Salt

Salt is sodium chloride, and it occupies a position no other compound does: it is a rock, a food, and a bulk industrial chemical, and the three trades barely overlap.

About six per cent of world production is eaten. The rest is de-icing roads, softening water, and — the largest share — feeding the chlor-alkali industry, which dissolves it in water and electrolyses it to get chlorine and sodium hydroxide. The salt in that process is mostly a way of buying chlorine, and the sodium comes along.

Biologically it is not optional. Sodium carries the electrical gradient every nerve impulse and every muscle contraction runs on, the body cannot make it, and it is lost continuously in sweat and urine. That is the whole explanation for salt's extraordinary historical value: a substance nobody can go without, distributed very unevenly, and easy to tax.

Three routes, producing three grades, and which one a region uses is a question of geology and climate rather than preference.

Solar evaporation runs seawater or a salt lake brine through a sequence of ponds and lets the sun do the work. Cheapest, needs a dry sunny coast, and gives the sea salt trade its cachet — though the chemistry is the same and the difference is trace impurities and crystal size.

Rock salt mining works bedded evaporite deposits by room-and-pillar, hundreds of metres down and in the driest working conditions in mining. This is where road salt comes from, unpurified because nobody minds.

Solution mining drills into a bed, pumps water down, and brings saturated brine back up. It is the route that feeds the chemical industry, because brine is what the electrolysis cell wants anyway and purifying a liquid is easier than purifying a solid. Vacuum evaporation of that brine gives the fine, pure salt sold as table salt.

Salt was strategically important for as long as food preservation depended on it, which is until refrigeration — so, essentially, until the twentieth century. Salting is one of very few ways to keep meat and fish through a winter, and a settlement without access to salt could not store a surplus.

That made it taxable in a way few things are. The French gabelle was among the most hated taxes in Europe and survived to the Revolution; the Chinese salt monopoly dates to the seventh century BC and ran, on and off, for two and a half thousand years; the British salt tax in India was answered by Gandhi walking to the sea at Dandi in 1930 to make salt from it, which is one of the most effective pieces of political theatre ever staged and turned on the fact that everybody understood what salt was.

The folk etymologies are worth being careful with. 'Salary' does derive from Latin salarium and the connection to salt is real, though the claim that Roman soldiers were paid in salt is not well supported. 'Worth his salt' and 'salad' — salted greens — are straightforward.

Road salt is the environmental issue, and it is a much larger one than its reputation suggests.

Chloride does not degrade, is not removed by any wastewater treatment in ordinary use, and accumulates. Freshwater streams and lakes across the northern United States, Canada and northern Europe have risen in chloride concentration over decades of winter application, some to levels toxic to freshwater life, and the salt that reaches groundwater stays there. It also corrodes vehicles, reinforcing bar and bridge decks, and the infrastructure cost is substantial and rarely charged to the salting budget.

Alternatives exist and each has its own problem. Calcium and magnesium chloride work colder and are still chlorides. Beet juice and other organic brines cut the salt needed but add oxygen demand to the water they reach. Sand adds grit and no melting. The genuinely effective measure is using less — pre-wetting, brine rather than crystal, and better spreading control — rather than substituting.

Solar salt ponds, meanwhile, are among the more benign extractive operations there are, and several are managed as bird habitat.