MINERAL-TANNED LEATHER.

The invention of the earliest form of mineral tanning, that with alum and salt, dates from remote antiquity; but as it is in large measure the type of all that has been since done, it deserves examination in some detail, at least as regards principles. In practice it is used alone in curing skins with the hair on, and for making white leather for laces and other purposes; and, in combination with oil and albumen, which, as we have seen, are the tanning agents in the case of "crown leather," it forms the process for producing calf and glove kids, as will be described under those headings (pp. 223, 225).

Careful researches by Reimer (Ding. Polyt. Jour., 205, p. 143 et seq.) show (what has long been known in practice) that alum alone is not capable of making a pliable leather. The salt, nevertheless, does not enter into combination with the alum, or even with the hide. Its function is partially physical, increasing the diffusion of the solution, and partially chemical, as in the presence of acids (and salts of acid reaction) it precipitates the coriin, and prevents it from gluing the fibres into a horny mass as it dries. Prof. Knapp has shown that this is the first essential in producing leather, and that raw hide may be converted into a pliable material with all the properties of white leather by simply withdrawing the water with alcohol, in which coriin is not soluble, and by which it is therefore precipitated. This leather, containing when dried no added constituent, is of course at once reconverted into raw hide by soaking in water. Both the salt and a portion of the alumina is removed from tawed leather by soaking in water, and it then dries hard and horny, and by boiling in water will yield a considerable percentage of gelatin. The alum is not absorbed as a whole. It is a double salt (alumina and potash sulphate or alumina and ammonia sulphate), and only the alumina sulphate is absorbed, potash (or ammonia) sulphate accumulating in the liquor. The alumina salt retained by the hide, especially in presence of much salt, contains slightly more than its normal proportion of alumina to acid, or in chemical language is to some extent basic. This is caused partly by the lime remaining in the skin from the unhairing process, which neutralises a portion of sulphuric acid, but in part is the result of the affinity of the hide-fibres for alumina, a certain small proportion of free sulphuric acid being left in the liquor. The accumulation of this and of potash sulphate is the reason why such liquors cannot be used perpetually by mere strengthening with alum, but must be frequently renewed. The attraction of hide-fibre for alumina sulphate is so strong, that in presence of a sufficient excess of hide it may be completely removed even from dilute solutions. Alumina acetate or sulphate may be substituted for alum with equally good results in practice, the only advantage of the latter being its easier preparation. Ferric and chromic salts and iron or chrome alum, may be substituted for common alum, and are absorbed in a similar manner, and in presence of common salt give equally pliable leathers, of a buff and pale greenish tint respectively. Without salt, the leathers are hard and brittle. In all these cases, the tanning agent may be to a large extent removed by simple washing with water. The tannage may be rendered more durable by passing the leather before drying through a weak bath of sodic carbonate or even lime-water, which precipitates the alumina, iron, or chrome in a basic form on the hide-fibres. Soap baths may also be used, by which aluminic, ferric, or chromic stearates and oleates are formed, possessing considerable toughness and resistance to water. So far as the writer is aware, no mineral tannage has yet been produced which will not yield gelatin when treated, first with dilute acid and then with boiling water; but this is rather a gain than otherwise, as leather scraps might be utilised for glue. There seems no reason why good and durable leather, for boot-uppers and for many mechanical purposes, should not be fabricated with salts of iron and chromium in conjunction with salt. If eggs and flour were also used, products similar to calf-kid would be obtained. Iron-leathers may of course be blacked with infusions of galls or many tanning materials, or with logwood. Ferrous salts have no tanning properties.

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.