Abstract: To sealingly retain a radiant heat exchange tube extending through an opening (5) in a wall (3) of a furnace, and to isolate the atmosphere within the furnace space (2) from outside ambient space, a thin-wall sleeve (25) has a first portion (40) shrinkfitted around the radiant heat exchange tube (7) which, for high temperature resistance, is made of a ceramic, preferably silicon carbide. A second portion (42, 44, 46) of the sleeve (25) which conically expands, is secured to a flange element (23), for example by welding, which, in turn, is secured for examples by screws, to a flange structure (8) on the wall (3) of the furnace. The flange element (23) has an inner diameter which is greater than the outer diameter of the tube (7). The sleeve (25) is preferably made of iron-nickel and has a wall thickness which is about 1/10 of the wall thickness of the ceramic tube (7).