Long Anneal Soaks

Credit: Bullseye https://www.youtube.com/watch?v=-8hK9Klprvc

Long anneal times seem to be becoming popular. At least they are being recommended frequently by people in Facebook groups. They are recommending very long anneals to solve stress and breakage problems.

Are they effective?

This Bullseye video on some results from annealing research shows (at 13:00 minutes) that excessively long soaks can cause more stress than the recommended length does. The video shows a 1” slab annealed for 4 hours (the recommended time) has less stress than one annealed for 16 hours.

The thoughts are that this effect results from the cooler space under the shelf and glass than the top during long soaks. This induces temperature differences between the bottom and the top of the glass, if not across the surface. The recordings show that during the anneal soak the temperature at the bottom of the kiln is less than above the glass.  This difference on a long anneal soak is larger than the ΔT=5˚C required for a good anneal.

The remedy shown by the video is to introduce heating elements under the shelf, which are separately controlled. This is impractical and is not needed in smaller kilns. The solution for these smaller kilns is to use slower and graduated cooling rates from the end of the annealing soak – not longer annealing soaks. 

The slower rate can be selected from the table. Whether you choose the rates for one or two layers thicker, relates to your perception of risk. Do not extend the length of the annealing soak when you use the slower cooling rates.

This cooling process has been researched by Bullseye and is effective to keep the glass within the temperature distribution requirements. It is a three-stage process. Stage one is to 427˚C/800˚F. The rate for the actual or calculated thickness is given in the Bullseye table (see below).

Stage two is to 371˚C/700˚F. This is normally 1.8 times the rate of the first cool.

Stage three is the cooling from 371˚C/700˚F to room temperature. This can be up to 6 times the first stage cooling rate. However, I find that a final cooling rate of 330˚C/600˚F is faster than most kilns can achieve.

I do programme this final cooling into the schedule. 

• It does not use more electricity unless the kiln is cooling faster than programmed. 
• It does not cause the relays it click in and out if the cooling rate of the kiln is slower than programmed. 
• It does protect the glass from too rapid cooling, or peeking. This is so especially from 100˚C/212˚F, when we are inclined to want the glass to cool faster than the closed kiln allows. 
• The sound of the relays operating indicates the kiln is open too much for the safety of the glass.

 

The Bullseye table Annealing Thick Slabs shows the recommended soak times and cool rates for each cooling stage. It is applicable to all fusing glasses. After annealing for the appropriate time at the temperature for your glass, use the rates and temperatures from this table.