Float glass

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

[edit] History

In earlier centuries, window glass was made by blowing either large bottles or large disks. The bottles 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 bottle method (these 'bottles' were 6 to 8 feet (2 to 2.5 m) long and 10 to 14 inches (250 to 350 mm) in diameter).

The float glass process was patented in 1848 by Henry Bessemer, an English engineer. It was also patented in the United States in 1902 by W. E. Heal and again in 1925 by Hitchcock (a revised version of Heal's patent). In 1953 Sir Alastair Pilkington and Kenneth Bickerstaff developed the first successful commercial implementation of this flat glass production technique.

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.

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 solidified 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 pressurized by a mixture of nitrogen and hydrogen to prevent oxidation. The glass flattens out, forming a perfectly smooth glossy surface on both sides with an even thickness of approximately 6 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 6 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.