The Principles of Leather Manufacture

by H. R. Procter

Various theories have been proposed to explain the reaction which takes place in the production of oil-leathers. Knapp supposed that it was merely a case in which the smallest fibrils of the hide were coated with the products of the oxidation of oils, and so prevented from adhering together, and protected from the action of water by the sort of waterproof coating which was formed. This explanation is scarcely feasible in the face of the fact that chamois leather can be treated even with hot dilute solutions of the caustic alkalies without destruction, while cotton fibres waterproofed by treatment with drying oils have their coating entirely removed by treatment with alkalies. Lietzmann supposed that the whole of the gelatinous fibres were removed in the liming and subsequent treatment, and that the finished leather consisted only of the skeleton of yellow or elastic fibre which exists in the skin, and which is remarkable for its resistance to heat, acids and alkalis. Unfortunately the proportion of these fibres does not exceed about 6 per cent. of the total, so that they are quite insufficient to account for the production of the leather. We now know, however, that aldehydes, including the acryl-aldehyde, which is evolved in the oil oxidation of chamoising (and which is covered by Messrs. Pullman’s patent) are capable in themselves of converting gelatinous substances into a material identical in its properties, and especially in its power of resisting hot water and alkaline solutions, with the fibre of chamois leather. In all cases where perfect chamoising is produced, intense oxidation takes place, and oxidisable oils are used which will evolve acrylic and other aldehydes. Where oils of little drying power are employed, as in the case of Crown- and other fat-leathers, only an imperfect chamoising occurs, and we are therefore justified in attributing the special qualities of chamois leather to a natural aldehyde tannage. On the other hand, there is no doubt that the coating of the fibres with oxidised oil-products really occurs, and is probably a powerful factor in the leathering of Crown-leather, and other similar products which are not washed out with alkaline solutions. Knapp proved by treating raw pelt which had been dehydrated with alcohol ([p. 74]) with a very dilute alcoholic solution of stearic acid, that a thin coating of stearic acid on the fibres would confer great softness and considerable resistance to water. Even where no stearic or other fatty acid is purposely added to alcohol used for dehydrating pelt, traces are present from the decomposition of the natural fat of the skin, and there is little doubt that this is the cause why such alcohol-leathers are much more difficult to wet back again to the state of pelt than would a priori be expected; and why hide-powder dehydrated in this way is unsuitable for use in the hide-powder filter ([p. 311]) from its non-absorption of water.

Fig. 85.—Scouring large Seal-Skins by Hand.

It is not within the scope of the present volume to describe in detail the processes used in currying, many of which are purely mechanical, and of no theoretical interest, whatever their practical importance; and with which the writer hopes to deal fully in a future book. The leather is usually scoured with stone, brush and sleeker to free it from “bloom” and loose tan ([Fig. 85]); or by machines such as [Fig. 86]; and is often reduced in thickness by shaving by hand ([Fig. 87]), or by machine ([Fig. 88]). In place of shaving, hides and skins are frequently split into two or more thicknesses. This is done by various machines, of which the “bandknife” shown in [Fig. 89] is the most important; the cutting tool being a thin steel belt stretched like a bandsaw and sharpened on one edge by an emery-wheel.

Fig. 86.—Scouring Machine.

Currying Shop, Leather Industries Department, Yorkshire College.

Something must be said here about the function of the oils and fats used in currying, and their general method of application. It is obvious that the possibility of coating the finest fibrils of leather with a fatty layer is not restricted to raw hide, but is present, sometimes even in a higher degree, in tanned or tawed leathers, in which the fibres are already so far isolated as to make the access of the fat easy. Even the possibility of an aldehyde-tannage is not excluded, where the fibre is not already completely saturated with other tanning agents or where these agents, from their nature, have not so firm a hold on the fibre as to be incapable of being displaced by the action of aldehydes. It is therefore obvious that we may apply some of the ideas which we have formed with regard to oil-tannages to the action of fats upon tanned leather. In the first place, it must be remembered that gelatinous matters are as a rule insoluble in fats; and vice versa, that fats are incapable of penetrating dry and solid gelatinous fibres. If the skin becomes dry in the chamoising process, that part remains raw. It may therefore be concluded that fats and oils have little power in themselves of isolating the fibrils, and that this must be accomplished by other agencies, since if they are still adhering together, the fats cannot penetrate them. Hence the necessity of moisture, which keeps the fibres soft and divisible; and with raw hide, the importance of powerful mechanical treatment, which will work the minute globules of fat between the fibrils. In the case of tanned leathers, the last condition is less important, since the fibres are already isolated by the tannage, and capillarity assists the penetration. Even in this case the distribution of the fat is much assisted if it is already in a state of fine division (emulsification), and if the surface-tension ([p. 76]) between it and water is low, as is the case with dégras and other partially oxidised oils. On this rather than on any special chemical affinity probably depends the importance of the “dégras-former” and other products of oxidation which are present in dégras; and the difference in penetrating power of different oils. So long as oil remains in an undivided condition, so long can it be squeezed out, and the leather will feel and appear greasy; while, when it is thoroughly emulsified, and adherent to the fibre, it can no longer be expelled by mechanical means. No doubt the different power of different tannages to “carry grease” without appearing greasy, is also related to the degree of isolation of the fibrils, and their surface tension with regard to fats. We may judge that the more readily an oil can be emulsified, the more freely and completely it is likely to fix itself on the leather fibre.