1085 © 2014 ISIJ

ISIJ International, Vol. 54 (2014), No. 5, pp. 1085–1092

Production of Silver across the Ancient World

Paul CRADDOCK*

Dept. of Conservation and Science, The British Museum, London WC1B 3DG.

(Received on November 30, 2013; accepted on March 3, 2014)

The quest for silver through antiquity encouraged a succession of major developments across a varietyof technologies. Silver and its minerals occur in a variety of ores, but rarely in more than trace amountssuch that in order to discover and extract them various special technologies had to be developed, result-ing in the first separation of small quantities from the fourth millennium BC. Through the first millenniumBC there was a steady increase in the demand for silver, much accelerated by the introduction of coinagefrom the Mediterranean to South Asia that led to the major developments in all aspects of mining tech-nology. The continuing demand led to new technologies to extract silver from copper and to recovermetal from smelting debris.

KEY WORDS: silver; ore; mining; smelting; process; Rio Tinto; Dariba; Agucha.

1. Introduction

Silver occurs as a native metal in limited amounts andalso as mineral ores such as the sulphide, argentite, Ag2Sand as the chloride, ceragyrite, AgCl, but once again in onlyvery limited quantities.1) Silver is far more prevalent as aminor component in other metal ores, gold and copper, butparticularly in those of lead. The presence of minor quanti-ties of silver in lead ore in the Middle East seems to havebeen known, and extracted from the fourth millennium BC.This was man’s first attempts to separate trace amounts ofone metal from another. This was done by the process ofcupellation, by which the argentiferous lead was subjectedto an oxidising blast at around 1 000°C, oxidising the leadto litharge, PbO but leaving the silver as a separate metalphase2,p.221-8,3) The evidence for this was the presence of lith-arge, which does not occur as a natural mineral, at smeltingsites in northern Syria and in eastern Anatolia.4,5) PieterMeyers6) has studied the composition of early silver arte-facts in the Eastern Mediterranean for indications of thelikely ores and processes used. Silver extracted by cupella-tion should contain traces of lead (at least 0.05%, but typi-cally at least an order of magnitude higher), but only negli-gible amounts of volatile elements such as zinc. For example,the silver artefacts of the Early Bronze Age Argaric culturein Spain of the second millennium BC have very low leadcontents and are thus likely to have been smelted directlyfrom silver ores.7,8) The silver artefacts from Mahmatlar ineastern Anatolia, dated to the third millennium BC, have sub-stantial traces of zinc and thus cannot have been cupelled.9)

Thus it is possible to begin to construct a scenario for ear-ly silver production. At first the very limited amount ofnative silver could be utilised, and some would inevitably

have been associated with argentite and ceragyrite, whichalthough not especially colourful or shiny compared to cop-per or even lead ores, are dense and thus of potential inter-est, and would have been easily smelted. The discovery ofcupellation is a little more problematic, but the silver min-erals are often associated with lead ores and the resultingmixed metal after smelting would have been decidedly dis-appointing in appearance. However carrying out the usualmetal refining technique of melting in an open cruciblewould have progressively oxidised the lead, enabling it to beskimmed off. From this it would soon become apparent thatsmall quantities of silver were sometimes to be found in leadores, but that both high temperatures and an air blast werenecessary if the lead was to be oxidised in a reasonable time(They also probably soon noticed that the practitioners ofthis process often became ill, but this never seems to haveconcerned anyone right up to the 20th century!).

2. Minerals and Mines

As noted above, native silver or minerals concentratedenough to be smelted directly only occur in very limitedquantities, and already by the first millennium BC virtuallyall silver must have come from ores containing only a fewthousand parts per million at most. The lead ores couldeither be the primary sulphide, galena, PbS or the secondarycarbonate, cerussite, PbCO3, formed by the weathering ofthe sulphides, and thus tending to occur near the surface.Meyers6) noted that the gold content of the oxidised ores isvery much higher than in the primary ores. This is becausein the weathering process some of the lighter more electronegative metal ions tend to dissolve and drain down into thedeposit but the gold is unaffected and left behind, and thusbecomes concentrated.

