For millennia, the value of salt derived from its preserving power and its rarity in certain regions. Yet, by the end of the eighteenth century, the rise of chemistry began to dismantle this ancient economic structure. This new field of science, which became an independent field of research in the late 1600s, finally revealed the true nature of salt, confirming that common table salt was merely one component of a larger, complex family of substances.

In 1744, Guillaume François Rouelle, a member of the French Royal Academy of Sciences, provided the enduring definition of a salt: any substance resulting from the reaction of an acid and a base. This theory rested on the observation that nature seeks completion, and like a well-balanced couple, acids and bases make each other more complete. Acids search for a missing electron, while bases try to shed an extra one. In common table salt, the electron donor (base) is sodium, and the electron recipient (acid) is chloride.

This understanding led to the isolation of salt’s constituent elements, forever linking the industry to the most dangerous and fundamental forces of nature.

  • Sodium: In 1807, Sir Humphry Davy, a largely self-taught English chemist, isolated sodium (the seventh most common element on earth) through electrolysis by connecting potash to the poles of a battery. Sodium is an unstable metal that can spontaneously burst into flame. Davy, realizing he had isolated a new element, reportedly began dancing around the room in ecstasy. He also isolated potassium (the base component of potash).
  • Chlorine: Davy then isolated chlorine (a greenish gas) in 1810, naming it after the Greek word for greenish yellow. Chlorine gas is a deadly poison. The element quickly became the basis for liquid bleach, a major substance in the British textile industry. More ominously, chlorine became the foundation for chemical warfare, including the compound known as mustard gas, which caused 800,000 casualties during World War I.
  • Bromine: The purple dye of the Roman Empire, derived from the murex snail secretion, was chemically identical to a blackish-purplish, foul-smelling liquid residue found in salt marsh water. In 1826, Antoine Jérôme Balard, a young pharmacy student, concluded this residue was a new element and named it muride, after the murex. The Académie Française, however, renamed it bromine, a word meaning “stench”.

The knowledge that common salt was merely the coupling of sodium and chloride transformed the industrial landscape, making common salt the least valuable of a specific group of substances. Saltworks, once contaminated by coal smoke and pan scale, rapidly expanded their product line and became increasingly toxic.

Act II: The End of Preservation Necessity

The Industrial Revolution, which accelerated in the seventeenth and eighteenth centuries, quickly developed inventions in other fields that radically changed the demand for massive quantities of preserved salt. The ability to preserve food without layers of salt effectively dethroned the salt miner and eliminated the need for salt as the single most critical provision commodity.

The first major blow was delivered by a Paris cook, Nicolas Appert, who believed that tightly sealing food in a jar and heating it would destroy the substance—which he called “ferment”—that caused food to spoil.

  • Canning: Appert’s book, The Art of Preserving All Kinds of Animal and Vegetable Substances for Several Years, was published in 1809. Only months later, Peter Durand was granted a patent in London for preserving food, listing “tin and other metals” alongside glass and pottery as possible containers. Bryan Donkin, an early British industrialist, immediately recognized the commercial potential of tin and co-founded the first British canning plant, Donkin, Hall, and Gamble. By the 1830s, the British Navy, provisioned by this company, had adopted canned food as a general provision, despite the fact that the can opener had not yet been invented.
  • Market Collapse: The economic impact was swift. A canning plant built in La Turballe in 1830, a sardine fishing town near the Guérande swamp, flourished, leading to the gradual collapse of the local salt fish business. Similar fates soon befell the salted anchovy and herring industries.
  • Freezing: The final blow to mass salt preservation came with Clarence Birdseye’s development of fast freezing in the 1920s.

With these new technologies, salted foods, once absolute necessities, transitioned into mere delicacies. Products like salt cod are now sometimes so lightly salted they must be kept frozen, sacrificing quality for consumer convenience. Even hams are now marketed as “fresh Christmas ham” if frozen in September and thawed and salted right before the holiday, legally circumventing older Swedish laws on cured meat.

Act III: The New Industrial Landscape

The new chemical knowledge meant that salt, once coveted for preservation, became a crucial, but now chemically defined, industrial feedstock.

