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Rough Castings


#1

I have been centrifugal casting with a silver /copper mix
(50/50). The castings come out rough/ pebbly.

The investment is the standard Kerr material, formulated pretty
much to the instruction sheet. The burn out is much longer as I
have been burning out twigs and little plastic pieces. Had a lot
of bad castings when only burning out for eight hours as I still
had sections of wood in the casting (lost a bunch of three ounce
castings).

I cannot tell if the problem is overheating of the alloy or
residuals of plastics/organics in the flask. The extended
burnout is the same for plastics/organics for sterling silver
castings without getting the rough surface. The pieces polish up
well enough. Seeing this problem in writing makes me think that
the burnout cycle is still too short or not at high temperature
soak long enough.

Type of plastic or organic material is anything that has a shape
of interest.

I guess a wax could be made of the plastic pieces and the problem
would go away except that is an unwanted interim step.


#2

William,

This isn’t an answer but a question: Why the 50/50
silver/copper alloy? What kind of metal does this produce? Just
curious.

Jill
@jandr
http://members.tripod.com/~jilk


#3

From what you described it sounds like the problem is with the
50/50 Ag/Cu mix. I presume you have to use that alloy. I’ve
never cast that alloy but it sounds like it would give a nasty
casting since even 7.5% in sterling causes problems. Use plenty
of flux (50% Boric acid 50% Borax) and a really reducing flame.

You say that when you cast the same things in sterling you don’t
get a rough surface, so I think the burnout is probably alright,
although a three ounce casting would indicate a lot of wood to
get rid of. I would suggest a real long cycle, if you use Kerr
hold 1350f for 10 hours. Some sulphate bonded investments can go
to 1400f but Kerr starts to break down.

Brett