[CHAPTER XII.]
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.