From IEEE Spectrum, March 30:
You need a lot of fossil fuel to float molten glass on a lake of tin
Plate glass, like steel, is all around us. It is taken for granted, and most people have no idea how it is made. Although both materials are ancient, their production was fundamentally improved only after World War II—primary steelmaking with the introduction of basic oxygen furnaces, and plate-glassmaking by the successful application of the old idea of floating molten glass on a large bath of molten tin. The tin provides a perfectly level surface.
Alistair Pilkington, of Great Britain, was the first to operate a continuous production line. The achievement was announced in January 1959, the first licensee was Pittsburgh Plate Glass, and the new method quickly conquered the market.
The ingredients are sand (SiO 2, about 73 percent of the charge), sodium dioxide (Na2O, about 13 percent), lime (CaO, about 9 percent), and magnesium (4 percent). The combustion of natural gas or a liquid fuel heats the mixture to about 1,050 °C, and the molten glass is then poured from a ceramic spout onto a 6-centimeter-thick layer of molten tin, at a temperature of 230 °C. The baths range from 4 to 8 meters in width and up to 60 meters in length, and they are kept under a protective blanket of nitrogen and hydrogen, to prevent the oxidation of the tin.
The glass spreads out in a way that can be regulated to produce sheets as thin as 0.12 millimeters and as thick as 25 mm. After the sheet cools to about 600 °C it is passed through a long container, called a lehr, that cools it gradually, an annealing process that prevents cracking. When cooled it can be cut into rectangles as large as 6 x 3.2 meters.
Like blast furnaces, float glass plants work nonstop for 11 to 15 years, which obviates the need to add large amounts of energy to restart the process. A plant produces anywhere from 50 to 1,200 tonnes or more of glass per day. Most plants have a single line, with the largest float glass factories in China operating 10 or more lines at one site....
....MUCH MORE
