TANNING. The formation of leather from skins.
It is founded on the fact, that the tan nin principle precipitates gelatine, or animal jelly, in an insoluble state. Tan nin also precipitates the sulphate of iron, lying between the hair and the skin. By strong extracts of the bark, Seguin tan ned calf in 1 day, and oxhides in 7 or 8 days. 1 lb. of catechu is equal to 21 galls, 3 sumach, 81 oak-bark, 71 willow-bark, and 18 elm-bark.
The outer coating of the hemlock, and various species of the oak are the princi pal materials generally used in the United States. The former for the great body of sole leather, the latter, for the various harness and upper leather. The trees are felled in the season when the sap is ascending, from 1st May to 1st Septem ber, though usually only from May 15th to August ; and the bark is easily peeled off in sheets of any required length, but usually four feet long. It should be suf fered to liewitla the inner surface expos ed to the sun one or two clear days, to dry up the sap on that surface, when it should be gathered into piles of a square form, in a dry place, on poles above the ground, and be protected by large pieces, laid carefully on the top of the pile. The body only is peeled in this country, except the larger branches of the oak ; while in England the small limbs, and even twigs, all that will peel, are saved, and thought to be stronger than the body bark. Thirty days of dry weather will cure the bark sufficiently for use. But in a large business it is drawn to a road side, after harvest, and piled in like man ner, and is suffered to remain until fall or winter, when it is drawn into the tan nery, and stored in large piles in the open air or • in cheap open sheds, and taken into the tannery as wanted. At the North this is usually done in winter, which makes good sleighing, almost as important to the tanner as bright skies in June and July. Chemical tests give to hemlock bark only 31 to 6 per cent. tannin. American oak not more than half as much, while English hedgerows is 16 per cent. Various other foreign substances contain tannin. Valonia, of Turkey, or the acorn cup and ball, gath ered in a green state, is the favorite in England, and it is believed that the great burr oak of the Middle States yield an annual crop of the same material, which if gathered would be sufficient for all the tanning of America, and save the destruc tion of our noble forests now going on at the North so rapidly. The strongest ar ticle known is Dutch, imported from the East Indies, evidently an extract boiled down to salts, which contain about 55 per cent. pure tan. It is too expensive tor common use in this country, but is much used in England, in liquors for heavy stock. It is computed that for
every cord of hemlock bark four trees are peeled, and one cord will tan five hides. If the whole quantity of leather is 1,000,000 sides, 200,000 trees are an nually destroyed to furnish the bark. The skins of animals are immersed, for several days, or even weeks, in water with bark, mostly of oak or larch ; and other astringent substances, as terra ja ponica, are employed, which shortens the time, but renders the substance more hard and brittle. Another method is by (awing. They are left to soak, for six weeks, in water, with fresh slaked lime, changed twice, rinsed, again soaked in water mixed with wheat bran, until they float, but, when beaten down, do not rise again. The bran is then scraped off, and a liquid paste is prepared, for 100 sheep-skins, 8 lbs. of alum and 8 lbs. of salt are dissolved, in warm water, and added to 20 lbs. of fine wheat flour and 96 yolks of eggs. A ladle full of this paste is put into a trough of warm water, in which 12 skins remain for some time, and are then pulled and stretched ; and this is repeated twice. They are then left six days, and afterwards quickly dried.
Slow tanning makes leather softer and stronger.
Time and labor are both materially reduced, and the quantity and weight of the leather increased, by the substitution of water power for manual labor, in many of the most laborious parts of the process ; viz. to soften and cleanse the hide preparatory to the bark being appli ed to it ; to grind the bark ; to move pumps for transferring the decoction of the bark from one vat to another (much of which is necessary to be done daily in an extensive tannery), and to roll the leather preparatory to its being sent to market ; also the least possible quantity of lime is now used to facilitate getting off the hair. This has been found greatly to add to the weight and quality of the leather. The application of heat to bark, in leaches, is found to be very important, and more particularly the application of the decoction (usually termed liquor) to the hide, rather than the bark.
Tanning by Bilberry or Whortleberry.
a lbs. of this tan dress 1 lb. of leather, while 6 lbs. are required from the oak, and tanners also gain four months.
After tanning, the currying takes place. This consists in removing all excrescences, soaking and trampling, covering with oil, and pummelling, to produce pliancy. They are then colored, white with white-lead, black with a solution of iron, and a second of soot, vinegar, and gum.