AD 260: the year the empire broke apart
Within a few months an emperor was captured by the Persians, Gaul seceded, the East passed under Palmyrene control, and the silver coinage stopped containing silver. No other year of the period leaves so clear a mark on the coinage.
Edessa
Valerian was taken prisoner by Shapur I at the end of a disastrous campaign in Mesopotamia. It was the first time a Roman emperor had fallen alive into the hands of a foreign enemy, and the event was staged on the Persian side: the rock reliefs at Naqsh-e Rostam show the king on horseback and the Roman kneeling.
Gallienus, his son, had been reigning jointly since 253. He found himself alone, without the eastern army, facing frontiers left open on the Rhine and the Danube.
Two secessions
On the Rhine, Postumus was proclaimed by his troops and founded what modern historians call the Gallic Empire. He did not attack Rome: he administered Gaul, Britain and part of Spain with his own consuls, his own Senate and his own mints, at Cologne and Trier. His coinage, moreover, is of better quality than Gallienus's, which says a good deal about the legitimacy he was claiming.
In the east, Odaenathus of Palmyra drove back the Persians in Rome's name, then his widow Zenobia turned that de facto autonomy into a separate power, going as far as striking coin in the name of her son Vaballathus. The empire was in three pieces for some fifteen years.
What the coinage records
It is exactly at this moment that the antoninianus ceased to be a silver coin. Metallurgical analyses carried out on whole series show the fineness falling to a few per cent: the flan is bronze, whitened at the surface by a chemical treatment that gave it the look of silver for as long as the first handlings lasted.
The hoards tell it better than any text. Deposits buried after 260 contain almost nothing but recent, debased pieces: the good money had been taken out of circulation and hoarded, exactly as one observes in every economy that has lost confidence in its currency.
Further reading
J. F. Drinkwater, The Gallic Empire. Separatism and Continuity in the North-Western Provinces of the Roman Empire, Stuttgart, 1987.
K. Butcher and M. Ponting, The Metallurgy of Roman Silver Coinage. From the Reform of Nero to the Reform of Trajan, Cambridge, 2014, for the method of analysing fineness.
R. Bland, ‘The Development of Gold and Silver Coin Denominations, A.D. 193–253’, in Coin Finds and Coin Use in the Roman World, Berlin, 1996.