When clay is heated to around 1100 ° F or higher, it transforms. Chemically linked water molecules are removed from clay particles at such temperatures. The clay particles are conjoined together, resulting in a lasting modification in the ceramic piece. When clay items are re-introduced to water before transformation, the clay particles slake (fall apart) then re-dissolve into the water.
After that, they can be reconstituted into usable clay and reused. Low-fired pottery is just minimally heated beyond this point, and it is less lasting than high-fired pottery.