Lake Grassmere Salt Works

Most of New Zealand's salt comes from here despite the high latitude compared with most successful works. Commercial viability was achieved only after many years of trying. The pink colour which contributes to the remarkable landscape is due to the same algae that inhabits the Red sea. Listen to the audio guide or read on for more of the story.

In such a rain-soaked country as New Zealand it is difficult to believe there is a place where the evaporation rate exceeds the precipitation rate, but this occurs at Lake Grassmere. Here you will find a plant extracting salt from the sea and relying only on solar radiation and the wind. In 1892, the Government, eager to encourage self-sufficiency in the colony, offered a bonus of £1 per ton for the first 500 tons of salt manufactured in New Zealand. But nothing came of it.

Then in 1942, at the time of the Second World War, the urge for self-suffiency resurfaced. Rubber industry entrepreneur, George Skellerup, wanted salt to make caustic soda for a rubber reclaim plant and he embarked on what would be a protracted venture. Grassmere’s qualities, as a site near the sea and railway where long sunshine hours and driving winds evaporated more water than rainfall provided, had been identified in the 1920s. However, establishing the precise conditions for successful salt recovery did not prove easy. It would be 13 years before the first worthwhile harvest and 23 before the venture was commercially successful. By then, George Skellerup would be dead and the challenge was with his son Peter.

Seawater contains salts of calcium (0.13%)1 sodium (2.7%) and magnesium (0.6%). These salts crystallise progressively as the concentration increases. Using concentrating ponds in sequence, it should be possible to first separate the calcium salts, then the sodium chloride and, finally, to wash away the so-called bittern containing the magnesium salts.

So much for the theory, in practice the ‘salt makers’ had a lot to learn. They had to take into account that in some months the brine was diluted because the rainfall exceeded the evaporation rate, that the Cook Strait winds stirred up the brine so that the lighter rainwater mixed with the concentrate, and that the high evaporation season only occurred over a small portion of the year.

Finally, with the help of mathematical modelling and the advice of an Israeli with experience in other plants a successful system was arrived at. This involves a number of preliminary and final concentrating ponds, storage ponds for the winter period 16 to 20 feet deep, reconcentration ponds and crystallising ponds. Yields vary from year to year with the average recovery about 70,000 tonnes. An El Nino year can see an increase to 110,00 or more tonnes. The pink of the crystallising ponds is the bloom of an algae which turns red in concentrated brine.

There is an obelisk overlooking the saltworks with the inscription, “The winds of Lake Grassmere, aided by sunshine and science, work today in the service of man. G W Skellerup was never dismayed by the many setbacks and disappointments, as his vision went far beyond them. They were but another challenge to the ingenuity of a man who said, ‘I am doing this because I believe it is worth doing and I have a chance to do it’.”

Further Reading: Pollard J S, “Salt from the Sea: Achieving the Near Impossible” in “New Zealand is Different – Chemical Milestones in New Zealand History”, Clerestory Press 1999. (Footnotes) 1 Percentages by weight.