There is an relationship between how heat is applied and the temperature required to achieve the wanted result. Heat can be put into the glass quickly, but to achieve the desired result, it will need a relatively higher temperature. If you put the heat into the glass more slowly, it will require a relatively lower temperature.
For example, you may be able to achieve your desired result at 814C with a 400C/hr rise and 10min soak. But you may also be able to achieve the same result by using 790C with a 250C/hr rise and 10min soak. The same amount of heat has gone into the glass, as evidenced by the same result, but with different kinds of schedules. This can be important with thick glass, or with slumps where you want the minimum of mould marks. Of course, you can also achieve the same results with the fast rise with a longer soak at the lower temperature, e.g. a 400C/hr to 790C with a 30 min soak.
In short, this means that heat work is a combination of time and temperature. The same effect can be achieved in two ways:
- fast rates of advance and high temperatures
- slow rates of advance and low temperatures.
You obtain greater control over the processes when firing at slower rates with lower temperatures. There is less marking of the back of the piece. There is less sticking of the separators to the back and so less cleanup. There is less needling with the lower temperature.
The adage “slow and low” comes from this concept of heat work. The best results come from lower temperature processing, rather than fast processing of the kiln forming.