A quick study of the types of coins in circulation during Roman times will show that, far from being static, the money supply was quite fluid, with new denominations being added, and others being removed from circulation.
These changes, however, rarely took place overnight. They tended to be in response to the needs of the authorities, or as a result of a general reform of the monetary system as a whole.
During the chaos that resulted from the Civil Wars that marked the change from the Republican period to the Imperatorial period of Roman history, the various factions fighting for control of the Roman world were required to produce a huge supply of money to pay for their troops, and to supply those troops in the field. In doing so, they also sent the Roman economy into a period of rising inflation. With so much money in circulation, the common bronze coins were no longer produced, and the denarius became the coin that kept the Roman economy, such as it was, in operation.
Following the death of Julius Caesar, and the rise of Octavian, now called Augustus, to power, the Roman mints once again began producing base metal coins, using different alloys for various denominations. Augustus also introduced four new denominations of coin. Two large denominations: the sestertius and the dupondis, and two smaller denominations: the semis, and the quadrans.
The table below shows the types of coins in circulation following the reforms of Augustus, as well as their comparative values:
These standards remained remarkably stable, until around the reign of Caracalla (198-217 AD), with the exception of the denarius.
During the reign of Nero, a small change was made: the dupondis began to be issued with the portrait of the emperor wearing a radiate crown in order for it to be differentiated from the As, which was of a similar size and weight. Also, the silver content of the denarius slipped from about 98% purity to around 94%, a small debasement that would continue through the Severan era.
By the time of the emperor Caracalla, the silver content of the denarius had been reduced to around 50%, and public confidence in the value of the coin had fallen along with it's bullion value. Around 214 AD, Caracalla introduced a new denomination: a large silver coin bearing the portrait of the emperor wearing a radiate crown. These new coins, being around 24mm in size, and near 5 grams in weight, contained around 60% silver, and were most likely valued at 2 denarii.
Today, numismatists often call these coins the "double-denarius", or the "antoninianus".
The denarius itself continued to circulate, but its silver content continued to drop, until it was finally discontinued toward the end of the 3rd century AD, by which time it was a small bronze coin with no silver content at all.
The purity of the new antoninianus, however, continued to fall. By the time of the emperor Claudius II (268-270 AD), the coin had become a small, rather crude bronze coin, with a mere plating of silver. It wasn't until the reign of Aurelian (270-275 AD) that the coin regained some (but not all) of its former glory. Under Aurelian, the coin again was struck on a full flan, and with a content of around 5% silver. His coins also began to carry the letters XXI or the Greek numerals KA, most likely to indicate a bronze to silver ratio of 20:1.
XXI in the exerge
(Note: this subject is still known to cause cyber-fights and flame wars among numismatists who disagree on its meaning).
During the time of Diocletian (284-305 AD), Roman coinage was once again went through a general reform.
The emperor, interested in returning stability to the Roman economy, set the weight of the aureus (also known as the "solidus") at 5.3 grams, and also began issuing a new silver coin of high purity, the argenteus. He also introduced a new base metal coin, containing around 5% silver. Although we don't know for certain what the coin was called at that time, today they are commonly referred to as a follis, after coin bags called folles.
Another new coin also appeared. This coin, as with the antoninianus, showed the emperor with a radiate crown, although it contained no silver content. Today, we call this coin the post-reform radiate, or just radiate.
Note that the post-reform coin no longer contains the XXI, but a mintmark, indicating where it was struck.
The table below shows the coins in circulation following Diocletian's monetary reforms.
Constantine I, the Great also reformed the monetary system during his reign (307-337 AD). He discontinued the production of the aureus, and replaced it with the solidus, a gold coin weighing about 4.5 grams. He also issued two other gold coins, the semissis and the 1.5 scripulum. In addition, he issued two new denominations of silver coins, the miliarense and the siliqua.
Precious metal coins of the reign of Constantine I
Base metal coins also underwent changes during Constantine's reign. The follis, issued under Diocletian with a 5% silver content had steadily decreased in size and precious metal content, until they became rather small coins with no silver content at all. Under Constantine, they once again were raised to 5% silver content. Once again, this was not to last for long. Through gradual debasement, the silver content was gradually reduced, until the coins once again contained no precious metal at all. As their relation to the precious metal coins is not known today, numismatists have placed them in 4 rather broad categories, based on their relative sizes: