Sodium chloride - common salt - is an essential for life. It is found in all body fluids, from sweat and tears to semen and snot. It makes your heart beat, your blood flow and your neurons fire. Your muscles use it when they move and your stomach uses it to help digest your food. You have nine grams of salt in every litre of blood: sweat and urine get rid of the excess - your body's way to keep the balance right.
Too much salt can be fatal as it causes fluid retention which can lead to high blood pressure. It is thought that as many as 35,000 people in the UK die each year from strokes and heart attacks associated with excess salt.
However, too little salt can also be dangerous: early symptoms of a salt deficit are muscle cramps, dizziness, nausea and tiredness and, in the long term, salt deprivation can be fatal. It is particularly dangerous because there is a gap between the physiological need and the brain's perception of the problem - we don't develop a craving for salt when we are deprived of it.
Salt also plays a key role in preserving food, especially in the days before refrigeration. It absorbs moisture (which is why it clogs up your saltcellar on a humid day) and any bacteria in food will dry out and die in its presence. Meat and fish were salted to keep over long winters when fresh food was not available. Salted food was also vital for explorers on long voyages - it is said that salt-fish was a vital part of Christopher Columbus's discovery of America. In many parts of the world, salt crystals have been harvested from the surface-evaporation of shallow pools or lagoons for thousands of years - the Chinese are known to have collected salt in this way in 6000bc. Another common technique, also recorded in ancient China, was to boil sea water in clay pots, and this practice became widespread in Europe under the Roman Empire. Chinese ruler Li Ching went on to drill deep wells, using bamboo pipes to pump up the brine, and by 200ad bamboo was also being used to supply natural gas for boiling the brine in iron pans. Salt production in clay pots was widespread in the British Isles in the Iron Age. The pots were supported on pillars: when the water had evaporated, the vessel was broken open to obtain the lump (or "briquette") of salt. This process made it easier to transport.
The need for timber for fuel in this early industry resulted in the extensive destruction of forests around Nantwich and other salt-producing areas of Cheshire (place-names ending in "wich" indicate local salt-making in Anglo-Saxon times). Later, coal was used as fuel, brought from Lancashire on packhorse trains, which were then laden with salt for the return journey. In South America and the Sahara, salt bricks are still produced for transportation by llama or camel train.
Salt has also figured in power struggles, wars and revolutions from the earliest days of recorded history. It may have been the first traded commodity, and horses, camels and boats laden with salt opened up trade routes. In the Sahara, the rich civilisation centred on Timbuktu was based on trading, with caravans of thousands of camels transporting salt and gold across the desert. In some places, it was even used as money: in Ethiopia, salt bars were for centuries used as currency; Marco Polo reported that in Tibet the ruler's image was impressed on cakes of salt; and a ration of salt made up part of the salary for Roman soldiers (the word soldier itself comes from the Latin sal dare - to give salt). Slaves, criminals and prisoners of war have traditionally been put to work producing salt.
Banishment to the "salt mines of Siberia" was a byword for punishment by hard labour in Russia under the Tsars and Joseph Stalin. And in empires across the world, enslaved people have been exchanged by their captors for salt.
By the early 17th century, Cheshire salt was being processed in Liverpool.
Many Liverpool merchants who later became rich through their participation in the Atlantic slave trade, started out by exporting the processed salt to the Newfoundland cod fisheries for making salt-fish, which was then transported to the West Indies. The ships would return to Liverpool laden with rum, tobacco and sugar for the home market. In 15th-century France, the salt tax - known as the "gabelle" - was a major source of state revenue. A system of custom posts and inspectors enforced the tax with severe penalties for "faux saunage" (salt fraud) - in the Camargue shepherds whose sheep drank from salty pond water faced heavy fines. By the 1780s, thousands of troops were deployed across the country to prevent salt smuggling, and thousands of French men, women and children were being jailed (or even executed) each year for salt-related crimes. The salt tax by this time symbolised everything that was odious about the regime, and is often cited as one of the main reasons for the French Revolution.
