Vegetable tanning uses tannins, or tanning agents, that are found naturally in plant-derived sources, including tree bark and various plant species. During this method, the tannins are placed alongside the hides in water-filled pits that create a tannic acid after a few days. Over time, the hides are immersed in vats that have increasing concentrations of tannin. These chemicals produce a leather that is supple and brown. Vegetable-tanned leather does have a low resistance to water. If submerged, the material will shrink and harden, and can even become brittle.