A schedule was presented for a slumping problem of a 6mm/0.25” blank. It consisted of three segments each of a rate of 277C/500F with short holds up to 399C/750F and then a rapid rise to 745C/1375F. The cool was done with two long holds at 537C/1000F and 482C/900F followed by cooling rates for 12mm/0.5”
My response was
that, yes it was fired too high. Not
only that, but the firing strategy, as shown by the schedule, is odd.
Strategy
The general
strategy for slumping follows these ideas.
·
Glass
is slow to absorb heat, and in one sense, this schedule accepts that by having short
soaks at intervals. As glass is slow to
absorb heat, it is necessary to use slow ramp rates and without pauses and
changes in rates. This should be applied
all the way to the slumping temperature.
·
Holds
of short durations are not effective at any stage in a slumping firing. The objective is to allow the glass time to
form to the mould with as little marking as possible. This implies slow rates to low temperatures
with significant holds at appropriate stages.
This about putting enough heat work into the glass that higher
temperatures are not needed.
·
This
kind of firing requires observation for new moulds and new arrangements of
glass to ensure the slump is complete. Once
you know the mould requirements and are repeating the layup of the glass, the
firing records will tell you what rates and times to use to get a complete
slump with minimum marking.
·
The hold
at annealing temperature is to equalise the temperature throughout the glass to
produce a stress-free result. Any soaks
above are negated or repeated by the necessary soak at the annealing
temperature. The hold there must be long
enough to complete the temperature equalisation that is the annealing.
·
My work
has shown that annealing for one (3mm/0.125”) layer thicker produces a piece
with less stress. This indicates that a
6mm/0.25” piece should be annealed as for 9mm/0.35” to get the best result.
The summary of the
firing strategy for slumping is:
- ·
A
single ramp of a slow rate to the slumping temperature.
- ·
Observation
of the progress of the slump to determine the lowest practical temperature and
hold time.
- ·
Annealing
for one layer thicker that being slumped.
- ·
Three
stage cooling of the piece at rates related to the annealing hold.
Critique
This is a critique of the schedule. For comparison, my schedule for a
full fused 6mm blank would be different.
- ·
140ºC/250ºF
to 677º/1250ºF for 30 to 45 minutes.
- ·
9999 to
482ºC/900ºF for 1.5 hours
- ·
69ºC/124ºF
to 427ºC/800ºF, no hold
- ·
125ºC/225ºF
to 371ºC/700ºF, no hold
- ·
330ºC/600ºF
to room temperature, off.
The rate of the
published schedule is fast for a full fused blank and extremely fast for a tack
fused blank. This needs to be slowed. The
schedule provides a single (fast) rate of heating, but with unnecessary holds. The holds are so short as to be ineffective, anyway. There
is no need for the holds on the way up to the slumping temperature. In general slumping schedules are of fewer
segments. This is because glass behaves
well with steady slow inputs of heat.
Then strangely, the
schedule increases the rate to top temperature. It does so with a brief soak at 593ºC/1100ºF. This fast rate of 333ºC/ 600ºF begins at 400ºC/750ºF. This is still in the brittle phase of the
glass and risks breaking the glass. The
brittle stage ends around 540ºC/ 1005ºF.
This rapid rate
softens the surface and edges of the glass without allowing time for the
underside to catch up. This explains uneven
edges. It also risks breaking the glass
from too great expansion of the top before the bottom.
Additionally, the
schedule uses a temperature more than 55ºC/100ºF above what is a reasonable highest
slumping temperature. The top
temperature of this schedule is in the tack fusing range.
There is no need
for a hold 55ºC/100ºF above annealing soak. It is the annealing soak that
equalises the temperature before the cool begins. The higher temperature equalisation is
negated by the cooler soak at annealing temperature. So, the hold at the higher
temperature and slow cool to the annealing temperature only delays the firing
by about two hours. It does not have any
effect on the final piece.
The schedule is cooling for a piece of 12mm/0.5”. This is slower than necessary. As noted above, cooling for one layer thicker
than the piece is advisable to get the most stress free result. The annealing soak could be 1.5 hours
following this idea. Cooling with a
three stage schedule reduces the risk of inducing temporary stresses that might
break the glass. Although the initial
cooling rate I recommend is very similar to this schedule, it safely reduces the
total cooling time.
- ·
69ºC/124ºF
to 427ºC/800ºF, no hold
- ·
125ºC/225ºF
to 371ºC/700ºF, no hold
- ·
330ºC/600ºF
to room temperature, off.
Using my kind of
schedule for the first time will require peeking once top temperature is
reached to determine when the slump is complete. It may take as much as an
hour. Be prepared to either extend the hold, or to skip to the next segment if
complete earlier. The controller manual will explain how.
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