Method and apparatus for discharging glass from a melting furnace

In a method for periodically discharging batches of a glass melt from a ceramic glass melting furnace in which localized heating energy limitable in time is supplied to the glass melt by means of electrodes in the furnace, the furnace being provided at its bottom with an induction heatable outlet member of ceramic material presenting a vertically oriented outlet passage for discharge of such batches, means defining a resistance heating region extending between the furnace electrodes and the bottom of the outlet member, and an induction heating source generating induction heat in a region located below the outlet member and below the resistance heating region, the furnace being operated to cause the outlet passage to be closed by a plug of the glass when no discharge is to take place, the improvement comprising melting such plug for discharging a batch of the glass melt via the outlet passage by the steps of:

• (a) preheating the outlet member and the glass plug by induction heating produced by said source up to a preheating temperature which is below the melting temperature of the glass and above which the rate of increase of the electrical conductivity of the glass plug as a function of increasing temperature is greater than that of the ceramic material of the outlet member;
• and
• (b) then further, heating the plug, while retaining the induction heating power, by means of resistance heating in said resistance heating region in which only the glass plug and the melt in the furnace serve as resistances, until the plug becomes almost liquid and molten and is suddenly pushed out of the outlet passage by the hydrostatic pressure of the glass melt thereabove.