Late
Roman antoniniani and folles, apart from any surface silver, are basically copper. There is a British Museum study that shows them all (Occasional Paper no. 120, 1997). Remember that before electricity was understood, and conducted, copper, always common and easy to use, was not so highly valued. A glance at the charts of all the
Gallienus coins they analyzed shows over 80% copper: just impure enough to make them hard enough.
Bronzes (note the plural) vary, according to what casting characteristics, what color, etc., are desired, also according to the available metals for alloying. So they vary according to use (clamps and dowels, coins,
statues, vessels to be
plated, partigilt—if I spelled it right—like the
Sassanian plates and bows). So dear old
Caley called 'em 'copper alloys' which, metallurgically, is correct. Easier to call 'em all
aes. Where available, tin is the usual primary alloy-metal.
Copper should be reasonably pure to be so called; it often has lots of trace elements native to it.
Orichalcum of the best period, in
Rome usage, is about 80% copper with about 20% zinc.
Modern brass
comes in two kinds: like
church brass, often under 20% zinc and so warmer-colored, and machine brass, often well over 20% zinc, so more lemon-yellow and more brittle, but harder.
Good brass screws are machine brass.
The trace elements should cumulatively be less than 1%.
From
Marcus Aurelius onwards, the
Rome mint seems to have
had a zinc problem; probably its cost, to smelt it from the ore and for transportation (but we don't really know), when the
dupondius and
sestertius lost their former
buying power, made the
Rome mint not only melt down worn coins and even scrap but also substitute other metals for the zinc.
Sestertii of the later periods are seldom really
orichalcum and really interesting in their varying alloys.
Pat L.