Leather: From the Raw Material to the Finished Product

by K. J. Adcock

PIM'S SYSTEM OF FLAYING

Even the apparently trivial matter of removing the hide from the cheeks and face is economically important, for, unless those parts are removed so as to get the maximum surface, they are only fit to be cut off and thrown in a pit with other pieces and roundings which are made into glue.

Imperfect preservation is another serious form of damage to hides and one that cannot be easily remedied in some of the hot climates. Hides and skins may be simply dried, salted and dried, wet-salted, treated with arsenic solution and dried, brined or pickled with acid and salt; of these methods the last-named is the most effective, but is not practicable, or, at least, has not yet been applied practically to the cure of hides and calf skins. Salt is not available, or is too costly in most tropical countries; hence, Chinese, Indian, Mexican, Colombian, and Arabian hides and skins are generally exported in a dry condition. Even when the hides are dried under the best conditions in a cool and shady place, they are of less value than a fresh or a wet-salted hide, owing to a certain loss of gelatinous matter in softening them before they can be placed in the lime liquors. But it not infrequently happens that hides are dried by exposure to the hot sun, or perhaps in a strong current of air. In either case, the hide is much reduced in value and may be irreparably ruined. The effect of submitting hides to these conditions is that the exterior surface becomes rapidly dry and, naturally, contracted, so that the air or heat cannot reach the interior, which retains moisture. This moist inner layer may be quite thin, but it contains sufficient nutrient to develop putrefactive organisms, so that when the hide is soaked in water it practically falls to pieces. The effect of hot sun or heat of any kind is, of course, disastrous to raw hides and skins, and there have been not a few claims on shipping companies as the result of storing hides near the boilers of ships.

Salt is almost invariably used for curing both hides and calf skins, but though it is a good preservative it has one or two minor defects. It contains too much water, and is liable to contain traces of iron which is inimical to both raw hides and leather in process of manufacture. Common salt is also liable to cause stains which cannot be removed in later processes, and which are even accentuated in the tan liquors. One trade chemist attributes these stains to the presence of calcium sulphate (Ca. SO₄) or gypsum in the salt, which is converted into calcium phosphate by the action of the phosphoric acid in the nuclei of the hide on the sulphate of calcium. Another well-known technical chemist is certain that stains are produced by the growth of bacteria, and to prove his assertion prepared in gelatine several cultures from salt-stained skins. Practical men generally attribute the stains to the presence of blood on the hides or skins at the time of curing, and the majority of the stains are probably due to this cause, although the presence of calcium sulphate as an impurity of the salt would undoubtedly contribute to this defect. Blood contains a percentage of iron, and, with other extraneous matters, should be washed from the hides before salting them.

Fortunately, chemists have lately paid attention to the advantages of the use of pure salt in various industries, with the result that at least two chemically pure products are now available. The use of these salts should be general for the cure of hides and skins, as they are quite dry, and, therefore, easily spread. As a curing agent, they are much more effective and lasting than common salt.

The use of glauber salts (Na₂ SO₄) is recommended by the International Commission for the Preservation, Cure, and Disinfection of Hides and Skins instead of ordinary salt, where the latter is unobtainable. The preparation of a sterilised salt, however, renders its export a practical proposition to almost any part of the world.

Although the loss due to bad curing and flaying is very great, it is quite small compared with the damage caused by natural defects.

The ravages of disease cause a great wastage of hides and skins, as animals infected with anthrax are immediately destroyed and cremated in all civilised countries, while, in Great Britain, foot-and-mouth disease is kept in check by the same drastic method. In many other countries, the infected cattle are isolated, treated with an antiseptic hoof-and-mouth wash and generally cured, as it is a mild fever which soon runs its course, although it is very contagious. The germ of foot-and-mouth disease has not yet been discovered, for the most powerful microscope fails to reveal its presence, but cattle readily show the complaint, as their hoofs and mouths become covered with swollen lesions.

Another kind of fever, known as "tick," was prevalent in the southern part of the United States, but this disease was eventually eliminated by systematically "dipping" the cattle three or four times a year. The cattle "dip" used effectually prevented the ravages of the fly which caused the disease. A similar method has of late years been adopted in South Africa, with the result that cattle-raising in that country is developing rapidly.

Anthrax is due to the presence of bacillus anthracis, a vegetable organism of Siberian origin. Dry Chinese and Russian hides are specially liable to contain the spores of anthrax, and, as the disease proves fatal to workmen infected by it unless treatment with anti-anthrax serum be given in the early stages, hides and skins from infected areas should be disinfected before shipment. The method proposed by Mr. A. Seymour-Jones, which consists in treating hides with very dilute formic acid and one part of bichloride of mercury in 1,000 of water, and afterwards with a saturated solution of salt solution (.02 per cent.) of bichloride of mercury, seems to be the most effective without damaging the hides.