Page 90 - Copper and Bronze in Art: Corrosion, Colorants, Getty Museum Conservation, By David Scott

ual copper corrosion products as a negative impression of the original material. Since copper
ions are strongly biocidal, their impregnation of the organic material may preserve it from
biodeterioration during burial.
Positive pseudomorphs are much more common with copper than, for example, with iron.
Gillard and coworkers (1994) examined mineralization processes using samples of both cel-
lulosic and proteinaceous material. Cotton, linen, silk, and wool were examined after alumi­
num potassium sulfate mordanting, followed by exposure to solutions containing cupric sulfate,
cupric chloride, copper powder, or 6% tin bronze. Mineralization of wool samples had already
begun after two days of exposure. Wool was the most reactive natural fiber because of the large
number of reactive sites in the protein helix. Silk was almost as responsive; like wool, it bonds
to copper at the carboxyl and amide linkages. The more easily degraded wool is influenced by
the reactive disulfide linkages in the cystine residue, which is predominant in the amorphous
regions of the wool fiber (Alexander, Hudson, and Earland 1963). The rate of mineralization
for silk and cellulosic fibers is slower than that of wool. This can be attributed to the greater
crystallinity of silk and cellulose, since regions of crystallinity are more resistant to attack.
Cellulosic materials bind to copper primarily through the hydroxyl groups, although the
details have not been elucidated. During these studies, the addition of sodium chloride to
the reactants caused a fine mineral layer to be deposited on the fabric surface. The mineral
proved to be botallackite, which rapidly recrystallized to atacamite; on continued exposure, this
was further recrystallized to paratacamite—probably clinoatacamite, in fact (see CHAPTER 4 on
botallackite).
Exposure of raw wool samples to copper powder and sodium chloride for one and a half
years resulted in almost complete mineralization: a carbon loss of 89% occurred with parataca­
mite formation. Malachite was also found in some simulated-wool samples as well as in mate­
rial from archaeological contexts. As for cellulose fibers, breakage by hydrolysis can occur
under acidic conditions at the ether linkages.
Proteinaceous materials, however, are joined by peptide linkages. In wool, the result­
ing polypeptide chains are bound by hydrogen bonding, carboxyl and amide end-group inter­
actions, and disulfide linkages. Gillard and Hardman (i996) carried out electron-spin resonance
(ESR) studies that showed that where the pH < 9, the bonding of copper to wool is pre-
dominandy due to cupric carboxyl complexes. Cupric ions catalyze the oxidation of disulfide
bonds but bind elsewhere. The smallest peptide unit exemplifying these reactions is biuret,
NH 2 CONHCONH 2 , which, with mildly acidic copper ions, forms a blue-green, bis-biuret cop­
per (II) dichloride complex. The biuret molecule acts as a bidentate chelating material via its
amide oxygen atoms; distorted octahedral coordination is maintained by chloride ions. This
work suggests that a positive mineralization cast of the original fiber develops during corrosion
in two stages: First, copper ions infiltrate the fiber at certain positions in the organic matrix to

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