It is remarkably permanent for an organic substance, suffering no alteration on being combined with other pigments, and withstanding the effects of exposure to air and light. Though slightly transparent, and not quite constant in tint, it possesses very great colouring power. Being of extremely fine texture it can be worked up equally well as an oil colour or as a water colour, but it is especially in the latter capacity that it forms an indispensable artists’ colour, and permits the production of a great range of shades and tints. ULMIN.--The pigments grouped under this name are also of organic origin; but though they possess good colour, mix well, and flow readily from the brush, they lack the durability which is essential to their successful use. The following methods have been employed in their preparation:--(1) Fused caustic potash is digested in alcohol, and the liquor filtered and heated till a brown powder is thrown down, which is filtered and washed with acidulated water; (2) Waste cotton, peat, or brown coal, heated with an alkali; (3) Farinaceous matters carbonised by mineral acids. UMBERS.--These form a large class of natural earths of a brown colour, differing widely in the proportions of their chief constituents, but closely allied to the ochres and siennas in general composition, and owing their colour mainly to the presence of hydrated oxides of iron and manganese, the latter prevailing in the umbers to a greater degree than in the ochres and siennas, which consequently belong to the yellow group (_q. v._). Beds or veins of umber of varying thickness and extent are found in many places, especially in connection with magnesian limestone (dolomite). Apparently they are often derived from decomposition of this rock, perhaps due to the infiltration of carbonated water, which has acted upon the calcium and magnesium carbonates in the dolomite, and left the silica and the iron and manganese as oxides, forming the bulk of the umber. Usually these beds of umber are near the surface, though covered by an overburden of vegetable soil, and the operation of working them may be called quarrying rather than mining, being of a superficial and simple character, often only amounting to small pits. As no umber is a definite body, but rather a mixture of various substances, so the composition of every kind is peculiar to itself, and very wide differences are noticeable. Even the same bed will not necessarily produce always the same class of umber. The following figures show the extent to which the proportions of the several ingredients may vary:-- Per cent. Water given off at 212° F. 4 to 65 Water in combination 5 to 11½ Silica 4½ to 29½ Manganese dioxide 7 to 27 Ferric oxide 6 to 36