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Glass Tips from Verrier
Information on stained glass, fusing, kilnforming and glass working
Wednesday, 23 April 2025
Heat Shielding Glass
Wednesday, 16 April 2025
Testing for Stress
Wednesday, 9 April 2025
Wednesday, 26 March 2025
Wednesday, 19 March 2025
Bubbles on Single Layer Fusing
“I'm making 3mm French Vanilla sconce covers; …
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[initially they were] fine, but now 1.5"
bubbles form during the full fuse.
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I pop the bubbles and fill the holes with
frit and refire,
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[The]… edges draw in and distort the design…
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The shelf is flat,
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I fire on Bullseye paper, and
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the 13.5 hour long firing schedule [in F] is:
200 to 1150, hold 30 minutes.
50 to 1225, hold 30 minutes.
300 to 1490, hold 30 minutes.
9999 to 990, hold 60 minutes.
100 to 750, hold 1 minute.
Does anyone know what I can do to avoid the large bubbles?
A critique of the schedule.
This is for a single sheet of 3mm glass, so the hold at 621˚C/1150˚F is unnecessary as is the slow rise to and hold at 663˚C/1225˚F, because it is a single sheet and does not need the traditional bubble squeeze.
The hold of 30 minutes at 810˚C/1490˚F is excessive.
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The temperature may be too high.
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Ten minutes at top temperature is sufficient in
most cases.
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A soak of 1 minute would be enough.
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The anneal soak at 990˚F is most probably a
misprint for 516˚C/960˚F.
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The anneal soak is longer than the half hour
necessary, but not a bubble creating problem.
It
means the schedule could have been:
111˚C/200˚F to 796˚C/1465˚F for 5 minutes
AFAP to 516˚C/960˚F for 30 minutes
83˚C/150F˚ to 370F˚/700F˚, 0 minutes
Off
Different firing strategies are
possible.
- Reduce the time at top temperature to no more than 10 minutes.
- Reduce top temperature by 55˚C/100˚F or more and extend the soak to 20 minutes, if necessary. Peek frequently to see when the kiln work is complete.
- Fire on fibre paper covered with Thinfire to allow air out from under the glass.
These strategies can be mixed as desired,
and the reasoning for the strategies is:
- Excessive time at the top temperature allows the glass to thin as it migrates to form thicker areas/edges. This makes the glass too thin to resist the air pressure from below.
- Reducing the top temperature will increase the viscosity, so resisting the migration of the glass, and maintain the original thickness.
- Also, single layers are prone to dog boning, but there are ways of reducing it.
Ways to reduce the risk of bubbles appearing in general are:
- Reduce the time at the top temperature,
- Reduce the top temperature,
- Provide ways for the expanding air to migrate from under the glass.
Friday, 14 March 2025
Draping over steep moulds
Draping over a narrow or small supporting ridge with large areas of glass is difficult.
One solution might be just to invert the whole piece and let the glass slide down into the mould. However, there rarely is enough height in a glass kiln for deep slumps, especially with a “V” shaped mould. It has to be high enough for the edges of the glass to be supported at its edges. You could also approach this by having a first mould with a shallower angle or broader support at its centre. Drape over this first, then use the steeper mould as the second draping mould. This makes the balance less critical.
The idea of supporting the glass is the key to doing this kind of slump that seems to require an impossible balancing act, if it is to be done in one go. Place kiln washed kiln furniture at the edges of the otherwise unsupported glass. Fire the kiln, but watch until the glass begins to slump. Then reach in with a wet stick and knock the kiln furniture aside to allow the glass to continue its slump and conform to the mould shape.
The lower temperature you use to do the draping and the slower your rate of increase is, the less the glass will be less marked by the mould. Frequent brief visual inspection during the drape is vital.
Also have a look at a suggestion for the kind of firing required for this here.
Wednesday, 12 March 2025
Dog Boning in Slumps
I have done a few experiments on rectangular moulds with 3mm and 6mm thickness. I could not eliminate dog boning with larger rims, slower rates, or lower temperatures in any combination - although they did reduce the effect.
Square single layers dog boned even with increased rim width,
and reduction of slumping depth made little difference in the amount of dog
boning.
Rectangular single layers shapes persisted in dog boning on
the long side regardless of the rim dimension, and exhibited more dog boning on
the long side than in the equivalent single layer square. Two layer slumping had a decrease in dog
boning with increased rim width, but with less effect on the long side.
In general, glass slumped in rectangular moulds is more
sensitive the shape of the rectangle than the size of the rim, and very
sensitive to symmetrical placing on the mould.
The depth of the mould has less influence than the size of the rim,
especially for single layers. The wider
the rim, the less dog boning, in general terms.
