Page 314 - Copper and Bronze in Art: Corrosion, Colorants, Getty Museum Conservation, By David Scott

dition than f exposed to ambient light. The discoloration of the transparent greens in the van
i
Eyck painting was not linked to the presence of remnant blue-green grains, which did not show
any sign of attack by the medium. Therefore, it appears that photochemical activity was an
important aspect of the deterioration of these copper resínate layers. This brown discoloration
differed from the brown glaze over the resínate layers by its continuity with the green and by its
characteristic hue, often a light orange brown in transmitted light. Leonardo mentions the use
"
of aloe for the purpose of conferring additional stability on the copper greens: If you have
finished a work with simple green [a copper green] and you glaze it thinly with aloe dissolved
in water, then the work has a very beautiful colour" (Leonardo [i65i] 1956:88).
Alternatively, a protective layer or varnish could have been applied on surfaces painted with
verdigris pigment, as suggested by Merrifield (1849).

The chemistry of A series of experiments were conducted by Kühn (1970) in
copper résinâtes which prepared samples of copper resínate were exposed at
20 °C and 55% RH to high-intensity shortwave ultraviolet radia­
tion and to light from xenon lamps, fluorescent tubes, and daylight, with the exposure equiva­
lent to 120, 60, and 5 years. A brown discoloration was noticed in the samples exposed to the
high-intensity shortwave ultraviolet radiation. This was also noticed in tests carried out at the
National Gallery of Art, London.
As with the verdigris pigments, some of the recipes for copper résinâtes call for chemical
additions that may produce a series of complex reactions with the original verdigris component.
Birelli (1601), for example, mentions that the preparation of a "green like the emerald" starts
with linseed oil, rock alum, and good quality verdigris, with the subsequent addition of pine
resin. In recipes such as this, the addition of potassium aluminium sulfate is commonly used as
a reagent with copper salts (see CHAPTER 5 for further uses of sulfates).
A variety of resinous mixtures could have been mixed with verdigris to create copper
resínate pigments. One recipe recounted by Kühn (1993) calls for purified neutral verdigris to be
well ground with one part walnut oil, two parts turpentine distillate, and one part pine resin. f
I
resins from conifers are used, the principal copper salts expected to form are those of abietic
acid, C 1 9 H 2 9 COOH; the major components of colophony, which is the primary residue from
the distillation of turpentine, are abietic, dehydroabietic, pimaric, isopimaric, and sandara-
copimaric acids.
Detailed chemical studies of copper résinâtes used in works of art demand sophisticated
organic analysis techniques. Mills and White (1987) used such techniques when they examined
Raphael's Saint John the Baptist Preaching in the collections of the National Gallery of Art, Lon­
don. The paint medium was oil, but the green paint, after saponification and methylation,
showed a gas chromatogram that revealed the presence of methyl dehydroabietate and methyl

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