The purpose of this research was to study fracture initiation in a glaze-strengthened magnesia ceramic substance used as a core material for all-ceramic crowns. The fracture surfaces of modulus of rupture bars were studied to locate the fracture initiation point. These points were located at the center of hackles that are oriented parallel to the direction of the crack advance. The distance from the fracture initiation point to the surface of the test bars was measured. A direct relationship was found between the flexure strength and the depth of the fracture initiation point. This is consistent with the theory that residual compressive stresses at the glaze-body interface inhibit the spread of flaws and are responsible for the increases in strengths observed.