Chrome is not only of importance in tanning, but in dyeing; on account of its power of forming insoluble colour-lakes with many mordant colouring matters. For this purpose normal or basic chromic salts are sometimes used, sometimes chromic acid or dichromates, the latter acting not only by yielding chrome-oxide on reduction, but as oxidising agents to the colouring matters. Most of the colours produced with chrome mordants are of dark shades, that with logwood being deep violet or black. The mordanting power of chromium is important in the dyeing of chrome leather. Bichromate of potash is often used in dilute solution for darkening the shade of leather dyed with other materials, but is not to be recommended on account of its destructive action on the leather.
Numerous patents have been taken for processes of chrome tannage. The first practical method was described by Professor Knapp in 1858 (see [p. 210]), though he did not recognise its value. Some of the patents have a historical interest, though of no importance. Among these may be mentioned that of Cavallin, a Swedish apothecary, whose object was dyeing rather than tanning, but who treated raw hide with a solution of bichromate, which was afterwards reduced on the fibre by one of ferrous sulphate. The leather produced is dark reddish brown, and tender from the amount of basic ferric salt formed at the same time. Mr. J. W. Swan, well known in connection with photographic processes, and electric lighting, also patented a process of chrome tannage (as an addendum to a patent on carbon printing), in which the chromic acid first fixed in the pelt was reduced by “oxalic, or other suitable acid.” Although it is possible to produce leather within the lines of the patent, the strongly acid reaction of the reducing agent renders it unsuitable for practical use. The first chrome tanning process which made any show of practical success, was that patented in 1879 by Heinzerling, which was acquired in this country by the Eglinton Tanning Company, and also worked under their license for a short time by the Yorkshire Tanning Company at Leeds. Though the process was not commercially successful on any considerable scale, it possesses points of interest which make a brief description desirable. The hides or skins, after preparation in the usual way, were treated in a mixed solution of salt, alum (or aluminium sulphate), and potassium bichromate, but no systematic attempt was made to reduce the chromic acid to a tanning form, the product being, at first at least, merely an alum tannage, coloured, and perhaps somewhat hardened with chromic acid, though on keeping for a length of time, reduction gradually took place at the expense of the hide-fibre, and of the fats employed in currying, so that the leather internally became greyish-green, and really chrome-tanned. Specimens of the early products of the process, preserved in the museum of the Leather Industries Department at Leeds, have now all undergone this change, but are still tough and flexible, showing that the rapid tendering of the Heinzerling leather, which was one of the causes of its failure, must have been due to some error in manufacture, and was not inherent in the process. Interesting, historically, is the fact, that at an early stage in the life of the patent, a specimen of the leather was submitted to the late Professor Hummel, in order that he should suggest some means of overcoming the disagreeable yellow colour of the product. He reduced it with a bisulphite, and coloured it with an aniline dye, and a piece is still in the possession of the Yorkshire College, and in perfectly sound condition. If legal publication of this experiment could have been proved, it would have invalidated the important Schultz patents under which most of the chrome-kid of the United States has been manufactured. As bearing on modern chrome-tanning, the most important reaction in the process is that of the alum with the bichromate. It has been shown by Heal and Procter[115] that pelt absorbs practically no chromic acid from bichromate, unless it has been previously set free by acidification. When however alum, or sulphate of alumina is added, its sulphuric acid liberates the chromic acid, leaving a basic alumina salt in solution, and this fact has been utilised in some modern tanning processes.
[115] Journ. Soc. Chem. Ind., p. 251, 1895.
