Page 146 - Copper and Bronze in Art: Corrosion, Colorants, Getty Museum Conservation, By David Scott

atacamite is favored over paratacamite. With still higher copper complexes, such as CuCl 2 ,
CuCl 3~, and CuCl 4 ~, paratacamite becomes the favored species. Sharkey and Lewin (1971)
2
found that there was no dimorphic interconversion between paratacamite and atacamite under
the conditions they investigated.
It has been suggested that the relative proportions of the copper trihydroxychloride isomers
on an artifact could provide clues to the provenance of the object or perhaps to the authenticity
of the patina. This is not really feasible, however. As discussed, the proportions of the isomers
can vary, depending on whether the object has incipient bronze disease, which may show fresh
outbreaks of one of the copper trihydroxychlorides, or whether the original burial patina con­
stituents are being examined, which may show autochthonous chlorides. Even in the laboratory,
I
the mode of production of the basic chlorides is critical. f cupric chloride solution is added to
i
calcium carbonate and stirred, then atacamite is produced; f the solution is left unstirred, then
botallackite forms, as demonstrated by the experimental work of Tennent and Antonio (i98i).

Role of chloride The factors that control the conditions under which the differ-
ions in corrosion ent copper chloride products form are subde. Some reactions
are more repeatable than others. For example, an experiment
conducted by the author on the reaction of cuprous chloride, copper foil, water, and air
produced mosdy paratacamite, in agreement with most previously reported results. The same
reaction replacing cuprous chloride with cupric chloride, however, yielded a mixture of par­
atacamite and atacamite, with more atacamite. This last reaction invariably produces parata­
camite (or, rather, clinoatacamite, as is now known). Unless all parameters of the reaction, such
as pH, temperature, time, and molar concentrations, are carefully controlled, the end prod­
ucts cannot be predicted with certainty. This was confirmed by the work of Pollard, Thomas,
and Williams (1992b), who found that the rate of crystallization of atacamite was greatest
in solutions containing high ratios of (CuCl )/(Cu ) and (Cl~)/(H ) and a lower ratio of
+
2+
+
(Cu )/(H ). They were unable to determine exacdy what the critical step of the reaction is,
2 +
+
however, and the complete series of reactions is still unknown, despite all the research that has
been carried out to date.
The Pourbaix diagrams shown in FIGURE 4.1 have been drawn for a variety of chloride ion
activities. FIGURE 4.IB has a chloride ion concentration of 350 ppm, which is roughly equiva­
lent to saline groundwater. At a potential of 0.238 V and pH 3.94, the crystallization of atacamite
is much slower than that of paratacamite, and this reaction would be anticipated to proceed via
botallackite to paratacamite. To get atacamite to form, a greater concentration of copper and
chloride ions at the metal surface is needed, and two different routes can bring this about. First,
corrosion can occur in highly saline environments or in fairly arid environments, where a solu­
tion can become markedly concentrated on the metal surface as it dries out. Second, differences
in electrode potential on the copper surface might cause pitting corrosion, and within the pits

C H L O R I D E S AN D BASI C C H L O R I D E S
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