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Object lesson

The tell-tale tang in our blood, sweat and tears shows us that salt is essential to healthy life. Ancient, nomadic people got their fill from eating animals, but agricultural, then industrial, societies needed to add salt supplements to their cereal and vegetable-based diets. Its usefulness as a food preservative, additive and flavour enhancer soon made it a valuable commodity.

The Via Salaria was one of Italy's oldest trading routes, and the Romans paid their soldiers an allowance of salt called a salarium - hence our word salary. Our language is sprinkled with salty allusions. The compliment of calling someone "salt of the earth" alludes to the condiment's age-old associations, first mentioned in the Bible. Greek slaves who didn't meet the asking price were deemed not worth theirs.

Salt is still used as a currency in parts of Africa and has been taxed by governments (instrumental in the French revolution), preserved in place names (Salzburg in Austria and Saltburn in Cleveland) and fought over by armies. In peace time too, salt can be a matter of life and death. Even a small dose in a single helping of processed food can kill a baby. Too muchsalt in your diet can have long-term health implications but treacherous roads can be made safe with a liberal sprinkling - it lowers the freezing point of water and melts ice.

Every chef's favourite - sea salt - is extracted by evaporating the 3 per cent salt content of seawater in shallow pans. Layers of rock salt, called halite, were left underground when seas receded millions of years ago and where this lies close to the surface it can be mined like coal. Cheshire's salt mines have existed since pre-Roman times, but flooding and subsidence mean it is now extracted from the brine pumped from underground.

As a seasoning partner for pepper and a sharp-tasting sidekick to sour old vinegar, common salt - so called to distinguish it from other crystallised chemical salts - is a tabletop staple. But 90 per cent of salt produced is used in industry in the manufacture of dozens of everyday products. Soldiers once rubbed it into wounds to stop infection even though, metaphorically, that means making things worse, and superstitious souls throw it over their shoulders to appease evil spirits - a practice best taken with a pinch of the stuff.

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