Strontium isotopes in the investigation of early glass production 19Archaeometry 45, 1 (2003) 19–32. Printed in Great Britain

* Received 29 November 2001; accepted 6 August 2002.© University of Oxford, 2003

STRONTIUM ISOTOPES IN THE INVESTIGATION OF EARLYGLASS PRODUCTION: BYZANTINE AND EARLY ISLAMIC

GLASS FROM THE NEAR EAST*

I. C. FREESTONE and K. A. LESLIE

Department of Scientific Research, British Museum, London WC1B 3DG, UK

M. THIRLWALL

University of London Radiogenic Isotope Facility, Department of Geology,Royal Holloway University of London, Egham, Surrey TW20 0EX, UK

and Y. GORIN-ROSEN

Israel Antiquities Authority, P.O.B. 586, Jerusalem 91004, Israel

87Sr/86Sr ratios have been determined for glasses from four production sites, dated tobetween the sixth and the 11th centuries, in the Eastern Mediterranean region. On thebasis of elemental analyses, the glasses at each location are believed to have been meltedfrom different raw materials. Two glass groups, from Bet Eli‘ezer and Bet She‘an, in Israel,are believed to have been based upon mixtures of Levantine coastal sands and natron,and have 87Sr/86Sr ratios close to 0.7090, plus high elemental strontium, confirming a highconcentration of modern marine shell (87Sr/86Sr ~ 0.7092) in the raw materials. The isotopiccompositions of these two groups of glasses differ slightly, however, probably reflecting avarying ratio of limestone to shell because the sands that were utilized were from differ-ent coastal locations. Natron-based glasses from a workshop at Tel el Ashmunein, MiddleEgypt, have 87Sr/86Sr values of 0.70794–0.70798, and low elemental strontium, consistentwith the use of limestone or limestone-rich sand in the batch. High-magnesia glasses basedon plant ash, from Banias, Israel, have 87Sr/86Sr values of 0.70772–0.70780, probably re-flecting the isotopic composition of the soils that were parental to the plants that were ashedto make the glass. Strontium and its isotopes offer an approach to identifying both the rawmaterials and the origins of ancient glasses, and are a potentially powerful tool in theirinterpretation.

KEYWORDS: GLASS, TECHNOLOGY, PRODUCTION, PROVENANCE, STRONTIUM, ISOTOPE,THERMAL IONIZATION MASS SPECTROMETRY, BYZANTINE, ISLAMIC, ISRAEL, EGYPT

INTRODUCTION

The major element compositions of ancient glasses have been extensively studied over the pasttwo decades (Henderson 1985; Wedepohl 1997; Brill 1999), while recently emphasis has begunto focus upon the trace elements (e.g., Mirti et al. 1993; Hunter and Heyworth 1998; Aertset al. 1999; Freestone et al. 2002b). In spite of the significant advances that have been made,in many areas our understanding of early glass production remains limited. In most cases we

20 I. C. Freestone et al.

are unable to determine where the glass was made, while even the nature of the primary rawmaterials may be unclear. New approaches to these questions are required.

Wedepohl and Baumann (2000) have recently utilized the isotopic ratio of strontium, 87Sr/86Sr, to argue that modern marine shell was deliberately added as a source of lime in the manu-facture of fourth-century Roman glass from Germany. Strontium isotopes used in this way offera powerful tool to identify the nature of the raw materials used to make glass. In the presentpaper, we further explore the potential of strontium isotope analysis by comparing the composi-tions of glass from known production centres, mainly unworked or ‘raw’ glass.

