Up: Glassmaking
Sheet of Glass
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of the air and gas flow are reversed every twenty minutes, or at
some other selected time interval in order to attain the required
thermal efficiency.
In feeding the tank, the raw material, which
consists of the actual ingredients and broken glass, falls from a hopper
carried on an overhead crane, into what is called the filling pocket.
This pocket is a jutting out portion of the actual glass bath.
On tank design alone it would be possible to
spend a whole evening. Imagine a large box which may be as much as
120 ft. long, 36 ft. wide, and up to 5 ft. in depth. Its sides and
bottom are made of lay blocks and its roof of silica brick. It may
contain anything up to 900 tons of molten glass, with temperatures
varying from 1200°C. to 1450°C. in different parts of its
length. Then begin to play about with the shape of this box; alter
its depth at some point, lower its crown at another, give it a taper
from end to end. There is no end to the variations that can be
applied so to melt the frit at one end that one gets seedless
and homogeneous glass at the other. It is comparatively easy to
forecast the convection currents in a beaker, but the convection
currents in a glass tank of large dimensions are more difficult, and
to understand them is to know how to make good glass. Actually the
amount of glass flowing down the middle of the tank due to convection
currents is about twenty times as much as that being withdrawn at
the working end.
This prologue brings me to the description
of the actual processes which are involved in the making of a sheet
of glass, and to-night I will only deal with--
- Sheet or window glass.
- Plate glass.
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