When firing glass in ceramic moulds, and especially ceramic pots for pot melts, you should be aware of the temperatures at which the ceramic material quickly expands and contracts.
There are refractory ceramics which are not as sensitive as the kind of ceramics we are using in most kiln work. The ceramics we use are not refractory materials and contain, among other things, quartz and crystobalite. These two elements are important, as they have considerable effect on the survival of the pot or mould during the firing.
The effects are called inversions. This is because the rapid expansion experienced upon the heating is reversed as rapid contraction on the cooling of the ceramic.
The first element to be affected by the heat up is crystobalite. This element has a sudden expansion of 2.5% at 226°C. This does not seem to be much, but compare it to the expansion of glass at this temperature - .0085% - almost 300 times that of glass at the same temperature. And of course, the ceramic contracts by that amount when it reaches 226°C on the cooling.
The second element affecting the heat up is quartz. There is quite a bit of this in clay. The critical temperature for this is in the 570°C to 580°C range. The expansion and contraction is not so great here – only 1% - but it is still more than 100% that of the glass, and in a critical range for the glass on the cooling.
More information on the quartz and crystobalite inversions are given here.
The importance of these inversions for us are to remind us to be careful at these temperatures of about 225°C and 570°C - 580°C to prolong the life of the ceramic pots and moulds that we use.
It is probable that 150°C per hour is as quickly as we should increase the temperature when using ceramic moulds or pots. Some thought should be given to the cooling of the moulds too. They should not be taken from the kiln while hot nor subjected to draughts of relatively cold air.