5. Excrescences.—Gall Nuts, Chinese Galls, Pistacia Galls, Tamarisk Galls.
Tanning Extracts
1. Woods.—Oak, Quebracho, Hemlock, Chestnut, Mimosa, Mangrove, Spruce.
2. Barks.—Oak, Wattle or Mimosa, Larch.
3. Shrubs, Leaves, etc.—Gambier, Cutch, Catechu, Kino, Sumach.
4. Fruits.—Myrobalans, Valonia.
5. Roots.—Palmetto.
Of these materials, only about twenty are of much importance commercially, the principal being oak, chestnut, quebracho, hemlock, valonia, gambier, myrobalans, mimosa or wattle, sumach, mangrove, divi-divi, spruce, larch, and babool.
Oak Bark (quercus robur) is still an important material, but is rarely used alone. The bark from English oaks contains from 8-14 per cent. of tannin (quercitannic acid) as estimated by the impregnation of a standardised hide powder in a given quantity of the tanning material in solution. Owing to its weakness in tannin compared with other materials, oak bark tans very slowly. Used for sole leather, it would not produce the essential quality of firmness and solidity, and it is now customary to use a stronger tanning material, such as valonia, or valonia extract, or gambier in the latter stages of the process. This is the nearest approach to the pure oak bark tannage of former days, and, if carefully regulated, is a great improvement on the old method.
If dressing hides and calf skins required for boot upper leather are bark-tanned, the tannage is often completed in a sumach liquor, the object in this case being to lighten the colour so that the leather can be dyed evenly.