The very earliest leather tanning was carried out using tannins (a class of polyphenol astringent chemicals) which is how the process came to be known as ‘tanning’. Tannins bind themselves to the collagen protein found in the hides making them less water soluble and more decay resistant, by decreasing their vulnerability to bacteria. This also makes the hide more flexible.
In modern vegetable tanning, the most common barks used are chestnut, oak, redoul, tanoak, hemlock, quebracho, mangrove, wattle (acacia), and myrobalans such as Terminalia chebula. During this process, the hides are stretched on frames which are then immersed in vats which have increasing concentrations of tannins. The resulting leather is supple and brown in colour, with the exact shade dependent upon the tannins used and the colour of the hide.
Vegetable tanned leathers are not very resistant to water and becomes discoloured. If the leather is soaked in water it will then shrink, becoming harder and less supple. If it is soaked in hot water, then it will partially gelatanize, become rigid and eventually brittle. When this was discovered, the hard, toughened leather that could be obtained by boiling vegetable tanned hides (either in water or in wax) was used in the making of armour and book bindings.
Nowadays, vegetable tanned leather is used to make luggage, furniture, footwear, belts, and other clothing accessories. It is also the only type of leather that is suitable for carving or stamping.