Strontium isotopes have been widely used in geochemistry to investigate the petrogenesis ofigneous rocks and processes in the Earth’s mantle. However, of particular interest in the presentapplication is the variation in the isotopic composition of marine carbonates (e.g., shell andlimestone) which, when deposited, reflect the composition of the ambient seawater. The stron-tium isotopic composition of seawater is believed to reflect predominantly the input from twocompeting fluxes: one from rivers, transporting eroded material from the continents, and theother from hydrothermal exchange with oceanic (basaltic) crust at the mid-ocean ridges (e.g.,Dickin 1995). The balance between these sources is itself a reflection of the ocean spreadingrate on the one hand, and the rate of continental weathering on the other, which itself dependsupon the disposition of mountain ranges, global climate and so on. It follows that there hasbeen a variation in the isotopic composition of seawater with time, and this is recorded in thecompositions of limestones, as shown in Figure 1, which is based upon Burke et al. (1982). Itshould be noted that the residence time of strontium in the oceans is long, relative to the rate ofocean mixing, so that geographical variations in the strontium isotope composition of seawaterare negligible. For example, the mean and standard deviation of 15 Holocene shells collectedworldwide are given by DePaulo and Ingram (1985) as 0.709234 ± 0.000009. The chronologicalvariation has therefore led to the development and application of strontium isotope stratigraphyfor the dating and correlation of sediments on the basis of the strontium isotope composition oftheir carbonates (e.g., McArthur 1992). From the present perspective, a particularly noteworthyfeature of Figure 1 is the rapid increase in 87Sr/86Sr over the past 40 million years or so. This isattributed to the uplift and erosion of the Himalayas, which contain rocks with high 87Sr/86Sr,

Figure 1 Variation in the strontium isotope composition of seawater versus time, based on Burke et al. (1982)(drawing by A. Simpson).

Strontium isotopes in the investigation of early glass production 21

which is transferred to the oceans via groundwater and surface run-off (e.g., Basu et al. 2001).Modern marine shell has a 87Sr/86Sr value equivalent to that of modern seawater, from which itis precipitated, and this is significantly higher than, for example, the value for Cretaceouslimestone, where the latter has undergone little diagenetic alteration (Fig. 1).

The application of strontium isotopes to the interpretation of ancient glasses depends prim-arily upon the assumption that the bulk of the strontium of many glasses is incorporated withthe lime-bearing constituents in the glass (Wedepohl and Baumann 2000). These lime-bearingcomponents are likely to be, on the one hand, shell or limestone, comprising a mineral polymorphof calcium carbonate (i.e., aragonite and/or calcite) or, on the other, plant ash, which is usuallylime-rich (e.g., Brill 1970; Verità 1985). Where CaCO3 was derived from Holocene beach shell,the 87Sr/86Sr ratio should reflect that of modern seawater and be close to 0.7092 (Fig. 1). If, onthe other hand, the strontium was incorporated in the glass in the form of limestone, then it willhave an isotopic signature that reflects that of the seawater at the time the limestone wasdeposited, modified by any diagenetic alteration that might have occurred to the limestone overgeological time. For a glass made using plant ash, the 87Sr/86Sr value will reflect the bio-available strontium from the soils on which the plants grew. Ideally, this would give a guide tothe geological origins of these materials, a principle that has been exploited in attempts to tracethe habitats and movements of humans or animals from the analysis of archaeological bone ortooth (e.g., Horn et al. 1994; Price et al. 1994).

A limitation of this approach is the tendency of 87Sr/86Sr of limestone to change as thelimestone undergoes diagenesis. Although limestone is a large reservoir of strontium, cationexchange with groundwaters and clay minerals will shift the isotopic ratio from the idealseawater curve, as shown by the analysis of geologically very old limestones (i.e., Devonian toCretaceous rocks—see Table 3 of Hoefs and Wedepohl 1968). To ensure that, for example,glasses containing strontium derived primarily from beach shell are correctly distinguishedfrom those containing strontium derived primarily from limestone, with 87Sr/86Sr that had beendiagenetically modified, a second approach is required.

Not only the isotopic ratio but also the elemental content of strontium in a glass can be auseful indicator of the source of the lime, a characteristic also utilized by Wedepohl andBaumann (2000) in their discussion of Roman glass-making in the Eifel area. During diagenesis,metastable aragonitic CaCO3, in the form of shell and other biogenic material, is dissolvedand reprecipitated as calcite, which is the mineral characteristic of most limestones. While theoriginal aragonite may incorporate several thousand ppm of Sr from seawater, a lower distribu-tion coefficient, coupled with a lower concentration of Sr in meteoric water, means that thereprecipitated calcite incorporates only a few hundred ppm of Sr. Conversion of aragonite tocalcite during the diagenesis of ancient limestone therefore results in the loss of a substantialproportion of Sr from the calcium carbonate. Thus, in principle, the Sr content of a glass mayindicate if the lime was added as shell (aragonite ± calcite, high Sr) or limestone (calcite only,low Sr).

