Grey, Damp, Bitter Salt: Should the Family Switch to Boiling?
On the south coast a family makes sea salt in shallow clay-lined pans. They pump seawater, density about 1.025 g/cm³, into the pans and let sun and wind evaporate it. After about six days the brine reads 1.20 g/cm³ on their hydrometer and crystals begin to form. They keep evaporating until the pan is almost dry, then shovel out everything. The salt is grey, damp and slightly bitter, and a shop in town has rejected the last two sacks. The family sells about 40 kg a week at $2 a kg. The eldest son tries boiling brine in a pot on a gas stove. He gets crystals in about two hours, but they are small and still taste bitter, and he uses a lot of gas. He wants the family to give up the pans and boil all their brine. Rain is forecast for Thursday.
1. Explain why boiling produces crystals quickly but gives small crystals and a bitter taste, using the idea that crystal formation has two stages.
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Crystal formation happens in two stages: nucleation (tiny seed crystals appear) and growth (material adds to those seeds).
Boiling drives water off very fast, so the brine becomes strongly supersaturated - far more concentrated than it needs to be for salt to come out. A high supersaturation triggers a burst of nucleation, so a huge number of tiny seeds form at once. There is then little dissolved salt left per seed and little time, so each crystal stays small. In a solar pan, supersaturation builds slowly, few nuclei form, and those few have days to grow - hence large crystals.
Small crystals have more total surface area for their mass and more gaps between grains, so they trap the surrounding liquid. That liquid is the bittern: the concentrated magnesium, calcium, potassium and sulfate salts left after the sodium chloride has come out. Magnesium chloride and magnesium sulfate taste bitter, and magnesium chloride is deliquescent - it absorbs water from the air, which is why the salt feels damp even in a dry sack. Bittern left in the product causes both faults.
2. Explain why the family's current method gives bitter salt even before any boiling, and describe two changes to the pan process that would improve purity.
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Why the current salt is bitter: they evaporate 'until the pan is almost dry'. Sodium chloride is the first salt to crystallise as brine concentrates, but it is not the only one. As the last water goes, everything still dissolved - magnesium chloride, magnesium sulfate, potassium chloride, calcium sulfate - crystallises too, and it crystallises around and inside the salt grains already on the pan floor. Shovelling out the whole pan therefore harvests the impurities along with the salt. The grey colour is fine mud and insoluble particles that were in the seawater.
Change 1 - stop early and drain the bittern. Harvest while a few centimetres of brine still cover the crystals, heap the salt so the liquid drains away, and discard or separate the bittern. The salt is never in contact with the concentrated impurity liquor at the end.
Change 2 - wash with saturated brine, not fresh water. Fresh water would dissolve the salt itself and cut the yield. Saturated brine already holds as much sodium chloride as it can, so it cannot dissolve more, but it rinses mud and bittern off the crystal surfaces.
Supporting changes: let the brine settle and filter it before it enters the pan (removes mud), and cover the pan with a translucent cover to keep dust and rain out.
3. The son wants to boil all the brine. Give a reasoned recommendation to the family, taking account of cost, quality, quantity and the Thursday rain.
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Recommendation: do not switch to boiling as the main method. Keep the pans, fix the process, and use boiling only for a small premium batch if a buyer wants it.
Quantity: a household gas stove produces a few kilograms per batch, against 40 kg a week sold. Scaling up would need a furnace and a lot of fuel - far beyond a pot on a stove.
Cost: gas is a continuous cash cost; sun and wind are free. The family would be trading a free energy source for a paid one to make a product that is still bitter, because boiling does not remove bittern - it simply traps it in smaller crystals. The son's real problem is the bittern, not the speed.
Quality: solar evaporation, done properly with draining and washing, gives the large, bright, free-flowing crystals the shop is asking for.
The rain: Thursday's rain will dilute the brine and can re-dissolve crystals already formed, so on Wednesday evening they should cover the pans with a translucent sheet weighted at the edges, or harvest the ripest pans early. They should not respond to the rain by boiling, which dodges the problem rather than solving it.
A fair compromise: run one small boiled batch per week for a boutique/hotel buyer who wants fine salt, and label it accordingly.