You Can’t Anneal Too Long.
Can you anneal too long?
Yes, you can.
It’s not just the possible temperature differences in the kiln. If you have temperature differentials across your kiln, any piece that crosses those boundaries will have temperature differences locked into the glass. If you know you have temperature differentials and your glass by circumstance must be in both the cooler and the hotter regions, you need to do a standard length of soak only. Then reduce the rate of cooling a little more than normal, so that a slower cool occurs. This should avoid most of the stress that can be induced by very long soaks in a kiln with hot and cool spots.
The other factor against annealing too long has been revealed by Bullseye research on annealing. This video at about 13:00 minutes into the film explains. This complicating factor in annealing is about the difference in temperatures of the surfaces of the glass. The research shows that the longer you anneal the greater the differential in temperature becomes between the upper and lower surfaces of the glass. This means that you can introduce stress across the whole piece, rather than just a section as in an unevenly heated kiln.
What is more, the longer you soak, the cooler the bottom becomes in relation to the top. The reported research does not state the reasons for this. It just commented this as an observational fact. It can be assumed that the air temperature differences are the cause. Even during cooling the air is hotter on top of the shelf than under. This would allow the bottom surface to cool more than the top. This assumption is borne out by the fact that the effect is reduced or eliminated by having elements under the shelf.
There are two reasons to avoid long soaks. Uneven temperatures across the surface are locked into the glass. And long soaks at annealing induce an unwanted temperature differential between the top and the bottom of the piece.