The shape of an aperture drop can be controlled by the speed
of the slump. The speed at which the glass drops is a combination of heat and
size of the hole. Patience is required.
Rapid drops result from high temperatures. Rapid slumps
cause a thinning of the glass at the shoulder where the glass turns over the
inner rim of the aperture. The pattern is distorted and the colours are also
diluted. And a relatively large rim is left around the fired piece.
A much slower rate of drop spreads the strain of the slump over the whole of the unsupported area of glass. This tends toward a bowl with a gentle slope toward the bottom, reduced distortion of the pattern, maintenance of the colour densities, and a more even wall thickness all over the piece.
The slumping temperature for a shallow angled slump is less
than that used for normal slumps, and takes a lot longer – up to five hours
typically. This means that observation is required at intervals, say every half
hour.
A starting point for the slumping is around 100ºC above the
annealing temperature for the glass. So for Bullseye and System 96 the
temperature is about 615ºC. If after the first half hour, there is no movement,
increase the temperature by 10ºC. Check again in another half hour and if the
slump has begun, leave the temperature at that level and observe at the half
hourly intervals until the desired slump is achieved. Otherwise, increase the
temperature by another 10ºC with the check after half an hour, and repeat until
the slump has begun. After you have done the first one of these with a
particular size of aperture, you will know the temperature to start the slump.
The temperature you need to use is affected by the size of
the hole. The smaller the aperture, the higher the temperature will be needed.
But be patient. If the temperature is increased too much, you will get the
thinning of the sides that you are trying to avoid.
Additional information on aperture drops can be found
in this
series.
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