Tanning (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 ”

“ 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. ”

“ The operation of tanning leather consists in exposing a hide to the action of a chemical ingredient, called tannin, for a length of time sufficient to allow every particle of the hide to become saturated with the solution. ”

“ The Scientific American, in alluding to Morfit’s excellent work on the “Arts of Tanning,” had the following notice which we here insert:
“In looking over this book, and reading the different plans for improving leather, and for reducing the time occupied in tanning a skin or hide, we are more and more convinced of the important fact that the tanning art has been greatly improved by modern discoveries and application—a contrary opinion to that held by the universal mass of the people. ”

“ 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 ”

“ 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. ”

“ It is, however, now very common for the tanner who produces such leather also to curry it, and, as this effects a « 192 » considerable economy, both in labour and material, it is likely to become universal. When leather is to be sold rough, it is necessary to tan it in such a way as to give it a white appearance, from the deposit of "bloom" already mentioned; this being regarded by curriers as an essential mark of a good tannage, although the first step in the currying process is to completely scour it out. ”

“ 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. ”

“ 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. ”

“ In olden days, it was customary not only to wash the hides much more thoroughly in water, but to "scud" them (i.e. work them over with a blunt knife) , to remove lime, and the detritus of hair-roots and fat-glands, and this should never be omitted from sole-leather treatment where bright colour and clean buff are desired. Some tanners go so far as to bate best butts slightly with hen-dung, but with such treatment firmness and weight are lost. Washing in weak solution of sugar, or ammonic chloride or sulphate, or of sulphuric, or hydrochloric acid, may be adopted. ”

“ 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. ”

“ 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.) ”

“ If, as is often the case, the tanner is his own currier, he does not tan the goods so heavily, or trouble about adding superfluous weight, but otherwise the after processes, the art of the currier, are the same.
Currying consists of working oil and grease into the leather to render it pliable and increase its strength. It was once thought that this was a mere physical effect produced by the oil, but such is not the case. Currying with animal oils is a second tannage in itself; the oils oxidize in the fibres and produce aldehydes, which are well-known tanning agents ”

“ We claim the process of tanning with the use of lime, salt, bran, sumac, and cutch, or any other tanning in room of cutch, substantially in the manner described, whereby we commence tanning, at the same time that we commence reducing, as the salt and bran overpowers the lime, the tan takes the place of the lime, and converts the hide into more perfect leather, and in less time than can be made in any other way.
Hides are not liable to get damaged by our process, as we do not use an article that is injurious to leather. ”

“ Their solubility in water depends on their degree of hydration, many tannins giving a series of anhydrides of which those containing only one molecule of water less than the original tannin are quite soluble in water, while the higher members of the series become less and less soluble as they lose water. Those which are soluble form the colouring matters of tanning materials, and generally are practically tannins, precipitating gelatin and combining with hide to form leather. ”

“ To overcome these evils, some American tanners now pass the hides, after sweating, through a weak lime. This, to a great extent, prevents the hair fixing itself in the flesh, and tends to counteract the injurious effect of the vitriol (which is almost invariably used in plumping sweat stock) on the colour of the leather. By this process, 10,000 Texas and New Orleans wet-salted hides gave an average yield of leather of 73 per cent. on their green weight, and the leather was excellent in quality (Schultz) . ”

“ From this it would seem that time is an essential element in the process of tanning, and that we cannot make up for the want of it by increasing the strength of the liquor, or raising the temperature at which the process is conducted, any 305more than we can fatten an ox or horse by giving him more than he can eat.
Hides that are treated with liquor below the proper strength become much relaxed in their texture, and lose a portion of their gelatine. The leather necessarily loses in weight and compactness, and is much more porous and pervious to water. ”

“ This treatment imparts great firmness, or more properly speaking, hardness; but the leather is deficient in toughness, and the grain usually cracks on bending sharply. Extract properly used is, however, capable of making excellent leather; it is employed in at least one of the highest priced tannages in the country.
It may be noted here, that when Continental writers speak of extracts and extract tannage, what we should call liquor tannage only is meant, and not specially the use of the concentrated extracts, to which alone in England the term is applied. ”

“ For the best heavy tannages, 6-8 layers are required. Each time the butts are raised, they should be mopped on the grain, to remove dirt and loose bloom. Strong valonia liquors, or heavy valonia dusting, causes a brown sandy crust to form on the freely exposed parts of the butts. This is removed in striking, but is sometimes very troublesome on rough dried dressing leather. In pure bark tannage, which, however, is gradually becoming extinct, the liquors used are of necessity much weaker, as it is extremely difficult to obtain liquors of more than 25°-30° Bark. ”

“ 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 materials that are used to tan hides and skins act upon the hide fibers in such a way that the hides are rendered proof against decay and become pliable and strong. There are many vegetable tans; they are used for sole leather, upper leather, and colored leather for numerous purposes. The bark of hemlock trees is one of the principal tans. The woods and barks of oak, chestnut, and quebracho trees are often used. Palmetto roots yield a good tan. Large quantities of leather are treated with gambier and various other tanning materials that come from foreign countries. ”

“ Usually, however, the term "combination tannage" is confined to those cases where the main tanning agents not only differ in type, but where none are in predominant quantity. A typical case is that of "semichrome leather," in which a vegetable tannage is succeeded by a chrome tannage. E.I. tanned sheep and goat skins are rather heavily "stripped" of their vegetable tannage and heavy oiling, by drumming with warm soda solutions, and after washing with water are chromed with the one-bath process; they are neutralized, dyed, fat liquored and finished for glacé upper leather. ”

“ 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. ”

“ The extract of bark in liquor form is used today by all large tanneries.
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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. ”

“ 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. ”

“ 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. ”

“ The warm water soaks, softens, and swells the roots of the hair, and much the same result is obtained as in "scalding" pigs. So little lime really permeates the inner fibre that, after « 145 » a slight wheeling, the hides may be thrown into cold water, and allowed to cool and plump, preparatory to taking their places in the handlers. The process is strongly recommended for sole-leather, particularly where great firmness of fibre is desired. The tanner who tries it must be satisfied if he gets 20-30 sides a man unhaired and fully ready for the liquor per diem. ”

“ Morocco leather is produced from goat-skins. Rough-haired or "blue-back" seal-skins are also used, and produce an excellent article; while an inferior description, called "French morocco," is produced from sheep-skins. The skins are unhaired by liming in the usual way, and are then baited with a mixture of dogs' dung and water. The tanning is done chiefly with sumach, at first in paddle-tumblers, and then in handlers, lasting about a month in all. ”

“ Generally, the best mixed tannages for solid leather consist of a combination of both pyrogallol and catechol tannins. Reviewing the characteristics of some of the principal tanning materials, it may be said that oak bark produces a nice, fawn-coloured leather of strong texture, but tans slowly; valonia makes the leather solid, durable, and waterproof; myrobalans quicken the process and lighten the objectionable colour of other tannins; divi-divi and algarobilla are very rich in tannin, and are, therefore, useful in later stages of the process ”