Tannin (Tannic acid). Cotton and linen strongly attract tannin and when prepared with it they are able to retain dyes permanently. Cotton saturated with tannin attracts the dye stuff more rapidly, and holds it. Tannic acid is the best tannin for mordanting as it is the purest and is free from any other colouring matter; it is, therefore, used for pale and bright shades. But for dark shades, substances containing tannic acid are used, such as sumach, myrobalans, valonia, divi-divi, oak galls, chestnut (8 to 10 per cent tannin), catechu