Paul T. Craddock Mining and Metallurgy (OH of Engineering and Technology in Classical World)

Print Publication Date: Dec 2009 Subject: Classical Studies, Greek and Roman Archaeology, Material Culture StudiesOnline Publication Date: Sep 2012 DOI: 10.1093/oxfordhb/9780199734856.013.0005

Mining and MetallurgyPaul T. CraddockThe Oxford Handbook of Engineering and Technology in the Classical World

Oxford Handbooks Online

Abstract and Keywords

The first millennium bc saw great developments in all aspects of metal production. The introduction of coinage in the mid-first millennium bc created anenormous demand for precious metals: gold and, above all, silver. Metal ores in Northern and Western Europe, Balkans, Greece, Cyprus, Anatolia,Syria, Palestine, and Egypt are discussed. The main innovation in mining technology in the first millennium bc was the introduction of iron and steeltools, completely replacing the stone mining hammers and array of bone, antler, and bronze tools that had been used previously. The benefits of oresare also described. The article then addresses smelting and refining technologies. The uses of iron, steel, and brass are presented. The production ofmetals also survived the collapse of the Roman Empire in the west.

Keywords: mining, metallurgy, iron, steel, brass, gold, silver, smelting, refining

Tradition and Innovation

The first millennium B.C. saw great developments in all aspects of metal production, resulting from changing patterns of demand brought about by thespread of iron usage and the introduction of coinage. The tremendous increase in the production of metals across the Old World, particularly in theRoman, Mauryean-Gupta, and Han Empires, is recorded in the heavy metal content of the cores taken through the Greenland ice sheet (Hong et al.1996). The established economic foundations of the Bronze Age cultures of the Mediterranean, based on long-distance trade in copper and even moreon tin, were disrupted forever by the emergence of iron. By the beginning of the period covered by this book, iron had supplanted bronze, extendingmetal usage well beyond what had previously been feasible. Iron above all else was available; most regions had some ore deposits.

The introduction of coinage in the mid-first millennium B.C. created an enormous demand for precious metals: gold and, above all, silver. The greatmines of the Bronze Age had been worked for copper, but in the succeeding millennium they were worked for silver. Rio Tinto, Laurion, and others to theeast through to the mines of northwest India flourished in the first millennium B.C. and were primarily producing silver destined for coinage (Craddock etal. 1989). This new demand for deep-mined ore encouraged developments in mining technology aided by new developments in knowledge.

The first millennium saw the rise of the study of secular knowledge, or philosophy, and its application to the world, with theories to cover the nature andorigins of matter and the material world. This was an obvious area where observations made during mining could help the formulation of theory, andconversely where a real understanding of geological matters could have helped with mine strategy. Although theories on the origin of rocks, and inparticular on the origins of metals, were developed, there is very little mention of phenomena observed during mining. This lack of oversight is reflectedin the mines, where there is very little evidence of an understanding of the nature and extent of the ore bodies (see below).

It is in the developing knowledge of mathematics and all aspects of mechanics to move large quantities of air, water, and other materials in difficultconditions that the application of the new knowledge is most evident. There was probably a synergy between the problems posed by winning ore fromever deeper and more difficult deposits, and the possible solutions offered by the new knowledge. The familiar adage of the Industrial Revolution,“Science owes more to the steam engine than the steam engine owes to science,” could well be expressed in the classical world as “Science owesmore to mining than mining owes to science.”

Sources of Metal Ores

Northern and Western Europe

On the peninsula of Italy, the main source of metals seems to have been in Tuscany, with many mines in the Colline Metallifere (the ancient MassaMetallorum) inland from Piombino (Badii 1931; Davies 1935: 63–75; Cambi 1959; Minto 1955). On Sardinia there are many copper and silver/leaddeposits where, although activity is now mainly associated with the Bronze Age, there was certainly Punic and probably Roman production (Tylecoteet al. 1983).

Rather surprisingly, with the exception of gold, there does not seem to have been much Roman activity in the mineral deposits of the Alps compared tothat in the prehistoric and medieval periods. The Romans exploited copper in southern France (Davies 1935: 76–92). In the Central Massif region ofcentral France there were important gold-workings in the first millennium B.C. (Cauuet 1995, 1999), and there were also tin workings (Penhallurick

Mining and Metallurgy

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1986: 85–94). Further north, it seems likely that the lead/zinc deposits at Stolberg near Aachen in Germany were worked from the beginning of theRoman occupation for zinc ore to make brass (see below). Together with the deposits just over the border in Belgium at La Vieux Montainge, theseprovided the zinc minerals for the major early medieval and medieval dinanderie, or brass industry (Dejonghe et al. 1993). The distant province ofBritannia certainly produced some iron and copper, and large quantities of lead (noted by Pliny, HN 34.164), although the silver content was probablydisappointing. The gold mines at Dolaucothi in central Wales are well known, and recent work has supported their probable Iron Age origins andemphasized their scale and importance in Roman times (Lewis and Jones 1969; Burnham and Burnham 2004).

Iberia was probably the greatest source of mineral wealth through the first millennium B.C., certainly in the western Mediterranean (J. C. Allan 1970;Davies 1935: 94–139; Domerque 1990). Silver was the metal that generated the first outside interest, attested by the Phoenicians in the south from theend of the second millennium B.C., and it continued to be the most important metal produced through the first millennium B.C., but gold, lead, copper,and to a lesser extent tin and mercury were also produced. The silver occurred in the form of both jarosite and argentiferous lead ores, especially inthe south. Gold was mined principally in the northwest (J. C. Allan 1970: 29–33; Lewis and Jones 1970; Domerque and Hérail 1999), and tin (J. C.Allan 1970: 23–29; Penhallurick 1986: 95–104) and mercury were worked in northern and central Iberia. Copper and lead were mined all over thepeninsula (Davies 1935; Domerque 1990).

The silver and copper mines at Rio Tinto were first operated on a substantial scale by the Phoenicians and their successors, the Carthaginians(Blanco and Luzon 1969; Kassianidou et al. 1995). The famous mine of Baebelo described by Pliny (HN 33.97) is almost certainly to be identified withRio Tinto. This was one of the most important mines of the early Roman Empire (J. C. Allan 1970: 3–9; Rothenberg et al. 1989). The undergroundworkings explored so far probably date from the end of the first millennium B.C., but were of considerably earlier origin (Willies 1997).