The near surface deposits would be expected to be thefirst exploited. At the great Mauryan-period silver mines in

* Corresponding author: E-mail: PCRADDOCK@britishmuseum.orgDOI: http://dx.doi.org/10.2355/isijinternational.54.1085

© 2014 ISIJ 1086

ISIJ International, Vol. 54 (2014), No. 5

the Aravalli Hills (see below), the heavily oxidised, knownas gossanised, surface deposits suggest that in the upper lev-els of the mines the ore would have been principallycerussite10) (although the primary argentiferous galena wascertainly also worked from the lower levels). The few con-temporary early punch-marked silver coins that have beenanalysed have gold contents between 0.7 and 1.3%, sugges-tive of oxidised ores.11) A somewhat similar situation isfound at Laurion in Greece where Conophagos12) believedthat the upper levels would have been mainly of cerussite.However, if so then these must have been worked out earlyin the mine’s long history, probably already in the BronzeAge. Analysis of the blebs of metal in some of the Classical-period slags led Photos-Jones and Jones13) to believe thatcerussite was still the main ore used, but more detailed studyshowed that the original mineral in the slag had been galena,and which had subsequently oxidised.14) This conclusion issupported by the relatively low gold content of the contem-porary Athenian drachma coins.6)

Meyers further postulated that although most Greek silvermust have come from primary galena ores, elsewhere,through the Middle East the high gold content of the silversuggest that cerussite was the more common ore. However,there is another possible source- the legendary silver fromTartessos. From the beginning of the first millennium BCthe Phoenicians crossed the Mediterranean and began toexploit the resources, particularly in Sardinia and the IberiaPeninsula. In their search for metals they found the vastmineral wealth of the Iberian Pyrites Belt that stretches forhundreds of km across Andalucia and into Portugal.15) Inparticular they discovered the jarosite ores, rich in silver.These form at the base of the weathered horizon at its junc-tion of with the primary deposit where the more solublemetal salts that percolated from above will have precipitatedand where early mining concentrated (Fig. 1).16)

They are thus a decomposition product of very variablecomposition and appearance, but are mainly the sulphatesand oxides of iron, potassium, aluminium of general formu-la Fe3(OH)6.X(SO4), where X can be a number of metalsincluding arsenic, antimony, copper, lead, bismuth and sil-

ver.17,2,p.216-21) As they are from an oxidised ore, the silvertypically has a high gold content. Jarosites can be a soft clayor hard rock, with a wide colour range and are not particu-larly dense such that they do not have the appearance of apromising ore. As such they do not seem to have beenexploited by the indigenous Late Bronze Age inhabitants,and the great achievement of the Phoenicians was to discov-er the jarosite’s potential and to develop ways of successful-ly smelting them.18) Many of the jarosites have a low leadcontent and thus lead had to be added to the smelting chargein order to collect the small amount of silver contained ineach smelt. Sometimes the lead came from considerable dis-tances as evidenced later in Roman times at Rio Tinto.19)

There was a later legend that the Phoenician sailors had somuch silver to carry back that they had to throw away theirlead anchor stocks and replace them with silver. Could thisbe a fanciful reference to the Phoenicians bringing in leadfrom across the seas to smelt the jarosites?

In addition other complex argentiferous lead ores weresmelted often adding barites and quartz as fluxes. This pro-duces a mobile slag through which the argentiferous leadcould drain easily. This seems straightforward but at manysites some of the slags contain many quite macroscopicfragments of the crushed fluxes that have appearance ofhaving been added very shortly before the slag solidified(Fig. 2), and are accordingly referred to as free silicaslags.20) These slags also routinely retain much more argen-tiferous lead. Thus it would seem that for some reason theslags were solidified by adding the crushed flux whichwould both stiffen and cool the melt, prior to being removedfrom the furnace whilst the process was still in progress. Thesmelting site at Monte Romero, near to Rio Tinto, in Huelva,in the south of Spain, dated to the 7th century BC has beenexcavated and studied.21,22) There some of the free silicaslags were in the form of balls or buns, typically 20–25 cmin diameter, and had been seemingly stored for further pro-cessing, although here and at other sites most of the free sil-ica slags were abandoned on the slag heaps with no furtherprocessing intended. A possible explanation could be thatafter the majority of the argentiferous lead had drained

Fig. 1. Rio Tinto. East end of Corta Lago: Junction of the primary sulphidic pyrites (white) with the oxidised gossan (dark)above. Note the ancient gallery (circled) exposed in the enriched material just below the contact. (P. T. Craddock 1977).

