Compatibility relations of Al203in the quaternary system Zr02‐Al203‐Si02‐CaO were studied by firing and quenching followed by microscopy and energy‐dispersive X‐ray examination. A projection of the boundary surface of the primary crystallization volume of Al203was constructed in terms of the CaO, Si02, and Zr02contents of the mixtures recalculated to 100 wt%. Two invariant points, where four solids coexist with a liquid phase, are defined, and the positions of the isotherms were es