Page 91 - Copper and Bronze in Art: Corrosion, Colorants, Getty Museum Conservation, By David Scott

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form copper-organic complexes. Second, when all available sites are filled, controlled mineral
deposition occurs, initiated by the already occupied sites. In contrast, the formation of negative
casts relies on higher concentrations of copper ions sufficient to exceed the solubility product of
the deposited mineral. This process is therefore more random in effect than the formation of
positive pseudomorphs. Studies using Fourier transform infrared spectroscopy show that min­
eralized fibers are rarely totally mineralized; they often retain sufficient organic components to
allow fiber identification and, in some cases, dye characterization.
Copper is used today to facilitate commercial mordant dyeing processes. It is especially use­
ful for dyeing polyacrylonitrile fibers (Orion), which cannot be mordanted with traditional
cations such as aluminum. Copper in the cuprous state complexes with the nitrile groups and
penetrates the fibers, providing dyeing sites for anionic dyestuffs (Roberts and Caserío 1964).
Chen, Jakes, and Foreman (i996) examined X-ray diffraction data for copper mineraliza­
tion of plant fibers. They found differences in the physical microstructure between modern and
mineralized fibers; the crystallite sizes of the mineralized fibers were larger than those of mod­
ern examples of the same fiber. The presence of malachite within the fiber was determined by
X-ray diffraction.
Some examples of the preservation of organic materials are shown in PLATE 6 and in FIG­
l
URE .io. PLATE 6 shows the preservation of a marine organism over the corroded surface of a
copper alloy object retrieved from the Mediterranean Sea. The marine organism had formed

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