Lead Isotopes and Aegean Metallurgy

The purpose of the study of the isotopic composition of lead in archaeological artefacts is not to date them directly. Instead, the potential is as a 'fingerprint' technique bearing on such matters as the provenance of the artefact, trade routes and questions of authenticity. Artefacts suitable for study by this method include gold, silver, bronze and lead coins, statues, jewellery, weapons and tools, and glass, glazes and kohls. All such ancient artefacts normally contain small amounts of lead, which is not surprising in view of the simple refining methods used. Modern techniques permit the isotopic analysis of as little as 1 microgram of lead (1 microgram = 1 millionth of a gram) from an object which may contain as little as 0.01 % of lead.

It must be stressed that the technique is primarily negative, in that it provides evidence helping to eliminate an ore source, if the isotopic composition of the artefact differs from that of the ore, but it alone can rarely prove unequivocally that lead in an artefact came from one particular ore source. Nevertheless, the elimination of mineral areas may have far-reaching archaeological implications for assumed trade routes and the movement of peoples. Further the lead isotope technique together with chemical analysis and archaeological probability may allow proof of the provenance of lead in an artefact.

The chemical analysis of antiquities, aimed at providing information on exactly such topics, dates back about 150 years, and in the last 20 years the techniques have been greatly improved in the direction of ability to analyse for smaller and smaller concentrations, and in devising non-destructive methods of analysis. Indeed the value of chemical analysis to archaeology can hardly be doubted. Nevertheless the attempt to relate artefacts to their parent ores by chemical analysis alone suffers from the defect that trace elements (and some minor elements) may well be added or subtracted in rather arbitrarily varying amounts during the refining and smelting processes, even on a day-to-day basis. An example is the cupellation of argentiferous lead in silver refining, where elements such as tin and zinc can survive at any level between 0.001 % and 0.1 % (but rarely more) in bulk silver, whatever their original concentration in the ore (McKerrell and Stevenson 1972, 195).

In contrast, the isotopic composition of lead is not measurably affected by extraction or refining processes, so that whether the lead occurs as the major element in an antiquity (lead sheet, lead pigments, kohl etc.) or as a minor element (bronzes, silver, gold, glasses and glazes etc.) the isotopic composition of that lead will be the same as the composition of the lead in the ore from which it derives.

This characteristic would be of no utility were it not for the fact that terrestrial lead is one of the few elements which varies markedly in its isotopic composition in a manner dependent upon the geological and geochemical history leading to the formation of a particular lead ore (Doe 1970). Hence the isotopic composition of lead may vary from time to time and may be unique for each of a number of mines. It is apparent, therefore, that the isotopic composition of the lead in an antiquity may in principle be matched with the isotopic composition of lead in lead ores from ancient mining areas so as to trace the mine from which the lead in the antiquity was derived. Apart from this application to questions of provenance, ancient objects can be grouped together on the basis of the isotopic composition of their lead with obvious applications to questions of authenticity.

The reason for the variation in the isotopic composition of lead is that three of its four isotopes are partly radiogenic (i.e. partly derived from radioactive decay). The isotopes of ²⁰⁶Pb, ²⁰⁷Pb and ²⁰⁸Pb (but not ²⁰⁴Pb) in any present day lead may conveniently be thought of as composed of a mixture of two parts:

Present day lead = Primaeval lead + Radiogenic lead

The isotopic composition of lead existing at the time when the Earth was formed is termed primaeval lead. The radiogenic lead consists of the isotopes generated by the decay of naturally occurring radioactive isotopes of uranium and thorium in the time interval between the formation of the Earth (about 4.6 X 10 ⁹ years ago) and the formation of the lead ore body. Long radioactive decay chains are involved in the production of radiogenic ²⁰⁶, ²⁰⁷, ²⁰⁸Pb, but they may convenientiy be summarised as:

²³⁸U ---------- T½ ~ 4.5 X 10⁹ yr. ---------> ²⁰⁶Pb + 8 ⁴He + energy 

²³⁵U ---------- T½ ~ 0.7 X 10⁹ yr. ---------> ²⁰⁷Pb + 7 ⁴He + energy 

²³²U ---------- T½ ~ 3.9 X 10⁹ yr. ---------> ²⁰⁸Pb + 6 ⁴He + energy 

In contrast, ²⁰⁴Pb is not produced by radioactive decay.

