CHAPTER XVII.

_COMBINATION OF VEGETABLE AND MINERAL TANNAGE._

In very early times leathers were produced, which were partly tanned with alum, and partly with vegetable materials. One of the earliest of these was probably the Swedish or Danish glove-leather. The principle has long been applied to the production of certain very tough and flexible leathers known as “green leather,” and used for “picker-bands” for looms, laces for belting, “combing-leathers” and some other purposes where softness and toughness are of principal importance. About twenty-five years since, it was applied in America by Mr. Kent to the manufacture of an imitation of glazed kid, which he named Dongola leather; and since that time, the method in various modifications, has taken a considerable place in the manufacture of the finer leathers for shoe purposes, especially in the United States.

Alum-tanned leathers, as has been already stated, are remarkable for softness and toughness, and the mineral (crystalloid) tannages have the power of penetrating and isolating the individual fibrils of the skin in a much greater degree than the vegetable tannins, and hence are less dependent than the latter on a previous isolation produced by liming. On the other hand, they give much less plumpness and solidity, and more liability to stretch, and are less resistant to the action of water; and are, as a general rule (to which some chrome-tannages are an exception), incapable of producing a soft leather without mechanical softening (staking) after the tannage is completed. Purely mineral tannages have always a woolly fibrous structure, and never the firm and compact flesh which is required in leathers which are to be “waxed,” or finished on the flesh side to a smooth surface, and as they communicate more or less of these peculiarities to combination-tannages, the latter are mostly used, either for grain-finish, or for uses where a soft and velvety flesh-side is required, as in the case of “ooze-” or “velvet-” calf. On the other hand, the partial use of vegetable tannage communicates to them a degree of plumpness, fulness and resistance to water which is not possible to alum-tannages pure and simple, and a softness which is not easily obtained in vegetable tannage without the use of large quantities of fats or oils. A preliminary mineral tannage also greatly increases the rapidity of the penetration of the vegetable tans, by isolating the fibres, and rendering them less gelatinous. Once a leather is _thoroughly_ tanned by vegetable materials, it is little affected by subsequent treatment with alumina, or even with chrome; and on the other hand, though chrome and alumina leathers are still capable of absorbing considerable quantities of vegetable tannins, they always retain, in a degree, the qualities which the mineral tannage has communicated to them. The resulting leathers are thus not only modified by the different proportion of vegetable and mineral tannages which have been given, and by the properties of the particular vegetable tannage used; but by the order in which the several treatments have been given, and always retain, to a considerable extent, the characteristics of that which has been first applied. We have thus in our hands a powerful means of modifying the character of our leather to suit the special requirements which it is to fulfil.

So long as tanners were restricted on the one hand, to the ordinary methods of stuffing tanned leathers with oils and fats, and on the other to the use of egg-yolk, which had long been common in alum-tannages, combination-tannage remained of but secondary importance, and it was the application of the method of “fat-liquoring” by James Kent to his Dongola leather, which gave them the place they now possess, by providing a cheap substitute for egg-yolk, and enabling the tanner to obtain softness and resistance to water, without producing the greasy feel which is common to curried leathers. The process of fat-liquoring has already been mentioned in connection with chrome leathers, to which it was subsequently applied, and we shall return to it, after having given some further details of the methods of tannage.

In the first place we must consider briefly the mutual action of the mineral and vegetable tannages on each other. It has been pointed out by Eitner, and also mentioned (p. 339) in connection with the decolorisation of extracts, that the addition of say ¹⁄₂ per cent. of alum, or aluminium sulphate to tanning liquors, lightened their colour, not only by giving a degree of acidity to the solution, but by precipitating a portion of the less soluble and more darkly coloured tannins. Chrome-alum, and basic chrome salts produce a similar effect, though from their marked colour, the lightening of the solution is not so easily observed. It is therefore advisable if these salts are to be used in actual mixture with the vegetable tans, to allow the solution time to subside, or to filter off the dark-coloured precipitated matters. Larger quantities than ¹⁄₂ per cent. of the alum do not appear materially to increase the effect just described.

A second effect produced by these mineral salts on vegetable tannins, is in many cases to develop mordant colouring matters which are present; and thus, since most of these colouring matters are yellow, to produce a yellower leather than would be obtained with the vegetable material alone. This effect is very marked in the cases of sumach, gambier and quebracho. The compounds which these colouring matters form with chrome are mostly of a darker shade than those with alumina, tending to olive, and therefore chrome-combination leathers are generally dull in colour. Potassium dichromate, especially if acidified, generally oxidises and precipitates tannins, and darkens their colours, so that it is not practical to follow a vegetable tannage by the two-bath chrome process; and though the reverse order may be pursued, the single-bath chrome process, and that following and not preceding the vegetable tannage, generally gives the best results. If lightly tanned leathers, such for instance as the imported East Indian tannages, with babool or turwar barks, be treated with a basic chrome tanning liquor, such as is described on p. 215, so large a proportion of chrome will be absorbed, that the leather will possess most of the characteristics of a genuine chrome leather.

Combination-tannages for glove-leathers, such as the Danish and Swedish leathers already alluded to, are generally first tawed with alum and salt, with or without addition of flour and egg-yolk, and are then coloured, and more or less tanned with vegetable materials. That employed on the original Danish leather was willow bark (of _Salix arenaria_). In France, where this willow is not found, the bark of the commoner _Salix caprea_ was substituted; and as it is much weaker in tannin, additions of oak-bark or sumach to supply the deficiency, and of madder to give a redder colour were made to it. The dyeing of these leathers is frequently combined with the tannage, dyewoods or dyewood liquors being mixed with the tanning liquors. In the manufacture of glazed French kid, indeed, the process is so arranged, by brushing on dye-liquors mixed with tannins, as merely to tan the grain-surface, which is necessary to enable it to be glazed by friction, leaving the substance of the leather of purely alum tannage.

