Stephen Richard

From your description and the fact that you don’t mention annealing, I am betting on inadequate annealing.  Annealing consists of two stages – first a soak at the annealing temperature for a length of time adequate to equalise the temperature throughout the glass, second – a slow steady cool down to about 370C  (for 6mm, 80C/hr should be adequate).


If the piece has not been adequately annealed, it will break easily upon the heat up.

Stephen Richard

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