Tan-bark is obtained from many trees, including hemlock (Fig. 263), oak (Fig. 243), willow (Fig. 228), chestnut (Fig. 24), larch (Fig. 259), and spruce (Fig. 260), which are rich in tannins. These substances, as already explained in sections 57 and 60, are astringents which are present in various parts of many plants, and agree in forming an inklike product when combined with an iron salt. Though chemically more or less diverse they mostly resemble indican and haematoxylin in being glucosides, and are believed to be usually waste products of the plant producing them. A property of tannins which renders them especially valuable to the dyer is that they are readily absorbed in solution by cotton, linen, and silk, and will then precipitate various dyes within the fiber, thus serving as a mordant. But the chief property which gives industrial importance to plants rich in tannins is the power which these substances have of so combining with animal skins as to render them permanently pliable and resistent of decay. Hence it is that a hide soaked, under proper conditions, in an extract of tan-bark becomes leather. At the same time, the staining powers of the tannin and associated substances may be taken advantage of to impart a strong color to the product.