Page 381 - Copper and Bronze in Art: Corrosion, Colorants, Getty Museum Conservation, By David Scott

although the reaction is slow. The solid products, confirmed by X-ray diffraction, are cuprite and
paratacamite (clinoatacamite). With sodium sesquicarbonate, the OH" ion concentration is
higher than in water alone, and the following reaction occurs:
2CuCl + OH" = Cu 2 0 + 2C1" + H + 12.2

As a result, there may be some cuprite formation as well as dissolution of cuprous chloride.
Treating bronzes with carbonate solutions has resulted in some interesting chemical prob­
lems. Horie and Vint (1982) drew attention to the formation of chalconatronite as an alteration
product resulting from treatment. They had found chalconatronite crystals on Roman copper
and iron armor from an excavated site in Chester, England, and attributed its formation to the
conservation work done many years earlier when the metalwork had been treated with sodium
sesquicarbonate. The researchers noted that in the laboratory, sodium copper carbonate (chal­
conatronite) can be prepared by precipitating the crystals from a concentrated solution of
sodium carbonate containing bicarbonate and copper ions.
Earlier, chalconatronite had been reported as a bluish green, chalky crust within the hollow
interior of an Egyptian bronze figurine of the deity Sekmet in the Fogg Museum of Art; on an
Egyptian bronze group of a cat and kittens in the Gulbenkian Collection, Lisbon; and on a Cop­
tic censer in the Freer Gallery of Art (Gettens and Frondel 1955). Chalconatronite was subse­
quently identified on a copper pin from St. Mark's Basilica in Venice (Staffeldt and Paleni 1978).

Localized In the treatment of some bronzes, localized cleaning of parts of
chemical treatments a surface may be necessary. Often this is directed at the removal
of chlorides in an attempt to improve the stability of the bronze.
For example, Organ (196i) quotes Nichols, who attempted the local stabilization of copper chlo­
rides in pitting corrosion at the British Museum in 1924. Nichols had used a dilute solution of
silver nitrate to immobilize the chlorides as silver salts. Organ improved on this method by rub­
bing a paste of silver oxide and ethanol into the corrosion pit and allowing the silver chloride
that formed to plug it. Sharma, Shankar Lai, and Nair (1995), however, reported several failures
with the silver oxide paste on bronzes with severe chloride corrosion. They suggested that a pos­
sible detrimental aspect of the silver chloride paste is that this compound acts as both an elec­
tronic and an electrolytic conductor, resulting in the seal being only partially protective. These
investigators then studied the efficacy of using zinc dust instead of silver oxide. After removing
the copper trihydroxychloride eruptions, they applied zinc dust moistened with aqueous
ethanol (1:10 v/v) to the excavated pits with a small artist's brush. To ensure good contact with
the nantokite (CuCl) and the excavated edges, the moist zinc dust was depressed with the tip of
a scalpel. The treated spots were then moistened with aqueous ethanol at one-hour intervals
ten to twelve times per day for the next three days. A relatively tough seal of gray zinc reaction

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