PAINTING MATERIALS-
made in different ways have concluded (p. 365)3 on the basis of microscopic
examination, that differences in color are primarily dependent upon the amor-
phous or crystalline nature of the sulphide and on its state of division. They say
further (p. 366) that particles of the orange amorphous cadmium sulphide are
in the order of 50 times as great in diameter as the yellow particles (see Cadmium
Orange). Many of the light or lemon shades of cadmium yellow are really cad-
mium lithopone (see Cadmium Yellow Lithopone) which are co-precipitates with
barium sulphate. All of these cadmium yellows are very finely divided and even
in particle size and at high magnifications can be observed in tiny globules.
Cadmium sulphide has a high refractive index and, hence, good hiding power.
It is permanent and fast to light. The modern product, because of freedom from
excess free sulphur, is compatible with most other pigments. When strongly
heated, cadmium sulphide burns to yellowish brown cadmium oxide. It is in-
soluble in cold dilute acids and alkalis but readily soluble in concentrated mineral
acids with evolution of hydrogen sulphide gas. Although cadmium sulphide was
first observed by Stromeyer as early as 1817 and was introduced into oil painting
by Melandri in 1829 (Eibner, Malmateriallenkunde^ p. 128), it did not become
commercially available as a pigment until about 1846 (see Weber, p. 29). Laurie
(The Pigments and Mediums of the Old Masters> p. 16) says that the cadmium
yellows were first shown in the 1851 Exhibition. It is now perhaps the most
important yellow pigment on the artist's palette and is widely available in numer-
ous shades. It appears that, until recently, the cadmium lithopones (see Cadmium
Yellow Lithopone) were the only cadmium yellows manufactured in this country,
the straight commercial cadmium sulphides being chiefly a German product.
Now, however, the latter are beginning to be manufactured here.
Cadmium Yellow Lithopone (see also Cadmium Yellow) is a co-precipitated
mixture of cadmium sulphide and barium sulphate (from cadmium sulphate and
barium sulphide), made in a way similar to zinc lithopone (see Lithopone). The
precipitate is washed and calcined; it contains about 38 per cent cadmium sul-
phide and can be produced in a variety of shades ranging from lemon yellow to
orange and at a cost considerably less than that for pure cadmium sulphide. It
is a very finely divided pigment and its properties are similar to the straight
sulphide. When this pigment was first introduced, H. W. D. Ward ('New Cad-
mium Pigments/ The Chemical Trade Journal and Chemical Engineer, LXXX
[1927], pp. 59-60) reported that cadmium yellow lithopone had all the fastness
to light and heat of the pure sulphide but did not quite equal it in covering power,
On a cost basis, however, the ratio was in favor of the 'cadmopone,' as he called
it. At the time of his report (January, 1927), he said that this pigment had been
on the market three or four months. Until very recently the cadmium lithopones
were the only cadmium yellows manufactured in the United States.
Carbon Black (see also Lamp Black, Ivory Black, Charcoal Black, Vine Black,
Graphite) includes various pigments that are derived from the partial burning