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The Antoninianus was a coin used during the Roman Empire that was valued at 2 denarii. It was initially silver, but was slowly debased to bronze.

The coin was introduced by Caracalla in early 215 AD and was a full silver coin similar to the denarius except that it was slightly larger and featured the emperor wearing a radiate crown, indicating that it was valued twice as much. Although valued twice as much, the antoninianus never weighed more than 1.6 times that of a denarius. The denarius continued to be issued along side the antoninianus, but during the middle of the third century AD was debased rapidly to fund the constant warfare of the period.

The antoninianus replaced the denarius after the reign of Gordian III, and the latter was no longer struck in appreciable quantities. As political and economic conditions worsened and the coins were first debased simply by adding copper and tin to produce a billon alloy that looked similar to silver. By the middle of the reign of Gallienus new methods were introduced so that coins continued to appear silver. The flans were produced of very low silver content (about 5-10%) and then pickled so that the copper on the surface of the coin was leached away. When struck these coins had a thin silver layer that quickly wore away to reveal the copper beneath.

These coins are usually referred to as "silvered" as opposed to "silver" by numismatists. Eventually even these measures weren't enough to maintain a silver appearance of the coins so Aurelian introduced a reform and produced an antoninianus that was set at a distinct fineness which was twenty parts copper to one part silver that was marked on the reverse of the issues (XXI in the west and KA is the east). The silvered antoninianus continued to be issued until the monetary reforms of Diocletian at the end of the third century AD.

During the third century (and perhaps also during the fourth century) many locally made imitations of the antoninianus were produced. These are usually referred to as barbarous radiates, although most were probably produced within the empire and probably filled the need for small change. These coins are characterized by blundered and poorly engraved portraits and designs on small flans of copper. The most frequently imitated coins are those of the Gallic emperor Tetricus I.

The word antoninianus is a modern term based on the name of Caracalla (Marcus Aurelius Antoninianus), who was the first to issue such a coin; the ancient name of the coin is not known. The coin is also referred to as a radiate, from the radiate crown worn by the emperor. Since they were issued in large numbers, they are second only to Constantinian bronzes in abundance on the collector's market.

row 1: Elagabalus (silver 218-222AD), Trajan Decius (silver 249-251AD), Gallienus (billon 253-268AD Asian mint);
row 2: Gallienus (copper 253-268AD), Aurelian (silvered 270-275AD), barbarous radiate (copper), barbarous radiate (copper)

See also: Roman currency.