What is TANNING?

is an important industry, as indicated by the fact that in 1918 about 8 million hides and calf skins of the value of 30,000,000 were tanned in the United Kingdom. After soaking to remove blood and lymph, the skins are painted on the fleshy side with a mixture of slaked li me and sodium sulphide or calcium hydrosulphide, and then the wool is pulled, leaving the skin, or “pelt.” This, after washing, is passed through a lime liquor, and, after rubbing off the hair, is then subjected to a mechanical operation to remove all adhering flesh, when the skins are ready for “bating,” or “puering,” to render them soft and supple by removal of hair sheaths, sebaceous glands, muscles, sweat ducts, etc., held together by elastic fibres a process which is of a fermentative character, carried out by dressing with dung. After drenching, the skins are then ready for treatment with the tan liquor, which may be an infusion of sumac other tanning materials being barks, gambier, myrobalans, valonia, and extracts, such as kino, and those of oakwood, chestnut, and quebracho. In the process of tanning, the astringent principles enter into combination with the collagen part of the skins constituting the fibrous tissues, producing leather. Formaldehyde has been used in tanning, by reason of its coagulating and preservative effect on the collagen molecule. In another process of tanning, chromium salts are used for treatment of the prepared pelts, the bath consisting of a mixture of potassium or sodium dichrnmate with hydrochloric or sulphuric acid, followed by the application of sodium thiosulphate to effect the reduction of the chromium salt to a basic state or oxide in combination with the tissue. It is stated that after unhairing and pickling by immersion in a bath of ai.ute sulphuric acid and sodium chloride, skins can be completely tanned in two days by immersion in a bath of chrome liquor, consisting of basic chromic sulphate diluted to contain 17 grms. of chromic oxide per litre The impregnation of the materials to be tanned is sometimes effected i>v a process of electric endosmose, which is said to be applicable to the impregnanation with chromium or other metallic salts, of fabrics which have been treated with glue or gelatin, and that this method permits of the utilization of very dilute tanning liquors and greatly facilitates the process in point of time.

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Definition of  TANNING