How do I get a glass jar out of refractory plaster?
My approach would be to carefully drill holes into the glass very near one another, and then gradually break pieces away with long nosed pliers, or if you have a dremel with a tip for grinding glass remove the glass that way once the holes are dilled. I'm assuming you can get to the glass, perhaps not.

A photo of the challenge would be a big help. 

Am I assuming correctly the foundry can take a lot of heat? If so, then would a blowtorch on the glass melt it? (obviously the foundry will become exceptionally hot).