The first really important advance in practical chrome tanning was made by Augustus Schultz, in 1884. Schultz was not a tanner, but a chemist, employed by a New York firm of aniline colour merchants, and his attention was accidentally drawn to leather by a friend who asked him if it were possible to produce a leather for covering corset steels, which would not rust them as ordinary alumed leathers do. The process which he adopted was probably suggested by a method then recently patented for the mordanting of wool by chrome oxide, and depended on the power of the pelt to absorb free chromic acid (as it does all other free acids), and the subsequent reduction of the latter on the fibre to a basic chrome salt, which produced the tannage. The reducing substance employed was the free sulphurous or thiosulphuric acid of an acidified solution of sodium thiosulphate (hyposulphite), and as it was not certain which of the two acids was the really active agent, Schultz duplicated his patent, so as to cover both. Though he made no claim in his patent to having discovered the best proportions of his ingredients, those which he specified have proved practically useful after allowing for the modifications required by different skins, and slightly different methods of working. His first bath consisted of a solution of 5 per cent. of bichromate of potash, and 21⁄2 per cent. of concentrated hydrochloric acid (or 1·25 per cent. of concentrated sulphuric acid), reckoned on the wet weight of the prepared pelt, and dissolved in sufficient water for convenient use in the paddle or drum which was to be used in the process. In this bath the skins were worked till they took a uniform yellow colour throughout, but without any tanning effect being produced. They were now freed from superfluous chrome liquor by draining or “putting out,” and transferred to the second bath, which consisted of 10 per cent. of “hypo” and 5 per cent. of hydrochloric acid similarly dissolved. In this, they rapidly took a duck-egg green colour from the reduction of the chromic acid; and when this was uniform throughout the skin, the tannage was complete. The exact quantity of water is not of great importance, and good results can be obtained with anything varying from 20 to 50 gallons per 100 lb. of pelt (200 to 500 per cent.) if time be allowed for the weaker solution to act. The quantities of “hypo” and hydrochloric acid given for the second bath are often somewhat insufficient, and have to be slightly increased to complete the reduction. The reactions which take place are represented by the following formulæ, in which the weights of the materials taking part in the reaction are also given below the symbols. In the first bath—
| Potassium dichromate. | Hydro- chloric acid. | Potas- sium chloride. | Chromic acid. | Water. | ||||
| K2Cr2O7 | + | 2HCl | = | 2KCl | + | 2CrO3 | + | OH2 |
| 294 | + | 73 | = | 149 | + | 200 | + | 18 |
As ordinary concentrated hydrochloric acid does not contain more than about 30 per cent. of actual HCl,[116] about 2·5 parts would be required to completely decompose 2·94 parts of dichromate, while in Schultz’s formula 2·5 parts of hydrochloric acid are used to 5 parts of dichromate. This excess has been found useful in the production of a good leather, both to prevent accidents from an overdose of hydrochloric acid, and because of the modifying effect of an excess of neutral salt on the action of the chromic acid (cp. [p. 82]).
[116] Acid of S.G. 1·16 (32° Tw.) contains 31·5 per cent. of HCl by weight or 36·6 grm. per liter, and therefore is practically 10 × normal strength. Acid of S.G. 1·2 (40° Tw.) contains 39·1 per cent. or 469 grm. per liter.
The reactions which take place in the second bath are somewhat complicated. Eitner, in a valuable series of articles on chrome tannage, which have been appearing in the ‘Gerber’ since January 1900, states that even better results are obtained by using the hydrochloric acid in slight excess, as the action of chromic acid (in the presence of the potassium chloride of the chrome-bath) is not swelling but hardening to the skin, and the slight swelling action of the hydrochloric acid tends to counteract this, and also to facilitate the subsequent reduction. The two views are not contradictory, as the excess of bichromate behaves to the hide as an alkaline salt, which also produces a slight swelling effect, and it is quite probable that better results are attained when the solution is either alkaline or acid, than when the potassium chromate is exactly decomposed. Eitner recommends the use of four parts by weight of bichromate, and four parts of the strongest hydrochloric acid, dissolved in 400 parts of water, for each 100 parts of wet pelt, which should yield about 40 parts of dry leather. He states that if such a bath be used, it may be safely and economically exhausted by a second pack of skins, which is impossible in a bath containing excess of unacidified bichromate. He gives[117] the following explanation of the successive changes which take place when acid is gradually added during the reduction, but points out that in practice the reactions always to some extent go on simultaneously.
[117] Gerber, p. 297, 1900.
In the first stage, very slight acidification is required, and if the skins have been chromed with excess of hydrochloric acid, may be altogether dispensed with. The skins become brownish from the conversion of the chromic acid into so-called “chromium dioxide” (probably really a basic chromic chromate, Cr2CrO4(OH)4, which on ignition leaves Cr3O6); no sulphurous acid is liberated, or sulphur deposited, but sodium tetrathionate is formed in the bath, and the reaction may be represented as follows: