Fig. 22.—Faller Stocks.
Beside merely soaking the hides, it is sometimes necessary to work them mechanically, to promote their softening; this was formerly accomplished by “breaking over” the hides on the beam with a blunt knife. This process is still in use for skins of many sorts, but for the heavier classes of leather is now usually superseded or supplemented by the use of “stocks,” or drums. The former consist of a wooden or metallic box, of peculiar shape, wherein work two very heavy hammers, raised alternately by pins or cams on a wheel, and let fall upon the hides, which they force up against the curved end of the box with a sort of kneading action. The ordinary form of this machine is shown in [Fig. 22]. A more modern form, which seems to possess some advantages, is the American “double-shover,” or “hide-mill,” seen in [Fig. 23]. “Crank stocks,” similar in form to the faller stocks, but driven by cranks, are sometimes used for softening, but are better adapted to lighter uses.
Fig. 23.—American Hide-Mill.
The number of hides which can be stocked at once naturally varies with the size of both hides and stocks, but should be such that the hides work regularly and steadily over and over. The whole number should not be put in at once, but should be added one after another, as they get into regular work. The duration of stocking is 10-30 min., according to the condition and character of the hides. Hides should not be stocked until they are so far softened that they can be doubled sharply, without breaking or straining the fibre. After stocking, they must be soaked again for a short time, and then be brought into an old lime. A small quantity of sodium sulphide added to the soaks or in the stocks has been recommended as of great value in softening obstinate hides, and probably with justice, from its well-known softening action upon cellular and horny tissues.
Tumbler drums of various forms may also be used with good effect for softening purposes, especially for skins, and are much less detrimental than stocking, both as regards the weight and quality of the goods.
For sole leather, and even for kips, the use of stocks has in recent years been entirely discarded by many of the more advanced tanners. If mechanical work is required at all, the drum is preferred, and is sometimes employed after a few days’ liming, the goods being first merely softened in fresh water. The use of caustic soda, sodium sulphide, or sulphurous acid renders mechanical softening almost unnecessary.
Fig. 24.—Drum for Washing or Tanning.
The drums employed are in principle like a barrel-churn, and are large cylindrical wooden chambers 6 to 12 feet in diameter, and fitted inside either with shelves like the floats of a water-wheel, or with rounded pegs on which the hides fall. The American wash-wheel figured on [p. 111] is a machine of this kind, and one of a more elaborate description is shown in [Fig. 24]. Drums are not only used for softening, but for tanning, dyeing, and many other purposes in leather manufacture. It is advantageous to be able to reverse the direction of their rotation to prevent the rolling up of the hides.