With so many depilatories available, it is not surprising that several patents have been granted and numerous suggestions made with the object of trying to improve on the old process of liming. While there may be some objections to lime, it has a few advantages which are lacking in other depilatories. These advantages are not perfectly understood theoretically, but the tanner recognises them in practice. Hence, there are very few tanneries where lime is not used at all, and the only progress that seems to have been made in the process of liming consists in the admixture of sulphide of sodium or arsenic to hasten the process, reduce the loss of gelatine, and, in the case of arsenic, to improve the fineness of the grain of skins for boot upper and glove leathers.
One patented method consisted in forming the calcium hydrate within the hide by treating it with a 1 per cent. solution of caustic soda and then with a 1-1/2 per cent. solution of calcium chloride, the reaction of these two chemicals forming calcium hydrate (lime) and sodium chloride (salt). This method, however, does not loosen the hair at all and has to be supplemented by soaking the hides in putrid water. It is a good method of liming hides or skins dressed in the hair, as it opens up the fibres without weakening the hair roots and prepares the hides in a suitable condition for tanning.
Another method consists in mixing a small quantity of soda ash with the lime, thereby hastening the process by increasing the alkalinity of the liquor.
A somewhat complicated method was introduced a few years ago, but, although it seemed advantageous from a chemical point of view, it has not proved successful in practice so far as is known, probably because of its expense. In consisted of four distinct processes. The hides were first mopped on the hair side with a thin paste of lime and arsenic, and dehaired after twenty-four hours. In the second process, they were treated in a drum for twenty-four hours with a solution of sodium sulphide, they were then drummed for twenty-four hours in a solution of hyposulphite of soda, and finally placed in a vat or pit containing a solution of lime and a little arsenic for two to five days. After washing, etc., they were ready for tanning. The sulphide of sodium swelled the hides or skins by distending the fibres, and the natural fat is converted into a soluble soap. The hyposulphite arrests the action of the sulphide and acts as an antiseptic.
In modern yards it is a growing practice to use mechanical power to keep the hides in motion, instead of handling them. The hides are attached to poles joined to a strong cross beam, which, in its turn, is connected by stout iron rods with the main pullies and shafting. The installation is expensive, but it saves an enormous amount of manual labour and time, while ensuring uniform treatment. This method is not so much used for liming as for the tanning process.
After loosening the hair and opening up the fibres, the next operation is dehairing or depilation, or, as it is generally described in the trade, "unhairing." The hair must slip quite easily before beginning this operation, otherwise a number of them will be left in the hide and will be difficult, or almost impossible, to remove later on if the hides are being made into sole leather. These hairs present an unsightly appearance and lower the value of the leather. Depilation may be done by hand or machine; but the latter method is rapidly superseding the former, as it is in nearly every other process of leather manufacture.
In the manual process, the hair is removed by scraping it off in a downward direction with a blunt, convex-shaped knife, fitted into two wooden handles (Fig. [17]), the hides being placed on a sloping convex beam (Fig. [15]) supported by a trestle. A series of grooves under the beam permits it to be placed at any angle. The beam (different from that used by curriers) may be of wood, iron, steel, or zinc-covered wood.
Fig. 17