ISIJ International, Vol. 54 (2014), No. 5

1087 © 2014 ISIJ

through the slags more crushed flux was added to halt theprocess. Some of the free silica slag could be crushed andassayed to determine the amount of argentiferous leadremaining. Where sufficient was present the slag would beworked into balls and stored for further processing, other-wise the slag was just dumped on the heaps. The resmeltingof the free silica slags produced free-flowing tap slags. Thesilver would have been extracted from the argentiferous leadby cupellation. At Monte Romero a stack of used cupella-tion vessels, now mainly composed of lead oxide and sili-

cates were found, presumably intended for resmelting torecover the lead.

3. Production in India

The introduction of coinage from the mid first millenni-um BC created an enormous demand for silver across theOld World from Spain to India. This resulted in major devel-opments in all aspects of mining technology.23) Indeed manyof the most famous mines of antiquity, Rio Tinto, Laurionetc. achieved their maximum production being worked forsilver in the later first millennium BC. This is also true in

Fig. 2. Section through a typical free silica slag from Cerro de laTres Aguilas, Spain. Note the large quantity of quite macro-scopic quartz fragments. Although they are cracked due tosudden heating, their profiles, especially at the edges arestill quite sharp showing that the iron and barium oxides insurrounding molten slag had little or no time to react andbegin to dissolve the quartz before the slag had set. (cf theDariba slags Fig. 13). (P. T. Craddock).

Fig. 3. Ancient mines in the Aravalli Hills of North West India, AAgucha, D Dariba and Z Zawar, together with B, theancient port of Bharuch or Broach. (T. Simpson).

Fig. 4. Plan and section of Dariba. (HZL).

© 2014 ISIJ 1088

ISIJ International, Vol. 54 (2014), No. 5

India where the introduction of the silver punch-markedcoinage in the mid first millennium BC24,25) created an enor-mous demand for silver. The sources of the metal have hith-erto been obscure with many claiming that silver was notproduced on any scale within ancient India.26) However,fieldwork in the Aravalli Hills of Rajasthan in north westIndia has shown that the Mauryan state developed severaltruly enormous mining enterprises, both for zinc minerals atZawar27) and for silver at Dariba and Agucha10,28) (Fig. 3).

The Aravalli Hills are formed of Precambrian rocks tiltedalmost vertically. At Dariba and Agucha the metalliferousore is contained in the hard calc silicate rocks and the adja-cent and much softer graphite mica schists. The latter weremuch easier to mine but their complex mineralogy withmica, sillimanite, potassium feldspars etc. made them diffi-cult to smelt (see below). The primary ore minerals are ofmixed iron, zinc and lead sulphides, of which the lead isargentiferous at Dariba and Agucha.29) The near-surfacedeposits at both mines are extensively gossanised and it islikely the argentiferous lead ore would have been principallythe carbonate, cerussite, PbCO3 and the sulphate, anglesite,PbSO4. At Agucha the complex antomonide ore, freibergite,([Cu, Ag, Fe]12 [Sb, As]4 S13) was also a significant sourceof silver.

At Dariba a series of small near-surface mines survive,principally in the graphite mica schist (Figs. 4, 5 & 6) dugboth for their silver and to access the ore in the calc silicatedeposits below, and there is evidence that similar workingswere also once present at Agucha. These are likely to havebeen the earliest workings and mining continued at depthfollowing the ore bodies down. This created a series of mas-sive near vertical workings, known as stopes, penetratingdown for hundreds of metres, well below the water table(Fig. 7). The graphite schist and surrounding country rocksare pervious which created a major drainage problem. Asthe workings at both mines are well below the surroundingplain there was no possibility to drive a drainage adit fromthe workings out to an adjoining river valley as the Romanshad dug at Rio Tinto. Instead the water had to be raised tothe surface and a series of bailing ponds were encountered

at the top of one of the stopes at Dariba (Fig. 8), rather sim-ilar to systems recorded in Japanese mines in the 19th cen-tury.30)

At Agucha the recent open cast mining of the Mauryanmine has exposed a complex series of workings beneath the

Fig. 5. Looking north along North Lode (photo position at arrow A on Fig. 4). Note the quartzite chert ridge with workingsin the graphite mica schist to either side (see Figs. 6 & 7). In the foreground is a later opencast in the calc-silicatewhich has gone through earlier galleries now exposed on the left hand side. (P. T. Craddock).

Fig. 6. Mining in the graphite mica schist alongside the upstandingquartz dykes at North Lode. (P. T. Craddock).

