The Float Process
At the heart of the world’s glass industry is the Float Glass Process, developed by Pilkington in 1959 and now the world standard for high-quality glass production. The process manufactures clear, tinted and coated glass for buildings and clear and tinted glass for vehicles.
In the Float Glass Process, silica sand is the main component by weight of the ‘batch’ (raw material mixture). Soda ash is one of the most expensive raw materials used in glass manufacturing and represents about 16 percent of batch weight, but around 60 percent of batch cost.
Recycled glass (cullet) represents, on average, around 15 percent of the materials used. Its addition helps reduce the energy required in the process.
Although primary glass-making is energy-intensive, the ‘energy balance’ means that the energy expended in manufacture is quickly paid back over the lifetime of the products.
The Float Process
At the heart of the world’s glass industry is the float glass process. Invented by Sir Alastair Pilkington and announced in 1959, the process manufactures clear, tinted and coated glass for buildings, and clear and tinted glass for vehicles. The process, originally able to make only 6mm thick glass, now makes it as thin as 0.4 mm and as thick as 25 mm.
Molten glass, at approximately 1000ºC, is poured continuously from a furnace onto a shallow bath of molten tin. It floats on the tin, spreads out and forms a level surface. Thickness is controlled by the speed at which solidifying glass ribbon is drawn off from the bath. After annealing (controlled cooling) the glass emerges as a ‘fire’ polished product with virtually parallel surfaces.
A float plant, which operates non-stop for between 10-15 years, makes around 6,000 kilometers of glass a year. The process produces clear, tinted and coated glass for buildings and clear and tinted glass for vehicles.
Globally, over 380 float lines are in operation, under construction or planned, with a combined output of about 1,000,000 tonnes of glass per week. We operate, or have interests in, 46 float lines worldwide.
Modified basic manufacture
Extra ingredients can be added to the glass raw materials at the melting stage to produce tinted products. Modified properties can be produced by means of surface coating (on or off-line).
Plies of glass are bonded or laminated together with a layer of polymer film in between for use in safety and security applications. Glass can also be heat-treated (toughening), shaped, bent, silvered (mirrors), surface-worked, installed in multiple glazed units and, in Automotive, assembled in modular systems. Energy accounts for around 20 percent of total cost in the float process.
Float glass and environmental impact
The manufacture of one square meter of low-e double glazing leads to the emission of 25 kg of CO2. However, industry studies undertaken for the European flat glass trade association Glass for Europe show that the CO2 saved by replacing one square meter of single glazing with low-e double glazing in a typical European building is 91 kg per year, offsetting the CO2 emitted during manufacture after only 3.5 months in use. If ordinary double glazing is replaced by low-e double glazing, the offset period is typically 10.5 months.
For more information go to www.glassforeurope.com