The potential application of the isotopic composition of lead as a tracer for the origin of archaeological objects was recognised about ten years ago by Wampler and Brill (1964, 109 - 110) and by Grögler et al. (1966, 1167 -1172). A large amount of data has been accumulated by Brill and his associates on artefacts, together with a relatively small amount of data on the isotopic compositions of lead ores from regions which might have been exploited in ancient times (Brill and Wampler 1967, 63 - 77; Brill 1970, 143 - 164; Brill et al. 1971, 73 - 83; Brill and Shields 1972, 279 - 303; Barnes et al. 1974, 1 - 10; Brill et al. 1974, 9 - 25). It has gradually become apparent that the application to archaeology requires the determination of the isotopic composition of lead with high precision and accuracy, often in small samples (10^-6g. Pb), and that the analysis of large numbers of samples requires a simple low-blank method for separating the lead for isotopic analysis by mass spectrometry. The necessary technical developments have been made only in about the last three or four years (Barnes et al. 1973, 1881 - 1884; Arden & Gale 1974, 2 - 9), with the result that papers in this field prior to 1972 are largely of historic interest only.

The isotopic composition of a given sample of lead, as determined with high precision by modern mass spectrometry, is usually specified by one of the two sets of atomic ratios:

²⁰⁸Pb / ²⁰⁶Pb            ²⁰⁷Pb / ²⁰⁶Pb          ²⁰⁷Pb / ²⁰⁴Pb            A

or

²⁰⁸Pb / ²⁰⁴Pb            ²⁰⁷Pb / ²⁰⁴Pb          ²⁰⁶Pb / ²⁰⁴Pb            B

The ratios of set B increase whilst a lead sample exists in an environment containing significant amounts of uranium and thorium. For a particular sample the present-day ratios will be determined by the integrated effects of all associations with these radioactive elements between some initial time t and the time t after which the lead is no longer associated with uranium and thorium. To avoid a possible misconception, it should be re-iterated that the ratios ²⁰⁷Pb / ²⁰⁴Pb and ²⁰⁶Pb / ²⁰⁴Pb depend not only on the geological age of the lead ore but also on the U / Pb ratio.

Likewise, the ratio ²⁰⁸Pb  /²⁰⁴Pb depends on age and the Th / Pb ratio. In set A above, ²⁰⁸Pb / ²⁰⁶Pb depends on age and the Th  /U ratio, whilst ²⁰⁷Pb / ²⁰⁶Pb and ²⁰⁷Pb / ²⁰⁴Pb depend on age and on the U / Pb ratio.

The simplest possible history for, say, a galena is that the parent material for the galena suffered no chemical fractionation of U from Pb from the time (t_p) for formation of the Earth until production of the galena (t) : such a history is referred to as single-stage growth of lead in a closed chemical system and can be characterised by a single-stage growth curve with associated μ-value [(²³⁸U / ²⁰⁴Pb), as measured today] for the parent U-Pb system.

It was first proposed by Stanton and Russell (1959, 588 - 607) that certain, possibly syngenetic, ore leads, apparently associated in a conformable way with rocks of island-arc-volcanic affiliation, fitted remarkably well to a single-stage growth curve. This curve has come to be known as the primary growth curve for terrestrial leads. Since near-surface concentrations of U, Th and Pb are very heterogeneous, the conformable Pb ores were considered to be the product of the lower-crustal or upper-mantle environment. Recent very precise lead isotope measurements of conformable ores (Stacey et al. 1969, 15 - 25) have established very well the Pb-isotope patterns of such ores, which are plotted in the way conventionally adopted by isotope geologists in Figure 1, but it should be mentioned that modern analysis of the systematics of the primary growth curve (Stacey and Kramers 1975, 207 - 221; Russell and Birnie 1974, 158 - 166) requires a more complex history for the galenas involved.

For the application to archaeology, however, the simplicity of the gross systematics of these conformable galenas remains, together with the relative constancy of the isotopic composition within each of these galena bodies. In contrast some ore leads, including those from vein deposits, exhibit more complex isotopic patterns; such leads often have isotopic ratios which vary within an ore body or mining district and that fit along straight lines, (so-called secondary isochrons).

These leads are often termed 'anomalous' or (better) multistage leads. Russell et al. 1964; 680 - 705 have published a compilation of such leads, and show that the Th / U ratios of the source regions inferred from them form a distribution similar to that observed by direct measurement in crustal rocks. Lead compositions falling on such secondary isochrons are considered to be mixtures in various proportions of more primitive crustal lead with more highly evolved lead. The array of lead compositions would define chords in Figure A between the two end-member compositions which would normally lie on the primary growth curve.

