Figure 3b
presents the time required for gas bubbles to reach the
glass melt surface, depending on the height of the melt
in the fining zone. Very important, however, is that
during the primary fining processes, all existing bubbles
will grow and that generation of new seeds or bubbles
is restricted. Thus, bubble formation at the glass melt -
refractory interfaces in the fining section and downstream
the fining zone should be prevented by the
choice of suitable refractory materials and controlling
the interface temperature to values below a certain critical
level. The glass melt entering the fining zone
should be free of grains (sand, alumina, aluminum-silicates)
that still have to be dissolved, since these grains
may generate (nucleation, or a decrease of CO2 solubility
in melt surrounding dissolving grains) new CO2
seeds interfering with the fining process.