ULTRAMARINE occurs in nature as the colouring princi ple of lapis lazuli, from which it was originally obtained by crush ing the stone and separating the pure material by levigation. The scarcity of the mineral, which is a double silicate of sodium and aluminium with some combined sulphur, and the low yield ob tainable rendered genuine ultramarine extremely costly and the pigment to-day is manufactured solely by artificial means. The raw materials used consist, briefly, of (I) kaolin (China clay) free from iron and manganese, (2) soda ash, (3) anhydrous so dium sulphate, (4) finely powdered sulphur, (5) charcoal, ash free coal or colophony, and (6) silica or quartz. In the "indirect process" these materials are intimately mixed and ground and heated in a furnace at a bright red heat for 7-10 hours with exclusion of air, then allowed to cool slowly. Sulphur is added to the green ultramarine so formed and the mixture is converted into blue ultramarine by further heating at a low temperature. In the "direct process" the raw materials, as above, after mixing and grinding are roasted in an oven with access to air.
Artificial ultramarines are divided into three classes distin guished as the lighter "sulphate ultramarine," "ultramarine poor in silica" (dark blue) and the "ultramarine rich in silica"; the last, which is made by the direct process, is darkest in colour with a reddish tinge and, unlike the first two, made by the in direct process, is very resistant to alum solutions (hence is largely used in the paper trades). Ultramarine is fast to light and alkalis
but is readily destroyed by mineral acids. Owing to the beauty of its colour, its cheapness and permanence, it is extensively used as a water, size, lime, oil and varnish colour ; in printing inks ; as a corrective for whites; for colouring paper; and in the ceramic industry. As it is liable to contain free sulphur, it is incompati ble with lead pigments. The sodium in ultramarine may be re placed by lithium, potassium and silver, and the sulphur by selenium or tellurium. Ultramarine violet is obtained by heating ultramarine blue in a furnace with ammonium chloride. Ultra marine red is prepared by the action of hydrochloric acid on ultramarine violet at a high temperature. These colours are rarely met with in commerce. See PAINTS, CHEMISTRY OF.
(R. S. M.)