I find that air-acetylene torches are really at or over their limit
when trying to metal bronze, even with a #5 or 6 tip. The crucible
just soaks up too much of the heat. Also the curved shape of the
crucible helps to reflect some of the flame. Do you preheat the
crucible bottom to a red heat? That helps with gold, silver, or
bronze. Remember that Sterling melts at 1645 and the bronzes usually
around 18-1900. BIG difference. I also use Boric Acid powder instead
of Handy Flux. I am not sure Handy Flux is works well at the
temperature that bronze requires. Have found Twenty Mule Team Borax
good as a brazing flux. The blacksmiths use it for forge welding so
it can handle the temperature range of bronze too.
I have been casting small bronze pieces (30+ to a tree) and found
that even using Oxy-acetylene and a hand held crucible gave me
problems getting an even heat through the metal. Set up the furnace
and am now pouring from a #2 graphite crucible. No problems as the
furnace generates more than enough heat. (25lb capacity). The lead
picture on my website homepage shows a pour in progress. Can we say
fun? Love this stuff! There is something so primal in that stream of
molten metal. That stream and a hammer is at the heart of all of our
work. Nothing has changed in the last 8,000 or so years.
Hope this helps.