I0g PAINTING MATERIALS
Coal-Tar Colors are made from the distillation products of coal tar, a by-
product of coke and coal gas manufacture, and are compounds which contain
chiefly carbon, hydrogen, nitrogen, and sometimes sulphur. Benzene, toluene,
anthracene, naphthalene, phenol, and pyridine are all direct coal-tar distillation
products. By processes cf synthetic organic chemistry, these distillation products
may be changed to dye intermediates like aniline, phthalic acid, etc., which, in
turn, may be synthesized to color products which are dyes. Since the discovery
of the first aniline dyestuff, mauve (see Mauve), by William Perkin in England
in 1856, many thousands of coal-tar dyes have been prepared. Some have become
important in the preparation of lake pigments, being valued for their richness
and brilliance in color. Many coal-tar lakes lack permanence and have rightly
caused the whole range of lake pigments to be looked upon with suspicion by the
artist. In recent decades, however, there has been a very decided improvement
in the permanence of coal-tar dyestuffs; like the dyes of natural origin, those in
the red region of the spectrum are the more permanent, but there has been a
great improvement in the stability of lake pigments for other regions of the
spectrum, examples of which are the Hansa yellows and the phthalocyanine
blues. For the future, there may be developed organic colors which will rival the
inorganic colors in light stability and general permanence.
Cobalt Blue (Thenard's blue) is now the most important of the cobalt pig-
ments. The simplest form is made by calcining a mixture of cobalt oxide and
aluminum hydrate to form, in part, cobalt aluminate (CoO'AlgOs). One modern
manufacturer gives the composition as CosC^ = 32 per cent and AUOs = 68 per
cent (see Gardner, p. 1359). •"•* ma7 ^e ma<ie in other ways: the original Thenard's
blue was said to be cobalt phosphate on an aluminum base (see Church, p. 211).
The color varies slightly with different methods of manufacture and with the
amount of impurities present, but it is usually a pure shade of blue, especially
in natural light. Microscopically, the particles are characteristic; they are mod-
erately fine, irregular in size, and rounded; the surface of some of the larger
particles has a crusty texture; they are bright blue by transmitted light, and are
isotropic; the refractive index is medium, about 1.74 in blue light (Merwin).
Chemically, cobalt blue is very stable; it is insoluble in strong acids and
alkalis and is unaffected by sunlight; it can be used in all painting techniques,
even for the blue coloring of ceramic glazes, in much the same way as cobalt
oxide is used.
Cobalt blue was discovered by Thenard in 1802. De Wild gives a brief account
of its history and says (p. 28): 'Since the new pigment satisfied a recognized
demand, it was employed everywhere relatively soon after its discovery especially
in France, as was natural/ The earliest picture painted in Holland on which it
was identified by De Wild (see p. 30) was 1840 and he adds, 'Hence its use did
not penetrate into Holland directly after its discovery/ It has been identified
on a water color painting by R. P. Bonington, 1801-1828. Since it is one of the