Petalite is unique for its thermal properties; like many lithium-bearing compounds, it has a very low rate of thermal expansion, making it a valuable addition for cookware-intended clay bodies (such as flameware). Petalite has close to 0% thermal expansion, which makes it one of the best additives for this purpose. This low thermal expansion may affect glaze fit, however because most glazes do tend to expand and contract (regardless of lithium content), when they are placed on a Petalite bearing clay body. Crazing will likely result as the clay body won't move much and the glaze will.
In glazes, Petalite tends to create silky white surfaces at high and low temperatures and in both oxidation and reduction atmospheres. Because it has such a low iron content, it is a popular glaze additive for white and clear glazes as it melts without delivering a noticeable tint.
Petalite
Petalite is a lithium feldspar used in clay bodies and glazes to help reduce thermal shock problems. It and imparts lower expansion and gives unique color response to copper and cobalt in glazes.