The development of macroscopic residual stress, as in glass-tempering processes, was studied for porcelain tile. Mechanical strength was observed to increase less than might be theoretically expected, owing to deterioration of the sintered tile microstructure. A model has been developed, using linear elastic fracture mechanics, to estimate the natural flaw size in the tempered material. The study shows that as the cooling rate raises, the macroscopic residual stress and flaw size increase. This microstructural deterioration is mainly attributed to the allotropic transformation of quartz in the presence of thermal tensile stress at the porcelain tile surface.