Almadén, the mercury mine

The mercury that refined the silver of the Americas

Almadén sits 589 metres up in the Sierra Morena, in the province of Ciudad Real in Castile-La Mancha, over the world's greatest reserves of cinnabar, the red ore of mercury. Its name is the Arabic al-maʻdin, the mine, and in roughly 2,000 years of working it has produced about 250,000 tonnes of mercury, more than anywhere else on Earth and, by one reckoning, a third of all the mercury humanity has used.

The Romans dug cinnabar here for vermilion, the intense orange-red pigment of their paints, dyes and cosmetics. Under Islamic rule the miners of al-Andalus built furnaces that drove the mercury out of the ore as vapour, collected it in clay pots called xabecas, and exported quicksilver across the Mediterranean. The geographer al-Idrisi described the mine and gave its depth as 250 fathoms, and Abd al-Rahman III is said to have set a basin of mercury, probably from Almadén, in a hall of his palace at Madinat al-Zahra, where it threw the sunlight about the room. Alfonso VII took the town in 1151 and gave it to the Knights of Calatrava.

The New World made it indispensable. From the mid-16th century silver and gold in the Americas were extracted by amalgamation with mercury, and most of Almadén's quicksilver went by way of Seville across the Atlantic. The German bankers of the Fugger family ran the mines in the 16th and 17th centuries in return for loans to the Crown, and when they could not find men willing to work poisoned ground, the king sent them convicts. The first 30 forzados, drawn from prisoners waiting in Toledo for the galleys, arrived at the end of February 1566. Enslaved people, mostly from North Africa and bought cheap because their owners did not want them, followed, and by 1613 they outnumbered the convicts two to one. The author Mateo Alemán, investigating as a royal commissioner in 1593, found rations of meat, bread and wine, an infirmary and an apothecary, and also that 24 percent of the convicts sent there since 1566 had died before their release, mostly of mercury poisoning; men who bailed water were whipped if four of them failed to lift 300 buckets without rest. The state took the mines over in 1645 and later built a Royal Prison for Forced Labourers and a miners' hospital.

The mine stopped working in 2002 under the European ban on mercury mining. In 2006 it opened to visitors, who can go down to the first level 50 metres underground, and in 2012 UNESCO listed Almadén with Idrija in Slovenia as the Heritage of Mercury, the two greatest mercury mines in the world.