A key step in the ancient process of preserving skins and hides and turning them into leather; tanned leathers have been known to last thousands of years. Skins are first cured (with salt) and cleaned before being soaked in a tanning agent, and then finished. Un-tanned leather is called rawhide.
Communities the world over have their own particular tanning recipes and techniques, but broadly speaking this can be achieved either by using vegetable tannins (from the bark of trees such as chestnut, oak, tanoak, hemlock, qubracho, mangrove…) or mineral tannins such as chrome in combination with salts and acids; tanning with alum is referred to as tawing (see taw), and is specifically used for pig and goatskins. Chrome tanned leather is faster to make and tends to come in a wider range of colours, but from a quality, durability and environmentally-friendly perspective, vegetable, organic tanning methods produce the finest leathers.
Tanned leathers were, and still are used to make shoes, bags, waterskins, rugs, floor tiles , walling and for any upholstery. Calf and kid skin are the softest; goat skin is mostly used for floor rugs, pig skin is tougher so readily accepts all manner of finishes, cow skin is used extensively as hide and as leather for upholstery.