Roman Coins

[Mithradates]

What im interested in is the life span of Roman coins. For instance i was part of an archaeological dig which found a silver denarius in the north of England. This coin was minted in the reign of Nero. Would this coin have been in Roman currency much later for instance after the Romans had occupied most of England. Or would it have been a remnant of trade taking place before the Romans. What im saying is how long were they used for did people keep old coins with old emperors on them long after they were dead? Or were there re-issues and replacements? Any interesting information regarding Roman coins and the like as well would be appreciated as well.