The use of vegetable tannins and dyes is unquestionably as old as civilisation itself, having been used for preparing leather from animal hides and skins and for colouring purposes, respectively. The art of tanning and dyeing seems to have developed independently by the people of both the Old and the New Worlds using different materials and methods. The vegetable dyes have lost ground largely to the synthetics, such as alizarin or aniline dyes obtained from once ‘useless’ coal tar substances. Only a small fraction of the dyes used today is of natural origin. Those who still use vegetable dyes, however, believe that no synthetic dye has a comparable lustre, the underglow of rich colour or the soft light and shadow so appealing to the eye. They consider synthetic dyes as garish and harsh. Vegetable tannins, on the other hand, have retained their prominence and are still in great demand since, the synthetic products (syntans) could not be made available at competitive prices. Although nowadays, more and more mineral tannins, inorganic salts (such as those of chromium and zirconium) and other tanning agents are being used. In addition, the demand for leather substitutes, such as synthetic leather and composition shoe soles is steadily increasing. Most of the synthetic and mineral tannins now in use lack certain attributes necessary to obtain heavy leather and, therefore, cannot be considered as true substitutes for vegetable tannins. Nonetheless, vegetable tannins are still sufficiently important to merit some discussion whereas the vegetable dyes are treated for nothing more than their historical importance.

Chemically, tannins and dyes are far from simple, usually being composed of carbon, hydrogen and oxygen (along with some nitrogen in the case of dyes). They are secretion products found in small or large quantities in all plant tissues. In spite of their wide occurrence, they are not concentrated adequately enough to warrant economical extraction; in consequence, relatively few plants have become important from a commercial standpoint.

Vegetable Tannins

The usefulness of tannins stems from their ability to combine with the proteins of hides and skins to yield stable leather that is soft, pliable, very strong and resistant to air, moisture, temperature changes and bacterial attack.