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Melted, then cooled too fast to crystallise

How Do You Turn Sand Into a Window?

Glass is sand melted and then cooled so quickly that its atoms never get the chance to line up into crystals.

1 Sand, sodaandlimestone2 Melted atabout1,700°C3 Cooled fast— no timeto line up4 A clearsolid sheet
The trick is not the melting. It is the cooling.

  1. Step 1 of 5

    It starts as ordinary sand.

    A handful of sand Quartz — silicon andoxygen

    Beach sand is mostly quartz, which is silicon and oxygen. That is the main ingredient of nearly every window and bottle you have ever seen.

  2. Step 2 of 5

    Two extra ingredients make it possible.

    Sandabout 70% — theglass itselfSoda ashabout 15% —lowers themelting pointLimestoneabout 10% — stopsit dissolving inwater

    Pure sand melts at about 1,700°C. Adding soda ash drops that a long way, and limestone is added back in to stop the result dissolving in water.

  3. Step 3 of 5

    Cooling slowly would give you crystals.

    Cooled slowly Atoms line up — cloudyCooled quickly Atoms freeze where theystand — clear

    Given time, atoms settle into tidy repeating patterns. That is what a crystal is, and it is what makes a solid cloudy.

  4. Step 4 of 5

    Glass is a liquid's arrangement, frozen still.

    A window Atoms jumbled, butlocked in place

    The atoms in glass are jumbled exactly as they were when molten. They simply stopped moving. That is why it has no grain and no crystals.

  5. Step 5 of 5

    Flat glass is floated on molten tin.

    1 Molten glassleaves thefurnace2 Poured onto abath of liquidtin3 It spreads deadflat and cools

    Pour the melt onto a bath of liquid tin and it spreads out perfectly level all by itself. Nearly every window in the world is made this way.

    Glass does not sag over centuries. Old cathedral windows are thicker at the bottom because that is how they were made, and the glaziers fitted the heavy edge downward.

The short version

Glass is sand melted with soda and limestone, then cooled too fast for its atoms to line up into crystals.

Try it yourself

Freeze one ice cube tray fast in the coldest part of the freezer and one slowly in the fridge door. The fast one is cloudier — trapped air, frozen in before it could escape.

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