An insight into ancient mining history:

The Roman furnaces at Cerro de los Almadenes, Segovia, Spain

Aerial shot of the excavations at Cerro de los Almadenes

Cerro de los Almadenes, in the Segovian town of Otero de Herreros is the best representation of Roman mining in the center of the Iberian Peninsula.

Even though the existence of these mining works was known of old, due to the presence of numerous smelting slags, they have not been studied in depth until 2009 and following.

Although there are indications of works prior to Roman presence, and settlements since the Bronze Age, copper deposits exploitation took place between the second half of the first century BC and the first half of the first century AD. Subsequent works have concluded that the bulk of the exploitation took place between 10 BC and 40 AD. The excavations carried out in 2013 have exposed Visigoth furnaces (Fifth and Sixth centuries) which show that the exploitation lasted until this century at least.

The excavations carried out in 2011 revealed the existence of several Roman furnaces between 5 and 7 meters high. These furnaces were of three types: toasting, reduction and reverberation.

Excavated Roman furnaces at the Cerro de los Almadenes site

The process of obtaining copper consisted of:

• Extraction of the raw material, washing and crushing.
• Toasting, to remove toxic sulfides and water. They were simple ovens without forced ventilation. Toasting ovens gave rise to petriform slags with rough texture and abundant vacuoles.
• Reduction or casting, the resulting material was processed in reduction furnaces, with forced ventilation and generating cordiform slags with metallic appearance and smooth texture that took the negative form of the container
• Finally the copper concentrate was separated from the slag in a reverberatory furnace. The final result was ore cakes 30-40 cm in diameter.

Slags occupied a large area in the ovens’s vicinity. It is estimated that they could reach 250,000 m³. These were important more recently when part of them were used in the works of San Rafael to Segovia highway in 1965.

This week we have an auction running for a piece of slag from this Roman field, very representative of the reduction process: cordiform and with a nice metallic ocher color. Not your average mineral specimen but a good piece of ancient mining history.

We hope you like it!.

Roman slag waste on the surface at Cerro de los Almadenes