On the basis of the foregoing arguments, the relative 87Sr/86Sr and Sr contents expected fromvarious glass-making raw materials are summarized in Table 1.

For this exploratory work, glasses from four different locations in the Near East, dating fromthe sixth to the 11th centuries ad, were selected. All are known glass-working or glass-makingsites, and the majority of the samples were chunks of raw glass. These samples were chosen onthe basis that their origins and technologies are reasonably well understood, so that the sourcesof calcium and strontium in the glasses were anticipated. Furthermore, each group is believed,on the basis of the archaeological context and elemental analysis, to represent a single set of

22 I. C. Freestone et al.

raw materials, even if the raw glass was not made in the workshop where it was found. Finally,the use of raw or chunk glass is expected to have minimized complications due to the mixingand recycling of old vessels (cullet), as glass in chunk form is believed in general to representglass made direct from raw materials in a primary workshop, for distribution to secondaryfabrication workshops (e.g., Freestone et al. 2002b).

THE SAMPLE

Low-magnesia or natron-based glass

The soda-rich glasses of the ancient Mediterranean and Near East fall into two main groups,as originally recognized by Sayre and Smith (1961). Glasses with MgO and K2O each below1.5%, termed low-magnesia, low-potash glasses (LMLK), are believed to have been preparedusing an evaporitic source of soda, such as the trona deposits of the Wadi Natrun, Egypt.Glasses of this type are commonly referred to as natron-based or, simply, natron glasses.Although use of the term ‘natron’ for the sodium carbonates of the Wadi Natrun is not strictlycorrect, as they are predominantly of trona, a different mineral, it has become entrenched in theliterature and will be used here. At the present time it is unclear just what proportion of the sodaused in LMLK glass was derived from the Wadi Natrun deposits. Pliny the Elder (Naturalhistory XXXI, 109—see Healy 1991) recorded that in Egypt soda was obtained by pouringNile water into pits (presumably inducing efflorescence of the soda in the soil); natron istherefore retained as a general term for inorganic soda, with no specific mineralogical orgeographical connotations intended.

Brill (1988) showed that natron-based glasses from the Roman workshop at Jalame, northernIsrael, could be made by mixing soda with a shell-bearing coastal sand, to produce a soda–lime–silica composition:

natron (Na) + sand (Ca, Si) ⇒ LMLK soda–lime–silica glass (Na, Ca, Si)

Other workers have concurred with this model (e.g., Nenna et al. 1997; Freestone and Gorin-Rosen 1999). In contrast, Wedepohl and Baumann (2000) have inferred that some fourth-century Roman glasses from the Hambach Forest, Germany, were made from a three-componentmixture, of natron, silica-rich sand and shell:

natron (Na) + sand (Si) + shell (Ca) ⇒ LMLK soda–lime–silica glass (Na, Ca, Si)

It should be noted that the account of Pliny (Natural history XXXVI—see Knoll et al. 1979;Healy 1991) may be read to support both models, as he refers both to the manufacture of glass

Table 1 Inferred relative strontium isotope compositions and total strontium in selected glass-making raw materials

Material 87Sr/86Sr Sr content

Beach shell Equivalent to that of modern seawater, c. 0.70917* HighLimestone Variable = ancient seawater, generally < 0.70917, plus diagenetic solutions LowPlant ash Equivalent to that of parental soil, variable ?

* Mean of 15 Holocene shells collected worldwide and measured by DePaulo and Ingram (1985), adjusted to 87Sr/86Sr for NBS

standard SRM 987 of 0.710248, as measured here.

Strontium isotopes in the investigation of early glass production 23

using natron and the sand from the beach near the mouth of the Belus River (Bay of Haifa)which is calcareous, and also to the addition of shell to glass.

