The bronze or orichalcum (orichalcum - a metallic alloy, regarded by the ancient as a sort of bronze. How little acquainted they were with its true formation is shown by the fact that, deceived by its color, they supposed gold to be one of its constituents, and then perpetuated their error by a false orthography, [aurichalcum]) Semis is fraction in the Roman monetary system. It is worth half As. The weight of the Semis varied from 3.0 to 4.0 grams but the weight may be lower depending of the weariness of the coin.
The denomination system Early Empire* has the following structure:
Aureus = 375 Asses Gold AV 7.2 - 7.9 g. 19 - 20 mm **
Quinarius AV = 187.5 Asses Gold AV 3.6 - 4.0 g. 13 - 15 mm **
Antoninianus = 32 Asses Silver AR 4.7 – 7.0 g. 21 – 24 mm **’ ***
Denarius = 16 Asses Silver AR 2.7 - 3.8 g. 17 - 19 mm **
Quinarius AR = 8 Asses Silver AR 1.8 – 2.0 g. 14 – 15 mm **
Sestertius = 4 Asses Brass AE 24.0-30.0 g. 25 - 35 mm **
Dupondius = 2 Asses Brass AE 9.0 – 13.0 g. 26 - 28 mm **
As = As Copper AE 8.0 – 11.0 g. 24 - 28 mm **
Semis = ½ As Copper AE 3.0 – 4.0 g. 17 - 19 mm **
Quadrans = ¼ As Copper AE 2.0 – 3.0 g. 14 - 16 mm **
* The Monetary System of Ancient Rome was a conservative but dynamic structure. For additional brief information please review also The Monetary System of Roman Republic, Monetary Reform of Diocletian, Monetary Reform of Constantine the Great.
** All sizes and weights are given for regular, well preserved issues in FDC, Mint State or Extremely Fine condition. Depending of the thickness of the coin the diameter may varied out of the given above dimensions.
*** The Antoninianus was introduced by Caracalla during his reign in early second decade of Second Century AD. Designed as a coin with high purity of the silver (about 60 %) the antoninianus was quickly debased and the silver content during the reign of Aurelian was only 5 %. The stages of debasement of this type reflect in its name Silver Antoninianus, Billon Antoninianus, AE Antoninianus. Typical for the male personification on the obverse is the Radiate Crown and for the female – situating the bust on a Crescent.