<p>This study reports the effect of iron oxide impurity in aluminosilicate ceramics (3Al₂O₃2SiO₂) on the effectiveness of use as refractory filler in manufacturing foundry mould coatings. The research unfolded the association of Fe₂O₃ with the thermal stability of the ceramic through dilatometry study. The study considered six commercially available mullite samples and examined for thermal expansion from the dilatometry data. The fragile propensity of the coating under thermal shock has been tested in laboratory condition. Also, the coating performance was tested in actual industrial trials. A comprehensive analysis of dilatometry results, thermal shock behaviour and performance of application of coatings on casting mould surface in real life production trials is discussed in the article. Coefficient of thermal expansion is observed to be declining with increasing iron oxide content in the filler material at higher temperature of 900&#xa0;°C. The study suggest that the least Fe₂O₃ content (0.18%) sample shows crack free specimen surface under thermal shock study. Also, coating made from the mullite sample containing 0.18% Fe₂O₃ revealed acceptable level of surface finish of castings in actual industrial trail.</p>