The Balkans and Greece

Although base metals were produced, the region was more famed overall for precious metals. In the mid-first millennium B.C., silver from the mines atLaurion in Attica and gold and silver from Siphnos and Thasos (Wagner and Weisgerber 1988) in the Aegean were of great importance, but thereafterdeclined. Latterly the Balkans supplied much of the gold used in the Roman and Byzantine Empires. The gold mines of Illyria, at Urbas in Dalmatia,operated until about the second century A.D., and thereafter Dacia, supplying gold from the Aurariae Dacicae, must have been one of the mostadvanced and productive mining regions of late antiquity (Wollmann 1976).

Cyprus and Anatolia

Cyprus, whose very name is derived from “copper,” was one of the great sources of that metal through the Bronze Age and classical antiquity (Muhlyet al. 1982). Comments by Galen (Walsh 1929) suggest it may also have been an early source of zinc minerals for the brass industry. Anatolia wasanother important source of metals (de Jesus 1980; Pernicka et al. 1984). In the first part of the first millennium B.C., there was clearly a major goldstrike in the alluvial deposits of the Pactolus River in the immediate vicinity of Sardis. Geophysical surveys carried out in the early 2000s (as yetunpublished) have revealed major buried trenches in the gravels of the Pactolus just to the north of the city. These have been interpreted as docks, butit seems more likely that they are the old gold workings. By the first century B.C., Strabo described these gold mines as exhausted (13.1.23). While theylasted, they were the source of the fabled Lydian wealth and the gold for the new innovation of coinage (Ramage and Craddock 2000; Craddock et al.2005).

Copper is relatively abundant at many places, with evidence of major early mining along the Black Sea hinterland from Küre to Rize, and also at Erganiin the southeast (Wagner and Öztunali 2000). Silver/lead/zinc ores are also found in the Black Sea region as well as further west, as is exemplified bythe Bayla Madan mine (see below).

Syria and Palestine

The principal mines on both sides of the Wadi Arabah exploited copper. These are the mines in the Wadi Feinan, now in Jordan, which include the well-preserved Roman mining system at Umm el Amad (Hauptmann and Weisgerber 1992), and further south on the other side, the mines in Beer Ora andthe Wadi Amran (Rothenberg 1972: 208–23; Willies 1991). The deposits are somewhat unusual in that they are either of malachite (copper carbonatehydroxide) or of chrysocolla (hydrated copper silicate), rather than the more common sulphidic deposits.

Egypt

The mines of the eastern deserts of Egypt were a major source of gold from Pharonic times through classical antiquity, as is attested by the writings ofAgatharchides (Burstein 1989) and their physical remains (Klemm and Klemm 1989).

Mining Technology

The first millennium B.C. saw developments in mining technology from Spain to China that were radical in both scale and concept. Mining hadpreviously been constrained by problems of ventilation and drainage, which meant that a mine could not penetrate far below the water table and agallery could not extend far from the shaft into the deposit. Thus a major Bronze Age mine such as Timna, in the Negev Desert of southern Israel, wasno more than a large collection of very small mines, with thousands of shafts often only a few meters apart (Conrad and Rothenberg 1980). In the firstmillennium B.C., much of this changed. At just the time when mathematics and mechanics were beginning to receive serious study through the Greekworld, major mines were being systematically laid out for the first time (Craddock 1995: 69–92; Healey 1978). Deep shafts were now sunk thatpenetrated far below the water table, with galleries stretching out for many hundreds of meters. The sciences of hydrostatics, pneumatics, andmechanics were being applied, if only empirically. (For a collection of relevant Greek and Latin literary sources, see Humphrey et al. 1998: 173–92.)








Where the deposits lay in mountainous country, there was always the possibility of driving a passage (known in English as an adit) into the hillside upfrom the valley bottom to link with the workings inside. This would drain the workings above the adit. Rio Tinto provides good examples of a variety ofdrainage systems (Palmer 1926–1927; Salkield 1987: 10, 40; Willies 1997; Guerrero Quintero 2006). An adit was driven for over 3 km through barrenrock just to meet up with and drain the mine workings. When the mines were reopened in the nineteenth century, this adit was cleared and for the nextcentury continued to drain the modern workings.

Where the workings were below the level of the adit, the water had to be raised up to it. In antiquity this could be achieved in several different ways(Oleson 1984). A series of small reservoirs could be built up an incline and water bailed from a lower to the next higher reservoir and so on until thedrainage adit was reached. Such systems were still in use in Japan in the nineteenth century (Gowland 1899). Pliny (HN 33.97) seems to describesomething similar: “The Baebelo mine … now dug 1,500 paces (2,200 m) into the mountain. Along this distance watermen are positioned day andnight, pumping out the water in shifts measured by lamps, and making a stream.” There is probably some conflation here. It is more likely that thewatermen bailed the water up to a long adit, which, if it was anything like the existing adit at Rio Tinto, sloped down, allowing the water to flow away outof the mine.

A variety of mechanical devices for the raising of water seem to have been developed in the Hellenistic world from about the third century B.C. (Oleson1984, 2000; Wilson 2002; see chapter 11). Water-wheels were another method of raising water used in the mines of the south of Spain and elsewhere(Palmer 1926–1927; Weisgerber 1979). The series of wheels at Rio Tinto raised water through about 30 m. They were operated by slaves treading therim, and it is estimated that one person could raise about 80 liters of water per minute through about 4 m. Another method of raising water at thisperiod employed the Archimedean screw, examples of which have been found in ancient mines in France and Spain. A force pump with moveablenozzle was found in the Roman mine at Sotiel Coronoda (figure 13.5).

The movement of air through the mine workings was probably achieved mainly by the use of carefully placed fires at the bottom of shafts inconjunction with shuttering and doors within the mines. Air could be drawn up one shaft by the fire at its base, pulling air through the workings drawn infrom another shaft. This result could be achieved with a single shaft with a partition down the middle, sucking the air in one side, through the workings,and up the other side (Healy 1978: 83). Pliny (HN 31.49) also describes the use of linen sheets, shaken in the manner of an Indian punka.

