Pyrite

Iron pyrite (FeS₂), "fool's gold" — a common, brassy-yellow mineral mined and roasted not for its iron but for its sulphur, the raw material of the sulphuric acid that underpinned much of Victorian Britain's chemical industry.

Historical usage

Demand for sulphuric acid ("oil of vitriol") grew rapidly through the 18th and 19th centuries as it became central to bleaching textiles, refining metals and manufacturing fertiliser, initially made by burning sulphur imported from Sicily. As Sicilian sulphur remained expensive under a long-standing monopoly, British chemical manufacturers turned increasingly to roasting pyrite ore instead from the 19th century onward, which had the added advantage of leaving a residue that could itself be processed for iron or copper. Cae Coch, in the hills above Trefriw in north Wales, is the only mine of its kind worked in the country, first recorded in 1607 and reaching its peak in 1918 — wartime demand for sulphuric acid, itself needed for explosives manufacture, meant Cae Coch alone was responsible for over half of all the pyrite mined anywhere in Britain that year.

How it worked

Pyrite was mined by conventional shaft and level methods, then roasted in a furnace to drive off sulphur dioxide gas, which was converted into sulphuric acid; the iron-rich roasted residue ("cinder") could be sold on for ironworking, so a well-run pyrite operation produced two saleable outputs from one ore.

Why it was used

Pyrite offered a cheaper, more reliably available domestic alternative to imported Sicilian sulphur for acid manufacture, at a time when sulphuric acid had become one of the most important bulk chemicals in the industrial economy — a status often used as a rough measure of a nation's industrial output.

Value over time

Pyrite's value tracked the wider sulphuric acid market rather than having any independent price history of its own — demand, and with it British pyrite mining, spiked sharply during major wars when explosives manufacture required vast quantities of acid, as at Cae Coch in 1918. That demand structurally disappeared after 1945, as the petrochemical industry began recovering sulphur as a cheap by-product of oil and gas refining, undercutting mined pyrite for acid manufacture almost everywhere.

Hazards & challenges

Pyrite reacts with air and water to generate sulphuric acid directly, meaning worked-out pyrite deposits and waste tips continue producing acidic, heavy-metal-laden drainage for decades after mining ends — an effect studied in unusual detail at Cae Coch itself, where the mine is internationally significant to geologists precisely because its ongoing pyrite oxidation is driven largely by bacteria rather than simple chemistry, offering a natural laboratory for understanding acid mine drainage generally.

Risks & limitations

Pyrite roasting was tied directly to demand for sulphuric acid and, during wartime, explosives — production at Cae Coch peaked sharply in 1918 and declined once that demand eased, and pyrite mining generally struggled to compete once cheaper sulphur sources, including sulphur recovered from oil and gas refining, became available after the Second World War.

Regional variation

Cae Coch (Conwy, north Wales) stands alone in Britain as a mine worked specifically and solely for pyrite; elsewhere pyrite occurred more often as an incidental mineral within other metal veins rather than a mining target in its own right.

Images

Mines associated with this term

Included only where MineArchive's own research gives a reasonable evidential basis — not every mine where this might plausibly apply.

• Cae Coch Sulphur Mine — Conwy (well documented example)
  Wales's only pyrite mine, responsible for over half of all pyrite mined anywhere in the UK at its 1918 wartime peak.

Sources

1. (secondary) Neglected Bradford Industries: Vitriol (sulphuric acid) making — Bradford and District Local Studies
2. (secondary) The Many Faces of Fool's Gold — American Scientist
3. (secondary) The trouble with pyrite — Deposits Magazine

Record created: 22 August 2026 · Last researched: 22 August 2026