Abstract: A refractory brick and system of bricks for constructing flue walls of a ring furnace. The brick and the bricks of the system have opposed sides and upper and lower surfaces when disposed in a vertical wall structure, with one of the surfaces having at least one crosswise groove while the other surface has a corresponding crosswise tongue for seating in the groove of an adjacent brick, the tongues and grooves terminating short of the ends or sides of the brick when the bricks are disposed in a vertical wall structure. The crosswise tongues and grooves have rounded configurations sized to mate with bricks having the same tongue and groove configurations when a plurality of such bricks are stacked one upon the other in constructing a vertical wall.