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Wollastonite (Vansil-W30)

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by the pound:
Vansil W-30 Wollastonite

Appearance: Brilliant White
Shape: Acicular
Specific gravity: 2.9
Refractive Index: 1.63
GE Brightness: 90
pH (aqueous solution): 9.9
Water solubility (gms./100cc): 0.0095
Density (lbs./cu.ft.): 181 (2.9 g/cc)
Moh's hardness: 4.5
Coefficient of expansion: (in/in/degree C): 6.5 x 10 -8
Melting point: 1410
on 325# sieve: 1% (Vansil W-30: 0.05%)
Hegman Fineness of Grain: 3 (Vansil W-30: 4)
Median Particle Size: 14
Surface Area (m2/g) (BET): 1.5
LOI (1000C): 0.5% (Vansil: 1.6%)

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. The SiO2 and CaO it contributes react more readily to form silicates, thus wollastonite is used as a major flux in high temperature sanitaryware, tile and electrical insulators (augmenting feldspar and silica, pushing their percentages down very significantly).

Wollastonite exhibits a slight solubility in water, but slips containing it can become more alkaline (potentially affecting rheological properties). In casting bodies this can be a benefit, its gellation can decrease casting rates.

The powdered form has a tendency to form agglomerates during storage (which create lumps in glazes necessitating sieving). Manufacturers warn that stock should be rotated to prevent it getting to 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, especially at lower temperatures (since the wollastonite is sourcing SiO2). Wollastonite's greater readiness to melt at higher temperatures has a curious side effect: Glazes can tolerate higher SiO2 (from the Wollastonite) and melt just as well or better.

In glass and fiberglass making wollastonite melts more readily (lower energy costs) and microbubble generation is lower than limestone-sand mixes.

Wollastonite has the ability to seed crystals (in glaze melts of sympathetic chemistry), and can be valuable to create special effects which depend on devitrification (crystallization during cooling). Since CaO tends to devitrify in high temperature slow cooled glazes wollastonite can be employed to make faster cooled lower CaO content ones exhibit the same effect.
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