Most inorganic substances possess tanning properties when in the colloidal state, e.g., sulphur, halogens, chromium salts, iron salts, silver oxide, and the salts of mercury, copper, bismuth, zinc, lead, platinum, cesium, vanadium, and the rare earths (salts of cerium, lanthanum, didymium, neodymium, thorium, and zerconium).
For practical purposes, however, only sulphur, chrome, and alum salts are used, the latter two being of the greatest importance.
SECTION IV
METHODS OF EXAMINING TANNING MATTERS
Whereas the evaluation of vegetable tanning matters necessitates determinations of their practical applicability in addition to qualitative and quantitative analyses, the latter two determinations are of practically no value when dealing with synthetic tannins. The way in which tanning matters obtained by chemical means exert their action, in addition to the intensity with which they convert pelt into leather, is the only criterion of their quality for practical (tanning) purposes; both may be demonstrated by experimental tests.
When dealing with the natural tanning materials it is desirable to know their contents of actual tanning matter, from which their special qualities as tanning agents may be deduced. Where the vegetable tanning materials have already been converted into extracts, it is essential to establish the identity of the original material used by the qualitative reactions of the extract in addition to the quantitative estimation of actual tannin contents. It is frequently necessary to examine whether the extract in question has been actually prepared from the material giving the extract its name, or whether the extract has suffered the addition of other extracts of tanning materials of but low quality. Such determinations may be undertaken by microscopical observations and by means of qualitative and quantitative reactions; for this purpose many colour reactions and precipitation methods are available in addition to the determination of the molybdenum figure (Lauffmann),[Footnote: Collegium, 1913, 10.] the alcohol and ethyl acetate figures and microscopical examination (Grasser).[Footnote: Ibid., 1911, 349.] Of other adulterants tending to reduce the quality of extracts may be mentioned sugars, mineral salts, and coal-tar dyes; [Footnote: Grasser, Collegium, 1910, 379.] for the determination of these, the special literature should be consulted. [Footnote: Grasser, "Handbuch f. gerbereichem. Laboratorien" (Leipzig, 1914); Procter-Paessler, "Gerbereichem. Untersuchungen" (Berlin, 1901).]
Two methods are devised for the purpose of quantitatively determining the tannin contents, both of which employ hide powder, and which are known as the "shake method" and the "filter bell method" respectively: the former is adopted as the official method of the "International Association of Leather Trades' Chemists" (I.A.L.T.C.). [Footnote: And also by the Society of Leather Trades' Chemists.-Transl.]
The original method, [Footnote: Leather Manufacturer, 1894, No. 9 J.S.C.I.,1894, 494.] worked out in the laboratory of the Yorkshire College (now the University of Leeds), essentially consists in introducing 6-9 gm. of hide powder in a shaker, washing it at least twice with distilled water and carefully squeezing out the powder in a linen cloth between each washing. 100 c.c. of the solution to be examined, which may not contain more than 1 per cent, total solids, are introduced into the shaking bottle which is then weighed. About one-third of the washed hide powder is then added, and the bottle shaken ten to fifteen minutes; another third is then added and, after shaking, the third portion. The bottle plus contents is now weighed, and the amount of hide powder introduced ascertained by difference of the two weighings. The liquid is then filtered through filter paper, 50 c.c. of the clear filtrate evaporated in a basin, dried and weighed. The residue in the original solution is then obtained by multiplying the former by 100 (plus weight of water added with hide powder), and dividing by 100.
This method was closely investigated by a large number of leather trades' chemists, was considerably improved, and in its final form presented a method of the highest degree of accuracy; the method was therefore adopted as The Official Method of Tanning Analysis by the I.A.L.T.C., which body, at the same time, gave precise instructions as to the details of the method. The latest instructions, which are reprinted below, permit of any method of analysis which observes the following conditions:—
1. The solution for analysis must contain between 3.5 and 4.5 gm. of tanning matter per litre, and solid materials must be extracted so that the greater part of the tannin is removed at a temperature not exceeding 50° C.