Sheet of Glass
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There are two other raw materials which are
important in the flat glass industry, coal and rough sand for
grinding. When it is realised that coal, sand, both for glassmaking
and for grinding, are to be obtained locally, that limestone comes
from Derbyshire, and that St. Helens was a centre of chemical activity,
it will be understood why this town was selected in the first place
for the manufacture of glass.
M E L T I N G P R O C E S S E S
Having mixed the raw materials, or frit as it is called, the next step
is to melt it, and to do this there are two well-known processes:--
1. The older method in which the materials are put into one of a number
of clay pots inside a gas-fired (or oil-fired) furnace, and a definite
time temperature cycle is allotted to melting, founding, refining and
cooling off to the working temperature.
In filling the pots one man spades the raw
material into a scoop, which, when filled, is pushed by a second man
through a hole in the furnace wall and dropped into the pot inside the
furnace. A model of a recent pot furnace of this type is to be seen
at the Science Museum, South Kensington.
2. The alternative melting method is the tank furnace. The Siemens
regenerative furnace was invented about 1856, and we were almost the
first people to apply this principle to the manufacture of glass.
In this case the mixture is fed at one end
where it is melted, and the glass then flows through controlled
temperature zones which ensure the founding and refining, and finally
arrives at the working end at the required temperature. The direction