In summary, each mine or mining district will be characterised either by a single lead isotopic composition or by a small (linearly related) range of compositions.

As mentioned above, once the ore deposit has been emplaced the lead isotopic composition will not be changed by weathering, extraction or refining, so that the isotopic composition of the lead in an antiquity will in principle give a unique indication of its provenance.

The most serious problem in the application of lead isotope ratios to archaeology is the re-melting and mixing of metals originating from different geographical areas. Such mixing will invalidate both lead isotope and chemical analyses. For example, at an early date many coins of the Roman Empire were struck in Rome, where silver and copper from all parts of the Empire were mixed. With glazes, one does not have this problem of mixing and re-melting.

A further difficulty is that it is geologically probable that lead ores from some quite different geographical regions will prove to have lead isotopic compositions  that are indistinguishable even when the most precise modern techniques are used; indeed our own work (see Figure 2) has now demonstrated this to be the case. It may prove in some cases (silver cupelled from lead is an example) that chemical analyses will provide extra evidence to eliminate some isotopically possible ore sources, but in other cases an appeal to archaeological plausibility may be essential.

Yet another difficulty is that there may be isotopic variation within a given mining region, although no examples of this have yet been proved in archaeologically important areas. There are two well-documented modern studies - Laurion (Barnes et al. 1974, 1 - 10) and Bleiberg (Köppel and Kostelka 1976) - where excellent isotopic homogeneity has been demonstrated throughout the mining district. From our own studies we have limited data which suggests that there is isotopic homogeneity within the Laurion, La Union, Cartagena, Pontokerasia (Macedon) and Siphnos mining regions. 

We have analysed galenas from Greece, Turkey, Italy, Spain, Austria, Persia, Yugoslavia, Egypt, Cornwall etc., as a guide to the variation of isotopic composition to be expected. Figure 2 shows some of the data. It is important to realise, in attempting to interpret the data recorded in Figure 2, that the mass spectrometric technique employed (which utilises silica gel as an ionisation enhancer) permits the measurements of isotopic ratios to about ± 0.1 %, as demonstrated by Chamberlain and Gale (1976) by measurements on a lead isotopic standard.

In other words, ratios falling within about 0.2 % of each other are isotopically indistinguishable - though we have found that repeat analyses on artefacts usually give the same results within ± 0.05 %.

It is clear from Figure 2 that considerable caution will need to be exercised in interpreting lead isotope data as applied to archaeology. The figure shows numerous examples of isotopically indistinguishable ores from quite different geographical regions; one can cite, inter alia, the groups Yukari Maden (Turkey), Derbyshire and Joachimsthal (Bohemia); Pontokerasia, Madem Lakos, Olympias and Vouno (Euboea).

On the other hand five yellow glasses and one yellow glaze from Amarna (XVIII Dynasty) have unusual (unradiogenic) isotopic compositions which (of the ores so far analysed) are similar (but not identical) only to Egyptian lead ores; for example the Um Gheig, Eastern Desert, galena analysed by ourselves and by Brill et al. (1974a, 9 - 25) and the Zog-el-Bohar galena. The Egyptian kohls analysed by Brill et al. (1974a) also fall in this field of isotopic composition. Interestingly enough, a pin from the Middle European Unetice culture also falls in this region of isotopic composition, and is the first example to be reported of an apparently non-Egyptian artefact with an "Egyptian" lead isotope composition.

One of the most satisfactory conclusions from the work of Chamberlain and Gale (1976) was that archaic silver Athenian owl coins (pre 475 B.C.) from the Asyut hoard contain lead which is indistinguishable from that of the Laurion (Attica) mines, giving a clear confirmation of their provenance. This work has now been extended and a brief account of it is in order because of the possibility that lead (and perhaps silver) from Laurion (or Thorikos) may have been used in the Aegean region in earlier times. The work of Brill et. al. (1974b, 1 - 10) first demonstrated that the isotopic composition of lead from modern Laurion galena and oxide samples, though essentially constant, had a small range of about 0.3 %.

