Roman metallurgy

Many of the first metal artifacts that archaeologists have identified have been tools or weapons, as well as objects used as ornaments such as jewellery. These early metal objects were made of the softer metals; copper, gold, and lead in particular, either as native metals or by thermal extraction from minerals, and softened by minimal heat (Craddock, 1995). While technology did advance to the point of creating surprisingly pure copper, most ancient metals are in fact alloys, the most important being bronze, an alloy of copper and tin. As metallurgical technology developed (hammering, melting, smelting, roasting, cupellation, moulding, smithing, etc.), more metals were intentionally included in the metallurgical repertoire.

By the height of the Roman Empire, metals in use included: silver, zinc, iron, mercury, arsenic, antimony, lead, gold, copper, tin (Healy 1978). As in the Bronze Age, metals were used based on many physical properties: aesthetics, hardness, colour, taste/smell (for cooking wares), timbre (instruments), resistance to corrosion, weight (i.e., density), and other factors. Many alloys were also possible, and were intentionally made in order to change the properties of the metal; e.g. the alloy of predominately tin with lead would harden the soft tin, to create pewter, which would prove its utility as cooking and tableware.

Las Médulas, remains of the most important gold mine in the Roman Empire. The spectacular landscape resulted from the Ruina montium mining technique

Iberia (modern Spain and Portugal) was possibly the Roman province richest in mineral ore, containing deposits of gold, silver, copper, tin, lead, iron, and mercury).[2] From its acquisition after the Second Punic War to the Fall of Rome, Iberia continued to produce a significant amount of Roman metals. [3]

Britannia was also very rich in metals. Gold was mined at Dolaucothi in Wales, copper and tin in Cornwall, and lead in the Pennines, Mendip Hills and Wales. Significant studies have been made on the iron production of Roman Britain; iron use in Europe was intensified by the Romans, and was part of the exchange of ideas between the cultures through Roman occupation.[4] It was the importance placed on iron by the Romans throughout the Empire which completed the shift from the few cultures still using primarily bronze into the Iron Age.

Noricum (modern Austria) was exceedingly rich in gold and iron, Pliny, Strabo, and Ovid all lauded its bountiful deposits. Iron was its main commodity, but alluvial gold was also prospected. By 15 BC, Noricum was officially made a province of the Empire, and the metal trade saw prosperity well into the fifth century AD.[5] Some scholars believe that the art of iron forging was not necessarily created, but well developed in this area and it was the population of Noricum which reminded Romans of the usefulness of iron.[6] For example, of the three forms of iron (wrought iron, steel, and soft), the forms which were exported were of the wrought iron (containing a small percentage of uniformly distributed slag material) and steel (carbonised iron) categories, as pure iron is too soft to function like wrought or steel iron.[7]

Dacia, located in the area of Transylvania, was conquered in 107 AD in order to capture the resources of the region for Rome. The amount of gold that came into Roman possession actually brought down the value of gold. Iron was also of importance to the region. The difference between the mines of Noricum and Dacia was the presence of a slave population as a workforce.[8]

Roman ingots of lead from the mines of Cartagena, Spain, Archaeological Municipal Museum of Cartagena

The earliest metal manipulation was probably hammering (Craddock 1995, 1999), where copper ore was pounded into thin sheets. The ore (if there were large enough pieces of metal separate from mineral) could be beneficiated ('made better') before or after melting, where the prills of metal could be hand-picked from the cooled slag. Melting beneficiated metal also allowed early metallurgists to use moulds and casts to form shapes of molten metal (Craddock 1995). Many of the metallurgical skills developed in the Bronze Age were still in use during Roman times. Melting—the process of using heat to separate slag and metal, smelting—using a reduced oxygen heated environment to separate metal oxides into metal and carbon dioxide, roasting—process of using an oxygen rich environment to isolate sulphur oxide from metal oxide which can then be smelted, casting—pouring liquid metal into a mould to make an object, hammering—using blunt force to make a thin sheet which can be annealed or shaped, and cupellation—separating metal alloys to isolate a specific metal—were all techniques which were well understood (Zwicker 1985, Tylecote 1962, Craddock 1995). However, the Romans provided few new technological advances other than the use of iron and the cupellation and granulation in the separation of gold alloys (Tylecote 1962).

While native gold is common, the ore will sometimes contain small amounts of silver and copper. The Romans utilised a sophisticated system to separate these precious metals. The use of cupellation, a process developed before the rise of Rome, would extract copper from gold and silver, or an alloy called electrum. In order to separate the gold and silver, however, the Romans would granulate the alloy by pouring the liquid, molten metal into cold water, and then smelt the granules with salt, separating the gold from the chemically altered silver chloride (Tylecote 1962). They used a similar method to extract silver from lead.

While Roman production became standardised in many ways, the evidence for distinct unity of furnace types is not strong, alluding to a tendency of the peripheries continuing with their own past furnace technologies. In order to complete some of the more complex metallurgical techniques, there is a bare minimum of necessary components for Roman metallurgy: metallic ore, furnace of unspecified type with a form of oxygen source (assumed by Tylecote to be bellows) and a method of restricting said oxygen (a lid or cover), a source of fuel (charcoal from wood or occasionally peat), moulds and/or hammers and anvils for shaping, the use of crucibles for isolating metals (Zwicker 1985), and likewise cupellation hearths (Tylecote 1962).

