Cecily Grzywacz

The Getty Conservation Institute

A NEW BLUE ON THE BRONZE: SODIUM COPPER ACETATE CARBONATE

With permission of the author of the original paper, this is a summary of an Oral Presentation for the Objects Session of the AIC Annual Meeting, St. Louis, June 1999, "The Hidden Secrets of The Hidden Secrets of Copper Alloy Artifacts in the Athenian Agora", by Alice Boccia Paterakis, Head of Conservation, Agora Excavations, American School of Classical Studies, Athens, Greece.

A small percentage of the bronze collection at the Agora had a blue corrosion product that did not match previously identified compounds. All of these objects had been chemically treated at one point in their history. The Metropolitan Museum of Art analyzed samples and found copper acetates by XRD & FTIR. Additional analysis was needed to determine the exact composition. Scientists at the British Museum in London used XRD, FTIR, IC, SEM and (Energy) Dispersive Analysis X-Ray (SEM-EDAX). David Thickett reported that the corrosion product was sodium copper acetate carbonate. Approximately 500 ppb or 1267 µgm^-3 of acetic acid were detected at the Agora.

The acetic acid emissions from construction materials contributed to corrosion. It is further postulated that previous chemical treatments, such as sodium sesquicarbonate contributed to the corrosion of the bronze alloys. However, British Museum scientists stated that acetic acid concentrations of 500 ppb were not sufficiently high to lead to corrosion. This is yet to be proven, and higher initial concentrations or fluctuations in temperature, RH and many other factors may have led to the damage. One air quality measurement is not sufficient data to dismiss the effect of organic carbonyl pollutants.

Thickett describes the following possible mechanism to form the sodium copper acetate carbonate: sodium copper carbonate hydrate forms on copper alloys treated with sodium carbonate or sodium sesquicarbonate. Long-term exposure to acetic acid may result in the formation of sodium copper acetate carbonate. Relative humidity is an important parameter and fluctuates can cause further damage to the bronze objects.

Paterakis, A. B. (1999). "The Hidden Secrets of Copper Alloy Artifacts in the Athenian Agora." AIC Annual Meeting, St. Louis, MI, American Institute for Conservation: Washington DC, Objects Session Post Prints, in press.

Thickett, D., Bradley, S., and Lee, L. (1998). "Assessment of the risks to metal artefacts posed by volatile carbonyl pollutants." METAL 98, International Conference on Metals Conservation, France, (eds. W. Mourey and L. Robbiola), James and James Science Publishers Ltd: London, pp. 260-264.