Griesser/Traum abstract 2007
Unexpected risk in coin collections: causes for the infestation of gold coins and medals by ‘brown spot corrosion’ and first attempts for handling the phenomenon, I and II: Martina Griesser and Rene Traum, Kunsthistorisches Museum, Vienna
Altogether 202 “Austrian” gold coins from the collection of the Münzkabinett (Coin Cabinet) at the Kunsthistorisches Museum (KHM), Vienna, minted between 1790 and 1938, exhibit red-brown to brown-black spots (so-called “brown spot corrosion”) to varying degrees. The oldest coins were minted under Leopold II, the newest in the last year of the “Ständestaat.” In between, gold coins from all sovereigns up to and including Karl I as well as those of the First Republic are represented. A further 199 medals from the Hapsburg medal series in the Vienna Cabinet, from the period 1738-1879 – that is, from Emperor Karl VI to Emperor Franz Joseph – also show “brown spot corrosion.”
Using a series of modern analytical methods for scientific investigations the phenomenon of “brown spot corrosion” could be intensively studied on these coins and medals at the KHM within a four years research project sponsored by the Jubiläumsfonds der Oesterreichischen Nationalbank (OeNB) (“Investigation and Restoration of Corrosion Damage on Minted Gold Coins and Medals,” project number 9117). The incidence and degree of the brown spots was first light-microscopically documented for all objects. Selected pieces were further investigated by Scanning Electron Microscopy (SEM), Scanning Acoustic Microscopy (SAM), and electrochemical methods. The results continuously showed that the brown spots were caused by embedded silver or silver/copper particles, which had oxidised over time due to sulphur-containing air pollutants (as present in the historic wooden (oak) storage cases for objects held in the Viennese collection) to silver or copper sulphide – and thus become brown. Only in single cases could other metallic inclusions such as iron, or embedded soiling particles, be identified.
In collaboration with Münze Österreich AG (the Austrian Mint), test ducats were struck which deliberately contained added silver or silver/copper impurities. These test ducats were then both stored in the (oak) wooden storage cases in the Coin Cabinet of the KHM, and artificially corroded using potassium polysulphide (K2Sx). As both cases showed a corrosion phenomenon comparable with the original objects, they served as test and comparison pieces for the further analyses, particularly for those with Secondary Ion Mass Spectrometry (SIMS). The SIMS investigations executed in cooperation with the Vienna University of Technology confirmed the existence of thin layers (up to 100 nm thick) primarily composed of silver sulphide on the surfaces of the foreign inclusions, both on the test ducats and on two selected historic coins. In addition to carbon, hydrogen, and oxygen, chlorine, an element likewise potentially responsible for corrosion, could also be identified. An attempt using Time of Flight SIMS (TOF-SIMS) primarily to further investigate the influence of organic components on the development of the “brown spot corrosion” – such as the remains of microcrystalline wax coatings on the object surfaces or increased acetic acid concentrations from the woods used in the storage cases – was unfortunately unsuccessful due to the complexity of the signals.
Initial cleaning tests on a few selected pieces showed finally that the brown spots could best be removed using electrochemical methods, in this case with sodium hydroxide. As was shown with the test ducats, this superficial cleaning combined with further storage of the objects in their wooden cases would only bring a temporary solution of the problem. The silver or silver/copper inclusions remaining in the object surfaces after the treatment would over time tarnish to brown through renewed sulphide formation.
As considering the question as to the further handling of the demonstrated foreign inclusions, i.e. their possible removal, is inseparably linked to their probable cause, studies of the historic literature on coin production in the relevant time period were carried out as part of the project. Surprisingly, references to possible contamination of the coin metal through the parallel processing of silver and gold in the primarily Austrian, Italian, and German mints considered were repeatedly found in contemporary reports. Especially the – documented – use of the same rolling and machining tools for both metals led to embedded impurities in the form of foreign particles of silver in gold strains, sheets, and blanks. For the production of medals there exists still greater danger of impurities through the use of a single die for medals from different metals (gold, silver, and bronze as well as possibly other non-precious metals).
Although they are aesthetically disturbing, one should not impute a solely negative aspect to the corrosion spots investigated here, as the information content of these defects is not to be underestimated. For researchers and scholars, the existence of such foreign inclusions, as could be shown, provides new insights into industrial techniques, contemporary materials, tools employed, and so on. The few historical references from the archival materials with the results of our work yielded new correlations, which made us aware of how the damage phenomenon came to be. This not only enabled us to practically confirm the spot formation on gold objects for the ducats struck during the project, but also the selection of the optimal investigative and analytical methods to obtain reliable results.
The understanding of the corrosion processes taking place on the metal inclusions is not only decisive for the conception of the best possible conservation or restoration, but is also an important component in the development of optimal storage conditions for these historically valuable objects in the collection. Only when all deciding factors are known can effective measures against renewed corrosion be met, or it is at least possible to minimise this. Thus within the project presented here, an integral scientific basis for the decision regarding the further treatment of the relevant objects in the collection of the Viennese Coin Cabinet could be gained, which will inform concrete measures in the future.