Coinage has a marked propensity to debase.

"For in every country of the world, I believe, the avarice and injustice of princes and sovereign states, abusing the confidence of their subjects, have by degrees diminished the real quantity of metal, which had been originally contained in their coins."

-- Adam Smith, Wealth of Nations

https://en.wikipedia.org/wiki/Denarius#Debasement_and_evolut...

https://en.wikipedia.org/wiki/Denarius#/media/File%3AFinenes...