Selenium (Se)

Grey selenium is a semiconductor whose electrical resistance collapses when light falls on it. Put it in the dark and it is a poor conductor; shine daylight on it and current flows tens of times more freely. Almost every notable thing selenium has done in the past hundred and fifty years traces back to that single response, and so does its strange career of being extremely important for a while and then being replaced.

Named for the moon because it looked like the earth

In 1817 Jöns Jacob Berzelius and Johan Gottlieb Gahn were investigating a reddish sludge that accumulated in the lead chambers of a sulfuric acid works at Gripsholm in Sweden. Burning it gave off a smell of rotten horseradish, which was the recognised signature of tellurium, and Berzelius initially reported it as such. Closer work showed the substance was a new element that merely behaved like tellurium's twin.

Tellurium had been named from Latin tellus, the earth. Berzelius took Greek selene, the moon, so the two names would stand in the same relation as the bodies. It is one of the few element names chosen explicitly as a comment on another element, and it fixed a pairing that chemistry has honoured ever since — the two sit adjacent in group 16 and turn up in the same ores, the same refinery residues and often the same paragraph.

A submarine cable resistor that would not behave

The photoconductivity was found by accident. Willoughby Smith, an engineer working on transatlantic telegraph cables, needed high-resistance elements for testing insulation and used selenium rods. In 1873 his assistant Joseph May noticed the readings changed depending on where the rods sat in the room — sunlight through a window was altering their resistance.

The discovery escaped the telegraph industry immediately. Alexander Graham Bell built the photophone in 1880, transmitting speech on a beam of light to a selenium receiver, and considered it a greater invention than the telephone. In 1883 Charles Fritts coated selenium with a transparent film of gold and produced a device that generated current from light with no external power at all: the first solid-state solar cell, about one per cent efficient, more than seventy years before silicon made photovoltaics practical.

Selenium then became the light meter. Before batteries were small and cheap, the exposure meter on a camera was a selenium cell driving a moving-coil needle directly, needing no power source of its own. Millions of cameras carried one, and a well-kept example still works, because there is nothing in it to go flat.

The drum inside every office

Selenium's biggest industrial moment was xerography. Chester Carlson's 1938 process needed a photoconductive surface that could hold an electrostatic charge in the dark and lose it wherever light struck; amorphous selenium plated onto an aluminum drum did the job better than anything else available, and it became the heart of the office copier through the 1960s and 1970s. Latent charge patterns attracted toner powder, and toner was fused to paper.

Organic photoconductors and later laser printing displaced selenium drums, for cost and manufacturing reasons rather than performance. The pattern repeats across selenium's history: it gets there first, works well enough to build an industry, and is superseded by something cheaper.

Making glass look like nothing

The largest current use is far less exciting and much more durable. Ordinary soda-lime glass carries a green cast from iron impurities in the sand. Selenium in small quantities produces a pale pink that sits opposite green on the colour wheel and cancels it, giving the neutral "water-white" appearance expected of a window or a bottle. Slightly more selenium, with cobalt, gives bronze architectural glazing; combined with cadmium it gives the deep ruby of traffic signals and art glass.

The other bulk consumer is electrolytic manganese production, where selenium dioxide added to the electrolyte lowers cell voltage. Beyond that, selenium hardens the lead in some batteries, makes free-machining steels and copper alloys cut cleanly, and appears in copper indium gallium selenide thin-film photovoltaics.

The twenty-first amino acid

Selenium is genuinely essential to human life, and it is built into proteins by a mechanism found nowhere else. Selenocysteine is inserted during translation at a UGA codon, which in every other context means stop; a specific stem-loop structure in the messenger RNA tells the ribosome to read straight through it and load selenocysteine instead. Roughly twenty-five human proteins use it, including the glutathione peroxidases that break down peroxides and the deiodinases that activate thyroid hormone.

The margin between too little and too much is narrower for selenium than for almost any other nutrient. Keshan disease, a cardiomyopathy affecting children and young women, was traced to selenium-poor soils in a belt across China and takes its name from Keshan County in Heilongjiang; supplementation largely ended it. At the other end, livestock grazing seleniferous rangeland in the American West develop alkali disease and blind staggers, because plants such as Astragalus concentrate the element from the soil.

The most instructive case is Kesterson Reservoir in California's San Joaquin Valley. Irrigation drainage from seleniferous soils was ponded there through the early 1980s, selenium concentrated up the food chain, and waterbird embryos were found deformed at rates that closed the site. It remains the standard example of an essential trace element becoming an ecological disaster purely through concentration.

Anode slime, and a decay measured against the age of the universe

There is no selenium mine. Effectively all of it is recovered from the anode slimes left when copper is electrolytically refined — the same sludge that yields tellurium, silver and gold — so world supply tracks copper production and is concentrated in the countries that do large-scale electrorefining: Japan, Germany, Belgium, Russia and Chile.

Among selenium's six natural isotopes, selenium-82 is the one physicists care about. It decays by emitting two electrons and two antineutrinos simultaneously, with a half-life near 10^20 years, and the NEMO-3 detector in the Fréjus tunnel used thin foils of it to hunt for the neutrinoless version of that decay. Selenium-79, meanwhile, is a long-lived fission product that turns up in spent-fuel repository assessments for the same reason technetium does: it is mobile in groundwater.