Page 338 - Copper and Bronze in Art: Corrosion, Colorants, Getty Museum Conservation, By David Scott

ENAMEL ON C O P P E R

Copper has long been used as a substrate for the application of enamels. Maryon (1959) notes
that the copper sheet or plaque is prepared for the enamel by being domed up slightly in the cen­
ter, forming a small lip around the edge to hold the enamel in place. f the plaque is to be cov­
I
ered with enamels of different colors, the design is first scratched into the copper plaque with a
steel point. The copper is traditionally cleaned of any oxide scale by being boiled in 5% sulfuric
acid, then scrubbed thoroughly with pumice powder and water. By coating the copper plaque
with a clear flux before the enamels are applied, the colors show much greater depth than f they
i
were applied to the copper directly.
Smith, Carlson, and Newman (i987) report on the preparation layers that were tradition­
ally used on copper for fine enamels, such as those made in Limoges, France, from 1470 to 1530.
The copper plate was either flat or slightly convex and from 0.5 to 1.0 mm thick. The copper sur­
face was roughened to provide a key for better adhesion of the enamel ground or preparation
layer. A thin white layer of opaque white enamel was fused to the front of the plate. A counter-
enamel, made from scraps of waste glass, was applied to the reverse of the plate and fired
together with the white enamel ground. The counter-enamel helps to prevent differential
expansion and contraction of the enamel decoration on the front during firing. The different
colors of the enamel design were then fired over the white enamel ground. Deterioration of
these enamel layers is much more common than corrosion of the copper substrate, which is
usually well protected by the glassy coatings.

Notes
1 Reactions between substrate and media are some­
what different from the corrosion of copper itself.
In the first situation, copper ions might diffuse
into an organic medium and produce chemi­
cal changes to it that could be deleterious. In
the second case, corrosion of the copper might
occur directly if, for example, the medium were
very acidic.
2 The painting, which is signed "F. v. Mieris/Ao
I661" to the right of the woman's shoulder, has
an extraordinarily complex provenance stretching
back to around 1690. In 1849 the painting was in
the possession of William Williams Hope of Rush-
ton Hall, Northamptonshire, before passing to the
first Baron Revelstoke, London, by 1875.

C O P P E R AS A S U B S T R A T E F O R PAINTING S
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