As of June 2009, Bullseye has changed its chart for annealing thick slabs. Specifically, the recommended anneal soak temperature has been lowered from 960°F/516°C to 900°F/482°C.
Stress Out!: Avoiding Painful Breaks and Strains
Derived from Ted Sawyer's talk at BECon 2009, this 20-minute presentation focuses on the theory and process of getting the stress out of fused glass work and acts as an introduction to the new annealing cycle recommended by Bullseye.
Why the change?
For several years we have used the new 900°F/482°C soak temperature for everything from simple fused pieces to large-scale castings, with tremendous success. We now consider it more practical than 960°F/516°C—especially on larger, thicker projects, for two reasons:
- More effective: After the stress has been relieved by holding the glass at an anneal soak temperature of 900°F/482°C, the glass cools over a shorter span of temperature in which annealing stress could be introduced than it would be if held at 960°F/516°C.
- More efficient: It takes less time to cool over a shorter span of temperature.
What about past work that has already been made with the anneal soak at 960°F/516°C?
There is no need to worry about this work. Effective annealing has been and can be accomplished when the anneal soak is performed at 960°F/516°C. It just takes longer, especially with projects that are thicker or very large. If your past projects have been successful using cycles with an anneal soak at 960°F/516°C, you may continue to use that temperature. Or, if you wish to save time, you can revise your cycle to have an anneal soak at 900°F/482°C.
Why can most works be annealed successfully with either anneal soak temperature?
This is because when annealing glass, the most important factor is not the temperature at which one performs the anneal soak (within reason). Rather, the most important factor is the ability to achieve uniform temperature throughout the body of glass during the anneal soak and subsequently cool the glass in such a manner that it does not develop more than a 10°F/5°C temperature difference throughout the body of glass during the first anneal cool to 800°F/427°C.
For more information on this topic, see TechNote 7: Monitoring Kiln Temperatures for Successful Annealing.