Page 336 - Copper and Bronze in Art: Corrosion, Colorants, Getty Museum Conservation, By David Scott

adhesion problems between the paint film and the substrate. The potential corrosion of the cop­
per, especially in humid environments, may have influenced artists to employ tinned copper
panels, but this is not necessarily the only reason, since tinning would also have given a quality
of luminosity with thin paint films.
Some artists may have chosen copper supports because they appreciated the metal's
dimensional stability against changes in relative humidity and temperature; this is especially
important for small, delicate paintings. Craquelure is reduced with copper, which has a linear
coefficient of expansion of 14 X 10 ~ , whereas wood may reach 35 X 10 ~ .
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The surface of the copper plate would often be mechanically roughened for better adher­
ence of the paint film. Garlic is sometimes mentioned as an additional means of surface prepa­
ration (Horovitz 1999); it would be applied either as a cut clove rubbed over the surface or as
strained garlic juice. This treatment probably helps to wet the surface and provide a sticky,
slightly grainy base over which the paint can be applied. Some copper organometallic salts are
probably formed during this process, and they may also help to adhere the paint film.
Most paintings on copper have a conventional ground of lead white, although occasionally
this is absent, allowing hues from the underlying copper metal to enhance the visual effects of
the painting.

Analytical techniques Bridgman, Michaels, and Sherwood (i965) successfully carried
out electron-emission radiography of a painting on copper;
conventional radiography was completely ineffective, however, because the copper masked any
radiographic contrast of the pigments used in the painting. They examined Susanna Van Collen,
a small oval portrait in oil, attributed to the Dutch artist Cornells van Poelenburgh (i586-
1667). The portrait had an underdrawing, which was successfully revealed by electron-emission
radiography. This technique requires that the surface to be radiographed be in very close con­
tact with the film to minimize image distortion. Because this copper support had a slight con­
cavity, the painting was not mounted in a vacuum holder; instead, a thin foam-rubber lining was
used to gently but firmly maintain the film in close contact with the paint surface. The painting
was placed face up with a sheet of Kodak type M industrial single-emulsion film with emulsion
side contiguous with the painting. The X-ray tube was positioned ι m above the film cassette. A
hard X-ray beam is required to minimize the effects of the direct exposure and to provide a good
yield of energetic electrons to penetrate the outer layers of paint. To accomplish this, an 8 mil
thick copper filtration plate was used to remove softer X rays that would have fogged the film.
An exposure of 220 kV at 10 mA was made for three minutes. This technique created a satisfac­
tory image of the painting and revealed its underpainting.

Poelenburgh created many paintings on copper sheet. Landscape with Bathing Nudes in the
collections of the J. Paul Getty Museum was imaged with X-ray radiography to examine the cop­
per substrate, but the painting's lead white ground made it difficult for the beam to penetrate

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