I cast some charms and pendants out of the Rio Grande “Ancient
Bronze” for the first time yesterday. I was quite confused by what
little info I could find on bronze casting in books and online. I
found several widely varying melt/flow temperatures–anywhere
between 1800F and 1950F. The variance was due to differences in the
makeup of different types of bronze. I don’t know what the exact
makeup of “Ancient Bronze” is, so I had to guess.
And I only found one reference to flask temperature–1250F. I checked
the grain every 500 degrees as the electromelt heated. It finally
melted at a temp very close to 2000F (top end of the gauge on the
electromelt). I set the kiln to keep the flasks at 1250F and cast at
that temp.
The resulting bronze was all covered in thick firescale and it
took a lot longer to get the stuck-on investment off the pieces than
it does with silver. Is this normal, or did I do something wrong?
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.
Ancient bronze is copper and tin (I think it is 85/15). 2000 is
about right temp wise, maybe a bit low. The black scale is due to the
copper, a dirty metal to cast and weld. We found that a small bead
blast cabinet worked VERY WELL to remove any investment on jewelry
castings.
Try a lower flask temperature like 900-1000. Most bronze melts at so
high a temperature that it is really right at the limits of what the
gypsum bonded investment can take before it breaks down. When it
breaks down sulfur dioxide is released and that causes severe tarnish
/ firescale and the investment can “flux” or glassify and this
resulting hard white deposit will be very difficult to remove and
will leave a terrible surface under it after removal.
Best results will be from keeping the flask as cool as necessary to
fill the mold cavities.
James Binnion @James_Binnion
James Binnion Metal Arts
Since Rio Grande calls it "Ancient Bronze", than we can assume
that it is an alloy of copper, tin, and lead.
Our Ancient Bronze has no lead content, or less than 1%. It is
comprised of 90% Cu and 10% Sn. It has a flow (liquidus) temperature
of 1905 f so a casting temp of around 2000 f would be ideal for most
applications.