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Flat plates with water between them

Why Can You Shape Clay but Not Sand?

Clay holds a shape because its grains are flat plates that slide on a film of water and then cling where they stop.

Wet sand Round grains — theshape collapsesWet clay Flat plates — the shapestays
The grain shape is the whole difference.

  1. Step 1 of 5

    Sand grains are round, so they roll.

    A heap of sand Round grains that rollapart

    Round grains have almost nothing holding them to their neighbours. Pile them up and they simply roll down again, which is why a sandcastle is so fragile.

  2. Step 2 of 5

    Clay grains are flat plates, thousands of times smaller.

    A sand grainabout 0.5 mmA clay plateunder 0.002 mm —a thousand timessmaller

    A clay particle is a plate, and it is so small that a thousand of them side by side would barely cross a grain of sand.

  3. Step 3 of 5

    Water between the plates does two jobs at once.

    A shaped potWater lets the plates slideWater also pulls them together

    A film of water lets the plates slide, so the clay is soft. But the same water pulls the plates together, so they cling where they stop.

  4. Step 4 of 5

    That is why clay remembers a shape.

    1 You push2 Plates slidepast each other3 They stay whereyou left them

    Push it and the plates slide to a new arrangement and stay there. Nothing springs back, because nothing was stretched — the plates simply moved house.

  5. Step 5 of 5

    Too much water and the clinging loses.

    Barely dampcrumbles —plates cannotslideJust rightslides and holds— you can throwa potSoakingslip — pourable,holds nothing

    Add more water and the plates get too far apart to hold each other. That is slip — clay thin enough to pour, useful for gluing pieces together.

    The oldest known fired clay object is a small figure from the Czech Republic, about 29,000 years old — roughly 20,000 years older than the oldest known pot.

The short version

Clay can be shaped because its grains are flat plates sliding on a film of water, which then pulls them together wherever they stop.

Try it yourself

Try building the same small tower from wet sand and from modelling clay. The sand slumps and the clay stands — nothing to do with how wet either one is.

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