The usual method of breaking up hard rock since the inception of mining had been firesetting, and this continued long after the introduction ofgunpowder for blasting in seventeenth-century Europe (Craddock 1992, 1995: 33–37; Willies 1994; Weisgerber and Willies 2000). The traditionalperception has been that it was necessary to extinguish the fire suddenly with water, but more recent experimental work suggested that this wasunnecessary and often impracticable. However, some classical sources, including Pliny (HN 23.57, 33.71), not only describe dousing but also suggestit should be done with vinegar. This seems inherently unlikely, but experiments on limestone showed it worked remarkably well (Shepherd 1992). Pliny(HN 33.72–73) also describes a method of exposing mineral deposits by undermining them with tunnels allowed intentionally to collapse (arrugiae).

The main innovation in mining technology in the first millennium B.C. was the introduction of iron and steel tools, completely replacing the stone mininghammers and array of bone, antler, and bronze tools that had been used previously. By the end of the first millennium, the complete premodern kit ofmining tools was in use: picks, hammers, wedges, chisels, hoes, and rakes (figure 4.1). Metallographic examination of a selection of Roman tools fromRio Tinto has shown that many are of heat-treated steel, with properties comparable to their modern counterparts (Maddin et al. 1996). Pliny (HN33.72) described the use of machines with hammerheads of iron weighing 150 Roman pounds (49 kg) for breaking the silices (quartz, not flint, in theseinstances). He could be referring to the iron-shod stamps for crushing the ore (see below), but the passage implies that they were used undergroundagainst the rock face, so possibly large battering rams are meant, such as have been found in the Indian mines of the same period. The minedmaterial was removed in small baskets or trays. The broad straight galleries in some of the larger mines would be suitable for wheeled transport, andat Três Miñas parallel grooves in the floor seem to indicate some form of controlled trackway (Wahl 1998: fig. 3). The wheelbarrow was unknown to theRoman world.

In addition to the deep mines, open cast pits were also worked, especially for gold. Good examples survive in Spain (Lewis and Jones 1970;Domerque and Hérail 1999; Bird 2004). The overburden was washed from these workings by the sudden release of millions of liters of water stored inreservoirs above the workings, known in English as hushing. Pliny (HN 33.74–76) provides a dramatic description of the Spanish operations. Thismethod of mining is apparently a Roman innovation, relying on their practiced ability to collect, store, and control the release of huge volumes of water.







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Figure 4.1. Selection of Roman iron and steel tools from Rio Tinto mine, including wedges or gads (bottom left), picks (bottom right) and hoes (top).Huelva Museum. (Photograph by P. Craddock.)

In contrast to the application of the mathematical and engineering sciences, there seems to have been but little appreciation of the geology of thedeposits. From the post-medieval period on, miners attempted to define the extent of a deposit, then to sink a shaft or to drive an adit beneath it. Theythen excavated upward (known as overhand stoping), bringing down the material to the base of the shaft or adit from where it could be convenientlyraised to surface. In antiquity an ore body, once located, was dug out directly (known as underhand stoping), the ore and waste being brought tosurface through the worked-out deposit. Very often deposits are faulted, which means that the deposit has slipped and continues some distance away.Sometimes there clearly was an understanding that the vein was likely to continue somewhere in the vicinity and efforts were made to try and relocateit. At the Roman mines in the Wadi Amran (Israel), a series of exploratory holes each several meters in length had been cut in several directions at theend of gallery where the deposit terminated at a fault (Willies 1991).

Mine Organization

The organization and administration of mining in classical antiquity have been much discussed but are still imperfectly understood (Davies 1935: 1–16; Healy 1978: 103–32; Hopper 1979: 164–89). The evidence is derived from a large number of disparate sources, chance references, andinferences that are now difficult to interpret and from which it is difficult to obtain an overall coherent picture, if such a concept is even realistic.Contemporary accounts that do survive often seem to be in serious error, or seriously prejudiced. Hagiographers, for example, recorded theapparently appalling working conditions suffered within the mines by the Christian martyrs. Few direct records of real legislation and administrationexist, the famous bronze tablets from a mine of the second century A.D. at Aljustrel in Portugal being an important exception (Elkington 2001), fromwhich a very different picture of a more organized and workable management regime emerges.

In most societies, the ruling authority lays claim to all valuable mineral deposits in the ground, and in classical antiquity the state took an active interestin getting the maximum yield, especially from the deposits of precious metals. It seems that the state sometimes operated the mines directly, at least intheir developmental stages in newly won territories, as exemplified by the apparent military involvement in both the lead and gold mines in Britain (cf.Tacitus, An. 11.20; Aristotle, Ath. pol. 47.2).

From some surviving descriptions and contracts, particularly from Greece and Spain, it appears that the mines were often leased to private concerns,although they were still answerable to the state mining department. At Laurion, for example, several different concessions operated within one mine(Crosby 1941, 1950). This system could explain why the mining operations seem to have been confined to digging in the ore. There would be littleincentive to develop the infrastructure of the mine if a concession of only a few years was in operation. It is clear, however, from the large-scaledrainage works in the mines at Rio Tinto and elsewhere that long-term development work away from the immediate ore body was carried out, and itdoes seem that state involvement went far beyond just collecting the dues, ensuring that the deposits were worked to the best overall advantage. Thesecond-century A.D. Aljustrel tablets from Spain are important because they list all sorts of detailed regulations regarding the maintenance of drainagesystems, support timbers, and ore dumps (Mommsen et al. 1909: 293–95, no. 113). This was not done out of any altruistic concern for the welfare ofthe miners, but pragmatically to ensure maximum efficiency. In the major silver and gold mines, the state monitored the operations quite carefully andprobably sometimes ran the mines directly. Pliny, who had been a fiscal procurator in Hispania Tarraconensis, wrote what are clearly detailedfirsthand descriptions of the gold mining operations, and his comprehensive and technically accurate descriptions of the silver smelting processesshow that state officials could be fully conversant with the processes involved.

