I'm not sure when the denarius
stopped circulating at bullion value and started circulating at face
value, but it must have been
by the end of the Republic. Nero
the weight of the denarius
(but not the value), so it must have been circulating at face
value at that point. In theory it then would then have been worth the mint's while to melt down the Republican
and Augustan denarii
and re-mint them as lighter coins. However, they had
to get hold of them first. The only way to do this would be from the flow of taxes through the treasury. People tended to keep back coins that appeared to be superior and so they would have tried to pay their taxes in the newer, lighter coins. Of course, the older and newer coins still had
the same value as each other and Joe Public wouldn't have been able to make a profit by melting down the old because the denarius
was overvalued compared with raw silver. Anyway that's the reason denarii
stayed in circulation for such a long time. Doubtless when the older coins became worn they were less attractive to mint anyway.
Mark Antony's legionary
coins were supposed to have been badly de-based. Whether they survived because they were Republican
era coins and therefore "good" or whether they were de-based and therefore "bad" and were ignored when used to pay taxes is unclear. The coins were apparently made in very large numbers, so that may also account for their survival.
Clearly the continued de-basement of the denarius
was an opportunity for the mint to melt down older coins, but did they? When the antoninianus
was introduced (at one and a half the weight of the denarius
, but twice the value) both coins continued to be minted, whereas you would have expected the denarius
to be taken out of circulation altogether.