Chemical treatment of raw animal hides or skins to convert them into leather .
Vegetable tanning (using bark, wood, roots, or berries) has been practiced since prehistoric times. After removal of hair, flesh, or fat, a tanning agent displaces water from the interstices between the protein (mostly collagen ) fibres in the skin and cements the fibres together. The agents most widely used are vegetable tannin , salt s such as chromium sulfate, and fish or animal oil. The tanning of fair skin in humans by sunlight is completely different: ultraviolet light causes production and redistribution of the pigment melanin in epidermal cells.