How is sea salt produced?

1. Seawater tastes salty because salt is dissolved in it: broken into bits too small to see. Every liter of seawater holds about two big spoonfuls of salt.
2. At a salt works, wide shallow ponds lie right beside the sea. A gate is opened, often at high tide, and seawater flows in. Then the gate is shut.
3. Sun and wind make the water evaporate: it turns into vapor and drifts away. The salt cannot go with it. So the water that is left, called brine, gets saltier every day.
4. The strong brine is moved to the last pond. When the water can hold no more salt, the salt has to come out. It forms little white cubes on the bottom: salt crystals.
5. Workers rake the salt into heaps before the pond is completely dry. Then the salt is rinsed with very salty water, which washes off the mud without dissolving the salt.
6. The clean salt dries in the sun. Then it is sieved, and sometimes crushed, and packed into boxes. From sea to table, it can take many weeks.

Follow the seawater from the gate to the box, one step at a time.

• Let the sea in. The sea gate opens and seawater floods the first shallow pond. Then the gate is closed again.
• Evaporation pond. Sun and wind carry the water away as vapor. The salt stays, so the brine gets stronger, and it often turns pink.
• Crystallizer. The strong brine flows into the last pond. Here the water is so full of salt that crystals form on the bottom.
• Raking. The salt is raked into heaps by hand or scooped up by machines. A little bitter water is left behind in the pond.
• Washing. The salt is rinsed with very salty water. It washes away mud, but it is too full of salt to dissolve any more.
• Drying. The washed salt is left in heaps so the last water can drain off and dry in the sun.
• Packing. The dry salt is sieved into coarse or fine grains and packed into boxes and bags.

Push the cloud away from the sun and watch both pans. Try a windy day too. Which pan makes salt first?

Experiment: Two pans hold seawater from the same sea. One is shallow and one is deep. Which makes salt crystals first?

1. Push the cloud away so the sun shines on both pans.
2. Watch the water levels and the colors. Where do crystals appear first?
3. Let in new seawater and try a windy day.

What we found: The shallow pan makes salt first, because it has less water to lose. That is why salt ponds are wide and shallow. Sun and wind speed things up; clouds slow them down.

Turn on JavaScript to try it yourself.

Run the salt works and pack as much salt as you can. Move the brine on when it is strong enough, and choose the right rinse.

If you stir salt into water, it seems to vanish. It has dissolved: it has broken up into bits far too small to see, mixed in among the water. The sea has been collecting salt this way for a very long time. Rain and rivers wash tiny amounts of salt out of rocks and carry it down to the sea.

To get the salt back, you have to take the water away. That is what the sun does. Warm water turns slowly into water vapor, a gas that floats off into the air. This is called evaporation. But salt cannot evaporate. Every bit of salt stays in the water that is left, so that water, called brine, gets saltier and saltier.

Water can only hold a certain amount of salt. When the brine is completely full, any more evaporation squeezes the salt out, and it forms solid crystals. Salt crystals are little cubes. Salt makers help this along by using very wide, shallow ponds, because shallow water warms up quickly and has less water to lose.

Then comes the work. The salt is raked into heaps and rinsed with very salty water. Fresh water would dissolve it again, but water that is already full of salt cannot. The clean salt dries in the sun and is packed. People have made salt this way for thousands of years, and the sun still does most of the work for free.

Key words

Dissolve

To break up into bits too small to see and mix evenly into a liquid.

Evaporation

Liquid water slowly turning into water vapor and drifting into the air.

Brine

Very salty water.

Saturated

So full of salt that the water cannot hold any more.

Crystal

A solid whose particles are stacked in a regular pattern. Salt crystals are cubes.

Bittern

The bitter liquid left when most of the table salt has crystallized. It is rich in magnesium salts.

Check your thinking

1. What happens to the water in a salt pond?

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   It evaporates into the air. The sun warms the water and it turns into vapor that drifts away.

2. Why are salt ponds made wide and shallow instead of deep?

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   Shallow water warms quickly and has less water to lose. A shallow pond reaches the crystal stage much sooner than a deep one of the same size.

3. Why is harvested salt rinsed with very salty water and not with fresh water?

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   Fresh water would dissolve the salt, but very salty water cannot. Water that is already full of salt has no room for more, so the crystals stay solid.

4. Which weather makes salt fastest?

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   Sunny, dry and windy. Sunshine warms the water and wind carries the damp air away. Rain would add water back.

5. Why is the salt harvested before the crystallizer pond dries out completely?

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   The last liquid holds bitter magnesium salts that would crystallize into the salt. Magnesium and potassium salts are very soluble and stay in the bittern until nearly all the water is gone.

6. At what moment do salt crystals start to form?

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   When the water holds as much salt as it possibly can. Water can hold only so much salt. Once it is full, more evaporation pushes the salt out as crystals.