Page 93 - Copper and Bronze in Art: Corrosion, Colorants, Getty Museum Conservation, By David Scott

samples was depolymerized, with the level of polymerization ranging from 60 to 80, compared
with 200-500 in undamaged areas. This difference in polymerization levels can be explained
by the cellulose chain being split at the gluocidic linkage. The cellulose is attacked by the cop­
per ions under both alkaline and acidic conditions. Under acidic conditions, the new end groups
are aldehydes; under alkaline conditions, the new end groups are carboxyl groups with the
amorphous regions of the cellulose fibers attacked first.
According to studies by Shahani and Hengenihle (i986), transition metals such as copper
are capable of catalyzing the oxidation of cellulose over a wide pH range. This interaction of
copper ions and cellulose occurs under both acidic and alkaline conditions, with the cellulose
absorbing copper ions. The copper ions are exchangeable with protons of the carboxyl groups
of partially oxidized cellulose (Bicchieri and Pepa 1996). The binding of copper by cellulose is
also possible by complex formation with hydroxyl groups. According to Blattner and Ferrier
(1985), the absorption of copper increases with the formation of carboxyl groups in cellulose;
this results from oxidation under alkaline conditions. The chemical interaction of copper with
polysaccharides, such as cellulose and cellulose derivatives, forms a variety of complexes; addi­
tional references to this are found in Gmelin (i966).
Williams (i967,1993) and Williams and da Silva Frausto (i996) did extensive research on
the compounds that may form between copper (and other metallic ions) and proteinaceous
materials, and their cited works should be consulted for further background information. Inor­
ganic ions such as copper facilitate the decarboxylation of RCOO - groups in proteins, thus induc­
ing deterioration. Guthrie and Laurie (i968) studied the reactions of cupric ions with mohair
keratin, whose amino acid components show relatively high amounts of aspartic and glutamic
acids. Their results indicate that the side-chain carboxyl groups are the principal binding sites
for cupric ions in keratin, with some possible amide contributions through nitrogen atoms.
Daniels and Leese (1995) studied the effects of verdigris on silk and calculated an activation
energy for the resulting degradation, noting that the copper pigment must be in direct con­
tact with the silk for the deterioration to be accelerated. The direct action of copper ions on
a silk substrate may be slowed by suspending the verdigris in a gum binder. For example,
alum-deerskin glue is commonly found as a binder in Japanese paintings on silk, and this
may play a role in preserving the verdigris pigment used in these works. The gelatin in the glue
on sized silk paintings may act as a scavenger for copper ions, since the -S-H groups form
a strong linkage—as was noted earlier for wool—and this may help prevent the chemical
interaction between the silk and copper ions. Daniels and Leese found that benzotriazole and
aqueous solutions of magnesium bicarbonate were both effective in helping to stabilize the
degradation of damaged silk fibers. Since magnesium bicarbonate proved effective in previous
conservation treatments of other fibers, the researchers recommend it as the preferred choice
for stabilization of silk.

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