Annealing Range

NOTE: completely revised 31 December 2021

After Bullseye published annealing tables for thick slabs, some people feel they need to use the lower part of the annealing range for all their glass. To determine whether or when to use these tables needs some understanding of the annealing range.

Range

The annealing range of a glass is approximately 40ºC/72ºF on either side of the annealing point, but for practical kiln forming purposes it is normally taken as 33ºC/60ºF. The annealing point is around 510ºC/950ºF for System 96; 516ºC/962ºF for Bullseye and Uroboros for example. The range for a fusing glass will be around 549ºC to 477ºC/1020ºF to 890ºF for fusing glasses. Although the upper half of that range is merely theoretical. The lower end of the range is the strain point.

The annealing soak is to equalise the temperature throughout the glass to within 5ºC. Once the annealing soak is complete, the first stage of cooling begins. This first 55ºC/100ºF below the annealing soak is essential to the adequate annealing of the glass.  And this illustrates the impracticality of annealing in the upper part of the range.  The first cool rate needs to be maintained to at least 55ºC/100ºF below the low end of the annealing range.

To exemplify this. It would be possible to start the annealing at about 550ºC/1020ºF for any of these glasses. But the slow rate of decline in temperature, following the equalisation soak, would need to be maintained for the whole range of 550ºC/1020ºF to 429ºC/805ºF, rather than just the 55ºC/100ºF from the anneal soak point. This would more than double the annealing cool time. 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. It is also why the Spectrum 96 soak above the annealing point was not essential.

Soak

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 cool needs to be continued at a steady slow rate.

Lower part of annealing range

Bullseye now recommends the use of 482ºC/900ºF for  the temperature equalisation soak, but have increased the soak time from 30 minutes to one hour. Choosing to start the annealing process at the lower part of the annealing range speeds the process for thick slabs and is very conservative for thinner glass. Bullseye have not changed the composition of their glass so the anything annealed at 516ºC/960ºF for things 6mm/0.25" or less is still properly annealed.

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 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/900ºF – with a long soak reduces the total time in the kiln, but continues to give a good anneal. In the case of Bullseye, 461ºC/863ºF is the bottom end of the annealing range according to the calculations indicated above.