Effects of Multiple Layers

Stacking layers of glass fully or partially over the base layer has significant effects on the firing of the whole piece.

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.

* Firing Schedules for Glass; the Kiln Companion, by Graham Stone, ISBN 0646 39733 8