Tannin

a substance much used by the tanner in the manufacture of leather. There are several tannins, all of vegetable origin. Scientists divided them into two groups. Tannins of the first class are obtained from abnormal growths caused usually by the sting of insects. The most important source of this sort is nut galls. Other tannins are obtained from healthy portions of the plants, as from the leaves and twigs of the sumac and from the bark and leaves of various oaks and the hemlock. Other tannins are obtained from divi-divi and catechu. It is the particular province of tannin to prevent bacteria from rotting the leather. For this reason the hides are steeped after proper preparation in an extract of tannin.