Need explanation for melted alumina lining in lab muffle furnace

[jajeanne]

Please provide possible explanations for following scenario:

During a routine lab procedure, the alumina-based lining in a countertop muffle furnace we use for burning off liquid asphalt binder incurred three small holes by what appears to be melting. The fuses did not blow, and the lab's circuit breaker did not trip. The manufacturer of the furnace informed us that the lining has a melting temperature of 3800C, well above the maximum operating temperature of 2000C.

Please see attached photo. Coil diameter < 1cm.

 

[Compositepro]

The asphalt binder wouldn't happen to be on fiberglass? Glass and many other substances can melt at a relatively low temperature and dissolve alumina (acting as a flux). Molten glass is also electrically conductive. This can short your coils or continue to carry current even after the wire has melted.

 

[EdStainless]

There are a number of metals that will also for low melting eutectics with alumina.
If you get some arcing or corona you can melt the surface of the alumina and then it is electrically conductive enough to continue the process.

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