Page 175 - Copper and Bronze in Art: Corrosion, Colorants, Getty Museum Conservation, By David Scott

Microenvironment Selwyn and colleagues (i996) found unusual mixed salts on
and corrosion the outdoor bronzes they studied in Ottawa. One was dev-
illine, a basic hydrated sulfate of calcium and copper,
CaCu 4 (S0 4 ) 2 (OH) 6 -3H 2 0. This mineral, first identified in material from Cornwall, England,
is monoclinic with a vitreous pearly green to bluish green color and a low Mohs hardness of 2.5.
It was found in a patina that also contained gypsum and probably formed from the evaporation
of solutions containing copper sulfates and calcium sulfate. The other compound was an ammo­
nium copper sulfate hydrate, Cu(NH 4 ,S0 4 ) 2 -2H 2 0, found in a surface sample taken from an
exposed area on a statue of Sir George Etienne Cartier that was unveiled in Ottawa in 1885.
Tutton's salt, Cu(NH 4 ,S0 4 ) 2 · H 2 0, a mixed basic ammonium sulfate hydrate of copper (II)
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similar to the one found by Selwyn's group, was reported by Bernardini and coworkers (1992)
on two Italian statues located in sheltered areas. This mineral would be expected to be found
only in sheltered areas because it is very soluble and would be washed away in more exposed
regions of the patina.
Bernardini and colleagues studied Uincredulità di San Tommaso, a bronze sculptural group
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by Andrea del Verrocchio (1435-88) that was erected around 48i in a niche on the eastern facade
of the Orsanmichele church in Florence. The rear of the bronzes is roughly finished; the front
is chased and partially gilded. A total of fifty-six different sampling locations were examined
with up to four samples isolated from each location. The following discrete layers could be dis­
tinguished in the samples: an outer gray-to-black, coarsely textured crustal material; a greenish
or yellowish, earthy, predominantly compact layer overlying a thin, reflective patina with red­
dish grains; a compact reddish or brown layer; and finally the interface between the corrosion
and metallic phases. Twenty-one different compounds were identified during the study. The
most surprising result was the prevalence of Tutton's salt, determined in fifty samples; and of
moolooite (cupric oxalate hydrate), Cu(COO) 2 -nH 2 0, found in eighty-two samples. Nantokite
was found in twenty-eight samples, which suggests that active chloride corrosion of the bronzes
may pose a serious problem with long-term stability. This study is a valuable reminder that dif­
ferent compounds can be isolated from discrete regions of a patina, and these identifications
were possible because of the large number of samples removed for examination.
The specific microenvironment of exposed bronzes obviously has a tremendous impact
on the nature of the corrosion products. The partially protected niche housing the Verrocchio
sculpture is potentially more deleterious, as far as corrosion is concerned, than f the bronze
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group were fully exposed to the elements, since urban pollution in Florence, the proximity of
the building, and the presence of bird excrement, rain runoff, and particulate matter will all
accelerate corrosion. The appearance of moolooite in substantial amounts and the presence of
the calcium oxalates— whewellite, Ca(COO) 2 -H 2 0, andweddellite, Ca(COO) 2 -2H 2 0 — clearly

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