Leather Tanning

The process of tanning leather is the preservation of animal hides or skins by use of a chemical which 1) makes them immune to bacterial attack, 2) raises the shrinkage temperature, and 3) prevents the collagen fibers from sticking together on drying, so that the material remains porous, soft, and flexible. The main active constituent in tanning chemicals (tannins) is tannic acid. The tannins penetrate the skin or hide after long periods of soaking during which the molecular aggregates of the tannin form cross links between the polypeptide chains of the skin proteins; hydrogen bonding is an important factor.

In mineral or chrome tanning the sulfates of chromium, aluminum and zirconium are used. The reaction is of a coordination nature between the carboxyl groups of the skin collagen and the metal atom.

Tanning by any method is a time consuming and exacting process, requiring careful control of pH, temperature, humidity, and concentration factors. Bicarbonate increases the alkalinity of chromium sulfate solutions to favor the reaction cross-linking chrome complexes with hide collagen. The reaction stabilizes the hide collagen fibers so they are no longer biodegradable. Sodium bicarbonate is commonly used as a pH control in basic leather tanning and ammonium bicarbonate is used for finer leathers where the sodium ion may be too harsh.