Tanning (industry)

“ The varied requirements of the trade render it difficult to give any practical information as to the selection of tanning materials. As a general rule, it is important at the outset to give the required colour; and if materials undesirable in this respect are to be used for the sake of cheapness, they should be introduced in the form of liquors in the middle stages of the process, i. e. in the later handlers or earlier layers. Materials used as dust generally have more effect in producing bloom and colouring the leather, than those used in liquors at this stage. ”

“ 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.) ”

“ 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. ”

“ 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. ”

“ 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 ”

“ Another question which might occur to the non-technical reader is, why should not the process be hastened by placing the goods in strong liquors? The reason is simple. 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. ”

“ 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. ”

“ 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. ”

“ 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 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. ”

“ 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. ”

“ 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 ”

“ If the tannage is pure bark, it may perhaps be advisable to let the strength be somewhat less, but something depends on whether the exhausted liquors are returned with all their impurities to the "taps" or liquor-brewing pits, or whether the liquors are made with water, and hence purer. In any case, the free acid in the suspenders should always be sufficient in quantity to neutralise the lime brought in by the butts, or bad colour will certainly result, making itself visible in the shed, or as the tanning proceeds. ”

“ 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. ”

“ 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 ”

“ 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. ”

“ The hair can be removed by lime, sweating, and other means, but the employment of a substance or substances that will combine with the gelatine of the skin to form a new substance, insoluble in water and incapable of being injuriously acted upon by the atmosphere, offers a wide field for the historian of the tanning art, and presents a subject for the study of every tanner at least in our country. ”

“ 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." ”

“ The usefulness of such a combination will be evident, for while vegetable tanning produces fullness, plumpness and resistance to water, the mineral dressing produces a softness unnatural to vegetable tannages without the use of large quantities of oils and fats. It may also be noted that once a leather has been thoroughly tanned with either mineral or vegetable materials, although it will absorb large quantities of the material which has not been first used, it will retain in the main the characteristics of the tannage first applied. ”

“ 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. ”

“ 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. ”

“ 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. ”

“ 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. ”

“ 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. ”

“ 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. ”

“ But the quality of the tannin, or rather the quality of the leather produced by these different kinds of tanning materials, is a matter of quite as much 298importance as the relative or absolute quantity of tanning contained in each of them. While terra japonica possesses the greatest quantity of tannin, it is considered as producing the most inferior quality of leather. So hemlock, which, excepting the Sicily sumac, possesses the next highest quantity, produce the next worst quality of leather; while the oaks, which are the lowest in the scale of quantity, afford the most superior in quality. ”

“ Ure says:—“Various menstrua have been proposed for the purpose of expediting and improving the process of tanning; among others, lime-water and a solution of pearlash; but these two substances form compounds with tannin, which are not decomposable by gelatine; it follows that their effects must be prejudicial. There is very little reason to suppose that any bodies will be found, which, at the same time that they increase the solubility of tannin in water, will not likewise diminish its attraction for skin.” ”

“ 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. ”