Strontium (Sr)

Strontium is the element that gives fireworks their red, and it is essentially alone in doing so. The excited strontium monochloride and monohydroxide species formed in a burning pyrotechnic composition emit in a narrow band around 650 to 680 nanometres — a deep, saturated crimson that the eye reads as unambiguously red rather than orange. Lithium can produce a red, but it is paler, costlier and hygroscopic. So strontium nitrate ends up in almost every red star in a firework, every marine distress flare, every highway fusee and the tracer element in tracer ammunition, and there is no serious competitor for the job.

A lead mine in Argyll

The name comes from Strontian, a small village on the shore of Loch Sunart in Ardnamurchan, on the west coast of Scotland, where lead was mined from the 1720s. Among the gangue minerals brought up was a heavy, pale one that looked like a barium mineral and behaved differently. It became strontianite, strontium carbonate, and the element took its name from the rock, which took its name from the village — which in Gaelic, Sròn an t-Sìthein, means the point or nose of the fairy hill.

No other chemical element is named after a British settlement. It is a peculiar honour for a place of a few hundred people that also happens to hold the first miners' cottages built with state money in Scotland.

Who found it, and which William Cruickshank

The attribution is more tangled than the single credit above suggests. In 1790 Adair Crawford and William Cruickshank concluded, from the way strontianite behaved with acids, that it contained an earth distinct from baryta; Crawford wrote the paper and Cruickshank did much of the analytical work. Thomas Charles Hope at Glasgow examined it independently and in 1793 reported the crimson flame colour and argued clearly for a new earth, proposing the name strontites. Martin Heinrich Klaproth reached similar conclusions in Berlin at about the same time. None of them had the metal: Humphry Davy produced that in 1808 by electrolysing moistened strontium salts with a mercury cathode, in the same run of work that gave him barium, calcium and magnesium.

There is a further wrinkle that trips up references. Two contemporaneous men were called William Cruickshank — the chemist and lecturer at the Royal Military Academy at Woolwich, and a surgeon of the same name. Sources have confused them for two centuries, which is why the chemist's entry is usually disambiguated explicitly.

The colour television that used a hundred thousand tonnes

Through the 1990s the dominant industrial use of strontium had nothing to do with colour. The electron guns in a cathode-ray-tube television accelerate electrons hard enough to generate soft X-rays when they strike the screen, and the glass faceplate has to absorb them. Strontium carbonate in the glass melt does that efficiently without darkening the glass, and every colour television and computer monitor sold carried several hundred grams of it.

Flat panels ended that market in under a decade. Strontium demand did not collapse to nothing, but it moved and it shrank, and the industry that had been built around supplying television glass had to find other buyers.

What it found were two quieter uses:

• Strontium ferrite magnets. Hard ceramic magnets made from strontium hexaferrite are weak compared with rare-earth magnets and cost a tiny fraction as much. They are in loudspeakers, small motors, magnetic separators, door catches and the great majority of magnets by mass produced anywhere.
• Aluminum casting. A few hundred parts per million of strontium added to aluminum–silicon casting alloys changes the silicon from coarse plates into a fine fibrous form, which transforms the ductility of the casting. Almost every aluminum wheel and engine block is modified this way, and the additive is invisible in the finished part.

Enamel remembers where you grew up

Strontium follows calcium into bone and tooth enamel, and the ratio of strontium-87 to strontium-86 in the local geology varies from place to place — old granite terrains are rich in strontium-87 because rubidium has been decaying into it for billions of years, while young volcanic rocks and marine limestones are not. Groundwater and plants inherit that ratio, and so does anyone who eats locally.

Tooth enamel forms in childhood and is not remodelled afterwards, so it locks in the signature of wherever a person was living while their teeth were forming. Bone, which does remodel, records the last years of life. Comparing the two tells archaeologists whether an individual died where they grew up. The technique identified the Amesbury Archer, buried near Stonehenge around 2300 BC, as someone who had spent his childhood in the Alpine region, and it has been used on everything from Viking mass graves to the crew of the Mary Rose.

The teeth that ended atmospheric testing

Strontium's chemical resemblance to calcium is also the reason strontium-90 is dangerous. Produced in high yield by nuclear fission, with a half-life of about 28.8 years, it travels from fallout to grass to milk to children, and once absorbed it deposits in bone rather than passing through.

Between 1959 and 1970 the Baby Tooth Survey in St Louis collected some 320,000 shed baby teeth from children and measured their strontium-90. The results were unambiguous: children born in 1963 carried levels roughly fifty times those of children born before large-scale testing began. The data reached the White House and contributed materially to the Partial Test Ban Treaty of 1963, after which the curve turned over. It is one of the clearest cases in history of a public health measurement changing a treaty.

The isotope has one legitimate application, as a compact heat source. Soviet radioisotope thermoelectric generators powering remote lighthouses and navigation beacons ran on strontium-90, and locating and securing the abandoned units has been a long-running problem in the Arctic.

The clock that may take the second from cesium

Strontium's most likely future is metrological. An optical lattice clock holds tens of thousands of strontium-87 atoms in a standing wave of laser light and probes a transition at 429 terahertz — some fifty thousand times higher in frequency than the cesium microwave transition that currently defines the second. A higher frequency means a finer division of time, and the best strontium clocks now keep time to better than one part in 10^18, good enough to detect the gravitational time dilation from raising the apparatus by a centimetre.

Strontium is among the leading candidates for a redefinition of the second, which would move the world's timekeeping from a microwave transition in a heavy alkali metal to an optical one in an alkaline earth. The main obstacle is not accuracy but agreement, since several optical species are competing and comparisons between distant laboratories are harder than the clocks themselves.

Where it is dug

The commercial mineral is celestine, strontium sulfate, which forms in evaporite and sedimentary settings. Spain has long hosted the largest producing deposits, with China, Mexico and Iran supplying much of the rest. In the sea, strontium sulfate is used as a structural material by acantharians — single-celled marine protozoa that build intricate radiating skeletons from it, and are the only organisms known to do so.