The dependence of flexural strength on crystalline phase content, residual porosity, and sintering temperature has been investigated for ceramic cores based on a mixture of fused silica and zircon. The rule of binary particle mixture, adopted for three particle sizes of fused silica, predicts a critical weight fraction of fine (zircon) particles of 46.9 wt% for the best packing. However, the optimal composition of a 37.8 wt% zircon-fused silica mixture for the flexural strength of ceramic cores found experimentally is lower in zircon content. The content of alpha-cristobalite + alpha-quartz crystallized from infilling silica sols exceeds similar to3 wt% when the strength decreases from similar to10 MPa to similar to6 MPa in samples sintered at 899-927degreesC. The degradation is ascribed to microcracking and the loss of coherency between the fused silica grains and the infilled silica due to beta- --> alpha-phase transformation on cooling and differential densification during sintering.