Tanning

“ When men were skin-dressed animals they knew little or nothing of tanning. Tannic acid is found in nearly every plant that grows, and its combination with the fresh skins spread or thrown thereon, may have given rise to the observation of the beneficial result and subsequent practice. But whether discovered by chance, accident or experience, or invented from necessity, the art of tanning should have rendered the name of the discoverer immortal. ”

“ There was no other alternative; so kneeling down, I presented my bare back for the application of the lash.
"How do you like tanning?" he exclaimed, as the rawhide descended upon my flesh. "How do you like tanning?" he repeated at every blow. ”

“ Tanning.] [F. tanner, LL. tannare. See Tan, n.] 1. To convert (the skin of an animal) into leather, as by usual process of steeping it in an infusion of oak or some other bark, whereby it is impregnated with tannin, or tannic acid (which exists in several species of bark) , and is thus rendered firm, durable, and in some degree impervious to water.
The essential result in tanning is due to the fact that the tannins form, with gelatins and albuminoids, a series of insoluble compounds which constitute leather. ”

“ But you may ask why the tanner does not put the skins at once into a strong liquor? The reason is, that the exterior surface of the skin would soon become tanned, and the central part would remain untanned, which, in a short time, would begin to rot and decay, and the leather so treated would soon fall to pieces. The tanner, therefore, judges of the perfection of the tanning by cutting through the leather ”

“ The whole science of tanning depends on two principles, one the removal of the hair from the skin, 291with the least injury to the gelatinous matter of which it is composed, and the other is the rendering of the skin insoluble in water, and to resist the action of the atmosphere, and yet be flexible. ”

“ Second, I claim the process of tanning hides and skins by the use of any kind of tannin, in combination either with the muriatic acid of commerce, or with muriatic acid generated by a mixture of sulphuric acid and salt in water, with the tannin, in the manner substantially as above described.
317 CHAPTER V.
TANNING PROCESSES. More than one hundred tanning processes have been patented in the United States and Europe, and the cry is, “still they come.” New methods, and new works upon old methods are constantly presenting themselves. ”

“ The shrinking points of skins dressed by the various tanning methods are constant within certain limits, depending on the nature of the skin and on the conditions of tanning, and it is possible by observing the shrinking point, in conjunction with other characteristics of a given pelt, to determine what method of tanning was used. ”

“ Here from morning till evening paunchy men with shaven heads lie naked or almost naked in the sun, acquiring first a shrivelling of the cuticle which amounts to flaying, and then the tanning which is so triumphantly borne back to the Fatherland. ”

“ Strong tanning solutions have the effect of “drawing the grain” of pelt, i.e. contracting the fibres, and causing the leather to assume a very wrinkled appearance which cannot afterwards be remedied; at the same time “case tanning” results, i.e. the outside only gets tanned, leaving the centre still raw hide, and once the outside is case-hardened it is impossible for the liquor to penetrate and finish the tanning. This condition being almost irremediable, the leather would thus be rendered useless. ”

“ There is a primitive kind of tanning often practised, the tan being the bark of several trees that grow on the hills, such as the waggonhout-tree, or, failing that, the bark of the common mimosas from the river banks. Those who make a trade of tanning purchase the undressed skins from the hunters ”

“ In all the above processes, as the animal fibres on the surface of the skin absorb most readily the tanning principles, and thereby obstruct, in a certain degree, their passage into the interior fibres, especially of thick hides, it becomes an object of importance to contrive some method of overcoming that obstacle, and promoting the penetration of the tan. ”

“ The slowness of the process in tanning leather, and the imperfect manner in which it has hitherto been accomplished, arise from the difficulty in bringing the tannin or tannic acid into contact with the gelatinous tissue, or fibre of the skins ”

“ Tanning under these conditions is at its easiest; it is almost more difficult to spoil the goods than make them right. Under such conditions tanning deteriorated rather than improved in method. When neglecting it made little difference to the finished leather, it was neglected.
This state of affairs, however, was embarrassing whenever a tanner wished to try any other tanning material. The expense of gambier and oak bark made valonia and mimosa bark into obviously desirable alternatives and substitutes. ”

“ It is an essential qualification that nothing of a fatty nature be used in their preparation. The tanning process of the best skins, whether with a glossy surface artificially obtained by rolling or with a dull surface, should be a very simple one. Oak and sumach tannins (used especially for morocco leather) are those which best preserve the suppleness and capillarity of leather. The susceptibility of its colour to alteration from exposure16 to light is greatly influenced by the materials used in tanning. Oak wood and chestnut are the tannins least influenced by light. ”

“ Many are inclined to believe that both changes take place, and this view is probably correct, as a pure tannic acid produces a thin, empty leather, while a tanning material which contains a fair proportion of non-tannin matters, yields a full and firm leather. The practical tanner who adapts his work to the theory that tanning is a process of feeding the hides with gradually increasing strengths of tannin, is, in any event, on the right track. The suspender liquors are always weak in tannin, and are pumped from "handler" liquors through which packs of hides have already passed. ”

“ When the tanner curries his own leather, he of course aims at putting in as little bloom as possible, thus economising both tanning material and labour. In addition, the leather goes direct from the tan-house to the currying-shops, thus saving both drying and soaking again, and, it is said, giving better weight and quality. The tanner, too, is enabled to shave his hides or skins more completely, utilising the material for glue-stuff, which, had the leather been for sale in the rough, must have been left on to obtain a profitable weight. ”

