Las Médulas

Mining technique

Pliny the Elder, who was a procurator in the region in 74 AD, described a technique of hydraulic mining that may be based on direct observation at Las Médulas:

What happens is far beyond the work of giants. The mountains are bored with corridors and galleries made by lamplight with a duration that is used to measure the shifts. For months, the miners cannot see the sunlight and many of them die inside the tunnels. This type of mine has been given the name of ruina montium. The cracks made in the entrails of the stone are so dangerous that it would be easier to find purpurine or pearls at the bottom of the sea than make scars in the rock. How dangerous we have made the Earth!^[7]

Rock-cut aqueduct in La Cabrera

Pliny also describes the methods used to wash the ores using smaller streams on riffle tables to enable the heavy gold particles to be collected. Detailed discussion of the methods of underground mining follows, once the alluvial placer deposits had been exhausted and the mother lode sought and discovered. Many such deep mines have been found in the mountains around Las Médulas. Mining would start with the building of aqueducts and tanks above the mineral veins, and a method called hushing used to expose the veins under the overburden.

The remains of such a system have been well studied at Dolaucothi Gold Mines, a smaller-scale site in South Wales. Opencast methods would be pursued by fire-setting, which involved building fires against the rock and quenching with water. The weakened rock could then be attacked mechanically and the debris swept away by waves of water. Only when all opencast work was uneconomical would the vein be pursued by tunnelling and stoping.

The metallurgic "Orellán" town in Las Médulas (fl. 1st–2nd centuries BC)

Pliny also stated that 20,000 Roman pounds (6,560 kg) of gold were extracted each year.^[8] The exploitation, involving 60,000 free workers, brought 5,000,000 Roman pounds (1,640,000 kg) in 250 years.