Alumina has long been known to enhance the strength of porcelain; however, the mechanism of strengthening is not completely understood. The strengthening in alumina-strengthened porcelain is examined on the basis of the change in thermal shrinkage of the porcelain matrix upon addition of various amounts of calcined talc. The increase in flexural strength of the porcelain due to the alumina addition increases with increasing difference in the thermal shrinkage between an alumina particle and the porcelain matrix. The increase rate of the flexural strength as a function of the difference in thermal shrinkage is described by the increase in nominal tensile stress required for the fracture of the porcelain due to the internal stress on the porcelain matrix. However, the strengthening due to the internal stress is not enough to describe the actual strength of the porcelain. Further strengthening appears to be achieved by the relatively large thermal shrinkage of alumina particles suppressing the formation of large cracks around quartz particles in the porcelain matrix.
