A white nonmetallic, natural mineral identified chemically as calcium metasilicate, CaSiO3. It is the only commercially available pure white mineral that is wholly acicular (needle-like crystals). Wollastonite is available in fine particle size powders as well as fibrous 'high aspect ratio' products (20:1). This material has a very unusual texture; it does not flow at all (a hand full can be picked up with fingers downward). They vary in purity; some require almost no beneficiation; others may require removal of up to 80% impurities such as garnet, diopside, limestone, and dolomite (e.g. by magnetic separation, froth flotation, optical sorting). Synthetic wollastonite is also made by combining quicklime with quartz, calcium carbonate and calcium hydrate.
The fibrous form of wollastonite can be very beneficial in bodies. In low fired ceramics wollastonite reduces drying and firing shrinkage and drying and firing warpage. It also promotes lower moisture and thermal expansion in the fired product and increases firing strength. It fires with no LOI and its fibers help vent out gassing. These factors have made it a valuable component in tile bodies, especially for fast fire. It is common to see 10% wollastonite in low fire earthenware recipes. Vitreous and semi vitreous bodies can also show reduced shrinkage with small additions (2-5%), however wollastonite becomes a stronger flux as temperatures go above 1100C.
Wollastonite exhibits a slight solubility in water, but slips containing it can become more alkaline (potentially affecting rheological properties). This affects its tendency to form agglomerates during storage (which create lumps in glazes necessitating sieving). Manufacturers warn that stock should be rotated to prevent it getting too old, that it should be stored in dry conditions and that pallets should not be stacked more than two high.
At higher temperatures the powdered form is valuable as a source of CaO flux in glazes (and bodies). The other main raw source of CaO is whiting but it releases a high volume of gases of decomposition which produce suspended micro-bubbles that demand slow firing to clear. Also, since wollastonite sources silica as well, glaze recipes employing it do not need as much raw silica powder. Further the SiO2 and CaO react more readily to form silicates. Thus wollastonite is used as a major flux in high temperature sanitaryware and electrical insulators.
In glass and fiberglass making wollastonite melts more readily (lower energy costs) and microbubble generation is lower than limestone-sand mixes.
Wollastonite is also used in stain and frit formulations to supply CaO in a more easily melted form. Even though powdered samples may appear quite similar to feel and the naked eye, pay attention to the amount of +325 material if you need to switch brands or suppliers.