You'd have to do some research there. All coinage was minted to serve some elite (non-democratic) ruling dynasty/government, so right off the bat you're not dealing with "free market" in any ordinary sense when speaking of coins (bullion is arguably a different matter). And evidently a much larger portion of pre-modern economies was conducted in bartering raw goods for goods.
Probably especially with copper/bronze coins you have a proto- paper/fiat thing going on. Those coins were presumably used mainly at "face" value, not weighed for their intrinsic bronze value. I could be mistaken, but that level of coinage was already debased very early on -- reduced basically to a low fiat value. Rome and other governments had plenty of forced laws (
http://en.wikipedia.org/wiki/Roman_currency)
"Later, during the Roman Empire, there was a division in the authority of minting coins of particular metals. While numerous local authorities were allowed to mint bronze coins, no local authority was authorized to strike silver coins. On the authority to mint coins Dio Cassius writes, "None of the cities should be allowed to have its own separate coinage or a system of weights and measures; they should all be required to use ours." Only Rome itself struck precious metal coinage, and the mint was centralized in the city of Rome during the Republic and during the early centuries of the Empire."
One of the troubles with a non-fiat relationship between gold/silver is that it's quite difficult to conduct trade if both the goods themselves and the coinage are fluctuating, voyages take weeks/months, and there is no quick communication system of word-of-mouth to sensibly compute supply/availability. So the idea that coins (even silver + gold) have some early fiat aspects, and a fiat ratio to one another, is as old as coins themselves. It's all about Empire and the ruling dynasty. Again, raw gold or silver bullion is a different matter.