Page 410 - Copper and Bronze in Art: Corrosion, Colorants, Getty Museum Conservation, By David Scott

material remaining inside the object will receive a radiation dose, which renders any attempt at
I
thermoluminescence dating quite meaningless. f such dating is required, the casting core of the
bronze must be sampled before any radiographic work is done.
I IMAGING WITH χ RAYS X-ray imaging is capable of record­
ing differences in both the physical and chemical density of an object. The object's elemental
composition and thickness determine the range of kilovolts (kV) and milliamperes per sec­
ond (mA/s) to be used, as well as the characteristics of the film, filtering, or lead intensifying
screens, all of which improve image quality. Although the kilovolts will determine the radio­
graphic contrast—that is, which areas will be penetrated—the milliamperes per second deter­
mines the density of the image. It is possible to penetrate through objects using different ranges
of kilovolts, milliamperes, and time, so that the image quality can be adjusted by altering these
exposure conditions. Reducing the kilovolts somewhat while still maintaining penetration gen­
erally improves the contrast of the image, while short exposures at high kilovolts achieves bet­
ter penetration but less contrast. The J. Paul Getty Antiquities Conservation Department X-ray
radiography system in Malibu uses an IRT/Nicolet system with an X-ray tube made by Comet
of Switzerland. Imaging with this system indicates that most hollow bronze castings require
between 200 kV and 300 kV to penetrate the copper alloy. For exposures in the 120-2000 kV
range, lead-foil screens in direct contact with the film aid in intensifying the contrast, and by
absorbing scatter, they greatly improve image definition. Common X-ray film brands include
Agfa DP4, Kodax cx2, and Kodax Industrex M. These films all have emulsions on both sides,
unlike some medical X-ray film. This should be taken into account f fine-grain medical film
i
is to be used for examination of bronzes because the exposure conditions will be quite dif­
ferent. High-contrast medical films, such as Agfa D, have also produced good results in con­
servation work.
A customary setup for X-raying artifacts is to load the film into cassettes lined with 2-5 mil
of lead foil (1 mil = 0.0254 mm, or 1/1000 in.) at the tube side of the film and a 10 mil sheet
at the back, with a 2 mm thick aluminum filter over the tube window. The aluminum filter is
especially useful when X-ray radiographs are taken of three-dimensional objects at greater than
40 kV, since it reduces the scatter of soft radiation and greatly improves image resolution.
A range of exposure conditions are necessary to achieve good X-ray images. For example,
a bronze sheet 1 mm thick may require 8O-100 kV at 5 mA for 150 seconds, while a very heavy
casting 3-6 mm thick would need 250-350 kV at 8-10 mA for 150 seconds in order to pene­
trate the alloy. f the alloy is a leaded tin bronze, then even higher accelerating voltages may
I
be required. To X-ray small bronze antiquities and Renaissance statues at the GCI Museum
Research Laboratory, the X-ray tube was placed 1 m from the object with a 2 mm aluminum
filter on the tube. The film cassette was lined with 2 mil of lead foil at the front and 10 mil at the
back. In some cases, better results were obtained using a double layer of 5 mil thick copper sheet
in front of the tube rather than the 2 mm thick aluminum filter.

C O N S E R V A T I O N T R E A T M E N T S F O R B R O N Z E O B J E C T S
393