Calcium

Calcium was isolated by Humphry Davy in 1808, though its compounds are among the most familiar substances on Earth. The name comes from the Latin calx, meaning lime, the material of limestone, chalk, marble and quicklime. Calcium is a soft grey alkaline-earth metal, far too reactive to occur free, but bound up as carbonate, sulfate and phosphate it forms mountains, shells, coral and the bones and teeth of animals. The fifth most abundant element in the crust, calcium is vital to living things, hardening bone and shell and triggering muscle contraction and the clotting of blood, while its compounds lime and gypsum remain foundations of building and agriculture.

Vast formations of limestone, including the White Cliffs of Dover, are compressed calcium carbonate laid down over millions of years from the microscopic shells of marine organisms, so that entire landscapes are, in effect, fossilised calcium. In the body the element reaches its hardest form in tooth enamel, built from a calcium phosphate mineral called hydroxyapatite, the toughest substance the human body produces. Gypsum, another common calcium mineral, has been carved as alabaster and burned to make plaster of Paris, used for centuries in sculpture, wall finishing and, in modern medicine, for setting broken bones.

The parent substance lime, calcium oxide, is one of the oldest worked materials of human culture: burnt from limestone in kilns, it made mortar and plaster for building, was spread on fields, and served in tanning and in the preparation of pigments. Limestone and marble themselves are calcium carbonate, the stone of temples and statuary. The metallic element is a late discovery with no alchemical or mythological identity of its own, but through lime, chalk and marble it underlies a vast swathe of pre-modern craft and architecture, and through bone and shell it is woven into the very structure of living bodies.

Lime mortar bound the Roman Empire's greatest monuments, including the dome of the Pantheon and the Colosseum, and modern materials scientists have found that Roman concrete, mixing lime with volcanic ash, contains self-healing mineral structures that let cracks reseal themselves over centuries, a durability modern concrete still struggles to match. Lime also appears in scripture as whitewash: the Gospel of Matthew condemns hypocrites as whitewashed tombs, outwardly bright with lime yet full of decay within, a Judaean burial custom of coating tomb exteriors with lime that both marked graves as ritually impure to avoid and let the image carry its moral force.