PIGMENTS 109
most costly of artists' colors, it is liable to adulteration and to substitution by
ultramarine and even blue lakes.
Cobalt Green (Rinmann's green, zinc green) is similar to cobalt blue, except
that zinc oxide replaces wholly or partly the aluminum oxide in the latter. In
one of the ways of making it, a solution of a cobalt salt is added to a paste of
zinc oxide and water; the mass is then dried, calcined, and prepared for pigment
purposes by usual methods (see Church, p. 196). In the final product, there is
only a small proportion of CoO to ZnO, but the color, which is a bluish green,
remains much the same with widely varying proportions of cobalt. This indicates
that the two oxides, zinc and cobalt, form a solid solution and not a definite
compound like CoO-A12O3 (see De Wild, p. 82). Cobalt green is semi-transparent
and does not have great hiding power. It is fine and regular in particle size; the
grains are rounded and transparent, bright green in transmitted light, and they
are highly refracting and birefracting. It is a stable and inert pigment and can
be used in mixtures and in different techniques. Although it is soluble in concen-
trated acids, it is unaffected by alkalis and by moderately high temperatures.
Church says (p. 196):c Cobalt green is, in fact, one of the too-rare pigments which
is at once chemically and artistically perfect/ It has not had, however, great
favor with artists because in oil it covers only moderately well, is costly, and
because its color can so easily be imitated by mixtures. Although it was dis-
covered by Rinmann in 1780 (see Rose, p. 290), it was not until after the middle
XIX century, when zinc oxide became available in large quantities, that cobalt
green in turn became commercially possible. Laurie (The Pigments and Mediums
of the Old Masters, p. 16) gives 1835 as ^e date of the first literarv mention of
cobalt green as a pigment.
Cobalt Violet has been made in various ways, but the violet cobalt pigment
on the market today appears to be either anhydrous cobalt phosphate, CosCPO^,
or arsenate, Co3(AsO4)23 or a mixture of the two. The darker variety is the phos-
phate. It is made by precipitating a soluble cobalt salt with disodium phosphate,
washing, and then strongly heating the precipitate. The color is reddish violet;
it is transparent and weak in tinting strength, and this fact, in addition to its
high cost, seems to be the reason why it is not more generally used as a pigment.
It is stable and unaffected by most chemical reagents and can be used in all
techniques. Microscopically, it can be seen to consist of irregular particles and
particle clusters which are red-violet in transmitted light, highly refracting, and
brilliantly birefracting. Samples from different sources differ quite a little in
microscopic character. They are still listed by artists' colormen. The preparation
of cobalt phosphate as a pigment was first described by Salvetat ('Matieres
minerales colorantes vertes et violettes,' Comptes Rendus des Seances de Y Aca-
demie des Sciences > XLVIII [1859], pp. 295-297).
Cobalt Yellow (aureolin) is a complex, chemical compound, potassium co-
baltinitrite, CoKsCNC^VHaO. It is made by precipitating a cobalt salt in acid