Theoretical equations have been developed for crystal growth rate in layers of small frit (glass) particles during firing, Throughout the process, the crystalline and the glassy phases have different compositions; therefore, the system can be considered a pseudo-two-component system consisting of a crystallizable component (structural unit) and a noncrystallizable mixture of several components. The concentration of the crystallizable component decreases in the residual glassy phase during the crystal growth process, On integrating at the surfaces of crystals having the same composition. Throughout the crystal growth process, a concentration gradient of the crystallizable component Is therefore produced in the glassy phase, which results in mass transport by diffusion of this component from the bulk residual glassy phase to the surfaces of the crystals. Equations have been derived assuming that the diffusion step of the crystallizable component through the residual glassy phase is the overall crystal growth process rate-controlling step.