Large quantities of clarification sludge are produced, in alum works, as the sediment from the red liquors. This sludge consists mainly of ferric oxide, with small quantities of other oxides and sulphuric acid (basic ferric sulphate), and would be an entirely worthless by-product except for the fact that it can be manufactured into pigments, some of them of great beauty. All alum makers should treat this residue and convert it into pigments, which they could put on the market at a low rate, the cost of preparation being small. Since the material is chiefly composed of ferric oxide, the resulting colours are very similar to those obtained from iron ores; and all shades, from yellow-brown, through red, to the darkest brown, are represented. _Mine Sludge_ The water frequently present in iron mines sometimes contains large quantities of sediment, which consist mainly of iron ochre and can be advantageously worked up into pigments. There is scarcely any need to mention that all substances containing ferric oxide can be used for making any of the pigments obtainable from the oxide itself, the only difference between the various raw materials being their degree of purity, so that it is not always so easy to obtain a certain desired shade from a given material in such beauty as is furnished by another material, the small quantities of impurities associated with the ferric oxide having, in many instances, an important influence on the colour. (D) BLUE EARTHS Only two minerals are known which are capable of direct use as blue pigments, viz. vivianite (native Prussian blue) and copper carbonate (azurite, ultramarine), and as neither of them is particularly handsome, they are only used for unimportant work. Lapis lazuli is no longer employed. _Azurite, or Ultramarine_ This mineral, which is of frequent occurrence with malachite and other cupriferous minerals, forms small crystals of a beautiful deep azure blue consisting of cupric oxide in combination with carbon dioxide and water, expressed by the formula 2CuCO{3}, Cu(OH){2}, or Cu{3}(OH){2}(CO{3}){2}, and containing 69·19% of cupric oxide, 25·58% of CO{2} and 5·23% of water. The colour of the powdered mineral is much paler than that of the crystals. The pigment, which is used for cheap paints, is not particularly stable, and loses much of its beauty when applied to plaster. _Vivianite_ This mineral occurs in many places as crystalline masses, but also forms earthy deposits, some of which, especially in certain bogs, attain considerable thickness. The colour is between indigo and blackish blue; and the freshly won mineral often has an unsightly whitish appearance, which, however, soon changes into the pure blue. The cause of this peculiarity is due to the fact that vivianite originally consisted of hydrated ferrous phosphate, which is white, this compound being transformed, under the influence of the air, into the blue ferric phosphate.