CHAPTER XII.
DEPILATION.
After the softening and cleansing of the hide or skin is completed, and before proceeding to tan it, it is usually necessary to remove the hair or wool. The earliest method of accomplishing this was by means of incipient putrefaction, which attacks in the first instance the soft mucous matter of the epidermis, and thus loosens the hair without materially injuring the true skin. This loosening of the hair often takes place accidentally in hides which have been kept too long without salting, and is known as “slipping,” and is apt to be accompanied by some degree of injury to the grain. The old method of loosening the hair by putrefaction, or, as it is generally called, “sweating,” was to lay the hides in piles, usually in some warm and damp place. Occasionally a slight preliminary salting was given to prevent too much putrefaction of the hide. The action in this case, however, was very irregular, and it has been quite abandoned in all civilised countries.
Fig. 25.—Sweat-Pit.
The method which is now used is to hang the hides in a closed chamber, generally called a “sweat-pit,” [Fig. 25], but usually constructed above the ground-level and protected from sudden changes of temperature by double walls, or by mounds of earth. The hides are hung in the sweat-pit, in small chambers each capable of holding 50 or 100 hides. The temperature is kept at about 15° to 20° C., the air being warmed, if necessary, by the admission of steam below a perforated floor, or cooled by a shower of water from sprinklers, so arranged as not to play directly on the skins, and is thus always kept saturated with moisture. Little if any ventilation is allowed, and a large quantity of ammonia is given off from the decomposition of the organic matter, and no doubt contributes to the solution of the epidermis and the loosening of the hair, as the writer has found that ammoniacal vapours alone very speedily produce this effect.
After 4-6 days of this treatment, the hair is sufficiently loosened to be removed by working the skin over the beam with a blunt knife, or by means of the stocks or hide-mill (see [p. 116]). Great care and watchfulness are required to avoid injury to the grain by putrefaction.
The hide is in a slimy and completely flaccid and “fallen” condition, and some trouble is occasioned by the hair being worked into the flesh by the hide-mill, to obviate which, a slight liming is frequently given after the sweating. Hides which have been unhaired in this way require to be swollen by acid in the liquors in order to produce a satisfactory sole-leather, as the sweating process does not swell or split up the fibres.
In some European tanneries a similar process, but at a higher temperature, is employed, and it is also largely used for sheep-skins under the name of “staling,” but in this case is sometimes conducted in a very rude and primitive manner, and frequently with the result of considerable injury to the pelt.
The great objection to the sweating process, however carefully conducted, is the liability of putrefaction to attack the skin itself, causing “weak grain.” Its most advantageous use is for sole leather, as, although the solution of the hide-substance may not be very much less than in the case of liming, the dissolved matter remains in the hide instead of being washed out, and being fixed by the tannin, contributes to the solidity of the leather.