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Blue Carmine








In a former edition of this work there appeared the following:--"Blue
carmine is a blue oxide of molybdenum, of which little is known as a
substance or as a pigment. It is said to be of a beautiful blue colour,
and durable in a strong light, but is subject to be changed in hue by
other substances, and blackened by foul air: we may conjecture,
therefore, that it is not of much value in painting." In his estimate of
this colour the author was certainly right. It is formed when a
solution of bichloride of molybdenum is poured into a saturated, or
nearly saturated, solution of molybdate of ammonia. A blue precipitate
falls, which is a molybdate of molybdic oxide, hydrated, and abundantly
soluble in water. When dried, it furnishes a dark blue powder,
resembling powdered indigo, having a bitter, rough, metallic taste, and
reddening litmus strongly. The solubility of this hydrated oxide is
alone fatal to its employment as a pigment. It may, indeed, be rendered
comparatively insoluble in water by ignition; but the anhydrous oxide so
obtained is nearly black, and as a colour worthless.

A more eligible preparation is the molybdate of baryta, produced by
mixing solutions of molybdate of potash and acetate of baryta. A white,
flocculent precipitate results, which rapidly condenses to a crystalline
powder, and turns blue on ignition. It is, however, a costly compound,
of little merit, and not likely to come into use. It is insoluble in
water.





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