Much of the labor force seems to have been made up of slaves, and criminals could be sentenced to the mines as a form of essentially capitalpunishment (Xenophon, Vect. 4.14–18; Plutarch, Nic. 1.1–2; Diodorus 3.12–3.14.4). As a result, there are very few representations of miners (figure 2.12;cf. figure 2.1). In remote areas, the local population could be conscripted to work in the mines on a corvée system. As well as this enforced labor, therewere clearly also specialist technical staff who would be necessary to operate the mine and who, even if not free, were living a more tolerable life thanis popularly perceived (Mrozek 1989).









Beneficiation of Ores

This is an important stage in the production of metal, as Merkel (1985, 1990) and Maréchal 1985 have stressed. The ores of most metals needed tobe reduced to between the size of a pea and a walnut for smelting and would usually have been broken up manually with hammers. Trip hammerswere possibly in use by the Roman period, as the distinctive stone anvils (figure 4.2) survive at a number of mines in Iberia and elsewhere (Wahl 1993,1998; Sánchez-Palencia 1989), and they may have been water-powered (M. J. T. Lewis 1997, and below). Gold ores needed to be finely ground torelease the tiny particles of metal, and for this grinding mills were used, based on the mills used for grinding food grains but with harder grinding stones(Diodorus 3.13.2–14.4; Wilson 2002). For other metals, where the ore had to be smelted, fine powders would probably have been mixed with animaldung to make discrete lumps to charge into the furnace. This was the practice in the Middle East and India until the nineteenth century (Craddock 1995:152, 161).

Figure 4.2. Anvil of an ore-crushing stamp battery from the Roman gold mines at Três Miñas. Deutsches Bergbau Museum, Bochum. (Photographby P. Craddock.)

Figure 4.3. Roman washing floor at Rio Tinto. (Photograph by P. Craddock.)

The crushed ores could be concentrated by washing and allowing the lighter dross to wash away. This process could be effected by shoveling the oreagainst a controlled flow of water, but more elaborate arrangements are also known, most famously the washeries from the mines at Laurion in Attica(Conophagos 1980; Jones 1984; Photos-Jones and Ellis Jones 1994). These are very sophisticated and well-preserved systems, but their true functionis still not fully understood, and they are unique to Laurion. At Rio Tinto, more simple washing floors for concentrating the jarosite silver ore prior tosmelting have been excavated (figure 4.3); in these, water was introduced from the left, washing dross down the large drain on the right.

Smelting

In contrast to mining, the smelting methods that had evolved in the eastern Mediterranean and Middle East during the third and second millennia B.C.remained largely unchanged in the classical cultures (for a collection of Greek and Latin literary sources, see Humphrey et al. 1998: 205–33). Thetypical smelting furnace through antiquity was a cylindrical shaft of clay and stone (Craddock 1995: 169–74). The dimensions were quite small,typically about 30 to 60 cm internal diameter, and 1–2 m tall. Taller and presumably broader furnaces are recorded by Strabo (3.2.8) for the smeltingof silver ores. Based on Strabo and the evidence from Laurion, Conophagos believed some of the furnaces could have been between three and fourmeters in height (see below).

Early furnaces were very impermanent structures; archaeologists usually have to reconstruct their form from little more than their bases. Furnaces arerepresented on some Greek vases depicting scenes of metalworking, but there are problems with interpretation. The furnaces usually appear to havea pot set on the top, sometimes complete with lid (Oddy and Swaddling 1985). In this type of furnace, however, the objective is to ensure anunimpeded flow of air through the chimneylike system to remove spent gases up the shaft, and a pot set on top would prevent this (figures 4.4, 16.2).Possibly these representations all illustrate furnaces used specifically for annealing purposes, but a simple hearth would seem better suited for thatpurpose. The illustrations remind us that there is still much to learn about early metallurgical processes.








Figure 4.4. Shaft furnace on a Greek oenochoe, 525 B.C. BM inv. B507. (From Oddy and Swaddling 1985; by permission of Swaddling.)

One of the few real improvements to take place in smelting technology was the introduction of more efficient bellows. Early bellows, in use from aboutthe third millennium B.C., were either bag-bellows or pot-bellows that could deliver no more than about 200 liters a minute of air (Tylecote 1981; Merkel1990). The well known carving of the workshop of Hephaestus on the north frieze of the Siphnian Treasury at Delphi depicts bag-bellows (Lullies andHirmer 1960: fig 48). In the first millennium B.C., the familiar hinged concertina bellows were introduced into the Mediterranean area (Weisgerber andRoden 1985, 1986), but they seem not to have spread beyond Europe. Although still manually operated, they were capable of delivering about 500liters of air/minute (Merkel 1990). The inability to deliver air at sufficient pressure using manually operated bellows was the major constraint on furnacesize. Only in the medieval period did powerful water-driven bellows allow much larger furnaces to evolve in Europe, initially in large part for the smeltingof iron.

There are no detailed contemporary descriptions of the smelting processes, but replication experiments, such as those carried out by Merkel (1990),coupled with scientific examination of the surviving smelting debris, have gone a long way toward reconstructing the operations. First, the furnace wasbrought up to a temperature of about 1,000ºC by burning charcoal alone for about an hour, after which roasted ore and more charcoal were added. Inthe smelting process, the metal minerals in the ore were reduced to droplets of molten metal, and the waste minerals formed a molten slag. Much ofthe success of the operation depended on the slag (Bachmann 1982). Even the richest ore after beneficiation would still be made up of about 60 to 80percent of waste material, typically silica and/or iron oxides. By themselves, these minerals have very high melting temperatures and would rapidlychoke the furnace. Silica and iron oxides, however, react to form iron silicates, which have a much lower melting temperature, enabling them to be runout of the furnace as a liquid, known in English as tap slag. Occasionally, the ore had just the right proportions of silica and iron minerals to remove allof the waste material without intervention, but usually there was an imbalance. If there was too much silica, then a little iron oxide would be added;conversely if there was too much iron then a little silica would be added (known as the flux) to create a free-flowing slag. The slag was periodicallydrained from furnace, and the process continued with the addition of more ore and fuel. The metal would drain through the slag to form an irregularmass at the base of the furnace. Typically a smelt lasted for about six to ten hours, allowing some tens of kilograms of metal to accumulate in the baseof the furnace. This activity would usually take place at night, both to take advantage of the cooler conditions and to enable those in charge of the smeltto be better able to judge the color of the flames at the top of the furnace, which was the best indicator of the conditions within.

