Marta Rodrigues1,2
M. Mäder1
M. Melcher1,2
M. Schreiner1,2
N. Schindel3
M. Alram3,4
1) Institute of Science and Technology in Art, Academy of Fine Arts, Schillerplatz 3, A-1010 Vienna/Austria
2) Institute of Chemical Technology and Analytics, Vienna University of Technology, Getreidemarkt 9, A-1060 Vienna/Austria
3) Numismatische Kommission, Academy of Sciences, Ignaz-Seipl-Platz 1, A-1010 Vienna/Austria
4) Münzkabinett, Kunsthistorisches Museum, Burgring 5, A-1010 Vienna/Austria
During excavations carried out by a team of archaeologists from Izmir University in Turkey, headed by Prof. Rahmi Ünal at the medieval site of Becin Kalesi during the summer 2000 a great number of coins could be found. It soon became clear that this archaeological find is one of the most important discoveries in Turkey. It is one of the most important findings of coins - named after its find-spot "Hoard of Becin" - and is even the largest coin hoard ever found in Turkey in total numbers and the most important Ottoman treasure ever discovered.
In a co-operation between the Turkish and the Austrian Academies of Sciences a project has been carried out in order to catalogue all the coins and to perform systematic investigations including the chemical analyses of the material used for the production of the silver coins. The coins of main interest stem from the Ottoman Empire and were struck during the 16th and 17th centuries under the Sultans Murad III, Mehmed III and Ahmed I, in 18 different mints. As most of the coins presented a green patina on their surfaces, it was agreed to take small samples of approximately 400 objects typical for the period as well as the mints. In the past, a great number of analyses on ancient silver coins has been carried out using XRF (x-ray fluorescence analysis), mostly on the surface of the objects. However, due to the corrosion of such coins, a depletion of copper and hence an enrichment of silver must be observed in the near surface regions, and so the majority of analyses performed up to now are not reliable. The determination of the chemical composition of the coin's core is essential, which means that either sample material has to be taken or the coin has to be cross-sectioned.
The small specimen were embedded in synthetic resin, cross-sectioned and polished in order to achieve a flat surface. Investigations were carried out by means of micro-x-ray fluorescence analysis with a self-built instrument*), where a diameter of the primary x-ray beam of approximately 150 µm can be achieved. Further analyses were performed with energy dispersive x-ray microanalysis in a scanning electron microscope (ESEM/EDAX). The spectra obtained were evaluated quantitatively concerning the main components silver and copper as well as the minor constituents such as Pb, Bi and Au. In some cases also mercury could be detected. In average the silver content is between 90 and 95%. A statistic evaluation of the results by principal component analysis (SPSS-software) enabled a clustering of the objects as well as a clear correlation of the various coins to specific mints.
*) COPRA: A Compact Röntgen Analyser, EU Project No STM4-CT98-2237; Prof. Dr. K. Janssens, G. Vitiglio, University of Antwerp/Belgium; Dr. J. Heckel, Dr. P. Klinger, SPECTRO Analytical Instruments, Kleve/ Germany; Prof. Dr. M. Schreiner, Academy of Fine Arts Vienna/Austria