DRESSING LEATHER.

Hides which are intended for purposes where softness and flexibility are required, as for instance, for the upper-leathers of boots, and for saddlery purposes, are called "dressing" or "common" hides, or, if they are shaved down to reduce their thickness before tanning, they are denominated "shaved" hides. Hides for this purpose are limed much in the same way as has been described for butts; but if they are required very soft and flexible, a somewhat longer liming is permissible. After unhairing, fleshing, and washing in water, they are usually transferred to a "bate," composed of pigeon- or hen-dung, in the proportion of about 1 peck to 25-30 hides.

In this they are retained for some days, being handled frequently. They completely lose their plumpness, and become soft and slippery; the caustic lime is entirely removed; and the remaining portions of hair-sheaths and fat-glands are so loosened that they are easily worked out by a blunt knife on the beam. This final cleansing process is called "scudding." The theory of the action of the "bate," or "pure," as it is sometimes called, is somewhat imperfect. It is frequently attributed to the action of ammonia salts, and phosphates, contained in the fermenting dung. Ammonia salts certainly will remove caustic lime, free ammonia being liberated in its place, and weak solutions of ammonia sulphate or chloride will rapidly reduce hides, and remove or neutralise the lime. The phosphates in dung are mostly, if not entirely, in the form of lime phosphate, which is quite inert. In point of fact, the process seems to be a fermentive one, the active bate swarming with bacteria; to this, rather than to its chemical constituents, its action must be attributed. The bacteria act not only on the organic constituents of the dung, but on those of the hide, producing sulphuretted hydrogen, together with tyrosin and leucin, and other weak organic acids, which neutralise and remove the lime, and, at the same time, soften the hide by dissolving out the coriin, and probably also portions of the gelatinous fibre. The truth of this theory is supported by the fact that, in warm weather, the activity of the bate is greatly increased, and that, if one pack of hides is over-bated, the next following is much more severely affected, the hides having in fact themselves furnished food for the multiplication of the bacterian ferment from the destruction of their own tissues. It also explains the effective use (as a substitute) of warm water with a very small portion of glucose, which, in itself, would be insufficient to dissolve the lime, but with a small quantity of nitrogenous matter, forms an excellent nidus for the multiplication of these organisms. An American invention for bating is the use of old lime-liquor neutralised with sulphuric acid, an idea which is much more scientific than would at first sight appear. Old lime-liquors, as we have seen (p. 143) contain much ammonia and weak organic acids, such as caproic, amidocaproic (leucin), and tyrosin. On adding sulphuric acid, the lime forms an inert sulphate, and the sulphate of ammonia and the weak organic acids which remain dissolved are just what are required in a chemical bate. The lime-liquor should of course be filtered or settled clear before using, and enough acid added barely to neutralise the lime, and the liquor again settled or filtered. By this means both the dissolved gelatin and the iron of the acid will be got rid of. The liquor might then be slightly acidified before use. The writer has no experience of the method, but imagines that used as described it might be worth trying, although it would have a very unpleasant smell. In this connection may be mentioned the fact that, when bran drenches are used, in which lactic acid is developed, the butyric fermentation is liable, in hot weather, to take its place, and as butyric acid is a powerful solvent of gelatinous tissue, and the dissolved tissue itself feeds the fermentation, rapid destruction of the skins is the result. Cleanliness, scalding out of the drench vats, and washing the bran before using with cold water to remove adhering flour, are useful precautions.

If the removal of the lime be the only object aimed at in bating, the ordinary process is most wasteful, as well as disgusting, from the loss of pelt it entails. It is easy to find chemical reagents which will remove the lime; but the resultant leather has been found wanting in softness, and it is probable that the solution of the inter-fibrillar matter is in many cases advantageous. Probably one reason for the non-use of such chemicals is their expense. Maynard has patented the use of sulphurous acid for the purpose. If sugar, glucose, or ammonia salts be used, and the alkalinity of the solution nearly neutralised after each lot of hides by common vitriol, the same liquor may be used again and again. In this case, if iron is contained in the acid it will be precipitated by the ammonia and must be settled out. The writer is convinced, from his own experience, that with suitable tannage such bating would yield better weights and quite as satisfactory leather for many purposes as the ordinary mode. French tanners, by the free use of water, and careful working at the beam, and the employment of very weak liquors at the commencement of tanning, make excellent dressing leather without bating and this is also true of the celebrated French calf.

The bating required may be shortened, and probably with advantage, by washing the hides with warm water in a "tumbler," or rotating drum, Fig. 49, prior to putting them into the bate, or the whole bating may be done in the tumbler. After a short bating, also, the hides may be softened and cleansed by stocking for 15-20 minutes. Warm bates act much more rapidly than cold ones.

