Fusing multiple layers is prone to the creation of
multiple large bubbles. It also needs a
strategy to schedule for thick layers.
Avoid bubbles
A widely recommended strategy for stacks of glass is to fire
in pairs of layers. Then combine the fused two-layer pieces in a final firing.
It is easier to fire two layers of glass than 6, 8 or 10
layers. The heat up is easier and less time consuming for multiples of 6mm than
multiples of 3mm. The bubble squeeze schedule is simpler. It also allows inclusions between the initial two-layer
sheets and then between the layers of 6mm sheets.
This multiple firing strategy reduces the risk
of large bubbles in a stack of multiple pieces. It seems the weight of the 6mm
layers forces the air out from between the thicker glass more effectively than thinner
layers.
It is also a simpler set of firings. If you were to want to make up a 12mm thick
piece from 3mm sheets, your heat up will be very long compared to firing two
layers in three firings.
E.g. Stone* recommends a heat up for 2 layers of 3mm glass:
240C/hr to 250C, no soak
400C/hr to 500C, no soak (a bubble squeeze could be
inserted here by raising the target temperature to 650, with a 30-minute soak)
500/hr to top temperature.
This is about 2.3 hours to top temperature without the
bubble squeeze and 6.7 hours to cool.
This means that you could fire twice in one day, if organised well. If you are planning a final tack fused layer
that should be done in the last firing of the combined layers.
However, it is a much longer schedule recommended by Stone
for 6 layers of 3mm glass:
- 25C/hr to 125 for 20’
- 30C/hr to 250 for 20’
- 40C/hr to 375 for 20’
- 50C/hr to 520 for 15 (a bubble squeeze could be inserted here by raising the target temperature to 650, with a 30-minute soak before continuing at the same rate to the top temperature).
- 150/hr to target temperature
This is about 18 hours to top temperature without the
bubble squeeze and another 18 hours to cool.
This strategy requires 1.5 days, assuming all the layers are even. The same amount of time is required for both
strategies, but the chance of large bubbles is dramatically reduced.
He recommends for 3 layers of 6mm glass:
- 200C/hr to 250, no soak
- 340C/hr to 500, no soak
- 400C/hr to 600, no soak (a bubble squeeze could be introduced here by changing the target temperature to 650 with a 30-minute soak)
- 500C/hr to top temperature.
This is about 2.5 hours to top temperature and 18 hours
to cool without the bubble squeeze.
This means that it only takes 2/3 of the time to fire 3
layers of 6mm glass than it does to fire 6 layers of 3mm glass. Yes, you lose some time in firing the pairs
of 3mm glass, but you gain in reducing the risk of creating large bubbles that
will ruin your final piece.
Inclusions
If you are putting elements between the initial two-layer
pieces for fusing, you need to introduce a bubble squeeze. Putting elements between the fused pairs will
also require a bubble squeeze on the final firing.
Tack fusing the final layer
Note the times indicated above are for even layers. If you have uneven layers or are tack fusing,
the times will be extended much further than the ones noted there.
For a tack fused set of top layers, you will need to add
those in the last firing, or do a sharp tack firing before the last firing. In the case of a tack fused pair for the top
layers you will need to reduce the rates of advance for the last firing by about
1/3. This would mean:
- an initial rate of 135C,
- a second ramp of 230C,
- a third of 270C and
- the fourth of 335C instead of the rates for even layers.
You will also need to reduce the top temperature. Observation will be required to determine
when the correct profile has been achieved.
Further information is available in the ebook Low Temperature Kiln Forming.
When firing multiple layers of glass, the risk of
creating large bubbles can be reduced by firing pairs of 3mm sheets, and then
combining the results into one stack.
*Graham Stone. Firing Schedules for Glass, the Kiln Companion, 2000, Melbourne Australia. ISBN 0-646-39733-8
Once again, such a valuable tip. I am finally back in my studio after almost two years absence. I appreciate the advice and experience of those more experienced than I.
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