The antoninianus is a Roman double denarius coin denomination (pl. antoniniani) struck from 215 to 293 A.D. (or 274 if you consider the later issues, sometimes called aurelianiani, a new denomination). On the obverse of the antoninianus the emperor is depicted wearing a radiate crown, caesars are bare-headed, and empresses are shown with a lunar crescent behind their shoulders. On some later antoniniani the emperor wears a helmet. The ancient name for the type is unknown. Our modern name for it, antoninianus, names it after Marcus Aurelius Antoninus (nicknamed Caracalla) who introduced it in A.D. 215. Although the antoninianus was valued at two denarii, the weight was considerably less than that of two denarii. During the reign of Gordian III the antoninianus replaced the denarius as the primary Roman denomination. The first antoniniani were first struck with an alloy containing about 49.5% silver, but the denomination was continually debased until by 274 the alloy contained only 2.5% silver. In 274, Aurelian reformed the radiate denomination, striking on a copper core with a 5% silver coating, and valuing the new coin at 5 denarii commune. Older antoniniani no longer circulated after this reform and the radiate coins struck after the reform of 274 until 293 are also called aurelianianus. The antoninianus (or aurelianianus) was used until reform of Diocletian in 293.
Antoninianus is the modern name for the silver coin introduced by the Emperor Caracalla (q.v.) in the course of 215 AD. Its ancient name is not known, so it has been named after Caracalla's proper name. The name Caracalla, a nickname which was never used officially, came from a style of long Gallic robe which he made fashionable at Rome. He was given the name of Marcus Aurelius Antoninus by his wishful-thinking father Septimius Severus (q.v.) when Caracalla was elevated to Caesar in 195. Unfortunately for Rome Caracalla bore no resemblance to either Marcus Aurelius (q.v.) or Antoninus Pius (q.v.), and hence is better known to history as Caracalla.
Caracalla murdered his popular brother Geta (q.v.) in 212 and very soon afterward increased the soldiers' pay by approximately 50% to keep them loyal, since Geta was their favorite of Severus' two sons. The result was a financial crisis which Caracalla tried to meet by two measures: debasing the currency and increasing tax revenues. Under the Severans the silver coins had already dropped from about 71% fine to about 50% fine, and any serious further debasement there would give them the appearance of billon. Therefore an avenue other than debasement was needed.
In the course of 215 he instituted the new silver denomination known today as the 'antoninianus' which, although it was probably tariffed at two denarii, weighed only 5.10 grams as against the 6.30 gram weight of two denarii. Its typical diameter was approximately 22-23 millimeters. It is not really known what the legal ratio of antoniniani to denarii was, although the fact that the government was in a fiscal crisis and the even more compelling fact that Caracalla's and Julia Domna's portraits on the antoniniani were radiate lead most to believe that a 1:2 ratio applied. The standard gold coin, the aureus, dropped from a weight of 7.20 grams in 214 to 6.40 grams in 215. Caracalla also granted citizenship to nearly all of the provincial inhabitants, partially with a view toward establishing an empire like Alexander the Great's, but also with an eye towards subjecting them to taxes, such as inheritance and emancipation, which only applied to Roman citizens. The financial crisis drove him to create new taxes and increase old ones.
Caracalla was assassinated on April 8, 217 and his successor Macrinus (q.v.) issued the new denomination in very small quantities. His successor in May of 218, the depraved Elagabalus (q.v.), resumed striking considerable quantities of antoniniani, but when he was succeeded by his no-nonsense cousin Severus Alexander (q.v.) in March of 222, the production of antoniniani stopped (another sign that the legal ratio was probably 1:2). Michael Grant in his book "The Severans" made the sensible suggestion that Alexander's issuance of a series of coins bearing the inscription "RESTITUTOR MON" (Restitutor Monetae -- the restoration of the coinage, or possibly the restoration of the mint) may refer to the discontinuance of the antoninianus, which did indeed restore the integrity of the monetary system. When Alexander was succeeded by the crude soldier Maximinus I (q.v.) in March of 235 production was not resumed.
It was not until the accession of Balbinus (q.v.) and Pupienus (q.v.) in 238 that production resumed, probably due to the emergency cash needs of the civil war with Maximinus. Their three-month reign was succeeded by Gordian III (q.v.) who not only debased the silver to about 40% fine and reduced the weight of the antoninianus by 10%, but made the antoninianus his dominant silver issue.
The six-year reign of Gordian III was the death knell for denarii, which are extremely rare after his reign. The disappearance of denarii is simply another confirmation that the official ratio of antoniniani to denarii was probably 1:2, since if that were true it would simply prove "Gresham's Law" again. Sir Thomas Gresham (1519?-1579) was an English merchant and financier who founded the Royal Exchange while in the service of Queen Elizabeth I. His name was given to the long-known economic principle that "bad money drives out good money". Simply stated, if depreciated or debased coinage circulates concurrently with coinage of higher intrinsic value, the higher intrinsic value coins will disappear due to hoarding. Gresham's Law applied many times over during the course of the Roman Empire, with some notable examples being the debased legionary denarii of Marc Antony and the debased "post-reform" denarii of Nero (q.v.), which, according to hoard evidence, both continued to circulate long after their higher-intrinsic-value contemporaries had ceased to do so.