Various machines have been proposed to take the place of hand-labour in the beam work, and, at least as regards the smaller skins, with considerable success. As a type of these, may be mentioned Molinier's hide-working machine, Fig. 29, which consists of a drum covered with helical knives, rotating at a speed of about 500 rev. a minute, over a cylinder coated with india-rubber. The skin is allowed to be drawn in between these drums, and the two being pressed together by a treadle, it is drawn out by a mechanical arrangement in a direction contrary to the rotation of the knives, which scrape off the flesh, or work off the hair.

After bating, "shaved" hides are reduced in thickness in the stronger parts by a shaving-knife, on an almost perpendicular beam. The workman stands behind the beam, and works downwards. The knife is represented at A, Fig. 26, and is a somewhat peculiar instrument. The blade is of softish steel, and after sharpening, the edge is turned completely over by pressure with a blunt tool, so as to cut at right angles to the blade. There is an obvious economy in shaving before tanning, since the raw shavings are valuable for glue-making, while, if taken off by the currier, they are useless for this purpose. The hide also tans faster.

Instead of shaving, the untanned hide is frequently split, by drawing it against a rapidly vibrating knife. The piece removed is tanned for some inferior purpose, if sufficiently perfect. In sheep-skins, which are split by a special machine, the grain-side is tanned for French morocco or basil, while the flesh-side is dressed with oil, and forms the ordinary chamois or wash-leather (see p. 210). Such a machine is shown in Fig. 50.

Tanned leather is frequently split by forcing it against a fixed knife, as in the American "Union" machine, Fig. 51. This is however being gradually superseded by the band-knife splitting machine, Fig. 52, in which an endless steel blade travels over two pulleys like a belt, and is kept constantly sharpened by a pair of emery-wheels seen below the machine. I am indebted for the block to Messrs. Haley and Co., who have made great numbers of these machines.

After bating, scudding, and shaving, the hides are taken into the tan-house, where they are grained, either by frequent handling, or by working in a paddle-tumbler (a vat agitated with a paddle-wheel, and known in America as an "England wheel"), with a liquor of suitable strength. What this strength should be depends on whether a well-marked grain is required or not. The stronger the liquor, the more it contracts the hide, wrinkling the surface into a network of numberless crossing furrows, which form the well-known marking of "grain-leather." In bark tannage, the after management is much like that described with sole-leather, except that weaker infusions are employed, and acid liquors, which would swell the hide and produce a harsh leather, are avoided. In old-fashioned country yards, which produce some of the best bark-tanned shaved hides, the liquors rarely range above 10°-15° of the barkometer, and the time employed is 3-6 months. The hides, after passing through a set of handlers, of gradually increasing strength, in which they are at first moved every day, are laid away with bark liquor and a good dusting of bark, receiving perhaps 4-5 layers of 2-4 weeks each. Unfortunately, these tannages are so unprofitable that they are rapidly being supplanted by quicker and cheaper methods.

These more rapid and cheap tannages mostly depend on the use of "terra" (block or cube gambier) in combination with bark, valonia, mimosa, and myrobalanes. Liquors warmed to 110° or even 140° F. (43°-60° C.) are frequently employed, and a bright colour is finally imparted by handling in a warm sumach or myrobalanes liquor, which dissolves out much of the colour imparted by terra or extracts. The tannage is helped forward by frequent handling, by working in tumblers, or sometimes by suspension on rocking or travelling frames, after the American fashion.

To this class of tannage belongs that of East India kips, which is largely carried on in the neighbourhood of Leeds. These kips are the hides of the small cattle of India, and are imported in a dried condition, and with their flesh-side protected (and loaded) with a coat of salt and whitewash or plaster. They are usually softened in putrid soaks, and unhaired with lime, and are used in England for many of the purposes for which calf-skins were formerly employed. A variety of East India kips, called "arsenic kips," are treated (instead of plastering) with a small quantity of arsenic before drying, to prevent the ravages of insects, which are often very destructive to these goods. Many kips tanned in India have also been imported of late years, and have greatly interfered with the profits of English tanners.

In yards where the leather is intended to be sold uncurried, it is taken up into the drying-sheds, well oiled on the grain with cod-liver oil, and either simply hung on the poles to dry, or stretched with a "righter," a tool shaped somewhat like a spade-handle, and finally set out with it to a smooth and rounded form. As in the case of sole-leather, too much light or wind must be avoided, and it is very difficult to use artificial heat successfully in the early stages of the process. It is, however, now very common for the tanner who produces such leather also to curry it, and, as this effects a considerable economy, both in labour and material, it is likely to become universal. When leather is to be sold rough, it is necessary to tan it in such a way as to give it a white appearance, from the deposit of "bloom" already mentioned; this being regarded by curriers as an essential mark of a good tannage, although the first step in the currying process is to completely scour it out. When the tanner curries his own leather, he of course aims at putting in as little bloom as possible, thus economising both tanning material and labour. In addition, the leather goes direct from the tan-house to the currying-shops, thus saving both drying and soaking again, and, it is said, giving better weight and quality. The tanner, too, is enabled to shave his hides or skins more completely, utilising the material for glue-stuff, which, had the leather been for sale in the rough, must have been left on to obtain a profitable weight.