Blood, ale and egg-white, making pure salt

The following article was originally published in The Mundling Stick, the Newsletter of The Lion Salt Works Trust.

When brine was pumped to the surface, workers would examine its “twaddle” – or saline content. A new-laid egg would be dropped into the brine; if it floated with most of its volume above the surface, it was strong brine.
But as well as the typical 25% of sodium chloride (NaCl), other salts could be present in the liquid: traces of sodium bromide, sodium sulphate, sodium carbonate, sodium iodide, magnesium carbonate, potassium sulphate, manganese carbonate…... Natural saturated brine was 27% NaCl, but Northwich brine contained about 1.5% of “earthy salts.” These would give the end-product a sour or bitter taste and had to be removed.

Early salt-makers had their own methods of clearing impurities as they heated the brine, by the addition of organic compounds, as John Ray described in 1674:1

This was known as “doping the pan,”

Thomas Lowndes, a Cheshire gentleman, endorsed this process in a long submission to The Admiralty in 1746, recommending the use of English salt in preference to French bay-salt, then commonly used by the Royal Navy. He also proposed the addition of “a nutmeg of butter” to the pan, although this did not seem to work with Cheshire brine. The open-pan method produced salt which was “clean, sweet, and strong.” Lowndes compared this with the French salt, made by evaporating sea-water, which was2

Later additives included alum, soft soap – depending on the type of salt crystals preferred , dolly blue, etc. Some impurities precipitated out through boiling, particularly calcium carbonate, magnesium and potassium salts, which had to be scraped from the pans once a week. At the Lion Salt Works, soft soap or glue would be tossed into the pan if the salt formed in “soft or hard shoots”.3 These formed a useful by-product in the form of salt-licks for livestock. Later salt production, especially in vacuum processing, uses soda ash (Na2CO3) and hydrated lime (Ca(OH)2) or caustic soda (NaOH). The magnesium and calcium salts are precipitated as magnesium hydroxide (Mg(OH)2) and calcium carbonate (CaCO3) and fall to the bottom of a settling tank. The latest processes claim 99.9% purity of the white salt produced.

Ironically, today’s salt producers may add chemicals to the finished product. Potassium iodide may be added as a source of iodine in countries where deficiency of iodine may cause thyroid problems such as goitre. And America’s largest salt producer, Morton, has since 1914 based its top advertising campaign on the Morton Salt Girl – a little girl cheerfully walking through the rain under her umbrella, carrying a package of salt which leaks onto the pavement. And the caption? “When it rains, it pours.” Morton’s salt contains an additive to prevent it caking in a damp atmosphere. In early days, magnesium carbonate was used, but since 1970 the anti-caking agent is calcium silicate.