Abstract: This disclosure teaches a vertically adjustable water cooled dome ring between an upper rim of a crucible and its gas trap. The gas trap has a dome with a dome opening which substantially conforms in cross section to a mouth defined by the upper rim of the crucible. The upper rim is provided with a downwardly and inwardly inclined lip. The dome ring has a seating collar adapted to engage the upper rim in a substantially gas tight manner. The seating collar is adapted to engage the upper rim about the mouth outward and in proximity of the inclined lip. Manifolds for cooling water are kept out of contact with the upper rim. A labyrinth passage is provided for entry of air between the dome and the seating collar.