A Manual of Shoemaking and Leather and Rubber Products

by William H. Dooley

To a large extent the so-called chemical processes have supplanted the vegetable processes, that is, old tan bark and sumac processes; but in some tanneries both methods are used on different kinds of skins.

In the old bark process the tan bark is ground coarse and is then treated in leaches with hot water until the tanning quality is drawn out. The liquor so obtained is used at various strengths as needed.

In the newer method the tan liquor is displaced by a solution of potassium bichromate, which produces its results with much less expenditure of time.

When the hides or skins are ready for the tanning process, they are put into a revolving drum, known as a “pinwheel,” or into a pit in which are revolving paddles, with a dilute solution of potassium dichromate or sodium dichromate, acidified with hydrochloric or sulphuric acid. If the pinwheel is employed, it is revolved for seven hours or longer; after which time the liquor is drawn off and replaced by an acidified solution of sodium thiosulphate or bisulphite, and then the revolution is continued several hours longer. If the pit is used, the skins are removed to another drum containing the second solution, and kept at rest or overturned for a like period.

In removing the skins from the pinwheel or vat, and in handling them after treatment with lime for the loosening of the hair, the hands and arms of the workmen are seriously injured, becoming raw if not protected by rubber gloves; even with gloves it is difficult to prevent injury, and in some establishments the workmen are relieved by the substitution of a single-bath process, in which the liquor is less harmful to the skin.

Tanning Process

Showing the vats, the unhairing and liming processes. [Page 24.]

The hides are then removed from the pits, washed and brushed, followed by slow drying in the air. When partly dried, they are placed in a pile and covered until heating is induced. They are then dampened and rolled with brass rollers to give the leather solidity. Sole leather is oiled but little. Weight is increased by adding glucose and salt.

Various rapid processes of tanning have been devised in which the hides are suspended in strong liquors or are tanned in large revolving drums. It is claimed that this hastens the process, but the product has been criticized as lacking substance or being brittle.