Jubiläumsfonds der Oesterreichischen NationalbankAnalysis of corrosion phenomena and optimisation of the storage conditions for antique bronze coins showing a high lead content
On a number of antique Greek and Roman bronze coins made of high-leaded bronze and kept in the collection of the Coin Cabinet a white, powdery corrosion can be observed. Even coins showing an intact superficial appearance can be severely corroded within their bulk. This means that only a thin shell of metal or patina is still present, which can collapse anytime. To enhance the preservation of these antique coins as an invaluable numismatic and historic source, the composition of the objects and the corrosion products is studied within the research project by applying different modern analytical techniques.
1,202 antique Greek and Roman bronze coins from the eastern provinces of the Roman empire, showing different grades of whitish corrosion – in rare cases even already material loss – from the collection of the Coin Cabinet of the Kunsthistorisches Museum Vienna were chosen for analysis. Using different analytical methods it could be detected that the corrosion is mainly based on the formation of lead carbonate. The corroded areas were comprehensively documented using a UV fluorescence microscope; the data are now available for further investigations on the development of the corrosion phenomena. Additional investigations performed by electron microscopy and electrochemical analysis proved the presence of high-leaded bronze alloys and lead-based corrosion products.
Further studies at the Rutherford Appleton Laboratory, ISIS, Oxford, UK, using neutron diffraction enabled the detection of the alloy composition in a non-destructive way, again showing the high lead content of the coins. As the diffraction spectra received are very complex, data evaluation to identify single corrosion phases is still in progress.
In addition, neutron tomography studies were done at the Paul Scherrer Institute, Villigen, Switzerland, clearly showing an accumulation of lead-rich inclusions within the coins’ bulk, explaining the spotted appearance of the corrosion spots/areas. Because of the high lead content of the alloys (up to more than 40 weight%) during their melting no homogeneous mixture of Cu and Pb can be reached anymore, but the formation of a copper-rich as well as a lead-rich phase at the same time. The distribution of the lead-rich phase within the copper-rich matrix can be used as a hint for the manufacturing technique of the coins or coin blanks, which can be cast either using vertical closed or open horizontal moulds. For the investigated Greek bronze coins most probably a closed vertical casting mould was used in many cases – as can clearly be seen from the neutron tomography results. Only some single pieces show a differing distribution of the lead-rich inclusions (e.g. as a concentrated area in the middle of the coin).
Test castings were performed at the KHM to further study the casting technique; the test objects were also investigated using neutron diffraction and neutron tomography and the evaluation also of these results is still ongoing.
In parallel to the analysis of the coins the climatic conditions and environmental pollutions surrounding the objects were studied in and around the historic wooden storage cabinets still used for their keeping in the Coin Cabinet today. Because of the conditions detected, it became clear that the antique coins, sensible for corrosion, should not be kept in the historic cabinets any longer. The unfavourable climatic conditions and the high concentrations of organic acids would promote further corrosion on the coins. As in extreme cases this could even lead to substantial loss of the historically unique objects, they need to be transferred to more favourable environmental conditions. A first step to improve the storage conditions was done by installing a metal case flooded with nitrogen at the Coin Cabinet. The new storage case is completely made of steel, shows no pollutant emitting surface treatment on its inner surfaces, and is filled with nitrogen to exclude external pollutants and keep the oxygen concentration below 1 %. The most sensitive objects are in the process of being transferred to the metal cabinet and will be observed with regard to the development of corrosion in the future.
M. Griesser, R. Traum, K. Vondrovec, P. Vontobel, E. H. Lehmann, “Application of X-Ray and Neutron Tomography to Study Antique Greek Bronze Coins with a High Lead Content. in Proceedings XTACH 2011, IOP Conf. Series: Materials Science and Engineering 37 (2012) 012011;
DI Dr. Martina GRIESSER
René TRAUM (Münzkabinett), HR Dr. Günther DEMBSKI (Münzkabinett), Dr. Klaus VONDROVEC (Münzkabinett)