PIGMENTS 97
in dilute alkalis and in dilute mineral acids. It is decomposed by heat at high
temperatures but is little affected by light.' It may become slightly greener on
exposure to light because of formation of chromic oxide. Although Vauquelin,
the discoverer of the metal, chromium (1797), described the preparation of barium
chromate as early as 1809, little seems to have been recorded concerning its first
use as a pigment. J. D. Smith compared the solubilities of barium and strontium
chromates as early as 1836 (see 'On the Separation of Barytes and Strontia/
Philosophical Magazine, $d series^ VIII [1936], pp. 259-261). Barium chromates
and strontium chromates, which are quite similar, are both frequently sold as
* lemon yellow.'
Barytes (see Barium White).
Bentonite is a colloidal clay which consists chiefly of the mineral, mont-
morillonite, a hydrous aluminum, iron and magnesium silicate. It is of volcanic
origin and occurs rather widely. Large quantities are now mined near the Black
Hills in Wyoming and South Dakota and, also, in California. The outstanding
characteristic of bentonite is its colloidal behavior when mixed with water, in
which it swells as much as 22 times its absolute volume and forms a stiff, jelly-like
paste which is smooth and soft like soft soap. Best dispersion is obtained by
adding the bentonite to the water. It does not swell and form gels if water is
added to it. Bentonite is unaffected by acids and alkalis. When heated above
205° C., it begins to lose water and, also, its colloidal properties. Only special
forms of bentonite are pure white; it is generally a warm gray because of its iron
content. Microscopically, it is characterized only by its very fine grain size.
(This information is taken from the data sheets furnished by the American
Colloid Company.)
Berlin Blue (see Prussian Blue).
Bistre (see also Asphaltum) is a brown water color pigment which is derived
from the tarry soot of burned, resinous wood and beechwood. It is similar to
asphaltum in color and composition. The color varies from saffron yellow to
brown-black, depending upon the source and treatment of the raw material. It
was sometimes mixed with red ochre to give it a warmer tone (see Meder, p. 68),
Church says (p. 234) that the ground raw product is washed with hot water
before it is mixed with gum and glycerine in the preparation of water color. The
tarry nature of bistre (as with asphaltum) makes it an unsuitable pigment, except
perhaps in very thin washes. He also says (p. 234) that exposure to strong sun-
light oxidizes the tarry matter of bistre and the residual pigment becomes cooler
in hue and paler. Tarry materials of wood origin have probably been used for
centuries. Meder says (p. 66) that first literary mention of bistre (caligo) was by
Jehan le Begue in 1431; it had been used extensively, however, in Italian book
illustrations in the XIV century. It was used by Rembrandt for wash drawings.
It is still listed by artists' supply dealers, but is little used since it is admitted
by them not to be permanent.