IELTS Academic Reading Practice Test 18: Passage 1

Salt is now so cheap and so easy to buy that it is hard to imagine it was once among the most valuable substances on Earth. Yet for most of human history it was precious for two reasons. First, the body needs sodium to function, and while people who ate a great deal of meat could obtain enough from their food, farming communities living largely on cereals needed to add salt to their diet. Second, before the invention of refrigeration, salting, along with drying and smoking, was one of the main ways of preserving meat and fish, since salt draws water out of food and prevents the growth of the bacteria that cause decay.

The earliest methods of obtaining salt relied on the sun. In hot, dry regions, people collected the crusts left behind when shallow lagoons or salt lakes dried out in summer. Where the climate was cooler or wetter, salt water had to be boiled instead. In Iron Age Europe, salt-makers poured seawater or brine into coarse clay containers and heated them over fires until only salt remained. Because the containers usually had to be broken to remove the hardened block of salt, huge quantities of broken pottery built up at production sites. Along the coast of Essex, in eastern England, hundreds of low mounds of reddish earth, known locally as 'red hills', are the remains of this industry.

Salt was also mined. At Hallstatt, in the Austrian Alps, people were digging salt from deep inside a mountain more than 3,000 years ago. Because salt preserves organic material, archaeologists in the ancient tunnels have found objects that rarely survive elsewhere, including wooden tools, leather sacks used to carry the rock to the surface and even scraps of the miners' clothing. The village later gave its name to a whole period of early European culture. At Wieliczka, near Kraków in Poland, miners working from the thirteenth century onwards carved chapels and statues out of the rock salt itself.

Because everyone needed salt and its sources were limited, rulers soon realised that it could be taxed or controlled. In the second century BC, the government of China's Han dynasty took over the production and sale of salt and iron. Its value is even reflected in language: the English word 'salary' comes from the Latin salarium, which is linked to sal, the Latin word for salt, although the popular belief that Roman soldiers were paid in salt has little evidence to support it. In France, a hated salt tax known as the gabelle was abolished during the Revolution, reintroduced under Napoleon and not finally removed until 1945. And in 1930, Mahatma Gandhi set out with a small group of followers on a march of nearly 400 kilometres to the sea to make salt in defiance of a British law that taxed it, an act that drew worldwide attention to the campaign for Indian independence.

Some of the most remarkable technology was developed in Sichuan, in south-west China, where salt water lies far underground. From around the eleventh century, engineers there drilled narrow wells by repeatedly lifting and dropping heavy iron bits suspended from bamboo cables. By 1835 one well had reached a depth of more than 1,000 metres. The brine was raised in long bamboo tubes, and the natural gas that often came up from the same wells was piped to boiling houses, where it was burned to evaporate the water.

Today, only a small proportion of the salt produced worldwide is eaten. Most is used by the chemical industry, and large amounts are spread on roads in winter to prevent ice from forming. Some is mined as rock salt, and some is obtained by pumping water into underground deposits and evaporating the resulting brine in vacuum pans. In warm, sunny coastal regions, however, the oldest method of all remains one of the cheapest.

In a modern solar salt works, seawater is pumped, or allowed to flow at high tide, into the first of a series of large, shallow ponds. Over several months it is moved slowly from one pond to the next while the sun and wind evaporate the water, so that the brine becomes steadily saltier. As it does so, the less soluble minerals separate out first: calcium carbonate and then gypsum settle on the bottom of the ponds. In the later ponds the water often turns pink or even red, a colour produced by microscopic organisms that thrive in extremely salty conditions.

When the brine is close to saturation, it is transferred to crystallising pans, where sodium chloride forms crystals that gradually build up into a layer several centimetres thick. The liquid that remains, known as bittern, is rich in magnesium and potassium, and it is drained off before these minerals can crystallise, since they would give the salt a bitter taste. At the end of the season, the salt is gathered by harvesting machines and washed in brine, rather than fresh water, so that impurities are removed without the crystals dissolving. Finally, it is piled into huge stockpiles, where it is left to drain and dry.