How does clay change as it dries and is fired?

Clay undergoes significant physical and chemical transformations as it transitions from a plastic state to a finished ceramic object.

Drying

When clay is shaped, it contains water that provides plasticity. As it dries, this moisture—including water held in pores and adsorbed on mineral surfaces—is removed. Once the clay reaches "leather hardness," it can be turned or finished to refine its shape. Sun-dried clay remains water-soluble; if rehydrated, it will soften and collapse.12

Firing

Firing induces permanent chemical changes that prevent the clay from returning to a plastic state.2
* Dehydroxylation: At temperatures between 400°C and 700°C, hydroxyl ions are driven off, fundamentally altering the clay mineral structure.1
* Vitrification: As temperatures rise further, partial vitrification occurs, where crystalline compounds melt into an amorphous, glass-like structure. This process facilitates liquid-phase sintering, which bonds particles together, increases strength, and reduces porosity.3
* Coloration: The final color is determined by the clay's composition and the kiln atmosphere. For example, iron in the clay oxidizes to produce red or buff tones, while a reducing atmosphere can result in gray or black hues.2