In a furnace (10) for producing high purity fused silica glass boules, the refractory in the area of the burner holes reaches such elevated temperatures that cause impurities to leach out and the dissociation of the refractory, causing contamination of the silica glass. In order to reduce the temperature of the burner hole refractory, a porous insert or liner (16) having an outer housing portion (17) is positioned within the burner hole (14) in the furnace crown (12), and a suitable gas is supplied thereto. The insert has a closed-end flow passage (18) therewithin, having an inlet end (20) connected to a sources of gas under pressure. The gas diffuses through the porous liner and not only cools the liner and refractory, but also forms a boundary layer (22) along an inside surface (24) of the insert that prevents particle buildup. A burner (26) is sealed to the insert (16) with seals (28). The cooling of the insert and surrounding burner hole refractory prevents contamination of the silica glass from the refractory.