The experiments of Sir Humphrey Davy show, that 11b. of catechu is nearly equal to 2j of galls, to 3 of sumach, to 7i of the bark of the Leicester willow, 8. of oak-bark, 11 of the bark of theRpanish cheanut, 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 therefore, affects the quality of the 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 forme 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 pre pared by the process called taming; 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 akin is thoroughly cleansed, and reduced to the state of simple membrane, in which it is called pek, is especially the same, whether for Jawing 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 consi derable 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 nest 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 stop ping 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 par ticle 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 web. 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 exten sible white membrane, called a pelt, which is a condition that adapts it to any subsequent operation. of tawing, or dyeing, oil-dreseing, 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.
If the pelts are to be gassed, they are then put into a solution of alum and salt, in warm water, in the proportion of about three pounds of alum, and four pounds of salt to every 120 middle sized skins, and worked therein till they have absorbed a auflicieat quantity. This again gives the skin a remarkable degree of thickness and toughness. The skins are then taken out, washed in water, and then again put into a vat of bran and water; and allowed to ferment for a time, till much of the alum and salt is got out, and the usual thickening produced by them is for the most part reduced. They are then taken to a lofty room, with a stove in the middle, and stretched on hooks, and kept there till fully dry. The skins are thus converted into a flexible, and quite white leather; but to give them a glossy finish, and to take off the harshness of feel still remaining, they are again soaked in water, to extract more of the salt, and -put into a large pail containing the yolks of eggs beat up with water. Here the skins are trodden for a long time, by which they so completely imbibe the substance of the eggs, that the liquor above them is rendered almost perfectly limpid; after which they are bung up in a loft to dry, and finish by glossing with a warm iron. The essential diarence between tanning and tawing there fore, is, that in the former case the pelt is combined with tan or other vegetable matter, and in the latter with something that it imbibes from the alum and salt, probably alumine.