How was clay fired?

This site explains the changes, the excerpt is a small explanation:

The item is heated to temperatures up to 1170oC, during which time the clay undergoes

some chemical changes and the silica and feldspar undergo physical changes. The

reactions of the clay can be summarised as follows:

6Al2Si2O5(OH)4 ? 6Al2Si2O7 ? 3Al4Si3O12 ? 2Al6Si2O13

kaolinite metakaolinite silicon spinel mullite

Silica and water (from the crystal lattice) are also expelled during firing, resulting in a

further 5 - 7% volume reduction. This silica mixes with the silica already present and

melts to form a glass. It is this glass, which also includes metallic ions from the

feldspar, that makes the ceramic item non-porous and water-tight.

* The site: http://www.nzic.org.nz/ChemProcesses/inorganic/9C.pdf