Page 143 - Copper and Bronze in Art: Corrosion, Colorants, Getty Museum Conservation, By David Scott

The condition is manifest by light green, powdery excrescences or eruptions within the sur­
face, as shown in P L A T E 2 6 , loose material often falling from the object and dropping onto the
surrounding area. In extreme cases, very acidic light or dark green liquid may ooze from the
bronze and stain surrounding tissue or foam supports. Ultimately, bronze disease can reduce an
apparently solid object into a heap of light green powder.
The existence of some chloride-containing corrosion products within the patina of a bronze
does not mean that the object is necessarily suffering from bronze disease but may simply
represent a localized or superficial chloride corrosion process. Bronze disease, in contrast, is
characterized by the accumulation of deposits of cuprous chloride within the bronze itself or
underlying the surface patina, causing fundamental instability. Since chloride ions are often
driven by electrochemical reactions deep within the surface layers of corrosion, the chlorides
may be overgrown with cuprite that has developed as part of the corrosion process; this, in turn,
is often overlaid with basic carbonates or chlorides.

Bronze disease research Bronze disease has aroused a great deal of interest and specula­
tion over the last hundred years or so. In the late nineteenth cen­
tury, it was believed that bronze disease was microbiological in origin. Mond and Cuboni (i893)
attributed this corrosion to the presence of a common fungus, Cladosporium aeris, which they
were able to isolate from affected corrosion pits. Sterilization at 120 °C for twenty minutes was,
therefore, one of the first treatments recommended for stabilization of ancient bronzes. Around
the same time, Rathgen (i889,1898) proposed that bronze disease was due to the presence of cor­
rosive chloride salts, and this gradually gained acceptance as the correct explanation. A useful
review of the early history of bronze disease and its treatment is provided by Gilberg (1988).
One of the first scientists to study the problem from the chemical point of view was
the French organic and physical chemist Marcellin Berthelot (1827-1907), who recognized that
there must be an important cyclical component to the corrosion reaction (Berthelot 1894, 1895,
1901). He also recognized that one of the important products of the reaction was the basic cop­
per chloride atacamite, to which he assigned the formula 3CuO,CuCl 2 -4H 2 0. The modern
basic copper chloride formula is 3CuO,CuCl 2 -3H 2 0, which is surprisingly close. Berthelot's
achievement was remarkable for the chemistry of the time, especially since these basic copper
chlorides are not that well understood even today.
Berthelot's explanation for the process suggested that a small quantity of sodium chloride
reacted with the atacamite and the metallic copper. A slow reaction supposedly took place, form­
ing a double compound of cuprous chloride and sodium chloride and converting excess copper
into cuprous oxide: 5
3CuO-CuCl 2 -4H 2 0 + 4Cu + 2NaCl = Cu 2 Cl 2 -2NaCl + 3Cu 2 0 + 4 H 2 0 4.1

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