quartz

Quartz is crystalline silicon dioxide, SiO2, the second most abundant mineral in the Earth's crust and the clear or milky grains you see in sand and granite. Structurally it is the archetypal framework silicate: every tetrahedron shares all four corners with neighbours, weaving a rigid, fully connected three-dimensional net with no free ends and no easy cleavage. That total connectivity is why quartz is hard (7 on the Mohs scale), chemically tough, and one of the standard raw materials of traditional ceramics.

Quartz has a personality quirk every ceramist must respect: it changes crystal form on heating. Low (alpha) quartz flips to high (beta) quartz at 573 degrees C, a fast, reversible displacement of the same framework that is accompanied by a sudden volume change of about one percent. Because the change is abrupt and happens on both heating and cooling, passing a ware too quickly through 573 degrees C can crack it, a fault the trade calls dunting. Heated much higher, quartz slowly reconstructs into the other silica polymorphs, tridymite and cristobalite, though in real firings this is sluggish and often incomplete.

In a whiteware body quartz plays the filler or skeleton: milled to a fine flour (sometimes as flint), it is the non-plastic ingredient that opens up the plastic clay, reduces drying shrinkage, and gives the green and fired body its dimensional backbone. On firing, its surface partly dissolves into the feldspar-derived glass while a residual quartz core survives. Free crystalline quartz also carries a health note, breathing fine quartz dust causes silicosis, so quartz-handling in ceramics is done with dust control.