Clays, which represent one of the main components of the ceramic body, are sedimentary rocks that, depending on their depositional environment, can be classified as follows: marine sedimentary deposits (accounting for 80% of sedimentary rocks) and clays formed through the alteration and decomposition of rocks in situ (as a result of hydrothermal or atmospheric processes).

Their most important property is the ability to retain certain cations and anions, which can be exchanged with other cations and anions. This ion-exchange capacity (due to special bonds located at the apices of the silica-alumina structural units) is of utmost importance in all fields where clays are employed.

Clays are also characterized by a marked ability to absorb water — a particularly relevant aspect in ceramics.

Equally important is their plasticity, which can vary from sample to sample, and which makes the clay, once mixed with water, ductile and easy to shape (while allowing it to retain the given form over time). Once dried, the clay hardens, but if moistened again, it regains its original plasticity. The most plastic clays are also referred to as fat clays, while those with lower plasticity are generally called lean clays.

Among the main properties of clays is also their dehydration capacity, which is essential for the proper execution of the ceramic production process. Through heating (such as the high temperatures generated inside kilns), structural changes occur in clay substances that promote the formation of new mineral phases.