PAINTING MATERIALS
gen sulphide to form zinc sulphide, it is not darkened, because zinc sulphide itself
is white (see Llthopone) and, for this reason, the oxide has been an important
pigment for use where industrial atmospheres are prevalent. It is claimed to be a
mildew inhibitor for outside paints (see H. A. Gardner, L. P. Hart and G. G.
Sward, * Mildew Prevention; Fourth Report on Investigation with Conclusions
and Recommendations/ Circular no. -^75, National Paint, Famish and Lacquer
Association [January, 1935])- Zinc white, more than some other whites, seems
to accelerate the fading of certain coal tar colors that are mixed with it in tints
and exposed to strong sunlight. It has been widely used in paintings since the
middle XIX century. It continues to be popular for water color under the name,
€ Chinese white,' but it is also sold to artists in an oil paste.
Acicular zinc oxide is a special form in which the particles are needle-shaped
and crossed and joined in pairs to form X's. It has greater hiding power and
whitening strength than ordinary zinc oxide which contains mostly rounded
particles.
Zinc Yellow is zinc chromate, ZnCrCX, which is made artificially by adding a
hot solution of potassium dichromate to a solution of zinc sulphate. The pigment
has a bright, clean, lemon-yellow shade, much like strontium chromate. It lacks
the body and strength of lead chromate yellow. Since, however, it is not poisonous
and is not darkened by hydrogen sulphide gas, it has found favor for special uses.
It is partially soluble in water and this behavior has somewhat limited its use
It is also readily soluble in dilute mineral acids and in acetic acid, but is not
affected by dilute alkalis. It is not very permanent to light, having a tendency to
turn gray-green caused by the formation of chromic oxide. Microscopically, it
may be observed to consist of tiny spheroidal grains which have strong bire-
fringence. It has only a moderately high refractive index (n = 1.84-1.90 [Mer-
win]). Little is known about the occurrence of zinc yellow in paintings. It was
discovered by Vauquelin in Paris in 1809, but was not produced as a commercial
pigment until after 1850 (Trillich, III, 55). Apparently it has only been used as an
artist's color in recent years, and for this purpose only in oil and water color
mediums (see Weber, p. 133).
Zinaober Green is a term ordinarily synonymous with chrome green (see
Chrome Green) which is a processed mixture of chrome yellow and Prussian blue.
More specifically, it is given to mixtures that are olive in hue.