By treating peat with nitric acid, Jennings [Footnote: Jahresber. d. Chem., 1858, 666.] and Payne [Footnote: Chem. Centralbl., 1908, ii. 554; Ger. Pat., 200, 539.] have produced artificial tanning materials.
Skey [Footnote: Chem. News, 1866, 206; Zeits. f. Chem., 1866, 753.] obtained a dark brown extract, soluble in water and precipitating gelatine, by treating bituminous coal or lignite with nitric acid; by extracting coal with alkalies, Reinsch [Footnote: Pharm. Centralh., 1887, 141.] isolated a substance (pyrofuscine) which, when partly neutralised with carbon dioxide, was capable of converting pelt into leather.
In addition to these tanning materials the recovery of a substance possessing tanning properties from the so-called acid rosins has been made the subject of a patent; [Footnote: Ger. Pat., 36,019.] this rosin is formed when crude oil is treated with concentrated sulphuric acid in the oil refineries. The greasy substance is partly neutralised with alkali and is claimed to produce a very springy leather.
The waste liquors obtained in the manufacture of cellulose, the so-called sulphite and sodium cellulose waste, have, however, been the subject of numerous investigations, and several hundred publications have appeared and a great number of patents [Footnote: "Literatur überiSulfitablauge" 1910-13. (Reprint from WocheWochenblPapiePapierfabrikation)] taken out, the first one being that of Mitscherlich [Footnote: Jahresber. d. Chem., 1893, 890; Ger. Pat., 72,161.] and Hönig [Footnote: Chem. Centralbl., 1902, ii. 174; Ger. Pat., 132,224.]
The waste liquors contain large quantities of acids and lime, and in order to utilise the liquors for tanning purposes, the excessive sulphuric and sulphurous acids as well as the lime must be removed. The active tannin is no doubt the ligninsulphonic acid, and those cellulose extracts containing the largest amounts of free ligninsulphonic acid may also be considered the most efficient.
According to the author,[Footnote: Technikum, 1912, 20, 156.] such sulphitecellulose extracts precipitate gelatine, aniline hydrochloride, ammoniacal zinc acetate, and basic coal-tar dyes, and give a greenish-black coloration with ferric chloride. These reactions indicate the presence of tanning matters in cellulose extracts.
The official shake method of analysis gives the following results:—[Footnote: Ibid.]
Tanning matters 23.0 per cent. Non-tannins 30.3 " Insoluble matters 0.7 " Water 46.0 " ———————- 100.0 per cent.
Ash 4.3 "
Sulphurous acid 0.6 "
Many other substances have been used for tanning experiments, a number of them precipitating gelatine. Zacharias [Footnote: Zeits. f. Ang. Chem., 1907, 1645.] obtained leather by the action of many coal-tar dyes on pelt, similarly Herzog and Adler, by using Prussian blue, Neufuchsin, patent blue V, crystal violet, and colloidal gold.