Antimony (Sb)

Almost every metal shrinks as it solidifies, which is why a casting pulls away from the fine detail of a mould and why founders design in allowances for it. Antimony does the opposite: it expands slightly on freezing. Add it to lead and the alloy fills a mould completely and holds the corners.

That property built the printing industry. Type metal is lead with roughly ten to twenty per cent antimony and a few per cent tin, and the antimony is there for two reasons at once — it hardens the lead so a piece of type survives thousands of impressions, and its expansion makes the cast letter reproduce the matrix crisply enough to print a legible serif. Every book set in movable type for five centuries depended on a metal that gets bigger when it cools.

Stibium, kohl, and a name nobody can explain

The symbol comes from Latin stibium, which came from Greek stimmi and ultimately from an Egyptian word. What it referred to was not the metal but the mineral stibnite, antimony trisulfide — a soft grey-black solid ground and used as kohl, the eye cosmetic worn in Egypt and across the ancient Near East for several thousand years. Antimony's earliest human use was therefore not structural or medicinal but decorative.

Where the English name comes from is genuinely unresolved. One derivation reads Greek anti plus monos as "not alone", on the grounds that the element is essentially never found uncombined. Another traces it through Arabic al-ithmid, the same kohl mineral. A third, much repeated and almost certainly invented later, claims anti-moine — "anti-monk" — from a story about a monastery poisoning. That tale attaches to Basil Valentine, the supposed fifteenth-century monk credited with the alchemical treatise The Triumphal Chariot of Antimony, who did not exist; the book appeared around 1604 and was probably written by the Saxon salt-works official Johann Thölde. The element has an ancient symbol, an ancient use, and a name of uncertain age attached to a fictional author.

Why flame retardants need two elements

The largest single market for antimony today is fire safety, and antimony trioxide on its own does nothing useful there. It is not a flame retardant; it is a synergist.

The retardancy comes from a halogenated compound, usually brominated. What antimony trioxide does is react with the halogen released in a fire to form antimony trihalides and oxyhalides, which are volatile at flame temperatures and carry the halogen efficiently into the gas phase, where it scavenges the radicals that sustain combustion. The antimony also slows the release so the halogen arrives when the fire needs interrupting rather than boiling off early. Together the pair suppresses ignition at loadings that neither could achieve alone, which is why the two elements appear together in cable insulation, upholstery, electronics housings and textiles.

Over half of world antimony consumption goes into this. It is a use that exists entirely because of a chemical partnership.

Batteries, bottles and bullets

The remainder splits between three markets, all resting on antimony's effect on other materials rather than on anything it does alone.

• Lead alloys. Antimonial lead is far harder and more creep-resistant than pure lead, and it is what the grid plates of a conventional lead-acid battery are cast from. It also stiffens bullets, shot and cable sheathing. Maintenance-free batteries have moved to lead-calcium alloys, which has eroded but not eliminated the demand.
• PET catalysis. Antimony trioxide is the standard polymerisation catalyst for polyethylene terephthalate, so most drink bottles and polyester fibre in the world are made with it. Titanium and germanium systems compete, mainly in Japan and in applications where residual metal is scrutinised.
• Glass and ceramics. Antimony compounds act as fining agents, removing bubbles from a melt, and as opacifiers in enamels — a role documented in glass from the second millennium BC.

Indium antimonide and the three-to-five micron window

Antimony's semiconductor compound, indium antimonide, has the highest electron mobility of any known semiconductor and a very narrow band gap. That gap corresponds to infrared photons in the 3 to 5 micrometre band, which is where hot objects — engine exhausts, rocket plumes, gun barrels — emit most strongly, and where the atmosphere happens to be transparent.

Cooled indium antimonide focal plane arrays are therefore the sensor of choice for high-performance mid-wave thermal imaging: missile seekers, astronomical instruments and industrial inspection. They must be operated near liquid nitrogen temperature, because the same narrow gap that catches the photons also fills with thermally generated carriers at room temperature. This makes an instructive contrast with the uncooled long-wave cameras that look through germanium optics: different band, different physics, different price bracket.

Perpetual pills

Antimony compounds were a mainstay of pre-modern European medicine, prescribed as emetics and purgatives on the theory that violently emptying a patient did them good. Tartar emetic — antimony potassium tartrate — was the standard preparation, and wine left standing overnight in an antimony cup acquired enough of it to work.

The most memorable form was the perpetual pill: a small ball of metallic antimony swallowed to provoke the desired effect, recovered afterwards, washed and kept for reuse. Families passed them down. Whether they did anything beyond mechanical irritation is doubtful, and antimony's enthusiasts and opponents fought about it for two centuries, with the Paris medical faculty banning antimonial remedies in 1566 and reversing itself in 1666. The compounds are genuinely toxic; antimony has been raised repeatedly, and never settled, as a possible contributor to Mozart's final illness.

Antimony has one surviving therapeutic role: pentavalent antimonial drugs remain a first-line treatment for leishmaniasis in much of the world, an unusual survival of a pre-modern materia medica into current practice.

Eighty-one per cent from four countries

Antimony is a byproduct of gold, and of lead and silver, far more often than it is the reason for a mine. Roughly four-fifths of world mine production comes from China, Myanmar, Tajikistan and Russia together, and China holds around 90% of world smelting capacity.

That concentration turned into a policy instrument. China introduced export controls on antimony in September 2024 and banned exports to the United States that December, at a point when the United States had no primary domestic mine at all. Prices rose several-fold over the following year, and projects that had been economically marginal for decades — the Stibnite deposit in Idaho, various Australian prospects — became the subject of urgent attention. Antimony is on the critical minerals list of essentially every industrialised economy, and unlike some entries on those lists, its concentration is real and its substitutes are poor.

Antimony-124, and getting a reactor going

Antimony has two stable isotopes, antimony-121 and antimony-123, and one manufactured isotope with a very specific job. Antimony-124, made by irradiating antimony in a reactor, emits gamma rays energetic enough to knock neutrons out of beryllium nuclei.

An antimony-beryllium assembly is therefore a photoneutron source, and it has a property no other common source has: it can be switched on and off, in effect, by removing the antimony for re-irradiation. Such sources were used to provide the initial neutron population when starting a reactor from cold, where the natural fission rate is too low to give the instruments anything to measure. It is a use that exists only because of the coincidence between one isotope's gamma energy and one element's unusually loosely bound neutron.