Page 64 - Copper and Bronze in Art: Corrosion, Colorants, Getty Museum Conservation, By David Scott

increasing atmospheric concentrations of nitrates or organic acids may not have much effect on
the development of copper patinas, while a decrease in the sulfate or chloride-ion concentration
may be reflected in reduced rates of patina formation. Whether GraedePs hypothesis is correct
can be determined only after newly exposed bronze sculpture has been kept outdoors for many
years. Other environmental pollutants, such as carbon dioxide, have not had a particularly
noticeable effect on exposed bronzes despite the fact that global carbon dioxide emissions have
increased from an estimated 60 billion metric tons in i860 to 380 billion metric tons in 1997
{Guardian Weekly 1997). Increased ozone concentrations can act as a catalytic agent for bronze
corrosion, and complex interactions are possible with nitrogen oxides as well. Both gases are
found in present-day photochemical smog, which is generated by the reaction of sunlight with
automobile exhaust and industrial pollutants; undoubtedly, smog will prove to have a deleteri­
ous effect on exposed statuary. Strandberg and Johansson (1997a) point out that the presence of
i
both ozone and nitrogen oxides results n increased synergistic effects with sulfur dioxide.
Ozone in combination with sulfur dioxide results in a strong synergistic effect on the corrosion
of copper, producing cuprite and a variety of basic copper sulfates. At 70% RH, the effect of
ozone is considerable, whereas the nitrogen oxides do not have an effect. Ozone accelerates cor­
rosion at 70% RH by increasing the oxidation of adsorbed sulfite and, thereby, the rate of sulfur
dioxide deposition. Ozone in combination with N 0 2 and S0 2 inhibits cuprite formation, and
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a crust of the basic sulfates tends to form. This is a good example of how complex the initial
series of reactions are, since with S0 2 alone, a massive cuprite crust is formed.
I CORROSION STUDIES The deterioration of a copper-zinc-tin-
lead alloy exposed to sulfur dioxide, nitrogen dioxide, and sodium chloride was investigated in
laboratory simulations by Eriksson, Johansson, and Gullman (1993). For periods ranging from
twenty-four hours to twelve weeks, small samples of the alloy were placed in controlled envi­
ronments of SO 2 , NO 2 , and NaCl; the gases were added to purified air kept at 90% RH and with
extraneous additions of sodium chloride to the ambient environment. Both polished and artifi­
cially patinated samples were used in the study, which showed that nitrogen dioxide is more cor­
rosive to the bare alloy surface than is sulfur dioxide alone. Exposure of the polished samples to
the mixed gases resulted in a substantial increase in the rate of corrosion; a similar synergistic
effect was found on samples pretreated with sodium chloride and then exposed to the nitrogen
dioxide. In this study, HN0 2 (g) was released from the polished metal surfaces after exposure to
N 0 2 . In earlier experiments, Eriksson, Johansson, and Strandberg (1993) found that the reac­
S with NO2 on a metal surface could produce nitrous acid:
tion of 0 2
+ + 2 H 2 0 = 2 H + + S 0 4 " + 1.15
2
S0 2 2 N 0 2 2 H N 0 2
The sulfuric acid also formed by this reaction will tend to increase the corrosion rate of
bronze by dissolving any passivating film and forming a solution of hygroscopic metal sulfates.

C O R R O S I O N AN D E N V I R O N M E N T
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