Program on
Ancient Technologies and Archaeological Materials
at the
University of Illinois at Urbana-Champaign

ATAM is a Division of the Illinois Transportation Archaeological Research Program

A group of 40 Roman coins was borrowed from Loras College Library in Dubuque, Iowa to ascertain composition and minting technology. Participants in the project were Sarah Wisseman (ATAM Program, University of Illinois), Suzanne Seleem (Chemistry, Central State University of Ohio), James Mabon (Materials Research Laboratory, University of Illinois), and Karin Berkhoudt (ATAM and Anthropology, University of Illinois).

The coins belong to a larger collection, purchased by Father Kenneth Downing in the 1970s during his travels to Turkey and other parts of the Mediterranean. Many have been identified by Geoffrey Harrison of Classical Studies at Loras College. We selected 5-10 silver and copper-based coins from the reigns of Valerian (253-260 AD), Licinius (308-324 AD), Gallienus (253-260 AD), Maximinus II (305-313 AD), Constantine (307/312-337 AD), and Gordianus III (238-244 AD).

Roman coins were frequently recycled in antiquity by melting them down and making new objects from the combined alloys. While we cannot trace coins back to original ores without much more information on original provenance (meaning both the sources of metallic ores and the sites of coin manufacture), we can learn something about the composition, corrosion products, and manufacturing methods. For example, from the existing literature on ancient Roman coins, we know that many silver coins in later periods consisted of silver foil over a copper core.

The forty coins were examined non-destructively by binocular microscope at ATAM and scanning electron microscopy and energy-dispersive X-ray analysis (SEM/EDX) at the Center for Microanalysis in the Frederick Seitz Materials Research Laboratory (MRL) at the University of Illinois.**SEM gives high-resolution images while EDX identifies the major elements present.

One coin turned out to be brass (copper-zinc). Two coins were identified as forgeries. In addition to suspiciously clean surfaces, these coins displayed almost pure copper and silver instead of the mixed alloys of the ancient coins (either copper-tin bronze with small amounts of lead and silver, or silver with a little copper).

Most of the ancient coins showed small amounts of earth elements such as Fe, Ca, Al, Mg, O, presumably from dirt still adhering to the coins. At least two corrosion products were present, revealed by Cl (chlorine from copper choride) on copper and S (sulphur from copper sulphide).

Our findings are consistent with other published studies of ancient Roman coins. More data could be acquired using destructive sampling (e.g. metallography or PIXE) that would allow a better understanding of which coins were foil-wrapped. Any further study should allow for a thorough cleaning of each coin surface to get clearer readings of the constituent metals. This procedure would then require recoating with a protective substance to prevent corrosion.

Further Reading: Zwicker, U.; Oddy, A.; and La Niece, S. Roman techniques of manufacturing silver-plated coins. In book. Metal plating and patination. La Niece, Susan and Craddock, Paul T., Editors (1993), pp. 223-246, [English]. [ISBN 0750616113].

Carter, Giles F. and Razi, Hossein. Chemical composition of copper-based coins of the Roman Republic, 217-31 BC. In book. Archaeological chemistry IV. Advances in chemistry series. Allen, Ralph O. (Editor). American Chemical Society (1989), pp. 214-230, [English w. English summary]. 13 figs., 2 tables, 15 refs. [ISBN 0-8412-1449-2].

Klockenkämper, R.; Bubert, H.; and Hasler, K. Detection of near-surface silver enrichment on Roman imperial silver coins by x-ray spectral analysis. Archaeometry, 41, no. 2 (1999), pp. 311-320, [English w. English summary]. 3 figs., refs.

**This facility is partially supported by the U.S. Department of Energy under grant DEFG02-91-ER45439.