PETA
A lithium aluminium silicate, sold as a milled concentrate · Carries lithia in an insoluble form · Lowers the thermal expansion of a body and of a glaze · Converts to a lower-expansion phase at almost the same density, so the body holds its shape
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Petalite is a lithium aluminium silicate, and it carries lithia into a body or a glaze in an insoluble form. It goes into flameware and ovenware bodies, where it drops thermal expansion far enough for a pot to take direct heat.
Petalite is mined and milled as a concentrate, so a bag is largely but not purely the mineral itself. It is bought almost entirely for what it does to expansion: flameware and ovenware bodies are built around it, and it goes into glazes where crazing or thermal shock is the problem. Lithia is the lightest of the common fluxing oxides, so a given weight of it puts far more flux into a melt than the same weight of a heavier one. On the way up it converts to a lower-expansion phase, a change that gets going around 750°C. Petalite and the phase it becomes are packed almost identically, so the body gains no volume as it happens.
