Clay
Fine-grained weathered rock that is plastic when wet and rock-hard when fired — the oldest manufactured material.
Clay is what feldspar becomes when it weathers. The clay minerals are hydrous aluminium silicates built in sheets, and their extreme fineness plus their plate-like shape is what gives clay its defining behaviour: wet, the plates slide over one another and the mass holds any shape it is pushed into.
Firing makes that shape permanent. Above roughly six hundred degrees the chemically bound water is driven off irreversibly, and the clay minerals break down and begin to fuse. What comes out is ceramic, and it will not turn back into clay however long it is soaked. That irreversibility is why pottery survives in the archaeological record when almost nothing else does.
How it forms
Clay forms by the chemical weathering of silicate rock, principally feldspar, in the presence of water and dissolved carbon dioxide. Where the clay stays where it formed it is called residual, and it tends to be pure and pale — kaolin, the porcelain clay, is the type example.
Where it is carried away by water and redeposited it picks up iron and organic matter, becoming the darker, more plastic, lower-firing clay found in river terraces and used for brick and common pottery. The distinction between the two governs almost everything about how a clay behaves.
Uses
Brick and roof tile consume the greatest tonnage, followed by cement manufacture, where clay supplies the alumina and silica that combine with limestone in the kiln.
Refined clays go into pottery, sanitaryware and porcelain. Kaolin is also used in enormous quantity as a paper coating, giving a smooth printable surface, and as a filler in paint, rubber and pharmaceuticals. Bentonite clay, which swells dramatically in water, is used as a drilling mud, a foundry sand binder and a sealing layer in landfill liners.
How we know: checked recently · only one source, so there is nothing to cross-check it against · stated directly by the source.
How this connects
Where a connection has been confirmed by an outside reference, that reference is named beside it.
is sourced from
- Orthoclase — mineral · clay is chiefly what feldspar becomes when it weathers
- Plagioclase — mineral · plagioclase weathers to clay as readily as potassium feldspar does
is a source for
- Brick — material · formed, dried and fired until the clay minerals break down irreversibly
- Stoneware — material · a clay that survives 1200 °C and above, which an earthenware clay does not
- Earthenware — material · almost any clay, which is why it is the ceramic every early culture reached first
- Refractory brick — material · as fireclay for the general-purpose grades; the basic linings are magnesia and dolomite instead
is used as
- Refractory lining — application · fireclay brick, the oldest and cheapest lining material
- Glass and ceramics — application · the mineral that makes a ceramic shapeable before it is fired, which is the whole basis of pottery
is an input to
- Calcination — process · supplies the silica and alumina that combine with lime in the cement kiln
- Firing — process · shaped and dried, then heated past the point where its minerals break down irreversibly
- Extrusion — process · the standard way a brick is formed before firing: a continuous column pushed through a die and wire-cut into units
is a component of
- Shale — rock · 40–75% · the clay minerals that make up most of the rock, and whose flat particles give it its splitting
- Porcelain — material · 40–60% · kaolin, for the plasticity that lets the body be shaped before firing
- Bone china — material · 20–30% · china clay for the plasticity that lets the body be formed at all — bone ash has none, and a body of bone ash alone could not be thrown or cast
is an alternative to
- Talc — mineral · as a mineral filler in paper, paint and plastic, where the choice is between talc's platy particles and kaolin's stacked ones, and both are being bought for opacity per tonne
is used in
- Pencil — object · mixed with the graphite and fired, and the ratio is the grade: more clay is harder and lighter, which is what H means
Sources
- Material WorldOur own writing