On the other hand, in the “green leathers” (so-called from their greenish-yellow colour, and largely made in the West Riding of Yorkshire), the hides usually receive a light gambier tannage, extending over a week or so in weak gambier liquors in handlers, and are then “cured” by handling in hot and strong solution of salt and alum, in which they are finally left all night, and then dried rapidly without washing out the alum, much of which consequently crystallises on the surface. This is slicked off, and the leather damped back, and heavily stuffed with sod-oil. If, however, the combination-tannage is properly carried out, it will stand liberal washing without losing the necessary alum, and of course a tougher and more satisfactory, though somewhat lighter weighing leather results. It is in many cases a better plan to combine the two tannages in one bath, mixing the alum and salt with the gambier, and handling or paddling the goods in the mixture. This is the plan usually adopted for Dongola leather, in the United States. For skins which are to be glazed, it is important that the _surface_ should be tanned with the vegetable material, and the goods are therefore worked into gambier liquors, to which the salt and alum are only added after the tannage has made some little progress; while for dull Dongola, intended rather to imitate calf-kid, it is best for the alum and salt tannage to begin first. For goat-skins for glazed Dongola kid, about 4 lb. of block gambier, ¹⁄₂ lb. of alum, and ¹⁄₄ lb. of salt are used per dozen, and the tannage occupies in all about twenty-four hours.

After the skins are tanned, they are thoroughly washed out with tepid water, to remove loose alum and gambier, and are then ready for fat-liquoring. As in the case of chrome leather, it is of great importance that this washing should be done thoroughly, as any remaining alum which diffuses into the fat-liquor, will cause it to curdle. If the washing is thorough, the more neutral the fat-liquor and the better; but a somewhat alkaline soap-solution is less liable to curdle. The original fat-liquor used by Mr. Kent was the alkaline liquor which had been used in washing the surplus oil from chamois leather (see p. 380), but now soap- and oil-solutions are generally made specially for the purpose. Most of the remarks in the chapter on chrome tannages are applicable in this case, but probably fat-liquoring is somewhat easier than in the case of chrome. Mixtures of either soft soap or curd soap with cod, sod, and olive oil are frequently employed. Sesame oil also seems well adapted for the purpose. The better these are emulsified, and the more satisfactory is the result; a cylinder of zinc or copper fitted with a plunger, something like that of a “Lightning Egg Beater,” but covered with perforated zinc, or wire gauze, does very good service as an emulsifier on a small scale.

Another method is to melt the soap with just sufficient water to make it pasty, and to incorporate the oil thoroughly with the mass, which is afterwards dissolved in hot water. Oils are the most easily emulsified when they are somewhat acid. For this reason rancid olive oils are often used for the Turkey-red process, but a similar effect can be obtained by adding a small quantity of candlemaker’s oleic acid to the oil before mixing. The addition of sulphated castor oil (Turkey-red oil) also helps emulsification, and is in itself a very good softening agent. One of the commonest mistakes in fat-liquoring is the employment of too strong an emulsion--even so small a quantity as ¹⁄₂ per cent. of soap, and half that quantity of oil, reckoned on the wet weight of the well-drained leather, will produce a very notable softening effect. Of course, for dull finishes, much larger proportions may be used. Not only combination-tannages, but those entirely vegetable, can be fat-liquored with excellent effect, and the process is now largely used for coloured calf, and other leathers, which are required soft and nourished, but without any appearance of greasiness. Leathers absorb the fat-liquor most readily if put into it in a sammed or partially dried condition, but even if quite wet, they soon take up the whole of the oil and soap on drumming, leaving only a little clear water in the drum. Goods may be blacked while still wet with fat-liquor, but should generally (except in the case of chrome leathers) be dried out before dyeing, as this fixes the oil and soap in the fibres.

Many coloured leathers are now made by a process which may be considered a combination of the Dongola process itself with the ordinary process of vegetable tanning, the goods being coloured and partially tanned as if for a vegetable tannage, and then finished in Dongola liquors with alum, salt and gambier. Very good leathers are made in this way in the United States, with a tannage begun in suspension in hemlock bark liquors.

Imitations of Dongola leather are made by treating East India sheep or goat with alum liquors, and afterwards fat-liquoring (if necessary)[128] and finishing like genuine Dongola leather. The treatment is most effective, if a portion of the original tan be removed by washing with warm water, with a little borax, ammonia, or even soda, and the goods then alumed with a “neutralised” or basic alum solution such as that described on p. 187. Goods treated with a basic chrome-liquor, like that used for the one-bath chrome process, p. 212, are almost converted into chrome-tanned goods, and will even stand some degree of boiling. The use of a liquor made like the Martin-Dennis liquor, by dissolving chromic oxide in hydrochloric acid, was the subject of an American patent[129] which in this country is owned by Wichellow and Tebbutt, but which expires in 1903.

[128] East India sheep and goat are generally so heavily oiled with sesame oil (up to 30 per cent. of their weight), that it is desirable in many cases rather to diminish than increase the oil, which may be done by washing with soap solutions, preferably before aluming.

[129] Eng. Pat. Jensen 13126, 1889.

Chrome-combinations may also be made by retanning goods tanned by either of the chrome processes with vegetable materials, of which gambier seems the most suitable. The use, even of very weak liquors of sumach and most other tanning materials, deprives chrome leather of its stretch, and if carried to excess, readily makes it hard and tender.