calx

The Latin calx originally meant limestone, chalk, or the powdery residue produced when limestone was heated — what we call quicklime. The word's Indo-European root is shared with Greek khalix, meaning small stone or pebble, and through this root calx is distantly related to the Greek calculus (small stone) and to the English word 'calculus.' Roman builders depended on calx for making mortar: limestone heated in kilns lost its carbon dioxide and became calcium oxide (quicklime), which was then mixed with water and sand to make plaster and mortar. The English word 'chalk' is derived from calx, as is 'calcium' — the element named for the limestone it came from when Humphry Davy isolated it by electrolysis in 1808.

In medieval and early modern alchemy, calx acquired a second technical meaning: the powdery, earth-like residue left when a metal was heated strongly in air. When copper turned green and powdery, when iron rusted, when tin whitened — each produced its calx. Alchemists used the term 'calcination' for the process of reducing a substance to its calx by prolonged heating, and they understood it as a form of purification — the removal of volatile impurities, the reduction of a substance to its simplest earthly form. The calx of gold, notably, was essentially the same as gold itself; gold did not calcine in ordinary fire, which was one reason it was considered the noblest metal.

The calx became central to the great chemical debate of the late eighteenth century. Phlogiston theory explained calx formation as the loss of phlogiston from the metal: when iron calcined to form its rust-colored calx, it released phlogiston into the air. This produced an awkward problem: the calx of metals weighed more than the original metal, not less. If phlogiston had been expelled, why did the result weigh more? Some phlogistonists proposed that phlogiston had negative weight — an uncomfortable solution. Lavoisier's explanation was far simpler: calcination was not the loss of phlogiston but the gain of oxygen from the air. The calx was the metal plus oxygen — a metal oxide. This insight, built on precise weighing experiments, was the key move in Lavoisier's dismantling of phlogiston theory.

The word calx retreated from common chemical usage as systematic nomenclature took hold in the late eighteenth and nineteenth centuries. What had been 'calx of lead' became 'lead oxide'; 'calx of mercury' became 'mercuric oxide'; 'calx of copper' became 'copper oxide.' But the word persisted in derived forms: calcination, as a term for heating a substance to drive off volatile components, remains standard in ceramics, metallurgy, and chemistry. Calcium, derived from the Latin through Davy's naming, is the fifth most abundant element in the Earth's crust and the most abundant metal in the human body. The Roman lime-worker's word for white powder from heated rock became the name of the element in every bone.