Since ancient times randomly distributed quartz is routinely included in ceramic earthenwares as an aid to drying. Although it has been noticed that this practice also provides the ware with higher capability for flaw tolerance so that a crack does not become readily unstable in the event of overloading during service, no systematic study of the effect or a satisfactory model has been reported in the open literature. This work reports the results of such an investigation on the influence of grain size and volume fraction of quartz on the elastic modulus, strength and fracture energy dissipation of such composites based on fired clays. In addition, we have studied the fracture micromechanisms by dynamic in situ optical microscopy and scanning electron microscopy and introduced a simple model to explain the results, based on the local effects of the stress field of the inclusions.