Britannica Sources used in this answer

What chemical change prevents fired clay from becoming plastic again when wet?

Created from Britannica articles using AI. AI can make mistakes, so verify using Britannica articles.

traditional ceramics Stages in the slip casting of a thin-walled whiteware container. Clay powder is mixed in water together with a dispersing agent, which keeps the clay particles suspended evenly throughout the clay-water slurry, or slip. The slip is poured into a plaster mold, where water is drawn out by capillary action and a cast is formed by the deposition of clay particles on the inner surfaces of the mold. The remaining slip is drained, and the cast is allowed to dry partially before the drain hole is plugged and the mold separated. The unfinished ware is given a final drying in an oven before it is fired into a finished product.

The chemical change that prevents fired clay from becoming plastic again when exposed to water is known as dehydroxylation.1

During this process, which typically begins at temperatures around 500 °C (900 °F), the hydroxyl ions (OH⁻) contained within the clay mineral structure are driven off as water. This structural transformation is irreversible; once the clay has undergone this chemical change, it loses its ability to regain plasticity, regardless of how much water is subsequently added.12

While further heating at higher temperatures can lead to vitrification—the formation of a glassy phase that bonds particles together for increased strength and reduced porosity—the initial loss of plasticity is fundamentally tied to the dehydroxylation of the clay minerals.32