Page 411 - Copper and Bronze in Art: Corrosion, Colorants, Getty Museum Conservation, By David Scott

Renaissance bronzes at the Victoria and Albert Museum were imaged with a setup that
placed the X-ray tube ι m from the object. The exposure used 220 kV and 5 mA for a variety of
exposure times. This will penetrate bronzes of up to 25 mm in thickness. The Metropolitan
Museum of Art in New York has used 300 - 320 kV and 4-5 mA for 100-200 seconds under simi­
lar conditions and produced useful results with small Renaissance bronzes (Bewer 1996). The
new GCI Museum Research Laboratory at the Getty Center used 300 kV and 5 mA for 120 sec­
onds with an object-to-tube distance of 1.0-1.5 m.
I IMAGING WITH GAMMA RAYS Gamma-ray sources (such as
an iridium source), which are portable only under stricdy controlled conditions because of their
hazardous nature, provide a range of gamma ray energy roughly equivalent to 6 MeV (1 MeV =
100 kV), or 600 kV, while the usual range in energy of gamma rays can be from 500 kV to
2000 kV. The contrast and quality of the image that is achievable, however, are inferior to those
of a skillfully taken X-ray image.
Because of the time-consuming nature of developing film and getting the exposure con­
ditions right, there is an increasing use of digital imaging and computerized tomography in
the field of X-ray radiography, which allows digital images of the objects to be captured on
computer directly, where they can be manipulated and stored. In future work, these recent
technologies will become increasingly important in museum X-ray radiography as their cost
begins to decline.

Ultrasonic scanning Scanning with a similar ultrasound device as that commonly
used in medicine can be employed to survey the thickness of a
bronze sculpture in particular areas or to map thickness profiles over the entire surface. Ultra­
sonic scans allow nondestructive identification of areas of porosity, patches, soldered joins, and
cracks in an object. Scans of the equestrian sculpture of Marcus Aurelius in Rome (Marabelli,
Mazzeo, and Morigi 1991), for example, revealed that the thickness of the horse's four legs was
fairly constant, from 4 to 7 mm, with little variation from one leg to the other. In contrast, a band
of solder joining two sections of the statue had a highly variable thickness, which is indicative
of a substantial structural discontinuity between the castings and the joins.

Infrared imaging Infrared imaging has also been applied to conservation work. It
is normally used in a band between 2 and 5.6 μιη in the short
wave, which can achieve a resolution of about 0.2 °C. An infrared survey is carried out in the
absence of direct solar radiation, for example, when the object is cooling down in the early eve­
ning. With this technique, it is possible to identify structural inhomogeneity in a bronze cast­
ing that corresponds to a variable thickness or to the remains of core material. Marabelli,
Mazzeo, and Morigi (1991) also applied this method to the Marcus Aurelius statue and found

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