FLSI
Ground quartz — the main glass former in a glaze · A non-plastic filler in a clay body · Melts at around 1710°C on its own; the fluxes bring that down · Lowers a glaze's thermal expansion; raises a clay body's · Changes crystal structure at 573°C, expanding and contracting as it passes · Fired past about 1050°C it can form cristobalite, which inverts around 220 to 250°C · Called flint or quartz in most recipes
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Flint silica is ground quartz. It is the material that forms glass in a glaze, and in a clay body it is the filler that governs how much the fired piece moves.
Almost every glaze recipe carries some, because quartz on its own needs around 1710°C to melt, far past any pottery kiln, and the fluxes in a recipe exist to bring that figure down to something a kiln can reach. In a clay body the job is close to the opposite. Much of it comes through the firing undissolved, though not all — a good deal goes into the feldspathic melt at stoneware temperatures — and the free quartz that remains is a major control on the body's thermal expansion. Glaze fit follows from that, and silica moves expansion in opposite directions depending on where it sits: in a glaze it dissolves and lowers the glaze's expansion, in a body it stays crystalline and raises the body's. Correcting a crazing glaze is almost always easier from the glaze side than the body side. Quartz also changes crystal structure at 573°C, expanding on the way up and contracting on the way down, which is what makes a fast cool costly on thick ware. And silica fired past about 1050°C can convert to cristobalite, which inverts again much lower, around 220 to 250°C, with a larger volume change and on ware that has gone rigid by then. That second inversion is the commoner cause of dunting.
