Salt

The body needs salt daily and loses more in heat and hard work — sweat can carry away several grams a day. Table and cooking use is a few kilograms per person per year.

Preservation dominates: fish hard-cured at about 1 kg of salt per 3 kg of fish, meat dry-cured with several percent of its weight in salt, plus what is lost in brine and drips. Work out the autumn slaughter and the fish run first, and size the salt stock to that. A shortage in autumn means losing the meat. See preserving the harvest.

In order of cost:

1. Rock salt: mine and crush it. By far the cheapest if a deposit exists.
2. Salt springs: often marked by animal licks, bare patches where animals gather, salt-tolerant plants and place names containing salt, sal, wich or lick. Taste the water.
3. Seawater: about 3.5% salt — roughly 35 g per litre, so about 29 litres of seawater per kilogram of salt.

Some salt springs are far stronger than the sea, which is why inland salt towns grew around them.

No instrument needed: weigh exactly one litre.

• Fresh water: 1.00 kg.
• Seawater: about 1.025 kg.
• Saturated brine — 26% salt, about 360 g per litre of water: about 1.20 kg per litre. Salt begins to crystallise here.

The number tells you whether a spring is worth working, how far concentration has gone, and when to stop feeding fuel. A floating fresh egg shows the brine is strong, but eggs vary too much to measure by.

Boiling dilute seawater burns enormous fuel for a small yield. Take water out for free first.

• Solar pans: shallow, clay-lined ponds a few centimetres deep, fed in series from one to the next as the brine strengthens. Needs sustained dry, warm weather.
• Graduation tower: brine trickled over stacked brushwood in the wind, pumped back up and trickled again, evaporating most of the water before it ever meets a fire.

Use the litre-weight test to decide when the brine goes to the boiling pan: as close to 1.20 kg per litre as the weather allows.

Evaporate the concentrated brine in wide, shallow pans — iron or clay — keeping it just below a boil so crystals form rather than crust. As it goes:

1. Gypsum (calcium sulphate) comes out first, as a fine scale. Skim it or let it settle in an earlier pan.
2. Common salt crystallises next. Rake it out as it forms and drain it on a slatted rack.
3. Stop before the pan goes dry. The last liquor — the bittern — is rich in magnesium salts: bitter, laxative, and it makes salt damp. Discard it.

Salt pulls water out of the air and turns to brine in a damp store. Keep it in sealed, non-metal containers — glazed pottery, wood, glass — off the ground, in the driest building you have. Salt corrodes iron and ruins metal containers.

A handful of dry-roasted rice or baked clay pellets in the container helps absorb moisture in a humid climate.

Home-made salt contains almost no iodine, and a population that loses manufactured iodised salt loses its main supply. Deficiency causes goitre — a swelling at the front of the neck — and, far worse, cretinism: permanent brain damage in children whose mothers lacked iodine in pregnancy.

The food sources: seaweed — very rich — sea fish and shellfish, eggs and dairy from animals grazing iodine-rich land. Inland and mountain regions are where deficiency historically concentrated. If the group is inland, make sea food or dried seaweed a regular part of the diet, especially for pregnant women.

Salt sits in the chemistry domain for a reason:

• Salt + sulphuric acid → hydrochloric acid — see acids.
• Salt → soda ash by the Leblanc and later Solvay processes — see alkalis.
• Electrolysis of brine → chlorine, hydrogen and caustic soda, once there is electricity — and chlorine is what disinfects city water.

Plan salt production with the industrial demand in mind, not only the kitchen's.