The oldest system of relies on the chemical action of vegetable material contain.... tan acid

Vegetable tanning seems to have been practiced in prehistoric times. In historic times, the Hebrews tanned with oak bark, and the Egyptians, with babul pods. Tanning with salts, introduced at the end of the 19th century, was probably the first change in the chemistry of leather production in at least 2,000 years. In the double-bath method the hides are first bathed in a mild chromic acid solution. In the second bath, sodium thiosulfate and another acid react with the chromic acid to produce basic chromium salts, which are deposited on the fibres of the skins. Oil tanning is an ancient method that is used for such soft, porous leathers as chamois and deerskin, which can be repeatedly wetted and dried without detrimental effects. tan acid