The Vegetable Tanning Process - A Collection of Historical Articles on Leather Production. Various
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tannins” from insect galls will not make leather is absolutely without foundation. It is somewhat curious that these oak tannins are by no means chemically identical. The bark-tannins distinctly belong to the class we have called catechol tannins, though that of the common oak contains some mixture which gives blue-black with iron and produces “bloom.” The wood tannins belong to the pyrogallol class, and the gall tannin is the typical “gallotannic acid” which is a pure pyrogallol derivative.
Another important material of the pyrogallol class is myrobalans (30—40%), the dried fruit of an Indian tree, and divi-divi, the pod of Cæsalpinia coriaria, a tree allied to logwood (40—50%) and sumach, the leaves of Rhus coriaria (25—30%), while the most important sumach adulterant, Pistacio lentiscus, is a catechol tannin.
To the catcchol class belong also gambier, an extract from the leaves of Uncaria gambia (30—40%); the bark of the Australian mimosa or “wattles” (25—40%); quebracho extract from the very hard wood of a South American tree (dry 60—70%) and many others. It will be noted that all these materials are very much richer in tannin than oak-bark, and it is to the stronger liquors obtained from them, rather than to any “chemical tanning,” that the shortened time of modern sole-leather tannage is due. Whether the shortened process gives so durable a leather as the older method may be questioned, but the leather is honestly and thoroughly tanned with mixtures of natural tannins very closely approximating to that of oak-bark; while in the old process much time was positively wasted by ignorant mismanagement.
The way in which tannins tan and the chemical nature of the leather formed still need elucidation. The view most probable at present is that leather is rather a colloidal than a strictly “chemical” compound. The tannins all yield colloidal solutions, and gelatine and hide-fibre are typical colloids. It has been shown in the author’s laboratory and elsewhere that, in presence of the trace of acid essential to tanning, the particles of tannin have opposite electrical charges to those of gelatine, and it is well known that two colloid solutions in which this is the case, when mixed, are mutually precipitated as a colloidal compound.
Although not strictly vegetable materials, the synthetic tannins recently discovered by Dr Stiasny must be mentioned here, as they will probably have considerable commercial importance. They are coaltar products, produced by the condensation of sulphonated phenols with formaldehyde, and produce almost perfectly white and very soft leathers. Though not identical with any natural tannins, they possess most of the characteristic properties of the class, precipitating gelatine and basic dyes and giving blue-black inks with iron.
THE VEGETABLE TANNING PROCESS
THE tannage of sole-leather, though full of problems and difficulties in commercial practice, may in theory be regarded as one of the simplest; and a clear understanding of its methods will render easy the comprehension of the rest.
The preparation and character of hides for sole-leather have been described in Chaps. VI and VII. To recapitulate, they are somewhat rapidly limed in fresh limes generally somewhat “sharpened” with sulphide of sodium; the whole process being so conducted as to produce good swelling and easy unhairing, with the least possible solution of valuable hide-substance which should go to make a heavy and solid leather. After unhairing and fleshing, the hides are “rounded” or trimmed, as only the thicker and central part called the “butt” is suitable for soles. After washing in water the butts were (and in some tanneries still are) considered ready for tanning, but it is becoming more and more usual first to remove, the surface-lime by a bath of boracic or some other weak acid, not only to secure a brighter and more uniform colour, but to economise the natural acids of the tan-liquors, which in the modern rapid process are much less freely formed than in the old method, in which there was abundant time and material for bacterial and other fermentations.
The butts first go into “suspenders,” a set of 8 or 10 deep pits in which they are hung by string to sticks laid across the top of the pit. Sometimes these sticks are supported on a frame to which a gentle reciprocating motion is given by suitable machinery, which causes a constant flow of liquor between the butts and prevents their remaining in contact, which would cause stains; but otherwise they must be moved and shaken by hand, especially in the earlier stage of the process. The pits are generally arranged so that liquor can flow from the top of one pit to the bottom of the next, so that the fresh liquor is all pumped into the strongest pit, and flows away exhausted from the weakest into which the “green” butts from the limes are brought and moved forward daily in the opposite sense to the liquors, which are appropriately the oldest and most exhausted in the yard; though if fresh liquors must be used, gambier or myrobalans are among the most appropriate materials. The object of using the oldest liquors is not merely one of economy, but because such liquors are what the tanner calls “mellow”; that is, their action on the hide is gentle, and only mildly astringent. This arises from several causes. Even if only one material is used, the tannins contained in it are a natural mixture; and the ordinary liquors of “mixed tannage” contain a still larger variety, varying much in their affinity for the hide-fibre. It is obvious that when a liquor is brought in contact with partially tanned hide, those tannins which are most astringent, and have the greatest affinity for the fibre must be removed first, so that what remains at last is only the mildest and least active part. Most tanning materials also contain a considerable portion of what the leather-chemist styles “non-tannins.” These are partly sugars and other carbohydrates, which during the process are gradually fermented by bacteria and yeasts to organic acids; and partly derivatives of the tannins themselves which, though they do not actually tan, are yet to some extent absorbed by raw hide, and promote its conversion into leather.
We may add to this the gradual accumulation in the used liquors of organic salts of lime and potash which weaken the acidity of their corresponding acids, and so keep the tanning action in check, since a certain degree of acidity is necessary for the process, and sufficient addition of alkaline salts may even bring it to a standstill.
The first process which takes place in the suspender is the neutralisation by its acids of any lime which still remains in the hide, and the consequent reduction of alkaline swelling. The hides, which, if not previously delimed, were at the outset plump and elastic, become very soft, and easily impressed by the finger; but in a properly conducted process should not lose much actual thickness, since the liquid which escapes from the fibres remains between them, and, as the lime is gradually replaced by the weak acids of the liquors, the fibres again swell and the hide again increases in firmness. That this should take place is essential to the production of a firm and solid sole-leather, for pelt tanned in the flaccid and “fallen” condition remains soft and porous, as well as thin. Fibres which have once been swollen by lime are much more sensitive to acid swelling than where this has not been the case, and those unhaired by “sweating” or any other process in which swelling has not occurred, require much stronger acids to produce adequate swelling and differentiation of the fibre bundles. For this reason American “sweated” sole-leather is usually swollen by dilute sulphuric acid in an early stage of the process, when the surface only has been tanned and thus rendered insensitive to the action of acids. If applied without this precaution, mineral acids produce a dark and brittle grain, and though this is prevented by the slight tannage, a dark layer may usually be detected beneath the thin tanned surface. For this reason, if “acid” leathers are “buffed” or glass-papered in shoe-manufacture, the surface is usually blacked and burnished or covered with a coloured “fake” or paint to conceal the actual leather and imitate a better material.
At the same time that the alkaline is being replaced by the acid swelling, the surface of the leather is taking colour, most rapidly indeed if the hides are somewhat limey; and actual tannage gradually penetrates, so that by the end of the suspender period, the hide, if not tanned, should be nearly or quite “coloured through.”
At the end of the suspenders the butts, still soft, are advantageously laid flat to straighten out any lumps or creases which may have formed, before they go to the “handlers.” Though the time of suspension is short and the liquors weak, the latter must be abundantly supplied, for the green hides have great avidity both for tannin and acid, and if allowed to remain in exhausted liquors, putrefaction sets in, which brings down the hides as in a bate, and renders it impossible later to produce a firm and plump leather.