Page 133 - Copper and Bronze in Art: Corrosion, Colorants, Getty Museum Conservation, By David Scott
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large quantities of blue lumps, which consisted of very thin, anisotropic plates with spherulites
inside. Examination with a scanning electron microscope revealed that the principal elemental
composition of these plates was copper and calcium, with admixtures of iron, magnesium, and
aluminum. These plates are very likely an intermediate product formed during the synthesis of
artificial malachite pigment.
M I X E D - C A T I O N C O P P E R C A R B O N A T E S
There are a number of mixed-cation copper carbonates, some of which have been identified as
corrosion products and some of which have also been used as pigment additives.
Mixed copper-zinc Although zinc is often selectively lost in the corrosion of brass
carbonates in corrosion alloys, leading to plug dezincification and serious corrosion fail
ure, it is possible to find a mixed copper-zinc mineral forming
during the corrosion of brass objects or bronzes containing zinc. One of these minerals is rosa-
site, (Cu,Zn) 2 C0 3 (OH) 2 , which can occur as mammillary, botryoidal, or warty crusts with a
fibrous to spherulitic structure. This monoclinic mineral is somewhat brittle, has a Mohs hard
ness of about 4.5, and is sky blue to bluish green.
Rosasite has been identified on a Chinese bronze canister from the Han dynasty (206 B.C.E.-
220 C.E.). It was also identified on another Chinese bronze, known as a tut, from around
300 B.C.E. (Gettens 1963a), but there is some doubt about this age, since zinc is not known to
have been used as an alloying element at that time. One possibility is that the piece is a later copy
from the Song dynasty. The only recent identification of rosasite as a corrosion product is in the
patina on the Great Buddha in Kamakura, Japan (Matsuda and Aoki 1996).
Two other mixed copper-zinc basic carbonate minerals may be found: aurichalcite,
(Cu,Zn) 5 (C0 3 ) 2 (OH) 6 , and claraite, (Cu,Zn) 3 C0 3 (OH) 4 -4H 2 0. Only aurichalcite, however,
has been identified as a corrosion product; the one example is on a Roman coin of a copper alloy
that contains zinc. The coin is in a private collection. Aurichalcite occurs as delicate needlelike
crystals that are very soft (Mohs hardness 1-2) and pale green or greenish blue. Claraite is an
important member of this series of minerals, though there are no published accounts of claraite
as a corrosion product.
These mixed basic carbonate minerals arise from the substitution of copper atoms in the
crystal lattice with some zinc. Although each mineral phase has an assigned ICDD file, some
variation in composition, and hence in d-spacings, may be particularly evident with these car
bonates. Chemical analyses of aurichalcite, for example, show that copper and zinc can substi
tute for each other over a considerable range: Cu:Zn = 1:3.I6 and Cu:Zn = 1:1.57 are cited by
:
Palache, Berman, and Frondel (1951). For rosasite, Cu:Zn ratios ranging from 3:2 to 2 i have
been reported.
C H A P T E R T H R E E
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