The three historical methods of making leather are vegetable, oil, and mineral tanning. Oil tanning is considered to be the oldest process, probably employed in combination with smoke curing. Neolithic excavations have revealed elk and deerskins dressed with oil and smoked. Traditional oil tannage methods employ fish oils (of which cod oil is the most important) or animal fats worked ("stuffed") into the hides to bring about oxidation, transforming skin into leather. Variations of oil tannage include the milk and butter used by nomads of
Central Asia (Kyrgyz) and egg yolk tannage by peoples of northern China.
Native Americans of
North America are known to have tanned leather with a mixture of brains and oil. Smoke curing in pits has a long tradition in China.
Vegetable tanning, a 4,000-year-old process, was developed widely across the world by ancient peoples utilizing their own local flora. Plants containing tannins (compounds of
gallic acid) infused in water were discovered as early as the Paleolithic period to affix to skins forming an impenetrable substance. Egyptians preferred using the mimosa plant for tanning purposes, while peoples of the ancient Mediterranean employed sumach leaves. Oak (and pine) bark, nuts, and galls have been the most important sources for tanning compounds in Europe, practically until the advent of chemical processes in the nineteenth century.