mold coating composition of the invention functions to provide the above and other advantageous properties is not fully understood some observations have been made unless noted otherwise the weight percentages disclosed below are based on the total weight of all the ingredients in the coating composition except for the water added as the carrier for formation of a slurry zirconia provides the coating with refractory properties and because of its high melting point permits the molten metal to be poured into the mold at higher temperatures which minimizes surface defects in the casting the zirconia is in finely divided or comminuted form preferably at least the majority of particles being 325 mesh the amount of zirconia is about 25 to about 55 preferably about 30 to about 45 weight to provide a smoother coating surface and high heat resistance and still minimize cost a mixture of lower purity zirconia such as insuloxide contains approximately 95 zro 2 marketed by nl industries of highstown n j and a finer higher purity zirconia such as electrically fused zirconia oxide 441 contains approximately 99 zro 2 marketed by tam ceramics of niagra falls n y is preferred when such a mixture is used the amount of the lower purity zirconia can be about 15 to about 30 preferably about 18 to about 24 weight and the amount of the higher purity zirconia can be about.