Why does the metallic composition of the coins matter? It doesn't effect the point at all, so your use of "except" is confusing to me.
Only point I can see is that copper is not a traditional bullion metal. I'd argue that that's up to the trade partners to decide, not us centuries and an industrial revolution later. For instance, aluminum was considered a precious metal until the late 1800's when new smelting processes drastically increased supply.
Copper is as traditional a bullion metal as either gold, silver or iron, it's just a matter of what historical period we're talking about. Commodity money evolved organically over thousands of years from salt, to iron/copper, to silver+gold and finally to gold alone. Gold was better suited as money because it was rarer, and therefore carried more value per weight unit, and it's fairly useless for metal work because it's relatively soft.
That holds for coins used as currency, but not necessarily for coins used for bullion. Krugerands, the definitive bullion coin, don't have denominations printed. Their value is based only on their metal. With ancient coins there is a very fine line between a valuable piece of currency, and an attractive lump of silver. I suspect that by the time this coin is in Japan, it is worth more than its weight suggests. But does that intrinsic value appear because it is currency or simply novelty? I think the latter. This is a bullion coin worth more purely by circumstance.
There were more primitive systems of currency before authorities started endorsing coins. Hoards of bronze-age axeheads, few of which show signs of use, some that couldn't ever be, point to them as a means of moving wealth via standardized units. A standardized shape of metal used in trade... sounds like coinage to me.
Yes, that's true; I had been thinking about it in terms of traditional bullion, which I suppose is a Western-centric viewpoint. Even China only recently ramped up their copper production with industrialization.
Well, that amazing technology was the result of only having iron sands to work with, "Tamahagane". Lots of impurities you have to work out, hence the hammering and folding as well.
Which would explain why they weren't melted down. (They might have just been there in the mix, to make change for larger transactions among pieces no longer surviving).
