The use of extracts, and the demand for low-priced leathers, to compete with the American tannages, has introduced still more rapid methods than those described, and very fair-looking heavy leather has been tanned in 5-10 weeks. These tannages are very various, but their main feature is the free use of hot liquors, composed principally of extracts and gambier. This treatment imparts great firmness, or more properly speaking, hardness ; but the leather is deficient in toughness, and the grain usually cracks on bending sharply. Extract properly used is, however, capable of making excellent leather.
Treatment of in the butts, after being well mopped on both flesh and grain in a clear liquor, are taken into the drying-lofts, where they are hung on poles till about half dry. They are then laid on the floor in piles, and covered up till they heat or " sweat " a little, which facilitates the succeeding operation of " strik ing." This is per formed by laying the butt over a horizontal " beam" or "horse," and scraping its sur face with a triangular pin, shown at D in Fig. 903. This pin has an even, though tolerably sharp, edge, and is so used that it stretches and smooths out the grain, without breaking it ;-at the same time, it removes a portion of the white deposit called " bloom," which has been men tioned. Common goods are frequently struck by the machine introduced by Priestman, of Preston Brook, shown in Fig. 914 ; but the work is not very uniform, and the leather is much compressed and stretched. After a light oiling and a little further drying, the butt is laid on a flat " bed " of wood or zinc, and is rolled with a brass roller loaded with heavy weights.
Various machines are also in use for this purpose. In Fig. 915, is shown a roller adapted for rolling butts, in which the pressure is produced by springs immediately above the roller, which works backward and forward over a flat table. Fig. 916 represents a machine in which the roller is fixed, and works over a brass drum ; it is specially adapted for offal, and, when used for butte, is apt to make them " baggy." In both these machines, the reversing motion is obtained by using two belts, one being crossed. The leather is now frequently coloured on the grain with a mixture, for which each tanner has a recipe of his own, in order to hide uneven or dull colour, and, when sufficiently dry, is rolled a second time, and dried-off in a room gently heated by steam. This is the Bristol method of finishing. In the Lancashire district, butts are generally struck out much wetter, and "stoned," so as to remove the whole of the bloom, and show the natural brown " bottom " of the grain. When sufficiently dry, they are struck a second time, to set the grain, and rolled as described, the painting being omitted. This method has the disad vantage of requiring more labour, and causing a lees of weight ; but leather so got up brings a higher price, as the method is only applicable to such tannages as make a fair colour.
It is very important, and especially so with heavy mixed tannages, that the drying should be conducted in the dark, and not too rapidly. No artificial heat should be used, except in frosty weather, to wet leather ; and it should be carefully protected from harsh drying winds. After the leather is finished, it should be dried-off in a well-ventilated drying-shed, heated to about 21.° (70° F.). The same observations apply to the drying of rongh dressing-leather, except that
artificial heat should be avoided. Frost makes dressing-leather porous, and prevents its carrying a proper quantity of grease in currying.
Tannage of Dressing-leather, Common, and Shaved Hides.—Hides which are intended for purposes where softness and flexibility are required, as for instance, for the upper-leathers of hoots, 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 limeing 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, but, in this case, free ammonia is liberated in its place, which is just as powerful in swelling the pelt, and hence will not account for the rapid reducing effect ; while the phosphates of dung are mostly, if not entirely, in the form of phosphate of lime, 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 bide, producing sulphuretted hydrogen, together with tyrosine and leucine, and other weak organic acids, which neutralize 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 hate is greatly increased, and that, if one pack of bides 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. 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 fer mentation, rapid destruction of the skins is the result.