PART VI.—MISCELLANEOUS PROTEINS AND BYE-PRODUCTS
SECTION I.—BYE-PRODUCTS OF THE LEATHER TRADES
In the leather trades by far the most important and valuable bye-products are obtained from the hides and skins themselves, and all these are obtained before the tannage proper is commenced. The leather trades use only the dermis (corium) or true skin for the manufacture of leather, and as we have noted (Part I., Section II., p. [16]) this prepared and purified dermis is called "pelt." The cuttings and trimmings from the pelt form the most valuable bye-product of the leather trades, and are the raw material of the gelatine and glue industries (Part V., Section II., p. [221]). Many portions of the pelt, indeed, such as ears, noses, and cows' udders, are quite useless for any other purposes. Other portions, such as cheeks, faces, and even bellies, may be made either into glue or leather according to the state of trade. Hardly less important to the same industry are the cuttings of adipose tissue removed in "fleshing" the hides and skins. These, though yielding less protein, yield also, however, the valuable animal greases (Part V., Section II., p. [227]). To obtain both these products in a purer condition the removal of "flesh" after "soaking," but before "liming" (Part I., Section II., p. [18]), has been favoured by some, especially in America.
Amongst the epithelial structures of the hides and skins, we have several protein bye-products which have some commercial value. The horns of cattle are now almost invariably removed before reaching the leather manufacturer, but have some little value. This part of the epidermis is not solid keratin. A "pith" is easily removed after boiling in water. The outer parts, too, are often coarse and somewhat damaged, but if removed by scraping reveal often a rather beautiful structure of varying colour. There is some opening for this product in the manufacture of small articles of horn, but much of it, together with hoofs, is roasted and crushed for making fertilizers. The hair of cattle, goat, etc., has also a commercial value. This is removed after liming, and needs subsequent purification (Part I., Section II., p. [22]). The hair is well washed with water, using either repeated changes or a continuous supply, the operation being carried out in paddles or similar machines which stir up the hair in the water. When clean, the hair is transferred to a centrifugal machine or "spinner," in which much adhering water is removed. This is a great assistance in drying out, which is the next and final operation. In drying, the hair is laid upon steam-heated boxes or pipes, and a current of warmed air passed over or through it by means of a fan. It is better to have the hair "turned" occasionally. This ensures quicker as well as more even drying. The product is made up into large bales and sold for the manufacture of felts, mattresses, etc. White hair is usually kept separate and commands a larger price. The power consumed in driving the washing machinery, the centrifuges and the drying fan, together with the fuel required for the drying steam, and the labour involved throughout, make it doubtful whether this bye-product is worth either the capital outlay or the working costs necessitated. Many manufacturers avoid this treatment altogether, therefore, and the wet limed hair is sold direct to the fertilizer factory. A less price is obtained, but much expense is saved. Especially when the animals have only their short summer coats, this course is preferred.
In the case of the wool from sheepskins the product is much more valuable. The wool, indeed, is often the primary consideration. Unfortunately this sometimes results in the neglect of the pelt. The removal of wool from sheepskins forms a special industry known as "fellmongering," which has been previously described (Part II., Section IV., p. [110]). Pains are taken to clean the wool even before removal from the pelt, by the liberal use of water and the "burring machine." There is much variation in quality, and care is taken to keep the various grades separate, even during the "pulling" operation. From the fellmonger the wool passes to the "wool stapler," and forms the basis of one of our most important mechanical industries, the manufacture of woollen cloths. Wool is also removed from sheep by the periodic shearing, and in this case does not reach the fellmonger at all.
Apart from the raw material itself, there are few bye-products of the leather trades which are of commercial importance. The sludge from the pits of the limeyard contains, in addition to much lime and chalk, a certain proportion of protein matter. This is derived partly from the blood and dung associated with the hide, partly from the solution of the corium hide substance, partly from the solution of the softer keratins, and partly also undissolved and loose hair. This bye-product is rather difficult to deal with, as it will not easily dry. It is indeed sometimes a problem to dispose of it, except in rural districts, where the farmers appreciate its manurial value and will usually cart it away for a nominal fee. Where possible, it is better to let it drain and settle on land, and pile it up in heaps to dry further. Soak-pit sludge has a distinctly greater value as manure, on account of the greater proportion of dung proteins. As some lime is often used in these pits, the product is a really useful fertilizer.
The only other bye-product of the leather trades is waste leather itself. For small pieces of leather there is always some little opening in producing small articles, such as washers for taps, etc., and there is also the possibility of shredding or pulping and making an artificial leather. The best leather substitutes, indeed, are made from waste leather. Nevertheless, there is always a certain amount of waste leather which only finds an outlet in the fertilizer factory. Such material is usually steamed or roasted to make it brittle, and then crushed in a disintegrator. It is then mixed in with other materials, but is sometimes solubilized by the action of sulphuric acid. Leather seldom contains less than 30 per cent. protein.
REFERENCES.
"Chemical Fertilizers and Parasiticides," S.H. Collins, M.S., F.I.C. (Companion volume in this series on Industrial Chemistry.)
"Wool Wastes," Part II., Section V., p. 75.
"Hoofs, Horns, Leather," Part III., Section II., p. 115.
"Gelatine, Glue, and Allied Products," T. Lambert.