Seawater is about 3.5% dissolved salts by weight, and only about 27.4 of every 35 grams per kilogram of that is sodium chloride, the rest being sulfates and magnesium, calcium and potassium salts. Evaporation removes about 92% of the water before common salt starts to crystallise, so a salt maker needs a great deal of water to leave and a method that stops before the bitter salts come out.
Part of Salt Production & Sourcing
Seawater evaporation is the recovery of salt from the sea by removing its water with sun, wind or fire, in the order in which the dissolved minerals come out of solution.
What Is In A Kilogram Of Seawater
The parent article says seawater is approximately 3.5% salt by weight. That is the total of all dissolved salts, not sodium chloride alone. In standard seawater, a kilogram holds about 35 g of dissolved salts, of which sodium ions are about 10.8 g and the rest is chloride, sulfate, magnesium, calcium and potassium (standard composition after Dittmar and Millero; recalled). Pairing the sodium with an equal number of chloride ions gives the sodium chloride:
10.78 g × (58.44 ÷ 22.99) = 10.78 × 2.542 = 27.4 g of sodium chloride per kg,
where 58.44 and 22.99 are the molar masses of sodium chloride and sodium. So about 78% (27.4 ÷ 35) of the dissolved salts is common salt. The other 22% is the sulfates and the magnesium, calcium and potassium salts that taste bitter and absorb moisture, and they are why purification is a separate step (Removing impurities and bitter minerals).
The Minerals Come Out In A Fixed Order
As seawater loses water, the dissolved salts become more concentrated and the least soluble reach saturation first. Calcium sulfate (gypsum) separates early, then sodium chloride, and the very soluble magnesium and potassium salts remain in the final liquid, the bitterns. That order is why the parent advises 3-4 ponds in series: impurities such as calcium sulfate drop out in the early ponds, and the final pond is where the sodium chloride crystallises.
How much water has to go. Take 1 kg of seawater: it holds about 965 g of water and 27.4 g of sodium chloride. Sodium chloride dissolves to about 35.9 g per 100 g of water (recalled from standard solubility data), so the 27.4 g can stay dissolved in at most
27.4 ÷ 0.359 = 76 g of water.
The water to remove is 965 − 76 = 889 g, which is 889 ÷ 965 = 92% of it. This ignores the effect of the other salts, which shift the figure somewhat, but the order of magnitude is the point: more than nine-tenths of the sea has to be evaporated before the first crystals of salt appear, and all of that water is heat or sun the salt maker has to supply.
Choosing Sun Or Fire
The choice depends on climate, as the parent says. Solar evaporation needs strong sun, wind and low rainfall (Solar evaporation ponds). Where those are missing the water must be boiled off, at a large cost in wood (Boiling method for cloudy climates). The yield and the fuel can be worked out from the figures above (Yield calculations).
What You Need
- Access to clean seawater, away from sewage, river mouths with industrial runoff and harbours
- A method to evaporate: ponds in series or pans over fire
- A way to separate the crystals from the bitterns at the end
Method: Principles For Either Method
Step 1: Draw seawater from a clean place and let it settle to remove sand.
Step 2: Concentrate it in stages, dropping out calcium sulfate early.
Step 3: Let the sodium chloride crystallise in the final stage, and stop before the liquid becomes mostly bitterns.
Step 4: Drain off the bitterns and dry the salt.
Common Mistakes
- Taking the parent’s 3.5% as the sodium chloride content. About 78% of it is sodium chloride.
- Letting the pond go to dryness. The bitter salts crystallise last and spoil the salt.
- Collecting seawater near a drain or industrial outfall.
Safety
Seawater is not drinking water, and its salt may carry what the sea carries
Do not drink seawater or the unboiled concentrated brine. Salt made from polluted water carries the pollution into food, and boiling or evaporation does not remove metals or industrial chemicals. Use water from open, clean coast, and see Source Risk Rating for how to judge a source.
What’s Next
- Learn next: Seawater Salt Production — the two production routes in outline
- Combine with: Salt Sources — the first-month overview of finding salt