Method and apparatus for minimizing air infiltration in the production of fused silica glass

Fused silica boules (19) having improved radial homogeneity are produced by controlling the air flow around the boule (19) during its formation. The boule is formed in a cup-like containment vessel (13) which collects silica particles from a plurality of burners (14). The containment vessel (13) rotates and oscillates relative to the burners (14) as the boule (19) is formed. The containment vessel (13) is bounded by a cup-like containment wall (22), and a shadow or air flow wall (130) is spaced apart from and surrounds the containment wall (22) forming a gap or air flow passage (175) therebetween. A radially-outwardly extending deflecting wedge portion (23) is formed at the upper extent of the containment wall (22) and at an outlet end of flow passage (175). An upwardly and outwardly tapered surface (131) at the upper end of the air flow wall (130) complements an upwardly and outwardly deflection surface (123) formed on the deflection wedge (23) to form a radially-outwardly and upwardly flow passage (176) for discharging the flow from passage (175). The radially-outwardly and upwardly directed flow passage (176) deflects air infiltrated through the passage (175) outwardly away from the boule (19) so as to reduce and minimize the effects of infiltrated air on the boule.