TANNING LEATHER Leather is a combination of the hides of animals with substances which preserve it and make it useful in the arts and manufactures. The art of tanning is very ancient. Abra ham is described in the Old Testa ment as giving Hagar a water bottle of leather, which it is supposed must have been tanned with bark. Speci mens of leather tanned with alum have been found in China that are at least 3,000 years old.
Hide may be converted into leather in three ways: either by the use of astringents, as bark galls and the like; or by the use of some mineral—alum, chrome, or other—as a preservative; or by the use of oil. The leather of commerce consists of heavy grades in two classes, sole and upper; skins used for decorative purposes; and others. The hides of fur-bearing ani mals are also tanned with the fur on. Hides are bought by weight either " dry" or " green-salted," the latter being freshly skinned and salted down to prevent decay.
Both kinds before being tanned are soaked from four to ten days in the tannery to moisten the fibers and get them as near to the natural state as possible. The hides are then either sweated or limed. Sweating leather consists in hanging the hides in a warm, moist room to loosen the root hairs so that they can be taken off with a knife or machine. Liming con sists in immersing the hides from four to six days in quicklime, slaked and diluted with water. Limed hides are held superior to sweated hides. The hides are next immersed in a solution of dung, which contains some of the pepsin and pancreatin or digestive juices formed in the animals. This solution partly digests the hide. The object of soaking, liming, and bating is to remove all the soft substances which naturally occur among the fibers of the leather, leaving nothing but the network of clean fiber for the tanning liquor to act upon. When the hide is finally immersed in the tanning liquor, the substance called tannin penetrates into the fiber and changes it to chemically pure leather.
Or, if alum, chrome, zinc, or man ganese is used in the form of a metal lic salt, as copperas, sulphate of lime, bichromate of potash, sulphate of zinc, or permanganate of potash, it simply penetrates and surrounds the fiber in such a way as to keep the air from getting to it. The effect of oil is somewhat similar, as it penetrates the leather and surrounds the fibers with an air-proof and waterproof coating.
Tanning Materials. — Tannin is found in two classes of materials: one, the natural bark of various trees, as oak, hemlock, sumac, etc.; the oth er, in certain abnormal growths on various trees caused by diseases and the stings of insects, as, for example, galls. Oak bark contains about 10 per cent of tannin. Hemlock bark and pine bark, which are cheaper than oak and make an equally strong and durable leather, contain 7 to 12 per cent. Gambia, imported from Singa pore, consists of the leaves of a large bush boiled and formed into blocks. Sumac, which contains 15 to 25 per cent of tannin, consists of the leaves of the sumac bush ground to powder and packed in bags. These are the principal tanning materials used in the United States.
Galls are diseased excrescences pro duced on the leaves of oak, sumac, and other trees by the stings of in sects. Apple galls contain 60 to 75 per cent of tannin; istrien, 32 per cent; Persian, 30 per cent; Chinese, 80 per cent. Kopperin, which is similar to galls, is found on the acorn or leaf of the oak tree, es pecially the steel oak of Hungary, and is imported from Austria. It contains 27 to 33 per cent of tannin.
Tanning Extracts. — The tanning extracts dissolved in cold water give better results in color, quicker tan nage, and better weight than the barks themselves. Hence these are commonly used to strengthen weak bark liquors. Oak, chestnut, pine, hemlock, and other extracts are on fhe market and are used in very large quantities.