in the external neck portion 59 the purge gas fills the annular passage 71 and escapes downwardly out of the passage in a cylinder to form the second curtain of gas the velocity of the second curtain is less than that of the first curtain when used in conjunction with batch embodiments of the process the velocity of the second curtain decreases as the gas pressure over the melt increases the velocity of purge gas from the second curtain will preferably exceed about 250 cm second at about 30 torr about 75 cm second at about 100 torr about 35 cm second at about 200 torr and about 20 cm second at about 300 10 torr more preferably the velocity will exceed 280 cm second at about 30 torr about 80 cm second at about 100 torr about 40 cm second at about 200 torr and about 25 cm second at about 300 torr it is to be understood that the velocity of gas in the second curtain may be other than that described without departing from the scope of the present invention the first curtain is formed by the four purge gas nozzles 39 constructed of stainless steel and connected to the inert gas source by stainless steel flex hose not shown when used in conjunction with batch embodiments of the process the velocity of gas emitted by each jet is also diminished as the gas pressure over the melt 23 increases the velocity is preferably about 325 cm second at about 30 torr about 100 cm second at.