ISIJ International, Vol. 54 (2014), No. 5

1089 © 2014 ISIJ

ancient opencast (Fig. 9), the sophisticated layout of shaftsand cross cuts being partly determined by considerations ofventilation and drainage of the workings at depth as well asaccess to the ore (Fig. 10).

When the richest ore had been extracted by deep miningmajor opencast mining was undertaken at both mines. The

opencast pit over the East Lode at Dariba must surely be thelargest such mine working to survive from antiquity (Figs.4 & 11). At this mine there was an additional problem,because of the steep hillside on the west side, all the wastehad to be dumped on the other side which was of unstablealluvium. This had to be supported and thus a vast timbercomplex of benches was constructed (Fig. 12). The benchesrun along one side for several hundred metres and thebenches have been identified descending at four levels andfurther levels may continue down beneath the present fill ofthe opencast.

The mined ore would have been beneficiated by crushing,hand picking and washing. At Dariba a series of mortars sur-vive cut into the hard calc silicate rock surrounded by enor-mous heaps of bean-sized waste, and similar heaps exist atAgucha.

The excavations at both mines failed to discover anyremains of intact smelting units, but from the very manyfragments of refractory ceramics in the slag heaps it hasbeen possible to conjecture their form. There were manythick, crude curved pieces that could have come from hemi-spherical shapes of approximately 30 cm diameter. There

Fig. 8. Small bailing pond and timber dam at the top of the largestope (Fig. 7). The wooden ladder on the left is original andleads up to the next pool. Note the large beam across thestope, probably used for hauling water up to this level. (L.Willies).

Fig. 7. Dariba South Lode: Near vertical stope typical of the largescale workings in these mines. The arrow indicates the bail-ing pool (Fig. 8). (L. Willies).

Fig. 9. The ancient open cast mine at Agucha before modern mining commenced. A complex system of deep mines laybeneath (Fig. 10). (P. T. Craddock).

© 2014 ISIJ 1090

ISIJ International, Vol. 54 (2014), No. 5

were vitrified and slagged on their concave, i.e. their innersurfaces. These could either have come from a bowl furnaceor a smelting hearth. On some fragments which included theedge, it was clear that the slag flow was away from the edge,suggesting that the ceramic had been set in ground and thuswas a hearth lining, similar to those used in Japan until the19th century.30)

The smelting slags at both mines were very heteroge-neous, containing many fragments of unreacted gangue min-erals such as mica, and clearly must have been very viscous(Fig. 13), unlike the contemporary copper and iron smeltingslags at Dariba which were homogenous and clearly hadbeen fully mobile. The viscosity and high melting point isdue to the aluminium content of the gangue materials in thegraphite mica schists (see above and Fig. 14). However thesurvival of the gangue fragments suggests either that theywere added late in the process, recalling the free silica slagsfrom southern Spain, or alternatively a relatively short reac-

tion time at high temperature. The latter is reinforced by thestudies on the vitrification of the smelting hearth fragmentswhich suggest temperatures in the region of about 1 150°Cmaintained for only a little over an hour, a very short smelt-ing time. Despite this the slags contain relatively few blebsof argentiferous lead, certainly when compared to the Phoe-nician free silica slags discussed above, indicating a goodseparation of slag and metal, with very little silver being lostto the slags. It is possible that the slags were mechanicallyworked whilst still semi-molten to squeeze out much of thelead.

The next stage was the extraction of the silver from thelead by the process of cupellation. At Agucha some pits,also of the third century BC, were excavated that containeda great deal of cupellation debris. This included many verysmall cupels that must have been used to assay each batchof ore to discover the silver content. There were also frag-ments of furnace lining that had been attacked by lead oxide

Fig. 10. Agucha: Conjectured arrangement of the west end of the deep workings, based on drilling and development at themodern mine (the squares are at 10 m vertical and 100 m horizontal intervals). Note the paired shafts, probablyfor ventilation purposes. (HZL).

Fig. 11. East Lode opencast seen from the top of South Lode (photo position at arrow B on Fig. 4). with the exposed sec-tion of the timber revetment (Fig. 12) circled. This is certainly the largest opencast metal mine of antiquity. Notethe enormous quantities of waste dumped on the alluvium on the far side that had to be supported by the revet-ment. (L. Willies).

