Dear Michael,
Can you identify the exact issue of the coins; I am a little surprised at
brass as late as the 4th century although local issues might have
included it. Brass was on the whole not as important in the later Empire
as in the first two centuries AD.
The bimetallic coin almost certainly *is* the result of de-alloying. Over
one and a half millennia, the spongy mass of copper and cuprite left by
dealloying can itself corrode and leave asolid mass of redeposited
copper. In the Maiden Castle excavation report I very nearly published a
fragment as copper sheet when one more polish and etch revealed the last
remaining small area of brass in a solid mass of redeposited copper.
Polish and etch the copper and it should show you the characteristic
growth twins of that material.
Yours,
Peter Northover
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