Sometimes, people feel they need to use the lower part of the annealing range for all their glass, as Bullseye has published annealing tables for thick slabs. To determine whether or when to use these tables needs some understanding of the annealing range.
The annealing range of a glass is approximately 40ºC on either side of the annealing point, but for practical kiln forming purposes it is normally taken as 55ºC. The annealing point is around 510ºC for System 96; 516ºC for Bullseye and Uroboros for example. So the range for a fusing glass will be around 565ºC to 465ºC.
So it would be possible to start the annealing at about 560C for any of these glasses. But the slow rate of decline in temperature, following the equalisation soak, would need to be maintained for all 110ºC for the whole range, rather than just the 55º from the anneal soak point. This would double the time of the annealing cool. This high temperature anneal is a much slower process, which – together with the more rapid relief of stress at the annealing point – is why the top of the range is never used for the temperature equalisation point.
The annealing point is the temperature at which, if all the glass is at the same temperature, the most rapid cooling can take place. To achieve that equalisation temperature (+ or – 5ºC throughout), the glass needs to be soaked at the annealing point for varying lenghts of time relating to thickness and other variables. To complete the anneal and keep the glass within that tight range of temperature, the anneal needs to be continued at a steady slow rate of temperature change.
Bullseye still uses 516ºC as the annealing point for things up to 9mm thick, but chooses to use the lower part of the annealing range for thicker items. Choosing to start the annealing process at the lower part of the annealing range speeds the process for thick slabs. Bullseye have not changed the composition of their glass so the usual annealing point is 516ºC for things less than 12mm.
Using the bottom end of the annealing range for thick items, means there are a fewer number of degrees of very slow cooling to the strain point. But this lower soak, or temperature equalisation point, requires a much longer soak to equalise the temperature within the glass before the slow steady decline in temperature to maintain the temperature differentials within the glass to less than 5ºC.
Bullseye have found that using a temperature a bit above the bottom end – 482ºC – with a long soak reduces the total time in the kiln, but continues to give a good anneal. In the case of Bullseye, 476C is the bottom end of the annealing range. However, this low temperature equalisation soak is mostly applicable to thick slabs, and not necessary for things less than 12mm thick.