Drainage wheel from Rio Tinto mines

There is direct evidence that the Romans mechanised at least part of the extraction processes. They used water power from water wheels for grinding grains and sawing timber or stone, for example. A set of sixteen such overshot wheels is still visible at Barbegal near Arles and dates from the 1st century AD or possibly earlier, the water being supplied by the main aqueduct to Arles. It is likely that the mills supplied flour for Arles and other towns locally. Multiple grain mills also existed on the Janiculum hill in Rome.

Ausonius attests the use of a water mill for sawing stone in his poem Mosella from the 4th century AD. They could easily have adapted the technology to crush ore using tilt hammers, and just such is mentioned by Pliny the Elder in his Naturalis Historia dating to about 75 AD, and there is evidence for the method from Dolaucothi in South Wales. The Roman gold mines developed from c. 75 AD. The methods survived into the medieval period, as described and illustrated by Georgius Agricola in his De re metallica.

They also used reverse overshot water-wheels for draining mines, the parts being prefabricated and numbered for ease of assembly. Multiple set of such wheels have been found in Spain at the Rio Tinto copper mines and a fragment of a wheel at Dolaucothi. An incomplete wheel from Spain is now on public show in the British Museum.

The invention and widespread application of hydraulic mining, namely hushing and ground-sluicing, aided by the ability of the Romans to plan and execute mining operations on a large scale, allowed various base and precious metals to be extracted on a proto-industrial scale only rarely matched until the Industrial Revolution.[9]

The most common fuel by far for smelting and forging operations, as well as heating purposes, was wood and particularly charcoal, which is nearly twice as efficient.[10] In addition, coal was mined in some regions to a fairly large extent: almost all major coalfields in Roman Britain were exploited by the late 2nd century AD, and a lively trade along the English North Sea coast developed, which extended to the continental Rhineland, where bituminous coal was already used for the smelting of iron ore.[11] The annual iron production at Populonia alone accounted for an estimated 2,000[12] to 10,000 tons.[13]

Mural in the House of the Vettii in Pompeii depicting cupids using the tools and techniques of Roman goldsmiths

Romans used many methods to create metal objects. Like Samian ware, moulds were created by making a model of the desired shape (whether through wood, wax, or metal), which would then be pressed into a clay mould. In the case of a metal or wax model, once dry, the ceramic could be heated and the wax or metal melted until it could be poured from the mould (this process utilising wax is called the “lost wax“ technique). By pouring metal into the aperture, exact copies of an object could be cast. This process made the creation of a line of objects quite uniform. This is not to suggest that the creativity of individual artisans did not continue; rather, unique handcrafted pieces were normally the work of small, rural metalworkers on the peripheries of Rome using local techniques (Tylecote 1962).

There is archaeological evidence throughout the Empire demonstrating the large scale excavations, smelting, and trade routes concerning metals. With the Romans came the concept of mass production; this is arguably the most important aspect of Roman influence in the study of metallurgy. Three particular objects produced en masse and seen in the archaeological record throughout the Roman Empire are brooches called fibulae, worn by both men and women (Bayley 2004), coins, and ingots (Hughes 1980). These cast objects can allow archaeologists to trace years of communication, trade, and even historic/stylistic changes throughout the centuries of Roman power.

• Georgius Agricola
• Mining in Roman Britain
• Mining in ancient Rome
• Roman engineering
• Roman technology
• Malvesa

General

Output

• Butcher, Kevin, Matthew Ponting, Jane Evans, Vanessa Pashley, and Christopher Somerfield. The Metallurgy of Roman Silver Coinage: From the Reform of Nero to the Reform of Trajan. Cambridge: Cambridge University Press, 2014.
• Corretti,Benvenuti. "Beginning of iron metallurgy in Tuscany, with special reference to Etruria mineraria." Mediterranean archaeology 14 (2001): 127–45.
• Healy, John F. Mining and metallurgy in the Greek and Roman world. London: Thames and Hudson, 1978.
• Hobbs, Richard. Late Roman Precious Metal Deposits, C. AD 200-700: Changes Over Time and Space. Oxford: Archaeopress, 2006.
• Montagu, Jennifer. Gold, Silver, and Bronze: Metal Sculpture of the Roman Baroque. Princeton: Princeton University Press, 1996.
• Papi, Emanuele., and Michel Bonifay. Supplying Rome and the Empire: The Proceedings of an International Seminar Held At Siena-Certosa Di Pontignano On May 2-4, 2004, On Rome, the Provinces, Production and Distribution. Portsmouth, RI: Journal of Roman Archaeology, 2007.
• Rihll, T. E. Technology and Society In the Ancient Greek and Roman Worlds. Washington, D.C.: American Historical Association, Society for the History of Technology, 2013.
• Schrüfer-Kolb, Irene. Roman Iron Production In Britain: Technological and Socio-Economic Landscape Development Along the Jurassic Ridge. Oxford: Archaeopress, 2004.
• Young, Suzanne M. M. Metals In Antiquity. Oxford, England: Archaeopress, 1999.