Page 68 - Copper and Bronze in Art: Corrosion, Colorants, Getty Museum Conservation, By David Scott

Cu 2 S PbS INSOLUBLE INSOLUBLE ORGANIC
CuCI 2 PbS0 4 A l 2 0 3 F e 2 0 3
MINERAL SOOT ACIDS
n i l 12.0 n i l n i l 2.1 2.7 1.9 5.9 11.3
n i l 13.0 n i l n i l 6.6 4.2 1.1 5.8 10.0

trace n i l 3.4 2.6 1.0 4.2 0.7 0.8 1.0
0.6 n i l 47.9 6.3 1.9 0.7 1.6 4.8 2.5

The copper roof of Australia House, the London residence of the Australian ambassador,
still appeared black in 1930, after twelve years of exposure. In contrast, all the copper roofs
in the study that had been exposed for more than thirty years had developed green patinas.
A representative set of analytical data from the work of Vernon (i932a,b) is shown in TABLE 1.4,
which well illustrates the complexity of patinas formed in outdoor exposure. Vernon's work
showed that alloying elements such as lead, whose compounds are very insoluble, may make a
substantial contribution to the buildup of salts on the surface. These salts may have originated
from the use of a leaded bronze alloy or have come from contiguous lead sheeting on the roofs.
Insoluble sooty material and organic acid radicals were present in all of the patinas studied.
Some or all of this soot, which is derived from air pollutants, represents particulate matter incor­
porated into the patina; the organic fraction may represent copper oxalates and other copper
carboxyl compounds. The direction of the wind was an important factor in patina formation:
the surfaces facing south or southwest, the direction of the prevailing winds, formed a more per­
fect patina; those most exposed to washing by rain produced a streaked patina; and those in
sheltered areas tended to be black or dark in color.
A nice juxtaposition to Rathgen's patina observations is the summary provided by Strand-
berg (1997), who studied fourteen sculptures in Göteborg, Sweden, and noted the following:

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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