Page 94 - Copper and Bronze in Art: Corrosion, Colorants, Getty Museum Conservation, By David Scott

The pervasive influence of copper ions on degradation reactions extends to proteinaceous
paint media. Khandekar and Phenix (1999) studied molecular-weight changes in egg tempera
films to which lead white, vermilion, azurite, or verdigris pigment had been added, followed by
artificial aging. All samples containing azurite had crosslinked to a greater extent than those
containing vermilion, with those samples exposed to azurite having molecular weights above
250,000. The effect of verdigris on the egg tempera medium was very different: only low-
molecular-weight components, with weights under 14,000, were found, indicating that the
verdigris had initiated a breakdown of the proteins. This is probably due to verdigris being more
soluble than azurite.
Copper in an acidic environment gradually degrades amino acids until they are no longer
detectable by high-performance liquid chromatography (HPLC) (Halpine 1996). Masri and
Friedman (1974) found that wool, which contains about 3.5% sulfur, readily adsorbs copper ions
by reaction with the -S-H groups to form copper mercaptides.
Copper is involved in the polymerization rather than the degradation of Oriental lacquer,
an oil-in-water emulsion that makes up the sap of the lacquer tree, Rhus verniciflua. Copper is
found in the enzyme lacease, which initiates drying. Lacease, which has four copper atoms per
molecule, is a copper glycoprotein, p-quinol-0 2-oxidoreductase, with a molecular weight of
120,000. A subsidiary copper glycoprotein, stellacyanin (molecular weight 20,000), is also
present in the sap, although its function is not known. Polymerization of the sap is initiated by
the laccase-catalyzed oxidation of urushiol to a semiquinone radical; this is accompanied by the
reduction of Cu (II) to Cu (I) in the lacease. Subsequent or continuing oxidation of the urushiol
requires the oxidation of Cu(I) in the lacease by air and oxygen as follows:

Cu + (lacease) + A0 2 + 2 H + = Cu (laccase) + H 2 0 1.19
++
L
This process explains why Oriental lacquer is cured in a chamber with high relative humid­
ity, which encourages the diffusion of moisture and oxygen into the sap. The chemistry of these
reactions is reported in detail by Kumanotani (198i, 1982,1988).

THE M E T A L L O G R A P H Y OF C O R R O D E D C O P P E R O B J E C T S

Corrosion products on many ancient objects may be the result of deliberate patination at the
time of manufacture, or they can result naturally from burial or exposure to the atmosphere.
Many books on the subject do not discuss how corrosion processes, often multiple ones, can
transform ancient metals into practically composite materials consisting of metallic remnants
and mineral alteration products. Corrosion products, therefore, should not be cleaned from
the surface of antiquities before metallographic examination, f feasible. Loose material or soil,
i
and soil and mineral concretions, should be removed, however, because their continued loss
during polishing could create a badly scratched surface that would interfere with microscopic

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