Tungsten (W)

Tungsten is the metal that refuses to give in to heat. It melts at 3695 K — 3422 °C — which is higher than any other metal, and it boils at 5828 K, some forty degrees short of rhenium's 5869 K — the only element that stays liquid to a higher temperature. Iron is liquid at 1811 K; tungsten at that temperature has not begun to soften. That one property has decided almost everything about where the metal ends up, from the filament in a twentieth-century light bulb to the tip of a drill bit cutting through hardened steel.

The number that decides everything

A melting point is not usually the most interesting thing about an element. For tungsten it is the whole story, because nothing else comes close. The gap between tungsten and the metal in second place, rhenium at 3459 K, is more than two hundred degrees — and rhenium is rare enough to be priced like a precious metal. Among materials anyone can buy by the kilogram, tungsten stands alone at the top.

Two consequences follow. The first is that tungsten barely evaporates: its vapour pressure at high temperature is the lowest of any metal, which is why a lamp filament could glow white-hot for a thousand hours without thinning away. The second is that it hardly moves as it heats. Its coefficient of thermal expansion is close to that of borosilicate glass, which is the unglamorous reason tungsten wire could be sealed through the glass of a lamp or a vacuum tube without cracking the seal on the first switch-on.

It is also very heavy: 19.3 grams per cubic centimetre, within a whisker of gold at 19.28. That coincidence has a long tail of consequences, and not all of them are respectable.

Wolf's froth and heavy stone

The element carries two names because two different groups of people met it in two different ways, and neither name won outright.

German tin miners hit it first, and hated it. Smelting tin ore that contained the mineral now called wolframite gave them a poor yield — the ore was, they said, devouring the tin the way a wolf devours a sheep. The froth on the slag got the name Wolf Rahm, wolf's cream or wolf's froth, and the mineral kept it: wolframite. That is where the chemical symbol W comes from, and it is why German, Spanish and Swedish chemistry still call the element wolfram.

Meanwhile in Sweden a different mineral, unusually dense for a pale stone, was simply called tung sten — heavy stone. When the metal inside it was eventually isolated, English took the Swedish description of the rock and applied it to the element. So the periodic table ends up with an element whose English name means "heavy stone" in Swedish and whose symbol is the first letter of a German insult about wolves.

Who found it, and what they actually found

Attribution here is genuinely split, and the conventional single-name credit on the data card above flattens it.

In 1781 Carl Wilhelm Scheele examined the Swedish heavy stone and showed that it contained a new acidic oxide — tungstic acid. He had identified the element without isolating it. Two years later, in 1783, the Spanish brothers Juan José and Fausto Elhuyar showed that wolframite contained the same acid, reduced it with charcoal, and produced the metal itself. Scheele found the element; the Elhuyars made it exist as a substance you could hold.

The mineral Scheele worked on was later renamed scheelite in his honour, which is a tidy piece of scientific manners: the man who did not get the element got the rock.

Where tungsten actually goes

Almost everyone associates tungsten with light-bulb filaments, and almost nobody uses it that way any more. Incandescent lamps have been legislated and out-competed into a niche across most of the world. The metal did not become less useful; the demand simply moved.

The majority of tungsten produced today is not used as tungsten at all. It is converted to tungsten carbide, a compound of tungsten and carbon that is close to diamond on the hardness scale and holds an edge at temperatures that would anneal high-speed steel. Cemented carbide — tungsten carbide grains bonded in cobalt — is what the cutting tips of milling cutters, lathe tools, mining drill bits and circular-saw teeth are actually made from.

The rest of the demand splits into a handful of jobs that all lean on either the melting point or the density:

• Superalloys for turbine blades and rocket nozzles, where a few per cent of tungsten raises the temperature the whole alloy can survive.
• Non-consumable electrodes for TIG welding, which have to strike an arc without melting.
• X-ray tube anodes and radiotherapy collimators, where tungsten both absorbs the radiation and survives the heat of the electron beam.
• Radiation shielding and aircraft or motorsport ballast, where the job is to fit as much mass as possible into as little volume as possible.
• Kinetic-energy armour-piercing penetrators, where tungsten alloys have replaced depleted uranium in several armies for reasons that are as much political as ballistic.

The density coincidence with gold

Tungsten and gold differ in density by roughly one part in a thousand. No hand, and no simple weighing, can tell them apart.

This has an honest use and a dishonest one. The honest one is jewellery: tungsten carbide rings became popular precisely because they feel like a precious metal, resist scratching almost completely, and cost very little. The dishonest one is counterfeiting. Gold bars with a tungsten core have turned up repeatedly in the bullion trade, because a tungsten slug plated in gold matches both the weight and the volume of the real thing. Assayers deal with it by measuring something tungsten cannot fake — the speed of sound through the bar, or its electrical conductivity, both of which differ sharply between the two metals.

Five isotopes, one of them slowly leaving

Five isotopes of tungsten occur naturally, and the mixture is unusually even: tungsten-184 is the most common at about 31%, but 182, 183 and 186 are all present in substantial amounts, which is why the standard atomic weight sits at 183.84 rather than near a whole number.

The oddity is tungsten-180, which makes up only about a tenth of a per cent. It is not stable. It alpha-decays with a half-life of roughly 1.8 × 10^18 years, a hundred million times longer than the cosmos has so far existed. That is why tungsten is not classed as a radioactive element and why the decay was only observed directly in the twenty-first century. In practical terms every tungsten atom you will ever encounter will outlast everything else in the room.

The heaviest element life bothered with

Tungsten sits at the far end of an unexpected biological record. A handful of microorganisms, mostly archaea living in hot springs and deep-sea vents, build enzymes with tungsten at the active site — tungstoenzymes that catalyse reactions in place of the molybdenum-based enzymes most organisms use. Molybdenum sits directly above tungsten in group 6 and behaves similarly enough to be substituted for. No element heavier than tungsten is known to have a biological role anywhere.

Where it comes from

Tungsten is mined mainly as wolframite, (Fe,Mn)WO₄, and scheelite, CaWO₄. Production is strikingly concentrated: China supplies the large majority of the world's tungsten, with Vietnam, Russia and Bolivia making up much of the remainder. That concentration, combined with the fact that tungsten has no good substitute in cemented carbide, is why the metal appears on critical-minerals lists in both the European Union and the United States.

It is also one of the four so-called 3TG minerals — tin, tantalum, tungsten and gold — subject to conflict-minerals reporting rules, because artisanal wolframite mining has funded armed groups in central Africa.