Antoninianus - Carus SPES PVBLICA; Spes

While the coin is billon by virtue of being comprised of heavily debased silver, this level of debasement resulted in a coin that was more than 90% bronze with a very low-purity silver plating. The featured example has mostly intact silvering and retains a great deal of its original lustre, though a small mineral deposit is present on the reverse. At this point in numismatic history, the antoninianus was toward the tail-end of its roughly eighty-year run and essentially at its “rock bottom” during the Crisis of the Third Century, when chronic civil wars prompted the debasement of the coinage. 

The excess silver was used to raise and maintain legions to fight these wars. Of course, debasement could also occur on the whim of a profligate emperor with expensive taste. Or it could simply be due to regular silver shortages. Remember, silver was the coinage of trade. In the Roman Empire, the flow of goods went from East to West, while the flow of silver went from West to East. This means that Western silver was used to pay for luxury goods and exotic items from trading lanes such as the Silk Road, land and maritime routes. 

Starting with Caracalla in 215 CE and ending with Diocletian around 286 CE, the antoninianus simply fell victim to entirely too many civil wars, fiscally irresponsible emperors over a roughly fifty-year period of chronic usurpations from within, and staving off barbarian incursions from without. The antoninianus ended exactly as it began - debased.