- Jonathan Edmondson
In the Roman period most metals were obtained not in a natural state directly from mining, but as a result of metallurgical processing of compound mineral deposits (ores). Ores, once mined, were crushed with stone mortars and as much sterile rock as possible was removed by hand-sorting. Manual millstones, or occasionally handle-powered, ‘hourglass’ mills (similar to the grain-mills from Pompeii) were used to grind the ore to a powder, which was often further concentrated by washing. This was carried out using portable sievelike instruments or in a permanent installation (washery), where water was channelled over the ore, forcing the heavier metal-bearing grains to settle in basins while carrying off the lighter dross.
Few complete Roman smelting-furnaces have survived, and so knowledge of metallurgical techniques depends on scattered finds of parts of furnaces and on ancient authors such as Diodorus (3) Siculus, Strabo, and the elder Pliny(1), who describe the main techniques, sometimes conflating different processes. Scientific analyses of metal objects and slag-heaps, as well as experiments conducted in reconstructed furnaces, have revealed much about the nature and efficiency of Roman metallurgy. No major innovations in metallurgical techniques were introduced under the Romans, but slag-heaps show that the scale of smelting at many sites was significantly increased to match the increased scale of mining. The main types of furnace were the bowl-furnace and the shaft-furnace. The latter allowed continuous production, since fresh fuel and ore could be added at the top of the shaft while slag and molten metal were tapped at the base. These became more widespread throughout the empire under Roman rule. In both types charcoal was the main fuel used and the high temperatures required were achieved by channelling air from hand-worked bellows directly over the ore through a clay pipe (tuyère) fitted into the base of the furnace. Certain metals were more in demand than before, especially lead, which had previously been of interest mainly for its silver content. Orichalcum (brass), an alloy of copper and zinc, was probably only discovered in the Roman period. It played an important role in coinage from Augustus onwards, being used for sestertii and dupondii.
The Romans used various techniques to separate out individual metals from ores. Gold was refined by cementation (Diod. Sic. 3. 14; Strabo 3. 2. 8; Plin. HN 33. 84) or by amalgamation with mercury (Plin. HN 33. 99). In the former process ore was heated with salt or sulphureous substances in a clay crucible. Other metals and impurities were converted into chlorides or sulphides and burnt off or absorbed into the crucible, leaving behind the gold. Amalgamation was possible because all other substances float on mercury, while gold combines with it. The resultant amalgam was pressed on a leather hide. The mercury passed through it, leaving behind pure gold. Cupellation was used to refine silver from galena (lead sulphide). The galena was heated in a crucible (cupel) and oxidized by the air forced into the furnace. Some of the resultant lead-oxide (litharge) was absorbed into the crucible, while the rest was removed with cold iron rods. The silver and any gold present were left in globular form at the bottom of the crucible. Pyrites and copper-rich ores, especially chalcopyrite, often contained some silver and gold. Such ores were first smelted to produce an impure ingot, which was alloyed with lead and reheated. Since lead melts at a much lower temperature than copper, by the process of liquation the molten lead carried the silver and gold with it, leaving the copper in the ore. The precious metals were then separated from the lead by cupellation, and then from each other by cementation.
Iron has an extremely high melting-point (1,540 °C: 2,804 °F). Since furnaces could not achieve such temperatures until the 19th cent., wrought, rather than cast, iron was produced in Roman times. Iron ore was reduced in a furnace until it formed a lump (bloom) of iron, slag, and charcoal, which settled at the bottom of the furnace. A smith later hammered these blooms to remove pieces of relatively pure iron, which were then welded into the tools or implements required. The Romans understood the process of carburization, whereby iron, when heated with charcoal, absorbed carbon and gained strength, which could be enhanced if the hot iron was rapidly cooled by being plunged into cold water (i.e. quench-hardening). See mines and mining, Roman. See also under individual metals.
- O. Davies, Roman Mines in Europe (1935).
- R. J. Forbes, Metallurgy in Antiquity (1950),
- revised as Studies in Ancient Technology 8, 9.
- J. Ramin, La Technique minière et métallurgique des anciens (1977).
- J. F. Healy, Mining and Metallurgy in the Greek and Roman World (1978. rev. It. trans. 1992).
- W. A. Oddy (ed.), Aspects of Early Metallurgy (1980).
- R. F. Tylecote, A History of Metallurgy, 2nd edn. (1992).
- R. F. Tylecote, The Early History of Metallurgy in Europe (1987).
- P. T. Craddock, in J. P. Oleson (ed.), The Oxford Handbook of Engineering and Technology in the Classical World (2008), 93–120 (ch. 4).