After the denarius had been driven out of circulation, the cash-hungry government began to debase the antoninianus. By 258 in the reign of Valerian (q.v.) the antoninianus was about 20% silver, and the government began to treat the coins with a "surface silver enrichment process" (commonly though probably incorrectly called a "silver wash") to improve their appearance and make them seem to be of a higher fineness than they were. The "surface silver enrichment process" continued to be applied to the antoniniani until the end of their production, but the enriched layer was thin and quickly wore off. It is even believed that many of the newly manufactured antoniniani were chemically treated by enterprising citizens to "harvest" the surface silver. Only ten years later at the end of Valerian's son Gallienus' (q.v.) rule the antoninianus was only about 4% silver.
A result of the nearly total debasement of the antoninianus was the disappearance of the standard bronze coins of the first 250 years of the empire, the sestertius, the as, and the dupondius. The miserable antoninianus was essentially a very small bronze coin of about three grams, but the government claimed that it was worth much more than the large bronze denominations. Gresham's Law once again prevailed and all the large bronzes were removed from circulation and the government essentially discontinued their production during the reign of Valerian. Aurelian (q.v.) later tried without success to re-introduce them.
Aurelian about 271 increased the average weight of the antoninianus, but it remained at about 4% silver. Some believe that Aurelian re-tariffed the coin to four denarii, and thus created in essence a new denomination (called in modern times an 'Aurelianus' for lack of a better term), although the resemblance to the antoninianus is so striking that it is inevitable that the old denominational term is still employed. Aurelian also introduced the enigmatic mark XXI on the coins, the meaning of which is a matter of debate today. The two main theories are that it means the coin was valued at 20 sestertii and that the number is a ratio of copper to silver (20:1). Although the only people who knew for sure have been dead for over 1600 years, Numus leans to the ratio theory for the following eight reasons:
1) The XXI, if it is really a ratio of 20:1, means 4.76% silver, which equates admirably with the assayed value of these coins, being between 4% and 5% silver.
2) The multiples of 10 probably meant next to nothing to the Romans. There is no other Roman denomination that stood in a 10:1 or 20:1 relationship to another. The Roman system of weights or coinage never used any power of ten from the republic until the mid-fifth century.
3) There are coins marked XI which are interpreted as "doubles" but are not generally any larger and heavier than the normal XXI coins. If they were indeed doubles but of approximately the same size, the only thing that could have made them doubles is double the silver content. The phenomenon of ignoring a coin's bulk in its valuation, and only looking at the net silver content is well established in late Byzantine numismatics.
4) Aurelian's reform of the antoninianus really just set it back in line with the traditional relationship with the denarius. At the same time as he reformed the antoninianus he minted large quantities of denarii, the first since Gordian III. His denarii average about 2.51 grams, while his post reform antoniniani average 3.93 grams. Thus he was simply getting the antoninianus back into its original (in the time of Caracalla) relationship with the denarius (Caracalla's ratio was about 1.62:1, and Aurelian's was about 1.57:1)
5) Small quantities of silver were very meaningful to the ancients. The Romans had the capability to make copper with only a trace silver, and to find 4-5% is quite a deliberate action, as in the billon coinage of Alexandria which remained in the same range. After the Diocletianic reform of 294, the folles were about 4% silver while the "post-reform radiates" were totally devoid of silver. There we see simultaneous production of 4% silver and 0% silver copper coins. Later the majorina (q.v.) was about 3% silver, which led to its discontinuance, since the silver content was too small to be apparent to the eye, and enterprising forgers soon made a business of melting them, extracting the silver, and re-striking them from the remaining bronze. Constantius II and Magnentius tried to deal with that by decreasing the silver content of the majorinae to less than 1% sometime about 352, along with a weight reduction. Even that failed to solve the problem and in 354 Constantius II put the forgers out of business by de-monetizing all previous billon issues, and produced only bronze coins of about 2.5 grams.
6) Aurelian made a rather comprehensive reform of the coinage, issuing denarii, asses, sestertii, etc. in substantial numbers. Why would he only put a consistent plain mark of value on the antoninianus? If this is a mark of value in a new system, we would expect all of the coins to bear a mark of value, a la the 498 reform of Anastasius. Other late Roman coinage bears marks telling how many to the pound (the LXXII on the solidi, the XCVI on the earliest siliquae, etc) and Allectus used the 'Q' as a denominational mark on his quinarii, but none were values expressed in multiples of a base unit, although there is some speculation about some of the marks of the tetrarchy. But it's easier to recognize "traditional" Roman denominations in Aurelian's reform than after Diocletian's, and thus there was perhaps a need then to mark them at that later date.
7) Why would Aurelian use a ratio for the mark of value rather than just the number of units as earlier and later coins did? But if he meant to express the silver percentage, the XXI mark is natural.
8) The fact that the coin was an antoninianus was visually obvious, and the coin would not need a value mark any more than other denominations. The fact that it was 4-5% silver was significant, but not obvious and would need indication.
After Aurelian the antoninianus remained at about 4% silver and roughly the same weight until the great coinage reform of Diocletian (q.v.) in 294 led to its discontinuance. Diocletian began the substantial issue of silver again and thus eliminated the need for an "ersatz" silver coin, although he instituted the issue of a coin similar to the antoninianus but with no silver content called "post-reform radiates". Even those were discontinued in the west by about 299 and in the east by about 307. The place of the billon antoninianus was taken by the "surface silver enriched" follis, which weighed about ten grams (as opposed to the antoninianus' three) and also had a silver content of about 4%.