Patent ID: 7427327

Claim:
A method for making a fused silica crucible comprising: feeding bulk silica grain along an inner surface of a rotating mold to arrange the bulk silica grain in a crucible shape having a radially inner surface open to an interior space of the mold and a radially outer surface adjacent an inner wall of the mold; heating the bulk silica grain from the interior space of the mold; drawing air through the bulk silica grain and into ports distributed in the inner wall of the mold; evolving gas from the heated grain; establishing a fusion front that begins at the radially inner surface and proceeds toward the radially outer surface; maintaining a pressure differential between the fusion front and the inner wall of the mold that draws gas away from the fusion front at a rate faster than it is introduced until the silica grain forms an intermediate glass layer greater than approximately 2 millimeters and having less than about 1% bubbles by volume in cross-sectional area after a vacuum bake test in which the fused silica crucible is heated at approximately 1650 degrees C. for approximately three hours at a pressure of approximately 0.1 Pa; thereafter reducing the pressure differential between the fusion front and the inner wall of the mold to a rate at which gas is drawn away from the fusion front at a rate slower than it is introduced; feeding barium-doped silica grain onto the intermediate glass layer; and fusing the barium-doped silica grain onto the intermediate glass layer to a depth of less than 0.2 mm.