If the pelts are to be tawed, 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 sufficient 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 tough, 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 hung up in a loft to dry, and finish by glossing with a warm iron.
The essential difference between tanning and tawing therefore, 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, propably alumine.
The Morocco leather (so termed from its being the same description of article as was formerly imported from the kingdom of Morocco,) is distinguished into two kinds; one being made from deers' and goats'skins, which kind is by far the most durable and beautiful in appearance, and often called "real Morocco;" the other from sheep skins, which, from being only about one-third the price of the real, and being artfully made to imitate the other, by the dressing and finish, is most extensively used for book-binding, shoes, coverings to desks, furniture, and an infinite variety of purposes. The leather is thus made: - The skin, cleansed and worked in the way already described, is taken from the lime water, and the thickening thereby occasioned is brought down, not by bran liquor, as in tawing, but by a bath of dogs' or pigeons' dung diffused in water, where it remains until sufficiently suppled, and until the lime is quite got out, and it becomes a perfectly white clean pelt. If intended to be dyed red, or any other colour, the opposite edges of the skin are brought together and sewed up very tight, forming an irregular close bag, with the grain side of the skin outwards, as this side alone receives the dye; therefore, if there are any holes in the skin, they are also sewn up that the dye may not get inside the bag and dye both sides of the skin.
The temperature of the bath should not be greater than the hand can bear, when the skin bags may be thrown in, which float upon the surface, the dyer working them about with a rod until they have imbibed the dye uniformly. The proper management of this process requires much skill and experience, some colours, particularly the compound, requiring two or more baths to obtain the required hue. The cochineal and Brazil reds are usually passed through a weak bath of saffron, which heightens the brilliancy of the colour, and gives an agreeable odour to the skins. After dyeing, the skins are tanned in a large vat containing a warm infusion of sumach, wherein they are kept for some hours, until they are sufficiently tanned. Those skins that are intended to be black, are first tanned in sumach, without any previous dyeing, as the sumach (or the gallic acid contained in it) acts as a mordant, to strike a black colour by the addition of a solution of iron, which is rubbed over them by a workman with a stiff brush.
The next processes are polishing and graining; they are performed either by hand or by machinery, and are technically called finishing. When performed by hand, the workman takes a skin and lays it before him upon an inclined mahogany table, the highest side of which is upon a level with the workman's middle, and the opposite side about a foot lower, in order that the weight of the body may assist in giving effect to the polisher; this is a ball of glass cut into polygonal surfaces, with which the workman, holding it between his fists, rubs the surface of the skin uniformly from the higher part of the table to the lower, the weight of the upper part of the body being the principal force applied: the skin being held by its edges overhanging the highest side of the table against which the man presses during the work. This polishing or glazing of the surface, (which greatly improves the appearance of the article,) being done, the graining is proceeded in. For this purpose the workman employs a ball of hard wood, usually box or lignum vitae, around which, equa-torially, are cut a series of equi-distant parallel grooves, producing thereby an alternating series of projecting parallel ridges; with these ridges the workman scores the skin all over in parallel lines, and when that is done he shifts the skin a little, so as to cross the first lines at a very acute angle, with his ridged ball; which he does uniformly over the skin, and thus produces a regularly corrugated surface.
In the application of machinery to the operations of polishing and graining, the principal difficulty to be overcome was to make the action accommodate itself to the varying thickness, hardness, and texture of the skin; for the necessary quantity of force to grain the firm parts of the skin, would, if applied to the tender parts, tear them; and unless the machine possessed a very sensible degree of flexibility, the prominent parts would get severely rubbed or struck, while the depressed parts would not get touched, or be but slightly acted upon. We shall annex a description of the earliest invention (about twenty-five years ago) for this purpose, which has been in use ever since. Hebert's Patent Leather-finishing Machine. - This essentially consists of a very stiff circular frame or wheel, 8 feet in diameter, revolving horizontally on a vertical axis. On the under side of the periphery of this wheel are fixed, in suitable carriages, a series of circular polishers or grainers, according to the nature of the work to be done: the carriages being provided with proper means of adjusting the position of the rubbers with great exactness, and of readily fixing, unfixing, and changing them, according to circumstances.