Flint Silica

Almost every glaze recipe contains silica, because it is the glass former. Quartz on its own needs around 1710°C to melt, far beyond any pottery kiln, and the fluxes in a recipe are there to bring that temperature down to one a kiln can reach. In a clay body silica does something different. Some of it dissolves into the melt at stoneware temperatures, but much of it comes through the firing as quartz, and that free quartz is a major control on how much the body expands and contracts. Glaze fit follows from this, and silica moves expansion in opposite directions depending on where it is. In a glaze it dissolves and lowers the glaze's expansion. In a body it stays crystalline and raises the body's maturity. Correcting a crazing glaze is almost always easier from the glaze side than from the body side. Quartz also changes crystal structure at 573°C, expanding on the way up and contracting on the way down, and this is why a fast cool is risky on thick ware. Silica fired past about 1050°C may convert to cristobalite, which changes structure again at around 220 to 250°C with a larger volume change, on ware that is rigid by then. That second change is the more common cause of dunting.