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A reported 90% of the world's flat glass is produced by the float glass
process invented in the 1950s by Sir Alastair Pilkington of Pilkington Glass.
Molten glass is “floated” onto one end of a molten tin bath. The glass is
supported by the tin, and levels out as it spreads along the bath, giving
a smooth face to both sides. The glass cools as it travels over the molten
tin and leaves the tin bath in a continuous ribbon. The glass is then
annealed by cooling in a lehr. The finished product has near-perfect
parallel surfaces.

An important characteristic of the glass is that a very small amount of the tin is embedded into the glass on the side it touched. The tin side is easier to make into a mirror and is softer and easier to scratch.

Float glass is produced in standard metric thicknesses of 2, 3, 4, 5, 6, 8,
10, 12, 15, 19 and 22 mm. Molten glass floating on tin in a nitrogen/hydrogen atmosphere will spread out to a thickness of about 6 mm and stop due to surface tension. Thinner glass is made by stretching the glass while it floats on the tin and cools. Similarly, thicker glass is pushed back and not permitted to expand as it cools on the tin.

Edited from Wikipedia and other sources

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