Building on earlier work (e.g., Brill 1988; Gratuze and Barrandon 1990) at least five majorproduction groups of natron-based (LMLK) glass have been recognized to have been in use inthe Levant and Egypt in the second half of the first millennium ad (Freestone et al. 2000,2002a,b). Three glasses from each of three of these groups were selected for the present study:• Bet Eli‘ezer. Seventeen tank furnaces, each of which would have produced an estimated

8 tonnes of glass in each firing, were excavated at Bet Eli‘ezer, near Hadera, Israel (Fig. 2:Gorin-Rosen 1995, 2000; Freestone and Gorin-Rosen 1999). These glasses are high in silicaand low in soda relative to many Levantine glasses of Roman and Byzantine date, but vesselsof this composition are known from the early Islamic administrative centre Ramla (Freestoneet al. 2002a), which suggests that production was ongoing in the early eighth century,although the dates of the beginning and the end of the production are currently unclear.The glass is thought to have been made from its raw materials using local coastal sand fromthe Bet Eli‘ezer region (Freestone and Gorin-Rosen 1999). The compositional type is alsocommon in early Islamic glass from Sepphoris, Israel (data of Fischer and McCray 1999) andpreliminary reports suggest that a similar high-alumina, low-soda natron-based glass alsooccurs at Raqqa, Syria, dating to the eighth–ninth centuries (Henderson 1999), although it isnot yet clear if this glass is identical to that from Bet Eli‘ezer.

• Bet She‘an. A glass workshop has been excavated in Bet She‘an, Israel (Fig. 2: Mazor andBar-Nathan 1998) dating to the sixth–seventh centuries ad (Gorin-Rosen 2000). Large num-bers of raw glass chunks, probably made elsewhere and brought to the site to be worked intovessels, were found. These are of a compositional group which has been termed elsewhereLevantine I, and is found in northern Israel from the fourth to the seventh centuries (Freestoneet al. 2000). This group includes glass from the workshops at Jalame and, following thearguments presented by Brill (1988), we assume that they were made using the famous Belussand source, or a similar sand from the Levantine coast. A key question relates to whetherthese glasses may be distinguished from the Bet Eli‘ezer glasses on the basis of their isotopiccomposition.

• Tel el Ashmunein. Major element data for glasses from an Islamic workshop at Tel elAshmunein, Middle Egypt, were reported by Bimson and Freestone (1985). These natron-based glasses are rather high in lime and low in alumina, and correspond to the Egypt IIgroup of Gratuze and Barrandon (1990; see also Freestone et al. 2000). Since the originalreport, the glass has been re-dated on typological grounds to the eighth–ninth centuries,contemporary with coin weights made of similar glass (Gratuze and Barrandon 1990).Although it is questionable if the raw glass was made at Tel el Ashmunein, it was certainlyworked there, and in addition to two vessel fragments a lump of glass fused to the limestonefloor of the workshop has been analysed (Bimson and Freestone 1985). Glass of this com-position is not only found widely in Egypt and Israel, but it has been identified in smallquantities from a ninth-century workshop in Italy (Freestone and Henderson in progress).

High-magnesia or plant-ash glass

The second major category of early soda–lime–silica glass identified by Sayre and Smith(1961) contains MgO and K2O in excess of about 1.5% each, and is termed high-magnesia,high-potash (HMHK) glass. HMHK glass was produced using the ash made by burning plantsfrom salt-rich environments, either desert or brackish water. These plant ashes (Brill 1970;

24 I. C. Freestone et al.

Figure 2 A map showing the sites in Israel mentioned in the text (drawing by A. Simpson).