The smelting of silver was somewhat more complicated, because by the first millennium B.C. even the richest remaining silver ores in theMediterranean region contained no more than traces of silver, which would certainly have been lost in the conventional process outlined above. Muchsilver was obtained from the traces contained in lead ores (Bachmann 1991; Conophagos 1980, 1982). The lead was smelted in the usual manner,and the silver went into the lead. The argentiferous lead was then heated to about 1,000ºC and exposed to a blast of air, causing it to oxidize to leadoxide, in the process known as cupellation (Conophagos 1980, 1989; Craddock 1995: 221–31). The molten silver, however, did not oxidize, but floatedon the molten lead oxide “like oil on water,” according to Pliny (HN 33.95, 34.159). After cooling, the litharge was broken and a lump of silver released.

Ancient silver regularly contains from several hundred ppm to a few percent of gold, leading Meyers (2004) to suggest that the usual sources exploitedin antiquity were oxidized ores such as cerussite (hydrated lead carbonate) or jarosite (a mixed sulphatic ore), both ores that have enhanced goldcontents. It has long been debated whether the main ore smelted at Laurion was cerussite or galena. Photos-Jones and Jones (1994) stated the orefound in the washeries was cerussite, but Rehren et al. (2002) claim that it had been galena originally. With ores such as the jarosites from Rio Tinto,which contained little or no lead, it was necessary to add lead to the smelting charge (Craddock et al. 1985; Craddock 1995: 216–21; Dutrizac et al.1983).

Gold usually occurs as metal, so it could be separated by finely crushing the rock in which the tiny particles were embedded and then washing theresult to separate the heavier metal. There is some evidence that the Romans were smelting auriferous iron pyrites, in which the gold is very finelydistributed almost at the molecular level. Silica was added to the roasted iron pyrite to form a slag allowing the gold to separate and coalesce. Plinymentions gold smelting (HN 33.69, possibly 33.79), and slag heaps from the process have been found at the Roman mines at Três Miñas in northernPortugal (Bachmann 1993).

Metallurgy and the Environment

A new area of archaeometallurgical research concerns the effects of mining and smelting on the environment and on the health of those involved in thework. The fuel used in antiquity was invariably charcoal, and the major smelting establishments required enormous quantities, necessitating theestablishment of large areas of managed woodland to supply the timber (Craddock 1995: 193–95; J. C. Allan 1970: 9–11; Fulford and Allen 1992). Indesert areas, such as Palestine, it is possible that smelting operations had to be curtailed periodically because the charcoal sources in the vicinity hadbecome exhausted. Strabo (14.6.5) mentions deforestation on Cyprus resulting from copper refining. The conclusion of the studies of Mighall andChambers 1993 on smaller, prehistoric iron and copper mining and smelting operations in temperate Europe was that they did not bring about long-term degradation.




Another potential source of environmental pollution was noxious fumes. Most of the nonferrous ores smelted in classical antiquity were sulphidic, andthe roasting and smelting operations produced sulphur dioxide in quantity. In the absence of tall chimneys, these fumes were not dissipated butremained in the vicinity of the mines. Xenophon (Mem. 3.6.12) and Strabo (3.2.8) both allude to the problem of fumes and consequent environmentaldevastation. This aspect of ancient metal refining is difficult to study now, but sulphur dioxide pollution certainly was a major problem in manynineteenth-century mining areas, such as Rio Tinto (Salkield 1987: 43–44). The highly acidic groundwaters that resulted effectively controlled themosquito population, keeping the area free of malaria. Yet another environmental concern is the poisoning of the surrounding land with heavy metals,particularly with lead and cadmium from silver production by means of the cupellation of argentiferous lead, and to a lesser extent from copperproduction. This very persistent pollution has been studied by Mighall 2003 for prehistoric copper mining sites and by Pyatt et al. (2000) for the muchlarger Roman operations at Feinan. At the latter site there are enhanced levels of heavy metals in the soil that must inevitably have entered the localplants, including those grown for food. The ores are near surface, however, and in places are cut by the streams; thus the local alluvial fans wouldhave enhanced levels of heavy metals anyway. Similarly, attempts to study how this pollution affected the local population by analyzing bones fromthe cemeteries are compromised by the propensity of bone to absorb heavy metals from the soil.

Refining

Copper and the other base metals were refined by remelting in an open crucible and allowing impurities such as iron, sulphur, arsenic or antimony tooxidize. Some elements, such as the arsenic and sulphur, evaporated from the molten metal; others, notably iron oxide, could be removed byskimming the surface, possibly aided by adding a little silica to form a slag. Firerefining remained the usual method of refining until the twentiethcentury.

One of the major technical developments in classical antiquity was the introduction of gold refining in response to the introduction of gold coinage by theLydians. The remains of a gold refinery of the sixth century B.C. have been excavated at their capital, Sardis (Ramage and Craddock 2000; Craddocket al. 2005). The study of the refinery at Sardis, coupled with Agatharchides' description of gold refining as practiced in Ptolemaic Egypt in the secondcentury B.C. (Diodorus 3.13.2–14.4; Burstein 1989), has illuminated the ancient gold refining process (known in English as parting). Native gold typicallycontains between 5 and 30 percent silver. The gold granules and dust were mixed with salt and possibly alum in an earthenware vessel. This wasplaced in a furnace and heated at a carefully controlled moderate heat to between 600º and 800ºC for many hours or even days. The astringentvapors generated by the hot salt penetrated the gold and removed the silver as a vapor of silver chloride, leaving behind pure gold. The silver could berecovered from the clay parting vessels and furnace walls by cupellation.

The necessity of producing metal of uniform weight and purity required a change in the perception of the very nature of metals and introduced theconcept of elements. Our idea of gold as an element with precisely defined properties is based on relatively modern scientific concepts, in particular onthe Law of Constant Composition, which states that each element has precise and invariant properties. To us this seems no more than stating theobvious, but the ancients did not have such a concept of materials as elements. The widely held belief, as set down by Empedocles, was that metals“grew” in the ground. Thus it would seem logical to expect the properties of the metal to depend on their environment; metals such as gold, coming fromvarious sources, could have widely differing properties but still be gold. For example, a light-colored gold from a given locality would be regarded asthe intrinsic gold of that place, rather than as a naturally occurring alloy of gold with a rather high silver content.

