Page 171 - Copper and Bronze in Art: Corrosion, Colorants, Getty Museum Conservation, By David Scott

(Zn,Cu) 4 (S0 4 )(OH) 6 -4H 2 0, has been identified as a corrosion product on brass clocks from
the Wallace Collection in London (Scott and Bezur 1996).
Selwyn and colleagues (i996) found the following copper sulfate hydroxide minerals
from both sheltered and exposed areas of the statues they studied: Cu 3 (S0 4 ) 2 (OH) 2 ,
Cu 5 (S0 4 ) 2 (OH) 6 -5H 2 0, and Cu 3 (S0 4 ) 2 (OH) 2 -2H 2 0. The existence of these other basic cop­
per sulfate minerals as patina components on exposed surfaces is hardly surprising, given the
complexity of the formation conditions for these minerals. Indeed, this complex series of pro­
cesses was recently confirmed by Strandberg and Johansson (1997c), who carried out research
into the mode of formation of the basic copper sulfates. They found that when copper in humid
air is exposed to very low (sub-ppm) levels of sulfur dioxide, a new compound forms that has
never before been detected and that does not correspond to any of the previously described cop­
per hydroxysulfate salts. An X-ray diffraction pattern of this newly characterized compound,
Cu 2f5 (OH) 3 S0 4 - H 2 0, is shown in APPENDIX D, TABLE 4. The formula for this unnamed min­
2
eral is similar to the compound Cu 3 (S0 4 )(OH) 4 -2H 2 0, which Pollard, Thomas, and Williams
(1992 a) proposed based on their investigation of the stability regions for antlerite. In the case of
Strandberg and Johansson's research, however, the new compound was fully characterized, and
its structure determined. Of all the basic sulfates, this new compound has the highest ratio of
sulfate to hydroxide ions, which suggests that the mineral would be stabilized by low pH and
high sulfate activity. Strandberg and Johansson found that the mineral decomposes with time,
even in solutions with high sulfate ion content, forming brochantite, posnjakite, or anderite.
Finding this compound in the patina of outdoor bronzes could indicate recent and active corro­
sion, since it is rapidly converted to the other sulfates in humid air.
Strandberg and Johansson (i997d) proposed that this mineral is a metastable precursor in
brochantite and antlerite formation. The scarcity of antlerite in rainwashed regions may be
explained by the washing away of this precursor, which has a slightly greater solubility than the
other sulfates. The researchers had noted that the compound is rarely found in outdoor envi­
ronments but that it is frequently formed during laboratory studies. In recent developments,
however, Strandberg reports finding several occurrences of the new sulfate on exposed bronzes
8
from sites in the province of the former East Berlin.

E N V I R O N M E N T AND C O R R O S I O N

Atmospheric sulfur dioxide Laboratory studies by Strandberg and Johansson (1997a) and
by Strandberg 9 showed that humid air containing only traces
of sulfur dioxide formed an opaque black patina consisting principally of cuprite. At high
sulfur dioxide concentrations, the surface of copper is passivated, which implies that the depo­
sition velocity of sulfur dioxide on copper increases significantly with decreasing concentration

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