By Harold McGee
Salt comes from seawater, and seawater contains significant quantities of several bitter minerals, the chloride and sulfate salts of magnesium and calcium. Producers have a couple of ways of dealing with these minerals. They can remove them from rock salt by dissolving the salt, then adding sodium hydroxide and carbon dioxide to the brine to precipitate magnesium and calcium. They can remove them from seawater by the same means, or else by slow and gradual concentration in open-air pans, during which the calcium salts become insoluble, crystallize, and settle before the sodium chloride does, and so can be separated. The sodium chloride in turn crystallizes before the magnesium salts, whose slight residue on the crystal surfaces can then be washed off in new brine.