CHAPTER IX. SOLE-LEATHER.--Tanning Materials. Before describing the management of the hides in the tan-house, it is necessary to say a few words about one or two of the principal materials used, and the methods of preparing them for use. Further details of their nature and origin have been given in the section on Tannins, p. 23. Oak-bark is one of the oldest of tanning materials, and the leather produced by its aid is still considered for many purposes the best. For sole-leather, its weakness in tannin (8-12 per cent.), the slowness of its action, and the light weight of the leather produced, render it unavailable alone except for the very finest class of work. It is, however, generally used in admixture with stronger and cheaper materials, such as valonia. Valonia, the acorn-cup of an evergreen oak growing in Greece and the Levant, is perhaps the most important of materials to the English sole-leather tanner. It contains 25-35 per cent. of a tannin somewhat similar to oak-bark, and, like it, communicating a light-coloured bloom to the leather, but giving much greater firmness and weight, and a browner colour. Myrabolanes or myrobalans, the fruit of an Indian shrub, contains about as large a percentage of tannin as valonia, and gives a similar bloom, and excellent colour; but it can only be used very sparingly on butts, since it produces a soft and porous leather. Divi-divi is a South American bean, which contains much of a brown tannin in the pod, being considerably stronger than valonia. It makes a heavy and solid, but somewhat horny leather. Its great danger arises from a tendency to sudden fermentation in thundery weather, which, produces brown or red stains on the leather. At all times it is liable to give a bluish or violet colour, which is most obvious in the interior of the leather, and which resists both acids and alkalies. Mimosa-bark is the product of several Australian acacias, and is probably nearly as strong as valonia. It gives a hard and heavy leather, but of a dark-red colour. Hemlock-extract is a deep-red syrupy extract of the bark of the hemlock pine of America. Chestnut-extract is a similar product from the rasped wood of the Spanish chestnut. Its colour is paler and yellower than that of the hemlock, and hence it is often employed to correct the red tone produced by the latter. Oakwood extract is an analogous preparation from oak saw-dust. Grinding and Exhaustion of Tanning Materials.