The addition of temper was a common practice in the fabrication of traditional ceramics. The present work shows the effects of limestone or quartz added as temper to a kaolinitic clay, on the strength of the ceramic body. With this purpose ceramic tests were prepared adding the 5, 15 and 25% of each temper to a kaolinitic clay. Unimodal skewed grain size distributions (GSDs) for temper were used instead on single grain sizes used in previous experimental works. Furthermore, samples were fired at 500, 750 and 1000 °C in order to analyse the effect of firing temperature on ceramic strength. A correlation between the strength and the porosity, the mineralogy and the microstructure of the samples is presented. Results shows that while temper improves the strength of 500 °C and 750 °C-fired bodies, it worsens that of samples fired at 1000 °C. Moreover, while at 500 and 1000 °C limestone-tempered materials are less strong than quartz-tempered ones, at 750 °C the opposite occurs. For quartz-tempered bodies fired up to 750 °C, no difference in strength changing the temper GSD is observed, while in other cases the coarser the temper, the less strong the ceramic.