The gasification of coal can be accomplished more efficiently if it is operated at a temperature high enough so that the mineral constituents of the coal will form a molten slag which can be continuously removed from the gasifier during operation. Proper control of such a process requires careful temperature control, but the molten coal slag has been found to be corrosive to most thermocouple materials and to conventional refractory ceramics which are used as thermocouple protection tubes in other environments. It has now been found that magnesium chromite serves satisfactorily as a corrosion resistant refractory material in molten coal slags, and that thermocouple protection tubes of this material can be made by isostatic pressing of granular magnesium chromite and subsequent firing.