The bark of various willows, especially Salix arenaria and Russeliana, is used for tanning in Russia, and for Danish glove-leather. Some contain up to 12-14 per cent. of an iron-blueing tannin. They impart a strong odour to leather, but different to that of birch-tar oil, and the scent of genuine Russia leather is due to a combination of both. In many cases the bark peeled off osiers for basket-making is employed. A Russian willow (species unknown), in the form of thin bark of osiers or small branches, gave 9·5 per cent. tannin when examined in the Leather Industries Laboratory of the Yorkshire College; and willow barks certainly demand more attention than they have received in England as tanning materials for fine leathers. Salix caprea has been used in France for glove leathers, but is weaker than S. arenaria.
Poplars belong to the same natural order, and have been used for tanning, but their barks at the most contain 2-3 per cent.
POLYGONACEÆ, Docks.
Most members of this family contain tannin.
Rumex hymenosepalum, Canaigre, Gonagra (Cana agria), Red Dock, wild pie-plant ([Fig. 52]). Common in sandy alluvial plains of Mexico and Texas, and considerably resembling rhubarb. Its tuberous roots resemble those of the dahlia, and contain, when air-dried, 25-30 per cent. of a catechol tannin, probably allied to that of mimosa. Undried, the roots contain about 68 per cent. of water and only 8 per cent. of tannin. When well harvested by slicing thin and rapidly drying, it gives leather a bright orange colour, and, it is said, considerable weight and firmness, and is thus specially suitable for use in retanning and finishing light goods and harness leather. Besides tannin, the root contains a yellow colouring matter, and about 8 per cent. of starch, of which the granules are very variable in form and size, but mostly oval or elongated. They do not stain readily with iodine till they have been well washed, or treated with dilute sulphuric acid. Both the starch and tannin are contained in large and somewhat thin-walled cells, and the sliced material is easily extracted at low temperatures. Greater heat gelatinises the starch, and extracts a darker colour. The best temperature for extraction is between 30° and 50° C. (see [p. 348]).
Fig. 52.—Canaigre (Rumex hymenosepalum). ‘New Commercial Drugs and Plants,’ T. Christy.
The root is most readily grown from tubers or portions including the crown, as the plant seeds sparingly. Sandy soils, subject to inundation or irrigation, seem best suited to its culture. In California and Arizona the growth begins in October or November with the winter rains, blooming about the end of January, while the leaves die down in May and the roots remain dormant during the summer. It is not important at what time the roots are harvested, and they seem to improve in percentage of tannin up to the second year, after which they become darker and deteriorate.
The harvested crop should be sliced into thin pieces and rapidly dried at a low temperature, or still better, converted at once into extract. This is already done on a considerable scale at Deming, New Mexico. The residue after extraction is used in America as cattle-food; and might no doubt be also applied to the production of alcohol.
Planting takes place in autumn, in rows, say 30 inches apart, with 10 inches between each root. Roots for “seed” should be kept in the ground or stored in dry sand. This should yield a crop of 10 tons per acre in an average season.