Leather: From the Raw Material to the Finished Product

by K. J. Adcock

Mineral Tannages

By far the most important of the mineral tannages is the chrome process, the merits and demerits of which have not only been freely discussed in the trade, but also in the lay press. Fanciful theories of the poisonous character of this kind of leather have been published from time to time in the daily press. Such absurd statements as that prussic acid and mercury are used in the manufacture of chrome leather hardly need refuting, as, even if they could convert skins into leather, the cost would prohibit their use. The only poisonous acid used in one of the many chrome processes is chromic acid, but this is converted into the oxide of chromium in a second bath and is, therefore, made quite innocuous.

The chrome tannage is effected either by the one-bath method or the two-bath. In the former case, the tanning agent, either a basic chromium sulphate or chloride, is present in the one liquor used; in the latter method, the hides or skins are impregnated with a solution of chromic acid, which is reduced to chromic oxide in a second bath consisting of sulphurous acid and a small quantity of free sulphur. The properties of leather produced by the two-bath process render it especially suitable for vulcanising on rubber; hence its large use for non-skidding bands for motor tyres.

There are several recipes for making one-bath liquors. A favourite mixture consists of chrome alum and sodium carbonate (common soda). Another method consists of reducing a solution of chromic acid with glucose or grape-sugar. This liquor has a greater plumping effect on the leather than the chrome alum liquor has. A third process of making a one-bath liquor combines the use of bichromate of potash and chrome alum, which, when dissolved, is converted into a basic chrome salt by means of a reducing agent. The one-bath liquor can be easily and safely applied to hides and skins, and is used in much the same manner as a vegetable tan liquor, beginning with a weak solution and gradually increasing the strength until the process is completed. The two-bath method needs great care, as a slight difference in the proportions of the ingredients used may alter the character of the leather produced.

The formula now largely used is practically the same as that of the original patentee, Augustus Schultz, an American chemist. The first bath, the chromic acid solution, is made by treating bichromate of potash or soda with hydrochloric, sulphuric, or formic acids. Bichromate of potash and hydrochloric acid (commonly known as muriatic acid, or spirits of salts) are commonly used, and in the proportion of 5 per cent. and 2-1/2 per cent. of the weight of the drained pelts (5 lb. of bichromate and 2-1/2 lb. of acid for 100 lb. of pelts). The chemical reaction is represented by the following equation—

K₂Cr₂O₇ + 2HCl = 2KCl + 2CrO₃ + H₂O

Bichromate of potash + Hydrochloric Acid = Potassium chloride + chromic acid + water.

This process is most conveniently carried out in the paddle-vat (Fig. [25]), which, in this case, should be fitted with a wooden cover to exclude light, since the colour of the chromic acid liquor is affected by strong light. Some chrome tanners prefer to use the drum tumbler (Fig. [13]), but the pounding of the skins by this method is apt to make the grain loose. Whichever method be adopted, the hides or skins should be horsed up to drain for several hours before transferring them to the second bath. It is important that they be placed grain to grain and smoothed out, as creases and air bubbles between the skins become fixed in the second liquor, and depreciate the value of the finished leather. To avoid this danger, many tanners pass the hides or skins through a striking-out machine under light pressure. Another detail of importance is to cover the chromed skins with canvas or matting to keep them from the light. Although the skins are preserved by chromic acid, they are not made into leather, for in this condition they would dry quite horny. Hence, it is undesirable that the hides should be dried at this stage and sold as leather, although such a proceeding has been attempted. Further, chromic acid is an irritant poison which may cause an eruption on the hands and arms of workers handling the hides in this solution, unless they are protected by rubber gloves, or by coating the hands with a mixture of vaseline and lanoline. The second bath consists of chemicals which reduce the chromic acid to the oxide, which is quite inert, so that there is no danger whatever of contracting a poisoned foot as the result of wearing chrome leather. The chemicals largely used for the second bath are hyposulphite of soda and hydrochloric acid. Sodium sulphite may also be used without the addition of acid. The skins change from an orange colour to a pale, bluish-grey tint, but the process is not completed until a cross section shows that the colour has changed right through the skin. A suitable proportion is 10 per cent. "hypo" and 2-1/2 per cent. acid (28° Twaddell) calculated on the weight of the drained pelts. Calf, goat, and sheep skins are usually treated in the drum, but hides, especially if a certain degree of firmness is required, are preferably run in the paddle-vat. The reaction of the chemicals in the second bath are somewhat complicated, but the principal point is the reduction of the chromic acid (CrO₃) to chromic oxide (Cr₂₃).

The sulphurous acid produced acts as the reducing agent, but is not freely liberated until towards the end of the process, the skins first changing into a dirty brown colour which gradually gives place to a beautiful pale-bluish tint. The last stages of the process are also marked by the formation of free sulphur, which aids materially in softening the leather, and giving the two-bath chrome tanned leather its characteristic rubbery texture. The vats in which this process is carried out should always be fitted with lids to confine the strong sulphurous fumes. By adding an excess of "hypo," the skins can be bleached until they are nearly white. This has no harmful effect on the leather, but makes it softer, though somewhat looser on the grain. When completely tanned, the skins are horsed up again, left to drain for at least twenty-four hours, and are then ready for dressing and finishing.

The recipes for the one-bath process are numerous, but it is becoming a common practice for tanners to buy the liquors or extracts already prepared in chemical works, which are, naturally, better fitted up for the production of a more uniform material than it would be possible to make in most tanneries.