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Float glass is a sheet of glass made by floating molten glass on a bed of molten metal of a low melting point, typically tin, although lead was used for the process in the past. This method gives the sheet uniform thickness and a very flat surface. The float glass process is also known as the Pilkington process, named after the British glass manufacturer Pilkington, which pioneered the technique in the 1950s at their production site in St Helens, Merseyside.
Modern are usually made from float glass, though Corning Incorporated uses the overflow downdraw method.
Most float glass is soda–lime glass, although relatively minor quantities of specialty borosilicate and flat panel display glass are also produced using the float glass process.
The first advances in automating glass manufacturing were patented in 1848 by Henry Bessemer.Bessemer, Henry ; British patent no. 12,101 (issued: Sept. 22, 1848). His system produced a continuous ribbon of flat glass by forming the ribbon between rollers. This was an expensive process, as the surfaces of the glass needed polishing. If the glass could be set on a perfectly smooth, flat body, like the surface of an open pan of calm liquid, this would reduce costs considerably. Attempts were made, including by Bessemer, to form flat glass on a bath of molten tin—one of the few liquids denser than glass that would be calm at the high temperatures needed to make glass—most notably in the US. Several patents were granted,William E. Heal, "Manufacture of window and plate glass," U.S. patent no. 710,357 (filed: January 25, 1902; issued: September 30, 1902)Halbert K. Hitchcock, "Apparatus for manufacturing glass sheets or plates," U.S. patent no. 789,911 (filed: July 30, 1902; issued: May 16, 1905). but this process was unworkable at the time.
Before the development of float glass, larger sheets of plate glass were made by casting a large puddle of glass on an iron surface, and then polishing both sides, a costly process. From the early 1920s, a continuous ribbon of plate glass was passed through a lengthy series of inline grinders and polishers, reducing glass losses and cost.
Glass of lower quality, drawn glass, was made by drawing upwards from a pool of molten glass a thin sheet, held at the edges by rollers. As it cooled the rising sheet stiffened and could then be cut. The two surfaces were of lower quality i.e. not as smooth or uniform as those of float glass. This process continued in use for many years after the development of float glass.
Between 1953 and 1957, at the Cowley Hill Works St Helens, Lancashire, Sir Alastair Pilkington and Kenneth Bickerstaff of the UK's Pilkington Brothers developed the first successful commercial application for forming a continuous ribbon of glass using a molten tin bath on which the molten glass flows unhindered under the influence of gravity. The success of this process lay in the careful balance of the volume of glass fed onto the bath, where it was flattened by its own weight. US patent 2911759 – Manufacture of flat glass Full scale profitable sales of float glass were first achieved in 1960, and in the 1960s the process was licensed throughout the world, replacing previous production methods.
The molten glass is fed into a "tin bath", a bath of molten tin (about 3–4 m wide, 50 m long, 6 cm deep), from a delivery canal and is poured into the tin bath by a ceramic lip known as the spout lip.B. H. W. S. de Jong, "Glass"; in Ullmann's Encyclopedia of Industrial Chemistry; 5th edition, vol. A12, VCH Publishers, Weinheim, Germany, 1989, , pp. 365–432. The amount of glass allowed to pour onto the molten tin is controlled by a gate called a tweel.
Molten tin is suitable for the float glass process because it has a higher density than glass, so the molten glass floats on it. Its boiling point is higher than the melting point of glass, and its vapour pressure at process temperature is low. However, tin Redox in a natural atmosphere to form tin dioxide (SnO2). Known in the production process as dross, the tin dioxide adheres to the glass. To prevent oxidation, the tin bath is provided with a positive pressure protective atmosphere of nitrogen and hydrogen.
The glass flows onto the tin surface forming a floating ribbon of even thickness with perfectly smooth surfaces on both sides. As the glass flows along the tin bath, the temperature is gradually reduced from 1,100 °C until at approximately 600 °C the sheet can be lifted from the tin onto rollers. The glass ribbon is pulled off the bath by rollers at a controlled speed. Variation in the flow speed and roller speed enables glass sheets of varying thickness to be formed. Top rollers positioned above the molten tin may be used to control both the thickness and the width of the glass ribbon.
Once off the bath, the glass sheet passes through a lehr kiln for approximately 100 m, where it is cooled gradually so that it anneals without strain and does not crack from the temperature change. On exiting the "cold end" of the kiln, the glass is cut by machines.
Most forms of specialized glass such as toughened glass, frosted glass, laminated safety glass and soundproof glass consist of standard float glass that has been further processed.
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