The annealing temperature or annealing point is the stress-relief point glass reaches during the cooling down phase.
At this point, the glass is too firm to distort or bend but remains soft enough for any built up stresses to relax.
Soaking or holding the piece at this temperature helps to even out the temperature throughout the piece.
The time required for this soak or holding action can vary depending on the mass and type of glass.
Once this point has reached and the hold time has been accomplished, the glass is slowly cooled through the
. Following this process, the glass can carefully be cooled until it reaches room temperature.
This process takes time to accomplish and is necessary to have a successful fused glass piece.
If the residual strain or stress in the glass is not relieved the piece could break at any given time or place.
There have been pieces that are centuries old which were not take through this process and then suddenly break over what appears to be not obvious reason.
Of course, now we know it was because the glass was not properly annealed.
Annealing a piece highly increases the durability and lessens the likelihood of a failed piece by relieving all the internal stress and strains.
Once you have determined they COE of glass that you are going to use in your fusing projects, check with the manufacturer to find out the recommended annealing point or temperature.
Bullseye has just recently changed the recommended annealing soak temperature from 960°F/516°C to 900°F/482°C.
They claim that the lower temperature is more effective and efficient.
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Annealing Temperature to Glass Fusing Made Easy