Abstract: A method is provided for detecting the spalling off of a significant portion of buildup of alkali and/or metal from the refractory lining of a metallurgical furnace. The method includes monitoring the rate of change of temperature of the refractory lining at a plurality of spaced locations and determining when the rate of change of temperature at a predetermined number of such locations exceeds a first threshold level that a significant portion of buildup has spalled off. The invention is particularly applicable to blast furnaces and permits the addition of heat to be made to the molten metal in the furnace before the spalled off portion descends through the furnace shaft and reaches the molten metal.