HIDES AND LEATHER

Evening Post, Volume CIV, Issue 19, 22 July 1927, Page 14

HIDES AND LEATHER

THE SCIENCE OF TANNING

At a mooting of the Chemical Society held recently at Victoria College, two interesting lee turns wore given before a well-attended mooting. Miss M. Copping gave a paper entitled "The Nitration of Phenols," which embodied the results of a thesis performed at the University. The interesting and unusual behaviour of some of the new compounds prepared (as well as the conclusions finally.drawn from, the completed work) were described, but being of a highly technical nature, will not bo given here. The speaker was congratulated and accorded a hearty vote of thanks. The second speaker, Mr. 3ST. L. Wright, gave a paper illustrated' by slides on "The Scientific Aspect of Tanning." The art of tanning, he said, "is the'oldest cra.f't known, being mentioned even in the third chapter of Genesis. In those times animal grease was used, but later smoke-curing, and then alum-tannage were discovered. Strangely enough for such an ancient industry, almost nothing was known of its chemistry until about fifty years ago. This is because the scientific study of tanning lies in the realm of bio-chemistry, which is itself quito a modern branch of science. Generally speaking, tanning may be described as the conversion of putrescible hides into a non-putresciblc article of commercial value. for solo leather, the hides of larger animals (ox, cow, bullock, horse, buffalo) are used. They (ionic from all parts of the world, and their quality is largely affected by climate, and the usage of the animal. Animals from colder climates have thicker hides, and those of continental animals (mainly beasts of burden) are generally excellent. .South American animals, whioh furnish a large proportion of the world's total supplies, do not give such good hide as formerly, as they are raised principally for meat, and ; aro fattened as rapidly as possible, this fattening taking place.-at tho expense of the hide. Tho hides—coining from all over the. world—must first bo treated to prevent bacterial action, so those coming from a distance aro cither wet-salted' or dryHalted,^ though those -of beasts killed locally for meat may couie in. imtrcat- | cd. Microscopical examination shows that protective coverings of animals (including human beings, aro fundamentally similar in structure, but differ greatly in texture and thickness, so that tho following processes, referring specifically to sole-leather, aro in a general way representative of those applied in tho preparation of any other class of leather. As soon as the hido enters the tanyard it, in ovcry case, goes through a soaking process. This should restore if to its natural pliability, as well as remove blood and dirt, and if salted, the salt also. Tho hides are next huspoudod in a solution of limo and other chemicals, until ready for "dopilation" (tho roinbval of hair, etc., by scraping). Usually it is put through an unhairing • and fleshing machine, in which the connective tissue and fat aro also removed. The next process is "rounding." It is hero cut in pieces, which aro later given different tannages, as roquircd. The limo used for dopilation would euuso grave trouble if allowed to remain, and is next removed. This roquiros exact chemical control of tho strength of the acids used in "the process. ■• Tho goods arc now ready for tho tanning process proper, which consists in treating them with suitable natural tanning materials, these being chosen according to the products desired. Besides natural substances, synthetic tunning materials "synlans") aro now largely employed, and exemplify the chemist's effort to manufacture an artificial product to improve upon and replace thoso furnished by Nature. The tonnage proper consists in working tho hides through a scries of liquors of increasing strength. Jixact chemical control 33 required in this most important process. They aro now pilod into pits with tan-bark between tho hides, for two to six weeks. When this treatment is completed, they aro scoured and scrubbed to remove any solid matter, and then dried in special drying rooms. Concent rntnd extract of tannin is afterwards forced into tho goods (process of "weighing"). Finally, they are oiled with cod and minoral oils, and again dried very carefully, which ensures good colour, and grain. Tho process is completed with rolling, trimming, and sorting. The hido originally contained approximately 75 per cent, of' water, which is to a large extent replaced by tanning material. Slides wcro shown representing sections and surfaces of hides and leathers, and also the machinery mentioned. The chemical aspect o£ thu industry was stressed throughout (ho lecturi', and tho speaker was given a hearty voto of thanks by die meeting.