A stir plug of frusto-conical shape for introducing gas into a mass of molten metal and comprising a plug having an outer core formed of a first refractory material. A wear indicator in the form of a central core comprised of a second refractory material is located within a centrally located recess in the outer core adjacent the bottom end of the plug. The central core extends from the bottom end of the plug towards the top end of the plug and is of a predetermined height less than that of the outer core. The upper end of the central core when exposed by the erosion of the stirring plug provides a visual indication of when that plug should be replaced.