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 the glass will anneal without strain and will not crack just 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 from the bottle method (these 'bottles' were 6-8 feet long and 10-14 inches in diameter.)
Float glass became a goal after the American Civil War and was finally developed in separate processes about the time of World War I.
Prior to 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 side to smooth clarity, a very expensive process.
