Creation of microcracks in porcelain during firing

Quartz porcelain samples were tested by acoustic emission (AE) and sonic resonant methods (sensitive to nucleation and motion of structural defects) during a cooling stage of the firing process from temperature of 1250 °C. As a consequence of the mismatch of thermal expansion coefficients of the glass and the mullite phases, and the quartz particles, microcracking begun at the temperature of the glass transition (∼800 °C) and continued in several stages until the temperature reached 300 °C. A non-monotonous behaviour of the Youngʹs modulus and temporary vanishing of the AE signals on cooling between 573 and 500 °C confirm the significance of the β → α transition, which lies mainly upon the reversal of thermal stresses acting on the glass matrix and the quartz particles. Consequently, above this temperature, radial cracks nucleating at the quartz particles appear, whereas below this temperature, circumferential cracks around the particles are produced.