How window glass is made

Window glass is poured onto a lake of molten tin, which is why it comes out flat with no polishing.

stations: 7
things upstream: 23
raw sources: 13
steps from the ground: 5

The short version

Most window glass is float glass. Sand, soda ash and limestone are melted at about 1,500 °C and poured onto a long bath of molten tin. The glass spreads into a ribbon with perfectly smooth faces, then travels through a cooling kiln about 100 m long. Machines at the far end cut the ribbon into panes.

Batch the raw materials

Sand, soda ash, limestone and dolomite are weighed and mixed, with a controlled share of crushed waste glass added. Small amounts of other ingredients adjust color or help remove bubbles.

Goes in
Silica sand Soda ash Limestone Dolomite
Melt it down

A float furnace is about 9 m wide and 45 m long and holds more than 1,200 tons of glass. The batch is heated to roughly 1,500 °C. The glass is then settled at about 1,200 °C so its density is even all the way through.

about 1,500 °C
Float it on molten tin

The glass pours from a ceramic lip onto a tin bath about 3–4 m wide, 50 m long and 6 cm deep. Tin is denser than glass and stays calm at that heat, so the glass floats and levels itself. A nitrogen and hydrogen atmosphere over the bath stops the tin from oxidizing.

tin bath 50 m long

Goes in
Tin Nitrogen gas Hydrogen
Pull the ribbon off

As the glass slides along the bath it cools from about 1,100 °C to roughly 600 °C, firm enough to lift onto rollers. The speed of the rollers against the speed of the flow sets the thickness.

1,100 °C down to 600 °C
Anneal in the lehr

The ribbon travels through a cooling kiln about 100 m long. Gradual cooling lets the glass settle without strain, so it will not crack when the temperature changes.
Cut at the cold end

Once the sheet exits the kiln, cutting machines score and snap it into panes. Some panes are tempered or laminated afterward, since most specialized glass starts as plain float glass.
Seal two panes together

For double glazing, two panes are held about 1 cm apart. The gap is sealed permanently and filled with dry air or another dry nonreactive gas, which improves heat and sound insulation and resists fogging.

End of the lineWindow glass Next it goes into mirror, skyscraper, windshield and 1 more.

Worth repeating

Before float glass, large panes were cast as a puddle on an iron table and then ground and polished on both sides.
Sir Alastair Pilkington and Kenneth Bickerstaff developed the tin-bath process between 1953 and 1957, and full-scale profitable sales began in 1960.
In the early 19th century most window glass came from blown cylinders 6 to 8 feet long that were cut open and flattened.