BLUE PIGMENTS

Several metallic compounds po e a blue colour; especially those of iron, cobalt, and molybdenum. The metallic pigments, little if at all employed, but which may be found useful in particular cases, are the molybdate of mercury, the hydro-sulphuret of tungsten, the pru iate of tungsten, the molybdate of tin, the oxide of copper darkened with ammonia, the silicate of copper, and a fine violet colour formed from manganese and molybdenum. The blues of vegetable origin, in common use, are indigo, litmus, and blue cakes. The blue pigments of a metallic nature found in commerce are the following: Pru ian blue ; mountain blue , a carbonate of copper mixed with more or le earthy matter; Bremen blue , or verditer , a greenish blue colour obtained from copper mixed with chalk or lime; iron blue , phosphate of iron, little employed; cobalt blue , a colour obtained by calcining a salt of cobalt with alumina or oxide of tin; smalt , a gla coloured with cobalt and ground to a fine powder; charcoal blue , a deep shade obtained by triturating carbonized vine stalks with an equal weight of potash in a crucible till the mixture ceases to swell, then pouring it upon a slab, putting it into water and saturating the alkali with sulphuric acid. The liquor becomes blue, and lets fall a dark blue precipitate, which becomes of a brilliant blue colour when heated. Molybdenum blue is a combination of this metal, and oxide of tin or phosphate of lime. It is employed both as a paint, and an enamel colour. A blue may also be obtained by putting into molybdic acid, (made by digesting sulphuret of molybdenum with nitric acid,) some filings of tin, and a little muriatic acid. The tin deoxidizes the molybdic acid to a certain degree, and converts it into the molybdous, which when evaporated and heated with alumina recently precipitated, forms this blue pigment. Ultramarine is a beautiful blue pigment, which see. BLUE VITRIOL; sulphate of copper .