Deeper moulds, higher temperatures, longer holds, narrower
rims, all increased the dog boning. I conclude slumped square glass looks
better because the dog boning is symmetrical.
My solution is to make bigger rims and cut the piece square
after slumping. This approach needs cold work to the edges, of course.
The reason rectangular slumps dog bone is because the glass at the sides is drawn into the mould more easily than the corners, because there is more glass to draw in, just as in flat dog boning.
An alternative to the cold working is to round the corners
of the rectangles to reduce the amount to draw-in. A 1cm/0.375” radius curve will reduce the extent
of the dog boning, but does not eliminate the effect.
Wednesday, 5 March 2025
Pressing glass
I have been looking for a different way than flows or melts to mix
colours and thought glass pressing might be a promising way to achieve what I
wanted.
Weight vs Temperature
I conducted some experiments attempting to thin 1.25 kg/2.75 pounds of
glass to 3-4mm. One and then two 40x40cmx15mm
thick shelves were placed on top of the glass cullet with 3mm spacers at the
corners. The glass was fired at 220ºC/396ºF to 825ºC/1517ºF and initially held
for 30 minutes, later extended to 90 minutes.
The thickness stubbornly remained between 5 and 7mm.
A few other attempts with different times and temperatures gave
inconsistent results. Perhaps the uneven
piling of cullet had an influence on the outcomes, but I was still looking for
a flow and mixing of colours different to that obtained by melts.
Other experiments were being conducted in parallel, relating to
viscosity. These indicated that glass became thinner than 6-7mm at higher
temperatures without pressing. These
experiments lead me to think there are four elements controllable by
kilnformers in pressing: size, weight, time, temperature.
The same weight of press with the
same temperature and time will make small amounts thinner than large amounts,
and this is not surprising. More time with
the same temperature, weight, and amount allows some slight decrease in
thickness.
Higher temperatures with the same
weight, and time will allow thinner pressings of the same amount of glass. Viscosity decreases with temperature, so higher
temperatures make glass easier to thin.
More weight is required get the
same thickness when pressing a greater volume of glass. Of course, more time and temperature can be
added to increase the effect of the weight.
However, the main factor in pressing large amounts of glass
is higher temperatures, which results in reducing the viscosity and the resistance to thinning.
Annealing and Cooling
An important aspect of pressing is the annealing requirements. It is sensible to anneal for a longer time
than normal for thick glass, because of the heat retention of the pressing
weights.
This image shows the stress in an 8mm/0.3” (or 5/16”) after annealing as
for 16mm/0.63” (5/8”). There is widespread
low level stress with 30mm thick pressing weight.
Indications are that extending the annealing to at least 3 times the
target thickness is a minimum annealing soak requirement. Alternatively, if it is possible to remove
some, or all, of the weight from the glass at the beginning of the anneal soak, the annealing time can be reduced.
Veiling
The stress picture above shows there is visual element too. This veiling is most apparent in clear glass, and less obvious in coloured and opalescent glass. Small volume stacks, which are pressed thin will exhibit less of the veiling.
Four factors that kilnformers can control in pressing glass to less than
6mm are weight, size, time, and temperature.
The main one is temperature.
Wednesday, 26 February 2025
Stress Analysis of Broken Glass
Will stress still show with polarised filters on cracked and broken glass?
It's not a straightforward answer.
I was looking at some broken fused float glass a few years
ago. I had always subscribed to the idea
that a fracture relieves the stress. Not always. The broken float glass had been slumped, and the pieces still showed stress. This turned out to be a compatibility problem, although
both layers were float.
The stress of inadequately annealed glass is likely to
remain visible through the filters, because inadequately annealed glass will
have stress distributed across the whole piece.
But glass that has been cooled too quickly and suffered thermal shock,
is more likely to show minimum stress because the break relieved most of it.
It is likely stress will show on the tree piece pictured because
it has not completely broken a[art. And even when it does break, it may still
show a residue of stress.
It is sensible when trying to diagnose the problem to
perform a strip test of the glasses for compatibility of the glasses concerned to
be sure what is happening. If no stress shows on the test strip, the stress showing
on the cracked piece is unlikely to be from incompatible glass, and other factors need to be considered.
Photo credit: Debi Frock-Lyons
Sunday, 23 February 2025
Rapid Ramp Rates with Soaks
In both these cases, these are about the materials holding or contained in the glass, rather than the glass itself.
Cordierite/Mullite vs. pizza stones or tiles
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cordierite/mullite shelves |
This formulation of materials has an extremely low coefficient of thermal expansion that explains the outstanding thermal shock resistance of these kiln furniture materials. They are also strong although heavy. Cordierite/mullite shelves are sintered, to allow the mullite needles to form, and fired at 1400C+, higher than tiles.
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pizza stones |
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corelite shelves |