It is possible to begin your annealing at any point in the
annealing range.
The annealing point is the temperature at which the glass
most quickly relieves the stress within.
This occurs at the glass transition point.
The annealing range is between the softening point and the strain point of the glass. No annealing can be achieved above the
softening point, nor below the strain point.
This range, for practical purposes can be taken to be 55°C above and
below the published annealing point. For
thick slabs, Bullseye has chosen to start the anneal 34°C below the published
annealing point of 516°C.
High Annealing Point
A high annealing temperature, even up to 571°C,
the approximate strain point of the glass could have been chosen, but this is impractical. The effect of this is a greater slow cool range and so an extended anneal
cool. The reasons are as follows:
- The anneal cool range is greater as the first rate of cool needs to be maintained to the strain point.
- The anneal cool has to extend to at least just below the strain point.
- The highest practical annealing temperature is determined by the viscosity of the glass. Any soaks above that temperature are ineffective in production of soundly annealed glass.
- The purpose is to get all the glass at the same temperature in preparation for cooling. It is more difficult to maintain the small differentials in temperature achieved by the annealing soak over a large range of temperature.
Low Annealing Point
Starting the anneal cool closer to the strain point requires
a slightly longer soak to ensure the glass is all at the same temperature (+/- 2.5°C, often called the Delta T=5C)
before the anneal cool begins.
Typically, this initial soak would be for an hour before the initial
cool begins (for a 6mm/0.25" thick piece).
Effect of the Differences in Approach
The advantages and disadvantages centre around the need to:
- soak long enough to get all the glass to the same temperature, and to
- cool slowly enough to maintain the delta T throughout the glass.
Example
If you think of an example of a
piece of Bullseye glass 12mm/0.5" thick, it will show the differences in approach.
High temperature soak
A soak of 120 minutes at 571°C/1060°F (the highest possible start for an annealing soak) is still required to even the temperature. To
ensure the temperature differentials in the glass do not deviate from the Delta T,
the cool needs to be at 18°C/32°F per hour down to 427°C/800°F. It is possible then to increase the speed to
36°C/65°F per hour down to 370°C/700°F. This gives you a
total annealing cool of just over 11.5 hours.
Low temperature soak
Starting the anneal at 482°C still requires a two hour soak followed by a decrease in temperature of 18°C/32°F per hour to
427°C, and an increased rate of 36°C/65°F to 370°C/700°F.
This gives an anneal cool time of just over 6.6 hours.
The example shows how, although the annealing result may be
the same, there is considerable time saved (and especially for thicker pieces) in using the lower part of the
annealing range to begin the annealing.
It also will save some electricity.
However, an anneal of two hours at 516°C with a cool of 18°C/32°F per hour to 427°C/800°F and 36°C/65°F to 370°C/700°F will still give a perfectly adequate anneal for 12mm thick pieces even though it will take about 2 hours longer.
However, an anneal of two hours at 516°C with a cool of 18°C/32°F per hour to 427°C/800°F and 36°C/65°F to 370°C/700°F will still give a perfectly adequate anneal for 12mm thick pieces even though it will take about 2 hours longer.
Revised 30.12.24
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