Page 318 - Copper and Bronze in Art: Corrosion, Colorants, Getty Museum Conservation, By David Scott

foil that had been exposed to acetic acid vapor and corrosion from an Egyptian bronze in the
collections of Marischal College, University of Aberdeen, Aberdeen, Scotland, gave powder
X-ray diffraction patterns that could not be identified (Tennent et al. 1993), but as the data now
available are more detailed, it should be possible to at least identify the product from the expo­
sure of the copper foil to acetic acid. Ion chromatography was only partially helpful in identi­
fying the material; the corrosion on the foil was probably a basic copper acetate, but minimal
acetate or formate content was found in the light blue corrosion on the Marischal bronze.
X-ray diffraction data were collected for representative examples of light blue corrosion
products on bronze objects as part of a collaborative study at the GCI Museum Research Labo­
ratory, with samples provided by Tennent. The light blue to blue-green samples of corrosion
were from Egyptian bronzes in the collections of the Museum of Fine Arts, Boston; the Fitzwil-
liam Museum, Cambridge, England; Glasgow Art Gallery and Museum, Kelvingrove, Scotland;
and the Burrell Collection.
APPENDIX D, TABLE 20, presents data from the head of an Egyptian bronze of Min-Amun
in the collections of the Fitzwilliam Museum. The data reveal that some of these corrosion
products are of chalconatronite, or of compounds related to chalconatronite that may have
formed during storage and contain sodium, copper, carbon, oxygen and hydrogen, but whose
structure remains to be elucidated. Other entries show that at least some of the identified com­
pounds are, in fact, malachite, calcite, and connellite (the connellite found on this bronze is dis­
cussed in CHAPTER 6). Chalconatronite is not uncommon as a corrosion product on Egyptian
bronzes, although in the absence of detailed treatment records, it is possible that the chalcona­
tronite could have been produced as a result of patina alterations when the object was soaked in
sodium sesquicarbonate solutions, resulting in the slow crystallization of chalconatronite dur­
ing long periods of storage.
Another group of objects of interest are from Kelvingrove, which provide further examples
of the formation of unusual corrosion products as a result of alterations occurring during stor­
age or display in polluted showcases. A bright blue crystalline deposit from one object in Kelvin­
grove gave the X-ray diffraction pattern shown in APPENDIX D, TABLE 21 (GCI XRD no. 612).
The data suggest that the compound is closely related to potassium copper carbonate hydroxide
hydrate. Environmental scanning electron microscopy (ESEM) data for this salt also revealed
the presence of potassium, copper, carbon, oxygen, and a very small peak for sulfur. A number
of lines should be present according to the ICDD files that are absent in the sample. Although
the elemental analysis corroboration provided by the ESEM data appears reasonable, the iden­
tity of the compound cannot be determined with complete certainty. The GCI Museum Research
Laboratory was able to isolate some crystalline particles from the sample and, in collaboration
with Hardcastle from California State University, Northridge, single-crystal diffraction data
27
were obtained; the cell dimensions of the crystal lattice are currently under study.

T H E ORGANI C SALT S O F C O P P E R
301