Salt is so common today that it is easy to forget how valuable it once was.…

Salt is so common today that it is easy to forget how valuable it once was. Chemically, the salt we put on our food is sodium chloride, a compound of the elements sodium and chlorine. Although both elements are dangerous on their own, together they form a substance that is essential to life. The human body needs sodium to carry nerve signals and to control the balance of water in our cells. Without enough of it, muscles cramp and the body cannot function properly.

Salt is obtained in two main ways. The first is evaporation. In warm, sunny places, seawater is let into shallow pools and left to dry in the sun. As the water evaporates, salt crystals remain and are collected by hand or by machine. This method has been used for thousands of years and is still common along the coasts of many countries, including Vietnam. The second method is mining. In some parts of the world, ancient seas dried up millions of years ago, leaving thick layers of rock salt underground. This salt can be dug out like coal or dissolved with water and pumped to the surface.

For most of human history, salt's main importance lay not in flavouring food but in preserving it. Before refrigerators existed, meat and fish would quickly spoil. Salt draws water out of food, and since the bacteria that cause decay need water to grow, salted food could be kept for months. This made it possible for people to survive long winters, for sailors to make long voyages and for armies to travel great distances.

Because it was so useful, salt was also valuable, and in some places it was used almost like money. Roads were built to carry it, taxes were placed on it, and cities grew rich by controlling its trade. In several countries, unpopular taxes on salt even contributed to protests and political change.

Today, only a small proportion of the salt produced in the world is eaten. Much of it is used by the chemical industry to make products such as glass, paper and soap. In countries with cold winters, large amounts are spread on roads, because salt lowers the temperature at which water freezes and so helps to prevent dangerous ice.

Ironically, the substance that was once so precious is now something many of us consume too much of. Health authorities warn that eating too much salt can raise blood pressure and increase the risk of heart disease. The challenge for modern consumers is no longer how to obtain salt, but how to use less of it.