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Why can overfired structural clay products become brittle?

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

Structural clay products become brittle when overfired because the process creates an excessive amount of glass within the material's microstructure.1

While a certain amount of glass is necessary to bond particles together and provide strength, overfiring leads to an imbalance in the material's composition. This high glass content makes the finished product susceptible to failure under mechanical and thermal stress. Additionally, such overfired, glassy products are difficult to bond properly when used with mortars. In extreme cases, excessive heat treatment can also lead to other defects, such as deformation or bloating.12