How float glass gets perfectly flat

Float glass is made by pouring molten glass onto a long bath of molten tin. The glass is lighter than tin, so it floats and spreads into a ribbon whose top and bottom surfaces come out naturally flat and smooth. Left alone it settles at about 6 to 7 mm thick; rollers at the edges stretch it thinner or barriers hold it thicker, and an annealing oven called a lehr cools it slowly so it does not crack.

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MeltingSand, soda ash, limestone and recycled glass melt in a furnace at around 1,600 °C, then cool a little before leaving through a canal.
Floating on tinThe melt pours onto molten tin under a nitrogen and hydrogen atmosphere that keeps the tin from oxidizing. The glass spreads into a ribbon with both faces flat.
Thinner glassFor glass thinner than its natural thickness, toothed top rollers grip the ribbon edges and pull it wider and faster, stretching it thin.
Thicker glassFor thick glass, barriers at the edges stop the ribbon spreading, so it piles up thicker than equilibrium.
Annealing and cuttingThe ribbon slides off the tin at about 600 °C into a long lehr that cools it slowly to relieve stress, then is scored and snapped into sheets.

Why about 6 mm

A puddle of liquid on another liquid spreads until surface tension and gravity balance. For soda lime glass on tin that balance happens near 6 to 7 mm, so the float process makes that thickness almost for free. Every other thickness needs rollers or barriers working against it.

Output is ribbon width x thickness x speed x glass density. With a 3.3 m wide ribbon and a density of about 2.5 tonnes per cubic meter, the readout multiplies by 1,440 minutes per day.

Try it: hold the output near 600 tonnes per day while changing thickness. At 6 mm the ribbon runs around 8.5 m/min; drop to 3 mm and it must run about 17 m/min, stretched by the top rollers, to keep the furnace pulling the same tonnage.

What is simplified

The furnace check uses a rough teaching range of about 450 to 1,000 tonnes per day for one line. Real ribbons lose their thick edges when trimmed, and plants change speed and thickness together to keep the furnace pull steady. The drawn line is far shorter than a real one, which can run longer than a football field.

Sources

Flat glass production: the float process (lecture)Lehigh University, International Materials Institute
From Melt to Structure: The Science and Technology of Flat Soda Lime Silicate Glass for Structural EngineersBuildings (MDPI, open access)
Energy and Environmental Profile of the U.S. Glass IndustryU.S. Department of Energy

How to cite this model

Avid Solutions International. (2026, November 11). How float glass gets perfectly flat [Interactive 3D model]. Avid Field Lab. https://avidsolutionsintl.com/explore/float-glass

Last updated November 11, 2026

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