Of the numerous substances employed in tanning, oak bark is the chief in this country, not merely on account of its suitable properties, but from its comparatively low price, and the facility with which it may be obtained almost every where. In Russia, where the best of leather is made, the bark of the black willow is preferred, and next to that, the birch bark. Chesnut bark is now much esteemed for the purpose. A tanner at Bern-castle, on the Moselle, has lately employed the myrtle with great advantage; it is reported that by the use of it as a substitute for oak bark, better leather is made by it in much less time: a commission appointed at Treves, for the examination of leather so tanned, reported that they never before saw any article equal to it in quality. The Recueil Industriel has recently stated, that at Narbonne, the marc of grapes, after being distilled for the separation of the alcohol, had been found a most important substitute for oak bark in tanning. After the skins had been prepared in the usual way, they were placed in the pits containing the marc instead of bark; the skins were completely tanned in from thirty-five to forty days. The expected advantages are, shortening the process, reducing the cost, improved odour, and greater strength.
But of all the substances of recent introduction, the extract from the mimosa, known in commerce by the terms gum catechu, and Japan earth, is the richest in tannin matter. This tree grows in vast abundance in New South Wales; where preparations have been made for making the extract on a great scale for the tanning process. The leather made from it is of a beautiful colour, and an excellent quality.
The experiments of Sir Humphrey Davy show, that 1 lb. of catechu is nearly equal to 2 1/4 of galls, to 3 of sumach, to 7 1/2 of the bark of the Leicester willow, 8 1/2 of oak-bark, 11 of the bark of the Spanish chesnut, 18 of elm bark, and 21 of common willow bark, with respect to the tannin contained in them. He observes, too, that leather slowly tanned in weak infusions of bark, appears to be better in quality, being both softer and stronger than when tanned by strong infusions; and he ascribes this to the extractive matter they imbibe. This principle, there fore material employed in tanning; and galls, which contain a great deal of tannin, make a hard leather very liable to crack, from their deficiency of extractive matter.
The preparing and dressing of lambs, sheep, deer, goat, and other thin hides, closely resembles the method used with those of thicker or larger kind already mentioned, but it usually forms a distinct branch of business; and it is one that requires much practical skill and nicety of manipulation, to produce goods of the desired quality. The processes vary in many particulars, according to the nature of the commodity. This branch of the leather manufacture supplies the immense demand of white and dyed leather, the (so called) Spanish and Morocco leather, of different colours and qualities, and a great variety of thin leather for different purposes. Of these, the white leather alone is not tanned, but is prepared by the process called tawing; but the coloured leather receives always a tanning, which is usually effected by sumach, independently of the other dyeing materials. The previous preparation of each, or that in which the skin is thoroughly cleansed, and reduced to the state of simple membrane, in which it is called pelt, is especially the same, whether for tawing or dyeing.
The mode of performing these operations at Bermondsey, adjoining London, is as follows: - Lamb skins, are first soaked for a time in water, to cleanse them from the loose dirt and blood, then put upon the beam, (a half-cylinder of wood, covered with strong leather,) and scraped on the flesh side with the semicircular blunt knife with two handles, used for this operation; they are then hung up in considerable numbers, in a small close room heated by flues, where they remain to putrefy for a given time, during which a thick slime works up to the surface of the skin, and the wool is loosed, so that it readily comes off with a slight pull. Each skin is then returned to the beam, the wool taken off and preserved, and all the slime worked off with the knife, and the rough edges pared away. The skin is next put into a pit filled with lime water, and kept there from two to six weeks, according to the nature of the skin; this process has the effect of stopping the putrefaction of the skins, and renders them thicker and harder; after which it is again worked upon the beam, and much of its substance is pared down, and all inequalities smoothed with the knife.
Much skill and judgment are required in these operations; on the one hand, not to endanger the substance of the skin by the putrefaction, and on the other hand, to work out every particle of the slime, the least of which, if retained, will prevent the skin from dressing well in the subsequent processes, and from taking the dye uniformly and well. The skin is again softened and freed from the lime, by being plunged into a vat of bran and water, and kept there for some weeks in a state of gentle fermentation, being occasionally returned to the beam. All the thickening produced by the lime is thus removed, and the skin in this highly purified state, is a thin extensible white membrane, called a. pelt, which is a condition that adapts it to any subsequent operation, of tawing, or dyeing, oil-dressing, or shammoying.
The method of bringing kid and goat skins to the state of pelt, is nearly the same as for lambs, except that the liming is used before the hair is taken off, the hair, being only employed by plasterers, is of little value; but the lamb's wool, which is more valuable, would be injured by the lime. Kid's skins, being of a closer texture than lambs', take a long time in tanning.