WHIT
Calcium carbonate — the usual way calcium gets into a glaze · Fluxes at high temperatures: little melting below about cone 4 · Past about a fifth of the recipe it mattes rather than melts · Nearly half its weight leaves as gas during the firing
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Whiting is calcium carbonate, and the usual way calcium gets into a glaze. Calcium fluxes at high temperatures, so it does its work from about cone 4 upward.
Whiting is in a low-fire recipe for hardness, durability and opacity rather than for melting, since boron and the alkalis do the melting down there. From cone 6 it comes into its own, and at cone 9 and 10 it carries most of the melt. Past about a fifth of the recipe it stops fluxing and starts crystallising as the kiln cools, which is where calcium mattes come from: the same glaze can finish glossy or matte on the cooling curve alone. Nearly half its weight leaves as gas during the firing, though early enough that most glazes have not sealed over yet.
