Negative muons reveal the economic chaos of Rome’s AD 68/9 Civil Wars

During the AD 68/9 Civil Wars, Galba, Otho, Vitellius and then Vespasian fought for — and gained — control of the Roman Empire. Our textual sources suggest that this was a period of serious and sustained disruption. However, existing analyses of gold coinages produced in AD 68/9 show only a minor reduction in the purity of the gold coinage. Using X-ray fluorescence, we identify a number of heavily debased gold coins issued during the AD 68/9 Civil Wars, and many slightly debased coins issued in their immediate aftermath. We then confirm the interior composition of these coins totally non-destructively using muonic X-ray emission spectroscopy, thus eliminating hypothetical problems of ‘surface enrichment’ or compositional differences between ‘surface’ and ‘core’. Here we show that heavily debased Civil War gold coinages were indeed produced; that copper was used to debase Roman gold coins during this time, c. 185 years earlier than first shown; and that slightly debased gold coins were regularly issued in the years immediately after the Civil Wars. The metallurgical evidence from the gold coinage now allows us to show that the AD 68/9 Civil Wars caused significant and sustained disruption to the Roman economic system. More broadly, we have shown that muonic X-ray emission spectroscopy is a powerful tool for generating important archaeological conclusions from high-value cultural heritage objects that simply cannot be destructively analysed, but need to have their interior compositions sampled.