Strontium isotopes in the investigation of early glass production 25

Verità 1985) were rich in soda, but also in lime, and to a lesser extent potash and magnesia. Incombination with a lime-poor silica source, they were used to produce a soda–lime–silica glasswith moderate potash and magnesia:

plant ash (Na, Ca, Mg, K) + sand/pebbles (Si)⇒ HMHK soda–lime–silica glass (Si, Na, Ca, Mg, K)

HMHK glass from a single site was selected:• Banias. The remains of a glass workshop, including chunks and wasters, have been found at

Banias, Israel, a site being excavated by Tsaferis (Fig. 2: see Tsaferis and Israeli 1994) and aredated to some time between the 10th/11th and 13th centuries ad. These are high-magnesia,high-potash glasses, and were produced using plant ash as a source of soda, rather thannatron. Major and trace element data are provided by Freestone and Gorin-Rosen (1999),who estimate the composition of the ash used to make the glass, and Freestone et al. (2000).

METHODS

Three glass samples were selected from each of the four sites. With the exception of Tel elAshmunein, where two samples were of vessels, all were of raw or chunk glass. All have beenanalysed for major elements by X-ray microanalysis in the scanning electron microscope (EDXA).With the exception of Bet She‘an, the investigation of which is in progress, these data havepublished elsewhere (Bimson and Freestone 1985; Freestone and Gorin-Rosen 1999; Freestoneet al. 2000), but they are reproduced in Table 2. Eleven of the 12 samples have also beenanalysed for trace elements by inductively coupled plasma mass spectrometry (ICP-MS, Freestoneet al. 2000) and/or neutron activation analysis (NAA, unpublished analyses by M. J. Hughes).Comparison of the strontium contents of five glasses measured by both techniques gave a meanrelative difference of 11% in the strontium contents. Potential errors of this order are acceptablefor the present purpose, as will be seen. The remaining sample (23247S, from Ashmunein) wasanalysed for strontium using air-path energy-dispersive X-ray fluorescence analysis, calibratedagainst the Sr contents of the previously measured glasses; this result is considered semi-quantitative only. The selection of samples for isotope measurement was informed by theelemental analyses, care being taken to ensure that the samples were representative of theirrespective groups.

The surfaces of the glass sample were carefully removed with a diamond burr to eliminatecompositional changes due to weathering. Glass fragments weighing approximately 100 mgwere carefully cleaned to remove contaminants and then digested in nitric and hydrofluoricacids, evaporated to dryness and converted to chlorides using a two-step acidification withHNO3 and HCl. Solutions in 2.5M HCl were then run through conventional cation exchangecolumns, the Sr-bearing fractions being collected and dried on to degassed single tantalumfilament beads using phosphoric acid. The beads were analysed for strontium isotopes using amulti-collector VG354 thermal ionization mass spectrometer. Accuracy was monitored usingNBS standard SRM 987, which yielded 87Sr/86Sr = 0.710248 ± 15 (2σ on all analysed standards,N > 130) over the year in which the samples were analysed.

RESULTS AND DISCUSSION

The 87Sr/86Sr ratios are presented in Table 2, along with major element data. The initial observa-tion is that the Sr isotope data for glasses from each site form a tight, discrete group. The data

26I. C

. Freestone et al.

Table 2 Strontium isotope ratios and selected chemical data

Site Period Sample no. 87Sr/86Sr Sr SiO2 TiO2 Al2O3 FeO MnO MgO CaO Na2O K2O P2O5 Cl SO3

