Abstract: A process for the manufacture of a substantially bubble-free article of synthetic vitreous silica free from localised variations in refractive index (striae) and suitable for optical applications, wherein an ingot of synthetic vitreous silica containing unacceptable bubbles is submitted to a first heat treatment process consisting of hot isostatic pressing at a temperature in the range 1,250° C. to 1,500° C. at a pressure in the range 10 MPa to 250 MPa, followed by a second heat treatment process at a pressure in the range 0.01 to 1 MPa and at a temperature in the range 1,550° C. to 1,850° C.

Abstract: A method and apparatus for the manufacture of synthetic vitreous silica ingots involves the production of a melt of synthetic vitreous silica in a crucible (35) within a refractory furnace (31), and the continuous withdrawal of an ingot (43) through an orifice (40) in the wall of the crucible. The silica may be deposited in the crucible by a synthesis burner (33), which may also serve to maintain the silica above its sintering temperature. The emerging ingot is supported by an arrangement of moveable clamps (44).