“ As the decoction trickles slowly through the seams below, more is poured on from above, and from time to time the position of the hide is reversed in such a way that the tanning permeates each part in turn. Sometimes only one reversal of the hide takes place half-way through the process, which occupies as a rule some [417] eight days. But energetic Chamārs continually turn and refill the skin until satisfied that it is thoroughly saturated with the tanning. After a washing in clean water the hide is now considered to be tanned.”25
12. ”

“ Skins not valuable for art purposes were sent to the tanners to be converted into various kinds of leather. Tanning was highly developed, and the tanners turned out leathers which are to-day admired for their excellence. The tanners carried on their industries by chemical processes similar to those in use to-day. ”

“ Tan′nin or Tan′nic Ac′id is of great importance in tanning leather. The chief source of tannin is oak-bark, though in America hemlock-bark is much used, while the mimosa or wattle-barks of Australia are important. ”

“ The extract of bark in liquor form is used today by all large tanneries.
[581]
After the hides have been all prepared for tanning they are hung on rockers in the tanning vats, where they are kept in motion both day and night so that all parts of every hide are equally tanned. They are changed from time to time from weaker into stronger liquor until the tanning process is complete.
Tanning Vats
All sole leather is filled more or less to make it wear the better. ”

“ It is important, however, that the grain be good, and free from scratches and similar defects. The tannage must be sweet and mellow, i.e. contain no acid and little astringent tan. Hence myrabolans and gambier have always been the favourite tanning materials. A soft and mellow tannage is the more important, inasmuch as the leather is not heavily stuffed with grease in finishing. These types of method for tanning split hides will [Pg 87] now be outlined, and the nature of the currying then indicated. ”

“ Some information as to the respective qualities of the different tanning materials will be found in the chapter on Tannins; but even practical men are very deficient in accurate information on these points, since many materials are never used alone, but invariably in connection with others which mask their effects. ”

“ The change has been of great benefit to the health of the workers, for the continual stooping over beams and sloping tables, combined with the arduous nature of the work, was very injurious. The reputation that tanning had as a healthy occupation was due more to the work of the labourers than that of the skin workers. (The old-fashioned lime-yards and tan-yards were generally in the open, whereas modern tanneries are roofed.) ”

“ Tan′nage, act of tanning: browning from exposure to the sun: the act of steeping cast slabs of artificial marble in a solution of potash alum to harden it and make it insoluble; Tan′ner, one who tans; Tan′nery, a place for tanning; Tan′ning, the art of tanning or converting into leather; Tan′-pit, -vat, a vat in which hides are steeped in liquor with tan; Tan′-yard, a yard or enclosure where leather is tanned. [A.S. tannian; cf. Dut. tanen, or prob. O. Fr. tan—Bret. tann, an oak. If the latter, then Old High Ger. tanna (Ger. tanne) , fir, oak, is borrowed.] ”

“ White or light colored clothing permits the penetration of light rays to the skin, and dark clothing prevents this; the former, therefore, is of greater vitalizing value. The sun and light baths are of great therapeutic value with nervous or anemic children. The tanning of the skin gives remarkable resistance. ”

“ The proper mode of tanning lasts a year, or a year and a half, according to the quality of the leather wanted and the nature of the hides. A perfect leather can be recognized by its section, which should have a glistening marbled appearance, without any white streaks in the middle. The hair which is taken off hides in tanning, is employed to mix with plaster, and is often surreptitiously put into hair-mattresses. ”

“ When the tanning of the skins is completed according to their notions, they are finished by drying them, or rather smoking them, in a pit in the ground, which is covered with bark and some earth. We have seen very good brown leather made by this process. We are not able to give the exact proportions of the paste used by Preller, but this does not make much matter, for some of our tanners can surely make up a paste with flour, ox brains, and oil or grease, &c., and give it a fair trial, by kneading a skin or two in a tub, with a beetle, so as to test the principle of the process. ”

“ There is a prospect of some return to a renewal of the beneficent influences of the sun, from the sheepish followers of fashion. This fickle goddess has recently started the doctrine that, as a reaction from the tanning effects of a summer's out-of-door exposure, the winter's change adds new brilliancy and transparence to the complexion. Fashionable butterflies now seek for the most complete tanning that the summer's solstice can effect, in order to secure a corresponding reaction, and insensibly gather health and invigoration. ”

“ For preventing furring, most tanning materials or even waste tan-liquors are very effective, and the danger of any corrosive action is lessened by the addition of a portion of soda ash. So far as is yet known, from the tanning point of view, it is hardly necessary to make any distinction between lime and magnesia, which may be considered simply as "hardness." ”

“ Without salt, the leathers are hard and brittle. In all these cases, the tanning agent may be to a large extent removed by simple washing with water. The tannage may be rendered more durable by passing the leather before drying through a weak bath of sodic carbonate or even lime-water, which precipitates the alumina, iron, or chrome in a basic form on the hide-fibres. Soap baths may also be used, by which aluminic, ferric, or chromic stearates and oleates are formed, possessing considerable toughness and resistance to water. ”

“ The use of "syntans" in conjunction with vegetable tanning materials is also a combination tannage. The case of chamois leather is possibly a combination of aldehyde tannage with fatty acid tannage. Two-bath chrome leather is a combination of chrome, sulphur and fat tannage. Formaldehyde and vegetable tannage is also a known possibility. It is clear that there are possibilities of endless complexity, and that what normally may appear as a simple tannage is in reality a very complex combination tannage. ”

“ Before speaking of actual processes of manufacture, it is necessary to devote some attention to the structure and chemical constitution of hide or skin, which forms the raw material. Although a great variety of skins are employed in tanning, they are all constituted on the same general type, and an anatomical description of the hide of the ox will apply almost equally to those of the calf, sheep, and goat ”