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About Bronze Casting

Hello,

I recently took a class where we learned to do basic charcoal
casting with sterling silver. It was a lot of fun, and I ordered a
little fused-silica hand casting crucible & some more charcoal
blocks. I also bought sterling casting grain and bronze casting grain
(From Rio, the “ancient bronze” with no zinc, melt temp 1850F, cast
temp 1905F).

I played all yesterday afternoon with my new toys, and was sort of
surprised by some of my results, so I am hoping to learn more about
what I experienced.

I use a Smith propane/air torch setup, which has worked OK for me in
the things I have been doing so far. I started off by painting the
inside of the crucible with Handy Flux and burning it to the glassy
state with my torch - the instructor of my class had said to do
that. I switched to a bigger tip for melting than I use for
soldering. The silver behaved pretty much like it did in the class
setting. But in the crucible, the bronze would only melt enough to
fuse together, and would not flow to a liquid state so I could pour
it. Out of frustration, I took the fused chunk of bronze out of the
crucible, and put it directly on the charcoal mold & turned the torch
on it - lo and behold in just a minute or two it melted up into a
beautiful liquid state and I was able to squish it down into the
mold.

I was able to repeat this over & over - the bronze would not melt in
the crucible but did fine directly on the block. Which is really OK,
I guess, except it burns up the block pretty quickly and limits
re-use of the mold more than pouring on the melted metal does.

Why won’t the bronze melt in the crucible? Am I making mistakes that
prevent it? Any insight and advice are more than welcome.

Thanks,
Ann

Ann,

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.

Bill Churlik
@Bill_Churlik
www.earthspeakarts.com

 Why won't the bronze melt in the crucible? 

I have no idea-- someone else will, no doubt-- but one possible
solution would be to hollow out enough space in a charcoal block to
use as a crucible, and melt/pour from that. Be sure to wrap the
block very strongly with binding wire, or, better yet, fold a
piece of copper to enclose all but the top of your block, to prevent
accidents. Big block, small hollow!

–Noel