Roman coinage was slowly debased from the time of Nero until a point where most coins contained very little silver. The monetary reform of Aurelian in 274 set standard for silver in the antonianus at twenty parts copper to one part silver, and the coins were marked to indicate the silver content (XXI in Latin or KA in Greek). Further declines followed. Silvering was used to improve the silver appearance of these debased coins. The exact method used is uncertain. The blank flans were probably dipped in an acid before striking, which would leach away the copper from the surface resulting in a very thin coating of silver.