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.
Combination Tannages
Combination tannages have steadily grown in favour during the last few years, and will probably have an important bearing on future methods of tanning. Among those found practicable are: (1) Vegetable and chrome; (2) vegetable and alum; (3) alum and chrome; (4) synthetic and natural tannins; (5) synthetic and chrome; (6) alum and synthetic; (7) formaldehyde and chrome; (8) chrome and iron.
CHAPTER VII
THE DRESSING, DYEING, AND FINISHING OF LEATHER
The dressing and finishing of leathers of all kinds seems to be of growing importance every year, despite the fact that the durability of leather is often impaired by the chemicals and heavy machinery used in order to get a clear and bright colour or a highly-glazed finish. One exception to this rule, however, is japanned and enamelled leathers made by the new collodion-amyl acetate process, which not only produces the so-called "patent" finish, but also adds to the strength of the leather.
Dressing and finishing may be conveniently classified in four sections—
1. Finishing of boot sole leather.