ISIJ International, Vol. 54 (2014), No. 5

1091 © 2014 ISIJ

causing very extensive glazing together with long runs anddrips of lead silicate that enabled the orientation of eachfragment to be ascertained. Studies on these refractoriesshowed that they had been at temperatures of about 1 100°Cfor many hours suggesting an industrial-scale continuousprocess under ceramic hoods, similar to those envisaged byConophagos3,12) at the contemporary silver mines at Laurion.

4. Other Sources of Silver

Argentiferous copper was also a significant source of sil-ver in the past. The silver could be extracted by the processof liquation, in which the molten copper was mixed withlead, whereupon the silver transferred to the lead from

which it could be recovered by cupellation.2,p.232) Liquationwas used in Medieval Europe and in the Far East, but theorigins of the process are uncertain. Rovira and Renzi31)

believe that the process was used by the Tartessians early inthe first millennium BC, and it has been suggested that theprocess could have been used somewhat earlier in the LateBronze Age on Sardinia.32) The Romans certainly utilisedliquation,2,p.233) but it does not seem to have been used in theFar east until after European contact.33) The Romans werealso developing methods to recover silver lost in earlier pro-cesses, for example it is likely that the old Greek slags andtailings were reworked at Laurion.14) At Rio Tinto there isevidence for the processing of other smelting debris torecover the silver. When smelting ores that are rich in arse-

(a)

(b)

Fig. 12. (a) Excavated section of the East Lode timber revetment showing part of one of the lifts. Note the old ladder stilesreused as backing behind the vertical timbers. (P. T. Craddock). (b) Isometric drawing of the excavated section ofthe revetment. (B. R. Craddock).

© 2014 ISIJ 1092

ISIJ International, Vol. 54 (2014), No. 5

nic and antimony there is a strong possibility that a viscouslayer of iron arsenides and antimonides will form in the fur-nace, known as speiss. This is a problem in itself but unfor-tunately speiss absorbs the forming silver and thus there isa considerable potential loss. Fragments of speiss occur fre-quently in the slag heaps at Rio Tinto and usually containseveral thousand ppm of silver, but in one area well awayfrom the main heaps, large amounts of speiss are foundwhich is almost silver free, together with slags that are veryrich in arsenic. From another part of the site a large partiallyvitrified crucible was found which had a few percent of leadand large quantities of arsenic. This was reported as evi-dence for the treatment of the speiss to recover the silver.34)

Subsequently it was suggested that the crucible was in facta cupel,35) but reanalysis of the original sample confirms therelatively low lead content and the high arsenic, incompati-ble with use as a cupel.15)

Man’s quest for silver from the earliest times has involved

new technologies and great endeavours. From the develop-ment of sophisticated chemical treatments to the establish-ment of huge mining enterprises in remote locations alldemonstrate the determination to produce the maximumamount of silver possible.

REFERENCES

1) C. C. Patterson: Econ. Hist. Rev., 25 (1972), 205.2) P. T. Craddock: Early Metal Mining and Production, Edinburgh Uni-

versity Press, Edinburgh, (1995).3) C. E. Conophagos: Archaeometry, ed. by Y. Maniatis, Elsevier,

Amsterdam, (1989), 271.4) K. Hess, A. Hauptmann, H. Wright and R. Whallon: Metallurgica

Antiqua, ed. by Th. Rehren, A. Hauptmann and J. D. Muhly, DerAnschnitt, Beiheft 8, Bochum, (1998), 57.

5) E. Pernicka, Th. Rehren and S. Schmitt-Strecker: Metallurgica Anti-qua, ed. by Th. Rehren, A. Hauptmann and J. D. Muhly, DerAnschnitt, Beiheft 8, Bochum, (1998), 123.

6) P. Meyers: Patterns and Process, ed. by L. van Zelst, SmithsonianCenter for Materials Research and Education, Suitland, Md, (2004),271.

7) D. R. Hook, A. Arribas Palau, P. T. Craddock, F. Molina and B.Rothenberg: Bell Beakers of the Western Mediterranean, Part 1, ed.by W. H. Waldren and R. C. Kennard, BAR International, Series331(i), Oxford, (1987), 147.

8) M. Bartelheim, F. Contreras Cortés, A. Moreno Onarato and E.Pernicka: Trabajos de Prehistoria, 69 (2012), 293.

9) K. A. Yener, E. V. Sayre, E. C. Joel, H. Özbal, I. L. Barnes and R.H. Brill: J. Arch. Sci., 18 (1991), 541.

