A method and apparatus for determining the level of slag containing iron or iron compounds in the bottom of a glass melting furnace is disclosed. Electrically conductive electrodes or probes are placed in a predetermined position in the furnace. When the iron containing slag touches the electrodes or probes, the resistance between the electrodes or probes decreases or the current flowing between them increases activating a signal or alarm to warn the operator of the level of the slag in the furnace.