pigments	101
?• 73)9 i^n dealing with the preparation says: 'If the solution is made slightly
acid a yellow shade is produced and by changing the proportions of acid and
adding ammonium sulphide deeper shades may be made up to the deepest orange.'
The history of cadmium orange is about the same as that of cadmium yellow.
Cadmium Red (see also Cadmium Yellow) is a cadmium sulpho-selenide,
CdS(Se), which is prepared by precipitating cadmium sulphate with sodium
sulphide and selenium. By adjusting the proportion of sulphur to selenium and
by regulating the conditions of precipitation, shades varying from vermilion to
deep maroon may be obtained. Cadmium red is now a popular and favorite
pigment, and today it has, to a great extent, replaced vermilion on the artist's
palette. Microscopically, it may be seen as tiny red globules less than I p in
diameter, without appearance of crystallinity, and with very high refractive
index. The particles are strongly colored, deep red by transmitted light, and have
a characteristic appearance. The various cadmium sulpho-selenides are stable
and light-resistant under ordinary conditions. Their history is more recent than
that of the straight cadmium sulphides. Although a red-orange cadmium pig-
ment containing selenium was mentioned in a German patent (no. 63558; see
Rose, p. 107) in 1892, it seems that the commercial production of cadmium reds
did not begin until about 1910 (see Toch, Paints, Painting and Restoration,
p. 105; also Rose, loc. cit,}.
Cadmium Red Lithopone (see also Cadmium Yellow Lithopone) is a mixture
of cadmium sulpho-selenide co-precipitated with barium sulphate. It is made in
a way similar to lithopone; in one method the metal, selenium, is dissolved in
barium sulphide solution, the two co-precipitated with cadmium sulphate, and
the residue, after washing, is calcined (see U. S. patent no. 1,894,931 [1933], to
W. J. O'Brien). One manufacturer reports (see Gardner, p. 1274) that the barium
sulphate content in a variety of shades ranges from 55.0 to 58.5 per cent. In
microscopic appearance, it is very similar to pure cadmium red, except that at
high magnification irregular prismatic grains of barium sulphate can also be seen.
It is stable under ordinary conditions and is light-fast. It is a strictly modern
pigment, having been in use only since 1926 (see H. W. D. Ward, under Cadmium
Yellow Lithopone).
Cadmium Yellow is cadmium sulphide (CdS) which is prepared by precipita-
tion from an acid solution of a soluble cadmium salt (chloride or sulphate) with
hydrogen sulphide gas or an alkali sulphide. The color of the pure cadmium sul-
phide ranges in hue from lemon yellow to deep orange, depending upon the con-
ditions of precipitation. Cadmium sulphide is found in nature as the mineral,
greenockite, but the use of the mineral as a pigment has not been mentioned.
E. T. Allen and J. L. Crenshaw ('The Sulphides of Zinc, Cadmium and Mer-
cury; their Crystalline Forms and Genetic Conditions* [Microscopic Study by
H* E. Merwin], American Journal of Science, fourth series, XXXIV [1912], pp.
341-396), from an extensive study of the crystalline nature of cadmium sulphides