feldspar

Feldspar is the most abundant mineral group in the Earth's crust and, in the pottery workshop, the flux, the ingredient that melts to help everything else stick together. It is an aluminosilicate framework, a tectosilicate in which aluminium substitutes for some of the silicon in the all-corners-shared cage. Because Al3+ carries one less positive charge than the Si4+ it replaces, the framework needs a large cation, potassium, sodium, or calcium, parked in its channels to stay neutral.

That gives three end-members: potash feldspar or orthoclase (KAlSi3O8), soda feldspar or albite (NaAlSi3O8), and lime feldspar or anorthite (CaAl2Si2O8), with the alkali feldspars (K-Na) and the plagioclase series (Na-Ca) covering the natural range. The point for ceramics is what those channel cations do on firing: they lower the temperature at which the framework melts. A pure silica framework would need well over 1700 degrees C, but potash feldspar begins to melt near 1150 degrees C, and it does so incongruently, breaking down to solid leucite plus a viscous, sticky liquid rather than melting cleanly to a liquid of its own composition.

In the triaxial whiteware body feldspar is the vitrifying flux: on firing it forms a stiff glass that flows into the pores, wets and partly dissolves the clay relics and the edges of the quartz grains, and bonds the whole body into a dense, translucent porcelain. Too little feldspar and the ware stays porous and weak; too much and the glass gets so runny the piece slumps. The alkali cation matters too, sodium-rich feldspar melts sharply and runs, potassium-rich feldspar gives a longer, more forgiving firing range, which is why potash feldspar is the classic porcelain flux.