Page 95 - Copper and Bronze in Art: Corrosion, Colorants, Getty Museum Conservation, By David Scott

examination. It is usually possible to polish corroded samples in much the same way as more
robust specimens, although f the material is very friable, vacuum impregnation with a low-
i
viscosity epoxy resin should be considered before mounting; alternatively, the entire sample
should be mounted in resin under vacuum.
It is most important to examine all features of the corrosion products before etching a speci­
men, since many corrosion products are badly attacked by chemicals used to etch metal surfaces,
and they may be dissolved completely. For example, cuprite and malachite are strongly attacked
by alcoholic ferric chloride solutions or acidified potassium dichromate, which are often used
in the examination of microstructure. Under normal bright-field, reflected-light microscopy,
most corrosion crusts appear gray. Examination under reflected polarized light, however, often
reveals the true pleochroic colors of the corrosion products. This not only aids considerably in
the interpretation of many microstructures but also plainly reveals many crystalline or other
morphological details. PLATE 7 shows some typical microstructures incorporating different
corrosion features.
During metallographic examination of a corroded metal, the interface of interest may not
be only between the metal and the primary (or remaining) corrosion products. Important infor­
mation may also be preserved in some interfacial event between layers of corrosion products of
different composition or structure. It is sometimes possible to retrieve valuable structural infor­
mation concerning the authenticity of an object from metallographic examination of corroded
metallic fragments, even small chips. This is often because part of the structure is selectively cor­
roded, leaving one phase uncorroded, or because the phases have undergone pseudomorphic
replacement by corrosion products.
I POLISHING TECHNIQUES When a corroded sample is mounted
for polishing, difficulties may be encountered in preparing a scratch-free surface. Abrasive par­
ticles, which frequently become embedded in corrosion products, can be dislodged during pol­
ishing and may scratch the metal. At the other extreme, some corrosion layers may retain the
diamond polishing compound and loosened particles, resulting in an exceptionally good polish
to the cross section. Many other variables may also intervene.
Prolonged polishing with intermittent etching or ultrasonic cleaning in acetone or alcohol
to remove loose or embedded particles may help to produce a better finish. A danger, though, is
that small specimens will become rounded at the edges, causing part of the corrosion crust or
edges of the metallic constituents to be out of focus during optical examination. It still may be
possible, however, to view these specimens with an electron microscope, which has a greater
depth of field. The polishing hardness of corrosion products is usually quite different from that
of the metal that formed them. Surface relief effects are therefore common when examining cor­
roded samples. The author has found that detail in corrosion products is more sharply preserved
when diamond polishing compounds rather than alumina compounds are used. 23

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