Coinage introduced the concept of an invariant standard of purity (usually expressed by the metal refiners in antiquity in practical terms whenrepeated refining failed to reduce the weight any further). It was also noted that natural gold could be recreated by adding silver to the refined gold, andconversely that silver could be recovered from the refining waste. Thus, although natural gold was still “growing” and still contained silver that would intime turn into gold, there was an ultimate gold, and through refining the gold could attain that state.

This new perception, brought about by the need to refine precious metals for coinage, was already extending to other metals by Roman times. Forexample, Pliny (HN 34.94) stated that the Cypriot mines produced ductile “bar” copper that could be hammered without breaking, unlike other minesthat produced “fused” copper, which was too brittle to be hammered. But he continued, “in the other mines, this difference of bar copper from fusedcopper is produced by treatment; for all copper after impurities have been rather carefully removed by fire and melted out of it become bar copper.”That is, the difference was not just an intrinsic property of Cypriote copper but was also associated with the degree of refining.

Iron and Steel

In the first centuries of the first millennium B.C., the use of iron spread very rapidly throughout the Old World, supplanting bronze as the usual materialfor tools and weapons, as is exemplified by the tools used in mining (see above). Through most of its history iron has been used in a variety of states:wrought iron, phosphoric iron, cast iron, steel, and crucible steel (Craddock 2003).

In Europe, the Mediterranean world, and the Middle East, iron was always smelted by the bloomery process (Tylecote 1987; Craddock 1995). Thefurnaces and smelting conditions were not dissimilar to those used to smelt other metals, although some of the furnaces were of considerable size bythe Roman period (Crew 1998). The iron was produced as a solid pasty mass, called the bloom, invariably containing some slag. This was removedwhile still white-hot and vigorously hammered to squeeze out as much of the slag as possible, and also to consolidate it (Diodorus 5.13.1–2). Thewrought iron so produced could also contain small quantities of carbon, sometimes sufficient to be classified as steel.

If the iron ore contained phosphorus, as most bog ores do, then this could enter the iron. Small quantities of phosphorus have much the same effect ascarbon in iron, but can lead to serious brittleness. For this reason phosphorus is now regarded as a harmful impurity, but in Iron Age Europephosphoric iron was deliberately selected for blades such as sickles and knives, since they kept their edge, even if they occasionally broke (Craddock1995: 238; Ehrenrich 1985). The smith can have had no notion of the presence of phosphorus but knew that certain ores, when smelted in a particular




way, resulted in an iron suitable for blades.

If the temperature of the furnace was increased and the conditions made more reducing, then carbon could dissolve in the iron as it formed, reducingthe melting temperature down to about 1,200ºC. Thus, instead of a solid bloom, liquid iron could be run from the furnace containing about 3.5 to 4.5percent of carbon (called “cast iron”). Cast iron is not a very useful material, and it seems not to have been used in the classical world; the use of castiron was a great achievement of Chinese metallurgy. Iron with a small amount of carbon (typically between about 0.2 and 1 percent) is known as steel,and when correctly heat-treated it has properties much superior to wrought iron (see below); getting a regular amount of carbon into the solid iron,however, was very difficult.

The ideal material would be iron with about 1 percent of uniformly distributed carbon and containing no slag. This result could be produced by reactingwrought iron with charcoal and wood in small sealed crucibles at very high temperatures (1,500ºC) to produce crucible steel. This technology was inwidespread use through central and southern Asia over 2,000 years ago, and it seems that the classical world had some inkling of the process andpossibly imported sword blades made from it (see below). Crucible steel was always forged to shape, never cast.

Wrought iron could be shaped by hammering at red heat (hot forging) using methods and tools not dissimilar to those used by traditional blacksmithsto this day (Tylecote 1987; Sim and Ridge 2002). The iron could not be cast, but good metallurgical joins could be made by hammering piecestogether at red heat (hammer welding) or by hot riveting. The archaeological record shows that iron was a very familiar low-cost material. Estimateshave been made for iron production of 2,250 tons per annum in Roman Britain, and 82,500 tones per annum through the rest of the Empire (Sim andRidge 2002: 23; cf. Cleere and Crossley 1985: 57–86).

Iron was used on such a scale that technologists such as David Sim have speculated whether some more advanced production techniques mighthave already been in use. For example, it is usually held that the production of wire through a draw plate is a post-Roman technique, and that iron wirewould have had to be laboriously made by hammering a thin rod. Even a simple tunic of chain mail (the lorica hamata), however, contained hundredsof meters of wire that it would not have been feasible to make except by drawing. Draw plates have now been recognized, appropriately enough in thefloor of armorers' workshops inside Roman frontier forts (Sim 1997, 1998).

Wrought iron could be turned into steel by heating it for many hours in close contact with a variety of organic materials and charcoal dust (carburizing).There were two techniques: small pieces of iron could be totally carburized and then welded to the wrought iron of the remainder of the tool (the tip of achisel, for example), or the entire artifact could be forged and then the surface carburized all over (case hardening).

The famous Noricum iron praised by Pliny (HN 34.145) and others was a natural steel (Davies 1935: 173; Tylecote 1987: 168). The iron ores in thevicinity of Magdelensburg in Austria are rich in manganese, and thus the slag had a great deal of manganese oxide. Ordinary iron slag contains ironoxide, which reacts with any carbon dissolved in the iron in the furnace. Manganese oxide does not react in this way, and thus there is a better chanceof any carbon in the iron surviving; in the Noricum iron sufficient carbon remained for it to be classified as a steel. Steel can be greatly improved byheat treatments, as the Greeks knew as early as the eighth century B.C. The steel was heated to a cherry-red heat and held at that temperature forsome time before being plunged into water. The resulting steel was very hard, suitable for files but too brittle for other uses. To produce a steel moresuitable for chisels or saw blades, for example, the quenched steel was gently heated (tempered). The smith judged the degree of tempering by thecolor of the oxidized layer on the steel.