We have extended the geographical and geological extent of the sampling of Laurian galena, and our data (together with part of that reported by Brill et al. 1974b) is given in Figure 3. Also plotted is the composition of the lead in Thorikos sample C.4.i.2, which is a mixture of cerussite and litharge from a large dump found by H. Musche in the Thorikos excavations next to an ancient ore washery in the 5th century B.C. level. The Oxford results plus those of Brill et al. (1974b) define very well the field of Laurian lead isotope composition, and it is satisfying that the 5th century B.C. ore sample from Thorikos falls within this field. Analyses of further lead samples (lead sheet, lead pottery repair rivets etc.) from 6th to 4th century B.C. levels at Thorikos is in hand, and will serve to establish whether lead worked in earlier times all falls in the same isotopic field as that from modern ores at Laurion.

That it does is rendered highly probable by the analyses by Brill and Shields of 7 Athenian bronze coins (4th to 1st. century B.C., see Brill and Shields 1972, 279 - 303 for descriptions) and at Oxford of 7 Archaic silver Athenian tetradrachms (from the Asyut hoard, pre 475 B.C.) and 3 2nd century B.C. silver Athenian tetradrachms.

Figure 4 shows the data for these coins, all of which fall within the Laurian field within experimental uncertainty. The Athenian currency decree (circa 450 - 420 B.C.) and the use of the Delian League treasure brought into Athens large amounts of silver of very diverse geographical provenance. It would be interesting to investigate in some detail the isotopic composition of lead in Athenian silver coins of this period to see if these events are recorded in a change of isotopic composition from the Laurian composition so far observed.

In order to form a basis of comparison for our work on lead isotopes in Aegean metallurgy we have analysed a number of galenas from Aegean islands. The results for four samples from the mine at Cape Athinios in Thera (and the field they define) have already been presented in Figure 3; in Figure 5 these results are shown together with our data for the other islands so far sampled. (It would be of great value to our work if further galena samples, or archaeological lead or silver samples, from the same and other islands could be made available to us). The samples from Siphnos come chiefly from inside the ancient mines at Ayios Sostis and were collected in 1977 by a team from Heidelberg and Bochum.

Figure 5 shows that the Laurian field of lead isotope composition is quite distinct from that of the Aegean islands so far sampled, except perhaps for Seriphos for which more measurements are needed to establish more precisely the range of composition.

Three lead artefacts (from the Ashmolean Museum) from the Cycladic Early Bronze Age have been analysed; two leaden boat models and a roughly vee shaped lead fragment. The boat models were first published by Renfrew (1967, 1 - 20) and are of great interest both from a metallurgical standpoint and because of the light they throw on Early Bronze Age shipping. They show quite clearly that the prow of the boats rises steeply to a point higher than the stern, which is canted at a more gentle angle.

Boat model 1929.26 was reputedly found in Naxos with two folded-arm marble figurines typical of the Keros-Syros culture. The find is said to have comprised four boats and three figurines; the Ashmolean Museum has three boats and two figurines. Boats 1938.725 and 1938.726 may have been part of the original find which included the figurines and boat 1929.26, but there is now no archaeological or written evidence to establish this. However the isotopic composition of lead from boat 1929.26 is identical (within experimental error) to that from boat 1938.726, so that the boats were certainly made from lead from the same source and it therefore seems very probable that they were part of the same find. The source of the lead for these boats is clearly not Naxos or Laurion and may perhaps be Siphnos (in which case the ores were worked on this island earlier than is commonly supposed), but it will be advisable to wait for analyses of lead from more of the islands (and of the third boat) before attempting to draw any positive conclusions. The lead object AE243 from Amorgos has an isotopic composition quite different from the lead boats, and within error of a Laurian composition; however no lead ores from Amorgos have yet been analysed.

Samples of only two artefacts from Thera were available for analysis at the time of writing this article; one sample (CBE17 / 1973 / 3582) of arsenical copper from a jug and one sample (Δ9,1 / 27 - 8 - 71) of lead. Figure 5 shows that these samples plot quite inside the Laurian field so that it seems likely that they were made of lead from Laurion (so far no lead ores from other localities have proved to have the same isotopic composition as those from Laurion). It is most important to have access to other samples of lead, bronze and silver from the excavations at Thera so as to be able to investigate their provenance in some detail.

Renfrew (1967; 1972) has stressed the importance of metallurgy as an index of cultural development, and has remarked on the contrast between the remarkable cultural intensity of the Early Bronze 2 period (circa 2500 B.C. to 2100 B.C.) and the preceding phases. Early Helladic II, Early Minoan II, Troy II and the Cycladic Keros-Syros culture are societies on the threshold of urbanisation, whose wealth is illustrated by rich finds, including gold and silver, such as at Zygouries in Greece, Mochlos in Crete, Troy and Poliochni in the East Aegean and Syros and Amorgos in the Cyclades. There appears to be no comparable intensity of activity again until the Late Bronze Age empires of the Minoans and the Mycenaeans.  

