• Materials for Ceramics

Petalite

Notes

Petalite is a lithium aluminum silicate mineral (more simply a lithium feldspar) that is commonly used in clay bodies. It is valuable because it provides an insoluble source of lithium and has the highest Li₂O:Al₂O₃ ratio of any natural mineral. Lithium is a strong alkaline flux and is effective over all temperature ranges. It imparts lower expansion and gives a unique color response to copper and cobalt in glazes. Some commercial versions have a chemistry that fairly closely matches the theoretical chemistry given here.

Petalite is most prized for its mineralogical properties. 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

Petalite can be used to create low expansion glazes. It is suitable for white and transparents as well because of the low iron content. It does not present the frothing problems sometimes associated with spodumene and helps improve brightness. One formula suggested by Val Cushing is petalite 76.9, talc 14.2, whiting 3.4, kaolin 5.5. This glaze is claimed to have an expansion of 0.8 x 10 -6 and fires cone 9 to 12 with a silky surface.

Spodumene and lepidolite also provide insoluble sources of lithium. However, spodumene is more refractory, thus producing more porous fired results. If a body's range needs to be extended, spodumene can thus be added in preference to silica. Petalite is slightly more refractory than lepidolite.

P.C.E.: 15
P.C.E. 45% petalite:55% F4 Feldspar: 4
P.C.E. 45% petalite:55% Nepheline Syenite: 2

A petalite clay body can be made by combining petalite and ball clay or kaolin in proportions that provide a good compromise between high petalite content and necessary plasticity.

Petalite promotes the conversion of the quartz form of silica to the cristobalite form. This has implications on the thermal expansion of the fired matrix.

Spodumene is generally a better source of Li₂O for frits than petalite.

Related Information

Here is why Petalite and Spodumene will seldom substitute for Lithium Carbonate

Of course, if a recipe only calls for 1-5% lithium carbonate either of these might be candidates to supply the Li₂O. However, Petalite is eight times less and Spodumene five times less concentrated than lithium carbonate so to make either worthwhile the prices would need to be eight and five times cheaper. But if a recipe calls for more there is another problem: Petalite is extremely high in silica, which means supplying the needed Li₂O from it is almost certainly going to oversupply SiO₂. Spodumene will likely do the same. Both are also high in Al₂O₃ and likely to oversupply that (or at minimum supply the bulk preventing the presence of kaolin in the recipe).

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