Myrobalans is the unripe fruit of an Indian tree (terminalia chebula) and contains from 35 to 40 per cent. of tannin which gives a light colour to leather. This material is useful both for light and heavy leathers, but is generally used in admixture with other tannins. It deposits much bloom (ellagic acid) and is largely used for brightening the dark colour produced by other tannins. A large quantity of this material is now made into extracts, which are more convenient to handle and more uniform in strength of tannin. Natural myrobalans have the appearance of shrivelled nutmegs, except that they are yellowish in colour; they are very hard and require a special milling machine to reduce them to powder. The quality of myrobalan nuts varies in different districts, the best being Bhimlies and Jubbalpores.
Sumach is a valuable tanning material, and is used for a large proportion of the light and fancy leathers. It is a small bush plant which grows in Italy, Spain, Southern France, America, and Algeria, but of the numerous varieties the Sicilian (rhus coriaria) is by far the most important. Sumach is one of the few materials cultivated on an extensive scale; most tanning materials are derived from natural sources and, chiefly owing to the length of time before trees reach maturity, it would not be a paying proposition to cultivate them. The sumach shrubs are propagated from small cuttings and the leaves can be picked at the end of the first year, but it is better to allow the shrubs to become more firmly established before stripping them. The leaves are dried and sometimes exported whole, especially for the use of the silk manufacturers in Lyons; but they are more often ground to a fine powder. All sumachs should be ventilated to remove foreign matters and all traces of iron, which would cause dark bluish stains on the leather. "Ventilation" is effected by passing currents of air, preferably with a fan, through a narrow room, when the pure sumach is sent forward, while the heavier particles of dirt and small pieces of wood remain behind. Sometimes the process is repeated, and the best brands of sumach are generally described as "pure, extra ventilated." As far as possible, the male plants (mascolino sommacco) are cultivated in Sicily, where the best sumach is grown. Female sumach (femminello sommacco), grown in parts of Italy, is weaker in tannin than the male, but is rarely sold separately. The serious amount of adulteration formerly practised by the admixture of inferior plants, and particularly of lentisco (pistacia lentisco) led to the Italian Government taking strong action a few years ago, and it is now possible, for a very small sum, to have any consignment inspected and analysed by the Government. Lentisco is now sold separately and is used for common work.
Sumach has been successfully introduced into Australia, but its development is retarded owing to difficulties of labour, which render competition with the European product almost impossible. An inferior sumach (rhus glabra) is grown in America, chiefly in the State of Virginia. It contains from 15 to 20 per cent. of tannin, and produces a darker coloured leather than Sicilian, the best qualities of which contain 27 to 30 per cent. of tannin.
A useful test for finding out if a sumach has been adulterated is to treat a small quantity with strong nitric acid, which destroys the structure of the leaves. The mass is washed and neutralised with an alkali, when the appearance of the midrib and veins of the leaves of the common adulterants are easily recognised.
Sumach is not so much used for shoe upper leather as it formerly was, but it is the best tanning agent for many kinds of fancy light leathers, such as bookbinding, calf, and skivers (the grain of split sheep skins), moroccos, furniture leather, etc. It is also less subject to the action of the air and gaslight than any other tanning material, and is strongly recommended for tanning purposes by a special committee appointed by the Society of Arts to enquire into causes of the rapid decay of leather bindings. Sumach is very useful for brightening up the colour of leather tanned with darker tannins, and is frequently used for improving the colour of both dressing and sole leather. By itself, it yields an almost white leather which affords a good foundation for the most delicate shades.
Gambier or Terra Japonica (uncaria gambir) is a crude extract of a shrub indigenous to the Malay Peninsula. Nearly the whole of the production is shipped from Singapore. The leaves and twigs are boiled in an iron vessel, and when the mass has become syrupy it is strained through a rough sieve into a shallow tub, where it is cooled. The liquor is stirred while cooling and rapidly thickens. Before it sets, it is cut into 1 in. cubes and thoroughly dried. Good qualities contain from 50-65 per cent. of tannin. An inferior product, called "block gambier," is made by allowing the syrupy mass to set in large blocks weighing about 2 cwt. each. These are packed in coarse matting. The strength of tannin varies from 30 to 40 per cent. Gambier is a good tanning material, but its use has been declining for some years past owing to its being frequently adulterated with sago and other farinaceous plants. However, a pure gambier extract, manufactured on the latest scientific principles, has been placed on the market, and there will undoubtedly be a revival of the use of this valuable tannin. This pure gambier is prepared at Asahan, in Sumatra, and is guaranteed to contain a minimum of 38 per cent. of tannin.
Gambier can be used to advantage in keeping up the strength of bark liquors in the tannage of sole leather and hastening the process, while it may be used alone for the tannage of boot upper leather and dressing hides. It produces an exceptionally mellow and plump leather. It is preferable, however, to complete a gambier tannage with a little oak wood or quebracho extract, in order to fix the tannin principle of gambier, which, perhaps on account of its viscosity, does not readily combine with the fibres of the skin.
Mangrove or Mangle, a tree found on the coasts of several tropical countries, yields a useful bark for tanning purposes. At low tide, these trees show their great arched roots standing high above the ground. The best varieties, the ceriops species, are found in the East Indies and Bengal, and the bark of these is said to contain sometimes nearly 40 per cent. of tannin. Other varieties contain from 15 to 25 per cent. The bark is generally made into a solid extract, or "cutch," in which form it contains more than 60 per cent. of tannin, It is useful to blend with other materials, such as oak wood, chestnut, and quebracho extracts, but used by itself it imparts a strong reddish colour to the leather. Some of the Indian varieties are used as dyeing materials, and act as a satisfactory mordant in dyeing leather a dark shade.
Mimosa or Wattle trees, which belong to the acacia species, yield bark rich in tannin. Australia is the native country of several varieties, including the Black Wattle (acacia pycnantha), the Golden Wattle (A. longifolia), and the Green Wattle (A. decurrens).
The bark contains from 20 to 45 per cent. of tannin. The cultivation of wattle in Australia seems to have declined, owing to the high cost of labour and inability to compete with the mimosa bark imported from South Africa, where it is cultivated on a very large scale and where labour conditions are more favourable for the growers, as is clearly shown by the fact that the wattle growers in Australia successfully petitioned the Government a short time ago to place a duty of £1 10s. per ton on the imported bark.