a clay mineral

Clay is the single material that made ceramics possible: dig it, wet it, and it becomes a soft, mouldable paste that holds any shape you press into it and then hardens to stone when fired. Chemically, the clay minerals are hydrous layer aluminosilicates, a subgroup of the sheet silicates in which tetrahedral silica sheets are bonded to octahedral aluminium- or magnesium-hydroxide sheets, stacked into extremely small, thin, plate-like crystals, typically under a couple of microns across.

Three features make a mineral behave as a clay. First, the sheet structure, which forms flat plates. Second, the tiny particle size, which gives an enormous surface area and lets the plates carry a surface charge and adsorb water. Third, that adsorbed water, thin films of it between the plates, which lets the plates slide over one another yet be held together by surface tension. Together these give the defining property of clay, its plasticity: wet, it flows and takes a shape; leave it, it keeps the shape; dry and fire it, and it turns rigid and permanent. The main clay minerals a ceramist meets are kaolinite (the pure china clay of porcelain), the mica-like illite of common clays, and the swelling smectite montmorillonite.

Clays are never a single pure phase in nature; a working ceramic clay is a blend of clay minerals plus quartz, feldspar, iron oxides, and organic matter, which is why potters distinguish clean, weakly plastic primary (residual) kaolins from the dirtier, far more plastic secondary (sedimentary) ball clays. The clay mineral is therefore both a precise structural idea and the messy, indispensable raw material at the root of the entire pottery tradition.