Abstract

Furnace has a melting section and a withdrawal section bounded by side walls of refractory material which extend to a glass outlet formed by an overflow edge remote from the melting section. In operation a molten glass layer, a molten layer of undissolved sulfates, and a batch layer succeed one another from the bottom up in the melting section. The withdrawal section is separated from the melting section by a first dividing wall extending downwardly and terminating above the surface of the molten glass. A second dividing wall is disposed for the formation of an underside glass passage, and at least one closable opening for withdrawing the molten sulfates is disposed at the level of the sulfate layer between the first and second dividing walls. The opening has a bottom boundary whose height is adjustable, so that it can be adjusted to lie above the surface of the glass melt.