Page 267 - Copper and Bronze in Art: Corrosion, Colorants, Getty Museum Conservation, By David Scott

in fact, faustite (see APPENDIX D, TABLE 7). The presence of both copper and zinc in the beads
was confirmed by X-ray fluorescence. Faustite is a partially zinc-substituted turquoise mineral
not mentioned in any Old World contexts. There does not seem to be any mention of chal-
cosiderite either, which raises some doubts about the exactitude of the identification of
turquoise in the original texts. Foord and Taggart (i998) state that the number of known occur­
rences of faustite is quite small compared with occurrences of other members of the turquoise
group. This adds to the significance of this mineral's discovery from Moche cultural contexts.
Given the variations that may exist within the turquoise group of minerals, there is clearly a
need for the publication of more detailed X-ray diffraction data and for trace elements to be
examined as a possible distinguishing tool in the study of these minerals.

THE C O P P E R NITRATE M I N E R A L S
Copper nitrates are relatively rare as minerals, pigments, or corrosion products, since the usual
nitrate salts are all water soluble. The basic nitrates can occasionally be found, however, as
pigments or as corrosion products. The most common copper nitrate mineral is the basic salt
gerhardtite, Cu 2 (N0 3 )(OH) 3 , which has been found as a secondary mineral associated with
malachite and atacamite in massive cuprite deposits. Gerhardtite can occur as thick tabular crys­
tals, dark green to emerald green in color (Palache, Berman, and Frondel 1951). The crystal struc­
ture was determined by Imhoff (1953), although the nitrate was characterized in 1885 and named
after Gerhardt, who first synthesized the salt.
Other nitrate minerals include likasite, Cu 3 (N0 3 )(OH) 5 -2H 2 0, and buttgenbachite,
Cu 1 8 (N0 3 ) 2 (OH) 3 2 Cl 3 -H 2 0, which has a complex and unusual stoichiometry. Neither mineral
has yet been found as a corrosion product. Brief characteristics of the copper nitrate minerals
are given in TABLE 7.2.

Copper nitrate The most common occurrence of gerhardtite is as a component
corrosion products of artificially patinated copper alloys. Many recipes for green
patinas today contain ingredients such as nitric acid, potassium
nitrate, sodium nitrite, or ammonium nitrate (Hughes and Rowe 1982), which would create a
copper (II) nitrate salt.
Copper nitrates should be rare as a natural corrosion product since nitrate salts are usually
i
the most water-soluble group of copper corrosion products and, f formed, would invariably
be expected to be washed away from the surface. There are, however, some noted occurrences
of copper nitrates in corrosion. Otto (1959) identified gerhardtite as a patina component on
archaeological bronzes, and Riederer (19 8 8) reported an occurrence of this basic nitrate on
some ancient Mesopotamian bronzes in the collections of the Institut für Technologie der
Malerei in Stuttgart.

C H A P T E R S E V E N
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