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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