Page 63 - Copper and Bronze in Art: Corrosion, Colorants, Getty Museum Conservation, By David Scott

Exposure time (days)

Graph of exposure time in days against weight gain for copper
F I G U R E 1.7
samples exposed to S0 2 , N0 2 , 0 3, S0 2 with N0 2 , and S0 2 with 0 3 . There
is a strong synergistic effect for mixtures of S0 2 with N0 2 , and S0 2 with 0 3
(Strandberg and Johansson 1997a).

higher than 69 ppb, or the humidity is lower than 75%, the copper remains shiny. FIGURE 1.7
shows a graph of exposure time in days against weight gain for copper samples exposed to
sulfur dioxide, nitrogen dioxide, ozone, sulfur dioxide with nitrogen, and sulfur dioxide with
ozone. Strandberg and Johansson (1997a) showed that there is a strong synergistic effect with
mixtures of sulfur dioxide and nitrogen dioxide, and with sulfur dioxide and ozone, and that
this results in a substantial increase in the corrosion rate. The development of the black patina
was influenced neither by these mixtures nor by the degree of cold-working of the copper.
The results are explained by the development of a passivating chemisorbed sulfite film
at high SO 2 concentrations; at low concentrations and high humidity, the adsorbed sulfite is
destroyed by the formation of soluble species and by oxidation to sulfate. The thin oxide layer
is attacked by the acidic surface; copper is anodically dissolved; oxygen is cathodically reduced;
and cuprite is precipitated forming a dull black layer 200-300 nm thick. It is not clear why this
layer is black rather than red.
Although SO 2 levels in the West are generally declining, levels of nitrogen oxides are
slowly increasing. Estimates indicate that at the end of the nineteenth century annual emissions
were less than 1 X 1 0 1 2 g. A century later, in the 1980s, they had increased to approximately
18 X 10 1 2 g. In urban areas, ozone levels doubled over the same period, from about 20 ppb
during the I88OS to about 40 ppb in the 1980s. An analysis by Graedel (i987a) suggests that

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