Leather: From the Raw Material to the Finished Product

by K. J. Adcock

The disease of anthrax is generally contracted by workers through sores or cuts in the hand; the bacilli multiply rapidly in contact with the blood, and the first sign of disease is usually shown by a red swelling or pimple in the neck. If treated at this stage by anti-anthrax serum a cure is often effected, but if treatment is delayed the disease quickly proves fatal, the patient dying in awful agony. The germs of the disease may also be swallowed and the disease develop internally, but cases of this kind are rare.

Besides the danger to workers, there is the risk of cattle being infected. The effluent from tanneries where anthrax-infected skins are treated contains millions of bacilli, and it is doubtful if the latter are sterilised even when the effluents are precipitated and aërated before they are run into streams or municipal sewers. In any case, the sediment may be infected, and this ultimately finds its way to the land.

Until the various European Governments insist on imported hides from anthrax-infected areas being sterilised before shipment, the use of a disinfectant such as lysol or a similar cresylic compound, or bichloride of mercury, seems imperative in the first process of soaking. The use of these disinfectants would make the waste liquors fit for discharge into sewers or streams.

The English Public Health and River Pollution Acts have had a great effect in improving the hygiene of the tannery, although leather manufacturers have not welcomed them, as, in some cases, they have meant considerable expense in providing settling tanks for the treatment of waste liquors. The Public Health Act gives power to any town corporation to declare as offensive trades such businesses as tanning, hide and skin merchanting, fellmongering, tallow melting, etc., and several boroughs have taken advantage of this law. In such cases, anyone desiring to set up business in these trades must apply to the Town Council, who may or may not give their consent; in fact, a few applications to establish these businesses have lately been refused.

While the curing of wet hides with salt or in brine is more satisfactory than drying them, the use of ordinary salt is not an ideal method, as 10 per cent. brine dissolves hide substance. The recent introduction of a pure dry salt (99-98 per cent.) and of a sterilised salt for commercial purposes has to a large extent removed the objections to ordinary salt. Dry sulphate of soda is also a satisfactory cure. It may be that, as hygienic conditions are further advanced in the various industries, a suitable disinfectant will have to be used for all hides, in addition to the salt, except where the hides are sent direct to the tannery from the slaughterhouse.

The cure of hides in hot countries, especially where cheap salt is unavailable, is often unsatisfactory. A method of obviating this difficulty has been found in China, where, in one or two of the principal towns, hides and skins are preserved by freezing them in cold-storage. Although this process stiffens the hides, it is said to be fairly satisfactory if they are allowed to soften naturally before soaking them. If submitted to rough treatment before the stiffness relaxes, there is a great danger of the hide fibres being ruptured. Freezing removes the difficulty of softening which is experienced in treating dried hides, while it preserves the hide substance.

After the operation of soaking, hides and skins are ready to be treated for the removal of the hair. There are several ways of loosening the hair sheaths, but most of them consist of treating the hides in a solution of a caustic alkali. The use of a solution of common lime was practically universal until a few years ago, but nowadays sulphide of sodium, red arsenic sulphide (realgar), and caustic soda are also used, generally in admixture with the lime. Another process consists in sweating the hides in a heated room, preferably a damp cellar, where rapid decomposition of the hides soon loosens the hair.

This method is rarely used in England, but a few American tanners seem to prefer it for certain classes of hides. In the American process, the hides are first soaked, and then cut in half down the back, forming what are known as sides. Dry hides are subjected to the usual mechanical operations in the faller stocks, in which they are kneaded by two large hammers (Fig. [14]), or they are drummed in the tumbler (Fig. [13]). After the sides are thoroughly softened and drained, they are transferred to the sweat pit, which is, preferably, a dark underground chamber. The stock requires very careful attention as the process is risky. The temperature should never exceed 75° F., otherwise the hides may be irretrievably ruined. The process may take from one to four days, according to the varying conditions of the hides and of the weather. The loss of nitrogenous matter gives rise to the development of a strong odour of ammonia, which is sometimes even too pungent for the workmen. When the hair is judged to be sufficiently loose, the sides are washed in cold water and put in the stocks again for about ten minutes, when all the hair will be removed; or the hair may be scraped off in the unhairing machine. This method is not useful for sole leather, as it causes too great a loss of gelatine, but it saves time in the production of sides intended for boot upper leather, which is usually sold by measurement (superficial area).

A dehairing process has lately been invented and patented, however, which may supersede all of the methods just described. This process consists in treating the hides with various enzymes which loosen the hair so effectively that it can be removed more easily than the hair of a limed hide. The fine, short hairs underneath are also removed, whereas by the lime method a further process is needed to get rid of these hairs. The only drawback to its use is that the inventor has not yet been able to produce a material cheap enough to place on the market, but as soon as this difficulty is overcome the enzyme method may become fairly general. Neither the hair nor the gelatine of the hide can be damaged by this method.