<p>Investment casting is characterised by its ability to produce complex geometries with high-dimensional accuracy and excellent surface finish, relying on ceramic shells as mould cavities. This study, developed in collaboration with ZOLLERN &amp; Comandita Portugal, addresses an industrial need to balance shell strength, required to withstand metallostatic pressure during pouring, with controlled embrittlement during cooling to enable efficient knockout. The work investigates the influence that the formation of silica polymorphs, namely quartz and cristobalite, exerts on the mechanical behaviour of ceramic shell materials. Raw materials were characterised using X-ray diffraction, dilatometry and X-ray fluorescence, and ceramic shell powder with controlled additions of both polymorphs were prepared, compacted, sintered and mechanically tested under compression and biaxial flexure. A Design of Experiments approach was employed to statistically assess the influence of each phase. Results show that both quartz and cristobalite affect mechanical strength, with cristobalite playing a dominant role in promoting embrittlement. The 0–10% addition range was identified as the most relevant for optimisation, as higher additions do not yield proportionally greater strength reduction. These findings provide a foundation for optimising ceramic shell formulations to enhance knockout efficiency and improve process consistency in investment casting.</p>