they are worn out for his purpose, he sells them for jewels to be put
into watches."
"What is plate-glass?" Lawrence asked.
"That is not blown, but cast. The pot, or
cistern, containing the melted metal, is swung up by a crane over an
immense polished metallic table, and tipped. The table is heated,
and there is a rim to keep the glass which is poured on from running
over the sides. The glass is then rolled down to a uniform thickness
by a heavy copper cylinder, reaching across the table, and resting on
the rim, which is just as high as the plate is to be thick. For
bow-windows the plates are bent before cutting up into panes.
"For mirrors they are silvered in this way:--
A sheet of tin-foil is spread on a table, and a thin coating of mercury
is poured over it. Then the glass to be silvered-- sometimes an
immense plate, and it has been carefully annealed, ground, and polished,
of course-- is slipped on in such a way as to exclude all the air from
beneath it, the table being tipped just enough to let the superfluous
mercury run off. When the plate is in its place on the table, it is
kept for several hours under a press of heavy weights. The mercury
and tin-foil combine to form what is called the amalgam, which coats
the glass and makes the mirror."
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