How smartphone cover glass is made

Phone glass gets its toughness from a hot salt bath that crams bigger potassium ions into the surface.

stations: 5
things upstream: 20
raw sources: 12
steps from the ground: 5

The short version

Smartphone cover glass is a thin sheet of alkali-aluminosilicate glass, cut to size and then soaked in a molten potassium salt at about 400 °C. Larger potassium ions replace smaller sodium ions near the surface and press the outer layer into compression, which makes it resist cracks and scratches.

Melt aluminosilicate glass

The batch is built on silica and alumina with sodium added, so the glass has small sodium ions available for later swapping. Corning also found that adding aluminum and zirconium oxides improved the glass.

Goes in
Silica sand Alumina Soda ash
Draw it into thin sheet

The molten glass is formed into a thin, even sheet. Corning uses the overflow downdraw method rather than floating the sheet on a tin bath.
Cut the sheet to size

Panels are cut and ground to the shape of each screen. All cutting and grinding happens before the strengthening step, because a strengthened surface can fracture if it is cut afterward.
Soak in hot potassium salt

The glass is immersed in a bath of molten potassium salt at about 400 °C. Smaller sodium ions in the glass trade places with larger potassium ions from the bath. The extra bulk jams the surface into compression and keeps small flaws from spreading.

about 400 °C

Goes in
Potash
Check hardness and flaws

Finished glass is tested for scratch resistance. Corning rates its glass at 622 to 701 on the Vickers hardness test.

End of the lineSmartphone cover glass Next it goes into smartphone.

Worth repeating

Corning's chemically strengthened "muscled glass" of the 1960s went into about 100 Dodge Dart and Plymouth Barracuda racing cars in 1968 to save weight.
As of October 2017, Gorilla Glass was used in approximately five billion devices.
Replacing small sodium ions with larger potassium ones to toughen glass surfaces was first developed by Steven Kistler in 1962.