Page 137 - Copper and Bronze in Art: Corrosion, Colorants, Getty Museum Conservation, By David Scott

at this level of alkalinity, cuprous chloride is unstable and can be converted to cuprite. The
hydrochloric acid released is then neutralized by the carbonate, forming sodium chloride.
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The technique was ingenious, f not entirely desirable, since immersion in solution might
last for years, and the patina could thus be subject to complex alterations. The method was in
use in the Department of Conservation at the British Museum for a considerable period of time
until, in 1970, Oddy and Hughes felt the need to reassess it. They found that the long periods of
immersion that were recommended succeeded in washing chloride ions out of the corroded
bronze surfaces, as expected, but the treatment also caused mineralogical changes to the patina.
A secondary layer of malachite was sometimes found to occur as a precipitate, even though the
ion [Cu(C0 3 ) 2 ] 2 + is stable in the presence of the bicarbonate ion. Reactions leading to the for­
mation of malachite could be regarded as follows:
2CuCl + H 2 0 = Cu 2 0 + 2HC1 3.2

2HC1 + Na 2 CO s = 2NaCl + H 2 0 + C 0 2 3.3

Cu 2 0 + H 2 C 0 3 + H 2 0 = CuC0 3 -Cu(OH) 2 + H 2 3.4

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Conservators were advised to change the solution f the precipitation of malachite occurred, but
it was difficult to adequately monitor the treatment during a process that could take years.
When cuprous chloride is allowed to react with water, a steady increase in chloride ion
concentration occurs in solution as a function of time, although the reaction is slow. The solid
products, confirmed by X-ray diffraction, are cuprite and paratacamite. With added sodium
sesquicarbonate, however, the OH" ion concentration is higher than in water alone:

2CuCl + OH" = Cu 2 0 + 2C1" + H + 3.5

Consequently, there may be some cuprite formation as well as dissolution of cupric chloride and
the production of chalconatronite. Frondel and Gettens (1955) connect the formation of chal-
conatronite in the natural environment to copper alloy reacting with surface or subsoil waters
carrying alkali carbonates in solution: the water could produce chalconatronite either by direct
attack of the copper alloy or by reacting with intermediate alteration products, such as malachite
or atacamite. Atacamite, α-quartz, cuprite, and chalconatronite were identified by X-ray dif­
fraction on the outer part of the concretion covering the Riace bronzes (see CHAPTER 11)
that had been submerged for many centuries at a depth of 8 m at the bottom of the Ionian Sea
(Formigli 1991). This is of interest because chalconatronite is not usually considered as a marine
corrosion product, and there must be a possibility that it formed after excavation.

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