The effect of kaolin addition on the sinter-crystallization of a SiO2 – Al2O3 – RO (R = Ca, Mg, Sr) glass-ceramic glaze that devitrifies anorthoclase and diopside was studied using non-isothermal techniques (DTA and heating microscopy). It was found that increasing kaolin content makes the values of the starting temperatures and the maximum crystallization rate (peak temperature) increase. The following technological characteristics of pressed specimens fired at a heating rate of 15K/min up to different maximum temperatures, with a dwell time of 6min, were determined by the usual methods: linear firing shrinkage, water absorption capacity, and relative density. For some selected temperatures, the microstructure of the fired specimens was observed by SEM-EDX, and the total porosity and the pore size distribution were determined by microscopy and image analysis. In these test-pieces, hardness and deformation modules were determined by nanoindentation. It was found that on increasing kaolin content, the glass porosity, and the temperature at which the maximum densification is reached also increase, in other words, increasing the kaolin content in a glaze reduces its sinterability. Likewise, it was observed that when the kaolin content increases, the linear firing shrinkage decreases, due to the higher compactness of the unfired glaze. It was also proved that it is possible to obtain glass-ceramic materials practically free of porosity and with microhardness values up to 12.5±1 GPa by using frit and the mixture of frit and 8wt% kaolin. These microhardness values have been interpreted based on their microstructural characteristics.