Float Glass

Float glass is a sheet of glass made by floating molten glass on a bed of molten metal, typically tin, although lead and various low melting point alloys were used in the past. This method gives the sheet uniform thickness and very flat surfaces.
Modern windows are made from float glass. Most float glass is soda-lime glass, but relatively minor quantities of specialty borosilicate[1] and flat panel display glass are also produced using the float glass process.
Float glass uses common glass-making raw materials, typically consisting of sand, soda ash (sodium carbonate), dolomite, limestone, and salt cake (sodium sulfate) etc. Other materials may be used as colourants, refining agents or to adjust the physical and chemical properties of the glass. The raw materials are mixed in a batch process, then fed together with suitable cullet (waste glass), in a controlled ratio, into a furnace where it is heated to approximately 1500 °C. Common flat glass furnaces are 9 m wide, 45 m long, and contain more than 1200 tons of glass. Once molten, the temperature of the glass is stabilised to approximately 1200 °C to ensure a homogeneous specific gravity.
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.[8] The amount of glass allowed to pour onto the molten tin is controlled by a gate called a tweel.
Tin is suitable for the float glass process because it has a high specific gravity, is cohesive, and is immiscible with molten glass. Tin, however, oxidises 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.