The development of metallurgy stimulated commerce and an international spirit new to the Aegean. Renfrew (1967, 1 - 20) has emphasized the speed with which the knowledge and practice of metallurgy spread, and that the Aegean in Early Bronze II times was not an empire based on one dominant region, but consisted of a number of autonomous cultures in active commercial contact with each other, going far beyond the earlier contacts which are documented only by transport of obsidian from Melos. The proof of these contacts is to be found partly in the many Cycladic objects found in foreign contexts, in Crete, Thermi, Samos and Iasos. The Keros-Syros culture has many links with other Early Bronze 2 Aegean cultures in Crete, on the Greek mainland and in the East Aegean at Thermi, Poliochni and in Troy II (Renfrew 1967).

Both lead (which occurs on nearly as many sites as copper) and silver were widely used in the Cyclades in Early Bronze 2 times, though it cannot be said that there is as yet much certain information as to the geographical source of either.

Lead certainly derives from galena or cerussite, but the earlier accounts of their occurrence in the Cyclades are largely unreliable. However recent investigations by geologists from the University of Utrecht confirm occurrences of galena on Seriphos, Naxos, Kythnos, Antiparos, Santorini, Kimolos, Milos and Poliegos, and elsewhere in the Aegean galena certainly occurs on Thasos, Lesbos, Samos and Crete; the silver content of these galena occurences remains largely undetermined at present. Lead was frequently used in pottery repairs, but also was made into figurines, bracelets and the boat models already mentioned.

Finds of Early Cycladic silver objects (diadems, dishes, bracelets, pins and beads) are common in the Cyclades although gold is rare; the silver objects are to be set in the Keros-Syros culture (Renfrew 1967), and collectively they testify to the wealth of this culture.

In addition to the study of lead isotope composition as a guide to provenance one can expect chemical analysis of Cycladic lead and silver artefacts to provide valuable additional clues to the source of the silver. Silver in ancient times can in principle be derived from three sources: native silver; silver ores, principally cerargyrite (AgCl); argentiferous galena. Patterson (Patterson 1971) has shown that native silver from North and South America, Norway, Australia and Italy is very pure, containing essentially no gold, lead, tin or bismuth; thus silver containing gold in the few per cent level probably does not derive from native silver sensu stricto. Such silver might have been produced by the use of a naturally occurring silver/gold alloy, although alloys with say 99% - 98% silver and 1% - 2% gold do not seem to occur naturally today. Production of the silver from naturally occurring alloys of composition approximating electrum could yield silver containing a few per cent of gold, but this presupposes an earlier knowledge of, say, the chloride cementation process for parting gold and silver (and subsequent reduction of the silver choloride to silver by heating with charcoal) than is commonly supposed.

Copper occasionally ranges up to 0.5%, antimony up to 4%, in native silver, but both are usually < .01%. Cerargyrite from North and South America and Australia commonly contains 0.01% of Cu, Au, Sn, Bi, but .01% to 2.5% Pb and .01% to 0.5% Sb (Patterson 1971). In early times only galenas rich in silver (from higher temperature deposits) could have been roasted and then cupelled for silver. Bismuth and tin tend to follow silver in galena, so that galenas rich in silver tend to have relatively higher Bi and Sn concentrations. (El Shazly et al 1956; Fleischer 1955).

It has been shown that in cupellation Au goes with Ag, remaining at any chosen initial level under all conditions, whilst Pb and Bi typically remain at the 0.5 - 1 % level irrespective of starting concentrations (McKerrell and Stevenson 1972). At first sight the presence of lead but no bismuth in silver points strongly to an origin from cerargyrite, whilst lead and bismuth together in a silver point to an origin from silver rich galena. A complication is that cerargyrite can occur with native silver. Although the cerargyrite would have been blackened by exposure to light, on scraping it would exhibit a soft silver-metallic appearance easily confused with metallic silver. If heated in a furnace with charcoal cerargyrite yields metallic silver with no residue. A mixture of native silver and cerargyrite, when fused together, would yield silver with reasonably high lead but low bismuth content. Chemical analyses of silver for Au, Bi, Pb, Cu, Sb, Sn, As should give an important guide to the ore source of the silver, whilst lead isotopic analyses coupled with the chemical analyses may well enable the geographic location of the ores to be determined. Much further work remains to be done in the attempt to solve these problems.

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