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- February 26, 2009 at 1:38 am #9316thomas deckerParticipant
all of the fusing schedules that I find for bullseye glass only account for glass that is 3/8 ‘thick and up to 12”max . Is there a rule of thumb one can use for larger and thicker pieces? I can use the bullseye annealing schedule but I am at a loss as to the ramp up speed and holds for larger pieces. I am currently getting ready to tack fuse on to a full fused blank ( 2 -1/8 sheets of glass ) that is 24×16″ with design elements that in some places add another 3/8 of an inch. I have done this same design on smaller pieces 12×14 successfully but am not sure as to what my ramp up speed and holds for this larger piece should be. Thanks. TDFebruary 26, 2009 at 3:48 pm #11162AnonymousInactive
In theory, the only size that matters when annealing is the glass thickness. The schedule for a 6″ wide piece of 1″ thick glass and 24″ wide piece of 1″ thick glass should be the same.
That pretty much holds true in kilns that heat only from above – because all of the glass gets an equal “dose” of heat from the elements. Many of our kilns, though, have both top and side elements. In these kilns, the glass gets extra heat from the sides. That results in less even heating. The closer the glass edges to the elements the bigger the concern.
One way to mitigate this (without eliminating side elements) is to build a heat baffle (a small wall) out of kiln furniture, fire brick or fiber board around the piece. There are pictures of this in our tutorial on thermal shock (Technical Tutorials from the Learn menu above). This forces the heat from the side elements to travel farther to the glass and, in doing so, allows the heat even out around the piece. This not only allows for more even annealing, it can also significantly reduce the chance of thermal shock (where the glass breaks do to uneven heating).
As for bubble squeeze, air between larger sheets of glass has farther to travel from the center to the edge so a longer bubble squeeze can be a good idea.
We have about 20 firing schedules available on this site (under the “Tools” menu above). They address different thicknesses. Even better, every segment of ever schedule is explained to help you understand the choices and adapt them to your specific needs.
Hope this helps.
Helios Kiln Glass Studio
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