  • Soda Ash and Caustic Soda: Centuries of confusion existed between soda and potash. Potash (potassium carbonate) was historically made by cooking down wood ash and water. It was used in making glass, soap, and, before baking soda was manufactured, as a rising agent in baking—called “pearl-ash” in early American recipes. The Leblanc process, developed in the late 1700s, synthesized sodium carbonate (soda ash) from common salt. Soda ash became vital for manufacturing caustic soda and other basic chemicals.
  • Industrialization of Salt Centers: Old salt towns transitioned into chemical manufacturing hubs. Syracuse, New York, once a prosperous hub of Onondaga salt production and known as the “American Venice,” was turned into a chemical center to manufacture caustic soda, soda ash, and bicarbonate of soda. This conversion temporarily saved the industry, but nearly destroyed Lake Onondaga with pollution, including some of the deadliest industrial pollutants, polychlorinated biphenyls (PCBs). In 1918, the section of the Erie Canal that ran through Syracuse was closed and eventually covered over, creating Erie Boulevard, as the city struggled to clean up the polluted lake.
  • Cheshire’s Transformation: The Cheshire salt district in England, once notorious for its black coal smoke, also adopted modern methods. By the 1930s, the Stubbses (one of the surviving producers) imported the first magnificent triple-towered Art Deco vacuum evaporator. This machine used the steam from a first tank to heat successive tanks, allowing for up to six or eight tanks to evaporate brine efficiently. These evaporators made modern, white salt with crystals of a uniform size, forcing old open pans to close. The Thompsons, another traditional family, hung on, using lead boiling pans and steam engines, but eventually went out of business in 1986, unable to afford modernization.

Act IV: The New Mythology of Geology

The deep drilling techniques first developed by salt prospectors created the tools that would define the modern world, radically changing the understanding of underground salt deposits.

  • Drilling Technology: The ancient percussion drilling techniques used in Sichuan, China, and later in Kanawha, Virginia (now West Virginia), employed a chisel on a long shaft struck by a hammer. By the early nineteenth century, Americans had refined this method with the invention of the jar—an improved connection between the driving shaft and the drill shaft designed to withstand the constant pounding. However, the Chinese had already used similar elaborate techniques for centuries. The rotary drill, developed in Europe and used in China by 1943, eventually replaced the old percussion method. In China, the Shen Hai well, drilled in 1835 in Zigong, reached 3,300 feet, making it the deepest drilled well in the world at the time.
  • Theories of Origin: Geological debates persisted about the origin of salt. Theories included Descartes’s idea that freshwater evaporates and the hard salt particles remain, with fissures allowing seawater to seep and harden into rock salt. Another hypothesis was that gypsum (a soft mineral) turned into salt, or that salt came from the air or alkali (which turned out to be true, as alkali are bases).
  • Salt Domes and Petroleum: This focus on deep underground structures led directly to the age of oil. Edwin Drake, after studying salt drilling techniques, drilled 69.5 feet in Titusville, Pennsylvania, in 1859, and struck oil, shocking many who believed oil would not be abundant. The most significant event occurred in 1901 when Pattillo Higgins and Anthony Lucas ignored geological advice and drilled a Texas salt dome called Spindletop. Spindletop demonstrated that a single spot could produce enormous quantities of oil in a short period, leading the U.S. to surpass Russia as the largest oil producer. The three most important discoveries in American oil history—Titusville, Spindletop, and the East Texas Field—were all drilled against the advice of geologists.
  • Modern Rock Salt Mining: Today, massive salt domes, such as the one under Avery Island, Louisiana, are mined using modern equipment. This dome, estimated to be 40,000 feet deep, is mined in wide benches with 28-foot ceilings. Cargill, which took over mining in 1997, extracts 2.5 million tons annually. The mine’s equipment—trucks, bulldozers, and scalers (machines that munch away at the walls)—are assembled below ground, as it is not cost-effective to bring old equipment back up. Salt’s sealing ability also led engineers to propose using salt mines for nuclear waste storage, though the risk of incomplete sealing remains a serious concern.

Act V: The Last Days of the Old Guard

The ancient centers of salt production struggled to adapt to the new age where the chemical definition of the rock had surpassed the value of the rock itself.

The Chinese salt industry, famed for its ingenuity, eventually succumbed to modernization. In Sichuan, the traditional derricks (towers of weather-beaten tree trunks, symbolized by the character jing) dotted the landscape of Zigong. By 1960, the last percussion-drilled shaft was completed. The old brine-boiling houses, which used natural gas piped through bamboo, faded. Today, Zigong has largely modernized, using rock salt mining and vacuum evaporators, making uniform white salt crystals. The historic Shen Hai well, drilled in 1835, still operates but is considered a mere relic.

In England, the historical Cheshire salt district, once dominated by the black sky of coal smoke and scarred white with pan scale, is now green English countryside. The growing disaster of the ground collapsing due to salt extraction was euphemistically called subsidence, spawning religious sermons that compared the sinking boiling houses to hell. Though the area struggled to save its last saltworks as a museum, sinkholes continue to appear, covering the land in grass and shrubs.

The salt wars that had fueled empires for millennia had ended, defeated not by conquest, but by chemistry. As William Brownrigg had predicted in the mid-eighteenth century, “Old arts are improved and new ones daily invented,” permanently altering the quest for salt. Common salt, once the most critical commodity, had become a nuisance in the Dead Sea Works, constantly raising the height of evaporation pond bottoms and flooding hotel basements.