Page 29 - Copper and Bronze in Art: Corrosion, Colorants, Getty Museum Conservation, By David Scott

(1968),, Leidheiser (1979), and Samans (i963). A few selected topics of particular relevance are
briefly discussed later in this chapter.
The corrosion of copper compounds may be classified in many ways, and the organization
chosen strongly influences how the events that cause the corrosion and the resulting corrosion
products are discussed. One approach to categorizing corrosion products is to link them to their
environmental milieus as follows:

1. products formed during burial
2. products formed during exposure to the atmosphere
3. products formed in the indoor museum environment
4. products formed in marine conditions
5. products manufactured in the laboratory environment
for use in patination or as pigments
The first four of these topics are discussed in this chapter. The fifth one is covered in CHAPTER 9.
There are certain areas of overlap among the different types of environments listed here,
since one set of conditions does not exclude the same product from forming in another. For
example, marine exposure, land burial, or even atmospheric attack may all produce the same
corrosion products.
Robbiola (1990) has proposed that ion migration should be considered as a unifying prin­
ciple in the corrosion of bronzes. He divides corrosive events into two groups:
1. corrosion under cationic control
2. corrosion under anionic control

During corrosion under cationic control, the cations, such as copper or tin ions, diffuse to
the surface of the metal and control the rate of the corrosion reactions. This is often a slow pro­
cess that tends to produce patinas, especially cuprite layers, that may preserve the shape of the
original object.
Under anionic control, corrosion proceeds with a greater change of volume at the corrosion
interface; thicker and less coherent corrosion products may form as a result. Highly mobile
ions, such as chloride ions, may contribute to processes under anionic control because these
travel easily from the environment to the metal surface as anions, accelerating the rate of cor­
rosion and producing corrosion layers that are more disruptive.
Chase (1994) used the concept of ion migration to examine the corrosion of some Chinese
I
bronzes. In processes equated with cationic control, which Chase calls "Type I corrosion," a
smooth water patina enriched in tin oxides occurs on the surface. Cross-sectional studies show
that the alpha phase of the bronze has dissolved away, leaving behind uncorroded islands of
alpha+delta eutectoid (see APPENDIX A, on bronze). In some cases, these well-preserved pati­
nas may occur on bronzes that were intentionally colored or patinated in antiquity before being

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