Everywhere where salt was taxed, salt smuggling was rife. In 1792, a Royal Commission on the trade and revenues of the Isle of Man, reported that: "I it is notorious that considerable Quantities of (salt) are smuggled from the Island to Great Britain, to the great Injury of the Revenue of that Country." Suggested penalties included the seizure of horses, carriages and packages containing salt, and fines and imprisonment for people caught smuggling. Salt was taxed in Britain until 1825. But it was in its imperial ventures that Britain was able to exploit salt to the fullest. In the case of India, this exploitation had important historical repercussions. For thousands of years salt was produced and traded freely across the subcontinent. In the Rann of Kutch in Gujarat, on the west coast, the extensive marshland area is flooded with sea water in the rainy season; dry winds from the north help to evaporate the flood waters to leave massive salt fields. In Orissa, hundreds of miles of the eastern coastline are affected by spring tides which saturate the soil with salt up to 60 miles inland. When the water evaporates, the salt can easily be harvested - a traditional way of earning a living in the area. For centuries local people traded salt with merchants who used the river systems to transport it to inland areas. Taxes were low enough to be tolerable while giving local rulers a substantial profit. In the 18th century, the British East India Company began to relegate these ancient traditions to history. The Orissa saltfields contained large amounts of saltpetre, which was needed for making gunpowder, and Britain was at war with France. Within a few years, Orissa's salt became a British monopoly, and the destruction of the local salt industry undermined the entire social structure of the area. Salt taxes were also set at punishing levels across India, and much of the money was pocketed by East India Company officials for personal enrichment.
In the 19th century a customs line was set up around Bengal, and duty had to be paid on any salt that crossed it. This border was later reinforced by the construction of a 1,500-mile thorny hedge, extending across India from the Himalayas to Orissa, and many thousands of men were deployed along the line to enforce the salt tax and prevent smuggling. In spite of the effects of the salt tax, there was little protest against it. It was not until 1930 that the tax became a major political issue, described by Mahatma Gandhi as "the most inhuman poll tax that ingenuity of man can devise". The idea of reclaiming their traditional right to freely produce salt quickly appealed to the Indian people. When Gandhi picked up some crystallised salt on the sea shore in April 1930, this symbolic breaking of the law sparked off a massive campaign of non-violent civil disobedience, which finally forced the British to negotiate concessions and established the Indian National Congress on the road to independence.
If the history of salt reveals tyrannies and taxes, what about its future? In India, recent proposals to raise salt tariffs have had to be abandoned after they met with strong political resistance, with much reference to Gandhi's campaign. And in Japan, where the government had planned to liberalise the economy, pressure from salt producers has forced it to introduce a protective tariff on imported salt. In the Sahara desert, as in the Bolivian highlands, trucks and lorries are replacing camel and llama caravans, and old ways of life are disappearing. Once thriving communities are being left to an impoverished fate.
While in our domestic lives salt plays a greatly diminished role, it is still much in demand in the chemical industry. And it evidently still has the potential to raise the temperature of political and economic arguments around the world.
* To find out more about salt production, visit The Salt Museum and The Lion Salt Works in Cheshire
Salt: A World History By Mark Kurlansky Vintage, pound;7.99
* Salt: Grain of Life By Pierre Laszlo Ecco, pound;8.35
SALTY WONDERS OF THE WORLD
* Salt mining excavations can create ghostly, fantasy-like structures which can take on the dimensions of entire cities. In Detroit, for example, there is a gigantic salt mine more than 1,000 feet below the city surface. At the start of the 20th century, mules were lowered by rope down the narrow shaft for transporting the salt; once down, they were there to stay. In more modern times, huge lorries travelled the 50 miles of roads, transporting the salt from the rock face to the crushing machines. Next year, a museum will open in Detroit to remind future generations of its salty history.
* In Wielicza, Poland, where rock-salt mining goes back to the 13th century, you can visit hundreds of kilometres of excavation chambers, containing enormous chapels and galleries with art works, all intricately carved from salt.
* In Bolivia, you can stay in a hotel constructed entirely of salt (see picture, page 11), and perhaps see the llama caravans, laden with blocks of salt from the world's largest and highest salt lake.
* Salt pans, created by the pre-Inca Chanapata people and in use ever since, can be seen at Salinas in Peru, where more than 3,000 brine pools are worked by a co-operative of 400 families.
* For the Hopi tribe of Arizona, salt-gathering has long had a cultural significance, requiring an arduous journey to their mines in the Grand Canyon. They believe it was deliberately made inaccessible, as a reminder of how precious the salt is. The mines have been put off-limits to the general public by the US National Park Service out of respect for the Hopi people.
* The Zuni people of New Mexico believe a salt lake is the home of their Salt Mother deity. In August, 2003, the Zuni tribe, together with environmentalists and other supporters, won a 20-year campaign to prevent a massive coal stripmine being established near to the lake.
