I too am interested in what people think about reproducing copper
objects, as I myself was just asked to do the same thing with some
valuable copper alloy archaeological objects. My understanding is that
copper melts at such a higher temperature than the vulcanized rubber
(used to make the mold... 35O degrees F), that there will be no harm
metagraphically, however I was concerned about reside from the rubber. I
thought about coating the objects first with a barrier film, but I was
afraid that would melt as well.  I was assured by a jeweler as well, who
does this regularly, that there would be no harm to the objects.
However, in the case of archaeological metals which may appear stable,
there is always potential for intra-granular corrosion, or disruption in
the metal structure which is not always apparent. Also, heating any
metal to 100 degrees C can alter the metallographic structure of the
metal, which should be kept in mind.

I too am interested to hear if anyone has attempted to do this before,
or has had success making cast replications of copper alloy objects.

Thanks,
Lisa Young
Archaeological Conservator