Salt, or sodium chloride (NaCl), is a common mineral used in the manufacture of many products and as a food seasoning. Salt is the main mineral in sea water, making up three-quarters of the 3.5% of dissolved minerals in sea water. Salt also occurs naturally in mineral deposits, where it is known as rock salt. Because New Zealand has no rock salt deposits, salt was imported from the time of European arrival in the early 1800s until the country developed a sea-salt industry in the 1950s.
Almost 1% of human blood is salt. The salt-derived sodium in the blood helps regulate the body’s water balance, plays a central role in electrical impulses in the nerves, and assists cells to take up nutrients. Although some salt is vital to health, most people in the developed world eat too much of it.
Traditionally, New Zealanders sprinkle salt over a meal before even tasting it. This habit not only irks many chefs, it is unhealthy. Excess salt can cause health problems such as high blood pressure. On average, New Zealanders consume about 9 grams of sodium per day – the recommended amount is between 2.3 grams and 5.9 grams.
Salt is used to flavour food, and to preserve foods such as butter, meat and fish. During the 19th century it was added to soils as a fertiliser. It was also sprinkled onto hay during stacking so that livestock would get the benefit of sodium when the hay was fed out. Since the 20th century salt has been also been used in textile dyeing, bleaching paper, curing leather and in the dairy, food manufacturing and pharmaceutical industries.
James Cook saw Polynesians in Hawaii manufacturing salt in the 1770s, but it was never produced by Māori in New Zealand. Nevertheless, Māori had a conception of saltiness in their word mātaitai, meaning tasting of salt, or brackish. Their sodium needs were met through a diet of fish and shellfish. When salt was introduced by Europeans, some Māori believed that eating salted foods caused their people to contract illnesses previously unknown to them. However, the real reason was that Māori had no natural immunity to the infectious diseases introduced by Europeans.
In 1844 the Nelson Examiner reported that salt had been successfully evaporated from sea water in the local estuary. It seems the venture was carried out by a former whaling captain, who was unhappy about the price of imported Liverpool salt. In 1866 an import duty was imposed on salt in the hope that this would stimulate domestic production. It did not.
In 1892 the New Zealand government offered a bonus of £1 per ton on production of the first 500 tons (453 tonnes) of salt, provided it was done by 31 March 1893. The incentive prompted Aucklander John Stubbs to try his luck on the western side of Rangitoto Island, but the venture failed. In Gisborne in the early 1900s Francis de Latour tried pumping sea water onto towers of bundled sticks and strips of cloth that were exposed to the air. The method, which had been used in Japan, worked by using the large exposed surface area to increase the evaporation rate. However, it was not successful in New Zealand.
Before the 1920s many New Zealanders suffered from goitre, a swelling of the thyroid gland in the neck. It was the result of iodine deficiency, which can also cause mental disability in children. Because many New Zealand soils are iodine deficient, this trace element has been added to table salt since 1924 – a public health initiative which saw the virtual disappearance of goitre by the late 1950s.
From the end of the 20th century, speciality products such as flaky salt and coarse salt became popular. These lack iodine, and 1990s survey showed that many New Zealanders, especially children, were slightly iodine-deficient. This was probably due to a decrease in salt intake and a move towards non-iodised salt by food manufacturers and consumers.
Until the 1940s the lake bed at Lake Grassmere, Marlborough, was a mud bath in winter and a dustbowl in summer, with the occasional natural deposit of salt. The salt-making industry has arisen since then. In 2005 Lake Grassmere provided about half of New Zealand’s salt requirements.
In addition to low rainfall (about 585 millimetres per year), Lake Grassmere is perfectly suited to solar salt production. The large area of flat land that makes up the lake bed is near the sea so it can draw in salt water, and away from large rivers. High evaporation from sun and wind occurs during summer, and the site has ready access to both sea and rail transport.
The salt is produced from sea water by evaporation. Sea water is pumped through a series of large ponds, which concentrate the water, into a series of smaller ponds, which then crystallise the minerals out of water.
A traditional New Zealand saying has it that ‘nothing but salt water and ignorance separates North from South’. The salt water referred to is Cook Strait, which lies between the two islands. The strait is critical to salt making at Grassmere, acting as a wind funnel. Prevailing westerly winds are deflected by the Tararua Ranges to blow from a more northerly direction and with greater force through the strait. These drying northwesterly winds are the main reason for the success of the world’s highest-latitude solar salt works (at close to 42° south).
In 1942 Christchurch businessman George Skellerup became interested in making rubber, as it was in short supply during the Second World War. His business needed salt to make the caustic soda required to recycle old rubber. Skellerup threw his energy into constructing a salt works at Lake Grassmere. But no one in New Zealand knew much about salt making. Skellerup had plans drawn up for 20 concentration ponds covering over 400 hectares on the north side of the lake. However, wartime shortages of equipment and the worst winter floods in 30 years delayed construction.
In 1947 the government bought a share of the operation and the company changed its name from Skellerup Solar Salt Ltd to Dominion Salt Ltd. Two years later the crystallisation ponds were still not complete, but a thin salt crust had formed in the final concentration ponds. After years of setbacks, the first New Zealand salt of any quantity, some 45 tonnes, was collected by shovel and ‘washed’ with brine in a concrete mixer.
Early methods at Lake Grassmere were based on those of overseas saltworks, and trial and error. But other works were closer to the equator and relied mainly on the heat of the sun to evaporate sea water. Salt makers at Grassmere soon found that it was wind, rather than the sun, that did most of the work (Māori know the lake as Kāpara-te-hau – ‘wind-ruffled waters’). Scientists then worked out a process of moving the brines (salty water) from pond to pond as their concentration of salt increased. This special system, tailored to Marlborough’s unique evaporating conditions, increased the harvest year by year.
In New Zealand in the early 1800s skirmishes were commonplace and cannibalism was sometimes practised. Some Māori reported to the early English visitor Edward Markham that European sailors tasted too salty compared with the land’s indigenous people. He wrote in his journal of the 1830s:
‘Sailors the Gourmands pronounce to be too tough and Salt, and not so good as Mouries but still are eatable with a good appetite as Sauce and well done Potatoes’. 1
In 1962 new crystallisation ponds were built, with better road access and decanting troughs to run off rainwater. Because the ponds only covered around a third of the lake bed, the rest of the lake was used as a vast preliminary evaporation pond. This was done by pumping sea water into it. Two years later, conveyor belts replaced the light rail system used at harvest time. The vastly improved works produced 28,000 tonnes of salt in 1965. In the same year Cerebos, an English salt manufacturer, bought a one-third interest in the venture.
By 1970 the salt harvest had increased to almost 52,000 tonnes, but this could not match demand, which had grown with industrial development (especially from freezing works and the pulp and paper mills). It became evident that Grassmere could never meet New Zealand’s salt requirements. Bulk shipments of salt from the Caribbean and Australia were landed at Mt Maunganui, where a vacuum salt plant was built. This produced the high-purity salt needed by producers in the dairy and pharmaceutical industries. A second, smaller vacuum plant was also built at Lake Grassmere.
Manufacturing salt at Lake Grassmere is a process of concentrating the salt content of sea water by evaporation, then harvesting the crystallised 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 sulfate, 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.
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 sulfate. 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.
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, airline 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.
Pollard, J. S. ‘Salt from the sea: achieving the near impossible.’ In New Zealand is different: chemical milestones in New Zealand history, edited by D. Hogan and B. Williamson. Christchurch: Clerestory, 1999.
Reid, Jensen. Salt for New Zealand. Lake Grassmere: Dominion Salt, 1976.