Thursday, 25 July 2013

Annealing a Stressed Piece


If a project has not been sufficiently annealed, it is possible to re-fire to do a good anneal.

You need to establish a slow rate of advance; one that is much slower than for the heat up of a fully annealed piece of the same thickness. This is because you can heat shock the glass much more easily than one which is adequately annealed. I suggest going at about half the rate of a normal firing for a piece of the same thickness.

You have to make sure all the glass is above the annealing temperature so it is advisable to go up to near the slumping or softening point of the glass to ensure all the stresses are removed before beginning the annealing process. If you want or need to retain the level of texture of the previous firing, you will need to go to about 20ÂșC above the annealing soak point of the glass and soak there for at least as long as you plan to soak at the anneal point.

Anneal at a rate (after the anneal soak) that is considerably slower than previously used. Look at the Bullseye annealing schedules for thick glass, or their project notes for annealing bowls to get some good guidance on the rates of cooling.