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Cobalt Green

Synonyms: Cobalt Green, Rinmann's Green, Vert De Cobalt, Vert De Zinc, Kobaltgrün

It has long been known that the oxide or a salt of zinc, moistened with a solution of cobalt nitrate, and then strongly heated before the blow-pipe, gives a porous mass of a beautiful green hue. This compound or mixture of the oxides of zinc and cobalt may be prepared by: (1) Precipitating with an alkaline carbonate a mixture of the nitrates of cobalt and of zinc, and then strongly heating (after washing) the precipitate formed; (2) Making a paste of zinc oxide and water, and adding to it a solution of nitrate or sulphate of cobalt, or of roseo-cobaltic chloride; the mass is then dried, calcined at a dull red heat, thrown into water, ground, washed, and dried. Method No. 2 gives a finely coloured product, the depth of hue being proportional to the percentage of cobalt oxide. If the latter oxide amount to one-third of that of zinc the colour is a very deep bluish green; with no more than one-sixth the colour is still rich. Some specimens do not contain more than one-twentieth - sometimes even less - of cobalt oxide, and yet they are far from pale. An excellent deep sample contained 12 percent of cobalt oxide.

When properly prepared, cobalt green is a pigment of great beauty and power.1 The deeper tones of cobalt green are almost transparent in oil. The pigment works well, is quite permanent, and has no action on other pigments. Cobalt green is, in fact, one of the too-rare pigments which is at once chemically and artistically perfect; such at least is the conclusion I reached from my own trials, but Mr. J. Scott Taylor tells me that cobalt green fades a good deal when exposed to damp, although it stands light well. It must be admitted, however, that it may be exactly imitated by a mixture of viridian and artificial ultramarine with a little zinc white.

Cobalt green is again coming into artistic use, as it is equally adapted for all the methods of painting. It was discredited for awhile by the inferiority of the product obtained by Rinmann's original process (No. 1 above). It ought not to be an expensive pigment.

Sometimes cobalt green is prepared by precipitating a cobalt salt with an alkaline arseniate or phosphate, and then heating the precipitate with zinc white.



1 One sample of deep transparent cobalt green which I obtained from a Paris colour-manufacturer contained both viridian and ultramarine, added to enrich the colour of the cobalt green which formed the basis of the pigment.


Last Update: 2011-01-23