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Wollastonite

Calcium Silicate

Formula: CaSiO3
Alternate Names: Wolastonite

OxideAnalysisFormula
CaO48.28%1.000
SiO251.72%1.000
Oxide Weight116.20
Formula Weight116.20
Enter the formula and formula weight directly into the Insight MDT dialog (since it records materials as formulas).
Enter the analysis into an Insight recipe and enter the LOI using Override Calculated LOI (in the Calc menu). It will calculate the formula.
DENS - Density (Specific Gravity) 2.8-2.9
HMOH - Hardness (Moh) 4.5-5
MLPT - Melting Point (MP) 1540C
SLBY - Solubility Soluble in HCl

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). Wollastonite's unique qualities were first recognized in 1822 by an English scientist, Sir William Wollaston. However as a commercially available raw material wollastonite has only been available since the 1950s. Explosive market growth took place during the 1980s and 90s and major industrial sectors have adopted the material.

Deposits are mined mainly in US, China, India, Mexico, Canada, and Finland. 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.

No commercial products have the theoretical chemistry shown here. 325 mesh is used commonly in ceramics.

Some example physical data (US) of Nyad 325 (Vansil W-30 is very similar)

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.

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 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. 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 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.

Wollastonite is also used in stain and frit formulations to supply CaO in a more easily melted form. Even though a 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.

Mineralogy vs. Chemistry, Wollastonite vs. Calcium Carbonate

Wollastonite is an excellent demonstration of the fact that we must consider ceramic chemistry is a relative science and it is one piece in the glaze puzzle. The mineralogy of materials is another important factor to consider. For example, the melting temperature of a frit or glass is predictable, but since raw minerals are most often crystalline, the bonds holding the molecular structure together are more complex. The melting temperature of minerals of similar or even identical chemistry, for example, can be vastly different.

To demonstrate we took a reliable cone 6 calcium matte glaze (Wollastonite - 34.0, Ferro Frit 3134 - 21.0, Kaolin - 45.0) and used the above technique to calculate an equivalent recipe employing whiting to source the CaO. We fired the two glazes side-by-side on upright tiles and in a flow tester to cone 6 (picture shown below). You might expect these glazes to fire the same since they have the same chemistry. Not so. The wollastonite version runs much more on the flow tester. This is because the wollastonite melts at a lower temperature than whiting or is more easily dissolved in the melting frit glass. Also, the entrained bubble population is much higher in the whiting version (whiting has an LOI of 45%). Additionally the wollastonite version is a silky pleasant matte, the whiting version is glossy. The former more fluid melt gives the crystals much more freedom to grow during cooling.

In simply looking at the glazed tiles one might easily assume that the transparent glossy whiting version is melting more than the matte wollastonite one, however the opposite is clearly the case. This is a good reminder that ceramic calculations need to be viewed in perspective. They excel in ongoing predictions of how changes to existing material amounts in a recipe will affect fired properties. They are much less reliable as absolute indicators of properties of unknown glazes. Always remember that glazes are made of materials that have a chemistry, mineralogy and physical properties and you cannot ignore any of these.


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Pictures
Two glazes, same chemistry, different materials. Left has 34% Wollastonite, in other whiting sources same amount of CaO. See INSIGHT manual under: Mineralogy vs. Chemistry

Click for 164% larger

Screen a glaze to break down the agglomerates (e.g. of wollastonite) and remove other particulates.

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By Tony Hansen

XML for Import into INSIGHT

<?xml version="1.0" encoding="UTF-8"?> <material name="Wollastonite" descrip="Calcium Silicate" searchkey="Wolastonite" loi="0.00" casnumber="9056-30-8"> <oxides> <oxide symbol="CaO" name="Calcium Oxide, Calcia" status="" percent="48.280" tolerance=""/> <oxide symbol="SiO2" name="Silicon Dioxide, Silica" status="" percent="51.720" tolerance=""/> </oxides> </material>


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