Monetary alloys in Iron Age Armorica (Finistère, France): The singular case of the Osismi tribe

• No tri-metallic system.
• Debasement of gold alloys.
• Chronological chemical pattern.
• Presence of Sn and Sb.
• Attribution of unknown coins.

The analysis by PIXE and PAA of 64 coins struck in Iron Age Armorica by the Osismi tribe revealed the use of a different system from the usual Celtic Gaul tri-metallic system. The gold-based alloy (Au–Ag–Cu) firstly issued is debased over time to become a silver-based alloy (Ag–Cu–Sn). Based on the analytical data, two chronological phases were defined and dates of issuing could be ascribed to the coin-types. The presence of Sn and Sb in the alloys and the low contents of Pb were used in the attribution of 9 specimens of unknown origin to the Osismi monetary system. Considerations on the mints supplies could also be provided.