Potters start off with low quartz as their raw material.
As the temperature rises to 573°C this changes instantaneously to high quartz, with a volume change of 4.8%. Because of this sudden and relatively large change in volume, significant stresses are introduced if there are temperature differences across the pot, as different parts will expand (or contract) at different times. As such, it is important to fire slowly when approaching and oing through this temperature, so temperatures can even out and stresses are minimised.
If the silica is pure, then the next transition is to high crystobalite, which occurs from 1050°C upwards. Although this results in a volume increase of 11.6%, conversion typically takes many hours, so there is time for the pot to adjust to the thermal stresses.
If there are impurities in the silica, then from 870°C high tridymite will form (the starting temperature changes according to the impurities present, e.g. 872°C for Na, 883°C for K and 1005. This has a volume increase of 12.4%, but again the change is very gradual, and so can be accommodated by the pot. As the temperature rises, the tridymite may change to crystobalite - this occurs at 1420°C in isolation, but in the environment typical of clays and glazes may start as low as 1100°C. There is a small reduction in volume with this transition, of less than 1%.
If any amorphous silica is present, e.g. from wood or plant ashes, these will generally form high crystobalite on heating.
If cooling is slow enough, the crystobalite and tridymite may have enough time to change back to quartz. However at the faster rate of temperature change found in a kiln they will change to the low forms. Crystobalite changes between high and low at 270°C, with a contraction of 2.8%. Tridymite is more complex, having at least 7 phases between the its formation temperature and roome temperature, with the first phase change occuring at 477°C. Although the overall contraction is similar to crytobalite at 4.2%, each transition has a volume change of well under 1%, so there is time for the pot to adjust.
On future heatings, and crystobalite and tridymite will change from low to high forms, so caution needs to be taken when approaching and going through their transition temperatures.
Because the transitions in a kiln firing are not symmetrical, when pieces are fired more than once the amounts of tridymite and crystobalite are likely to increase with each firing, so increasing caution must be taken to avoid breakages.<HR>
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