Remove impurities from crude salt by dissolving it in the least boiling water that will take it up, filtering out sand and clay through clean cloth, settling and pouring off the clear brine, re-boiling it, and harvesting the first crystals, which are the purest. The bitter taste and the dampness of poor salt come from the magnesium and calcium salts left in the last liquid, called bitterns, which are drained away rather than eaten.
Part of Salt Production & Sourcing
Salt purification is the separation of sodium chloride from the insoluble matter and the other dissolved salts that come with it from sea, spring or rock, by dissolving, filtering and recrystallising.
What Is In Crude Salt
The parent lists the usual contaminants: calcium sulfate, magnesium chloride (which absorbs moisture), sand and clay. They behave differently and each needs its own step.
- Sand and clay do not dissolve. Filtering the dissolved salt through clean cloth takes them out.
- Calcium sulfate (gypsum) dissolves poorly and separates early as seawater or brine concentrates, before the sodium chloride crystallises (Seawater evaporation). In a boiling pan it is the scale that cakes on the hot base (Evaporation pan design).
- Magnesium and calcium chlorides, and magnesium sulfate, stay dissolved until the very end and are what makes salt taste bitter and go damp. The parent attributes the bitterness to calcium sulfate; as I understand it the bitter taste comes mainly from the magnesium salts.
- Organic matter and iron from pans and vessels discolour the salt.
The last liquid left after most of the sodium chloride has crystallised is the bitterns: concentrated magnesium and calcium salts. The parent says to discard it or to use it as a tofu coagulant, which is what magnesium chloride is (the traditional nigari). Do not use bitterns as a seasoning; magnesium salts are laxative in quantity.
Why Crystallise In Stages
Sodium chloride dissolves to about the same extent in hot and cold water, about 35.9 g per 100 g of water at room temperature (recalled), so you cannot purify it by cooling a hot solution the way you can with many salts. The separation comes from evaporation: as the brine concentrates, the sodium chloride crystallises while most of the more soluble magnesium salts stay in the liquid. Harvest the crystals before the liquid becomes mostly bitterns, and the first crystals are the purest, as the parent says. The cost is that some sodium chloride is left in the discarded liquid, which is the price of cleaner salt.
Washing
Crystals coming out of the pan are coated in a film of bitterns. Washing them with a little saturated brine (brine that already holds all the sodium chloride it can) rinses the film off without dissolving the crystals. Then drain and dry. A pile of freshly made salt left to stand on a slatted floor will also drain its bitterns on its own over days.
What You Need
- Crude salt, or brine, and clean water
- A pan or pot that does not react with brine (Evaporation pan design lists metals to avoid)
- Clean cloth for filtering, and a settling vessel
- A heat source, and a place to drain and dry the salt
- Some saturated brine for washing, made by stirring salt into water until no more dissolves
Method: Purify A Batch
Step 1: Dissolve the crude salt in the minimum of boiling water, as the parent advises.
Step 2: Filter through clean cloth to remove sand and clay.
Step 3: Let the brine settle and pour off the clear liquid carefully, leaving the sediment. The parent says calcium sulfate precipitates as the hot brine cools; as I understand it, gypsum is if anything less soluble hot, so expect some in the pan as well as the settling vessel. Either way, settle and decant.
Step 4: Re-boil the filtered brine until crystals form.
Step 5: Harvest the first crystals, wash with saturated brine, drain and dry.
Step 6: Keep the bitterns apart. Do not mix them back into the salt.
How To Tell It Worked
- Insoluble residue. Dissolve a weighed sample (say 100 g) in warm water, filter, dry the filter paper or cloth and weigh what remains. Very little should remain.
- Taste. Clean salt tastes simply salty. A bitter or metallic aftertaste means magnesium salts or metal.
- Damp test. Leave a spoonful on a dry dish in an open room overnight. If it turns damp while ordinary salt in the same room stays dry, it still has magnesium or calcium chloride (Keeping salt dry).
- What you cannot test: heavy metals and other poisons. Purification does not remove them. Know where the salt came from.
Common Mistakes
- Using road salt, de-icing salt or industrial salt. They may carry anticaking agents, additives and metals that are not food-safe.
- Stopping too late. Harvesting after the liquid turns to bitterns puts the bitter salts in your salt.
- Using lead, zinc or galvanised vessels to hold brine.
- Skipping the wash.
Safety
Hot brine scalds and some "salt" is not food
Boiling brine spatters and causes severe scalds; keep faces and children away and fill pans no more than they can hold. Salt sourced from road stockpiles, industrial deposits, water-softener supplies or contaminated shorelines can contain additives and contaminants that purification will not remove. If you do not know where it came from and what it is, do not eat it.
What’s Next
- Learn next: Grain size control — shaping the crystals for the use
- Combine with: Keeping salt dry — why purity decides how the salt stores