WOLL
Calcium silicate — crystals that grow as fine needles · Calcium and silica in together, with almost no weight lost as gas · A tenth to a quarter of a glaze; a few parts in a hundred of a body · In a body the needles toughen the fired ware and cut shrinkage
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Wollastonite is calcium silicate, and its crystals grow as fine needles rather than as grains. It brings calcium and silica into a recipe together, and almost none of its weight leaves as gas.
Wollastonite goes into a glaze as the calcium source, commonly between a tenth and a quarter of the recipe, and into a body at a few parts in a hundred. Almost none of its weight leaves as gas during the firing, so there is very little coming off underneath a glaze while it seals over. It brings its own silica as well as the calcium, so a recipe drawing calcium from it needs less silica added separately — not by equal weight, though, so the totals have to be redone. In a body its needles hold shrinkage down through drying and firing, and make the fired ware harder to crack, though they do nothing for its dry strength.
