A Metallurgical Provenance Study of the Marcus Herennius Military Diploma
The bronze used to make the military diploma for the Roman soldier Marcus Herennius in AD 109 is heterogeneous in texture and composition. In contrast to modern bronze, lead (Pb) inclusions are common, and the bronze shows a considerable range in copper (Cu) (73 to 92.7 weight percent) and tin (Sn) (6.1 to 26.5 weight percent). Lead isotopic compositions are identical to those of copper coins produced during the Imperial Era of Augustus and Tiberius and may indicate mixtures of ores from southeastern Spain and Sardinia. The combination of these analytical results provides permissive evidence that the plates are authentic artifacts manufactured at the turn of the second century AD.