the plasticity of clay

Plasticity is the near-magical property that lets a lump of wet clay be squeezed, thrown, coiled, or pressed into any shape and then hold that shape without cracking or slumping. It is the single behaviour that made pottery the first synthetic material of civilisation. A plastic clay is stiff enough to stand up under its own weight yet soft enough to flow under a potter's push, and it remembers the last shape you gave it rather than springing back or collapsing.

The mechanism is water plus flat plates. Clay minerals are tiny sheet-silicate platelets; add the right amount of water and each platelet is wrapped in a thin film of water only a few molecules to a few hundred nanometres thick. Those films let the plates slide past one another (so the clay flows when pushed), while the surface tension of the water and the van der Waals attraction between plates pull them back together and hold the new shape (so the clay does not slump when released). Too little water and the plates cannot slide, the clay is short and crumbly; too much water and the films are so thick the plates lose contact, the clay becomes a runny slip. Plasticity lives in a window of water content, roughly measured by the Atterberg plastic and liquid limits, and the span between them, the plasticity index, tells you how forgiving a clay is to work.

This is why particle shape and fineness matter so much: the flatter and finer the clay platelets, the more sliding surfaces and the more plastic the body, which is why swelling montmorillonite is intensely plastic while blocky, coarse minerals are not plastic at all. It is also why a porcelain recipe blends a weakly plastic kaolin with more plastic ball clays: the ceramist is tuning the plate-and-water system until the body throws well, dries without cracking, and still fires clean and white.