It would seem that if excessive temperatures were to be avoided, that the technique you recommended earlier - melting pure copper and then adding tin - would not be advisable. Pure copper melts at a much higher temperature than any bronze: 1084 Celsius or 1983F according to this handy char
Every time you alloy different metals this will be an issue. Low
melting metals will be over-heated. This cannot be avoided. What can
and should be avoided is heating patterns where problems associated
with over-heating are aggravated. The only way I know, is to melt
metal with highest melting point first and add the rest, as close to
the time of pour as possible. Incidentally, it is not only true of
bronze, but of any other alloy as well.