Page 322 - Copper and Bronze in Art: Corrosion, Colorants, Getty Museum Conservation, By David Scott

The growth of copper salts in the patina of exposed bronzes was reported by Burmester and
Koller (i987). They examined the state of preservation of the bronze doors on the Cathedral of
Augsburg in Augsburg, Germany, which dates to the second half of the eleventh century, as well
as the St. Georgsbrunnen, also in Augsburg. A complex array of corrosion products was found,
including a rare form of hydrated brochantite, CuS0 4 Cu(OH) 2 -2H 2 0. Brochantite was the
3 0
most stable and usual corrosion product, but some of the other compounds present in the patina
included a variety of copper salts and lead salts of organic acids. These organometallics resulted
from the application of unsuitable protective coatings based on beeswax, paraffin waxes, or
linseed oil during previous conservation treatments of the bronze surfaces. These coatings can
liberate free fatty acids or dicarboxylic acids, which will combine with copper or lead ions to
form metal soaps.
Copper soaps are especially chemically unstable and promote further corrosion of the
bronze. The influence of copper ions, oxygen, humidity, light, and heat causes oxidation of
double bonds in unsaturated fatty acids, as well as in alkenes, esters, alcohols, and ketones,
resulting in low-molecular-weight fatty acids and dicarboxylic acids, which are more aggressive
than the original products. These surface corrosion processes negate the purpose of using these
coatings as conservation treatments.
The interaction of bird droppings or other sources of phosphates and oxalates presents a
common problem for exposed bronze surfaces, since copper phosphates or copper oxalates will
form, resulting in disfiguring corrosion of the patina. Alunno-Rossetti and Marabelli (i976)
identified copper oxalates in two studies of bronze statuary in Venice; one occurrence was on
the famous gilded bronze horses of St. Mark's Basilica. The same mineral type was also identi­
fied on the Marcus Aurelius equestrian statue in Rome (Marabelli 1987), on two statues in Flor­
ence (Bernardini et al. 1992), and on the bronze doors of the central portico at Loreto (Mazzeo,
Chiavari, and Morigi 1989). Bernardini and coworkers were unsuccessful in determining
the exact crystal structure of the mineral. Nevertheless, the authors assigned it a formula of
Cu(COO) 2 - nH 20, which is identical to that usually given for moolooite.
Selwyn and colleagues (i996) identified moolooite in two samples from sheltered areas
on outdoor bronzes in Ottawa, Canada, and oxalate anions were found as a minor component
in the Statue of Liberty patina. Scott and Taniguchi (1999) identified moolooite as a major com­
ponent of some gray, discolored patches on Italian reproduction bronze sculptures of Roman
philosophers and statesmen. These were situated in a sheltered outdoor location along the
colonnade of the outer peristyle garden of the J. Paul Getty Museum in Malibu, California, from
1973 to 2000. For nearly twenty-seven years, birds had been using the heads of these statues as
3 1
convenient resting spots and leaving their droppings on the bronze surfaces, which resulted in
corrosion of the bronze and the formation of moolooite.

T H E ORGANI C SALT S O F C O P P E R
305