Absolutely true. I've used SiC and clay/graphite crucibles before, both
outgas some stinky concoction for a while. The SiC smelled worse for
longer. I would presume if you heat it too fast, these resins (or whatever
they are) could build pressure and crack the crucible. Also, being ceramic,
you should be gentle to them in general. Don't do anything that'll put a
big temperature difference across it - adding liquid much hotter or cooler
than the crucible's temp, etc.
Modern crucibles can take a lot more punishment than plain 'ol clay.
Anyways, the usual recommendation is to heat the crucible slowly over a
period of at least a few minutes to an hour or longer, to about red heat.
Just go slowly. I'm not aware of any benefit of doing this alone, i.e.
annealing, cooling and then using normally.
"I've got more trophies than Wayne Gretsky and the Pope combined!"
- Homer Simpson
I used both silicon carbide and graphite clay crucibles for years. It's
important to "season" the graphite/clay type if you don't want it to crack.
The idea is to expel any residual moisture it has accumulated, assuming it's
been stored on damp concrete, or in a moist environment. Silicon carbide
crucibles don't require it and I've never had one fail by putting it
directly in service by melting in it.
To season your graphite/clay crucibles, all you have to do is place them
where it's warm and dry before putting them in service. It's not a bad
idea to place it on the cover of your furnace when it's in operation,
avoiding the vent hole. It that's not possible, and your furnace runs
quite warm around the perimeter, place it near, sitting on a fire brick that
is known to be dry. Make sure you get it up to the boiling point for a
while before putting it in service. By doing that, you should avoid
SiC no, Clay graphite yes it does need to be seasoned, or more
properly heated before to rid it of any built up moisture. I always
place my crucibles be it SiC or CG or steel pipe on top of my furnace
for a bvit to heat them up and drive off any moisture......
Another thing you need to do is place a small piece of cardboard under
the crucible bottom (between furnace bottom or plinth block and
crucible. This will turn to carbon, and prevent a SiC type crucibnle
from sticking to the plinth blocks etc. Just a small piece of
cardboard from like a shoe box etc will work fine........and it does
not hurt the melt.
Nothing like having a SiC crucible stock to a plinth block.
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I used that trick for years when melting precious metals. Because flux was
used in abundance, it was common for the crucible rest to be quite sticky.
The carbonized cardboard prevented sticking perfectly. Soap boxes are an
excellent source because they're quire large. It was my practice to soak
the piece in water just before placing it on the rest if I did repeat melts
with the furnace. It prevented the cardboard from instantly igniting.