Page 125 - Copper and Bronze in Art: Corrosion, Colorants, Getty Museum Conservation, By David Scott

A Z U R I T E
Azurite is less common than malachite as a corrosion product, although it has been used exten­
sively as a blue pigment. In some older texts, azurite is also known as chessylite, a name derived
from a noted occurrence of the natural mineral in Chessyl, France.
In natural deposits and in corrosion products, azurite is usually associated with mala­
chite, and often with cuprite as well. Azurite belongs to the monoclinic system and has a
Mohs hardness of 3.5-4. Natural azurite may form by the action of carbonated waters on
other copper minerals or by the reaction of copper chloride or copper sulfate solutions with
limestone or calcite.

Azurite as a Both malachite and azurite form as corrosion products prin-
corrosion product cipally when copper alloy comes in contact with soil waters
or with water formed by surface condensation and charged with
carbon dioxide (Gettens 1963a). Azurite is less stable than malachite and may be converted to it
in the presence of moisture through loss of carbon dioxide. Lewin (1973) showed that this trans­
formation is promoted by an increase in temperature in the presence of alkali.
Azurite as a corrosion product rarely forms a coherent patina, although it is known to hap­
pen. More commonly, discrete crystals or patches of azurite are found in association with mala­
chite and cuprite. When mounted for polarized-light microscopy, particles of azurite corrosion
or pigment should show a good blue color with distinct angular edges. The particle size is
usually 5-40 μπι. The mineral is strongly biréfringent, and second-order colors can be seen
(Mactaggart and Mactaggart 1988). Strongly colored particles may be pleochroic. A mounted
preparation of azurite crystals viewed under plane-polarized light and under crossed polars is
shown in PLATE 23.

Azurite as a pigment Azurite's primary use since prehistoric times has been as a pig­
ment. Although Ucko and Rosenfeld (i965) state that blue and
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green pigments were never used in Paleolithic cave art, it is possible that some of the black pig­
ments observed on such drawings today could have been malachite or azurite pigments that
decomposed into tenorite by heating (a transformation described later in this chapter). The first
known example of Neolithic use of azurite pigment was found at Catal Hüyük in Anatolia,
dating from 6000 B.G.E. (Mellart 1967). Lucas (1934) reports the find of azurite from a shell con­
tainer used as a palette that was found at Medurn, Egypt, and that dates to the Fourth Dynasty
(2575 - 2465 B.G.E.). The artificial pigment Egyptian blue (calcium copper silicate) was also used
frequendy starting around this period because of the limited supplies of azurite.
The ancient Chinese were also familiar with natural deposits of azurite and malachite.
Detailed observations of malachite in nodular form with large holes and in stratified form with

C H A P T E R T H R E E
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