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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of glass flowing down the middle of the tank due to convection currents is about twenty times as much as that being withdrawn at the working end.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--
  1. Sheet or window glass.
  2. Plate glass.