This is a description of the analysis process to determine the possible causes of a split during a slump.
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| Credit: Maureen Nolan |
Observe the piece.
It is a tack fused piece, about 20cm (8") square, which
has been slumped.
The base layer is of clear. The piece has three additional layers,
but the fourth layer is only of small glass dots and rectangles. The central, heart, area is made of three
layers.
A split has appeared during the slump. It is split
irregularly through pieces rather than around them. It is split through the thickness but only
partially across the piece.
In one area the (brown) third of four layers spans the
split. Further to the left a brown
second layer seems to have broken, but still spans the split.
Threads and particles of glass are connecting across the
split.
The edges are probably sharp, although only so much can be
deduced from a description and one photograph.
History of the Piece
The tack fused piece has been put in a mould to form a platter
and has split during the slump.
The schedule in essence was:
139ºC/250ºF to 565ºC/1050ºF for a 30’
soak (some pauses but all at a ramp rate of 139ºC/250ºF)
83ºC/150ºF to 688ºC/1270ºF for 10’
222ºC/400ºF to 516ºC /960ºF for 60’
111ºC/200ºF to 427ºC/800ºF for 10’
167ºC/300ºF to 38ºC/100ºF, off
The assumption is that the tack fused
piece received a similar annealing soak and cool.
Diagnosis
Too fast
Slumping a tack fused piece of three
layers plus decorative elements on top needs to be fired as for 19mm (6 layers)
minimum (twice the actual). My work for the
Low Temperature Kilnforming* eBook showed best results are achieved by slumping
as for one more layer (21 mm/0.825” in this case). This gives a proposed schedule of:
120ºC/216ºF to 630ºC/1166ºF (not 688ºC/1270ºF) but for 30 to 45 minutes
AFAP (not 400ºF) to anneal 516ºC/960ºF for 3.5 hours (not 1 hour)
20ºC/36ºF to 427ºC/800ºF, 0
36ºC/65ºF to 371ºC/700ºF,0
120ºC/216ºF to room temperature
Commentary on the proposed schedule:
The slump is relatively shallow, so a low
temperature with a long soak is the most suitable schedule for this piece. The drop to anneal is at a sedate rate of 222ºC/400ºF. This is inappropriate, generally. Just as there is a rapid rate to top
temperature to avoid devitrification, so there needs to be an AFAP drop to
anneal, also to avoid devitrification. The
anneal soak was not the cause of the break, but it is worthwhile noting the
recommended anneal soak and cool rates are longer and slower than that
used. This is a note for the future.
Suspect Tack Fuse
If the tack fuse schedule was like the
slump schedule, the slump was started with inadequate annealing in the previous
firing. More importantly, the evidence
for an inadequate tack fuse is that the split under the brown rectangle was
through the clear and red on top, but the split left the brown intact. This means it was not securely fixed to the
red below it.
If the condition of the tack fuse is not
sound, it is probable that difficulties will be experienced in the slump. The poster commented “… why do [these splits]
happen only when slumping – it came through tack just fine.” It is probable the tack fuse was not “just
fine”. The way to be sure the previous
firing was just fine, is to test for stress.
There is enough clear in this piece that an inspection for stress could be conducted by use of polarising filters before the slump. Testing for stress is a simple viewing of the piece between two sheets of polarised light filters. Doing this test will give information on the amount of stress, if any, in the flat tack fused blank.
Slump Split
During slumping the glass is subjected to
more movement and therefore stress than while being fired flat. The glass is often only barely out of the brittle
zone when being slumped and that makes the stress more evident during the early
part of the slump. This requires careful inspection of the failed piece.
Look at the glass surrounding the split. My opinion is that the edges are sharp. If rounded, the threads of glass from the
edges of white would have melted to the edges of the split rather than spanning
it.
It appears the top layers were hot enough for less viscous glass on top to form stringers that span the break as the underlying layers split. It is probable that the split was during the plastic phase of the slump for the upper glass, but the lower layers were not as hot and suffered thermal shock.
This split of lower layers, while the
overlying ones are whole, is often seen in tack fuses, although the top ones do
slump into the gap as the firing proceeds. In a slump there is
not enough heat, time or space, for the brown piece to slump into the gap. Both splits appear to be a result of too
rapid firing. In the flat fusing work,
the split results from too fast a ramp rate during the brittle phase of the
glass. But the slumping splits appear to
occur after the brittle phase, almost as a slow tear in the glass. This may result
from the differential heating of the layers if not fully combined. It may also indicate the split developed
slowly.
One other observation is that these
splits seem to be more frequent during the slumping of tack fused pieces. As speculated above, it may be the inadequate
tacking together of the pieces of glass during the first firing, which can form a discontinuity in transmitting heat. And it may be that the different thicknesses
across the tack fused piece allow stress to build from differential heating of
the glass.
Rates
Whichever of these speculative effects
may be true, it appears the ramp rates are suspect. As mentioned elsewhere* (and in Kilnforming
Principles and Practice to be published soon), the reasons for these splits
are not fully known. Even microscopic
examination by Ted Sawyer has not produced a satisfactory explanation. The only practical approach that has been
successful is to slow the ramp rates. However,
the appearance of these splits is essentially random (with our current
understanding), so prevention is difficult.
Conclusion
The probable cause of the split in the
slump has been that the ramp rates were too fast. This may have been made worse by the too
short anneal soak, and the too fast cool of the tack fused blank.
Remedy
There is no practical rescue for this
piece. Prevention in the future is to
use ramp rates that are for at least one layer thicker, if it is full
fused. If it is tack fused, firing as
for twice the thickest part plus one additional layer is advisable to slow the
ramp rates, allowing all the glass to heat and form at the same rate.
*Low Temperature Kilnforming; an Evidence-Based
Approach to Scheduling. Available from:
and
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