PIGMENTS 95
to asphaltum in that they are tarry, organic substances, but their origin is quite
different.
Aureolin (see Cobalt Yellow).
Azurite (mountain blue) is a natural blue pigment which is derived from the
mineral, azurite, a basic copper carbonate, 2CuC(V Cu(OH)2. The mineral occurs
in various parts of the world in secondary copper ore deposits where it is fre-
quently associated with malachite, a green basic carbonate of copper (see Mala-
chite). Like other mineral pigments, this has been prepared from carefully selected
material by grinding, washing, levigation, and flotation (see Thompson, The
Materials of Medieval Painting, pp. 131-132). It has long since ceased to be of
importance in Western painting, and is rarely used today, except perhaps to a
limited extent in the East.
Azurite is crystalline and is fairly highly refracting and birefracting. For use
as a pigment, it is ground rather coarsely because fine grinding causes it to be-
come pale and weak in tinting strength. Ninety-mesh azurite, however, is deep
violet-blue in color. Areas of dark azurite on paintings can often be recognized by
their sandy texture and by their thickness. Traditionally, it appears to have
been most used in a tempera medium because in oil it would be dark and muddy
and would not have the sparkle that it has in tempera. The characteristics of
azurite blues in old paintings are well described by Thompson (loc. <://., pp. 132-
135). The penetration into azurite paint in European panel paintings of successive
layers of oil and varnish films has often caused such areas to become nearly
black. If cleaned, the particles are usually revealed unchanged. Although there
may be instances where the pigment has turned green (to malachite) by hydra-
tion, the more usual cause of the change in color is the optical effect of super-
imposed layers of discolored varnish. Azurite is blackened by heat and by warm
alkalis, and it is soluble in acids, even in acetic acid; but, under ordinary condi-
tions, it appears to be a remarkably stable pigment.
This natural copper carbonate was no doubt the most important blue pigment
in European painting from the XV to the middle of the XVII century and in
paintings of that period it is found more frequently than ultramarine. De Wild
(p. 23) lists nineteen early Dutch and Flemish paintings on which he identified
azurite. Europe had various sources of the mineral. There is evidence (see Laurie,
New Light on Old Masters^ p. 42) that Hungary was the principal source in the
XVI century, but the pigment disappeared from the painter's palette in the
middle XVII century when Hungary was overrun by the Turks. One of the early
names for azurite was azure ff Alemagna^ indicating that it came from Germany.
It is the azurro della magna of Cennino Cennini, and was known by numerous
other names in mediaeval times (see Thompson, * Trial Index for Mediaeval
Craftsmanship/ p. 415, f.n. 4 and 5).
Azurite was the most important blue pigment in the wall paintings of the
East. It was employed in the cave temples at Tun Huang in Western China and