Up: Glassmaking
Sheet of Glass
15 of 23
|
|
| |
Owing to the grinding and polishing it is immaterial
that the blanks should have a fire-polished surface, but it is still more
important that when polished the article should be free from seed, bubbles,
striæ and other faults.
POT PROCESS. Since
1774 when Plate Glass was first made in this country at St. Helens, though
not by us, and for some years after the war, the old pot process has carried
on with only minor modifications. The frit is filled into the pots through
an opening in the furnace door in the manner which has already been described.
Each pot contains about a ton of glass and yields a plate of about 300 sq.
ft. at a thickness of 7/16". When the melting, founding and
fining processes are complete, a pot is removed from the furnace and carried
on a bogie to the casting table.
Here the pot is raised by a "teeming" crane, which
turns it over and pours the molten glass on the table in front of the roller
in the form of a bolster. The moving roller converts it into a flat sheet,
which is then pushed into an annealing kiln, where it used to remain for
three days. This annealing kiln was superseded in England in 1902 by a
lehr, i.e. a series of kilns into which the plate is pushed, gradually
decreasing in temperature, until it is solid enough to travel on rails, by
means of which it is carried on a series of movements to the cold end. This
reduces the period of annealing for glass ½" thick from three days to
2½ hours.
BICHEROUX PROCESS.
The old pot process is not yet obsolete, but in most countries it has been
superseded by a method called the Bicheroux casting process, in which the
glass is made in a pot as before, but is poured between two rollers instead
of on to a table. The same kind of furnace
|
|