SUTCLIFFE SYSTEM OF DRYING
The natural chrome sole requires very little dressing. The chromed hides (usually rounded into butts or bends, shoulders and bellies) are allowed to remain on a horse two or three days after the completion of the tannage, so that the oxide of chromium may combine more firmly with the fibres. They are then placed in the drum tumbler again and washed in cold water for several hours. A constant stream is conveyed to the drum through a rubber pipe fixed to the tap, and escapes through small holes in the drum. The acid salts are not thoroughly removed by this washing and must, therefore, be neutralised with an alkali. Borax is the best for the purpose, owing to its mild action on leather and antiseptic properties. From 1 to 2 per cent. borax on the weight of the leather is generally sufficient, while, if soda be used for economical reasons, only half the quantity is required. The drum is revolved for about an hour; the liquor is then run off, and the leather receives a further washing in cold water for half an hour, when it is in a condition to receive the dressing, which is chiefly composed of materials which help to fill and lubricate the interstices of the fibres so that the substance is maintained and the natural strength of the fibres preserved. Gelatine, pure glue, Tragasol, paraffin wax, carnauba wax, spermaceti, Marseilles soap, are among the various ingredients used; while the delicacy of the tint may be improved with china clay or French chalk. Recipes vary considerably, each manufacturer claiming to have secret methods. Solutions are made of the ingredients, or, where one or more of the materials are insoluble, they are atomised in a mixing pan before use. To induce thorough penetration, the dressing liquid may be applied to the leather at a fairly high temperature, as chrome leather is unaffected by heat which would destroy the fibres of ordinary vegetable-tanned leather. Even so, it is inadvisable to use a higher temperature than 170° F. The leather is thoroughly impregnated with the dressing in about an hour's time, and is then piled on a wooden horse for several hours, or overnight, to drain and permit thorough incorporation of the dressing with the leather. When well-drained, the grain of the hides is smoothed by pressure with a special tool (Fig. [32]) or by machine, the operation being known technically as "striking out." The machine (Fig. [34]) gives quite satisfactory results and has replaced hand labour in all modern works. The hides are then "strained" (i.e., extended and nailed to wooden frames), or they are simply hung up to dry. In the latter case, there is a small shrinkage of the size of the hide, but the substance is maintained. With good straining, a satisfactory increase of surface measurement can be obtained.
Fig. 32
SLEEKER
When the leather is completely dried, it may be sold at once, but if, as is generally the case, a special "nap" finish is required on the flesh side, the hides are placed in damped deal sawdust until they are in the right condition of flexibility for the next process, known as "fluffing." This is done on a rapidly revolving wheel (Fig. [33]) covered with emery powder varying in the size of grain according to the effect desired on the finished leather. The abrasion of the leather on the flesh (i.e., the under side) raises a soft nap. Carborundum, a special abrasive made in America, by the fusion of sand, coke, and salt at a very high temperature, has to some extent supplanted the use of emery. As a final finish, the flesh side may be rubbed over with chalk or china clay to give it a clear saleable appearance.
Fig. 33