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.