Tanning is the ancient craft of transforming animal skins into durable leather. It was widely practiced in the southern mountains by both the Cherokee and early European settlers as part of their household self-sufficiency. The most common technique was vegetable tanning, which employs tannic acid leached from bark. This practice relied on abundant supplies of oak, hemlock and chestnut trees and was used to treat the hides of livestock, deer, and small mammals. Tanners used much of the leather they produced on their farms and sold the rest in the local community. Many small farmers supplemented their incomes by peeling bark and hauling it to nearby tanneries. A surviving specialty of this vanishing craft is the tanning of groundhog skins, which are prized as the best material for making banjo heads.