1. In ceramic tile, the development, during firing, of enclosed or broken macroscopic vesicles or bubbles in a body or in a glaze or other coating. 2. A bulging of the finish plaster coat as it separates and draws away from the basecoat; the resulting protuberances are often termed turtle backs. 3. Formation of bubbles on surface of paint or varnish film, generally caused by moisture behind the film or excessive heat.