painting materials
often characterized by their coarseness and irregularity in particle size and, also,
by the presence of impurities. Coarse and impure mineral pigments can impart
certain desirable qualities of texture and of tone to painted surfaces not attainable
with artificial, precipitated pigments. The inert pigments, like gypsum, clay, and
chalk, are of natural origin and have only to be purified to be used as paint
materials.
Ochre (yellow ochre, golden ochre, red ochre, brown ochre). An ochre is a
natural earth which consists of silica and clay, and which owes its color to iron
oxide in either the anhydrous or hydrous form. Red ochre is colored by anhy-
drous iron oxide, Fe_2O_3 (see Iron Oxide Red), but in yellow ochre the color is
caused by the presence of various hydrated forms of iron oxide, chief of which
is the mineral, goethite, Fe_20s- H_20. This is seen at high magnifications in strongly
birefringent spherulites less than I p in diameter. Brown ochre is nearly pure
limonite. In addition to iron minerals, yellow ochre may contain impurities of
gypsum, magnesium carbonate, etc. There is a wide variation in the iron oxide
content, but French ochre, which is one of the best varieties, contains about
20 per cent; it is low in aluminum and high in silica. The best ochre district of
France centers about Apt in the Department of Vaucluse in the south (see Ladoo,
p. 380). Ochre, however, occurs all over the world and ochre pits are worked in
many places. It is prepared for use by selection, grinding, washing, levigation,
and drying (see Beam, p. 60). Since it is a natural product, it is found in a num-
ber of shades which vary from dull, pale yellow to reddish brown. Some ochres
have good hiding power; others, like the siennas from Italy (see Sienna), are es-
pecially valued for their transparency. Microscopically, the pigment is hetero-
geneous in particle size and in composition; it is a mixture of colorless silica and
semi-opaque, pale yellow and brown particles which are sensibly isotropic. Like
other forms of iron oxide, yellow ochre is permanent in all techniques; it is not
affected by dilute acids or alkalis. It turns red (to red ochre) on burning from loss
of water of hydration.
Yellow ochre has been universally used as a pigment from earliest history.
It was known and used in ancient Egypt, in Roman times, and in the East. It
was important in the Middle Ages and in all periods of European painting.
De Wild has reported it (p. 53) in twenty specimens from paintings of all periods
of Flemish and Dutch art. Yellow ochre is now listed by all artists' supply dealers
and is a fairly dependable product. In the recent past, it was occasionally bright-
ened with chrome yellow or by natural or aniline dyes (see Weber, p. 94). Arti-
ficial iron yellows or ochres are now common (see Mars Colors).
Orange Mineral (see Red Lead).
Organic Mgments are those which belong to the organic division of chemical
compounds. They are compounds of carbon with hydrogen, oxygen, nitrogen,
sulphur, and other elements. They may be derived from vegetable sources or they
may be made synthetically. Vegetable coloring matters are those like madder,