Page 412 - Copper and Bronze in Art: Corrosion, Colorants, Getty Museum Conservation, By David Scott

that there were differences in the filling of the left foreleg and the right hind leg (the two weight-
bearing legs) compared with the left hind leg, where there was a cavity.

Acoustic emission analysis In acoustic emission analysis, the sound waves that emanate
from bronze objects as they undergo different periods of heat­
ing or cooling can be used to investigate structural characteristics of the object. Highly stressed
regions, for example, may emit a higher frequency or intensity of acoustic signals than areas that
are relatively stress free, allowing an acoustic map of the object to be made.
Acoustic emission analysis was used by Accardo, Canevá, and Massa (i983) on the bronze
portal of San Zeno church in Verona, Italy, to determine the periods of maximum environmen­
tal stresses on the work. The portal was studied in clear and cloudy weather and from the begin­
ning to the end of a cycle of direct radiation; for comparison, nocturnal readings were also
taken. The results of these studies showed that the maximum acoustic emissions, indicating
stresses on the work, occurred in the morning, during the dynamic phase of maximum heating
and drying of the wooden support on which the panels of the bronze portal were hung; at sun­
set, during the inverse phase; and between the hours of maximum exposure to the sun, from
2:00 to 3:00 P.M.

Other techniques Other advances in nondestructive testing include the work of
Bartolini and colleagues (1997), who used resistance of polari­
zation (Äp) measurements on the interior of the Riace bronzes. These electrical measurements
are taken from the surface of the bronze object and can reveal the resistance of the object to
the electrical stimulation applied. For example, after corrosion inhibition treatment, one might
expect the resistance of polarization measurement to increase. The measurements were taken
before and after the bronzes had been given a corrosion inhibition treatment with BTA, which
was applied with a paintbrush as a 10% ethanol solution at 40-50 °C for five minutes. The mea­
surements were repeated after a second BTA application. The data indicated that the BTA treat­
ment had a beneficial effect on the polarization resistance, which should mean better corrosion
protection than before the treatment.
There are a number of other nondestructive testing or monitoring approaches that can be
used to evaluate both structural condition and corrosion, such as sophisticated electrochemical
sensors to measure corrosion rates, and these will undoubtedly be used on metallic works of art
in the years to come.

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