Page 313 - Copper and Bronze in Art: Corrosion, Colorants, Getty Museum Conservation, By David Scott

and this may be caused by Venice turpentine being used to make the copper resínate pigment.
Mills and White (1977) note that although Venice turpentine is often mentioned in early copper
resínate recipes, it is not entirely satisfactory for this purpose because it dries to a yellowy,
brittle film. Venice turpentine contains a high proportion of neutral components; conse­
quently, there are fewer acid groups to react with the copper compounds to form a resínate pig­
ment. Pine resin, as used in a painting by Raphael (ΐ483-ΐ52θ) cited by Mills and White as a
salient example, consists almost entirely of resin acids and is much more suitable than Venice
turpentine as the pigment medium.
The usual medium for copper resínate, however, is one of the drying oils. In fact, Cennini
mentions that verdigris should be used ground in a drying oil even when the rest of the paint­
ing may be in tempera, as with the Raphael work mentioned by Mills and White.
There are many historical sources that draw attention to the use of glazes over the cop­
per resínate layer to change the color, but these glazes themselves may discolor, creating difficul­
ties for conservation practice and for the identification of the materials in these organic glazes
(Woudhuysen-Keller 1995).
Italian painter and author Giovan Battista Armenini (1530-1609), writing around 1580, also
refers to verdigris and mentions an interesting mixture with soot and oil to form a dark varnish.
He writes that soot of rosin is incorporated
most beautifully with verdigris, well ground with oil. One third part of verdigris is used to
two thirds of soot, and they are mixed together on the stone with some oil and a little com­
mon varnish. This varnish is of such quality that it gives strength to all colours that suffer
upon drying. (Armenini 1977:66)

Kockaert (1979) noted that dark greens and some gray or whitish tones on fourteenth- to
sixteenth-century paintings from the Netherlands often appear brownish, adding that these
transparent browns cannot be confused with general varnishes because they are limited to par­
ticular areas with a definite pictorial imagery. The most probable explanation for the presence
of these layers, according to Kockaert, is that the artist altered the color balance of the resínate
layer by the addition of other oil-based pigments, for example, that could have influenced the
degree of deterioration of the resínate layer. Kockaert also examined more than one hundred
cross sections, including thirty-three thin sections, from fifteenth- and sixteenth-century paint­
ings. In some paintings, the copper resínate was still perfecdy green; in others, it exhibited
extensive brown discoloration, mainly at the surface, to a depth of 1 μιη or more.
For example, a sample of resínate from Jan van Eyck's (1390 -1441) Mystic Lamb in St. Bavon
Cathedral, Belgium, appeared different from samples of resínate in other paintings by a certain
heterogeneity due to darker grains and by its retention of a good green color. Kockaert demon­
i
strated that f a copper resínate has been protected from light, it will survive in much better con-

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