Rudimentary forms of open-cast mining and metallurgy had been a mainstay of the economy since chalcolithic times. Bronze and copper artefacts were exported to markets as far away as the Danube while silver was shipped in vast quantities to the eastern Mediterranean countries. The arrival of the Romans brought a new technology which enabled an increased output to meet both domestic demand and the requirement of Rome for the payment of tribute.
The alternative to surface extraction was deep-vein mining which , although more difficult and dangerous, promised better yields for the extraction of gold and silver. A series of narrow exploratory shafts were dug as deep as 50 m. until a vein of the minerals was encountered. The shaft would then be widened so that horizontal galleries could be opened and the ore hoisted by pulleys to the surface. The lifting mechanism was controlled by large wheels of up to 5 m. diameter made from holm oak and operated manually.
Aqueducts were built to bring water from dammed rivers to cisterns which then unleashed a powerful flow through the surface mines to loosen the rocks . Machines known as “stamp-mills” and “trip-hammers” were also powered by this flow and used to crush the ore so that the metals could be extracted.
The manual labour was provided almost exclusively by slaves some of whom had originated as prisoners or hostages taken from Lusitanian tribes while others were brought from various provinces of the Roman Empire. Working conditions were abysmal with only leather aprons and hoods being provided for protective clothing while underground tunnels were dimly lit by oil lamps set in niches . Roof falls were common especially when the Roman supervisors deliberately lit fires to loosen the ore. The only implements available were stone hammers and iron picks . No wonder that the average mortality age for slaves was around 25 years as recorded by the few epitaphs found in the cemetery of the workers´ compound.
Much of the mineral extraction took place in the Iberian Pyrite belt which covered a huge geographical area running from Alcaçer do Sal to Seville with principal mining towns at Aljustrel , Castro Verde, Neves Corvo and Mertola. In the north by far the largest mining operation took place at Três Minas located close to Vila Pouca de Aguiar in the Serra da Pedala. This was perhaps the largest gold mine in Iberia with an enormous output that went almost exclusively to enhance the Imperial coffers. The scale of such production can be judged from the calculation of slag heaps of 18 million tons at Três Minas and 3 million at Aljustrel ; astounding figures when one accepts that all was moved by manual labour
All mining in Roman Portugal was controlled by the State with detachments of the army being stationed at all major centres . The movement of processed minerals was made by wagon with cavalry protection from bandits and severe penalties were imposed for theft either in transit or on site. For Aljustrel and some smaller sites, a system of concession was allowed with an exploration fee being paid initially and a share of production (usually 50%) of any ore successfully extracted. This and other regulations were famously recorded on two brass plaques discovered in the slag of Aljustrel . Other written records are few and accurate assessments of the industry are largely guesswork based on the relatively small degree of archeological excavation which has revealed an abundance of small sites where lead, tin, copper, iron and silver were extracted. There is little evidence of metallurgical activity on an industrial scale ; we must guess that this was confined to small localised foundries and blacksmiths which produced the artefacts and equipment in daily use.
The catalyst of Roman innovation and construction vastly expanded the manufacture of building materials , pottery and glass which were fired in kilns throughout the territory but little was intended for export except perhaps for some amphorae used for wine or oil. Similarly, the increasing population brought about a demand for better domestic furnishings and paraphernalia some of which were imported from other provinces of the Empire and paid for by the revenue derived from mining and excess agricultural produce.
In recent times, a number of engineering surveys have been undertaken in the Penamacor region of Central Portugal to evaluate the profit which might be expected by a resumption of mining. This has prompted interest from private prospectors who believe that “There´s gold in them thar hills” but it seems that modern cost/yield ratios reduce feasibility so local tranquillity will hopefully not be disturbed.
Two videos are available on YouTube which give an excellent presentation of ancient mining practise . The first is by Speleo-TV and titled “Mineração Romana em Valongo (Porto)” with sub.titles in English soon to be available while the second , made in 2017, can be found in the Turismo site for Três Minas.