PAINTING MATERIALS
sunlight but are said to retain their color well in artificial light (Colour Index,
p. 301).
Raw Sienna (see Sienna).
Raw Umber (see Timber).
Realgar is the natural orange-red sulphide of arsenic, AS2S2, and it is closely
related chemically and associated in nature with orpiment, the yellow sulphide of
arsenic (see Orpiment). The two minerals are often found in the same deposits.
Although it occurs perhaps as widely in nature as orpiment, realgar appears not
to have been used so widely. Like orpiment, it was known in ancient times. All
are agreed that the * sandarack ' of Pliny (see Bailey, II, 75-77) is identical with
the modern realgar. It was confused by the ancients with red lead because it
resembles it in color (see Bailey, II, note on p. 205). It is said to get its name from
the Arabic, Rahj alghar (powder of the mine) (see Dana, p. 357). Cennino Cennini
mentioned it but did not hold it in favor (Thompson, The Craftsman's Handbook,
p. 29).
The chemical and physical properties of realgar are similar to those of orpi-
ment. It belongs to the same crystal system (monoclinic), but has slightly lower
refractive index. Its color is orange by transmitted light but usually many yellow
particles of orpiment can also be seen. It may be made artificially, but the artificial
product is not used as a pigment in modern times, being too poisonous for that
purpose.
Realgar has not been identified in ancient paintings so frequently as orpiment.
It was observed, however, as the pigment in an orange-red area on a fragment
of wall painting from Kara Khoto in Central Asia (XI—XII centuries).- One may
expect to find it, along with orpiment, in Eastern miniatures and illuminated
manuscripts.
Red Bole (see Bole).
Red Lead (minium, orange mineral). The red tetroxide of lead, Pb3O4, is made
by heating litharge or white lead for some hours at a temperature of about 480° C.
(See Beam, pp. 114-117, for details.) Care must be taken that the temperature
does not get too high or it will be decomposed again into litharge, because the
reaction is reversible. Most of that now made commercially is from the direct
oxidation of lead through the monoxide stage and the product may contain 3
per cent or more of free litharge.c Orange mineral,* which is made by the roasting
of white lead, has the same composition as red lead, except that it contains less
free litharge. There is still some question about the structure of red lead but, for
analytical purposes, it is treated as PbO2- aPbO, in which the PbO2 functions as an
oxidizing agent.
^The pigment is bright scarlet, has good hiding power, and excellent texture.
It is finely divided but may be either crystalline or amorphous, depending upon
how it is made (see Merwin, p. 518). Microscopically, it is not very characteristic.
Some of the particles are transparent and orange-red by transmitted light. The