I2o PAINTING MATERIALS
Indigo (see also Woad) is a blue vegetable coloring matter which seems to
have been used in the Far East very early for dyeing cloth and for painting.
The dye is yielded by different plants of the genus Indigo/era, among which
I. tinctoria, probably of Indian origin, was the chief source of the indigo of com-
merce until the time of the discovery of the process for making synthetic indigo
by Baeyer in 1880 (see A. Baeyer, 'Synthese des Isatins und des Indigblaus/
Berichte der Deufschen Chemischen Gesellschaff, XI [1878], pp. 1228-1229; 'Ueber
die Beziehungen der Zimmtsaiire zu des Indigogruppe,' ibid., XIII [1880], pp.
2254-2263; English patent no. 1177). Indigo was formerly grown all over the
world, particularly in India and China, but since 1900 the synthetic product has
almost entirely replaced the natural. Bengal indigo was one of the best grades
and was used widely in dyeing textiles. In the plant indigo exists as a colorless
glucoside called 'indican.' For the preparation of the dye, the freshly cut plants
are macerated, packed into large vats, and allowed to ferment; in this process
the glucoside is hydrolyzed to indigo and sugar. The dark precipitate is strained,
pressed, and dried into cakes (see Perkin and Everest, pp. 475-524). For use as
a paint pigment, it is not precipitated with a mordant but is ground directly to
a fine powder suitable for mixture with artists' mediums. (For details, see Church,
pp. 217-223.) It is still used to a small extent for artists' water color. Cake indigo
is a deep violet-blue with a bronze-like lustre. The coloring matter is indigotin,
Ci6HioN202 (Colour Index, pp. 279 and 299).
Indigo has fair tinting strength; in thin films it is green and blue by trans-
mitted light. Physically, it is much like Prussian blue but, chemically, it is quite
different In an oil medium, no distinct particles can be seen at ordinary mag-
nification; the dye appears to dissolve in the film and to stain it, De Wild says,
however (p. 30), that at a magnification of i5ooX intensely blue discrete par-
ticles appear. Though it can be worked in oil, it is better in tempera or in water
color. Indigo may fade rapidly when thin and exposed to strong sunlight, yet
specimens of it are frequently seen where it has lasted for many centuries without
apparent change. Locked in tempera beneath varnish films, it is very stable. It is
also stable chemically, being insoluble in water, ether, alcohol, lyes, or hydro-
chloric acid. Nitric acid, however, decomposes it, with the formation of a yellow
compound called 'isatin.' It sublimes when heated at 300° C. Indigo is reduced
by several reducing agents to soluble indigo white, called 'leuco indigo/ This
reduction is an important operation in dyeing. The dye is taken up by the
fibres in this reduced soluble form and is then oxidized by the air to the insoluble
Indigo blue. It is bleached by hypochlorite solutions.
Indigo was known and used as a dye in early Egypt (Lucas, p. 313). It was
mentioned by Pliny, who called it 'indicum* (see Bailey, I, 145, and II, 87 and
89), It has been identified as one of the pigments used for decorating Roman
parade shields of c 200 A.D. found at Dura-Europos in Syria, was mentioned as
early as the XIII century in European commercial transactions (Church, p. 217),