Page 179 - Copper and Bronze in Art: Corrosion, Colorants, Getty Museum Conservation, By David Scott

A number of mixed-cation, copper-containing compounds were also identified. These
salts constitute unique characterizations in several cases and clearly indicate that localized
conditions on exposed bronzes can produce an extraordinary variety of substances, even f
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they are present only as minor patina components rather than major ones. Some of the com­
pounds found on the Buddha were mixed copper-zinc salts: a synthetic copper-zinc-silicon
sulfide, Cu 2 ZnSiS 4 , which is an unusual and perhaps dubious identification; rosasite, a mixed
copper-zinc basic carbonate, (CuZn) 2 (C0 3 )(OH) 2 ; and schulenbergite, a mixed copper-zinc
basic sulfate, (Cu,Zn) 7 (SO 4 ,CO 3 ) 2 (OH) 10 -3H 2 O, that is rhombohedral with a pearly, light
green-blue color. Schulenbergite was first identified in 1984 from the Glücksrad mine, Ober-
schulenberg, Harz, Germany. Mixed copper-lead minerals included a synthetic copper-lead
chloride, Pb 3 CuCl 2 ; a synthetic copper-lead carbonate sulfate, Pb 4 Cu(C0 3 )(S0 4 ); caledonite,
a copper-lead basic sulfate carbonate, Cu 2 Pb 5 (S0 4 ) 3 C0 3 (OH) 6 , which is an orthorhombic
mineral with a resinous green or bluish green color, first found n Leadhills, Lanarkshire,
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Scotland; beaverite, a complex basic lead sulfate with iron, copper, and aluminum,
Pb(FeCuAl) 3 (S0 4 ) 2 (OH) 6 . This rhombohedral mineral has an earthy yellow color and was first
named in 1911 from Frisco, Beaver County, Utah.
Other minerals identified on this sculpture include a synthetic copper-tin hydrated
phosphate, CuSn(P0 4 ) 2 -3H 2 0; spangolite, a copper-aluminum basic sulfate chloride,
Cu 6 Al(S0 4 )Cl(OH) 12 -3H 2 0, which is a vitreous dark green to bluish green mineral; a partially
hydrated copper sulfate, CuS0 4 -H 2 0; a calcium-copper oxide, Ca 2 CuO s ; a synthetic copper
phosphate, CuP0 3 ; the copper sulfides Cu 196 S, Cu 7 2 S 4 , Cu 2 S, and Cu 8 S 5 ; and calumetite
(see CHAPTER 4).
It is surprising that such a wide array of compounds should be identified on this particular
sculpture. Its sheer size, resulting in different zones of exposure, may explain some of them, but
it is not easy to deduce why the rest should form. The sculpture does have a high lead content
that has clearly influenced the occurrence of several mixed copper-lead minerals. The selective
corrosion of some of the lead globules has resulted in the formation of the primary lead sulfates
and phosphates identified in the study. The prevalence of a number of phosphate compounds
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must relate to a consistent source of phosphorus in the environment—for example, bird drop­
pings. The published analysis of the Buddha statue does not show any zinc, so it is superficially
odd that so many mixed copper-zinc minerals should be identified. Aoki wrote that no zinc-
containing alloys were used in Japan before the fifteenth century. 13 He noted, however, that
the Great Buddha underwent several restorations during the Edo period (16OO-1868). The fig­
ure was repaired in 1712, parts of the body in 1735, and the hair in 1736. X-ray fluorescence spec­
troscopy showed that the hair of the Buddha contains zinc, and this explains the origin of the
copper-zinc salts.

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