Page 134 - Copper and Bronze in Art: Corrosion, Colorants, Getty Museum Conservation, By David Scott

Pourbaix diagrams for the three mixed basic carbonates are not yet available, and an under­
standing of the likelihood of formation of these products will need to wait until such stability
diagrams are established.

Synthetic pigments with Because of their availability and low cost, zinc salts were fre­
i
copper and zinc salts quently used as additives n nineteenth-century commercial
pigment preparations based on copper. Riffault, Vergnand, and
Toussaint (i874), for example, describe the synthesis of a "mineral green lake," which, the
authors point out, is a misnomer because it is not precipitated onto an inert base like a dye. The
pigment was prepared from a mixture of copper and zinc oxides obtained from a saturated solu­
tion of copper in ι part nitric acid and 3 parts hydrochloric acid combined with a solution of zinc
in concentrated nitric acid. Potassium carbonate was used to precipitate a light green product of
mixed carbonates. This precipitate was washed, dried, powdered, and then heated in a crucible
until the "carbonic acid" (an early name for C0 2 ) was expelled, and the product acquired a fine
greenish hue. Ground very fine, the durable pigment was used for watercolors and for oil colors.
Depending on the proportions of copper and zinc salts dissolved in the acidic solutions, the
precipitated carbonate might represent any of the mixed copper-zinc salts discussed here.

C H A L C O N A T R O N I T E : A S O D I U M - C O P P E R C A R B O N A T E
Chalconatronite, Na 2 Cu(C0 3 ) 2 -3H 2 0, is the best known sodium-copper-carbonate mineral.
It was first identified by Frondel and Gettens (1955) as a bluish green, chalky crust found in the
hollow interior of an Egyptian bronze figurine of the deity Sekmet from the Saite-Ptolemaic
period 663-630 B.CE.) in the Fogg Museum of Art, Boston. The mineral was also identified
(
on an Egyptian bronze group of a cat and kittens in the Gulbenkian Collection in Lisbon and
on a Coptic censer, dating from about the seventh century, in the Freer Gallery of Art, Wash­
ington, D.C.
Chalconatronite was subsequendy identified on a copper pin from St. Mark's Basilica in
Venice (Staffeldt and Paleni 1978). Shordy thereafter, Horie and Vint (1982) found chalconatro­
nite crystals on Roman copper and iron armor from a site at Chester, England. The armor
had been conserved many years earlier, and the authors suggest that the chalconatronite is a
by-product of that treatment. In addition, the author (Scott 1999) identified chalconatronite as
an alteration product on a New Kingdom Egyptian gilt-bronze figurine of Osiris in the col­
lections of the Department of Near Eastern Studies, University of Southern California. The
alteration had penetrated to a considerable depth, with whole regions of the preserved surface
being transformed to chalconatronite.

BASI C C O P P E R CARBONATE S
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