Thanks for all the replies!
Aggie, I am backing my plaster with a sand/plaster mix or with
sand/pumice mix. How hot were you firing the burnout for the
sculpture? Did you have much trouble with cracking or flash? I
suspect I could push the temperature up a bit without too much risk.
Maybe a little cracking and the resulting flash will be easy enough
to deal with.
Andrew C., I have used Satincast as a core and for backup in my
trials. It is not hard enough to stand up to the type of carving I am
doing. The test is to make parallel lines, 2 vertical and two
horizontal about 3 mm apart (#)with a “V” graver about 60 degree
angle. If this will consistently leave a sharp pyramid, it works.
Doesn’t work with Satincast, clay or any plaster tempered with more
than just the smallest amount of an additive.
Andrew W., Unfortunately we don’t know the original alloy. The piece
in question, the Ardagh Chalice, has been extensively studied. There
have been assays of the silver and gold bits, but the bronze has not
been tested as far as I can learn. I am trying to reproduce the
cylindrical stem section. Not especially a good project for a
multi-part mold in soapstone. The four bronze cast pieces are all
gilded and all show some casting porosity. I don’t have to make a
perfect casting for this to be a success. Moisture can be driven off
OK at 500F if it is done long enough.
I have done often enough to have solved the problem.
Noel, steam might be a solution to get better pressure into the mold
but I am not going to try it. It seems like there is too much to
learn about how to make that work. I have had pretty good luck
gravity casting silver in these kinds of molds and I am almost there
Brenda, Brittle will not be a problem. Not planning to bend or forge
the piece after it is cast.
Ted, That was Cellini who tossed in all the household pewter in the
pot while stressing about getting his bronze to melt. It is a truly
great chapter in his autobiography. You can really feel his anxiety
as he tells about this over-the-top project that looks like it is
going to fail because he runs out of fuel.
Jim, I think what I will do is add some zinc to the mix by measuring
out some brazing spelter. I will do the math and add enough along
with some more tin by way of pewter, so that I have about 4 or 5%
zinc and around 15- 18% tin. There will be a small amount of antimony
also. I will pour a test into a sand mold first just to make sure it
isn’t to weird.
Elliot, Clay was what I first assumed the metal was when I first
twigged it that this style was done in carved mold. I tried many
kinds of clay and clay with dung as recommended by Theopholis. Tried
them all all different levels of semi-dry, low fired, etc. It is
great stuff for lost wax or making bivalve impression molds,
especially the dung/clay mix. Could not get it to carve on the level
I needed as described above in the reply to Andrew C.
Here is the plan for the next step. I have made three test molds,
two are carved in haste as tests to see if the detail will fill. The
third will be fully carved. Probably on Wednesday next week I will
try the bronze with some zinc and pewter. 1st I will pour one of the
test molds with gravity, getting the metal especially hot. (Just
getting it hotter might be all it takes) If it works I will pour the
fully carved mold the same way. If the test fails I will put the
second test in my centrifuge. If that goes OK I will finish off the
last one that way. Moving on to “plan C” if all else fails, I will
pour it with a lower melting silver alloy of around 60% silver so
that I can save my effort