Longer Anneal on Each Firing

 Need for an Extension of Anneal Soak on Subsequent Firings

 Sometimes people recommend extending the length of the anneal soak each time the piece is fired. If nothing significant is added, there is no reason to extend the anneal soak.  If the piece can be fired as fast as the previous firing, the annealing will be the same, not longer.

 The physics and chemistry of annealing glass are the same for re-fired glass - without additions - as they were for the first. Extending the annealing soak seems to be more about reassurance of the kilnformer than a necessity.

 Bullseye research has shown that it IS possible to over anneal, locking in stress. If concerned about inadequate annealing, it is best to reduce the cooling rate. Especially over the first 55°C below the anneal soak temperature.  The testing and recording that I have done for a book on tack fusing shows that any differences in the glass - at the end of the anneal soak - will be relieved in that first 55°C/100°F. The remainder of the cool to 370°C/700°F can be about 1.8 times faster, and the final cool ramp can be 3 times faster than the 2nd stage cool. 

 I have observed that a three stage anneal cool is important to successfully anneal a piece. This has been reinforced by the temperature recordings of many firings. Often at the end of the anneal soak there is a little more than the desired 5°C/10°F difference in temperature across the piece. The recordings show this is relieved during the slow firs stage cool and maintained over the next two cool stages. If the kiln is cooling more slowly than the schedule, no electricity is used.  No kiln time is lost.

An example of the first cooling stage

 The first stage cool is key to a successful stress-free result.  If there are concerns about inadequate annealing, two things are important.  Be sure the right length of soak is chosen for the piece being fired.  Second, reduce the speed of cooling by the rates for a piece at least 3mm thicker.  These rates are available from the Bullseye chart for annealing thick slabs.

 The rates are applicable to other than Bullseye glass.  Only the temperatures need to be changed.

 If no significant changes (other than powder, wafer or stringers) are made to the glass before the second firing, no lengthening of the annealing is necessary.

 More detailed information is available in the e-book: Low Temperature Kilnforming.