Page 260 - Copper and Bronze in Art: Corrosion, Colorants, Getty Museum Conservation, By David Scott

Copper phosphate corrosion BURIAL ENVIRONMENT Tomb burial of copper objects in
in different environments association with decomposing bone, horn, or some other source
of phosphorus, frequently produces copper (II) phosphates,
either as a corrosion product on the objects or as a blue-green stain on the organic material. The
occurrence of libethenite is probably quite common in both contexts, but its occurrence on bone
is rarely reported in the literature, most likely because conservators may assume that the stained
bone or other material is indurated with copper salts and that there is no particular reason to
submit these samples for analysis.
Pioneering studies by Geilmann and Meisel (1942) identified libethenite as a blue-green
corrosion on a bronze spiral from a Bronze Age tumulus in Germany. Mattsson and coworkers
(1996) report the discovery of three copper phosphates from their extensive study of the corro­
sion of Swedish Bronze Age and Viking finds. In addition to cornetite, they identified a hydrated
copper phosphate, Cu 3 (P0 4 ) 2 -xH 2 0, and an unidentified copper phosphate mineral that con­
tained copper, phosphorus, and oxygen as shown by scanning electron microscope - energy
dispersive X-ray analysis (SEM-EDAX). This study also revealed that the occurrence of copper
phosphates on bronze objects buried in cremation sites was deleterious to the preservation of
the objects, because the copper phosphates formed under these conditions provided surpris­
ingly poor protection of the bronze to further corrosive attack. It is interesting that cornetite was
found in this study and that libethenite was not, since the general assumption has been that
libethenite should be present in at least some of the analyzed material.
I OUTDOOR ENVIRONMENT Selwyn and colleagues (i996)
reported finding libethenite on Louis-Philippe Hébert's (i850 -1917) outdoor statue of Sir John A.
MacDonald that was unveiled in Ottawa in 1895. This mineral probably was formed by the
interaction of bird droppings with other copper compounds in the corrosion crust. A second
phosphate of copper, zapatalite, was also found on the same statue and marks the only known
occurrence to date of this mineral in corrosion products.

Sampleite and the The corrosion chemistry for the mineral sampleite indicates
arid environment that this copper phosphate should occur on bronze objects in
arid localities where groundwater has high concentrations of
soluble salts. This association is supported by archaeological finds in Egypt and South America.
Pollard, Thomas, and Williams (1992b) found that the presence of mixed ions such as
sodium, chloride, and phosphate in groundwater can produce sampleite. FIGURE 7.1 shows the
relevant stability diagrams for the copper phosphate minerals where the log of the activity of the
cupric ions is plotted against pH. Sampleite occupies the extreme left-hand corner at a log a C U 2 +
of about 5 and between pH 2 and 4. The stability diagram for varying phosphate ion concentra­
tions is shown in FIGURE 7.2. These equilibrium stability fields of sampleite strongly suggest

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