Fusing glass takes place at lower temperatures than those required for ceramics, However because glass is prone to thermal shock, the firing must take place in 5 stages. Therefore, the firing cannot be done manually. the kiln must be programmable to take this complex series of temperature rises, maintenance (soaking) and drops
Heating phase. Where the temperature increases from room temperature to fusing or slumping.
Soaking phase. Where the temperature is maintained at a given point for a period of time
Rapid cooling phase. when the temperature is rapidly dropped from its highest point to just above the annealing phase. The kiln can be opened at this point to allow the temperature to drop very rapidly (flash venting).. This is particularly used for glass which is prone to devitrification.
Annealing phase. The critical step that relives the stress in the glass. This may be a series of drops and soaks or a very slow decrease in temperature, Often the longest part of the firing, it is 8 hours for Float glass, and 6 hours for Spectrum 96.