Acetylene with pure oxygen burns at 3160°C. And there are things that burn hotter. It is not hard to melt even the pure refractories.
I would not recommend trying to make your own crucibles for these immense applications. The risk of harm from them splitting on unloading is high.
In addition, if the temperature is not high enough, and the melt complete, you will end up with a Gooch crucible. That is a porous crucible.
Another problem is purity. Alumina and other refractory crucibles are usually used when the temperature is not only high, but a pure environment is
needed to ensure the contents won't corrode the crucible and the crucible won't contaminate the contents.
[Edited on 16-8-2011 by peach]
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