Salt, also referred to as table salt or by its chemical formula NaCl (sodium chloride), is an ionic compound made of sodium and chloride ions. All life depends on its chemical properties to survive. It has been used by humans for thousands of years, from food preservation to seasoning. Salt's ability to preserve food was a founding contributor to the development of civilization. It helped eliminate dependence on seasonal availability of food, and made it possible to transport food over large distances. However, salt was often difficult to obtain, so it was a highly valued trade item, and was considered a form of currency by certain people. Many salt roads, such as the Via Salaria in Italy, had been established by the Bronze Age.
All through history, availability of salt has been pivotal to civilization. In Britain, the suffix "-wich" in a place name sometimes means it was once a source of salt, as in Northwich and Droitwich, although other "-wich" towns are so named from the Saxon 'wic', meaning fortified dwelling or emporium. The Natron Valley was a key region that supported the Egyptian Empire to its north, because it supplied it with a kind of salt that came to be called by its name, natron. Today, salt is almost universally accessible, relatively cheap, and often iodized.
Sources
Salt comes from two main sources: sea water and the sodium chloride mineral halite (also known as rock salt). Rock salt occurs in vast beds of sedimentary evaporite minerals that result from the drying up of enclosed lakes, playas, and seas. Salt beds may be up to thick and underlie broad areas. In the United States and Canada extensive underground beds extend from the Appalachian basin of western New York through parts of Ontario and under much of the Michigan basin. Other deposits are in Texas, Ohio, Kansas, New Mexico, Nova Scotia, and Saskatchewan. In the United Kingdom underground beds are found in Cheshire and around Droitwich. Salzburg, Austria, was named "the city of salt" for its mines. High-quality rock salt was cut in medieval Transylvania, Maramureş and southern Poland (Wieliczka). Tuzla in Bosnia and Herzegovina was named in Hungarian Só (salt) from the twelfth century on and later "place of salt" by Turks.
Salt is extracted from underground beds either by mining or by solution mining using water to dissolve the salt. In solution mining the salt reaches the surface as brine, from which the water is evaporated leaving salt crystals.
History
Ancient world
Early Neolithic salt production, dating to approximately 6,000 BCE, has been identified at an excavation in Poiana Slatinei-Lunca, Romania.
Solnitsata, the earliest known town in Europe, was built around a salt production facility. Located in present-day Bulgaria, the town is thought by archaeologists to have accumulated wealth by supplying salt throughout the Balkans.
Salt was of high value to the Israelites, Greeks, Tamils, Chinese, Hittites and other peoples of antiquity. In the early years of the Roman Republic, with the growth of the city of Rome, roads were built to make transportation of salt to the capital city easier. An example was the Via Salaria (originally a Sabine trail), leading from Rome to the Adriatic Sea. The Adriatic, having a higher salinity due to its shallow depth, had more productive solar ponds than the Tyrrhenian Sea, much closer to Rome. The word "salary" comes from the Latin word for salt. The persistent modern claim that the Roman Legions were sometimes paid in salt is baseless; a salārium may have been an allowance paid to Roman soldiers for the purchase of salt, but even that is not well established.
During the late Roman Empire and throughout the Middle Ages salt was a precious commodity carried along the salt roads into the heartland of the Germanic tribes. Caravans consisting of as many as forty thousand camels traversed four hundred miles of the Sahara bearing salt to inland markets in the Sahel, sometimes trading salt for slaves: Timbuktu was a noted salt and slave market.
Salt in Chinese history was both a driver of technological development and a stable source of revenue for the imperial government.
Cities and wars
Salt has played a prominent role in determining the power and location of the world's great cities. Liverpool rose from just a small English port to become the prime exporting port for the salt dug in the great Cheshire salt mines and thus became the entrepôt for much of the world's salt in the 19th century.