I stopped returning the crucible to the furnace years ago and just empty it and set it on an IFB to cool. The reason is I only cast aluminum, so the crucible isn't that hot after the pour anyway, and if I return an empty crucible to the furnace, it will turn all the residual aluminum skin to aluminum oxide. That Al2O3 will adhere tenaciously to the crucible whereas if allowed to cool outside the furnace, I can remove the skin in a single piece and have a pristine crucible for the next session.
These are clay graphite Salamander Supers. I know Morgan recomends returning them to the furnace but at aluminum temps, I just don't think the threat of thermal shock is enough to worry about and I've noticed no difference in crucible life.....I still get many, many melts....~100+. I could scrape out what I can after the melt and return it to the furnace, but you can never get it all and the scraping can cause some attrition of the inner crucible surface.
Now, if it was at iron temps that's likely a very different story but it's a tough life for a crucible in iron duty in a fuel fired furnace.
The other old debate is whether you preheat the crucible and charge it hot or cold. The argument for hot is a cold charge causes a large temperature gradient across the crucible wall which induces stress, but I think that happens anyway as soon as you put a cold charge in a hot crucible, unless you start from a small heal and add the charge a small chunk at a time. Again, for aluminum, and especially with resistive electric heating, I just fill it up and hit the switch. You just need to make sure a cold charge crucible is loosely packed so when the metal expands with heat it doesn't crack your crucible.
Best,
Kelly
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Peter,
I’m having the same problem, I just purchased an old Jelrus
Handy-melt (stamped 1969), similar to the Kerr 30 oz. Unfortunately
the crucibles are slightly different at the bottom from what Kerr
offers.
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My first attempt at using it was an attempt to make my own brass
from copper wire and pennies (97% zinc). Not really the best project.
The typed out instructions that came with the unit said not to take
it above 2000 F, which I thought would be high enough to melt the
resultant brass. Trouble is, that copper melts pretty close to 2000
and thus I’m left with a fused bundle of copper pieces stuck at the
bottom of the crucible and a haze of zinc. I’m tempted to melt some
aluminum on top and hope the copper will dissolve into the molten
aluminum so that I can pour it all out, similar to the way in which
salt (high melting temp) dissolves readily into water. Any
metallurgist have a helpful suggestion for this? Would tin be a
better option?
Graphite can be machined readily. You may want to lightly sand/bore
out any contamination that you can’t burn off or pour out. I think
the general practice is to have separate crucibles for each metal
and to replace them regularly. Note, that because graphite produces a
reducing environment you won’t need to use quite as much borax as
you’d expect (if any).
Scott
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