Tanning hide into leather involves a process which permanently alters the protein structure of skin, making it more durable and less susceptible to decomposition and coloring.
Before tanning, the skins are dehaired, degreased, desalted and soaked in water over a period of six hours to two days. Historically this process was considered a noxious or "odoriferous trade" and relegated to the outskirts of town.
Historically, tanning used tannin, an acidic chemical compound from which the tanning process draws its name, derived from the bark of certain trees. An alternative method, developed in the 1800s, is chrome tanning, where chromium salts are used instead of natural tannins.