TIDI

Titanium dioxide dissolves into a glaze melt and crystallises again as the kiln cools, and it is those crystals that scatter the light. How much goes in decides whether it variegates a surface or whitens it.

Titanium dioxide is the first thing to come out of solution as a glaze cools, so it seeds the crystals that everything else grows around, and those crystals are what scatter the light. A small addition dissolves and shows little; a few parts in a hundred give the mottled, streaked surfaces it is known for; a tenth or so of the recipe whitens a glaze outright, and past that it goes opaque and patterned with a yellow cast coming in. It sends cobalt green rather than blue, and the cooling curve counts for as much as the amount does, since the crystals grow on the way down.