Solidus.- See GOLD COINAGE.- See Exagium Solidi.
The "Solidus" (plural: solidi) was a revision instituted about 310 by Constantine I to the old Roman gold coin standard, the "aureus". The word "solidus" is a Latin adjective which means "trustworthy" and was probably applied to contrast with the wild gyrations in weight which the aureus had experienced in the third century. The aureus had been struck at 60 to the "libra" (Roman pound) since about 287, which meant that 5 aurei were struck from each "uncia" (Roman ounce). Constantine aligned the coinage with the normal system of Roman weights by reducing the weight of the gold coin so that 72 were struck from each libra, or 6 from each uncia. Some early solidi bore the mark 'LXXII' (Roman numerals for '72') on the reverse as a mark of value. The Romans had a name for a sixth (1/6) of an uncia, the "sextula". Since the Romans also had a smaller unit of weight known as the "scripulum" (sometimes known as "scrupulum" or scruple) which was a twenty-fourth (1/24) of a Roman ounce, the solidus is sometimes referred to as being on the "four-scruple" standard.
Although Constantine I instituted the new denomination within his domain, the other rulers of the Tetrarchal Period did not adopt the new standard and continued to strike on the old aureus standard. After the defeat of Maxentius at the Battle of the Milvian Bridge on October 28, 312, only Constantine I and the Licinii (Licinius I and his son Licinius II) were left to rule the Roman world. Even then the Licinii refused to adopt the new standard, so for another twelve years the Romans had two different gold coins in circulation, related to each other by the value ratio of 6:5 (six solidi were worth five aurei).
Only with the defeat of the Licinii in the Second Civil War in 324 and the emergence of Constantine I as the sole ruler of the Roman world was the solidus established as the standard Roman gold coin. The first post-Second-Civil-War solidus issue and thus its first emission as the absolute Roman standard was an anepigraphic-obverse issue from Sirmium which first introduced the diadem to Roman coinage. The solidus was to remain the standard of golden coinage for over 600 years.
As with almost all Roman gold, the solidus was maintained at a very high fineness, approximately 98%. The theoretical weight of a solidus as 1/6 of an uncia was about 4.55 grams, but since gold was over-tariffed by the authorities by about 3% (coined gold was worth slightly more than bullion gold), most solidi were issued at about 4.42 grams. Although the weight of the aureus had fluctuated greatly during the Empire, once the gold coin was 'pegged' to a definite weight standard the weight was held very constant for the entire life of the solidus denomination. A Roman solidus is typically between 20 and 21 millimeters in diameter.