[114] Gerber, i. (1875) p. 170; ii. (1876) p. 664.
The tawed skins are now dried by hanging on poles, grain inwards. Rapid drying in well-ventilated, but only moderately heated rooms is essential to the manufacture of a satisfactory product.
The dry leather is rapidly passed through tepid water, and after being hung for a very short time, to allow the water to drain off, is trodden tightly into chests, and allowed to remain in them for about 12 hours, so that the moisture may be uniformly distributed. It is then trodden on hurdles (German Horden) composed of square bars of wood, joined corner to corner, so as to make a floor of sharply angular ridges. The next operation is stretching with the “moon-knife”; after which the leather is dried nearly completely, and staked again.
This completes the tawing process. The goods are now “aged” as in calf-kid manufacture. Before dyeing they are washed with tepid water to remove part of the tawing mixture, and especially, superfluous alum and salt, and are re-egged much like calf-kid, before dyeing if the latter is done by brushing, and after if in the dye-tray or paddle. Aniline colours are more used than formerly, especially for topping and brightening the natural colours, but the dyewoods and other mordant colours are still largely employed. The leather is first prepared with an alkaline mordant (stale urine, ammonia, etc.) (cp. [p. 413]), then repeatedly brushed with or dipped in the dyewood liquor, and a wash (“striker,” German Ueberstrich) containing some metallic salt is generally applied, with the object either of bringing out the special tone required, or of making the colour more lively and permanent. The striker is usually a solution of one of the so-called “vitriols”: “white vitriol” (zinc sulphate), “blue vitriol” (copper sulphate), “green vitriol” (iron sulphate), or occasionally other salts.
After the dyeing, the skins, if dipped, are wrung out and re-egged; if brush-dyed, sleeked out with a brass or ebonite sleeker to get rid of superfluous water. They are then dried in an airy room. Before staking (stretching), the skins are laid or hung in a damp cellar, or in moist saw-dust. They are staked twice: once damp, and once nearly dry; and are finished by glassing or ironing.
Skins which are much damaged on the grain, or otherwise faulty, are smoothed with lump pumice on the flesh side, or fluffed with fine emery on the fluffing wheel. They are then dyed on the flesh side, mostly by dipping, but occasionally with the brush, in which case, the method described is slightly modified.
Tawing with alum and salt is frequently employed for commoner and stronger leathers, such as aprons (of sheep-skin), leather for whip-lashes, laces for belts, and “skivers” for capping chemists’ bottles. The process is practically the same as for calf-kid, except that no egg, and little flour is used. Often flour is entirely omitted, and the goods may then be alumed in tubs, in which they are merely handled, as the alum solution penetrates quickly. Goods which are required white are frequently handled or tumbled with a milk of “whitening,” both to improve the colour, and to neutralise any acid present, and fix the alum by rendering it more basic. Alumed goods can be stuffed with greases, either by hand or in the drum, after thorough softening by staking.
Alum, and other salts of alumina are frequently used in combination-tanning with vegetable materials (see [Chap. XVII.]). “Green” leather for laces, “dongola,” and “dog-skin” glove-leathers are made in this way. Glazed kid for ladies’ shoes must be slightly vegetable-tanned on the surface, or it will not glaze, but this is frequently accomplished by the use of materials in the dye-liquor containing tannins.