The sestertius (: sestertii), or sesterce (: sesterces), was an ancient Roman coin. During the Roman Republic it was a small, silver coin issued only on rare occasions. During the Roman Empire it was a large brass coin.
The name sestertius means "two and one half", referring to its nominal value of two and a half asses (a bronze Roman coin, singular as), a value that was useful for commerce because it was one quarter of a denarius, a coin worth ten asses. The name is derived from semis, "half" and "tertius", "third", in which "third" refers to the third as: the sestertius was worth two full asses and half of a third.
English-language sources routinely use the original Latin form sestertius, plural sestertii; but older literature frequently uses sesterce, plural sesterces, terce being the English equivalent of tertius. A modern shorthand for values in sestertii is IIS (Unicode 𐆘), in which the Roman numeral II is followed by S for semis, and the whole struck through; but because this symbol and striking through letters are not always convenient, HS may be used instead, with the horizontal bar of the 'H' representing the strike through the numeral II, rather than the letter H.
History
The sestertius was introduced as a small silver coin valued at one-quarter of a denarius (and thus one hundredth of an aureus). A silver denarius was supposed to weigh about 4.5 grams, valued at ten asses, with the silver sestertius valued at two and a half asses (1.125 grammes). In practice, the coins were usually underweight.
When the denarius was retariffed to sixteen asses (due to the gradual reduction in the size of bronze denominations), the sestertius was accordingly revalued to four asses, still equal to one quarter of a denarius. It was produced sporadically, far less often than the denarius, through 44 BC.
In or about 23 BC, with the coinage reform of Augustus, the sestertius was reintroduced as a large brass denomination, while the as, now made of copper, was worth one quarter of a sestertius. Augustus tariffed the value of the sestertius as one hundredth of the gold aureus. The sestertius was produced as the largest brass denomination until the late 3rd century AD. Most were struck in the mint of Rome but from AD 64 during the reign of Nero (AD 54–68) and Vespasian (AD 69–79), the mint of Lyon (Lugdunum), supplemented production. Lyon sestertii can be recognised by a small globe, or legend stop, beneath the bust.
The brass sestertius typically weighs in the region of 25 to 28 grams, is around 32–34 mm in diameter and about 4 mm thick. The distinction between bronze and brass was important to the Romans. Their name for brass was orichalcum, also spelled aurichalcum (echoing the word for a gold coin, aureus), meaning 'gold-copper', because of its shiny, gold-like appearance when the coins were newly struck (see, for example Pliny the Elder in his Natural History Book 34.4).
Orichalcum was considered, by weight, to be about double the value of copper. This is why the half-sestertius, the dupondius, was around the same size and weight as the bronze was, but was worth two asses.
Sestertii continued to be struck until the late 3rd century, although there was a marked deterioration in the quality of the metal used and the striking even though portraiture remained strong. Later emperors increasingly relied on melting down older sestertii, a process which led to the zinc component being gradually lost as it burned off in the high temperatures needed to melt copper (zinc melts at 419 °C but boils at 907 °C, copper melts at 1085 °C). The shortfall was made up with bronze and even lead. Later sestertii tend to be darker in appearance as a result and are made from more crudely prepared blanks (see the Hostilian coin on this page).
The gradual impact of inflation caused by debasement of the silver currency meant that the purchasing power of the sestertius and smaller denominations like the dupondius and as was steadily reduced. In the 1st century AD, everyday small change was dominated by the dupondius and as, but in the 2nd century, as inflation hit, the sestertius became the dominant small change. In the 3rd century silver coinage contained less and less silver, and more and more copper or bronze. By the 260s and 270s the main unit was the double-denarius, the Antoninianus, but by then these small coins were almost all bronze. Although these coins were theoretically worth eight sestertii, the average sestertius was worth far more in plain terms of the metal it contained.