[152] Cp. Report of Committee on Leathers for Bookbinding, Journ. Soc. of Arts, 1901, p. 14.
Wagner, a German chemist, attempted to classify the tannins into “physiological” tannins, which were produced in the natural growth of the plant, and “pathological” which were caused by the attack of insects such as the gall-wasps, and he further ventured the assertion that only the former class were capable of producing leather. It has since been shown that the tannins produced in galls are identical with some of those found in healthy plants, and galls themselves have been used in tanning from very ancient times. It is only necessary to remind the reader of the use of Turkish gall-nuts, in place of sumach, which was common in the East in the tannage of moroccos, and of the “Knoppern,” or oak-galls formerly so largely used in Austria as a tanning material for sole leather. It is true that the tannin of galls is not very suitable for the latter purpose, consisting as it does mostly of gallotannic acid, which, giving no solid deposit of bloom or reds, is incapable of making a heavy or solid leather. Pure gallotannic acid itself produces a very white and soft leather.
The class to which the tannins of the different tanning materials belong is mostly mentioned in the [Botanical List] ([Chap. XVIII.]), but it may be well here to specify a few of the most common. Galls and sumach contain gallotannic acid with a little ellagitannic; myrobalans, valonia, divi-divi, algarobilla, oakwood and chestnut are all pyrogallol-tannins giving ellagic and gallic acid among their decomposition products. All the pine barks, including the American hemlock, and the larch, all the acacias and mimosas, including the Indian Babul (Acacia arabica), the oak barks (though not the oak wood, fruits, or galls), quebracho wood, cassia[153] and mangrove barks, canaigre, cutch and gambier are catechol-tannins, and the two last contain phloroglucol, of which minute traces are also present in many other catechol-tannins ([p. 297]).
[153] Cassia auriculata, or “turwar” bark, is the ordinary tannage of the East Indian or “Persian” sheep- and goat-skins, largely used in bookbinding, but which redden and decay very rapidly.
Gallotannic acid, and several artificial tannins with the characteristic reactions of the class have been produced in the laboratory, but there is no present prospect of their manufacture at prices which can in the faintest way compete with those of natural production.
Tanning materials frequently contain mordant colouring matters, often derived from the same phenols as the accompanying tannins. They also usually contain gums, starch and glucose. Oak bark contains lævulose which is not combined with the tannin. Many tannins, however, exist in nature in combination with the sugars as glucosides, which are easily decomposed by the action of acids or by fermentation. These sugary matters are important as furnishing by fermentation the acetic and lactic acids of tanning liquors.
CHAPTER XX.
THE SAMPLING AND ANALYSIS OF TANNING MATERIALS.
Although the analysis of tanning materials falls more properly within the scope of a book for chemists than one intended primarily for tanners, and though it has been treated at considerable length in the ‘Leather Industries Laboratory Book,’ a slight sketch must now be given of the methods in general use, since it is of great importance that at least the principles on which they are based should be understood by all to whom they are of practical interest, and also because an approximate analysis of a tanning material by the hide-powder method is within the scope of any intelligent tanner who will provide himself with the necessary implements. Much attention has been paid to the subject area by the International Association of Leather Trades Chemists, and also by the American Official Association of Agricultural Chemists, and as the methods prescribed by one or other of these are with very little exception employed by all qualified chemists throughout the world, their directions, corrected up to date, are given in [Appendices A] and [C]. As, however, these directions are addressed to chemists already familiar with the usual course of analysis, a somewhat fuller explanation must here be given.
It must specially be insisted on, that absolute adherence to the methods given is essential to obtaining concordant results, and little points of manipulation which appear in themselves unimportant, are frequently the result of long experience and careful discussion. The members of the International Association, especially, are bound by their rules to make note in their analytical reports of any deviation, however small, from the prescribed process.