Glass
is a poor conductor of heat, so you need to be careful to allow the
heat to penetrate to the base layer to avoid thermal shock. There
also is the effect of the (very small) insulating space between each
sheet. The effects of multiple, even layers can be seen from this
table based on Graham Stone's* work:
3mm
layers
1
sheet – Initial Rate of Advance =1000ºC to 475ºC (less than half
an hour)
2
to 3 layers – IRA = 240ºC to 475ºC (ca. 2 hours)
4
layers – IRA = 100ºC to 475ºC (4.75 hours)
6
layers – IRA = 25ºC to 125ºC, then 30ºC to 250ºC, then 40ºC to
375, then 50ºC to 475 before 150C to top temperature (ca. 15.5
hours)
This
shows the dramatic effect increasing the number of layers has on the
firing schedule to make sure the heat gets to the bottom sheet
evenly. If you compare the initial rates of advance (IRA) with the
same thickness, but fewer sheets you can see the space between layers
is important.
6mm
layers
1
sheet – IRA = 320ºC to 475ºC (ca. 1.5 hrs)
2
layers – IRA = 240ºC to 475ºC (ca. 2 hrs compared to 4.75 hrs for
4 layers of 3mm)
3
layers – IRA = 200ºC to 475ºC (ca.2.5 hrs compared to 15.5 hrs
for 6 layers of 3mm)
These
are the fastest safe firing speeds for evenly covered sheets.
This difference in firing times for stacks of thicker glass, shows how important it is to fire sections of the stack before the final firing of all the layers together. It also reduces the risk of bubbles developing within the stack.
If you are thinking of tack fusing with thicker and thinner areas, you need to take account of the differences in thickness in the various areas of the piece when preparing your schedule. You will need to decrease your IRA by quite a bit. So you might want to be thinking of firing some of your pieces to be added to the base layers before tacking them in an additional firing to reduce the risk of thermal shock to the base layer.
This difference in firing times for stacks of thicker glass, shows how important it is to fire sections of the stack before the final firing of all the layers together. It also reduces the risk of bubbles developing within the stack.
If you are thinking of tack fusing with thicker and thinner areas, you need to take account of the differences in thickness in the various areas of the piece when preparing your schedule. You will need to decrease your IRA by quite a bit. So you might want to be thinking of firing some of your pieces to be added to the base layers before tacking them in an additional firing to reduce the risk of thermal shock to the base layer.
*
Firing Schedules for Glass; the Kiln Companion, by Graham Stone, ISBN
0646 39733 8
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