With so many imponderables in the production of good steel, it is not surprising that things often went wrong, and the proportion of tools and weaponsthat had been correctly heat-treated is low. Studies of prestige blades, however, have shown that they are regularly of good-quality, correctly temperedsteel (Lang 1995). Swords could be made by hot-forging and folding the iron. The folding could be repeated several times, resulting in a blade made upof many thin layers of iron (Lang 1988), a process known as piling; it was probably done to homogenize the iron. From the end of the first millenniumB.C. in Europe the practice began of twisting the iron sheets or rods into decorative patterns during the forging, sometimes alternating iron and steel.Pattern welding, as this is called, is usually associated with the post-Roman barbarians, but the technique was certainly practiced in the late RomanEmpire.

Studies of Roman armor have shown that steel and iron were intelligently and deliberately combined to produce a superior product (Williams 1977).For example, the lorica segmenta plates of Roman armor were fabricated from wrought-iron sheet to which steel sheet had been welded (Sim andRidge 2002: 96). The outer hard steel layer would stop penetration, and the inner tough iron layer would prevent the lorica from breaking and absorbthe energy of the blow.

As early as the first millennium B.C., the Chinese were producing iron castings on a prodigious scale, and crucible steel was being produced in centraland southern Asia (Craddock 2003). What knowledge the Romans had of these materials, or what access they had to them, is uncertain. Tylecote1987: 325–27), for example, listed finds of iron castings from Roman sites and suggested that they could have been imports from China. Ongoing workin southern Germany has shown that the blast furnace process developed from about the eighth century A.D., much earlier than previously believed,although the liquid iron so produced was not used for castings but instead was treated (fined) to produce solid wrought iron (Craddock 2003). Theancestry of the process in Europe is not known, but excavations of Iron Age and Roman iron-smelting sites have shown a hitherto unappreciatedvariety of processes (Crew and Crew 1997). At the major iron-smelting site at Oulche (France), there is evidence that granules of high carbon iron mayhave been separately collected from the ordinary iron and forged to produce high carbon steel (Dieudonné-Glad 1997).

It has been claimed that various passages in classical literature refer to liquid iron or steel, but these can be discounted (Craddock 2003), with theexception of the writings of the Alexandrian alchemist Zosimos, who lived in the third century A.D. (Berthelot 1893: 332). He gave a detailed andaccurate description of how the Indians produced crucible steel, and he indicated that this material was used by the Persians to make high-qualityswords that were then traded to the Romans. Blades from central Asia dating from the third century A.D. (Feurerbach 2001), and from SassanianPersia (Lang et al. 1998), have been identified as crucible steel, but so far no crucible steel blades have been reported from the Roman or ByzantineEmpires. Since literary evidence suggests that crucible steel was already known in the late Roman and Byzantine world, it is very likely that continuing





research will uncover more evidence of sophisticated processes and products.

Brass: the New Metal

Bronze—the alloy of copper and tin—appeared in the fourth millennium B.C. and was in universal use by the first millennium B.C. Throughout theclassical period, lead was added in ever greater quantities to bronze intended for castings (Craddock 1976, 1977, 1986). The major change in copperalloys in the ancient world, however, was the introduction of brass—an alloy of copper and zinc—sometime within the first millennium B.C. By the lateRoman and Byzantine periods, once the technology for its preparation had been developed, brass had largely replaced bronze as the usual copperalloy (Caley 1964; Bayley 1998; Craddock 1978; 1995: 292–302; 1998; Craddock and Eckstein 2003). Zinc ores are abundant compared to those oftin. The reason for the relatively late introduction of brass as an alloy is the volatility of zinc. At the temperatures necessary to smelt it, zinc is a veryreactive gas. As a result, the pure metal was not produced commercially until about A.D. 1000, in India (Craddock et al. 1998a).

Almost from the inception of metallurgy, some copper artifacts from all over the world have been found to contain varying quantities of zinc (Welter2003). They always seem to be isolated examples among items that otherwise consist of copper and bronze, and they are probably the unintentionalresult of smelting zinc-rich copper ores. From the end of the second millennium B.C. in Anatolia and the Middle East, copper artifacts containingbetween about 5 and 15 percent of zinc become more common, contemporary with references to a special copper known as “copper of the mountain”in Assyrian and other documents. From the mid-first millennium B.C. there were also references in Greek literature to oreichalkos, “copper of themountain,” as a distinct metal, and by the first century B.C. this term, and its Latin form aurichalcum, certainly referred to brass (Halleux 1973). Earlyproduction seems to have been centered in western Anatolia, although production in Persia and east as far as India is a distinct possibility. The firstseries of artifacts regularly made of brass that have been identified so far are the base metal coins of Mithradates VI, who ruled western Anatoliaduring the early first century B.C. (Craddock et al. 1980). These issues include both copper and brass coins and are directly ancestral to the Romanreformed coinage of 27 and 23 B.C., which saw the introduction of brass dupondii and sestertii (Burnett et al. 1982) (figures 4.5, 30.1). It seems likelythat through the first century B.C. brass rapidly gained in popularity and was extensively used by the military. Istenic and Šmit (2008) have shown thatthe Alesia type of brooches, associated with the military, are of brass from about 60 B.C., and by the first century A.D. military brooches, trappings,and fittings were regularly made of brass (Jackson and Craddock 1995; Ponting and Segal 1998). These brasses and the contemporary brass coins(Calliari et al. 1998) are usually an alloy of copper with about 20 to 25 percent of zinc and minor amounts of lead derived from the zinc ore. Thecontemporary civilian metalwork is often of an alloy containing less zinc but more lead along with some tin, suggesting the indiscriminate mixing ofscrap bronze and brass.

Figure 4.5. Early brass coins. Top: Early first century B.C., Pergamum (BMC 130, 144). Middle: Roman SC coin of the reformed eastern coinage of27 B.C. Bottom: dupondius and sestertius of the general reform of 23 B.C. (After Craddock 1995.)

