Page 130 - Copper and Bronze in Art: Corrosion, Colorants, Getty Museum Conservation, By David Scott

change to include both malachite and azurite under certain conditions (see FIGURE 2.1 D).
For example, slightly acidic water with 4 4 0 0 ppm of dissolved G 0 2 will favor the formation
of azurite rather than malachite (see FIGURE 2.1c). I f dissolved salts produce bicarbonate
ions, such as Na HC0 3 ~, the pH will rise higher than 7, producing alkaline conditions that
+
favor the production of malachite rather than azurite. The same is true for carbonate ions, such
as N a 2 C 0 ~ , which also tend to produce malachite. As the amount of dissolved carbon diox­
2
+
ide increases, azurite becomes the stable product rather than malachite (see FIGURE 2.1 D). Con­
ditions for azurite stability, therefore, will be favored in aqueous environments in which the pH
is low and the carbon dioxide content of the water is high.

D E C O M P O S I T I O N OF M A L A C H I T E AND A Z U R I T E BY HEAT
Interest has been shown in the decomposition temperatures for malachite and azurite
because of their use in wall paintings that may have been affected by fire. Rose (i85i) found that
3
2
malachite begins to decompose at 20 °C, and by 00 °C a considerable change in weight has
occurred. Rose reported that azurite transformed into tenorite at 00 °C. Simpson, Fisher, and
3
Libsch (i964) revised the data after concluding that the decomposition temperatures initially
reported for azurite and malachite were influenced by different heating rates. They found that
both malachite and azurite will begin to decompose at 2 0 0 °C, with azurite losing 2.76%
°
by weight over forty-five days at 190 C. Both minerals readily transform into tenorite over a
U
300 - 400 °C temperature range. In a more recent review, Rickerby (1991) examined existing data
on decomposition temperatures reported for malachite and azurite and found that they varied
over a wide range of temperatures—from 2 0 0 °C to 00 °C. Rickerby's own studies showed that
5
azurite was not altered at 0 0 °C but that it transformed into tenorite at temperatures greater
2
3
3
2
than 00 °C. Malachite decreased in luminosity at 0 0 °C, turned brown at 00 °C, and trans­
4
formed into tenorite at 0 0 °C and greater. As with all carbonates, these reactions depend on the
partial pressure of the carbon dioxide and water that the malachite or azurite liberates when it
begins to decompose. 12
Rickerby studied wall paintings done in the fresco technique and showed that even f
i
such paintings are affected by fire, significant information about the pigments used may still be
available. By studying cross sections of samples from damaged wall paintings, he found that
large pigment particles are still visible at the bottom of the cross sections and were the least
altered by the heat.

A R T I F I C I A L M A L A C H I T E AND A Z U R I T E

Supplies of malachite and azurite were never plentiful or universally accessible. Starting around
the fifteenth century, synthetic green and blue pigments were produced to replace these natural
copper carbonate minerals. By the seventeenth century, the artificial versions were replacing

BASI C C O P P E R CARBONATE S
113