Tylee Beaumont, of South New Brighton School, asks :-

What is sea salt?

Robert Kooistra, a chemist with the Dominion Salt Works in Marlborough, responded.

There are four main sources of salt. The first, sea salt, is obtained by evaporating sea water in large shallow ponds.

Rock salt is mined, much like coal. Great beds of salt are buried in many places around the world, particularly central Europe and Russia. The largest European deposit is at Cracow in Poland. These salt deposits were formed from ancient oceans so are therefore also effectively sea salt.

Brine-well salt is produced by sinking a well into a rock salt deposit. Water is forced down to dissolve the salt and the saturated salt solution is pumped up and the salt recovered by evaporation.

Lake salt comes mainly from Australia and the USA, where there are several saline lakes. Salt is recovered from these lakes in the same way as sea salt.

Two other terms associated with salt are Solar salt and Vacuum salt. Solar salt is any salt obtained by evaporating a salt solution naturally, using the wind and sun. Vacuum salt is salt produced from a solution, by heating it with steam while it is under a vacuum. All the table salt Dominion Salt produces is solar sea salt.

Dominion Salt has a solar works at Lake Grassmere that produces 65,000 tonnes per year. This is half New Zealand's requirements. The rest is imported into Mount Maunganui, and refined.

It takes two years at Lake Grassmere to extract salt from the sea. The first year is spent getting seawater to the point where salt is ready to crystallize. The second year is used to extract the salt from the saturated brine.