Chrome tannage has been chiefly developed in this country during the last twenty years and is now in general use. It consists in throwing an insoluble chromium hydroxide or oxide on the fibers of a skin which has been impregnated with a soluble chromium salt—potassium bichromate. Other salts like basic chromium chloride, chromium chromate, and chromic alum are also used. The hydrochloric or sulphuric acid acts by setting free chromic acid.
After several hours, the skin shows a uniform yellow when cut through its thickest part. It is then drained and the skin worked in a solution of sodium bisulphite and mineral acid (to free sulphur dioxide). The chromic acid is absorbed by the fiber and later reduced by sulphur dioxide.
In the making of chrome black leather each tanner has his own method. Contrary to the general belief, there are many different methods of chrome tannage. No two tanneries employ just the same process.
Tanners of chrome leather seek to produce leather suitable for the particular demands made upon it by the peculiarities or characteristics of the varying seasons. Summer shoes require a cool, light leather; at other times a heavier tannage is essential, with some call for a practically waterproof product.
All leathers, whether vegetable-or chrome-tanned, must be “fat liquored.” That is to say, a certain amount of fatty material must be put into the skin in order that it may be mellow, workable, and serviceable. This is very essential in producing calf leather. Fat liquors usually contain oil and soap, which have been boiled in water and made into a thin liquor. The leather is put into a drum with the hot fat liquor; the drum is set in motion, and as it revolves the leather tumbles about in the drum and absorbs the oil and soap from the water. It is the fat liquor that makes the leather soft and strong.
Leather used in shoes is divided into two classes: sole leather and upper leather.
Sole leather is a heavy, solid, stiff leather and may be bent without cracking. It is the foundation of the shoe, and therefore should be of the best material. The hides of bulls and oxen yield the best leather for this purpose.
The hide that is tanned for sole leather is soaked for several days in a weak solution (which is gradually made stronger) of oak or hemlock tan made from the bark. Oak-tanned hide is preferred and may be known by its light color. A chemical change takes place in the fiber of the hide. This is a high-grade tannage, and is distinguished principally by its fine fibers and close, compact texture.