PART III.—CHROME LEATHER
SECTION I.—THE NATURE OF CHROME LEATHERS
In these days the manufacture of chrome leather has attained a position hardly less in importance than that occupied by the ancient method of tanning by means of the vegetable tanning materials, and large quantities of hides and skins are now "chrome-tanned" after preparatory processes analogous to those described in connection with vegetable tannages (Part II., Section II.; and Part II., Section I.).
Chrome leathers are made by tanning pelts with the salts of chromium, and are typical of what are known as "mineral tannages," in which inorganic salts are the tanning agents. Tannage with alum and salt (see Part IV., Section I.) is one of the earliest mineral tannages, but is now of relatively minor importance. Chrome tanning was first investigated by Knapp (1858), who experimented with chromic chloride made "basic" by adding alkali, but his conclusions were unfavourable to the process. A patent was taken out later by Cavallin in which skins were to be tanned by treating with potassium dichromate and then with ferrous sulphate which reduced the former to chromic salts, being itself converted into ferric salt. The product, which was a combination of iron-chrome tannage, did not yield a satisfactory commercial leather. Another patent, taken out in 1879 by Heinzerling, specified the use of potassium dichromate and alum. This in effect was a combination chrome-alumina tannage. The alum had its own tanning action and the dichromate was reduced to chromic salts by the organic matter of the skin itself and by the greases employed in dressing. The process, however, was not a commercial success. In 1881 patents were obtained by Eitner, an Austrian, whose process was a combination chrome and fat tannage. The chrome was employed as "basic chromium sulphate" made by adding common soda to a solution of chrome alum until a salt corresponding to the formula Cr(OH)SO4 was obtained. Such a solution is now known to be perfectly satisfactory, but at first it proved difficult to devise satisfactory finishing processes, and to supplement the chrome tannage with the fat tannage.
The first undoubted commercial success in chrome tanning was obtained by the process of Augustas Schultz, whose patent was the now widely known "two-bath process," in which the skins are treated successively with a chromic acid solution and with an acidified solution of "hypo" (sodium thiosulphate). The first bath was made up commercially of potassium dichromate and hydrochloric acid, so that, strictly speaking, it contained potassium chloride also. The second bath contained, in effect, sulphurous acid, which reduced the chromic acid in the skin fibres to the tanning chrome salts. Free sulphur is also formed in this bath and in the skin, and contributes to the characteristic product obtained by this process of tanning. Many minor deviations from the original process of Schultz have been introduced, but the main features have been unchanged, and this method of tanning is widely employed at the present time for both light and heavy chrome leather. In 1893 tanning by basic chromic salts was revived and the use of the basic chloride was patented by Martin Dennis, who offered such a tanning solution for sale. The validity of the patent has always been doubtful on account of the previous work of Knapp and others, but the process itself was commercially satisfactory, and the many variants of this and of the basic sulphate tannages are now generally known as the "one-bath process" in contradistinction to the variants of the Schultz process, and are widely used for all classes of chrome leather. A one-bath process which deserves special mention was published in 1897 by Prof. H. R. Procter. In this the tanning liquor was made by reducing potassium dichromate in the presence of a limited amount of hydrochloric or sulphuric acid by adding glucose. Although a basic chrome salt is the chief tanning agent thus produced, there is little doubt that the organic oxidation products play an essential part in producing the fullness and mellowness of the leather thus tanned, but their nature and mode of action has not yet been fully made clear though lyotrope influence is probable.
More recently Balderston has suggested the suitability of sulphurous acid as reducing agent. A stream of sulphur dioxide gas is passed through a solution of sodium dichromate until reduction is complete. The resulting chrome liquor has been favourably reported upon by some chrome tanners. Bisulphite of soda has also often been used as the reducing agent. Other organic substances are also often used, instead of glucose, to reduce the dichromate.
Theory of Chrome Tannage.—As to the theory of chrome tanning there is still considerable difference of opinion and much room for experiment. Some leather chemists regard the tannage as differing essentially from the vegetable tannages. Mr. J. A. Wilson has even suggested that the proteid molecule is in time partly hydrolyzed with the formation of a chromic salt with the acid groups. The author, however, strongly favours the view that in chrome tanning changes take place which are closely analogous to those which occur in vegetable tannage, the differences being mainly of degree. Thus the hide gel is immersed into a lyophile sol—the chrome liquor—and there follows lyotrope influence, adsorption, gelation of the tanning sol, as well as diffusion into the gel, and finally also, probably, precipitation of the tanning sol at this interface (see pp. [41-47] and [200-219]).
In chrome tannage the lyotrope influence is much more prominent than in vegetable tannage, but the effect is in the same sense, viz., to reduce the imbibition of the hide gel. Thus the potassium sulphate in a chrome alum liquor has its own specific action of this kind and contributes to the leather formation. Unhydrolyzed chromium sulphate and the sodium sulphate formed in "making basic" act also in the same sense.
The tanning sol is probably chromium hydrate, formed by the hydrolysis of chromium sulphate: it is a lyophile or emulsoid sol and is in consequence very strongly adsorbed by the hide gel. This adsorption, involving a concentration of lyophile sol, is the first stage in gelation, which occupies a relatively more prominent place in chrome than in vegetable tannage. Some diffusion into the gel also occurs, and both the gelation and diffusion of the sol are affected by lyotrope influence, but to a greater extent than in the vegetable tannage. Thus far the analogy is almost complete.