Page 145 - Copper and Bronze in Art: Corrosion, Colorants, Getty Museum Conservation, By David Scott

(This equation has been written using -319.8 kcal/mol as the value of the free energy of forma­
tion of paratacamite.)
In reviewing the same series of reactions within the context of a geochemical environ­
ment, Pollard, Thomas, and Williams (1992b) came to a different conclusion. They determined
that the reaction

2CuCl + H 2 0 = Cu 2 0 + 2 H + 2Cl~ 4.7
+
will proceed under most oxidizing conditions; that is, that cuprous chloride and water will form
cuprite. This reaction has a positive Gibbs free energy of formation under standard conditions
of about 13.5 kcal/mol. Pollard, Thomas, and Williams argue that, despite the positive value, this
is the principal reaction that is occurring and that it must be an important reaction in burial
environments. Once an object is brought into the laboratory and is freely exposed to air, how­
ever, the cuprous chloride tends to react to produce one of the copper trihydroxychlorides;
cuprite is not formed under these circumstances. In fact, when cuprous chloride is placed on
moist filter paper, it slowly changes to produce principally atacamite (Tennent and Antonio i98i).
Sharkey and Lewin (1971) contended that one of the critical factors in determining which
copper trihydroxychloride isomer is formed during the possible transformations to atacamite or
paratacamite is the concentration of complex cupric chloride ions in solution. Pollard, Thomas,
6
and Williams (1992b) confirmed that paratacamite (actually clinoatacamite) is thermodynami-
cally the most stable phase. They found, contrary to Lewin (1973), that recrystallization of
atacamite to paratacamite can take place in aqueous solution at room temperature. With a dif­
ference of only 0.26 kcal/mol in the Gibbs free energy of formation between atacamite and
paratacamite, however, it is not surprising that they both may form under closely related con­
ditions. Botallackite is the least stable phase. By carefully controlling temperature, solution
concentrations, and time, Pollard, Thomas, and Williams found in the same study that it was
possible to isolate botallackite in all experimental conditions tested.
The results of the work by Pollard, Thomas, and Williams (1992b) show that crystallization
of the copper trihydroxychlorides is controlled by a series of competing steps. Botallackite is the
first phase to form, but it recrystallizes rapidly under most conditions to form either atacamite
or paratacamite. Botallackite might be expected to form only when the solution responsible for
its formation has been removed or has dried out during the reaction. The occurrence of botal­
lackite on an object, therefore, could indicate that the mineral recently formed and has not had
time to recrystallize; or that soon after the botallackite formed, the environment dried out, pre­
venting recrystallization.
Studies by Sharkey and Lewin (1971) and by Lewin (1973) suggest that the important factors
in determining whether atacamite or paratacamite forms are the hydroxyl-to-chloride ratio in
solution or the presence of the higher copper chloride complexes. When the CuCl + concentra­
tion reaches between 20% and 30% of the copper ions in solution at a pH of about 4, then

C H A P T E R F O U R
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