Leather: From the Raw Material to the Finished Product

by K. J. Adcock

Fig. 25

PADDLE VAT

However, the drum is now largely used on the Continent for the tannage of sole leather. A great saving in the cost of production is thereby effected, but the leather, although of satisfactory appearance, lacks the durability and waterproof quality of pit-tanned leather. The process is much simpler than the pit method, and less room is required. There are only two stages of tanning: (1) by placing the hides in colouring pits or suspenders, in which the hides are nearly struck through with tannin; and (2) running them in slowly-revolving drums containing solutions of tannin which are gradually strengthened until the process is finished. There can be no doubt that the tannin is rapidly forced between the fibres of the hide by mechanical action, but it is not so firmly combined as that slowly absorbed by the hide in the pit method. This can be proved by placing two pieces of weighed leather—one tanned in pits and the other in a drum—for twenty-four hours, drying the leather and reweighing it, when it will be found that the drum-tanned leather has lost the greater percentage of weight. To obviate this disadvantage as far as possible, use is made of special oils, which serve to fix the tannin more firmly between the fibres and render the leather more resistant to water. Other frequent drawbacks of drum tanning are looseness of grain and lack of substance. In sole leather tanning, the former is modified to some extent by extra pressure in rolling the leather during the finishing operations; while the latter difficulty is sometimes overcome by swelling the leather with acid and then fixing the swollen condition of the hide by treating it with a weak solution of formaldehyde. This chemical also has tanning properties, so that the process is hastened; but leather prepared in this way cannot be as durable as that produced by a natural tannage.

It seems that the aid of the engineer is necessary to overcome the difficulty of looseness of the grain caused by the severe pounding of the hides in the drums. The constant circulation of the tan liquor is required, but the hides should only be subjected to gentle motion. It is true there are tanning drums on the market which are said to obviate all the difficulty found in making sole leather in ordinary drums fitted with shelves or pegs, but a drum that meets the ideal conditions for the production of a solid yet flexible sole leather has not yet been invented. It seems that a kind of inner framework, to which the hides can be attached and which rotates much more slowly than the main drum, may solve the problem. The drum tannage permits the use of an excessive quantity of tannin, which, of course, adds to the weight of the leather. By the drum method of tanning, heavy hides can be tanned in two days after leaving the colouring pits; lighter hides are, naturally, tanned in less time, proportionate to their substance. The pit method occupies any time from one to fifteen months, although nowadays very few hides are left in the tan pits for a year.

The methods of tanning just described relate chiefly to sole leather, but there is an enormous production of leather known as dressing hides, which are tanned, dried in the rough state, and sent to curriers or leather dressers for finishing. These hides are used for numerous purposes, including bags, portmanteaus, harness, saddlery, straps, belting, and boot uppers. The tannage of dressing hides differs slightly from that of sole leather; the liquors must be mellower and contain less insoluble matter, in order to obtain the necessary pliability, and a good, clear colour on the grain. A satisfactory tannage is obtained by treating the hides in oak bark liquors, which, in the later stages of the process, may be strengthened with oak wood, or myrobalan extract, or pure gambier, and completing the process in a tepid bath of sumach, which clears and lightens the colour. The drum is more suitable for the tannage of dressing hides than it is for sole leather. A quick method of drum tanning would be to treat the hides first in a 5 per cent. solution of neradol, the artificial tannin, and then complete the process with oakwood, chestnut, or quebracho extracts, or even in mixtures of these extracts. In this way, fairly good leather could be made in about two days. Neradol prevents the drawn grain and dark coloration that would result from the use of vegetable extracts alone.

In the case of light skins, such as calf, goat, and sheep, the method of vegetable tannage again differs from those just described, although there is a fair quantity of calf skins tanned with oak bark, especially those used in the shoe trade. The tendency, however, is to complete the process rapidly by using extracts, such as oakwood, quebracho, or mimosa. A very good tannage for the production of a mellow and plump leather is that of pure gambier, the colour produced forming a good ground for brown shades. Where light, fancy colours are required on the finished leather, this tannage must be completed by placing the skins in a tepid bath of sumach.

A large proportion of the vegetable-tanned sheep and goat skins is produced by sumach alone, which was adjudged by the Commission appointed a few years ago by the Royal Society of Arts to investigate the cause of decay in bookbinding leather to be the best tanning material and the one least affected by exterior conditions, such as gas, sunlight, air and dust.

Many sheep skins are split into two sections by machine before tanning, the top portion, known as the grain, being tanned in sumach, and described as "skivers." The under section, the side near the carcase, is known in the trade as a "lining," and is usually made into the so-called "chamois" leather by means of the oxidation of fish oils.

Other noteworthy vegetable tannages are those used in the production of Russia leather, and a large proportion of East India leather. Real Russia leather, of which the raw material consists of small native hides and calf skins, has a characteristic and pleasant odour, which is derived from the birch and willow barks used in the tanning process. Birch bark contains an essential oil, which is permanently fixed on the fibres of the leather during the process of tanning. This leather is only produced in Russia, and chiefly in one large tannery, although imitations are made in Great Britain, America, and Germany. These are produced by the use of ordinary tanning materials, and the scent is applied, either during or after the dyeing process by the addition of birch tar oil, which is made by the distillation of birch bark. In some respects, for example, in brilliance of finish, smoothness of grain, and freedom from defects, the imitation is better than the real, but the latter has the great advantage that its perfume is of superior fragrance and permanent, whereas the imitation leather only retains the odour for about a year. It is somewhat remarkable, in view of the good demand that exists for the leather, especially in England and Germany, that no firm outside Russia has thought it worth while to produce the genuine article. The tannage would be particularly serviceable for bookbinding leather, as the oil of the birch has both insecticidal and antiseptic properties.