Anneal and Cool Relationship

Annealing and cooling are directly related. You cannot extend the anneal soak without also slowing the cooling rates and expect to have a sound piece. What I am seeing on the internet groups about annealing breaks is comments saying the anneal soak is not long enough. So, people add time to the hold at the annealing temperature and still get breaks. They get breaks because the cooling rates are not slowed when the annealing soak time is extended.

A recording of an anneal soak and cool

If you need 3 hours anneal soak, you cannot cool at a rate of 83C°/150°F to 371C°/700°F. An anneal of 3 hours implies you are firing a piece of effectively* 19mm/0.75”. This needs a cool rate of :

• ·        25°C to 427°C. (45°F/hr to 800°F),
• ·        45°C/hour to 371°C (81°F/hr to 700°F),
• ·        150°C/hour (270°F) to room temperature.

Put the other way around, if you can use a first cool rate of 55°C (100°F)/hr you can use a two-hour soak at anneal. That means that you are firing a piece effectively* 12mm/0.5” thick.

But you cannot expect to maintain the required small temperature differential of 5°C/10°F (achieved at the anneal) with a single cool rate. Tests by Bullseye and confirmed by my own recorded tests show that a three-stage cooling is necessary to maintain that small difference of temperature throughout the cooling without using excessive firing times.

A two-hour soak requires cooling in three stages of:

• ·        55°C /100°F to 427°C/ 800°F
• ·        99°C/179°F to 371°C/700°F
• ·        330°C/595°F to room temperature.

If that small 5°C/10°F temperature differential is not maintained in the first stage cooling, temporary stresses can be induced.  Slightly higher levels of temperature differentials can be withstood during the next stages. The stresses induced by larger temperature differences can be great enough to break the glass. In many schedules published online by kilnformers, very long soaks are being used in relation the effective* thickness. But the cool soaks are too rapid in relation to the anneal hold to avoid inducing excessive (although temporary) stress.

This practice presumes the anneal soak is all there is to the production of a sound piece of glass. It is not. The cool rates from annealing to room temperature are important. To repeat, a long annealing soak with fast cool rates can lead to breaks - breaks that are not related to the annealing time. The cooling rates must be related to the amount of time needed for the anneal soak. A fast cool can induce temporary stresses that are great enough to break the glass. The appearance of the break will often be similar to an anneal break.

Don’t worry about using additional electricity with the slower rates of cooling. If the kiln cools more slowly than the scheduled rate, no power will be used. The relays will not have to operate.

Annealing times and cool rates are intimately related. And must be scheduled in relation to one another to avoid unnecessary breaks.

A more extensive discussion of this issue can be found in the ebook Low Temperature Kilnforming.

*”Effectively” in this context means a flat piece of the given dimension. The “thickness” of piece that is of uneven levels - as for a tack fuse - can be calculated to need firing as though it was a multiple of the actual total thickness. The multiple is based on the tack fusing profile.