Clay holds its shape thanks to a thin film of water
Wet clay can be squeezed into any shape and then keeps it, because its microscopic mineral plates are glued by a film of water that lets them slide yet holds them in place. Fire the clay and the chemistry changes for good, which is why pottery is among humanity's oldest technologies.
Clay is a fine-grained natural soil rich in clay minerals, hydrous aluminium silicates such as kaolinite. Pure clay minerals are pale, but traces of iron oxide give many natural clays a red or brown tint. Clay is everywhere: shale, formed mostly from clay, is the commonest sedimentary rock. Where exactly clay ends and silt begins depends on who you ask. Geologists and soil scientists usually draw the line at particles of 2 micrometres, sedimentologists at 4 to 5, and colloid chemists at 1, while engineers classify by how the soil behaves when wet.
The secret is structure. Aluminium and silicon ions bond with oxygen and hydroxide into tiny, thin, flexible plates. In kaolinite, water molecules hydrogen-bond neighbouring plates: weak enough to let them slip during moulding, strong enough to keep the new shape. Drying removes most of that water, so the plates bond directly, leaving the piece rigid but brittle, and wetting it again restores plasticity. Firing to earthenware drives out more water and fuses the plates with covalent bonds, turning kaolinite into metakaolin, which stays hard even when wet. Hotter firing for stoneware and porcelain forms still tougher minerals such as mullite. Kaolinite is workable between roughly 36 and 72 percent water content.
Humans noticed all this early. Pottery shards among the oldest ever found go back roughly to 14,000 BCE, and people wrote on clay tablets before any other known medium. Today clay is used to make paper and cement and to filter chemicals, and between half and two-thirds of the world's people live or work in buildings whose load-bearing structure relies on clay, often as brick.
Farmers have mixed feelings. Clay soils drain poorly, making them harder to crop, yet they are fertile because the charged plates hold nutrients such as potassium and ammonium, swapping positive ions with the surrounding water. Some clays swell dramatically as they soak up water, a serious headache for civil engineers. A soil of sand, silt and under 40 percent clay is called loam.
Source: Clay
