The antoninianus was a coin used during the Roman Empire that had the value of 2 denarius (in fact, it is also known as double denarius). At the beginning it was a silver coin, but it was gradually devalued up to become a bronze coin.
The coin was introduced with the monetary reform of Caracalla at the beginning of 215 BC. It was completely in silver and similar to the denarius, unless it was slightly larger and it represented the emperor with a radiant crown, pointing out in this way its double value compared to the denarius, like in the dupondio that was worth two asses.
The word antoninianus derives from the name of Caracalla (Mark Aurelio Antoninianus); the ancient name of the coin is not known. The coin is defined also radiate, after the image of the emperor, even if the term is less precise due to its use for the dupondis introduced with the monetary reform of Nero.
Even if it had the double of the value of the denarius, the antoninianus didn't weigh more than 1.6 times the weight of the denarius. The denarius continued to be issued with the antoninianus, but in mid third century AD it was devalued to face the permanent state of war of the period.
During the principality of Gordian III, the antoninianus replaced completely the denarius. Due to the difficult political and economic conditions, the new coin was devalued adding copper and tin, producing in this way an alloy called billon, looking similar to the silver. At the half of the kingdom of Gallienus there were introduced new methods of production to make the coins similar to the silver ones. The coin was produced with a very low content in silver (around 5-10%) and treated with acids in such way that the copper was removed by the surface of the coin, leaving therefore a superficial layer of pure silver. When the coins produced in this way were used, the thin silver surface was easily removed, leaving the underlying copper.
Regardless the fact that these measures were not enough to maintain a silver appearance for the coins, Aurelian reformed the antoninianus with a percentage of 20 copper parts for every silver part. This was marked on the reverse of some coins with the Roman figures XXI in western regions and with the Greek figures KA in the eastern regions. These coins were named aureliani. The silvered antoninianus continued to be issued up to the monetary reform of Diocletian at the end of the 3° century AD.
During the third century (and perhaps also during the forth century) a lot of imitations of the antoninianus were locally issued in the Roman provinces. They are commonly named Barbarous radiate.