[Targan]

My mum's husband owns a salt field here in Australia so, following a short phone call, I now know quite a lot more about salt production!

Apparently the majority of large scale salt production around the world involves mining rock salt. Australia and Mexico are unusual in producing nearly all their salt from salt fields, where seawater is let into wide, flat salt pans and evaporated off.

Around 2.5% of seawater is salts. Around 60% of those salts is sodium chloride, the salt you want for cooking. If you evaporate the seawater in bowls or pans or whatever (using sunlight or heating it over a fire, it doesn't matter which) you'll easily get useable salt but it will be bitter because of the other, non-sodium chloride salts in it. I doubt such poorly refined, bitter salt would be as valuable a trade item as more palatable, better refined salt.

There are various methods that can be used (by those with the knowledge) to re-dissolve and re-evaporate out the salt so that you end up with purer sodium chloride. One low-tech method described to me is used on the island of Bali and involves leaving a bundle of sticks in an evaporating bowl of seawater. The salt crystals grow up the lengths of the sticks, and are then re-dissolved in fresh water and the process repeated to purify the salt.