All Glossary

Dehydration

Dehydration in clay refers to the process of the evaporation of water during air drying (physical) and the removal of chemically bound water during firing (chemical). As air drying proceeds, water between the clay particles evaporates, causing them to move closer together and resulting in shrinkage. Part way through the process, a "leather hard" stage is reached, the clay stiffer and non-plastic, but can still be carved or joined. At the bone dry state all atmospheric moisture has evaporated. The clay is no longer cold to the touch. Bone-dry clay can be rehydrated and reused. Different clays dry at different rates; fine-particle ones like shrink more.

Chemical dehydration occurs in the firing. At approximately 350°C–500°C (662°F–932°F), chemically combined water within the clay molecule is driven off. This is an irreversible chemical change; the clay will no longer turn back into plastic mud if water is added.

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