Page 82 - Copper and Bronze in Art: Corrosion, Colorants, Getty Museum Conservation, By David Scott

I BIOLOGICAL FACTORS Many artifacts recovered from the sea
are associated with shipwrecks. At such sites, marine life, algae, rotting timbers, coral, and so
on, may strongly influence the local environment of submerged bronzes; as a result, such arti­
facts may be heavily covered with concretions. The fact that copper is a natural biocide does not
necessarily stop copper alloys from becoming heavily encrusted.
Microbially influenced corrosion (MIC) is an important aspect of the environmental attack
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on metals, but it is seldom considered n detail. In natural environments, bacteria attach to
solids, including metals, where they colonize the surface and produce a biofilm. Microorgan­
isms within the biofilm are capable of producing an environment at the surface of the object that
is radically different from the bulk solution in terms of pH, dissolved oxygen, and other organic
and inorganic species. The sulfate-reducing bacteria are a particularly important group in the
corrosion of metals, since the production of the sulfide ion is usually damaging to metal surfaces
and can initiate a number of corrosion events at the metal surface. McNeil and Little (1992) note
that in natural environments, various types of bacteria may grow in conjunction with oxygen-
using microorganisms that can deplete the local environment of oxygen, driving the biofilm
layer toward Eh values that are more electronegative and providing conditions for the growth
of sulfate-reducing bacteria. These biofilms may then produce elevated levels of sulfide ions and
shift pH levels toward the acid regions, as well as scavenge for chloride ions to maintain elec-
troneutrality. Under these conditions, the microbial corrosion of copper, silver, and their alloys
becomes an important event; this may be applicable both to aqueous environments and to land
burials, f microbiological activity is very active at the site.
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Leidheiser (1979) pointed out that copper concentrations of 10~ 5 to 10~ molar stimulate
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marine bacterial communities in a seawater-agar gel containing 0.5% peptone and 0.1% yeast
extract. In some cases, these colonies tolerate copper concentrations as high as 10" molar. It is
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known that certain fungi are very resistant to both high hydrogen-ion concentration and high
copper-ion concentrations; in particular, Acontium velatum Morgan and a dark green organ­
ism belonging to the Dematiaceae family can happily thrive at pH 0.1 in a medium saturated in
copper sulfate.
A study by Rogers (i948) on the corrosion of copper pipes in which sewage effluent was
used for cooling water showed that bacteria and molds growing in contact with 70Cu30Zn
brass accelerated corrosion, resulting in the formation of deep pits below the colonies. Some
bacteria tolerated high concentrations of copper salts, and several nonsporing microorganisms
withstood copper concentrations higher than 100 ppm. One of these was a nonsporing, gram-
negative rod living in large reddish brown colonies; the microorganism appeared similar to oth­
ers previously isolated from seawater. In another culture, a spore-forming organism removed
from copper exposed to infected seawater resisted 2000 ppm of copper. The attack on the brass
was invariably intercrystalline, and bacterial nutrients such as mannitol and asparagine accel­
erated the rate of attack. Samples of seawater known to produce rapid pitting of brass contained

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