The ancient written sources do not state how brass was made. The only physical evidence is a number of distinctive small, lidded crucibles, heavilyvitrified and massively impregnated with zinc (figure 4.6) (Bayley 1998). It has always been assumed that the alloy was made by cementation, theprocess that was used in post-medieval Europe up to the nineteenth century (Rehren 1999). In this process the calcined zinc ore together with charcoalwas added to finely divided solid copper in a closed crucible and heated to temperatures of about 900ºC, whereupon the zinc oxide was reduced tozinc gas that dissolved into the copper. The first mention of cementation, however, is by Biringuccio in the 1540s (Smith and Gnudi 1942: 70–76), andthe only medieval accounts—those of Theophilus, dating from the twelfth century (Hawthorne and Smith 1963), and those of al-Hamdānī and al-Kāshānī, dating from the tenth and twelfth centuries, respectively (J. W. Allan 1979; Craddock et al. 1998b)—all make it clear that the zinc ore andcharcoal were added to molten copper in the crucible, rather than to solid shavings. This process was much simpler and quicker, but less zinc enteredthe copper (Craddock and Eckstein 2003). That this was the process used in classical antiquity is supported by the composition of the ancientbrasses. Roman and medieval brasses contain up to a maximum of 28 percent of zinc, but after the sixteenth century this proportion rises to amaximum of 33 percent.





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Figure 4.6. Small Roman lidded crucibles probably used to make brass; a-b from Culver, St. Colchester; c from Palace St. Canterbury; d fromextramural settlement at Ribchester. (After Bayley 1998; copyright English Heritage, by permission.)

There was a further complication to the production of brass. The ore added to the copper must contain no sulphur. The zinc deposits of western Europethat were very probably worked by the Romans (at Stolberg in Germany, Vieux Montaigne in Belgium and the Mendip Hills in Britain) are carbonatesand could have been used directly, but the zinc ores of Anatolia and the Middle East are sulphides. The sulphidic ores first had to be roasted toproduce zinc oxide. As zinc oxide had medicinal uses—the familiar zinc ointment or calamine lotion (Lehmann 2000)—there are detailed descriptionsof its preparation in contemporary pharmaceutical texts, especially that of Dioscorides (Gunther 1934; Riddle 1985). The sulphidic ore was roughlyroasted to convert most of it to the oxide, and then smelted in an ordinary shaft furnace. The zinc vapor ascended the furnace flue where it met oxygenand promptly reoxidized. Complex arrangements of iron bars suspended in the flue, or even of whole chambers above the flues, were set up to catchand condense the fumes of zinc oxide. Debris of this process has been excavated at Zawar in India, dating from the last centuries B.C. (Craddock andEckstein 2003). The process continued in use in the Middle East at least until the thirteenth century, when it was observed by Marco Polo at Kerman inPersia.

The zinc content of the dupondii and of the sestertii declined during the first century, leading Caley 1964 to believe that brassmaking had alreadybecome a lost art. It is true that coins of the central mint cease to be made of brass, but analyses of other classes of metalwork show that it continuedin production (Craddock 1978; Dungworth 1996). More interestingly, the major analytical study of thousands of brooches from Roman Britain (Bayleyand Butcher 2004) has shown that different types of brooch used different and very specific alloys, some of brass and others of leaded bronze.Similarly, local issues of copper-base coins in the eastern empire were often struck in brass, whereas the corresponding issues from the central mintswere of bronze (Cowell et al. 2000). After the collapse of the western empire, with the attendant loss of the tin producing provinces of Iberia andBritannia and easy access to the tin fields of the Erzgebirge in central Europe, brass became the usual alloy (Hook and Craddock 1996; Schweizer1994). The earliest Islamic metalwork reverted to bronze (Schweizer and Bujard 1994; Ponting 1999), presumably until the capture of the brassmakingsources in Anatolia, after which brass replaced bronze almost completely (Craddock et al. 1998b).

Survival and Development

The production of metals survived the collapse of the Roman Empire in the west, with every indication, for example, that tin production in Britain andbrass production in Flanders continued. The Byzantine successors of the Roman Empire in the East seem to have maintained production, and this inturn was continued by the Islamic regimes. These later mines and smelters have been poorly investigated, and so comparison with earlier classicalpractice is difficult. Not until the sixteenth century do detailed descriptions appear for comparison, especially those of Biringuccio (Smith and Gnudi1942) and Agricola (Hoover and Hoover 1912). It is clear that Agricola (a pseudonym for Georg Bauer) was familiar with Strabo's Geography andPliny's Natural History, and in some ways his De re metallica contains striking parallels (Domerque 1989b). There are also striking and significantdifferences, not least between the authors and their intended audiences. Strabo and Pliny wrote as interested and educated observers, and their workwas intended for gentlemanly diversion, not for practical instruction. Agricola was trained as a doctor with strong interests in geology and metallurgy,and he wrote his works with official encouragement as real working manuals.

Clearly, both the classical and Renaissance mining ventures could be on a considerable scale. Investigations at Rio Tinto, Très Miñas, and other siteshave shown that sophisticated drainage methods were employed. The major difference seems to have been the prevalence of water-powered, and toa lesser extent animal-powered, machinery (M. J. T. Lewis 1997). Water power certainly was harnessed in the classical world, but how widespreadwas its use? This has been a major issue in the history of technology for many years (see chapters 6 and 13); some scholars, such as Wilson (2002),believe that water-powered devices were quite common even in mining operations. A thousand years after Rome, the pages of De re metallica are fullof descriptions and illustrations of drainage pumps, crushing machinery, and bellows all powered by water wheels. These do seem to be generallyabsent in Roman mines, although it is dangerous to assume their absence. In the few years since the stamps at Très Miñas were announced, severalmore have been recognized, including one at Dolaucothi in Wales adjacent to a Roman water mill and surrounded by heaps of crushed debris(Burnham and Burnham 2004). Thus water power might have been more prevalent than believed at present, although it does seem that earlier mining









relied far more on large labor forces.

There are more clearcut differences between classical and medieval smelting practices because of the application of water power. As noted above,the early furnaces were necessarily small because of the limitations of manually operated bellows to produce a blast of sufficient volume and power topenetrate a larger furnace charge. With the introduction of water power the bellows grew to enormous dimensions, and the furnaces expanded to overa meter in internal diameter and several meters in height. Especially in iron smelting, a single operation could last for weeks, producing many tons ofmetal with just a few highly skilled operators, totally different from the small-scale, labor-intensive operations of the classical world.

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Paul T. CraddockDr. Paul T. Craddock, Department of Scientific Research, The British Museum.




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