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, salts 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.