Stress is induced into glass during cooling through the outsides of the glass cooling more quickly than the interior. This contraction causes residual stress. Annealing is the process to relieve that stress. The annealing soak temperature is determined by a number of factors, of which coefficient of expansion, viscosity, exposed surface, and thickness are some. “The relief from stress happens because of a process of viscous flow. At the annealing point it can take place within a few minutes whilst at the lower annealing temperature…. It can take a few hours.” (Dictionary of Glass, Charles Bray, p.27)
The above statement is applicable to glass of a single colour from one manufacturer. When combining colours in kiln forming, the colours absorb and give off heat at different rates and so you need to allow more time for the annealing – relieving of heat induced stress – to occur. So, in the case of our work, annealing needs to be performed over a range.
The annealing point soak has the purpose of allowing all the glass to be the same temperature from top to bottom, and side to side. The annealing occurs during the slow cool past the lower strain point – usually about 50C below the annealing point. The manufacturers give annealing and strain points for their glass. These should be observed, rather than anything pre-programmed into your kiln’s controller.
Note that the stress of incompatible glass cannot be relieved by annealing.
Also, each time the glass is taken to a temperature above the annealing point, it must be annealed again. There is no short cut to this.
There are more notes on annealing here.