Wollastonite is a mineral made of calcium silicate (CaSiO3). Wollastonite is a very useful source of calcium in glazes. Its physical properties make it more desirable than a combination of Whiting and Silica. It produces less gasses during firing, melts more aggressively, and adds more "room" to the glaze recipe, allowing for an increase in clay content. Wollastonite is also readily available and among the cheapest of glaze materials.
It is possible to reduce the maturation temperature of hard Porcelain and Bone China by several cones from high to midrange temperatures by using a combination of Feldspar and Wollastonite as flux.
In Effect of wollastonite addition on sintering of hard porcelain (doi:10.1016/j.ceramint.2014.12.126) Turkmen et al. reduce the firing temperature of a hard paste porcelain by partially replacing potash feldspar with wollastonite.
They start with a traditional porcelain recipe (50% kaolin, 25% silica, 25% feldspar) and replace up to 5% of the feldspar with wollastonite. They fire samples in various temperatures and test for maturity.
| Kaolin | Silica | K-Feldspar | Wollastonite | Water Absorption % (1250C) | |
|---|---|---|---|---|---|
| W0 | 50 | 25 | 25 | 0 | 2.03 |
| W1 | 50 | 25 | 24 | 1 | 0.77 |
| W2 | 50 | 25 | 23 | 2 | 1.34 |
| W3 | 50 | 25 | 22 | 3 | 0.18 |
| W4 | 50 | 25 | 21 | 4 | 0.16 |
| W5 | 50 | 25 | 20 | 5 | 0.11 |
I think the specific wollastonite additions are not very useful to hobbyist potters because Turkmen et al. use a very fast firing schedule only possible in laboratory kilns. They heat their kiln at 10C/min, soak for 60 minutes at peak temperature, and then cool at a rate of 50C/min. Multiple samples bloated and could not be measured for water absorption and porosity. However, this does imply that a cone 10 porcelain can be reduced to cone 5 or even lower with wollastonite.
In Use of Wollastonite as a Flux for Bone China Bodies (doi:10.5402/2012/701821), Carús et al. reduce the firing temperature of a bone china porcelain by partially replacing feldspar with wollastonite.
| Bone Ash | Kaolin | Feldspar | Wollastonite | Maturation Temperature (C) |
|---|---|---|---|---|
| 50 | 25 | 25 | 0 | 1260 |
| 50 | 25 | 15 | 10 | 1200 |
The traditional bone china body reaches maximum density at 1260C while the body with wollastonite reaches maximum density at only 1200C.
The samples were fired with a 150C/hr (2.5C/min) ramp ahd held at peak temperature for 30 minutes.
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