Coeruleum

Synonyms:

Coeruleum: Cerulium, Cerulean Blue, Coelinblau, Bleu Céleste

When oxide of tin is moistened with cobalt nitrate solution and strongly heated, a greenish-blue mass is obtained, which, after powdering and washing, constitutes one of the varieties of the pigment known as coeruleum. There are other ways of preparing this substance. One of these consists in precipitating potassium stannate with cobalt chloride, collecting and washing the precipitate, and then mixing it with some pure silica and heating it. A good specimen of coeruleum contained in 100 parts: 49.7 tin binoxide, 18.6 cobalt oxide, and 317 silica. Some samples contain calcium sulfate, or lead sulphate, in place of the silica; they are of inferior quality.

Coeruleum is a permanent pigment of a rather greenish-blue colour, without any tendency to the violet cast, so noticeable with other cobalt blues (page 235), when viewed by gas or candle light. It suffers little, if any, change by exposure to light or impure air, or by commixture with other pigments. It is a sub-opaque, rather earthy pigment, with a moderate tingeing power. Although some painters find it useful, coeruleum may be imitated so nearly by a mixture of ultramarine, viridian, and white that its presence on the palette can easily be dispensed with.