fluere

The mineral fluorite — calcium fluoride — had been used as a flux in metal smelting since at least the sixteenth century. Georgius Agricola described it in 1529, noting that it helped ore flow when heated. The Latin word fluere, to flow, gave the mineral its name, and the mineral eventually gave the element its name. For three hundred years, nobody suspected the mineral contained a new element.

In 1764, Andreas Sigismund Marggraf discovered that treating fluorite with sulfuric acid produced a corrosive vapor — hydrofluoric acid. The acid etched glass on contact, a property so unusual that chemists suspected an unknown element was responsible. But isolating it proved nearly impossible. Humphry Davy tried and was poisoned. The Irish chemists George and Thomas Knox both fell ill from exposure. Henri Moissan finally succeeded in Paris in 1886 using electrolysis at extremely low temperatures, and received the Nobel Prize in 1906, partly for this work. He died two months after the ceremony, his health ruined by years of fluorine exposure.

Fluorine's extreme reactivity is what makes it both dangerous and useful. It forms compounds with almost every element, including the noble gases that resist bonding with everything else. Teflon, the nonstick coating invented accidentally by Roy Plunkett at DuPont in 1938, is polytetrafluoroethylene — carbon chains wrapped in fluorine atoms so tightly that almost nothing sticks to them. Freon, the refrigerant that later punched a hole in the ozone layer, was another fluorine compound.

Sodium fluoride was first added to public water supplies in Grand Rapids, Michigan, in 1945 to prevent tooth decay. The fluoride ion hardens tooth enamel by replacing hydroxide groups in hydroxyapatite with fluorapatite, which resists acid attack. A flux that helped metals flow in Renaissance foundries now protects teeth in municipal water systems — the same element, the same family of compounds, repurposed across five centuries.