Page 367 - Copper and Bronze in Art: Corrosion, Colorants, Getty Museum Conservation, By David Scott

Both Larsen (i987) and Goodburn-Brown (i996,1997) have made significant contributions
to the study of tool marks and structural detail preserved in surface patina or corrosion. Care­
ful binocular examination under oblique or raking illumination may reveal details of tool marks
in a bronze patina. In particular, artifacts from freshwater anoxic sites may be covered
in a crust of copper (II) sulfides that adheres poorly to the surface. This crust can be readily
removed during surface cleaning to reveal a characteristically etched and shiny metal surface.
Selective natural etching at grain boundaries, twin lines, and nonmetallic inclusions or sec­
ondary phases may reveal an extraordinary corpus of detail concerning the manufacture of
the objects. Because of topotaxic transformations that may occur, iron objects are generally
too corroded for the observation of microstructural detail. Copper alloys, however, are often
remarkably preserved. Uncoated small objects can be examined directly with a scanning elec­
tron microscope. If an object is too large to fit in the chamber, its surface can be replicated with
silicon rubber; these small rubber impressions can be coated with a gold-palladium deposition
for study in the microscope. An illustration in Goodburn-Brown (1997) shows how microstruc­
tural detail—such as grain boundaries, inclusions, and twin planes —can be clearly seen by this
kind of surface examination. The object in the illustration is a third-century Roman Radiate
coin recovered by the Museum of London during excavations along the river Thames. Another
example shows distorted twin planes on the surface of a second Roman coin, indicating that it
was either cold-worked in the final stage of fabrication or hot-struck at a temperature below that
of the recrystallization of the alloy.
There is considerable potential for the surface examination of bronze patinas from other
freshwater anoxic sites that have been excavated over the last several years. These sites include
Trondheim, Norway; Coppergate, York, England (Ottaway 1992); the Araisi Lake fortress in
Latvia (Apals and Apals 1998); and sites in Denmark (Rieck and Crumlin-Pederson 1988). Not
all of this material is necessarily preserved in the same way as the waterfront material from the
Thames excavations in London, and much of it may not be capable of preserving detail. For
example, some bronzes from Trondheim have a heavy sulfidic corrosion that was created by
rotting organic materials, such as leather and wood; this preserves no surface detail whatsoever.
Other bronzes are reduced to a green-colored shell of corrosion with an internal core of black
powdery copper sulfides (Scott 1977). The corrosion on objects from such environments is likely
to undergo postexcavation alterations because of oxidation of the sulfides to sulfates such as bro­
chantite, antlerite, and posnjakite.

C H A P T E R E L E V E N
350