Sulfur (S)

Sulfur is the only major industrial commodity that essentially nobody produces on purpose. Almost all of the world's supply is removed from oil and natural gas because regulators require cleaner fuel, then sold because a yellow mountain of it has to go somewhere. Sulfur output responds to refinery throughput and emissions law, and not at all to what sulfur is worth.

The mine that heated the ore instead of digging it

It was not always like this. Herman Frasch patented a method in the 1890s that made sulfur mining one of the strangest extractive processes ever operated commercially: three concentric pipes were sunk into the caprock of a Gulf Coast salt dome, superheated water was pumped down the outer one to melt the sulfur underground, compressed air went down the innermost, and molten sulfur came up the middle. Nothing was excavated. The Louisiana and Texas domes supplied most of the world for half a century on that principle, and the last American Frasch operation shut in 2000.

What killed it was not exhaustion of the deposits. It was competition from sulfur that arrived free. Sour natural gas has to be sweetened before it can be put in a pipeline, crude oil has to be hydrodesulfurised before its products can be burned under modern air-quality rules, and both processes concentrate the sulfur into elemental form as a matter of course. Each tightening of a fuel standard — the removal of sulfur from road diesel, and then the International Maritime Organization's 2020 cap on marine fuel — released another wave of by-product onto the market.

The paradox is neat: the cleaner the world's fuel becomes, the more sulfur it produces.

A shortage that would arrive by subtraction

That dependency has an obvious tail risk, argued most prominently in a 2022 analysis led by Mark Maslin at University College London. If fossil fuel consumption declines substantially, the by-product declines with it, while demand for sulfur — which is dominated by fertiliser and is growing in battery and metal-leaching applications — does not. Their projections put the potential shortfall at somewhere between 100 and 320 million tonnes a year by 2040.

The estimate is a scenario rather than a forecast, and it has been challenged on the grounds that sulfide ore roasting and even the old Frasch deposits could be brought back if the price justified it. What is not in dispute is the structural oddity: a critical input whose supply is tied to an industry that policy is trying to shrink.

Liebig's index of prosperity

Around ninety per cent of mined and recovered sulfur is converted to sulfuric acid, which is by a wide margin the most-produced industrial chemical on the planet. Justus von Liebig is supposed to have remarked that a country's wealth could be judged by how much of it that country consumed, and the observation held well enough to be used as an economic indicator for a century.

The single largest destination is not the chemical industry as such. It is phosphate: sulfuric acid is what dissolves phosphate rock into a form plants can absorb, and fertiliser production takes the majority of world output. Metal leaching — copper, nickel, and increasingly lithium and cobalt — takes much of the rest, along with petroleum refining and the acid in every lead-acid battery.

The p and the h that never belonged

The element has been known since antiquity as brimstone, literally the burning stone, and the biblical fire and brimstone is a description of volcanic sulfur igniting.

The modern spelling dispute is less venerable than it looks. Latin had sulpur and sulfur, and the Greek-looking ph was a later affectation applied by scholars who assumed a Greek origin the word never had. IUPAC standardised on sulfur in 1990; the Royal Society of Chemistry followed around 2000, and British scientific publishing has largely fallen in line while everyday British usage has not. This is the rare case where the American spelling is the etymologically defensible one.

Heat it and it thickens

Sulfur is the element conventionally credited with the largest number of allotropes: something over thirty distinct solid forms have been isolated and structurally characterised, most of them rings holding between six and twenty atoms. The record is not perfectly clean. Carbon's tally becomes open-ended the moment fullerenes of different cage sizes and nanotubes of different chiralities are counted as separate forms, and authors differ on whether they should be, so several standard texts hedge sulfur's claim with "except perhaps carbon". Among elements whose allotropes are discrete, countable molecular species, nothing else is close.

What sulfur does on melting is stranger than the count.

At ordinary temperatures the stable form is built from S₈ molecules: puckered eight-membered rings often described as crowns. Melt them and you get a thin, straw-coloured liquid. Keep heating and, somewhere around 159 °C, the liquid abruptly becomes more viscous rather than less — by orders of magnitude — and darkens to a deep red-brown. Almost every other liquid does the opposite.

The rings are opening. Above that temperature the S₈ crowns break and link into long polymeric chains that tangle around one another, and the melt behaves like a polymer rather than a simple liquid. Heat it further still and the chains themselves start breaking, and the liquid thins again before it boils.

Two rubber patents and a dispute that outlived both men

Natural rubber is useless across a real temperature range: sticky in summer, brittle in winter. Charles Goodyear found in 1839 that heating it with sulfur transformed it permanently, apparently after dropping a sulfur-rubber mixture onto a hot stove.

What sulfur does is form short cross-links between the long polyisoprene chains, tying them into a single network. A few per cent of sulfur gives an elastic solid that returns to shape — a tyre. Around thirty per cent gives ebonite, a hard, black, machinable material that was among the first commercially important plastics.

Thomas Hancock in England obtained a British patent in 1844, some weeks before Goodyear's application, having examined samples of Goodyear's material that had reached Britain. Hancock always maintained he had worked out the principle independently; Goodyear always maintained otherwise, litigated widely, and died more than two hundred thousand dollars in debt. It was Hancock's colleague who supplied the name, after Vulcan.

Isotopes that prove the sky had no oxygen

The most consequential thing sulfur isotopes have done is date the oxygenation of the atmosphere.

Ordinarily, isotopes of an element fractionate in proportion to their mass difference, so the enrichment in sulfur-34 relative to sulfur-32 is about twice that in sulfur-33. In rocks older than roughly 2.4 billion years, that relationship breaks: sulfur-33 shows anomalies that mass-dependent chemistry cannot generate. The accepted explanation is ultraviolet photochemistry acting on volcanic sulfur dioxide high in the atmosphere — which can only happen if there is no ozone layer screening those wavelengths, and no ozone layer means no oxygen.

The anomalies disappear from the record at around 2.4 billion years and never return. That disappearance is the sharpest single marker for when free oxygen appeared, and it comes from sulfur rather than from anything oxygen-bearing.

Detectable at parts per trillion

The human nose is extraordinarily sensitive to volatile sulfur compounds. Thiols can be perceived at concentrations of a few parts per trillion, which is a sensitivity comparable to a trained detector instrument and far beyond our response to almost any other chemical class. This is the basis of a skunk's defence and of the smell of a struck match.

It is also a deliberate safety measure. Natural gas is odourless, and after a leak destroyed the New London School in Texas in 1937, killing close to three hundred children and teachers, gas odorisation with sulfur compounds became a legal requirement. The smell of a gas leak is a thiol added in trace quantity precisely because human noses are so good at it.

Life relies on sulfur more quietly. Two amino acids contain it, and the disulfide bridges formed between cysteine residues are the covalent staples that lock proteins into shape — including keratin, which is why hair holds a curl and why burning it smells the way it does.