The material is extracted in open vats or stills of copper, which take one ton or more of nuts. A battery of 4, 6 or 8 of such stills is usually employed, and the temperature is kept well below boiling-point except in the vats containing the nearly spent material. The liquors move forward quickly, and the material is quite spent in 24 hours. The material when cast contains less than ½ per cent. of tannin. The liquor obtained is 40°-50° Bkr. (6-7½ per cent. tan), and after settling is concentrated at 40°-50° F. in a single effect vacuum pan, which though more costly in steam is quicker than the multiple effects, and gives the low temperature required. For solid extract the more concentrated liquor is run direct into tarred bags, in which it soon solidifies.
Hemlock Extract is manufactured from the North American pines and imported into this country to some extent. It gives a very red colour.
Mangrove Extract is made from the bark of Rhizophona Mangle and other species of mangrove which grow freely in the tropical swamps of West Africa, Borneo, etc. Much solid and liquid extract has been made from this material, but is not very popular on account of its harsh tannage and dark red colour.
Pine Bark Extract (Larch extract) is made in Sweden from the Norway spruce (Pinus abies). It is slightly sulphited and gives a good colour. It is a liquid extract of about 30 per cent. strength, and is sometimes used as a chestnut substitute. It should not be confused with the so-called "spruce" or "pine wood" extract, which is a paper trade bye-product and contains ligneous matters rather than tannin.
American Chestnut Extract, made from the chestnut oak, is either a liquid or a solid extract in powder form. It gives a wretched brown-black colour, which is quite unsuited to the usual British needs.
THEORY OF VEGETABLE TANNAGE.
Vegetable tannage is a phenomenon of colloid chemistry. The old arguments as to whether tanning was a chemical or a physical process have been rendered obsolete by the advent of a new set of explanations, which, though shedding light on many obscure points, have enormously increased the complexity of the problem. In vegetable tannage an emulsoid gel (pelt) is immersed in a complex emulsoid sol (tan liquor), which immersion results, not in simple reaction or change, but in a series of changes.
One of these changes is adsorption. Pelt is a gel which possesses a great development of surface. It not only exhibits like gelatine the phenomenon of imbibition and dehydration to a very marked extent, but also possesses a very fine fibrous structure due to its organic origin; thus pelt possesses an enormous specific surface, further intensified by the preparation processes previously discussed, which split up the hide fibres into smaller bundles and into much finer constituent fibrils. Tannins, on the other hand, are hydrophile colloids which in water form emulsoid sols, and which may thus be expected to exhibit the phenomenon of adsorption. A tan liquor usually contains several tannins in addition to other closely similar substances, also in colloidal solution, and is therefore a sol of considerable complexity. The immersion of pelt into a tan liquor results in an adsorption, which consists essentially in an inequality of concentration in the sol, the greater concentration being at the interface. This inequality between the surface concentration and the volume concentration of the sol, is due primarily to considerations of surface tension and surface energy, and exists before the immersion of the pelt. The surface layer having excess over the volume concentration, any considerable extension of surface in a fixed volume of sol must produce a very considerable decrease in the volume concentration. This is what occurs when pelt is immersed in a tan liquor, the immersion being the considerable extension of surface. It should be especially remembered that the inequality of concentration is in the sol, on the liquid side of the interface. In adsorption, the substance adsorbed, i.e. the excess at the surface, is too frequently regarded as bound to the solid immersed. This is because the excess is in the layer which wets the solid and remains wetting it when the solid is removed. Thus the immersion of pelt produces primarily only a change in the distribution of the tannins in the liquor. It follows from this that the adsorption is an equilibrium, and that if the sol be diluted, the equilibrium will become the same as it would have been by immersing the pelt directly into the dilute solution. Thus, if pelt be first immersed in one tan liquor and then into a weaker one it will yield tan to the latter solution.
The chief object in heavy leather tanning is to obtain the maximum possible adsorption in the minimum possible time, or in other words, to obtain good weight quickly. The amount adsorbed is proportional to the actual extension of surface, i.e. the adsorption is a function of the specific surface of the adsorbent. Hence, to obtain good weight it is necessary to develop in the pelt its maximum possible specific surface. This is one of the objects of "plumping," which splits up the fibres. It is attained also by the solution of interfibrillar substance in limes and bates.
The amount adsorbed is also a function of the volume concentration in the sol after equilibrium is reached. Hence the better weights are obtained with stronger liquors.