10) L. Willies: Bull. Peak District Mines Hist. Soc., 10 (1987), 81.11) O. P. Agrawal, H. Narain and J. Prakash: J. Hist. Met. Soc., 24

(1990), 81.12) C. E. Conophagos: Le Laurion Antique et la Technique Grecque de

la Production del’Argent, Ekdotike Hellados, Athens, (1980).13) E. Photos-Jones and J. Jones: BSA, 89 (1994), 307.14) Th. Rehren, D. Vanhove and H. Mussche: Metalla, 9 (2002), 27.15) P. T. Craddock: Acta Congresso Tartesso, ed. by J. Alvar and J. M.

Campos Carrasco, Almuzara, Huelva, (2013), 231.16) L. Willies: Bull. Peak District Mines Hist. Soc., 13 (1997), 1.17) D. Williams: Trans. Inst. Min. Met., 59 (1950), 509.18) L. Anguilano, Th. Rehren, W. Müller and B. Rothenberg: Archaeomet-

allurgy in Europe 2007, ed. by A. Giumlia-Mair and A. Hauptmann,Associazione Italiana di Metallurgia, Milano, (2009), 21.

19) P. T. Craddock, I. C. Freestone, N. H. Gale, N. D. Meeks, B.Rothenberg and M. S. Tite: Furnaces and Smelting Technology inAntiquity, ed. by P. T. Craddock and M. J. Hughes, British MuseumOcc. Paper 48, London, (1985), 199.

20) S. Rovira and M. Hunt: 34th Int. Symp. on Archaeometry, CSIC,Zaragoza, (2006), 217.

21) V. Kassianidou: Mining and Metal Production through the Ages, ed.by P. T. Craddock and J. Lang, British Museum Publications,London, (2003), 198.

22) V. Kassianidou, B. Rothenberg and P. Andrews: ARX, World J.Prehistoric Ancient Stud., 1 (1995), 17.

23) P. T. Craddock: Engineering and Technology in the Classical World,ed. by J. P. Oleson, Oxford University Press, Oxford, (2008), 94.

24) J. Cribb: South Asian Archaeology 1983, ed. by J. Schotsmans andM. Taddei, Istituto Universitario Orientale Series Minor 23, Naples,(1985), 35.

25) J. Cribb: South Asian Stud., 19 (2003), 1.26) P. Prasad and N. Ahmad: Archaeometallurgy in India, ed. by V.

Tripathi, Sharada Publishing House, Delhi, (1998), 184.27) P. T. Craddock, I. C. Freestone, L. K. Gurjar, A. P. Middleton and L.

Willies: 2000 Years of Zinc and Brass, ed. by P. T. Craddock, BritishMuseum, London, (1998), 27.

28) P. T. Craddock, C. Cartwright, K. Eckstein, I. Freestone, L. Gurjar,D. Hook, A. Middleton and L. Willies: Brit. Museum Tech. Bull., 7(2013), 79.

29) W. Höller and S. M. Gandhi: Can. Mineral., 33 (1995), 1047.30) W. Gowland: Archaeologia, 57 (1901), 359.31) S. Rovira and M. Renzi: Acta Congresso Tarteso, ed. by J. M. Campos

Caerrasco and J. Alvar Ezquerra, Almuzara, Huelva, (2013), 473.32) P. T. Craddock: Instrumentum, 24 (2006), 40.33) E. Izawa: Metallurgy and Civilisation, ed. by J. Mei and Th. Rehren,

Archetype Publications, London, (2009), 163.34) P. T. Craddock, I. C. Freestone and M. Hunt Ortiz: IAMS Bull., 10

(1987), 8.35) V. Kassianidou: Metallurgica Antiqua, ed. by T. Rehren, A. Hauptman

and J. D. Muhly, Der Anschnitt, Beiheft 8, Bochum, (1999), 69.

Fig. 13. SEM photomicrograph of slag 32 274 from Dariba Site 7,showing large relict potassium feldspar (dark) with Zn–Fe–S and PbS inclusions (bright spots) surrounded bydendritic olivine and hyalophane (grey tones) (cf the free-silica slag Fig. 2).

Fig. 14. Plot of Al2O3 against FeO for the Dariba and Aguchaslags. The Dariba copper slags are post-Medieval andtheir aluminium contents are lower than those in the ear-lier silver lead slags as they tend to have come from thecalc silicates rather than the graphite schists.