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Petalite Powder for sale. (LiAlSi4O10), also known as castorite, is a lithium feldspar with high silica content. It is used both in clay bodies and glazes to help decrease thermal shock problems. If used in large quantities it can cause glaze fit problems.
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 flamewar). 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.
It is especially valuable in imparting thermal shock resistance to clay bodies because it has almost zero expansion when heated above 700C. Bodies with 60%+ petalite can take a direct flame and rapid water cooling without failure. To make a plastic clay body mix petalite with as much ball clay as needed. For a casting body, use as much kaolin as needed to achieve the desired casting properties. Bentonite can be added to either type of body to increase the petalite proportion.
One serious problem with low-expansion petalite bodies is that it is very difficult to achieve glaze fit. All common glazes will craze. This is compounded at lower temperatures where the limited low-expansion silica and alumina necessary for melting raises glaze expansion. For some low-expansion bodies, it is almost impossible to match a glaze.
