If, instead of using neutral iron salts, basic ferric salts (which may be obtained by dissolving ferric oxide in solution of neutral ferric salts, or by oxidising ferrous sulphate with manganese black oxide, or nitric acid) be employed, much larger quantities are absorbed by the hide, and if this be fixed with soap baths and finished with a moderate quantity of oil, a gain of weight—approaching 50 per cent. of the finished leather, or about the same as that given by bark, may be obtained. The leather, however, has by no means the same resistance to wet and decay as bark-tanned leather, and invariably has a tendency to crack when sharply bent. The process has been most carefully worked out by Professor Knapp, and was patented and worked commercially for a short time in Brunswick, but apparently without financial success. Professor Knapp's method is as follows:—The iron solution is prepared by adding nitric acid to a boiling solution of ferrous sulphate (green vitriol) till the iron is completely oxidised to the ferric condition. To this, ferrous sulphate is again added so long as it continues to cause effervescence. The resulting solution is a clear orange, and of more or less syrupy consistence, and may be evaporated without decomposition or crystallisation to a transparent varnish. The hides are unhaired and prepared for tanning in the usual way, and are then handled in solutions of the iron salt, which are at first weak, and are gradually strengthened. Skins are tanned in 2-3 days, and the heaviest hides in a week. After tanning, the hides are stuffed in a drum ventilated through the axes, very similar to that described under "crown leather," [p. 213], with an insoluble iron-soap made by precipitating soap solution with the iron-liquor; or the iron-soaps may be formed in the hide by the alternate use of iron and soap solutions, as already described. The leather is finally saturated with a solution of stearin and paraffin, to render it waterproof.
A process which has been worked on a larger scale, is that of Dr. Heinzerling, introduced about 1878, with the usual promise of "complete revolution" in the leather trade; but which, in spite of the most determined and persevering efforts of the Eglinton Chemical Company, who own the English patent, has failed to take any very prominent position in commerce. The tanning materials employed are alum and salt, with a varying proportion of potash, soda, or magnesia bichromate. These salts have a very marked hardening effect on animal tissues, and, when mixed with gelatin and exposed to light or acted on by acids, become reduced, and at the same time render the gelatin insoluble even in hot water, a property which is made useful in many photographic processes. This is probably due to the formation of salts of chromium, which, as has been stated ([p. 219]) have a similar tanning effect, but perhaps more powerful, than those of alumina. However this may be, the effect of potash bichromate when exposed to light with gelatin, differs from that of the addition of chrome salts ready formed, the gelatin in the first case becoming incapable of even swelling in hot water, while in the second, though rendered insoluble, it becomes soft and swollen. The use of potash bichromate in tanning had been previously patented by Cavalin, who used it in conjunction with alum and salt, and with the addition of a portion of green vitriol, to give the leather a colour more similar to that of bark-tanned.
Dr. Heinzerling uses metallic zinc in the salt and alum solution to assist in the precipitation of amorphous alumina on the hide-fibres. The same material was used in a similar way by Jennings (No. 2295, 1861), but with the object of whitening the goods. Yellow or red prussiates of potash (potassic ferrocyanide or ferricyanide), are also sometimes mixed with the solution, in order to enable the leather to be blacked with iron-liquor, with which they produce prussian blue. To fix the tannage on the fibre, and prevent its washing out, the use of barium chloride, lead acetate, and of soap solution is claimed; the latter having been also patented for similar purposes by Knapp, and subsequently by Jennings and others.
In order to render the leather waterproof, it is finally saturated with solutions of paraffin, stearin, and other fats and hydrocarbons (resin is employed, though not named in the patent), in petroleum spirit and similar solvents.
Such is the original patent, which, it will be seen, is rather a combination of older processes than an original discovery. Whether it is still worked on the same lines the writer is unable to say, but he is aware that considerable improvements have been made in the finish and appearance of the goods. The leather in its present form possesses considerable resistance to water, is free from the brittleness so common in mineral tannages, and like other alumed leathers, considerably exceeds bark-tanned leather in toughness and elasticity. These make it valuable for many purposes, and among others, for machine-belting, although it has the disadvantage of elongating considerably while in use.
[CHAPTER XX.]
CALF-KID.
Calf-kid is used for light upper-leather, and belongs to a different class from any yet described, being "tawed" instead of tanned. In this respect, and in most details of its manufacture, it resembles glove-kid.