Page 30 - Copper and Bronze in Art: Corrosion, Colorants, Getty Museum Conservation, By David Scott

preserved in a burial environment. Chinese mirrors, for example, have been found with patinas
of two different colors on the surface. The origin of these patinas is still not understood, but they
may represent a natural patina that formed over an artificial one.
During corrosion under anionic control, removal of the tin-rich phase can occur, forming
a surface corrosion layer of tin oxide, or tin oxide mixed with cuprite, often with malachite or
azurite as well, leaving behind the alpha phase of the alloy to corrode last. Gettens (i969) docu­
mented this type of corrosion, which is often associated with a cuprite marker layer and with
chloride attack; it leads to the presence of cuprous chloride in pits or under the more stable min­
eral growth as the patina develops.
The concept of ionic migration may be a useful one, since the effects of ionic transport may
predominate regardless of the natural environment. The environment per se is actually less cru­
cial than the chemical species involved in the corrosion, since they determine what kind of cor­
rosion products may form.
The chemical processes by which these corrosion layers may grow are also of interest in the
conservation of the object and the possible preservation of the original shape of the object
within the corrosion crust. These events are influenced by the transformations that may take
place between the original metal or corrosion product and subsequent chemical changes. As
these minerals grow or develop, they can be grouped into events, which are
ι. epitactic,
2. topotactic, or
3. reconstructive.

In epitactic transformations, there is a direct relationship between the crystallography of the
initial layer and the structure of the layer that may grow or develop from it. For example, cuprite
may preserve the pseudomorphic dendritic structures of the cast copper alloys as the corro­
sion penetrates into the metal. The preservation of structure within the corrosion may originate
from an epitaxial relationship between substrate and product, with a corresponding retention
of shape. Once a chemisorbed film of copper (I) benzotriazole has formed on a copper surface,
for example, subsequent layers of this copper complex are deposited epitaxially to produce a rel­
atively thick, oriented film with good protective properties.
Topotaxy results from the solid-state transformation of one corrosion product into another;
the crystal lattice of the original product may be changed, and the replacement product may not
have any direct structural relationship to the original material. Possible changes in this category
include the disruption of copper sulfide layers and their alteration to copper sulfates or other
products, and the alteration of cuprite to tenorite. Many topotactic relationships exist, as, for
example, between the iron oxyhydroxides (such as goethite) and iron oxides (such as hematite
or magnetite).

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