The as (plural asses) was a bronze, and later copper, coin used during the Roman Republic and Roman Empire, named after the homonymous weight unit (12 unciae = ounces), but not immune to weight depreciation.
After the as had been issued as a cast coin for about seventy years, and its weight had been reduced in several stages, a sextantal as was introduced (meaning that it weighed one-sixth of a pound). At about the same time a silver coin, the denarius, was also introduced. Earlier Roman silver coins had been struck on the Greek weight standards that facilitated their use in southern Italy and across the Adriatic, but all Roman coins were now on a Roman weight standard. The denarius, or 'tenner', was at first tariffed at ten asses, but about 140 B.C. it was retariffed at sixteen asses. This is said to have been a result of financing the Punic Wars.
During the Republic, the as featured the bust of Janus on the obverse, and the prow of a galley on the reverse. The as was originally produced on the libral and then the reduced libral weight standard. The bronze coinage of the Republic switched from being cast to being struck as the weight decreased. During certain periods, no asses were produced at all.
Following the coinage reform of Augustus in 23 BC, the as was struck in reddish pure copper (instead of bronze), and the sestertius or 'two-and-a-halfer' (originally 2.5 asses, but now four asses) and the dupondius (2 asses) were produced in a golden-colored alloy of bronze known by numismatists as orichalcum. The as continued to be produced until the 3rd century AD. It was the lowest valued coin regularly issued during the Roman Empire, with semis and quadrans being produced infrequently, and then not at all by the time of Marcus Aurelius. The last as seems to have been produced by Aurelian between 270-270 and at the beginning of the reign of Diocletian.