A glass furnace, e.g. for annealing or tempering glass, has a roller conveyor whose fused rollers rest upon stub rollers overshot by a belt which alternately overshoots and undershoots the stub rollers. A drive for the belt advantageously is controlled by a computer allowing display of the heater parameters etc. and includes a pair of motors connected to the belt with overrunning clutches and driven in opposite senses to alternately drive the belt and thereby prevent play in the movement of the latter. The heater overlying the conveyor has a matrix array of individually or collectively controllable infra-red heaters.