Skins is the general term for the skins of calves, seals, hogs, dogs, &c. These, after being washed in water, are put into lime-pits, as before mentioned, where they are taken up and put down every third or fourth day, for a fortnight or three weeks, in order to dilate the pores and dissolve the gelatinous parts of the skin. The hair is then scraped oft; and the flesh and excrescences being removed, they are put into a pit of water impregnated with pi geon-dung (called a grainer or mastring,) forming a strong alkaline ley, which, in a week or ten days, soaking out the lime, grease, and saponaccous matter (during which period they are several times scrap ed over with a crooked knife to work out the dirt and filth), softens the skins, and prepares them for the reception of the ooze. They are. then put into. a pit of weak ooze, in the same manner as the hides, and being frequently handled, are by degrees removed into a stronger and still stronger liquor, for a month or six weeks, when they are put into a very strong ooze, with fresh bark ground very fine ; and at the end of two or three months, according to the substance, are sufficiently tanned ; when they are taken out, hung on poles, dried, and fit for sale. These skins are afterwards dressed and blacked by the currier ; and are used for the upper-leathers of shoes, boots, &c. The lighter sort of hides, called dressing hides, as well as horse hides, are managed nearly in the same manner as skins; and are used for coach-work, harness-work, &c.
Having given some account of the pro cess, as commonly used in this country, we proceed to one recommended by MSe guin in France, who is supposed to have done much towards simplifying and ren dering perfect the art. In order to give currency to the knowledge which he had obtained by a long course of experiments and actual practice in the business, he exhibited without reserve all that he had discovered, and at the same time actually executed Isis processes on the large scale, furnishing gratuitously skins and tan, in order that others who were witnesses to his plans might repeat for themselves, and at their leisure, the experiments they had seen him go through. We shall give an outline of his plan and reasoning on this important subject.
Skins swell up, and become soft, by moisture, which renders them permeable to water. Hence they are easily destroy. ed by the putrid process which ensues, and they become dry and brittle when the moisture is evaporated. Accident, no doubt, occasioned the discovery of the means ofpreventing these inconveniences by the use of certain vegetable substan ces, particularly the bark of oak. It was seen that skins prepared with these sub stances acquired new properties; that, without losing their flexibility, they be came less permeable to water, more firm, more compact, and in some measure inca pable of putrefaction. These observations
gave birth to the art of the tanner. This art, no doubt of high antiquity, because founded on one of the earliest wants of man in society, comprehends a succession of processes which was executed by habit and imitation, without a knowledge of the essential objects. The preparation of skins accordingly required several years, and frequently, in spite of the care, ex. pence, and slowness of the operation, the tanning was incomplete ; the skin formed a soft and porous leather, which was soon destroyed by moisture. These defects essentially sprung from ignorance of the true principles of this operation, because no discovery had been made respecting the action of tan upon the skin, and the circumstances, or conditions, which might accelerate or retard the process.
To arrive at this knowledge in an accu rate manner, it is necessary to consider, first, the nature and properties of tan ; and secondly, the structure and composition of the skin. We shall not enter into the de tail of such precautions as are requisite in the choice of oak bark, the time and man ner of separating it from the tree,preserv ing it, or pulverising it. It will be sufa. cient for our object to remark, that water poured into a vessel upon tan acquires, af ter some hours infusion, at the common temperature of the atmosphere, a brown colour, an astringent taste, and becomes charged with the most soluble substances contained in the tan ; that by drawing off the water, and adding a similar quantity to the tan repeatedly, the whole of the solu ble parts may be successively extracted, the water ceases to acquire colour, and there remains in the tub a mere fibrous matter or parenchymatous texture, insolu ble in water, and no longer adapted to promote the operation of tanning. This residue is therefore always rejected in the manufactories as useless. It is only used by gardeners for their hot-beds, but might probably be advantageously applied in the fabrication of coarse paper.
It is therefore in the water of infusion, or the lixiviations of tan, that we must seek for the soluble substances which alone are efficacious in tanning. On examination of the water of the last fil tration, it is found to be not only clearer, less impregnated, and less acrid than the water of the first lixiviation, but likewise that it possesses all the proper ties of the gallic acid. It reddens the in fusion of tournsol, acts upon metallic solu tions, and more particularly it precipitates a black fecula from sulphate of iron, &c. And it is also found, that a piece of fresh skin, divested of its fat and sanguine hu mours, and macerated in this liqour, in stead of becoming compact, is softened and swells up.