I am trying do do a small bronze casting, 200 grams, gravity
poured in a vented plaster mold. My tests have filled the molds OK,
but failed to fully get the details. Seems like the mold or the
metal are not hot enough. I used a 90% Copper 10% tin alloy. The
plaster starts to crack if heated over 500F, so that is my limit
for the mold temperature.
200g is marginal for gravity casting. A little assistance from a
vacuum might help here. Failing that, adding a large void attached to
the thickest part of your intended casting would perform two
functions: it would raise the total quantity of metal, which would
help fill the mold by adding to the static pressure of the liquid
metal, and it would also act as a shrinkage reservoir, in case your
problem is due to shrinkage and not lack of pressure or fluidity.
What if I add some pewter to the bronze, maybe 10%. Will that make
the metal more fluid? Enough to make a difference? What about
other elements? Lead or zinc.
That would make it melt at a lower temperature, but I doubt it would
behave much better. Bronze is extremely fluid when it’s at proper
pouring temperature, enough so to fill any tiny cracks in a mold,
especially a hot one.
I am trying to make a mold that demonstrates how a certain ancient
object was cast. I have made these kinds of plaster molds work
with silver alloys, several hundred degrees lower casting
temperature than the bronze.
In that case, it would seem that you should use the same alloy as
was used in the original objects you’re investigating.
If my goal was simply to make the object, I would have no problem
just dropping the mold in the centrifuge. The great dread is that
I am going to have an enormous amount to time invested hand carving
the mold and then fail because the metal does not flow well enough.
A little more explanation. Additives to the plaster, clay, sand,
pumice or talc, all work great to prevent cracking at higher
temperatures. The problem is that they spoil the carving
properties, so I am using straight Pottery #1 plaster of Paris for
the part of the mold that is carved.
Have you tried silica flour? Straight plaster doesn’t make a very
good investment material, as you’ve discovered. If you could cast
with the mold hotter, the metal would remain fluid longer and fill
better. I wonder if you’ve even driven off the chemically-bonded
water at 500F.
If you are really into to it, there is a paper on an earlier
project in my investigation at http://www.ganoksin.com/gnkurl/ep81g4
That’s a nice paper, Stephan, and while I agree with your conclusion
that the pieces you examined were made with a combination of casting
and chasing techniques, I’m skeptical about your conjecture that
gypsum plaster was used to make directly-carved molds for bronze
casting during the period in question. For one thing, it just doesn’t
work that well.
Alternatively, soapstone does carve nicely and it makes a good mold
for bronze, surviving multiple casting attempts. Furthermore,
soapstone molds have been found that show signs of having been used
that way. Here’s a link showing how a modern practitioner has revived
the technique for use with pewter:
Here’s a video of someone using primitive equipment to cast a bronze
axe in a soapstone mold: - YouTube
All the Best