|Monthly Tech-Tip |
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. 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.
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).
Petalite at Wikipedia
Materials used in Denmark
Generic materials are those with no brand name. Normally they are theoretical, the chemistry portrays what a specimen would be if it had no contamination. Generic materials are helpful in educational situations where students need to study material theory (later they graduate to dealing with real world materials). They are also helpful where the chemistry of an actual material is not known. Often the accuracy of calculations is sufficient using generic materials.
The most common source of fluxes for high and medium temperature glazes and bodies.
In ceramics, cristobalite is a form (polymorph) of silica. During firing quartz particles in porcelain can convert to cristobalite. This has implications on the thermal expansion of the fired matrix.
Spodumene is a lithium sourcing feldspar, an alternative to lithium carbonate to supply Li2O to ceramic glazes. Contains up to about 8% Li2O.
Can you be exposed to fluorine gas in ceramics. Yes, if you are using materials that contain fluorine. These are not many, but worth knowing about.
|Oxides||Li2O - Lithium Oxide, Lithia|
|Co-efficient of Linear Expansion||0.15|
|Frit Softening Point||1350C M|
|Density (Specific Gravity)||2.45|
|By Tony Hansen|
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