The Principles of Leather Manufacture

by H. R. Procter

Fig. 38.—The Slocomb Staking Machine.

The manufacture of glove-kid is quite similar in principle to that just described, but varied in detail to suit the softer and more delicate skins employed, to give greater softness, and especially the quality of stretching in any direction without springing back, which is so characteristic of the leather. Lamb-skins are the principal raw material, though genuine kid is also employed for the best qualities. The manufacture varies much with the quality and character of the goods. The skins, which are mostly dried, are soaked in clean and cool water for three to four days, according to age and thickness. Common qualities (small imported slink lambs) are often unhaired by dipping in or painting with a paste of gas-lime, lime and sulphide of sodium, or lime and red arsenic, so as to destroy the wool. Better skins are sometimes unhaired by painting on the flesh with lime alone or in mixture, and in other cases ordinary lime-pits are used, with limes, which are most usually strengthened with red arsenic, which is added to the lime while hot from slaking (cp. [p. 142]).

The calcic sulphydrate (and perhaps sulpharsenite) thus formed hastens the unhairing, and preserves the gloss of the grain. Well conducted glove-kid establishments avoid as much as possible the use of old limes, which produce a loose, porous leather, with a rough, dull grain. The liming lasts on the average ten days, and is of the greatest importance. It is essential that the inter-fibrillary substance should be dissolved, that the leather may have the quality known in Germany as Stand, that is to say, may be strongly stretched in either length or breadth without springing back. It also depends upon the liming (and this is of special importance in the case of lamb-skins), whether the tissue of the fat-glands is well loosened, so that the fat, either as such, or as lime- or ammonia-soap, may be readily and completely worked out. Skins in which this is neglected can never be properly dyed.

When the hair (or wool) is well loosened, the skins are rinsed in water, and then unhaired on the beam with a blunt knife. The water employed in washing should not be much colder than the limes, or it will prevent the hair from coming away readily. The wool or hair is washed and dried for sale. The skins are thrown into water, to which a little lime-liquor has been added, to prevent precipitation of the lime in the skins by the free carbonic acid of the water, which would have the effect of making them rough-grained.

Next comes the first fleshing (Vergleichen) or “levelling.” By this, the loose cellular tissue on the flesh-side is removed, together with the head, ears, and shanks; and the flanks are trimmed. The skins are then again thrown into water softened with lime-liquor as above described, and then into a puer of dogs’-dung. This is prepared by stirring up white and fermented dogs’-dung with boiling water, and straining it through a sieve or wicker basket. The puer must be used tepid, and not too strong. The skins “fall” (lose their plumpness) in it rapidly, and become extremely soft and fine to the touch; and the fat-glands, remaining hairs, and other dirt, can now be very readily scudded out.

Too strong puers, or too long continuance in them, produce evident putrefactive effects on the skins. (See also [p. 181].)

When the skins come out of the puer, they are stretched and worked on the flesh with a sharp knife, and any remaining subcutaneous tissue is removed. This constitutes the second fleshing. They are then rinsed in warm water, and beaten with clubs in a tub, or worked in a tumbler-drum, in either case with a very little water only; and finally brought into a tank of water, not too cold, and kept in constant motion with a paddle-wheel.

The skins are next cleansed on the grain-side by working on the beam with plates of vulcanite set in wooden handles, so as to remove fat, lime- and ammonia-soaps, and other lime compounds, together with all remaining hair or wool. The skins are now a second time washed in the “paddle-tumbler,” first in cold, and then in tepid water; and after allowing the water to drain from them, they are transferred to the bran-drench.

This is prepared by soaking wheaten bran in water at about 50° C., and diluting with warm water. Sometimes the mixture is strained, and the bran-water only used, to save the trouble and cost of removing adhering particles of bran from the delicate skins. Sufficient of the liquid must be employed to well cover the skins, and the temperature may range from 50° F. (10° C.) to 68° F. (20°C.). These conditions are favourable to bacterial activity, which comes into play, and, on the one hand, evolves acetic and lactic acids, which dissolve any remaining traces of lime, and on the other, loosens and differentiates the hide tissue, so as to fit it to absorb the tawing solution. Much care is required in the management of the bran-drench, especially in summer, since the lactic readily passes into the butyric fermentation (see also [p. 167]). The tawing mixture is composed (like that employed in the fabrication of calf-kid, q.v.) of alum, salt, flour and egg-yolks, in a quite thin paste. A small quantity of olive oil is also generally used. The skins are either trodden in it with the feet, or more generally put into a tumbler-drum with it. Kathreiner pointed out, some years ago,[114] that a mixture of olive-oil and glycerine might be partially substituted for the egg-yolks, in both the tanning and dyeing of glove-kid leather.