[Illustration: SKIMMING THE POT.]
[Illustration: CASTING PLATE GLASS.]
~HOW THE HUGE PLATES OF GLASS ARE CAST~
The pot, having been first brought to the necessary high temperature,
is filled heaping full with its mixed “batch” of ground silica, soda,
lime, etc. Melting reduces the bulk so much that the pot is filled
three times before it contains a sufficient charge of metal. When the
proper molten stage is reached the pot is lifted out of the furnace
by a crane; is first carefully skimmed to remove surface impurities,
and then carried overhead by an electric tramway to the casting table.
This is a large, massive, flat table of iron, having as an attachment
a heavy iron roller which covers the full width, and arranged so as
to roll the entire length of the table. The sides of the table are
fitted with adjustable strips which permit the producing of plates of
different thicknesses. The pasty, or half-fluid glass metal is now
poured upon the table from the melting pot, and the roller quickly
passes over it, leaving a layer of uniform thickness. The heavy roller
is now moved out of the way, and then by means of a stowing tool the
red hot plate is shoved into an annealing oven. All of these stages
of the work have to be performed with remarkable speed, and by men of
long training and experience. The plates remain for several days in
the annealing oven, where the temperature is gradually reduced from an
intense heat at first, until at the end of the required period it is
no hotter than an ordinary room.
[Illustration: PREPARING THE GRINDING TABLE.]
When the plate is taken from the annealing oven it has a rough, opaque,
almost undulating appearance on the surfaces. It is only the surface,
however, for within it is as clear as crystal. First, it is submitted
for careful inspection, so that bubbles or other defects may be marked
for cutting out. It then goes to the cutter who takes off the rough
edges and squares it into the right dimensions; and thence to the
grinding room.
[Illustration: HOW THE GLASS PLATES ARE GROUND