Page:Encyclopædia Britannica, Ninth Edition, v. 14.djvu/403

LEATHER 385

consumer, as he pays for it on the outside of his leather to be worked off in the dressing and currying operations. By some tanning agents – mimosa, for example – there is little or no bloom deposited.

The theory of the formation of the bloom is this. As soon as ooze has penetrated into a hide it loses its tanning material, but by capillary attraction is detained; this exhausted ooze acts by maceration on the finer and more soluble interstitial gelatin, and dissolves it. In handling, about one-twelfth of this flows out; the remaining eleven-twelfths accompany the hide into the next stronger solution, of which only one-twelfth is absorbed directly, and a small portion is slowly exchanged by endosmosis and exosmosis. The small portion of strong solution which passes into the pores of the hide contributes to tan the hard fibrous portions not dissolved, and the small portion of weak solution passing out of the hide by exosmosis gives up its dissolved gelatin to the tin of the stronger solution outside to form tannate of gelatin, which partly adheres to the surface as bloom, and partly falls to the bottom of the pit as pitching.

From the time when the raw ox hide is taken in hand till the leather is fully dried, not less than a year is consumed in the case of the best qualities of sole leather. It was formerly the practice in England, as it still is on the Continent, to tan by the process of stratification, for which purpose a bed of bark is made upon the bottom of the pit; upon this is laid the hide, then bark, then a hide, and so on until the pit is full; water is sometimes pumped in, and the pit left for some months; it is then emptied, and the same hides returned with fresh bark and water for a few months longer; this is repeated again and again, until the tanning is completed, the time varying from one to four years for heavy leather.

The devices and processes which have been proposed and to some decree put in operation with the view of shortening the time occupied in tanning are beyond all enumeration. In scarcely any case have time-abridging processes proved successful in practical working, so far as the production of good leather is involved; and now the opinion appears to be completely established that, for the thorough tanning of heavy leather, a slowly operating influence and consequently long time are essential. The devices for the hastening of tanning have for the most part turned upon some plan for forcing the tan liquor into and through the pelt, or for alternate soaking and squeezing of the hides. Among the plans which have been tried on a commercial scale may be enumerated tanning by the application of hydrostatic pressure to force the liquor through the hides, a method which failed simply because the pressure was equal on both sides. The vacuum tanning principle is another which has been extensively tried, only to issue in disappointment. It consists in hanging the hides in a pit or cylinder so constructed that the air can be exhausted by an air pump, after which tan liquors are forced into the vessel, air readmitted, and again withdrawn. Hides, however, loaded with water swell little under diminished atmospheric pressure, and the practical difficulty of procuring and maintaining a vacuum in tan pits is very great. More promising results have been obtained by setting up in tan pits the physical process of endosmosis and exosmosis. This is done by sewing up hides two and two as bags which, being filled with solution differing in specific gravity from the tan liquor in which they are immersed, thereby set up transfusion through the hide. This process failed chiefly through the hardness of the leather it yielded. A plan of sewing hides into bags and suspending them filled with strong tan liquor, which as the fluid exuded was renewed, was also tried for some time. Again, it has been attempted to keep the hide suspended stationary in the pits and move the liquors instead of carrying over hides from one pit into another. A more recent device, which may not yet be fully tested, consists in keeping up the strength of the liquor by a continuous circulation through pipes from the stronger into the weaker infusions. By this system of circulation, instead of the oozes in which hides are immersed becoming weaker and weaker the longer they rest in the liquor, the ooze is kept up at least to its original strength, and it may indeed, if desirable, be increased in proportion as the tannin combines with the hide.

Heavy hides for sole leather, belting, and similar purposes do not require to undergo any elaborate dressing or currying. When finally removed from the tan pits they are piled grain to grain and flesh to flesh to drain, care

being taken that no tan liquor is allowed to lurk in the pile, which is covered over from the light. When sufficiently drained, they are brushed or scoured to free them from adhering impurities, and removed to the drying loft, where, after lightly rubbing over with oil, they are hung on poles to dry. In the loft steam-heated pipes keep a dry atmosphere during winter, and enable the attendants to regulate and control the drying of the leather. The leather when dried in this condition is rough tanned, and for finishing as sole leather it has to be struck out or "pinned" and compressed by rolling. For striking or pinning by hand the hide is dampened with water, thrown over a beam, and worked all over the grain side with a striking pin (fig. 3, c). This operation smoothes and levels the grain, removes smaller wrinkles, and to some extent compresses and solidifies the leather. Striking machines (fig. 7) are now very generally used for the operation. These consist of a drum or cylinder having a parallel series of projecting knives, or plates of gun-metal, set angularly across its surface. Underneath the drum is a brass bed, fixed on a yielding cushion, which can be pressed up or eased by means of a foot lever, according as the leather operated on is thick or thin. The drum is made to revolve at a very rapid rate, the blunt edges and external angles of the knives thereby striking the surface of the leather with great violence, and thus the grain is struck out, smoothed, and compressed in a very rapid and efficient manner. Finally, the leather is rolled and compressed on a level zinc-lined wooden bed by a heavy hand roller, such as is shown in fig 8, or on the platform of one of the numerous forms of machines designed for that purpose.

Fig. 7. – Leather Striking Machine. Fig. 8. – Hand Roller.

The yield of leather from a given weight of dry hide varies very much according to the different styles of tannage and materials used. As a mean outcome, it may be said that 100 lb of green hide, tanned with from 300 to 400 of oak bark, will yield 40 to 50 lb of leather; 100 lb of green hide, however, when deprived of hair, flesh, and moisture, will weigh only 18 lb, and, taking 100 lb of dry hide, which, fleshed and unhaired, weighs 85 lb, the yield of leather will be from 180 to 200 lb according to tannage.