Barium (Ba)

Barium is named from Greek barys, heavy — and it is not a heavy metal. Its density is below titanium's and only about a third of lead's, which makes it one of the lighter metals in the lower half of the table. The name never referred to the element. It referred to baryte, barium sulfate, a mineral that is startlingly dense for something that looks like a dull white or grey rock and was known to miners as heavy spar long before anyone knew what was in it.

The Bologna stone

Around 1602 a cobbler and part-time alchemist named Vincenzo Casciarolo picked up unusual heavy pebbles on Monte Paderno outside Bologna, and — pursuing the usual hope of finding gold — roasted them with charcoal. The stones did not yield gold. Instead, after exposure to sunlight, they glowed faintly in the dark for hours.

The Bologna stone became one of the most discussed objects in early modern natural philosophy. Galileo examined it, and its ability to absorb light and give it back later was argued over for a century by people with no framework for what phosphorescence could be — it seemed to show that light was a substance that could be soaked up and stored. What Casciarolo had made, without knowing it, was barium sulfide with trace impurities: the first synthetic phosphor, and the starting point of a line of research that eventually produced everything from fluorescent tubes to the coating on a radar screen.

Who found the earth and who made the metal

Carl Wilhelm Scheele recognised in 1774 that heavy spar contained an earth distinct from lime, working with pyrolusite; Johan Gottlieb Gahn isolated that earth, baryta, in a purer state. Neither obtained the metal, and neither could have with the techniques available. Humphry Davy produced barium in 1808 by electrolysis of molten baryta with a mercury cathode, part of the extraordinary run in which he also isolated calcium, strontium and magnesium.

The single credit that convention attaches to this element merges two separate achievements thirty-four years apart. Scheele identified a new earth; Davy made the element.

Chemistry could not be wrong

In December 1938 Otto Hahn and Fritz Strassmann, in Berlin, were bombarding uranium with neutrons and trying to identify the products. Everyone in the field expected elements just heavier than uranium, or just lighter. What their radiochemical separations kept indicating was barium, element 56, less than half of uranium's atomic number.

Hahn's difficulty was not analytical but conceptual. Nothing known could split a heavy nucleus roughly in half. He wrote to Lise Meitner, his collaborator of thirty years, who had recently been forced to flee Germany and was in exile in Sweden, saying that as chemists they had to conclude the substances behaved like barium, and asking whether she could suggest anything.

Meitner, walking in the snow near Kungälv with her nephew Otto Frisch over Christmas, worked out the answer using the liquid drop model of the nucleus: a uranium nucleus deformed by an added neutron could pinch in two, and the mass difference would appear as around 200 MeV of energy. Frisch borrowed the word fission from cell biology. Hahn received the 1944 Nobel Prize in Chemistry alone; Meitner's exclusion is among the most criticised omissions in the history of the prize.

Barium was the answer because fission splits a heavy nucleus into two unequal fragments, and one of the most probable fragment masses lands squarely on barium. It remains one of the most abundant fission products in any reactor.

Six hundred and sixty-two kilo-electronvolts

Barium's most useful isotope is not one of the seven that occur naturally. Cesium-137 decays, most of the time, not directly to stable barium-137 but to barium-137m, a metastable state that sheds its excess energy 2.55 minutes later as a single gamma photon of 662 keV.

That photon is the workhorse of gamma spectroscopy. It is intense, it is a single clean line, and its parent's 30-year half-life means a sealed source stays useful for decades with a predictable output. Essentially every gamma detector in the world is calibrated against it, and the peak at 662 keV is the first thing an operator looks for when checking that an instrument is working. The gamma ray universally attributed to cesium-137 is in fact emitted by barium.

The same parent-daughter pair, in a small ion-exchange column, is the standard teaching demonstration of radioactive decay: the short-lived barium is separated, its activity is timed as it falls away, and the column regenerates itself from the cesium left behind.

Mud, and the reason it weighs what it does

Around four-fifths of the barium mined is never converted into anything. Baryte is ground and suspended in drilling fluid, where its job is simply to be dense: the column of mud in a borehole has to exert enough pressure at the bottom to hold back the formation fluids and stop the well blowing out.

Baryte is chosen because it is heavy, cheap, chemically inert, soft enough not to abrade pumps badly, and available in millions of tonnes. World consumption tracks the rig count almost exactly, so barium demand is effectively a function of oil and gas drilling activity, and the major producers are China, India, Morocco and the United States.

The remaining barium goes into much smaller markets. Barium carbonate is used in brick and tile manufacture to lock up soluble sulfates that would otherwise bloom on the surface, and in ceramic glazes. Barium titanate is a ferroelectric with an enormous dielectric constant and is the basis of a great many ceramic capacitors. Barium nitrate provides the only practical green in pyrotechnics, from emitting barium monochloride in the flame. And barium is a component of the copper oxide superconductor that first broke the liquid nitrogen barrier.

Swallowed, because it will not dissolve

A barium meal works because barium sulfate is essentially insoluble. Swallowed as a thick white suspension, it coats the lining of the oesophagus, stomach and intestine, blocks X-rays effectively, and passes out again without a measurable quantity of barium ever entering the bloodstream.

Everything about the procedure's safety rests on that one word, insoluble. Soluble barium salts are acutely toxic, because the barium ion blocks potassium channels in cell membranes. The result is a collapse in blood potassium, muscle weakness and paralysis, and barium carbonate was for many years sold as a rodenticide on exactly that basis. Outbreaks of poisoning have followed contamination of flour with barium salts, and a radiological contrast preparation sold in Brazil in 2003 was found to contain soluble barium carbonate and killed a number of patients who had swallowed it believing it was inert.

The distinction between barium sulfate and every other barium compound is one of the clearest demonstrations in toxicology that the hazard belongs to a substance rather than to an element.

The mirror inside a vacuum tube

Look into an old radio valve or the neck of a cathode ray tube and there is usually a silvery patch on the inside of the glass. That is barium, flash-evaporated after the tube was sealed and pumped.

No vacuum pump gets everything out, and gas continues to seep from the internal metalwork for years. The barium film — a getter — chemically absorbs the residual oxygen, nitrogen, water and carbon dioxide as they appear, and keeps absorbing them for the life of the tube. Its colour is a diagnostic: a bright mirror means the vacuum is intact, and a white, chalky patch means the tube has been let down to air and is dead. Barium is used for the job because it reacts readily with essentially every gas it is likely to meet, which is also why the metal itself is stored away from air.