Page 315 - Copper and Bronze in Art: Corrosion, Colorants, Getty Museum Conservation, By David Scott

7-oxydehydroabietate as well as copper. Mass spectrographic analysis also gave results con­
sistent with the inference that the original organic component had been pine resin and that
the oxidation of the abietadiene acids in the resin had produced dehydroabietic acid and
7-oxydehydroabietic acid.
The most reliable method to detect the presence of copper resínate is gas chromatography-
mass spectrometry of the relatively stable dehydroabietate component of the resínate films,
according to Mills and White (i987). That copper resínate greens can be made from verdigris,
prepared, in turn, from bronze or bronze scrap was shown by analysis of Raphael's The Holy
Family in the collections of the J. Paul Getty Museum. Nondestructive microscanning X-ray
fluorescence spectroscopy revealed some tin content in the copper resínate used in the paint­
ing. 2 4 The Holy Family is shown in PLATE 58, and a cross section of the copper resínate used in
this painting is shown in PLATE 59.

Copper salts Apart from their occurrence as corrosion products (discussed
of higher acids later in this chapter), copper salts of higher acids, such as cupric
oleates, maleates, and stéarates, may also be found as pigments.
Using Fourier transform infrared (FTIR) spectroscopy, Moffett, Sirois, and Miller (1997) report
the probable identification of a mixture of the fatty acids copper palmitate and copper stéarate
on two late-eighteenth-century Naskapi Indian artifacts, a coat and a pair of leggings. The
Naskapi were an Algonquian-speaking people who inhabited the Quebec-Labrador Peninsula;
museum collections of this tribal group are widely dispersed. The green pigments on the arti­
facts gave no X-ray diffraction pattern, indicating that they are poorly crystalline or X-ray amor­
phous. The use of green colorants is not common in Naspaki objects, and the copper salts of
these fatty acids may have been prepared locally rather than imported as prepared pigments.

Copper proteinates A neglected topic in connection with the résinâtes is that of the
copper proteinates. Verdigris pigments and other copper salts
can react with proteinaceous media to form copper-protein complexes that can be used as pig­
ment preparations in much the same way as a copper resínate. This subject has not received a
great deal of attention in terns of artist's materials, although the chemical reactions between cop­
per ions and proteins has been the subject of a considerable body of research. Copper proteinate
pigments can be made from ingredients such as sturgeon glue, gelatin derived from vellum, or
perhaps the lees of wine, which are mentioned in some historical texts. Cennino d'Andrea Cen-
nini (ca. 1370-1440) , author of // libro delVarte (The craftsman's handbook) of 1437, mentions
that verdigris is a useful color on paper when mixed with egg yolk. He writes, "Work it [the
verdigris] up with vinegar, which it retains in accordance with its nature... and it is especially
good on paper or parchment, tempered with yolk of egg." 25

C H A P T E R NIN E
298