The two Roman plates are made of bronze, an alloy of copper and tin. The bronze used to make this military diploma is heterogeneous in texture and composition. In contrast to modern bronze, lead inclusions are common in ancient bronze, which manifests a considerable range in copper and tin.
Lead isotopic compositions in the two Roman plates are the same as those of copper coins produced during the imperial era of Augustus and Tiberius. Metallurgy indicates that the Romans may have mixed ore from Sardinia with ore from southeast Spain.
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