Page 371 - Copper and Bronze in Art: Corrosion, Colorants, Getty Museum Conservation, By David Scott

Norman (i988) recounts that in the 1920s the Fitzwilliam Museum in Cambridge, England,
was sending its bronzes to the Ashmolean Museum in Oxford for conservation. The Honorary
Keeper of Antiquities at the Fitzwilliam was so horrified, however, when the objects came back
with their surfaces a chocolate-brown cuprite or coppery metal that the museum devised its own
conservation methods. Norman infers that the damaging treatment used in Oxford was the
zinc granule and sodium hydroxide method that had been published first by Rathgen (1905)
and later by Scott (1926). Rathgen notes that the treatment, known as "Krefting's Method," had
been in use in Denmark since at least 1887 (Krefting 1892a,b). With this treatment, the object is
immersed in a 10% (w/v) solution of sodium hydroxide, usually warmed on a hot plate, to which
zinc granules are added. This reaction, given in equation 12.1, has long been used in the labora­
tory to produce hydrogen.

Zn + 2NaOH = Na 2 Zn0 2 + H 2 12.1

This reaction electrochemically reduced the corrosion products on the immersed object, and
these products would fall away as a precipitate in the sodium zincate solution. Because of the
electrochemical potential between copper and zinc, the end result might be a surface leached of
alloying elements over which zinc metal precipitated from solution; or the partial elimination
of a cuprite patina might produce the undesirable subfusc surfaces that elicited such horror
from the Fitzwilliam Museum curator.
The treated bronzes at the Fitzwilliam Museum were sometimes polished after electro­
chemical reduction, but they were never lacquered. Further deterioration may occur as a result
of these chemical-stripping techniques, as in the example cited by Norman, in which a leaded
bronze treated with this technique now has a thin gray-to-white deposit over most of its sur­
face. The corrosion of the lead component of this bronze was probably exacerbated by the use
of the patina-stripping techniques, which would have exposed the bare metallic surface to more
immediate deterioration as a result of organic acid pollutants within the display case.
Jaeschke and Jaeschke (i988) reviewed the treatment records of some Egyptian bronzes in
the Petrie Museum of University College, London. During the 1950s and 1960s, many of these
bronzes were treated using the sodium sesquicarbonate method, in which bronzes are immersed
for long periods of time in a 10% solution of sodium sesquicarbonate in distilled water, and the
release of chloride ions is monitored during the treatment. During the re-treatment of one such
object in 1982, small yellow metallic flakes occurred on the surface, which were probably par­
ticles of redeposited copper produced as a consequence of the earlier conservation treatment
with sodium sesquicarbonate. The change in appearance of the copper particles may have been
due to the highly alkaline environment of the treated surface of the bronze, which may have
resulted in the alteration of the redeposited copper to cuprite and partially to tenorite. This
could also explain why the conservators observed this surface feature changing color, from yel­
low to red to purple, within a few months of the re-treatment.

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