Page 151 - Copper and Bronze in Art: Corrosion, Colorants, Getty Museum Conservation, By David Scott

Pourbaix (i976) suggested that analogous corrosion pits are formed on copper that has been
attacked by domestic water. Not all waters, however, caused this pitting; surprisingly water
from the river Thames in London, for example, produced no corrosion pits. Pourbaix's initial
experiments produced a series of peculiar results. In corrosion trials, copper with nantokite
would sometimes produce a deposit of metallic copper rather than corroding further. Dur­
ing the first period of laboratory experimentation, the electrode potential of the copper surface
showed a low value, from -50 mVscE to -10 mVscE (+200 mVsHE to +240 mVsHE), and
the copper surface was covered with cuprite. During the second period, the potential increased
steadily to about +40 mVscE (+290 mVsHE), and some malachite formed. During the third
period, the potential fluctuated up and down. Illuminating the copper with light did not affect
the electrode potential during the first period. During the third period, at 145 days from the start
of the experiment, light promoted a considerable potential drop of about 320 mV, to a value of
-250 mVscE (0 mVsHE), but this was only temporary. Pourbaix's detailed environmental
analysis (1977), illustrated in FIGURE 4.2, indicates that the equilibrium conditions expected for
the pH in corrosion crusts where copper, cuprite, and nantokite are present are quite acidic and
can contain potentially high amounts of complex copper chlorides. The reaction conditions,
therefore, act contrary to the model proposed by Sharkey and Lewin (1971).
A simplified Pourbaix diagram showing the relevant fields of stability for the mineral
phases is shown in FIGURE 4.3, and a listing of selected thermodynamic data for some copper
minerals relevant to those discussed here is given in TABLE 4.3.

T H E B A S I C C O P P E R C H L O R I D E S A S P I G M E N T S

Reports exist of atacamite, paratacamite, and botallackite having been used as pigments. Usu­
7
ally, these are pale green in color, lighter than malachite, and some may be of a light turquoise
i
hue. It is unclear, however, f these minerals are original pigments themselves or alteration prod­
ucts derived from the transformation of original pigments.
Delbourgo (i98o) reported finding atacamite and paratacamite in eighth-century paintings
from Dunhuang, the People's Republic of China. Since X-ray fluorescence spectroscopy was
used in the study, however, the identifications cannot be considered reliable since this analysis
would be able to show only the presence of copper and chlorine and would not be useful in
identifying the light green salts. PLATE 27 shows a bodhisattva sculpture from cave 328 of the
Mogao grottoes at Dunhuang. Botallackite was also reported by Wainwright and colleagues
(1997) from Buddhist wall paintings in these grottoes, but further research is required to ascer­
i
tain f this and other pigments are original or formed by alteration of malachite, which is the
most likely explanation for the botallackite.
Piqué (1992) examined green pigments from a series of wall paintings in the Buddhist
cave temples at the Yungang grottoes near Dunhuang. The paintings were commissioned by a

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