This coin has higher copper content than silver.

The Antoninianus or pre-reform radiate, was a coin used during the Roman Empire. The coin was introduced by the Emperor Caracalla in early 215 AD.

Originally this coin style was high purity silver content but went down to a slow as 5%. Traders would keep the older Antoninianus coins for trading, and pay taxes with the new lower grade coins that had high copper content.

Roman billon coins

This term is applied to coins of silver mixed with much copper alloy, or to copper with a small alloy of silver. From the reign of Gallienus to that of Claudius II (ad 253 to 270), almost all the coins were made of billon.