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 salt-water wind funnel
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).
Salt and rubber
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.
A matter of taste
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.