Cadmium (Cd)

Cadmium sulfide is a brilliant, opaque yellow that does not fade, does not react with other pigments, and survives being baked into plastic at 300 °C. Swap some of the sulfur for selenium and the colour marches through orange into a deep, saturated red that nothing else quite matches. Monet, van Gogh, Matisse and Hockney all used cadmium colours, and painters have defended them against every proposed restriction, because the organic alternatives that match the hue do not match the permanence and the inorganic ones that match the permanence do not match the hue. The European Chemicals Agency examined a proposed ban on artists' cadmium paints in 2014 and declined to recommend it, largely on that argument.

Three chemists and an inspection of apothecaries

In 1817 German officials inspecting pharmacies found that zinc carbonate was being substituted for zinc oxide in preparations sold as medicine. The substituted material turned yellow on heating, and treating an acid solution of it with hydrogen sulfide threw down a yellow precipitate that looked, alarmingly, like arsenic sulfide.

Three men investigated more or less at once. Friedrich Stromeyer, professor at Göttingen and the official inspector, showed the yellow substance was the sulfide of an unknown metal and isolated it. Karl Samuel Leberecht Hermann and Johann Christian Heinrich Roloff, working at a zinc works at Schönebeck, reached the same conclusion independently and published in the same year. All three knew each other and corresponded about it.

The credit is genuinely contested. Most references give the element to Stromeyer, on the grounds that he characterised it most completely; the credit carried above names Hermann, which is the minority convention and reflects that he reported an anomaly in the same material first. The honest summary is that cadmium was found three times in one year by people investigating a pharmaceutical fraud, and that no one of them did the whole job.

Cadmus, by way of calamine

The name comes from cadmia, the classical name for calamine, the zinc carbonate ore — and cadmia comes in turn from Cadmus, the Phoenician prince of Greek legend who founded Thebes and was credited by ancient writers with discovering the ore near the city. The element therefore carries a mythological name inherited second-hand from the mineral it hides in, which is fitting, because cadmium sits directly below zinc, follows it through every geological and metallurgical process, and has to be deliberately removed.

Toyama, and a disease named for a cry

The Jinzū River in Toyama Prefecture drains a valley that had been mined for zinc and lead at Kamioka since the eighteenth century. Mining intensified sharply during Japan's industrialisation and again during the war, and cadmium-bearing tailings entered the river and were carried onto the paddy fields that were irrigated from it. Rice concentrates cadmium efficiently.

From the early twentieth century, villagers — overwhelmingly older women who had borne several children — developed a condition of extreme skeletal pain, bones that fractured under trivial loads or even coughing, and progressive kidney failure. It was called itai-itai byō, from the sufferers' cries: "it hurts, it hurts."

Cadmium damages the proximal tubule of the kidney, which impairs the conversion of vitamin D to its active form and dumps calcium and phosphate into the urine. The result is osteomalacia on top of the renal disease. In 1968 the Japanese government formally recognised the cause as cadmium pollution from the Mitsui mine — the first official recognition of a pollution-caused disease in Japan, and the case that established the legal framework applied to the mercury poisonings at Minamata and Niigata.

Cadmium's danger is that it accumulates. Bound to metallothionein in the kidney cortex, its biological half-life in the human body runs to decades, so a small daily intake climbs steadily for a lifetime. Outside occupational settings the largest single source of exposure is tobacco smoke, because the tobacco plant takes cadmium up from soil with unusual efficiency.

Batteries, fasteners, and a long orderly retreat

Through most of the twentieth century, cadmium's biggest customer was the nickel-cadmium battery. NiCd cells tolerate deep discharge, extreme cold and very high current, and they were the rechargeable battery until nickel-metal hydride and then lithium-ion arrived. The European Union's battery legislation from 2006 restricted them in consumer products, and they have largely vanished from households. They persist where nothing else is trusted: aircraft starting batteries, emergency lighting, rail signalling and standby power in cold climates.

The second big use was electroplating. A cadmium coating protects steel sacrificially like zinc, but it is galvanically compatible with aluminum, lubricates threads so that fasteners torque predictably, and does not swell with corrosion product. Aerospace and defence specified it for decades. It is being displaced by zinc-nickel alloys and vapour-deposited aluminum, slowly, because requalifying a fastener on an airframe is a years-long process.

Cadmium's newer applications are compounds rather than metal. Cadmium telluride is the leading thin-film photovoltaic technology and a large share of utility-scale solar in the United States, with the cadmium locked into a stable, insoluble compound and the modules collected for recycling. Cadmium zinc telluride makes gamma-ray detectors that resolve energy well at room temperature — the trade-off against high-purity germanium being resolution for the freedom from cryogenics, which is why CZT is what goes into portable radiation identifiers and into some nuclear medicine cameras.

And cadmium selenide gave the field of colloidal quantum dots its founding material. Particles a few nanometres across emit at a colour set purely by their size, and cadmium selenide was the system in which that was first controlled well enough to be useful. The 2023 Nobel Prize in Chemistry went to Moungi Bawendi, Louis Brus and Alexei Ekimov for the work; the displays that came out of it now mostly use indium phosphide dots instead, specifically to avoid the cadmium.

Cadmium-113 draws a line through neutron physics

Cadmium absorbs slow neutrons voraciously, and almost all of that is down to one isotope. Cadmium-113, about 12% of natural cadmium, has a thermal neutron capture cross-section of roughly 20,000 barns — one of the largest of any nuclide — thanks to a resonance sitting at very low energy.

The absorption switches off sharply above about 0.5 electronvolts. That cliff is so abrupt that it is used as a definition: neutron physicists speak of the cadmium cutoff, and measuring a sample with and without a cadmium cover is the standard way to separate the thermal neutron population from everything faster. The same property made cadmium sheet the control material in Chicago Pile-1, the first reactor, and it remains a component of control rods, usually alloyed with silver and indium.

Cadmium-113 has one further oddity. It is not, strictly, stable — it beta decays with a half-life around 8 × 10^15 years, which is a million times the age of the universe. The nuclide that soaks up neutrons better than almost anything is itself, very slowly, falling apart.