PIGMENTS 119
hiding power, fast to light (see Doerner, pp. 66 and 92), and suitable for artists'
purposes.
Harrison Red, which is said to get its name from the artist, Birge Harrison
(Weber, p. 62), is a brilliant red lake color or toner which is similar to, if not
identical with, Toluidine Red, Some other lake colors, however, may be sold under
that name.
Hooker's Green is not a single pigment but is a mixture of Prussian blue (see
Prussian Blue) and gamboge (see Gamboge), used for water color, and is called
Hooker's green after an artist who is said (Weber, p. 63) to have introduced it.
Different hues of green are obtained by varying the proportions of the blue and
yellow components. The mixture has the chemical properties of its components;
in strong light it is likely to turn blue because of the fading of the gamboge.
Microscopically, it may be seen as a distinct physical mixture of blue and yellow
particles. For oil colors, other yellows like cadmium yellow or yellow lakes are
used with Prussian blue to produce similar hues.
India Ink (see Chinese Ink).
Indian Lake (see Lac).
Indian Red was formerly a variety of natural iron oxide red (see Iron Oxide
Red) imported from India. It varied in color from light to deep purple-red and
contained, generally, over 90 per cent iron oxide. Although the term still indicates
a dark red oxide, it is now used for a pigment artificially made by calcining
copperas (ferrous sulphate) which is a waste material in certain industries. The
product must be carefully washed to get rid of soluble iron salts. Artificial Indian
red is pure, homogeneous, and dense, and has great hiding power. Other names
for it are 'rouge/ 'colcothar/ and 'caput mortuum.'
Indian Yellow (purree) is a yellow organic extract formerly prepared at
Monghyr in Bengal from the urine of cows that were fed on the leaves of the
mango. Its manufacture is now mercifully prohibited by law. The coloring matter
is principally the magnesium or calcium salt of euxanthic acid, CigHieOnMg* 5H2O
(see C. Graebe, 'Ueber die Euxanthongruppe/ Annalen der Chemie, CCLIV
[1889], pp. 265-303). The dried extract formerly came on the market in round
lumps, brown or dirty green outside and brilliant yellow-green inside. The crude
material must be powdered and washed and, when thus purified, it has a deep,
rich, translucent, orange color. Microscopically, it may be observed as a yellow
crystalline material with weak birefringence. This pigment was used in India in
the manufacture of paint and also as an artist's oil and water color because of
its fastness to light. Church (pp. 154-156) found that even direct sunlight only
bleached it slowly.
Indian yellow is slightly soluble in water, is decomposed by hydrochloric acid
with precipitation of white, flaky euxanthic acid. The color which is sold today
under this name, however, either in oil or in water color, is a synthetic substitute
that may be just as permanent or more permanent than the original Indian yellow*