Bullseye claims that you should only
fire a piece 3 times
No. They only say
the glasses are tested three times and that you are on you own after that.
There is not
a general answer that can be given for the number of times you can fire a
piece. In general, Bullseye glass (and
probably others, although they do not state what their limits of confidence are)
can be fired three times with confidence.
Beyond that you need to do your own testing.
Bullseye states:
At Bullseye, glasses known to be fairly stable are tested by firing to a top temperature of 1500°F (815°C) and soaking for 15 minutes before annealing. Once cooled, these tests are viewed for stress through polarized light and graded accordingly. We fire glasses known to be less stable three times to make sure they'll perform well under multiple firing conditions, such as those used to fuse and slump a plate.
If you have
plans for multiple re-firings, tests are needed. The tests should replicate the
temperatures, colours and thickness of the proposed project. You probably do not need to reproduce the
size of the project in these tests though.
Results from
each firing should be tested for stress and these
tests should include a test for annealing each time.
You may wish to note that I have fired up to 7 times on several two layer with powder pieces. Many people fire more times successfully. It is my belief, but I have no proof, that multiple firings of a piece to slightly lower than full fuse will be more successful than each of them being to the full fuse. My practice is to go to a rounded tack each of the firings subsequent to the first full fuse, but the final firing will be to a full fuse if I wish a gloss finish. If I do not, my final firing will be about 10C - 15C below full fuse.
You may wish to note that I have fired up to 7 times on several two layer with powder pieces. Many people fire more times successfully. It is my belief, but I have no proof, that multiple firings of a piece to slightly lower than full fuse will be more successful than each of them being to the full fuse. My practice is to go to a rounded tack each of the firings subsequent to the first full fuse, but the final firing will be to a full fuse if I wish a gloss finish. If I do not, my final firing will be about 10C - 15C below full fuse.
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