Page 62 - Copper and Bronze in Art: Corrosion, Colorants, Getty Museum Conservation, By David Scott

Some relevant concentrations of modern atmospheric pollutants are given in TABLE 1.2,
which shows that sulfur dioxide, nitrogen oxides, and ozone are the principal agents involved.
Levels of these pollutants can range from thousands of parts per billion (ppb) in heavily polluted
urban areas to less than 1 ppb in very remote regions, although most of the world's bronze sculp­
ture is not found in such isolated areas. The concentration of pollutant gases is often measured
in parts per billion or parts per trillion (ppt), but these units have tended to become replaced
with the unit microgram/m 3 ^ g / m ) . The conversion factors for gas concentrations of some
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common atmospheric pollutants in parts per billion and microgram/m are given in TABLE 1.3.
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Over the past two centuries, while most bronze sculptures have been standing outdoors,
significant changes have taken place in the composition of urban air, particularly in sulfur
dioxide concentrations. Data from I88O for New York City suggests an S0 2 concentration of
5-10 ppb; this increased during the 1950s to about 50 ppb and subsequently declined to about
20 ppb following environmental legislation in the 1970s. In London, where the urban radius
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increased from approximately 3 km in 1580 to 15 km in 1900, the extensive use of coal resulted
in very high levels of pollution, reaching an estimated 150-Ι8Ο ppb during the period from
1800 to 1900. These pollutant levels, combined with heavy concentrations of particulate mat­
ter, resulted in most exposed bronzes becoming very dark in color. 14
In some cities of the world, the levels of S0 2 and N 0 2 pollution are still rising and are a
cause of concern, not only for exposed bronzes but for exposed citizens as well. The presence
of SO2 in the atmosphere accelerates the corrosion of many metals, but the initial interactions
with copper are quite complex and depend on the relative humidity and the S0 2 concentration.
This could explain why Tidblad and Leygraf (1995) and Tidblad, Leygraf, and Kucera (1991)
found no correlation between the increase in weight of the corroded copper samples and the
levels of S0 2 concentration in the environment. Where low levels of pollutants were found,
a substantial weight gain on the exposed samples was observed, and cuprite was a principal
corrosion product (Strandberg, Johansson, and Lindqvist 1997). Ericsson and Sydberger (1977)
found that the initial corrosion products in humid air with 10 ppm and 100 ppm S0 2 were a
sulfite, Cu(I) 2 Cu(II)(S0 3 ) 2 -2H 2 0; and a sulfate, CuS0 4 -5H 2 0; and not cuprite. Eriksson,
Johansson, and Strandberg (1993) exposed copper sheets to 500 ppb S0 2 at 70% and 90% RH.
After four weeks, the appearance of the copper had not appreciably altered, although the sheets
exposed at 90% RH were a darker hue. Strandberg and Johansson (1997c), however, noticed that
copperplates exposed to an S0 2 concentration of 69 ppb or less at 90% RH often turned black after
only twelve hours from the development of a cuprite film. Formation of this black film depended
on the relative humidity, S0 2 concentration, and the pretreatment of the copper sheets. Forma­
tion of a black patina on copper exposed outdoors has been observed many times, but this study
was the first to describe it indoors. At RH values greater than 75% with 4-69 ppb S0 2 , a dull
black cuprite patina forms after twenty hours of exposure. When the concentration of SO 2 is

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