Solid Neradol Matters Tanning Abs. Increase
Argentine D. Calc. of Mixture Increase per
Quebracho Found. in Tanning 100 gms.
Extract. Matters. Extract.
Gm. Gm. Per Cent. Per Cent.
100 0 66.0 66.0 … …
0 100 32.5 32.5 … …
90 10 62.7 64.7 2.0 2.2
80 20 56.1 58.7 2.6 3.3
60 40 52.6 56.9 4.3 7.1
50 50 49.3 55.2 5.9 11.8
30 70 42.6 47.3 4.7 15.6
20 80 39.2 42.3 3.1 15.5
The maximum solubilising effect is exhibited in the mixture of 70 parts of Neradol and 30 parts of quebracho, with an additional percentage of tanning matters in the mixture of 15.6 per cent.—a figure which is very nearly identical with that of the insolubles present in the original Argentine quebracho extract.
The phlobaphene-solubilising property of Neradol D is closely connected with the influence of the latter on the colour of leathers tanned with natural tannins. If, on the one hand, a pelt is tanned with natural (i.e., non-treated) quebracho extract, a rather light coloured leather results, the fleshy colour of which is characteristic of quebracho. The dark coloured phlobaphenes present, on account of their insolubility, will have no influence on the colour of the leather. If, now, the quebracho extract be treated with sulphite and bisulphite in the usual way, the phlobaphenes are solubilised, but the reducing effect of the bisulphite tends to brighten the colour of the otherwise dark coloured phlobaphenes as well as that of the soluble tannins, and a reddish-yellow coloured extract results, imparting its own colour to the pelt. When, on the other hand, the quebracho extract is solubilised by means of Neradol D, the phlobaphenes are brought into solution without reduction taking place, and a dark brownish-red extract results, which imparts a similar colour to the finished leather. This darkening effect of Neradol D is most conspicuous in the case of mangrove, maletto, and chestnut, but is absent in the case of algarobilla, dividivi, gambir, sumac, and valonea. The varying phlobaphene contents of the tannins easily afford an explanation of the different properties above alluded to: the mangrove phlobaphenes are dark coloured bodies, those of mimosa, maletto, and chestnut are of lighter colour, and the last-named tanning materials enumerated above are either devoid of phlobaphenes or possess them only as very light coloured bodies. Algarobilla, sumac, gambir, dividivi, and valonea, on the other hand, are associated with large amounts of sparingly soluble ellagic acid, known as "bloom" or "mud" which imparts a light colour to the finished leather, and conveniently covers the dark colour imparted to the leather by other tanning materials; for this reason the former are often used in the lay-aways or in the finishing processes.
Similar effects to those of Neradol D are exhibited by other salts of sulphonic acids, e.g., sodium benzylsulphanilate (Solvenol B.A.S.F., or solution salt ("Solutionsalz") Hoechst); the author prepared mixtures of such salts and untreated quebracho extract in order to determine their solubilising effects, and arrived at the following results:—
30 parts Solvenol plus 70 parts quebracho extract: clear solution,
no deposit.
25 parts Solvenol plus 25 parts quebracho extract: clear solution,
very little deposit
20 parts Solvenol plus 80 parts quebracho extract: nearly clear solution,
very little deposit.
15 parts Solvenol plus 85 parts quebracho extract: slightly opaque solution
some deposit.
Leathers tanned with these mixtures were more or less dark coloured according to the amounts used of solvenol and the consequent solubilisation of the phlobaphenes.