Page 144 - Copper and Bronze in Art: Corrosion, Colorants, Getty Museum Conservation, By David Scott

The double salt was then oxidized by air to produce cupric chloride and atacamite:
3Cu 2 Cl 2 -2NaCl + 30 + 4 H 2 0 = 3CuO-CuCl 2 -4H 2 0 + 2CuCl 2 + 6NaCl 4.2

The cupric chloride that remained in contact with the air and the copper or cuprous oxide was
also converted into oxychloride:

+ 3Cu + 30 + 4 H 2 0 = 3CuO-CuCl 2 -4H 2 0 4.3
CuCl 2
This completes the series of reactions that, overall, converts copper, oxygen, and water
to cuprous oxide and atacamite in a cyclical process. Berthelot maintained that the contin­
ual recurrence of this cycle under the influence of oxygen and moisture is the cause of bronze
disease, and this conclusion is essentially correct. More is known about the process today, but
not all the details of the corrosion chemistry involved are understood.
The equations advanced by Berthelot are not completely accurate descriptions of bronze
disease, however. Although the idea of a cyclical reaction is accepted, the principal cause of
instability in excavated bronze objects, as we now know, is due to the existence of cuprous chlo­
ride, which is formed by corrosion processes during burial. This cuprous chloride is not usually
exposed to view but is present as a corrosion product often close to the metal surface.
In more recent studies, Organ (i963) proposed that the main reaction responsible for
bronze disease is the production of cuprous oxide (cuprite) by hydrolysis of cuprous chloride:
2CuCl + H 2 0 = 2HC1 + Cu 2 0 4.4

The hydrochloric acid generated by this reaction will then produce more cuprous chloride:

2HC1 + 2Cu = 2CuCl + H 2 4.5

In practice, when cuprous chloride is in contact with copper and a drop of water is added,
cuprite formation does not occur; copper trihydroxychlorides are the principal products. The
standard method for synthesizing paratacamite is to immerse a sheet of copper in a solution of
cupric chloride, which first produces a thin layer of cuprite over the copper, followed by a layer
i
of paratacamite. Cuprite can be formed as a thin layer adjacent to copper f cuprous chloride
and copper are mixed together and regularly moistened with water, but this is not the principal
reaction that occurs. On copper samples in the laboratory, cuprous chloride slurries develop
a pH of about 3.5-4, and the solution forms a green precipitate, which is one of the copper
trihydroxychlorides.
The reaction is one of oxidation and hydrolysis of the cuprous chloride. This takes place
with a free energy of formation of about -360.9 kcal/mol, which means that the reaction
should occur spontaneously, as follows:

4CuCl + 0 2 + 4 H 2 0 = 2Cu 2 (OH) 3 Cl + 2 H + 2C1" 4.6
+

CHLORIDES AND BASIC CHLORIDES
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