Natural History (Pliny) - Roman Mining

Roman Mining

Pliny provides lucid descriptions of many areas of Roman technology, some of which have been verified by scholarly research and archaeology. Thus, he gives a clear description of gold mining, which includes large-scale use of water to scour alluvial gold deposits. The description probably refers to mining in Northern Spain, especially at Las Médulas, shown at right, and the remains of water tanks and numerous Roman aqueducts has been verified on the ground at this vast site. Fieldwork in the surrounding area has discovered many more Roman mines where similar techniques were used on a large scale. At another location, Montefurado on the river Sil, the river itself was diverted to expose placer deposits in the bed of the river. It is likely that Pliny saw the operations of gold extraction himself, since the sections in Book xxxiii read like an eye witness report. He was a Procurator in Hispania Tarraconensis in the later years of his life, so would have had access to the many mines of the region.

However, similar remains have been found in Britain, especially at Dolaucothi in west Wales, where excavations in the modern village have confirmed the presence of a fort and settlement, as well as a bathhouse nearby. Field work has also established the extensive use of hydraulic mining to prospect for gold by construction of several aqueducts and many water reservoirs and tanks at the minehead, just as Pliny describes. The water supply was used for hushing the deposits, by releasing a full tank, the water wave scouring the ground below. Alternatively, the aqueduct stream could be simply released onto the deposit, the water wearing it down if of a soft and alluvial nature. Hard rock veins could be worked by fire-setting with the water used to scour away the rock debris. The same water supplies were probably used in a controlled way to drive watermills to crush the ore, and to wash the resultant powder for extraction of the gold dust.

His work supplements the De Architectura of Vitruvius, who describes many devices and engines for construction of buildings and aqueducts, as well as dewatering machines such as reverse overshot water-wheels and the use of the Archimedean screw. They were used in deep mining when shafts penetrated the water table, and examples have been found in many Roman mines when re-entered by modern mining attempts. The system found at the Rio Tinto copper mines in Spain comprised a set of 16 such wheels arranged in pairs in a vertical sequence with a total lift of 96 feet. The wheels were worked as treadmills by workers standing on the tops, and lifting would have needed careful co-ordination to remove the water effectively.

Pliny describes methods of underground mining, including the use of fire-setting to attack the gold-bearing rock and so extract the ore. It involved creating a fire against a hard rock working to weaken it sufficiently to be able to remove it effectively, followed by quenching with water or vinegar. The method was fraught with problems, not least of which was the formation of large volumes of toxic gases, so ventilation was essential in the confined galleries. One way of achieving a good flow of air was by means of adits, which would not only drain excess water but also allow air to circulate freely through the mine. Three such adits were driven through barren rock at Dolaucothi direct to the workings. Two remain open to this day, and the method was used widely in later mines in Britain. That it was widespread is attested by Diodorus Siculus describing the gold mines of Ancient Egypt.

In another part of his work, Pliny describes the use of undermining to gain access to the veins, but it probably refers to opencast rather than underground mining, given the dangers to the miners in confined spaces.

Pliny's description of gold mining methods has been confirmed by field work and archaeology, especially the use of water power in sluicing alluvial gold ores, both in Britain at Dolaucothi in South Wales and at Las Médulas and many other mines in northern Spain. His description of construction of the aqueducts needed to prospect for gold-bearing ore by removing overburden and work the alluvial deposits bears the hallmarks of the eyewitness, and he served as Procurator in northern Hispania when the region, in 73 AD, was experiencing a gold rush. The memory must thus have been fresh in his mind when he wrote Book xxxiii. As the mines grew, more water was supplied simply by building new aqueducts along the line of the original, and the remains of such multiple systems are still visible at Dolaucothi and Las Médulas.

Such methods of hydraulic mining were used widely during the gold rushes of California and Australia in the Victorian period. By contrast with aqueducts providing potable water for towns and cities, those used in mining had a higher gradient so as to provide a faster stream to speed operations, and consequently a shorter life. It seems clear that the methods of hydraulic mining such as hushing were a Roman innovation, nothing comparable being known in previous times. No doubt their skills at aqueduct building promoted their less well-known use in large-scale mining, as attested by Pliny.

The research at Dolaucothi has shown how aqueducts could be used not just for prospection, but also for removing waste rock. A large tank would be built at the end of the aqueduct, and once a vein found, it was attacked using fire-setting (building a fire against the rock, then dousing with water) and the precious ore-bearing minerals extracted by hand. The waste or barren rock surrounding the vein was then washed away, again by using the wave of water from a full tank to scour the waste away. Pliny actually recommends a particular size of tank (200 by 200 feet, and 10 feet deep), but those found on the ground at Dolaucothi vary greatly in size and are smaller than he says. The same water supply was then used as a gentle stream to wash the crushed ore, the gold particles being collected in riffle boxes. At least two of the tanks used at the gold mine still hold water, a tribute to their builders nearly 2000 years ago.

Pliny was famously scathing about the search for precious metals and gemstones, as evidenced by this statement: "Gangadia or quartzite is considered the hardest of all things – except for the greed for gold, which is even more stubborn."