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,
reduced 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.