[Footnote 1: In alcoholic solution.]
SECTION III
TANNING EFFECTS OF MIXTURES AND NATURAL PRODUCTS
1. Mixture of Phenolsulphonic Acid and Formaldehyde
The most important invention relatively to the search for new tanning materials was that of Weinschenk,[Footnote: Ger. Pat., 184,449.] who first showed that pelt may be converted into leather by the action upon it of mixtures of naphthols and formaldehyde. This process consists of two steps: the pelt is first immersed in a 0.25-0.50 per cent, formaldehyde solution, and secondly in an aqueous solution of -[Greek: a] or -[Greek: b] naphthol; this order may be reversed. If, on the other hand, a pasty mixture is made of formaldehyde and naphthol, and this is allowed to act upon the pelt, the latter is rapidly converted into leather, but the mixture must be administered very gradually or otherwise the insoluble methylenedinaphthol is formed outside the pelt and hinders any tanning effect.
Leather obtained through the action of [Greek: a]-naphthol is, when freshly tanned, pure white and sufficiently soft and firm, but quickly assumes a brown colour on storing; if, however, [Greek: b]-naphthol is employed, a cream-coloured leather results, the colour of which turns only slightly more yellowish even when exposed to the direct rays of the sun.
A similar process has recently (25, xii., 1915) been protected by Ger. Pat, 305,516, granted to the Deutsch-Koloniale Gerb—und Farbstofif Gesellschaft, in Karlsruhe. According to this patent, pelt is treated in separate solutions, one of which is formaldehyde, the other being that of such aromatic compounds or their salts which yield water-soluble condensation products with formaldehyde; for example, pelt is immersed in 2-5 per cent, solution of formaldehyde for a few days, and is subsequently treated with 1-2 per cent neutral or faintly acidified solutions of [Greek: a]-naphthylamine hydrochloride, resorcinol or sodium phenate or cresylate, for several days. The resultant leather is claimed to be soft and full and to possess good tensile strength.
The tanning properties of mixtures of phenolsulphonic acid and formaldehyde have been examined by the author with the following results:—
I. II. III.
Grammes formaldehyde 10 20 40 " phenolsulphonic acid 20 50 100 " caustic soda (sol, 40 per cent.) 10 20 40 " water 500 500 500