Page 249 - Copper and Bronze in Art: Corrosion, Colorants, Getty Museum Conservation, By David Scott

Sulfide formation from Copper sulfides are infrequently found as a component of
atmospheric exposure exposed copper patinas produced by atmospheric corrosion
because they will usually be oxidized to the basic copper
sulfates after they form. Sulfides are, however, sometimes found on exposed bronzes in urban
environments. One of the earliest reports is that of Vernon and Whitby (1930), who found
sulfides in a copper patina and thought that chalcocite might have later become oxidized to
covellite. Franey and Davis (i987) observed that the artificial sulfide patina applied to a bronze
statue began to transform into basic copper sulfates after the statue had been exposed outdoors
for only a few months. Selwyn and colleagues (i996) found that on exposed bronzes in Ottawa,
sulfides were present only as minor patina components: geerite, Cu 8 S 5 or Cu 1-6 S, was found
or Cu 196 S, once.
seven times; chalcocite twice; and djurleite, Cu 31 S 1 6

Sulfide formation The alarming appearance of what is called the "black fuzzies"
from pollution in the or "brown fuzzies" on bronze or brass surfaces in the museum
museum environment environment has generated much discussion and concern.
These disfiguring excrescences of copper sulfides result from
exposure of metallic objects to indoor pollutants, primarily hydrogen sulfide emanating from
inappropriate materials used for display, storage, or exhibition.
Madsen (1977) and Madsen and Hjelm-Hansen (1982) originally invoked microbiological
activity as the origin of these black copper sulfide spots, after finding that a mold of the Clado-
sporium family grew in the presence of these copper sulfides. Hjelm-Hansen (1984) later recom­
mended the use of i-amido-i-cyanoethylene,2,2-disodium thiolate as the preferred cleaning
reagent for the removal of these spots, which were identified as covellite, CuS. Other chemi­
cal reagents used to clean bronze may work as well. Oddy and Meeks (1982) published a paper
expressing doubts about the microbiological theory of attack and suggested instead that all
occurrences of black spots on a museum object could be accounted for by the presence of hydro­
gen sulfide, carbonyl sulfide, or similar pollutants in the air. This is now generally accepted as
the correct explanation.
When copper is exposed to air with hydrogen sulfide contaminants, it rapidly develops a
tarnish film that thickens, following a parabolic growth rate. An example of the growth of this
sulfide film is shown in FIGURE 6.2. Laboratory experiments demonstrate that f copper is first
i
exposed to an atmosphere containing small amounts of hydrogen sulfide and is then removed
from it, the copper will continue to corrode at an accelerated rate. If, however, the copper is
first exposed to pure air so that a thin film of cuprite forms, it will not tarnish f then exposed
i
to air containing some hydrogen sulfide (Leidheiser 1979).
Identifying the sulfides in these spots is difficult because of the poor crystallinity of the
compounds. Interpreting X-ray diffraction data also poses problems, even when a good array

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