Page 335 - Copper and Bronze in Art: Corrosion, Colorants, Getty Museum Conservation, By David Scott

painting by Guido Reni (1575-1642) in the collections of the National Gallery London, was
shown by electron probe microanalysis to be on tin-coated copper.
One possible advantage of coating copper was to prevent chemical reactions between sub­
strate and media, as well as to help prevent corrosion of the copper itself. There are cases of
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uncoated copper supports corroding; when this has occurred, small, usually randomly distrib­
uted dark stains or excrescences that are visually disturbing appeared on the front of the paint­
ing. This kind of random pitting corrosion is quite common in coated metal objects where small
anodic pits and large cathodic surfaces are a common problem.
A vivid green layer, undoubtedly due to the formation of copper organometallics, can
sometimes be seen between a painting's copper support and ground, according to Horovitz
(1986). Experiments by Horovitz with copper strips coated with linseed, poppy, and nut oils
showed that after periods ranging from twenty-four hours to ten days, all samples developed
a greenish tinge. Graaf (1972) has suggested that this corrosion might even aid retention of the
paint layers.
Copper supports were fabricated primarily by casting the metal into an inclined bed of
sand and finishing it by cold-working with a hammer or roller or possibly both, according to
Horovitz. Since it becomes progressively more difficult to hammer thin sheets of relatively pure
copper from cast blocks, the metal was probably annealed at various stages, although Horovitz
(1999) does not mention annealing in his review of European paintings on copper supports. The
earliest known rolling machines are depicted in Leonardo da Vinci's notebooks, although even
as late as the eighteenth century, the most accessible technology was to hammer and anneal the
copper into shape.
Horovitz uses Rest on the Flight into Egypt by the Bolognese artist Domenichino (Domenico
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Zampieri, 58i-i64i) as an example of a painting in which both rolling and hammering may
have been used to create the copper support. Hammering tends to form an uneven surface,
which sometimes takes the form of concentric rings that are visible when viewed in raking light.
In contrast, early rolling machines produced a rippling effect, characterized by a series of par­
allel undulations due to uneven deformation of the copper during cold-working. Roughly par­
allel striae in another painting by Domenichino suggested to Buck (1970) that a rolling process
had been used to shape the support. X-ray radiography may reveal details that are important
in deciding f a support has been hammered and then rolled, or cast and rolled, or simply
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hammered. These copper sheets are usually small, typically no more than 20 by 30 cm, though
occasionally large single sheets of copper, often rolled, were used as supports. For example,
The Archduke Leopold Willen in His Painting Gallery in Brussels by the Flemish painter David
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Teniers I (1610-90) measures 106 by 129 cm. Rare examples of large-scale altarpieces made
from several sheets of copper joined by riveting and soldering are known. One of them is
Domenichino's altarpiece for the Cappella del Tesoro of San Gennaro in Naples, Italy (Horovitz
1999). The copper plates were finished by tinning, but despite this protected surface, there are

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