutes, when the leather is well prepared to receive the black dye. Instead of the iron and copper salts, corichrome is often preferred, as it is quite safe to use, whereas iron salts have a destructive action on the fibres of the leather, unless the precaution be taken to mordant the skins with a good quantity of dye-wood extract. Following the application of the iron or corichrome striker, the leather is dyed with suitable aniline black (leather black, or corvoline) and finally fat-liquored to nourish the leather, and to fix and intensify the black. This recipe also gives good results where the skins are dyed only on the flesh side, the solutions being applied with a brush.