Yes, burnt.  It is carbon

#11960
Stephen Richard
Participant

Yes, burnt.  It is carbon from your glue.  Apparently it was unable to vent before the bottle closed. You need to do a long soak at a temperature below that at which the bottle begins to slump.  Is your frit compatible with the bottle glass?

Stephen Richard

blogs at: http://www.verrier-glass.blogspot.com/ and  http://www.glasstips.blogspot.com/

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