Manufacturing salt at Lake Grassmere is a process of concentrating the salt content of sea water by evaporation, then harvesting the crystallised salt.
Concentrating the salt
Sea water, which contains 2.8% salt, is pumped into Lake Grassmere. Wind and sun evaporate the water, leaving behind a progressively more salty solution. When the salt content in the lake has increased to 5% the brine is transferred to a series of large concentration ponds. Calcium sulphate, the first mineral to crystallise, coats the bottom of the concentration ponds.
When the brine has increased to 25% salt, it is called a saturated solution. At this point around 90% of the original sea water has evaporated. The solution is then pumped from the concentration ponds into deep holding ponds, where it is held over winter.
In October of each year the six-monthly salt-making process begins. The saturated solution is pumped from the deep holding ponds into smaller crystallisation ponds. The salt forms crystals on the bottom of these ponds over the summer.
Salt is salt
Sea water is essentially the same the world over, and any table salt produced from it is also the same – typically over 97% sodium chloride, with trace levels of calcium, magnesium and sulphate. There is little evidence that these traces can be detected in the flavour of salt. The perceived difference in taste between flaky, coarse and normal table salt is due to the difference in crystal size, surface area and shape.
In March, when the salt is ready for harvest, the remaining brine containing other unwanted minerals (called ‘bitterns’ or ‘mother liquor’) is pumped from the crystallisation ponds into the sea. Unseasonable rain during harvesting, as occurred in 1986, can mean that no salt is harvested. Usually only 50 millimetres of rain falls during the autumn harvest period, and as rain water is less dense than brine, it forms a surface layer which can be decanted.
After the crystallisation ponds are drained, mechanical harvesters work around the clock for about five weeks scooping up the crystallised salt crust or ‘cake’, which varies in thickness from 25 to 100 millimetres. The salt is loaded onto trucks and taken to the washery. There, it is washed with saturated brine to remove mud and other impurities. The salt is then stacked in huge piles. After it has been crushed and sieved to produce similar-sized grains, it is sent to industrial users.
Range of salts
A large range of salts with slightly different chemical compositions, grain sizes and shapes are produced. All table salt produced in New Zealand is solar salt, and both iodised and non-iodised table salt is available. Iodised salt contains added iodine (to prevent goitre) and silicon dioxide (to make the salt run). Speciality salts, including flaky salt, are also produced at Lake Grassmere. They are as good as any in the world, despite the New Zealand fashion for sprinkling imported salt on fine cuisine. Animal health products such as salt licks for farm animals are produced at Lake Grassmere and Mt Maunganui.
At the end of summer Lake Grassmere’s gleaming white salt piles are easily seen from State Highway 1. This seasonal landmark forms a vivid contrast to the burnt brown Marlborough hills. And from overhead, air passengers descending towards Wellington can gaze down at the series of pink-coloured ponds where drying winds help produce half of the country’s salt.