Many wild animals visit salt licks, such as rock salt or brine pools (domesticated cattle have it added to their diet). Elephants troop into the Mount Elgon caves in Kenya, where they carve salt out of the rock with their tusks. Elephants need up to 100g of salt a day, and it is believed that the extensive complex of caves has been excavated by these animals over thousands of years.
MAKE AN ICE CREAM
Each student will need:
Measuring spoons and cups Half a cup of milk (full cream or low fat)
1 tablespoon sugar
14 teaspoon vanilla
1 pint-sized plastic zip-lock bag
1 gallon-sized plastic zip-lock bag
A large bag of ice
6 tablespoons rock salt
1 Pour the milk, sugar and vanilla into the small zip-lock bag.
2 Squeeze out as much air as possible and seal. Place the bag inside the large zip-lock bag.
3 Add the crushed ice and rock salt to the large bag. Squeeze out the air and seal the bag.
4 Shake vigorously for five to 10 minutes.
When the ice cream is thick and creamy take the medium bag out of the large bag andrinse off. Squeeze the ice cream into smallcups and eat.
SALT AND THE ENVIRONMENT
* Salt lowers the freezing point of water, and thus turns ice and snow back into liquid water. This is why it is used on the roads to help clear snow and ice. But it can also have a harmful effect, and in Canada, where up to 20 million tons of salt are laid on the roads each year, it was recently declared an environmental toxin. Along the road sides, up to 10 per cent of trees have been damaged or destroyed by salt, and plants can be affected up to 100 feet away from a major highway. There is increased corrosion of metal surfaces in cars and bridges, and more people complain about saltiness in the drinking water when the snow starts to melt. The salt has an impact on animal life, both in the rivers and on land, where animals coming to lick salt on the roads often become road-kill victims.
* In 2000, after a long campaign by Mexican environmental groups, a joint venture by the Japanese Mitsubishi Corporation and the Mexican government to build the world's largest salt works was dropped. The San Ignacio lagoon, in the heart of one of the world's leading nature reserves, was to be the site of an open-air salt evaporation plant - the second in the area.
The area is a World Heritage Site, and there were fears about the increasing environmental threat to populations of gray whales, sea turtles and prong-horned antelopes.
* In Australia, increasing salinity looks set to cause a major environmental disaster. For most of the continent's history, extensive areas of forest held in balance an eco-system that includes high concentrations of salt. But with the introduction of European farming methods, the balance has been drastically upset in many parts of the country. It has been estimated that 15 billion trees have been ripped out of the continent's ancient forests to make way for farms and urban development since the 18th century. This has caused the water table to rise, soaking up salt from deep in the earth and making the topsoil extremely salty. The trees that protect the soil have not been replaced, and more are still being chopped down. The problem has been exacerbated in south-east Australia, where huge river systems have been exploited for irrigation. The course of the rivers has been diverted through areas where the earth is more salty, thus making the water more salty. Scientists say that large areas of Australia are now drowning in salt.
Salty science facts
* Common salt is sodium chloride, which makes up 3.5 per cent of sea water and is found in rock form, due to the evaporation of ancient seas.
Its two constituents are two highly reactive elements: sodium, a silvery metal which bursts into flames when it makes contact with water; and chlorine, a toxic gas with a very bad smell. Much of the salt in sea water comes from the erosion of rocks on the land, washed into the rivers and out to sea; some is leached from rocks beneath the ocean floor. Shells and scales help protect sea animals from the salinity, but for some Antarctic species salt is a protection against the cold.
* Salt plays a vital role in the water cycle. Foam bubbles continuously form on the ocean surface; when they burst, salty water particles, called aerosols, are ejected into the air and carried upward into the atmosphere, where they attract water vapour which condenses around them in the cold of high altitudes to form clouds. The tiny salt particles are the seeds from which clouds are formed over the sea.
* Rock salt takes the form of halite crystals, and this mineral has been found in meteorites. In 1998, a meteorite fell in Morocco - the second ever to be found carrying halite crystals. Using radioisotope dating, researchers at the University of Manchester and the Natural History Museum in London found that these are the oldest halite crystals ever found in the solar system.
* A "new" species of camel has been discovered in the high Tibetan mountains. These animals have survived in desert conditions by drinking water from salt-water springs. It has been suggested that their tolerance of salt water and ability to withstand harsh environments could be passed on by interbreeding with domesticated camels.