Page 261 - Copper and Bronze in Art: Corrosion, Colorants, Getty Museum Conservation, By David Scott

that the compound is expected to occur only on bronzes from environments where solutions of
extreme ionic strength permit it to form, particularly in low pH environments. Such conditions
are encountered in arid regions where periodic wetting followed by desiccation can create much
higher concentrations of soluble salts than would generally occur in groundwater solutions.
In the early 1980s, during three seasons of excavation in the extremely arid environment of
Memphis, Egypt, 350 copper alloy objects dating from the fifteenth to the twelfth century B.C.E.
were recovered. Most of these objects possessed a remarkably thick, soft, light-green corrosion
product that did not resemble any of the previously described copper corrosion products. After
the first season of excavation, this product was identified by X-ray diffraction as sampleite, the
complex copper phosphate first described as a naturally occurring mineral by Hurlbut (1942).
The identification was confirmed by studies of mineral specimens of sampleite and by addi­
tional analytical studies of the corrosion product itself (Fabrizi et al. 1989). Found in association
with sampleite on copper alloy objects from Egypt is the rare copper (II) chloride eriochalite,
CuCl 2 · H 2 0, which shares the same region of stability as sampleite. Because eriochalite is water
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soluble, it occurs only in arid conditions and appears as wool-like aggregates or small crystals.
A striking feature of sampleite corrosion is the thickness of the crust that can be created by
the mineral's formation; in many places, the corrosion product is thicker than the object itself.
For example, thin rods only 2-3 mm in diameter can have a surrounding corrosion layer that is
10-15 mm thick. A typical example, shown in PLATE 45, is a copper-alloy nail with a very thick
coating of sampleite that totally obscured the nature of the object underneath before partial
mechanical cleaning. When first excavated and still moist, sampleite is blue green in color with
a very soft, cream-cheese-like texture that can be easily pared away with a scalpel. When dried,
the corrosion product is still soft and very fine, but it is a paler green, almost the color of bronze
disease. A mounted preparation of sampleite from one of the Egyptian artifacts from Memphis
is shown in PLATE 46. The particle size is small, - 4 μπι. When a sample is viewed under plane-
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polarized light in a melt-mount of refractive index 1.662, the crystals have very little relief
because their refractive indices are all close to that of the medium; two of these indices are espe­
cially close. When viewed under crossed polars, the crystals show a gray-white birefringence.
The wave plate shows clusters of tiny grains with differing orientations in the usual second-
order, straw-yellow and sky-blue colors.
There is no question about sampleite undergoing changes in chemical composition follow­
ing excavation; both hydrated and dried postexcavation material from Egypt revealed the pres­
ence of sampleite, and the corrosion products did not show any alteration. An X-ray diffraction
study was carried out on the mineral, and the results are shown in APPENDIX D, TABLE 6. Col­
umn 1 of the table shows the ICDD data for sampleite (ICDD 11-349), which is compared with
data from the type sample in the British Museum (Natural History) and with data for corro­
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sion product from objects excavated at Memphis, Egypt. The X-ray diffraction data uniquely
identify the corrosion product as fairly pure sampleite.

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