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The appearance of enclosed or broken macroscopic cavities in a body or in a glaze or other coating during firing.
1. Small blisters, bubbles, or bulges in a plaster finish coat; results from applying a finish coat over too damp a base coat, or from troweling on plaster too soon; also called turtleback.
2. See blister.
3. The irregular raising of a thin layer at the surface of placed mortar or concrete during or soon after completion of the finishing operation, or, in the case of pipe, after spinning.
4. In the firing of a ceramic, the development of enclosed or broken macroscopic vesicles or bubbles in a body or glaze or other coating.