Page 92 - Copper and Bronze in Art: Corrosion, Colorants, Getty Museum Conservation, By David Scott

F I G U R E î.io Wood fragments
preserved on a Greek copper
tablet from the sixth century
B . C . E . , shown in A , photomicro­
graph (magnification x245); and
B, scanning electron photomicro­
graph (magnification χΐ25θ) ,
illustrating the typical cellular
structure of a softwood, such as
pine, that has survived biodeterio-
ration by being in contact with
copper corrosion products on the
surface of the tablet.

Β

itself closely to the contours of the copper alloy surface. FIGURE 1.10 shows two different mag­
nifications of positive fiber pseudomorphs of wood preserved on the surface of an inscribed
tablet of hammered copper containing an early Greek text from the seventh century B.C.E. The
biocidal environment of the copper had even preserved wood fragments, which remained essen­
tially unmineralized during burial.
The degradation of wood and other cellulosic material by verdigris and other copper
pigments has been extensively studied. Verdigris, like all copper acetates, is slightly soluble in
aqueous solution and consequently easily absorbed by cellulose fibers. Mairinger and colleagues
(i98o) attributed the degradation of paper to autooxidation of cellulose accelerated by the pres­
ence of copper ions in a free radical process; the amount of damage was increased by high lev­
els of light, relative humidity, and sulfur dioxide. In their work on paper, Banik, Stachelberger,
and Wächter (1982) showed that copper ions are concentrated in the lumen of the fiber where
concentrations of copper can reach 8-10% by weight.
The degradation of wood and cellulosic material was studied by Banik (1989), who found
that the stability of cellulose impregnated with verdigris could be enhanced by immersing the
material in magnesium bicarbonate solutions. Banik (1990) also found that paper showing
signs of deterioration contained copper in both the univalent and divalent oxidation states, not­
ing that the presence of univalent copper could only be explained by a reduction of the copper
ion in the original copper pigment on the paper. The cellulose in damaged areas of the paper

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