Mercury (Hg)

For most of recorded history mercury was the only metal anyone could pour. That single fact put it at the centre of alchemy, of medicine, of mining economics and eventually of the first international treaty written to eliminate a chemical element from commerce.

The reason it stays liquid comes from the same relativistic effects that colour gold, working in a different direction: mercury's 6s electrons form a tightly bound, contracted pair that the atom is unwilling to share, so metallic bonding between mercury atoms is feeble. Its cohesive energy is the lowest of any metal. This site treats the mechanism at length in why mercury is a liquid. What matters here is what people did with the consequence.

Two properties follow from that weak bonding and shaped four centuries of use. Mercury's surface tension is roughly six times water's, so it beads rather than wets and can be swept up as droplets. And its expansion with temperature is unusually close to linear across a wide range, which is why Daniel Fahrenheit's 1714 switch from alcohol to mercury produced a thermometer that could be trusted between its calibration points rather than only at them.

Almadén: two thousand years, one mine

The cinnabar deposit at Almadén in central Spain is the largest concentration of mercury in the Earth's crust by an enormous margin, and it has been worked since Roman times. Estimates of its cumulative output run to about a third of all the mercury ever produced anywhere. The Romans used the ore ground into vermilion pigment; later operators wanted the metal, and the work was done substantially by convicts and enslaved labourers, with terms at the mine functioning in practice as a death sentence.

Almadén mattered so much because of a process invented in Mexico. In 1554 Bartolomé de Medina demonstrated at Pachuca that silver could be extracted from crushed ore by amalgamation with mercury, spread and worked in open yards — the patio process. It transformed the economics of low-grade silver ore, and it made the silver mountain at Potosí possible. The Spanish crown held mercury as a monopoly precisely because whoever controlled mercury controlled silver. When Almadén could not supply enough, the Huancavelica mine in the Peruvian Andes made up the difference under forced indigenous labour, and acquired the name la mina de la muerte.

Carroting, and why hatters went mad

Felt for hats was made from rabbit and beaver fur, and the fur felted far better if it was first brushed with an orange-coloured solution of mercuric nitrate — a step the trade called carroting. Hatters worked over steaming vats in poorly ventilated workshops and inhaled the vapour for years.

The result was erethism: tremor, drooling, memory loss, extreme social anxiety, irritability and paranoia, a syndrome distinctive enough that "mad as a hatter" was a description of occupational disease before it was a figure of speech. In the United States the practice continued until December 1941, when the industry agreed to abandon mercury under pressure from the Public Health Service — and switched to a hydrogen peroxide process that had been available for years.

Minamata

Between the 1930s and 1968 the Chisso Corporation discharged waste from acetaldehyde production into Minamata Bay in Kyushu. The catalyst was a mercury compound, and the effluent contained methylmercury, which behaves nothing like the liquid metal: it is absorbed efficiently through the gut, crosses the placenta, and concentrates up the food chain into fish and shellfish.

Cats in the fishing villages began convulsing first. Human cases were formally recognised in 1956 — loss of coordination, constriction of the visual field, damage to hearing and speech, and severe congenital injury in children whose mothers showed few symptoms themselves. Chisso and the Japanese government resisted the conclusion for twelve years. Tens of thousands of people have since been certified or compensated, and the litigation has never entirely ended.

The treaty that emerged from that history is named after the town. The Minamata Convention on Mercury was adopted in Kumamoto in 2013 and entered into force in 2017. It bans new primary mercury mining, sets end dates for mercury in batteries, switches, cosmetics and most lamps, and phases down dental amalgam.

The largest source is now a gold rush

Mercury emissions did not stop with the treaty, and the biggest single contributor today is not industry. Artisanal and small-scale gold mining — millions of informal miners across Amazonia, West Africa and Southeast Asia — uses the same amalgamation chemistry the Spanish used at Potosí, and accounts for something like 38% of anthropogenic mercury released to the atmosphere. It is the one sector where use is rising rather than falling, and it is by definition the hardest to regulate.

Meanwhile the industrial uses have shrunk sharply: mercury-cell chlor-alkali plants have closed across Europe, thermometers and sphygmomanometers have gone from clinical use, and compact fluorescent lamps — which contained a few milligrams each and were criticised for it — have themselves been displaced by LEDs.

What the isotopes reveal

Mercury has seven stable isotopes, and their behaviour hides something useful. Chemical and photochemical reactions fractionate mercury's odd-mass isotopes in a way that does not scale with mass, an effect called mass-independent fractionation. It leaves a distinctive signature depending on how a given mercury atom last reacted.

Geochemists use this to trace pollution to its source, distinguishing coal combustion from mining runoff from atmospheric deposition. Palaeontologists use it on much longer timescales: sharp mercury enrichments in sedimentary rock at the end-Permian, end-Triassic and Cretaceous–Palaeogene boundaries are read as fingerprints of massive volcanic provinces venting mercury into the air, and the isotope pattern helps distinguish a volcanic source from a terrestrial one.

The rarest of the seven, mercury-196 at about 0.15%, is the isotope that captures a neutron and becomes gold — the one genuinely workable transmutation route into the precious metal, and a hopelessly uneconomic one.

Hazard by chemical form

The element's danger depends almost entirely on what it is bound to. Liquid mercury is poorly absorbed if swallowed; its vapour, which the metal releases steadily at room temperature, is absorbed efficiently by the lungs and attacks the central nervous system. Inorganic mercury salts damage the kidneys. Organomercury compounds are the extreme case: dimethylmercury is absorbed straight through skin and through several laboratory glove materials, a property established at fatal cost in the 1997 death of the Dartmouth chemist Karen Wetterhahn, whose symptoms appeared months after a brief exposure.