The art of tanning is that by which animal skins are converted into leather, a product possessing certain properties differing from those of the raw material, and eminently adapted to the purposes for which it is employed. Chemically considered leather is a compound of tannin and gelatine, possessing the requisites of durability, pliability, insolubility in water, and great power of resisting the action of chemical reagents.
The name of tan is applied to coarsely powdered bark which is obtained mostly from oak and hemlock trees, although all barks contain more or less tannin, and in some countries the extract of others is used.
To tan a skin is to saturate it with tannin in such a manner as to promote the slow combination of this principle with the gelatine, albumen, and fibrine contained in the former.
The principal steps in the manufacture of leather are,
1. The washing and soaking in pure water, for the purpose of cleansing and softening the skins, and preparing them for
2. The unhairing.--This is effected by the use of lime, or by sweating the hides, which dissolves or softens the bulbous roots of the hairs, and thus facilitates their removal by mere mechanical scraping with a blunt-edged knife.
When lime is employed, about 4 bus. are slacked and put into a large vat of clean water, capable of holding 40 hides or 2 hundred calf-skins; the lime is well mixed by a plunger, and the hides or skins are then put into it and allowed to remain from 7 to 10 days for the former, and 10 to 14 days for the latter, drawing them out daily to facilitate the process.
When the hair will slip they are taken out of the lime and plunged into clean water, from which they go to the beam where the hair is scraped off with a long curved blunt-edged knife; they are immersed in water again and taken back to the beam, and all the flesh removed from the inside of the hide or skin with a sharp knife similar in shape to the one used in unhairing, after which they are ready for
3. Bateing.--As it is all important to have the skins soft and in a condition to absorb the tanning liquor readily; this is accomplished by putting them in water impregnated with pigeon's dung, 1 bu. being enough for the number of hides or skins above named. This is called a bate, and acts by means of the muriate of ammonia which it contains, the lime taking the acid becomes muriate of lime, which is soluble and easily worked and washed out of the skins, while the ammonia passes off in a gaseous state.
Hides intended for sole-leather should remain 24 hours in this bath, when they may be worked out and are ready for the bark extract; calf-skins, or other upper leather, should remain in the bath from 3 to 5 days, according to the weather (a longer time being required in cold than in hot weather), and during this time they are taken out 2 or 3 times and placed on the beam where they are scraped, first on the grain side, and lastly on both flesh and grain with a worker similar to the one used in unhairing, after which they are ready for
4. Tanning.--When the hides or skins are taken from the beam-house they are put into vats containing a weak solution of ground bark, and should be handled two or three times a day until they are evenly colored, when they should go into a stronger liquor, or ooze, where they may remain a week, being taken out daily and allowed to drain off, at the same time strengthening the ooze. They may now be considered ready for laying away. For this purpose a vat is half filled with a very strong extract of bark, and the hides or skins are carefully laid in, one at a time, each being covered with finely-ground bark to the depth of half an inch, until all are thus laid away. About a foot in thickness of spent tan is put on for a heading, and the vat covered with boards.
The hides or skins may be allowed to remain in this their first layer for two weeks, at the expiration of which time they must be taken up, washed clean in the liquor, and the same process repeated, using a new liquor and fresh bark, as the strength has been absorbed from the other. As the tanning proceeds the extract is exhausted more slowly, and from 3 weeks to 1 month may be allowed for each successive layer, after the first--3 layers being enough for calfskins, and 4 to 6 for sole-leather, according to the thickness of the hides.
When the sole-leather is tanned it is taken out of the vats, washed clean, and hung up to dry in the rolling loft. When nearly dry it is rolled on the grain with a brass roller until it is quite smooth, hung up again, and thoroughly dried, and is ready for the market.
_Currying or Finishing Calfskins._
When calfskins are sufficiently tanned they should be rinsed in the liquor in the vats, and hung over poles and slightly hardened, being careful not to expose them to the direct rays of the sun in the summer months. Put into piles, so that they will not dry out, dampening any part that may have become too dry. They are now shaved over a currier's beam, during which process the rough flesh is taken off, and the butts and heads are leveled and the rough edges trimmed off. The skins are then rinsed off, slicked on a marble table with a steel slicker and stiff brush on the flesh side, the dirt and coloring matter of the bark stoned, brushed, and slicked out on the grain side. They should then be hung up, by a loop cut in the head, for a few hours, that the water may be partially dried out of them: they must be then taken down and placed in a pile, and are ready for stuffing.
The grease called dubbing is composed of equal parts of cod-liver oil and melted tallow, and when ready the skin is laid on a wooden table and slicked on the flesh side. The stretch is in this manner taken out, and the skin should be perfectly smooth on the table before the dubbing is coated on, for which purpose a brush or pad is used, the quantity put on varying, according to the thickness of the skin. They are next hung up by the hind shanks, and allowed to dry. When entirely dry they must be taken down, and piled flesh to flesh and grain to grain, and should remain for a week or two, so as to become an even color, and also to absorb the strength of the grease. When ready to finish the grease must to slicked off on a finishing table (made of cherry or mahogany wood), and the skins are softened by rolling them with a board having fine grooves cut in it. The surface of the flesh side is smoothed by shaving over it with a currier's knife which has a very fine and delicate edge turned on it, so that the smallest quantity only is taken off. This process is termed whitening. The skins are then stoned on the grain side, and all wrinkles and breaks taken out, and a fine grain is turned on them with a smooth board, or with very fine grooves cut in it. They are matched for size, laid down in a pile, the larger ones in the bottom, and blacked on the flesh side with a compound of lampblack, tanner's oil, and dubbing, and a small quantity of water, to prevent it striking through. As they are blacked they are laid over a strip. They must now be pasted, to prevent the black rubbing off. The paste is composed of wheat flour and boiling water, stirring in a small quantity of soap and tallow, and is applied with a brush, coating them with as small a quantity as possible. They are hung up by the loop in the head and dried, then glassed with a polished glass slicker on the flesh side, and are ready for the last process, gumming. The gum used is gum tragacanth, dissolved in water, and is applied with a sponge, on the flesh side. hung up, and when thoroughly dry, they are ready for sale, or cutting into boots and shoes.
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