Leather: From the Raw Material to the Finished Product

by K. J. Adcock

In England, oak bark is harvested in April and May, when the sap rises in the tree. Rings are cut round the tree soon after it is felled and the bark is peeled from the tree with a special tool which is forced between the bark and the wood. It is peeled in narrow strips about 3 ft. in length, and on delivery to the tannery is stacked in huge ricks. If harvested in a good, dry condition, the bark is said to improve with age, although analytical tests have shown that there is always a certain loss of tannin. An old rick is much darker in colour than a new one, owing to exposure to the air. Coppice bark from young trees is preferred by tanners, as it is free from ross and generally contains more tannin than the rough bark.

In view of the modern demand for materials in extract form, English oak bark would almost certainly be preferred in the form of a concentrated liquid, if the supply of the raw material was plentiful within a limited area. During the last few years it has not met with a ready sale, owing to the large supply of other materials, but it would doubtless regain some of its former popularity if it were prepared in the form of an extract containing about 25 per cent. of tannin. The only oak bark extract on the market is the American chestnut oak (quercus prinus).

Oak Wood is very largely used for the manufacture of tanning extract, especially in Hungary and Canada. The extract contains from 24-28 per cent. of tannin, and is extensively used in the tannage of heavy leathers, as it strengthens the liquors and hastens the process, while keeping the quality of the leather at a high standard.

Valonia (quercus aegilops) is the acorn cups of an oak tree which grows abundantly in Asia Minor and the Greek Archipelago. No other part but the acorn cups is exported. The harvest in Asia Minor takes place in August, when the fruit ripens and the cups can be easily beaten from the trees. They are left to dry on the ground and are then sent to stores in seaport towns, and principally to Smyrna. The drying is still further completed in spacious warehouses, where the cups are spread out and turned over until fermentation ceases. During this process the acorns shrink and are rejected. The cups should be perfectly dried and very hard before export. The Turkish valonia contains from 30 to 35 per cent. of tannin, and is of much better quality than the Greek, which is usually harvested before it is ripe, and, therefore, contains the acorn. As the acorn has practically no tannin value, the Greek valonia contains only 25 to 28 per cent. tannin. There are several grades of valonia, the best going to Russia, Austria, and Italy. English tanners seem to prefer the lower qualities at present, probably because the price is much less than that of the best grade.

The beard of the valonia cup is much richer in tannin than the shell, and, as several of the spines become detached during the storage of the material, there is always a certain quantity of beard (trillo) on offer. This may contain up to 42 per cent. of tannin, but its price is usually the same as that of ordinary valonia. Of late years, very large quantities of valonia have been made into extract at two works in Smyrna. The production of extract will no doubt increase, with a corresponding reduction in the export of the raw material. The great advantages of the extract over the natural cup are its superior strength of tannin (60 to 65 per cent.), easier solubility, uniformity of quality, lower cost per unit of tan, and guaranteed purity.

Valonia is well adapted for the tannage of sole leather in conjunction with oak bark, for it deposits a heavy bloom (ellagic acid), imparts weight and solidity, and increases the resistance of the leather to moisture.

The Chestnut Tree (castanea vesca) probably provides the next tanning material of importance. This must not be confused with the chestnut oak, an American tree which also yields a very useful tannin. The chestnut is indigenous to the South of France and Italy, where the forests have been considerably reduced in size to meet the great demand for this popular tanning material. The greater part of the denuded forests have not been replanted with the chestnut, as the land has been put under cultivation whenever possible. A further depletion has been caused by the ravages of an insect, which turns the interior of the wood quite black and renders it unfit for tanning purposes. It will, naturally, be several years before the supply is exhausted, even if no reafforestation is undertaken. As it is the most important tanning material grown in France, and the chestnuts are used as a food, steps may be taken to cultivate the trees on areas unsuitable for agriculture. Liquid chestnut extract contains from 30 to 32 per cent. of tannin, and, when decolorised, gives a light brown colour to the leather. It is rarely, if ever, used alone, but generally in conjunction with quebracho, valonia or myrobalan extracts.

The Chestnut Oak Tree is indigenous to America and the wood yields a very good tanning extract, containing up to 30 per cent. of tannin. This material is the principal tanning agent used in America, where the tannages are roughly classified in three sections: (1) oak, (2) hemlock, and (3) the union (i.e., a mixture of hemlock and oak). American tanners also use other materials to a smaller extent, chiefly for blending with the principal tannins. Quebracho and spruce extracts are specially favoured.

Quebracho Colorado is a tree indigenous to South America, the best wood for tanning purposes being found in the Gran Chaco district in the north of Argentina, and in Uruguay. The wood contains from 17 to 22 per cent. of tannin, and is so hard and heavy that it sinks in water. In fact, its name is derived from two Portuguese words meaning "axe-breaker."

After felling the trees, they are cut up into logs about 4 ft. in length and either exported in this state to Hamburg, Havre, and Liverpool, or sent to the numerous factories in close proximity to the forests to be made into extract, in which an enormous trade has of late years been developed. Very little of the natural material is now used, as, even after cutting the wood into chips, the tannin is extracted only with great difficulty, whereas the extract can be treated with sulphites, alkalies, or neradol (the artificial tannin) to render it easily soluble, besides which the concentration of the material raises the percentage of its tannin to 65 or even 70 per cent. Owing, perhaps, to faulty preparation, this tanning extract was not well received at first, but it is now among the principal tannins and increases in importance every year.