Float glass

Float glass is made by melting glass and feeding a thin layer onto a tank of molten tin in a nitrogen atmosphere . The glass flattens out forming a perfectly smooth glossy surface on both sides with an even thickness. This allows the production of smooth clear window glass very economically.

As the glass flows down a long narrow tunnel, the temperature is gradually reduced until the sheet can be lifted from the tin onto rollers, where it is further cooled gradually so that it anneals without strain and doesn't crack from the change in temperature.

The nitrogen keeps the tin from oxidizing. 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.

Before float glass, 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).

Float glass was invented by Alastair Pilkington of Pilkington Brothers in the United Kingdom in the mid 1950s and announced to the world in 1959. (Alastair wasn't related to the family that owned the company, just an employee.)

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.