This coin has high silver content

size apro

as per images

The Antoninianus or pre-reform radiate, was a coin used during the Roman Empire thought to have been valued at 2 denarii. It was initially silver but was slowly debased to bronze with a minimal silver content. The coin was introduced by 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 coin 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