Technological study of ancient byzantine 12 Nummi coins

The Coin Cabinet of the Kunsthistorisches Museum Vienna (KHM) holds a set of 856 ancient Byzantine bronze coins, so-called 12 Nummi, from an unknown coin hoard. These coins were minted in Alexandria and numismatic studies were able to date them to the 6^th century AD, i.e. the reigns of Justinus I. (518-527), Justinianus I. (527-565), Justinus II. (565-578), Tiberius II. (578-582), and Mauricius (582-602).

Currently the set of 12 Nummi is investigated with regard to technological questions within a research project of the Coin Cabinet. Microscopic documentation of the whole set has already been completed, further non-destructive investigations using X-ray fluorescence analysis (XRF), scanning electron microscopy (SEM-EDS) and µ-X-ray diffraction (µ-XRD) are still ongoing. The first results suggest the use of high leaded bronze alloys for the 12 Nummi. Therefore, the formation of inhomogeneous phase distributions during the production of the coin blanks (which are subsequently used for minting the coins) is expected: Two different phase mixtures (one rich in Cu-Sn and the other one rich in Pb) should form during the casting process.

Not only the production of the 12 Nummi in the mint of Alexandria during the 6^th century AD, but also their comparison with the techniques used for contemporary imitative coins is of high numismatic value. The coin hoard also contains 65 of these imitative coins, for which any information on their production either in Alexandria (i.e. similar composition and production technique to the original coins) or in a special workshop for forgeries (i.e. clear deviation in composition and/or texture) is still missing today.

For studying museum objects non-destructive analysis is an absolute requirement. Based on the earlier experiences of combining non-destructive neutron-based tomography and diffraction/texture analyses to get information on the bulk composition as well as the production techniques and the state of preservation of antique bronze coins, investigations on a statistically relevant number (ca. 50) of original coins will be performed at the Rutherford Appleton Laboratory, Oxford, UK. These studies will be complemented by the investigation of laboratory-prepared model blanks, cast in different techniques (i.e. cast-only, cast and minted, cast in vertical/ horizontal moulds, struck in cold and hot conditions) to allow for the development and the revision of the evaluation of tomographic and diffraction data.