Page 317 - Copper and Bronze in Art: Corrosion, Colorants, Getty Museum Conservation, By David Scott

Emanations harmful to copper are not always from the wood but may come from asso­
ciated materials such as flame retardants and preservatives. For example, African mahogany
tested many years ago at the British Museum Research Laboratory (Oddy 1974a) was approved
for use in display cases, but copper alloys placed in the cases tarnished. The problem was traced
to the wood preservative, probably a phosphate derivative used to treat that particular batch of
mahogany before it left Africa.
Concentrations of formaldehyde, acetic acid, or formic acid in the tens or hundreds of parts
per million range are not acceptable in the museum context; the levels of these pollutants should
probably be in the low parts per billion range for overall safety of the collections. This general
recommendation is based on numerous museum surveys conducted for the Getty Conservation
Institute by Grzywacz (1993) and by Grzywacz and Tennent (1994). Thickett, Bradley, and Lee
(i998) attempted to provide a wider perspective to the problem. Their experimental work
showed that at 100% RH, acetic acid created severe corrosion of lead coupons; formic acid was
less corrosive, and formaldehyde the least. At 50% RH, the acetic and formic acids were less cor­
rosive, and no corrosion was observed with formaldehyde, although the concentrations used in
the experimental work were not stated in the publication.
Thickett, Bradley, and Lee monitored the internal environment of a display case containing
leaded bronzes and found 175 ppb of acetic acid but no corrosion. Their examination of Egyp­
tian bronzes that had been stored in wooden drawers inside wood cupboards since the 1930s,
however, showed an unusual blue corrosion product. The relative humidity reached a maxi­
mum level of 45% inside the cupboards; the acetic acid level was about 500 ppb in one example
and greater than 2500 ppb in another.
A direct relationship was found between the levels of acetic acid and the percentage of
objects that had been damaged by the formation of the blue corrosion product. There was
i
no evidence that corrosion was ongoing, however; lead coupons placed n the cupboards
were uncorroded after two years of exposure. One possible explanation is that since the 1930s
museum heating has improved, the cupboards have become seasoned, and consequently the rel­
I
ative humidity has fallen to levels at which bronze corrosion cannot readily proceed. f this is
the case, it suggests that controlling relative humidity is crucial in cases where high levels of
these pollutants are present.
Another example of corrosion on displayed bronzes is an Egyptian torque (a necklace of
twisted bronze) from the Burrell Collection in Glasgow, Scotland, that was exposed for ten years
in a wooden cabinet that contained high levels of acetic acid. The bronze had developed patches
of light blue corrosion products.
It is very difficult to determine the exact constituents of the corrosion products formed on
artifacts in storage or on display and whether some products, in fact, represent new compounds
occurring as corrosion products, not previously described. For example, corrosion from copper

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