96 PAINTING MATERIALS
was used lavishly in wall paintings of the Sung and Ming dynasties in Central
China. With difference in the fineness of grinding, different shades were produced
(see Gettens, 'Pigments in a Wall Painting from Central China'). The source of
azurite in China is not known, but there are extensive copper deposits in the
provinces of Kwei-chou and Yunnan. Uyemura lists azurite among the ancient
pigments of Japan. It was known and used in ancient Egypt. Lucas says that it
occurs both in Sinai and in the Eastern Desert, and he cites (p. 283) examples
of its use in very early dynasties. Strangely enough, it seems not to have been
reported among pigments identified in Roman paintings.
Barium White (barytes, blanc fixe, permanent white) is barium sulphate
(BaS04), which may be obtained naturally from the mineral known as barite,
barytes, or heavy spar, or it can be made artificially. Barytes is found widely in
Europe and in the United States. It can be prepared for use as a filler or extender
in paints by the simple process of grinding and settling. Frequently it serves as
a base for lake pigments. An extremely inert material, it .is quite unaffected by
strong chemicals, by heat, and by light. In judicious quantities, it may improve
the wearing and weathering qualities of lead and zinc white paints (see Toch,
The Chemistry and Technology of Paints, pp. 110-114). Barytes is a heavy inert
(sp. gr. = 4.3 to 4.6), but it does not have enough hiding power for a pigment
because of its transparency and medium refractive index (ft — 1.637 [Larsen and
Bermanj). Barytes has low oil absorption; some colors, which alone have high oil
absorption, need much less oil when ground with it (Colour Index ^ p. 303).
Blanc fixe is the name given to the artificial barium sulphate made by pre-
cipitation from barium chloride solution with sodium sulphate. It is identical
with barytes, except that it is a finely divided powder and has much greater hid-
ing power than the natural material. When co-precipitated with zinc sulphide,
lithopone (see Lithopone) is formed and, with titanium oxide, titanium barium
pigment is formed. Like natural barytes, it is an important lake base. Blanc fixe
is not opaque enough to be ground alone with oil for a white paint. As an extender
it is sometimes put into artists' flake white and other artists' oil paints. Several
grades of both barytes and blanc fixe are available now, but most of them contain
98 per cent or over of BaSO4 (see Gardner, p. 1241). Barium sulphate has been
used in connection with paints since about the beginning of the XIX century
(see Trillich, II, 45-46).
Barium Yellow (lemon yellow) (see also Strontium Yellow) is barium chro-
mate (BaCr04), which is a pale green-yellow pigment made by mixing solutions
of neutral potassium chromate and barium chloride. The pigment formed is
deficient in brightness and hiding power. Microscopically, it may sometimes be
observed in nearly colorless, birefracting, rhombic plates. Other varieties are so
fine that crystal character and optical properties can not readily be observed.
Church says (p. 151): 'Of all the chromates which have been used in painting,
barium chromate is the most stable. It is nearly insoluble in water but soluble