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Sheet of Glass
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    During this century three commercial flat drawn processes have been evolved:--
  1. Fourcault process.
  2. Colburn or Libbey Owen Process.
  3. Pittsburg process.
    Time does not permit a description of the first two processes in any detail. Each has its own peculiarities and difficulties to overcome; the Fourcault draws its sheet vertically from a slot in a depressed fireclay float, and our enemy, devitrification, is our constant companion. The Libbey Owen process draws its sheet vertically from an open bath, thus avoiding intensive devitrification, but the sheet when formed is reheated and bent to the horizontal by passing over a bending roller which tends to spoil the surface of the glass. The third process, the Pittsburg process, is now extensively used by us, and although it is exceedingly difficult to operate, we are very satisfied with its results. The process avoids the inherent defects of the Fourcault and the Libbey-Owen processes. The sheet is drawn vertically from an open bath of glass in which a bar of submerged fireclay defines the position of generation of the sheet, and the width is maintained by a peculiar device known as the "Edge Bowl."
    The edge bowl serves two purposes; the edge of the sheet is restrained by the sides of the slit with which it comes in contact, and it acts as a screen to shield the edges of the sheet from the radiant heat from the glass in the tank. The edge, therefore, cools quicker and becomes rigid sooner.
    As soon as the sheet has set it is engaged between pairs of rollers which drive it vertically through an enclosed tower. When it emerges at the top it is