Since Rio Grande calls it “Ancient Bronze”, than we can assume that
it is an alloy of copper, tin, and lead.
Tin and lead melts at far lower temperature than 1250F and probably
what happened is that some of it burned out and that raised the
temperature of the alloy.
It is important to melt bronze as quickly as possible to minimize
the effect, but one cannot completely avoid it.
The proper way to cast bronze is to melt copper and add required
amount of tin and lead just before pour.
You can try the same. When bronze is melted add some lead or tin
into the crucible to compensate for the burned material right before
pouring. It is important to stir the alloy.
Also do not underestimate the importance of flux in casting bronze.
If everything else fails, try the ancient recipe to melt under the
layer of sugar and charcoal.