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Changing Crucibles?

I am having a lot of problems with graphite ending up in my castings
when I use my Kerr electro melt to vacuum cast. How often should I
change the crucibles?

Jennifer Jenkins

I am having a lot of problems with graphite ending up in my
castings when I use my Kerr electro melt 

Hi Jennifer;

The best way to understand when to change your graphite crucible is
to finally burn the bottom off one accidentally. If you’ve got an old
one, saw off the last half inch and take a look. But here’s a trick.
The crucibles are cylindrical, the walls go straight down. If you
measure the outside diameter, subtract the inside diameter, then
divide by two, you’ll get the wall thickness. Once they’re down to
about one third the original thickness, which you can determine by
the same measurements, you’re in the danger zone. The bottom won’t
burn through before the walls just above the bottom give way. You’ll
notice the outside walls getting pretty spongy near the bottom at
that point.

But here’s what I think is happening in your case. Strictly
speaking, if graphite were in the melt, the metal would slide past it
during the pour and what fell in the flask would float on top of the
button. What I think is happening is that graphite dust from around
the lip of the crucible is falling into your flask before the metal,
and it’s getting trapped in nooks and crannies. Get a narrow
paintbrush, and before you start your melt, brush out the crucible
and especially around the lip.

David L. Huffman

When you notice they are starting to degrade.

Hi Jennifer,

I have been using the Kerr Electro melt for about 25 years and have
never had graphite in my castings. I use the crucible until the upper
ring is almost gone. I only quit using the crucible because without a
ring on the crucible there is an exposed gap between the crucible
and the muffle.

I am concerned that silver might fall into the gap and melt in the
muffle. Because graphite is so light compared to the silver, free
graphite floating in the melt it would float to the top of the melt
and not get into the mold with the melt. Are you casting sterling’ I
once read the De-0x silver should not be melted in a graphite
crucible. Are you using Kerr replacement crucibles or no name
crucibles. Are you sure the graphite is from the crucible’ How long
do you maintain

the flasks at burn out temperature’ Could it be the graphite is
carbon from incomplete burnout’ Lee Epperson

Thanks for the info.

Should I coat the graphite crucibles with anything?

Thanks for the info. Should I coat the graphite crucibles with

No, first it is not easy to coat graphite with anything that will
stick to it well(although with persistence a layer of flux can be
built up on graphite). Most coatings tend to flake off as the
graphite under the coating sublimes away as it combines with oxygen
and becomes carbon monoixide and carbon dioxide those flakes fall
into the melt and end up as inclusions in your casting, not good.
Anything that coats the graphite keeps it from doing its job which
is reacting with oxygen and keeping it out of your melt. So the best
thing is not to use any coating. Also many folks cant resist the
urge to look inside the electro melt often to see how things are
coming along. Resist this temptation. do a few tests to see how long
it takes to melt your amount of metal you typically use then once
you have an idea of times just leave the top closed for at least as
long as you think it may take to melt the charge. You will do more
damage to the melt by opening the top too often than letting it sit
at a molten state for a couple of minutes( as long as you have the
right pouring temperature set into the electro melt). This will also
prolong your crucible life

James Binnion
James Binnion Metal Arts