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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