A similar process, by the same author (U.S. Pat. 798,293), is described by Dr. Allen Rogers[144] in which 5 per cent. of glucose syrup and 10 per cent. of sulphur, computed on the weight of the skins, is fermented with yeast for twenty-four hours, after which it is used as a bate. From a chemical point of view, it is difficult to express the reactions taking place.
When the bath is first prepared, it is neutral in reaction, but, after standing, gradually assumes a slight acid reaction. On introducing the skins, it becomes alkaline, owing to diffusion of the lime. After a short time, it becomes neutral again, and remains thus until the skins are in a perfect condition, when a slight acid reaction is observed. These changes are caused by the fermentation of the glucose, carbon dioxide and alcohol being generated. Owing to the presence of sulphur, a quantity of hydrogen sulphide is produced, which, in its nascent condition, combines with the alcohol to form mercaptans, which in their turn are slowly oxidized to thioacids. These mercaptans and thioacids, being of weak acid character, act in conjunction with any excess of hydrogen sulphide upon the lime, converting it into soluble compounds, which are readily removed by diffusion.
Lederer, L. (French Pat. No. 404,926 of 1909).—Method of Preparing a Substitute for Animal Excrement (dog dung) in Puering Skins for Glacé Leather. In this process a mixture of 10 parts finely powdered bone meal, and 1 part of crushed lupin seeds or beans is formed into a paste with water and allowed to undergo spontaneous fermentation from 3–4 weeks. The mixture is said to form a perfect culture medium for the micro-organisms indispensable in bating. The removal of the lime is facilitated by the addition of 2 per cent. of fat, 0·75 per cent. sodium polysulphide and 1 1/2 per cent. common salt reckoned on the weight of the bone meal.
In this patent the researches of Wood, Andreasch and Eitner are referred to, and also Benker’s process of bating with Peruvian guano to which soda has been added.
Turney, Sir J. and Wood, J. T.—Coming now to bacterial and enzymatic bates, the author in conjunction with Sir John Turney applied for a patent for a new and improved method of bating skins (No. 25,894, 1896), in which cultures of suitable bacteria were to be used in place of infusions of dog or pigeon dung. It was found however that this patent had been anticipated by the patent 21,720 of 1895, of Dr. Popp and Dr. Becker of Frankfort o/M, and the application was therefore not proceeded with. As has been related in Chapter VII. the late Franz Kathreiner, who knew of the work which was being done both by Dr. H. Becker and the author, made us personally acquainted, and most of our later work was done in conjunction. Like many other new ideas, it was arrived at quite independently by Dr. H. Becker and the author. (See Report of Third Conference of I.A.L.T.C., Copenhagen, 1899, Wiss. Techn. Beilage des Ledermarkt, 1899–1900, p. 8.)
We shall therefore give the original patents in full, and afterwards the translation of Dr. Otto Röhms’ patent, and the recent patent of Dr. Eberle, as these cover the whole ground of the bacterial and enzymatic bates up to the present time.
Dr. Popp and Dr. Becker’s English Patent (No. 21,720, 1895). Improvements in Lyes or Liquors for Preparing Hides for Tanning. An important role in the process of manufacture of many kinds of leather is played by the dung liquor or lye, which in particular for calf skins and skivers constitutes one of the necessary preparatory operations for the subsequent actual tanning process, and which has for its purpose to bring the skins or hides, which have been previously swelled by the liming process, into a condition which allows the easy and thorough removal of the lime and lime soap as well as the remaining hairs by the subsequent mechanical treatment, and to obtain a uniform shrinking of the swelled hides.
The process hitherto employed consists in treating the hides or skins for some time (such as for 4 or 5 hours) in a vat provided with a reel, and containing a lye or brine formed of solutions of dog or pigeon or poultry dung.
Generally speaking, the dog dung acts best in some cases, and pigeon or poultry dung in other cases, but no satisfactory and sound explanation has as yet been given as to the nature of the action.