I06 PAINTING MATERIALS
most important commercial green pigment. Chrome green is not very suitabl
for an artist's pigment, however, because it is not light-fast. It has a tendenc;
to become blue in strong light because of the darkening of the chrome yellow
component. It is sensitive to acids (turns blue) which dissolve the lead chromate
and to alkalis which cause it to turn dark orange because they effect the decom
position of the Prussian blue; it is, therefore, unsuitable for fresco. No date fo
the introduction of this mixed green can be given* It must have been availabli
and in use shortly after the introduction of chrome yellow in the first quarter o
the XIX century.
Chrome Red, a brick-red, crystalline powder, is basic lead chromate
PbCr04-Pb(OH)2. It is made by boiling a strong solution of potassium dichro-
mate with white lead and a small amount of caustic soda (see Beam, p. 74)
Many of the particles of the deeper shades may be seen as perfect rectangular
tabular crystals that are highly birefracting. It is stable under ordinary condi-
tions, but is not widely used as an artist's pigment because it lacks brilliancy
and is readily affected by sulphur gases. Chrome red was first mentioned bj
L. N. Vauquelin, the discoverer of the metal, chromium, in 1809 (Annales </<
Chimie, LXX [1809], p. 91). Little is known about its history as a pigment, bui
it probably came into use in the early part of the XIX century.
Chrome Yellow, the most important of the commercial yellow pigments, is
lead chromate (PbCrO4). It is made by adding a solution of a soluble lead sail
(acetate or nitrate) to a solution of an alkali chromate or dichromate. (See Beam
pp. 65-76, for full details.) Lead chromate is a crystalline material which car
vary in shade from lemon yellow to orange, depending upon the particle size
which, in turn, depends upon the conditions of precipitation. Lighter shades
usually contain lead sulphate, or other insoluble lead salts. The middle hues are
neutral lead chromate, and the orange leads are basic lead chromate. The pig-
ment is finely divided, dense, and opaque. At high magnification, its crystalline
character can be observed; it consists of small, highly birefracting, monoclinic
prisms.
When chemically pure, chrome yellow is fairly permanent to light, but it is
frequently observed to darken and become brown on aging (see Weber, p. 40,
~and Doerner, p. 63). Sometimes, especially when mixed with colors of organic
origin, it takes on a green tone (by reduction to chromic oxide). It is most satis-
factory when used in oil. In fresco painting, only a basic lead chromate (chrome
orange or red) can be used, for yellow chromes are turned by alkali. Much chrome
yellow is used with Prussian blue to make chrome green (see Chrome Green),
As a pigment, it dates from the beginning of the XIX century. L. N. Vauquelin.
the discoverer of chromium (1797), described the preparation and properties oi
lead chromate in his 1809 'Memoir' (Annaks de Chimie, LXX, pp. 90-91). He
mentioned that it could be prepared in different shades, depending on the con-
ditions of precipitation. Chrome yellow did not come into commercial production.