In the "layers" the goods may be allowed to remain for long periods without change, being lifted and re-transferred into in creasing strength solutions at the end of each one or two weeks. This treatment permits the deposition of bloom and conduces to the production of firmer, harder, heavier and better wearing leather. To complete the process, the goods are removed and drained and then scoured to remove the deposited bloom from the grain.
Tannage of Calfskins.—One of the essential differences in the manufacture of this leather by the vegetable process is in the preparation before tanning. After liming, unhairing, fleshing and scudding the goods are either most commonly bated, or very thoroughly delimed. The goods after bating are sometimes pickled; sometimes, however, this operation is omitted.
Usually the tannage is begun by suspending the skins in non astringent liquors consisting of used liquors originally prepared from a mixture of two or more of the following materials— Mimosa bark, myrobalans, gambier, quebracho, chestnut and oak wood extract. After the goods have been suspended for a period varying from two to seven days the tannage may be completed in a "handler" round, the goods being finally given a re-tannage with a view to lightening the colour by drumming, paddling or suspending in a warm sumach infusion. Alternatively, the tan nage is sometimes done entirely in paddles using gambier in the earlier stages and gradually strengthening up the tannin content of the solution by adding quebracho-myrobalan extract, chestnut extract, etc. As calfskins are required in the majority of cases to be dyed it is thz: usual custom to avoid "bloom-giving" tanning materials, the irregular deposition of bloom on the surface of the leather detracting from the production of a level dyed result.
Tannage of Skivers.—Skivers are the grain portions of sheep skins, which have been split in the limed condition. The universal tannage used in their manufacture is sumach (Rhos coriaria), this material being chosen because it readily produces a white leather capable of being dyed in any pale or brilliant colour.
The goods, as removed from the splitting machine in their limed condition, require to be washed, delimed and very thor oughly bated, after which they are most commonly pickled and then tanned in a paddle wheel. The goods are placed in the paddle wheel with the sumach in the ground leaf form and preferably after having been infused with water for a few minutes at a temperature of 140° F; the tannage being usually commenced at a temperature of about 9o° F. The goods are usually completely
tanned within 12 hours, when they may be removed, drained, rinsed in water and hung up to dry into the "crust" condition.
Bottle Tannage.—A number of goatskins and also some few calf skins and sheepskins are tanned by a primitive method com monly termed bag or bottle tanning. The goods after being bated are individually sewn into a bag by doubling the skin down the ridge or backbone and then sewing round by means of a power driven sewing machine. The skin bag is then filled by means of a funnel to about 3 of its capacity with a sumach infusion through an aperture left for the purpose. A quantity of air is blown into the bag and the aperture is closed by tying round with a cord. The skin bags are immersed in a shallow tank previously filled with a warm sumach infusion in which they float, the position of the bags being changed by keeping the goods in motion and turning them over with a wooden pole. The goods are withdrawn after 2-3 hours and are then placed upon a draining rack and left for several hours until the liquor has drained from them, when they are again re-filled with a stronger infusion of sumach liquor and immersed in a still stronger and warmer infusion. After two or three immersions the goods become thoroughly tanned, when the bags are cut open and then either rinsed and dried, or given a further re-tannage by paddling in a stronger infusion of sumach. The distending of the skins produces a thin leather devoid of contracted grain, and not liable subsequently to stretch.
Mineral Tannages.—The oldest process of tanning by min eral salts is the alum process. This is still very largely practised in the manufacture of glove leather. The proportions of the materials used vary, but the following may be taken as an exam ple:—alum, 8 lbs. ; salt, 8 lbs. ; flour, 3-5 lbs. ; egg yolk, 2-4 lbs., for Too lbs. weight of prepared skins. It is customary to add all of the ingredients at the commencement of the tanning and prolong the drumming for about two hours until the leathering process is completed when the goods are removed, drained, and dried.