Wollastonite
Calcium Silicate
Formula: CaSiO3
| DENS - Density (Specific Gravity) |
2.8-2.9 |
| HMOH - Hardness (Moh) |
4.5-5 |
| SLBY - Solubility |
Soluble in HCl |
Wollastonite is a naturally occurring calcium metasilicate. 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.
Example of Typical Data
Appearance: Brilliant White
Shape: Acicular
Molecular weight: 116
Specific gravity: 2.9
Refractive Index: 1.63
pH (aqueous solution): 9.9
Water solubility (gms./100cc): 0.0095
Density (lbs./solid gallons): 24.2
Bulking balue (gal./lbs): 0.0413
Moh's hardness: 4.5
Coefficient of expansion: (in/in/degree C): 6.5 x 10 -8
Melting point: 1540
Nyad 325 on 325# sieve: 1.0
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. 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. 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).
At higher temperatures the powder 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.
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.
Out Bound Links
- (Materials - Unspecified)
Fibrous Wollastonite - CaSiO3
- (URLs)
Wollastonite at Wikipedia
http://en.wikipedia.org/wiki/Wollastonite - (Typecodes)
1: GNM - Generic Material
- (Typecodes)
1: FLS - Flux Source
- (MDT - Member)
Latin and South America
Latin America and South America. We are working on... - (MDT - Member)
Europe
Countries of Eastern Europe and former Soviet Unio... - (MDT - Member)
Australia
We are working on this database and would apprecia... - (MDT - Member)
Asia
All of Asia including Turkey, Russia, Indosnesia, ... - (Library - Unspecified)
Two Sources of CaO
This is an chapter in the book (and matching video... - (MDT - Member)
Africa
All of continental Africa. We are working on this ... - (MDT - Member)
New Zealand
We are working on this database and would apprecia... - (MDT - Member)
North America
The decision about what materials to include in th... - (MDT - Member)
Ron Roy
This is the traditional Ron Roy materials file. He... - (MDT - Member)
UK
We are working on this database and would apprecia... - (MDT - Member)
Crystal Glazes
These materials are specially defined for makers o...
In Bound Links
- (Materials - Alternative)
Calcium Carbonate - CaCO3
Carbonate of Lime, Whiting, Aragonite, Calcite, CaCO3
- (Materials - Related)
Wollastonite Vansil - CaSiO3 - Calcium Metasilicate
Vansil
- (Materials - Parent)
Nyad 200
Nyad-200 Wollastonite
- (Materials - Parent)
USAKOS Wollastonite
- (Materials - Parent)
Pfizer Wollastonite - No longer produced
- (Oxides - Material source)
CaO - Calcium Oxide, Calcia
- (Glossary - Unspecified)
Matte Glaze
A glaze that is not glossy. Of course, unmelted gl...
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
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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