Titanium (Ti)

Titanium is more abundant in Earth's crust than carbon, chlorine or nickel, and roughly a thousand times more abundant than tin. It is also, kilogram for kilogram, many times the price of steel and several times the price of aluminum. Nothing about that gap is geological. Titanium is expensive because separating it from oxygen is genuinely hard, and because nobody has yet found a way to do it continuously.

The reduction problem

The usual way to win a metal from its oxide is to heat the ore with carbon, which takes the oxygen away as carbon dioxide. Try that with titanium and you get titanium carbide, because hot titanium bonds to carbon at least as enthusiastically as it bonds to oxygen. It will also take nitrogen out of the air, and hydrogen out of practically anything, and every one of those impurities makes the metal brittle at concentrations measured in hundreds of parts per million.

The Kroll process, devised by Wilhelm Kroll in Luxembourg and brought to industrial use in the 1940s, works around this by going the long way. The ore is chlorinated to titanium tetrachloride, a volatile liquid that can be purified by fractional distillation to a standard no solid-state process could reach. That liquid is then reduced with molten magnesium inside a sealed steel retort under argon, over a period of days.

What comes out is not an ingot. It is a porous mass called sponge, welded to the retort walls, which has to be broken out mechanically, crushed, sorted, blended for consistency and then melted under vacuum in a consumable-electrode arc furnace — usually twice, occasionally three times, to get the composition uniform. Matthew Hunter had achieved pure titanium in 1910 by a comparable route using sodium, and the fundamental structure has not changed since.

The economics follow directly from that description: a batch process with long cycle times, high energy input, an expensive reductant that must itself be recovered, and a great deal of handling. Continuous alternatives — the FFC Cambridge electrolytic route and the Armstrong process among them — have been demonstrated repeatedly at pilot scale over the last twenty-five years and have not displaced Kroll.

A Cornish clergyman and a Berlin chemist

William Gregor was the vicar of Creed in Cornwall and an amateur mineralogist. In 1791 he examined a black magnetic sand from the valley of the Manaccan, separated out the iron, and found himself with a residual oxide that matched nothing known. He reported it and called the presumed metal manaccanite.

Four years later Martin Heinrich Klaproth in Berlin, working on rutile from Hungary, identified the same new element independently and named it after the Titans of Greek myth — his stated reasoning being that a name from mythology committed him to no claim about the substance's properties, which he could not yet determine. Klaproth subsequently obtained a sample of Gregor's mineral, confirmed the two were identical, and publicly credited Gregor with priority. That is a rarer act than it should be, and it is why the data card names Gregor while the element carries Klaproth's word.

Nine tenths of it becomes white paint

Ask what titanium is for and the answer is almost never metal. Something over ninety per cent of all titanium ore mined is converted to titanium dioxide pigment.

The reason is optical. Rutile titanium dioxide has a refractive index around 2.6 to 2.7, higher than almost any other cheap, stable, colourless solid. Particles ground to roughly half the wavelength of visible light scatter it with exceptional efficiency, so a thin film of paint containing them becomes opaque and brilliantly white. There is nothing else that hides so well for the money.

It is in architectural paint, in paper, in plastics, in sunscreen — where the scattering extends into the ultraviolet — in toothpaste, and in the white filling of biscuits. Its twentieth-century rise displaced lead white, which was the standard opaque white pigment for two thousand years and poisoned the people who made and sanded it. That substitution is one of the larger unremarked public health improvements of the century.

Its status as a food additive has since diverged between jurisdictions. The European Food Safety Authority concluded in 2021 that genotoxicity could not be ruled out, and the EU banned E171 as a food additive in 2022. It remains permitted in the United States, where the FDA has not accepted the same reading of the evidence.

Light on it does chemistry

Titanium dioxide is a semiconductor, and ultraviolet light gives its electrons enough energy to cross the band gap and generate reactive species at the surface. Akira Fujishima and Kenichi Honda demonstrated in 1972 that a titanium dioxide electrode could split water under illumination, which launched an entire field.

The commercial applications are more mundane and quite widespread. Self-cleaning glass carries a titanium dioxide coating that breaks down organic grime under daylight and simultaneously makes the surface hydrophilic, so rain sheets off rather than beading and takes the residue with it. Photocatalytic concrete and paving are marketed on the same principle for breaking down nitrogen oxides in urban air, with field results that are real but considerably more modest than the laboratory numbers.

Bone grows onto it

In 1952 Per-Ingvar Brånemark was studying blood flow in rabbit bone using optical chambers made from titanium, and found at the end of the study that he could not get the chambers out. The bone had bonded directly to the metal.

That observation — he later named it osseointegration — turned into the modern dental implant. The first human patient, Gösta Larsson, received titanium implants in 1965 and kept them for the rest of his life. The mechanism sits in the oxide layer: titanium's surface film is stable, insoluble and provokes essentially no immune response, so osteoblasts treat it as a surface to build on rather than as a foreign body to wall off. Hip stems, spinal cages and skull plates all rely on the same behaviour.

The same inert oxide makes titanium outstandingly resistant to seawater, which is why it appears in desalination heat exchangers, offshore risers and the hulls of the Soviet Alfa-class submarines — boats that were extraordinarily fast and deep-diving, and so expensive that very few were built.

Bought from the adversary through front companies

The Lockheed A-12 and its successor the SR-71 were built almost entirely of titanium alloy, because skin temperatures at Mach 3 would have annealed aluminum. In the early 1960s the largest available supply of suitable titanium ore was in the Soviet Union.

According to Ben Rich, who ran the Skunk Works after Kelly Johnson, the CIA established third-country front companies to buy it, and the aircraft that flew reconnaissance over the Soviet Union were built from Soviet metal. Manufacturing the airframes required new tooling as well — titanium welded and machined badly with the equipment of the day, and the programme had to develop the processes as it went.

An isotope that showed a supernova was lopsided

Titanium-44 has a half-life of about sixty years and is produced in the innermost layers of a core-collapse supernova, close to the boundary between material that escapes and material that falls back onto the remnant. It is therefore a direct tracer of the explosion mechanism.

The NuSTAR space telescope mapped its gamma-ray emission across Cassiopeia A in 2014, a remnant roughly three and a half centuries old. The titanium was not distributed symmetrically. Its clumpy, one-sided arrangement is strong evidence that the explosion itself was asymmetric — that core-collapse supernovae do not blow up as smooth expanding shells, and that the sloshing instabilities seen in simulations are real. Very few observations bear so directly on how a star actually dies.