Float glass

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Float glass is a sheet of glass made by floating the molten glass on a bed of molten tin. This method gives the glass uniform thickness and very flat surfaces. Modern window glass is float glass.

[edit] History

In earlier centuries, window glass or flat glass was made by blowing either large cylinders or large disks. The cylinders were cut apart and flattened and then window panes were cut from the large surface. Most glass for windows up to the early 19th century was made from rondels, while most window glass during the 19th century was made using the cylinder method (these 'cylinders' were 6 to 8 feet (2 to 2.5 m) long and 10 to 14 inches (250 to 350 mm) in diameter).

The flat glass process was patented in 1848 by Henry Bessemer, an English engineer was the first attempt to make a continuous ribbon of flat glass by forming the ribbon between rollers. The forming of flat glass on a tin bath was described in patents in the United States in 1902 by W. E. Heal and again in 1925 by Hitchcock (a revised version of Heal's patent). Between 1953 and 1957, (Sir) Alastair Pilkington and Kenneth Bickerstaff of the UK's Pilkington Brothers developed the first successful commercial implementation of the forming of a flat glass continuous ribbon glass using a molten tin bath on which the molten glass glass flows laterally unhindered to the limit of its free flow under the influence of gravity and surface tension. Neither Heal nor Hitchcock had described this free flow process. Full scale profitable sales of float glass were first achieved in 1960.

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 grinding and polishing both sides to smooth clarity, a very expensive process. From the early 1920s, a continuous ribbon of plate glass was passed through a lengthy and expensive series of inline grinders and polishers, helping to reduce glass losses and production costs.

Glass of lower quality, sheet 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 less parallel and of lower quality than those of float glass. This process continued for many years after the development of float glass.

[edit] Manufacture

Float glass is made by melting raw materials consisting of sand, limestone, soda ash, dolomite, iron oxide and salt cake. The raw materials are mixed together and fed into a large furnace that is natural gas or fuel oil fired. The raw materials, referred to as batch, blend together to form a large pool of molten glass. The molten glass is fed into the float bath (tin bath) through a delivery canal. The amount of glass allowed to pour onto the molten tin is controlled by a refractory gate called a tweel. The tin bath is provided with a protective atmosphere consisting of a mixture of nitrogen and hydrogen to prevent oxidation of the tin. The glass flows out onto the tin surface forming a floating ribbon with perfectly smooth glossy surface on both sides with an even thickness of approximately 4 mm. Thinner glass is made by stretching the glass ribbon to achieve the proper thickness. Thicker glass is made by not allowing the glass pool to flatten to 4 mm. Machines called attenuators are used in the tin bath to control both the thickness and the width of the glass ribbon.

As the glass flows down the tin bath, the temperature is gradually reduced until the sheet can be lifted from the tin onto rollers. It then passes through the lehr where it is further cooled gradually so that it anneals without strain and does not crack from the change in temperature. The glass travels down the rollers in the lehr for about 100 metres and comes out at the "cold end" where it is cut by machines.

Some tin is absorbed into the glass, and with a proper ultraviolet light a sheen can be seen which differentiates the tin from the non-tin side.