A rather technical query, to say the least !
According to Harold Mattingly ( a very distinguished Numismatist Keeper for over 37 years of the British Museum's coin collection), the Augustan system was as follows.
In the late Republic/Civil wars period, coin standards understandably slipped and this was the situation Augustus inherited. He standardised the gold aureus at 42 to the pound and fixed it's value at 25 denarii; the silver denarii at 84 to the pound. These were the general coinage of the Empire.
The bronze As/Aes was more commonly used in Italy, and since a chaotic situation with no fixed rules had occurred, Augustus was free to innovate and introduce an entirely new system. Whereas one metal ( bronze - remembering ancient bronze was more akin to our brass) had been used, Augustus introduced two - orichalcum/brass and pure copper, the former for the sestertius and dupondius, the latter for the As/Aes and Quadrans.
There are several points of note here:
1. The two metals, orichalcum/brass and copper are based on different standards - viz, the sestertius, weighing one ounce
presumes a theoretical As/Aes of Brass/orichalcum weighing quarter of an ounce ( which never existed) whereas the
actual As/Aes was of copper and weighed two-fifths of an ounce.
2. The sestertius in silver disappears and is no longer struck.
3. The only division of the As/Aes now struck is the quadrans ( the earlier triens, sextans and uncia disappear)
For completeness, here is the whole Augustan system:
1 Gold Aureus (122.9 gr; 7.96gm) =25 denarii=100 sestertii =400 Asses
1 Gold Quinarius(61.46 gr;3.98 gm)=12.5 denarii=50 sestertii=200 Asses
1 Silver Denarius(61.46 gr;3.99 gm)= 4 sestertii = 16 Asses
1 Silver Quinarius(30.73 gr;1.995 gm =2 sestertii = 8 Asses
1 Orichalcum/Brass Sestertius (1oz) = 4 Asses
1 Orichalcum/Brass Dupondius ( 1/2 oz) = 2 Asses
1 Copper As/Aes (2/5 oz) = 1/4 sestertius
1 Copper Quadrans (1/16 oz) =1/4 As/Aes
The ratios are not market place values, but set artificially at:
Gold to Silver at 1:12.5
Silver to Orichalcum/Brass at 1:28
Silver to Copper at 1:45
Brass to Copper at 28:45 ( almost 5:8 )
Gold and Silver were struck almost pure and very true to standard weight; Brass and Copper much more carelessly at so many to the pound.
The Orichalcum/Brass was 80% copper:20% zinc and appears to be a natural alloy obtained first from the Bergomate territory in Germany, and later from Mons Marianus in Spain. The Copper was struck almost pure.
The Augustan system remained in use until the reforms of Nero. Hope this answers the question......
As to the Bronze coins of the 2nd C BC, a 'new' coin, the denarius is introduced around 169 BC after the Eastern Wars, and is accompanied by 'new' bronze Asses of two-ounce weight, often called sextantal, and often over-weight; at ten Asses to the denarius, re-tariffed at 16 to the denarius in 117 BC and the Ass/Aes became one ounce.
Thereafter the types constantly change through the period of the Social Wars. By 90 BC the As/Aes had been reduced to half an ounce ( the Lex Plautia Papria of the at year) due to inflation in the Social War. A few years later the issue of bronze in Rome ceased.
The 'bronze' used was a copper-tin alloy with traces of lead and silver which varied considerably in composition, roughly 70-80% copper; 4-10% tin;13-20% Lead......
