Graphite crucibles for hand furnace

Jewelry Discussion
1

After casting using various centrifugal casting machines for 30
years I have begun to occasionally also use a vacuum casting machine
and Kerr Electromelt furnace to cast.

My questions for the group are about the graphite crucibles for the
hand furnace. I like the graphite because it seems to absorb gases,
giving me few to no micro pits…I really like that. But after I use
the crucible for 5-7 pours, the hard surface of the graphite begins
to break down. Even if I carefully vacuum it before the melt I can
get annoying little bits of graphite in the casting. Coating the
crucible doesn’t seem to help as the flux just collects at the
bottom. Also judging when the crucible has reached the end of it’s
useful life is a little tricky, meaning I really want to avoid
having the bottom of the crucible drop out during a melt.

So how do you deal with the degrading crucible and avoid graphite
pits? And how do you judge when the crucibles wall are getting too
thin?

A somewhat unrelated observation about comparing centrifugal vs
vacuum casting is that I have noticed that the vacuum castings seem
a bit softer. I say that because if I cast 14K white, one spun and
another vacuumed, and thenput the two castings in a magnetic
tumbler, the casting that was vacuum cast if more noticeably dented
than the one that was cast in a centrifuge. Not a problem, just
interesting.

Thanks,
Mark

2

Hi Mark,

Interesting.

I’ve some of my clay graphite crucibles break down also, but these
were the cheaper crucibles, currently I only use gas to melt metal
and a direct gas flame onto the side of a crucible is far harsher
than electric melters. In the larger furnace when I used a cheaper
crucible, the outside of the crucible would crack, but thus far have
remained intact. The smaller micro furnace, where I’ve only used a
good quality crucible, there are no problems.

I don’t get graphite in my melts at all.

This is my procedure for a melt.

1)* Fill the crucible to the brim with a high quality flux, not
borax (I have 80 kg of flux used for an aluminium pot line, seems to
work well). Then empty the crucible.

  1. Place in your charge of metal (or alloy components in the correct
    order). Sprinkle in some more flux.

  2. Melt your metal.

4)* Before you pour, stir the metal with a green stick.

  1. Pour the metal.

Thus far my castings are porosity free, and clean.

The * next to the step above represents a trick given to me by an
old founder.

Regards Charles A.