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In many moments of Roman imperial history coin was debased, with its silver contents being reduced over time, and in general this was linked with increasing imperial expenditures. In Harper's The Fate of Rome (2017) it is stated that Caracalla needed to create a new kind of coin, the antoninianus, to be able to pay soldiers after he raised their pay. This new coin was said by the state to be worth two denarii, even though its silver contents only reached the mark of 80% of the silver contained in two denarii. It seems that this worked for a while.

But in the 250s and 260s the denarii and sesterces were progressively melted down, eventually disappearing, and the antoninianus, the sole remaining silver coin, was debased until it was a billon coin, almost pure vile metal. Harper says that people must have started holding on to good metal, taking coins out of circulation and accelerating the currency crisis. In the author's words, "no other era of Roman history is so productive of coin hoards." (p. 148)

My question is: How could Romans, in this period and in other periods as well, tell if their coins were being debased? How could those people that took coins out of circulation know that their silver content was getting lower? Did they keep measuring the density of new coins, or did the state advertise that new coins had less silver? (This would seem counterproductive if the objective of these measures was to pay for the state's debts.)

(My research so far didn't give any results, as the articles I found were only about debasement as a problem for the Roman Empire, and didn't tell me what I wanted to know)

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    Roman silver coins were debased by replacing some of the silver content with copper. If your 'silver' coin starts going green in your pocket, it's probably a good sign that the coinage has been debased! – sempaiscuba Jan 18 '18 at 21:46
  • Some professional jewellers and other metalworkers would certainly have noticed the debasement. And from there the knowledge would have spread. That said, it's far from certain that most Romans were aware of what's happening. My understanding is that it's not clear that prices actually tracked precious metal content, as you'd expect them to if everyone knew. – Semaphore Jan 18 '18 at 21:50
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    Same way as we know that our money is debased: inflation. – Alex Jan 18 '18 at 21:59
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    Why would they know? Why would they want to know? Merchants and Moneychangers could weigh the coins against standard weights. It isn't complex – Mark C. Wallace Jan 19 '18 at 0:34
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    @sempaiscuba: I up-voted your comment, then some of my chemistry classes started to come back to me. Alloys don't work that way - which is how and why galvanized steel works. Simply wrapping a little zinc around an iron object is often enough to dramatically slow oxidation. Actually alloying the two metals works even better. – Pieter Geerkens Jan 19 '18 at 3:38
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The density of copper (~9 g/cc) is almost 20% less than that of silver (~10.5 g/cc). It wouldn't take a genius of Archimedes caliber to repeat the well known "Eureka!" experiment and test coins for precious metal content.

And the moment one person in a community starts discounting certain coins, you can be certain that others would get suspicious as well. It just takes one well educated merchant to start the rumours flying.

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    Note, the Archimedes story itself may be a myth, but it was first mentioned by Vitruvius in the first century BC, so regardless density vs. volume was known in Roman times. – Steven Burnap Jan 19 '18 at 3:32
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I'd go with hardness as Silver is soft but becomes much harder the more copper content is present. Take a known silver coin and a coin in question and setup an experiment where a jewelry hammer hits the coin with the same force. A pure silver coin should indent with a significantly lower force than the silver/copper coin. It's a pretty steep curve with a 10% copper impurity being almost twice as hard as a pure silver coin would be.

hardness increase

image credit here http://riograndeblog.com/metal-hardness-how-to-measure-it-and-why-it-matters-for-silver-alloys/

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    As this is a history question, is there any evidence Romans did this, or were capable of the necessary precision? – Schwern Jan 19 '18 at 1:54
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    Never mind a hammer - use your teeth; it is actually more reliable and more easily calibrated without machinery. It is well known that this technique was used, and commonly. – Pieter Geerkens Jan 19 '18 at 1:55
  • @schwern - they were definately capable of it...as pieter points out, your teeth would work to determine pure silver vs 10% copper pretty readily. Not sure if i can find proof of this practice, though itd be interesting if this is where biting a coin came from – Twelfth Jan 19 '18 at 3:02

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