Page 72 - Copper and Bronze in Art: Corrosion, Colorants, Getty Museum Conservation, By David Scott

I STUDIES OF OUTDOOR MONUMENTS Studies of the condition
and conservation of the Statue of Liberty, notably those sponsored by AT&T Bell Laboratories
(now AT&T Labs), generated a number of interesting reports on patina formation in the outdoor
environment. One of these studies is by Graedel (i987a), who summarized data for fifteen atmo­
spheric pollutants to which this famous landmark was exposed. Many of these compounds are
present only at the parts-per-billion level, but their dissolution in fine droplets of fog or mist
will, on evaporation, substantially increase the relative concentration of some of these chemi­
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cals. Graedel also reported the concentrations of reactive components n fog, rain, and snow.
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The data show that concentrations of these reactive components n urban fog are noticeably
greater for important anions such as sulfate, chloride, and nitrate, while the pH may vary from
2.2 to 4; thus, fog can potentially initiate corrosion in the urban environment.
A simulated rain study was used by Graedel, Nassau, and Franey (i987) to investigate
some of the primary patina components that might form when a copper alloy is exposed to rain­
water carrying various concentrations of sulfate, chloride, nitrate, and sodium ions and hav­
ing a pH ranging from 3.4 to 4.7. In nearly all cases, cuprite formed, followed by brochantite,
CuS0 4 -3Cu(OH) 2 . Atacamite formation was not observed, which is consistent with the
concentration-pH stability diagram. The reduction of cuprite to metallic copper, which is not
very common in outdoor bronze exposure, was also observed. The conclusions that can be
drawn from this study are rather limited, since the experiment was conducted for only fourteen
days, which is insufficient time to allow for a meaningful result. The work is useful, however, in
showing that once a patina has formed, changes in rainwater chemistry are unlikely to affect its
mineral content, unless very acid conditions where pH < 2.5 are encountered. Sweden may rep­
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resent this extreme condition. The rainwater there has increased n acidity tenfold since the
1950s; consequently, the protons may be dissolving the brochantite patina ten times faster today.
Outdoor conditions are not an equilibrium system, as assumed by Graedel, Nassau, and Franey
(i987), which limits the predictive ability of these types of studies.
The stability regions for the formation of antlerite and brochantite can be gauged from
FIGURE 1.8, a stability diagram for the copper-sulfate-water system with the superimposed
domains of typical urban fog and rain. The diagram shows that brochantite should be the most
favored phase to develop in outdoor exposure to rain; fog produces more antlerite and even dis­
solution of the patina at low pH levels. Individual stability diagrams have limited applicability
because of the complex system involved. They do show, however, that brochantite should be the
major corrosion product to form in outdoor exposure and that basic copper carbonates and
chlorides should be minor or absent, a prediction that has been verified empirically.
I COMPONENTS OF EXPOSED PATINAS The work of Vernon
(1932a) clearly reveals that exposed copper patinas may contain a variety of organic compo­
nents that are difficult to characterize. In a recent study, Muller and McCrory-Joy (i987) used
gas chromatography-mass spectrometry and ion chromatography to characterize water- and

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