I
frequently advocate using slow rates of advance and low temperatures to achieve
the results desired with a minimum of marking in forming, or a minimum of
firing difficulties during the fusing part of kilnforming.
But there
are limits to this both in terms of physics and practicality. There are temperatures below which no amount
of slow heat input will affect the brittle nature of the glass, for
example. If your temperature is below
the strain point of the glass, virtually no change will occur even with very
long soaks. The graph below shows the slumping range is from the annealing point (glass transition temperature) to about 180C above the annealing temperature. After that temperature the glass is prone to devitrification (the beginnings of crystallisation).
 |
| This shows the the slumping range of a specialised glass rather than the soda lime glass that kilnformers normally use. |
In this graph, the glass has an annealing temperature of about 600C, which is higher than that for float glass and much higher than for kilnforming glasses. The glass transition temperature range for existing fusing compatible glasses is around 510C (+/- ~10C). Float glass has a higher annealing point of around 540C (+/- ~ 10C). Following the research behind this graph, stable slumping temperatures would be in the range of about 510C to 690C (+/- 10C).
It is important to be aware that the annealing point is determined mathematically as the glass transition point. This is the annealing point at which temperature the glass can be most quickly annealed. The practical research conducted by Bullseye has shown that a temperature equalisation soak in the lower part of the annealing range is a good solution to the the practicality of balancing adequate annealing with the use of the kiln time. The annealing point temperature and that which you use to equalise the temperature within the glass may be quite different.
Even where it is possible to achieve an effect at a low temperature, it can take too long to be practical. For example, I can bend float glass at 590°C in 20 minutes into a 1/3 cylinder. I could also bend it at 550°C (just 10°C above the annealing point), but it would take more than 12 hours. This is not practical.
In addition to practicality, there is the physical limitation. If you slump below the glass transition point, you will be unable to properly anneal the glass and therefore produce an unstable item. It will contain stress from this inadequate annealing leading to an increased fragility.
It is important to be aware that the annealing point is determined mathematically as the glass transition point. This is the annealing point at which temperature the glass can be most quickly annealed. The practical research conducted by Bullseye has shown that a temperature equalisation soak in the lower part of the annealing range is a good solution to the the practicality of balancing adequate annealing with the use of the kiln time. The annealing point temperature and that which you use to equalise the temperature within the glass may be quite different.
Even where it is possible to achieve an effect at a low temperature, it can take too long to be practical. For example, I can bend float glass at 590°C in 20 minutes into a 1/3 cylinder. I could also bend it at 550°C (just 10°C above the annealing point), but it would take more than 12 hours. This is not practical.
In addition to practicality, there is the physical limitation. If you slump below the glass transition point, you will be unable to properly anneal the glass and therefore produce an unstable item. It will contain stress from this inadequate annealing leading to an increased fragility.
The
balance required between the rate of advance and top temperature means that you
will need to do your own experiments to find where the practical limits to
using heat work are for you. The more patient you are, the lower temperature
you can use.
More detailed information is available in the e-book: Low Temperature Kilnforming.
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