Page 377 - Copper and Bronze in Art: Corrosion, Colorants, Getty Museum Conservation, By David Scott

objects, it is generally best to clean only what is absolutely necessary and to leave the cutaneous
layer of green-colored corrosion products intact.
Other problems may be encountered during the mechanical cleaning process itself. For
example, many ancient Peruvian copper alloys may have been gilded or silvered by the electro­
chemical replacement plating technology described in CHAPTER ι (Lechtman 1979; Lechtman,
Erlij, and Barry 1982). Because this subtle gilded layer, often no more than 2 μπι thick, is fre­
quently disrupted and discontinuous, it can easily be completely lost during cleaning.
Another frequent problem is the presence of textile fibers. Textiles play a crucial role in
many ancient Andean societies, for example; bronzes within tombs or burials are often placed
on the body, or next to it, in association with clothing, textiles, feathers, bone objects, and so on.
Fiber remnants are, therefore, commonly encountered during the cleaning of bronze objects
from such sites, and the associated organic residues or preserved fibers may be very significant
from an archaeological standpoint. Mechanical cleaning through hard layers containing these
textile fibers is possible, but the delicate strands may be destroyed in the process.
The patina or corrosion on outdoor bronzes, as opposed to those displayed or stored
indoors, presents an altogether different problem in terms of mechanical cleaning. Streaks of
light-green corrosion products and the presence of unsightly deposits destroy not only the sta­
bility of the patina but also the aesthetics of the bronze, creating a variety of problems for the
conservator. Such bronzes frequently require cleaning. The aim should be to remove as little
material as possible and compatible with the aesthetic aims of the proposed treatment and,
where possible, to analyze the corrosion products and record a history of the object and of the
corrosive events that shaped its condition up to the time of cleaning.
I GLASS-BEAD PEENING AND OTHER ABRASIVE TECHNIQUES
Weil (i976,1980,1983) made the controversial proposal that the patina that develops on a bronze
over time from outdoor exposure is actually aggressive to the bronze metal and should be
removed. For this purpose, Weil often used glass-bead peening, followed by an application of
7
Incralac (discussed under "Coatings for Copper Alloys") and a sacrificial outer wax coating. 8
This treatment regimen was used quite frequently in the United States during the 1970s. Many
metal conservators were troubled by the removal of patina and by the claim made by other con­
servators that glass-bead-peened bronze surfaces showed enhanced corrosion resistance com­
pared with the patinated metal. Veloz, Ruff, and Chase (i987) reviewed some aspects of this
approach. They noted that glass-bead microspheres, about 100 μιη in diameter and applied
at a pressure of 4 0 - 8 O psig, had been used to clean bronze sculpture since the middle of the
1970s. The glass spheres can damage the metal surface once the patina has been removed, and
Veloz, Ruff, and Chase suggested the use of a much softer abrasive, namely crushed walnut
shells. These had already been used in 1979 to clean, without complete patina removal, a bronze
statue of Admiral Richard Evelyn Byrd by Felix de Weldon (b. 1907) that was located in Arling­
ton, Virginia. 9

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