Page 319 - Copper and Bronze in Art: Corrosion, Colorants, Getty Museum Conservation, By David Scott

The match of this Kelvingrove sample to ICDD 28-746, potassium copper carbonate
hydroxide hydrate, K[Cu 2 (C0 3 ) 2 (OH)] 4 Η 2 0 , is far from perfect, which is a good illustration
·
of the problems encountered in these studies where single-crystal data may be hard to deter­
mine. Two other corrosion samples from the Burrell Collection have very similar spectra; the
data for one of them are shown in APPENDIX D, TABLE 22, along with data for the newly
characterized sodium copper carbonate acetate (hydrate) that has been identified by Thickett
and Odlyha (2000), who investigated a pale blue corrosion product found on 184 ancient Egyp­
tian bronzes during a survey of2,840 Egyptian artifacts in the British Museum. The storage cup­
boards in which these objects were kept contained significant amounts of acetic acid pollutant,
between 1071 and 288O g/m , and very low levels of other carbonyl compounds. In two cases,
μ
3
the pale blue corrosion matched ICDD 31-453, copper chloride acetate, C 2 H 2 ClCu0 2 . 2 8 By titra­
tion, ion chromatography, and thermogravimetric analysis (TGA), it was determined that the
unknown pale blue corrosion product had a stoichiometry of 1:1:1:1 in terms of sodium, cop­
per, carbonate, and acetate groups, TGA also indicated the presence of some water of crystal­
lization, and most likely oxide or hydroxide groups are present as well since these are very
common components of such compounds. Because the chemical valences can be balanced by
assuming a formula of CH 3 COONa- CuC0 3 , it is possible that this new compound is an acetate
analog of chalconatronite, the sodium copper carbonate, whose formula can be written as
Na 2 C0 3 -CuC0 3 . The logical conclusion is that this new corrosion product formed by reaction
of the bronze object with airborne acetic acid, sodium ions present on the surface of the object
(either from prior conservation treatments using sodium hydroxide or from naturally occurring
halite from the Egyptian soil), and carbonate corrosion products such as malachite. The combi­
nation of these four ions as a prerequisite limits the occurrence of this sodium copper carbon­
ate acetate (hydrate) to very localized instances on these Egyptian bronzes from the British
Museum. The X-ray diffraction data determined by Thickett and Odlyha are shown in APPEN­
DIX D, TABLE 23.

The two Burrell samples are probably more examples of this new compound. It is not yet
proved that these salts are actually a hydrate, but by analogy to other known salts of a simi­
lar type that are all hydrated, it is a fairly safe assumption for these samples as well. Scanning
electron microscopy-energy dispersive X-ray analysis determined that both unidentified salt
samples contained copper and sodium, with substantial peaks for carbon and oxygen and a trace
of sulfur. The occurrence of this pale blue corrosion product, a sodium copper carbonate acetate,
is probably more common as an alteration product than previously realized, now that refer­
ence data have been obtained by Thickett and Odlyha. There are undoubtedly other crystalline
deposits from the corrosion of bronze objects on display that await complete characterization.
Tennent and Baird (1992) investigated a white, acicular, crystalline efflorescence on an
Egyptian bronze tripod vessel from the Burrell Collection. The salt, which was associated with

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