Bet Eli‘ezer 6th–8th 6831-1-Q 0.708979 322 76.0 <0.1 3.3 0.5 0.1 0.6 6.0 12.1 0.4 <0.1 0.7 0.2Bet Eli‘ezer 6th–8th 6831-3-X 0.709050 346 75.0 0.1 3.4 0.5 <0.1 0.6 7.3 11.4 0.5 <0.1 0.7 0.1Bet Eli‘ezer 6th–8th 6831-2-T 0.709053 420 73.7 0.2 3.7 0.9 <0.1 0.7 6.7 13.0 0.6 <0.1 0.5 0.1Bet She‘an 6th–7th 6831-217-Q 0.708912 494 69.7 0.1 3.0 0.5 <0.1 0.6 8.8 15.5 0.6 <0.1 0.9 0.2Bet She‘an 6th–7th 6831-226-T 0.708937 503 70.6 0.1 3.1 0.4 <0.1 0.6 8.1 15.5 0.6 <0.1 0.9 0.1Bet She‘an 6th–7th 6831-221-W 0.708956 456 70.6 0.1 2.8 0.4 <0.1 0.6 8.0 15.8 0.6 <0.1 0.9 0.2Tel el Ashmunein 8th–9th 23249Z 0.707959 143 69.8 0.3 2.5 0.8 <0.2 0.6 9.5 14.7 0.2 <0.1 1.1 0.1Tel el Ashmunein 8th–9th 23247S 0.707940 200* 67.8 0.2 1.6 0.5 0.7 0.3 10.9 15.3 0.1 <0.1 1.1 0.2Tel el Ashmunein 8th–9th 23248Q 0.707979 158 68.6 0.3 2.3 0.8 <0.2 0.6 10.1 15.3 0.2 <0.1 1.2 0.2Banias 10/11th–13th 6831-61-T 0.707723 403 70.7 0.1 0.9 0.5 1.0 2.8 7.8 12.8 1.9 0.3 0.9 0.3Banias 10/11th–13th 6831-62-X 0.707797 393 71.7 0.2 0.9 0.3 1.0 2.7 7.6 12.8 1.8 0.3 0.9 0.3Banias 10/11th–13th 6831-63-K 0.707734 374 68.9 0.1 1.0 0.4 0.9 2.6 10.2 12.6 1.8 0.3 0.8 0.3

Notes: Major elements by SEM-EDXA (Bimson and Freestone 1985; Freestone et al. 2000). Sr by ICP-MS in Bet Eli‘ezer and Ashmunein (Freestone et al. 2000), except * by

semi-quantitative X-ray fluorescence (see text) and Bet She‘an and Banias by NAA (unpublished analyses by M. J. Hughes). Data in weight per cent except for Sr, which is in ppm.

Strontium isotopes in the investigation of early glass production 27

600

500

400

300

200

100

00.7075 0.7080 0.7085 0.7090 0.7095

87Sr/86Sr

Sr

(pp

m)

Bet Eli‘ezer

Bet She‘an

Ashmunein

Banias

Germany

Plant Ash

Limestone-bearing sand

CoastalSand M

odern Ocean W

ater

Figure 3 Strontium contents and isotopic ratios for the glasses analysed, related to sources of strontium/lime.The ‘Germany’ data are from Wedepohl and Baumann (2000) and are late Roman glasses from Hambach andGellep, which they attribute to glass furnaces at Hambach.

for Bet She‘an and Bet Eli‘ezer are close together, and well separated from Tel el Ashmuneinand Banias, which have lower 87Sr/86Sr. The data are plotted against the elemental Sr contentsof the glasses in Figure 3, which also shows the data of Wedepohl and Baumann (2000) forRoman glasses from Hambach and Gellep in Germany. The elemental data reflect the tightisotopic grouping.

The present interpretation of the data is based upon the assumption that strontium in thenatron-based glasses is derived almost completely from the calcium carbonate in the startingmaterials, and that the possibility of a major contribution from natron, which consists essen-tially of soluble sodium salts, is unlikely. Strontium can, of course, substitute for calcium inother minerals, such as feldspar. However, it should be noted that even if all of the aluminiumin a glass with around 3% Al2O3 had been present in the glass-making sand as the pure calciumfeldspar end-member anorthite (CaAl2Si2O8), this would mean that only about 1.6% CaO waspresent as silicate, which amounts to less than 25% of the total lime of the glasses analysed(Table 2). This is a maximum, as most of the glasses have lower alumina and/or higher limethan assumed here. In practice, detrital feldspar is rarely dominated by anorthite, and thealumina in the sand will have been additionally supplied by other phases, including clayminerals, oxides and mafic minerals such as amphibole and pyroxene. Finally, it is also notedthat, according to Emery and Neev (1960), the feldspar contents of Israeli beach sands are onlyaround 3%, implying a maximum contribution to the sand of around 0.6% CaO, even if all thefeldspars should be anorthitic. It would therefore be very surprising if more than 10% of thelime in the glasses, and by implication the strontium, was derived from non-carbonate minerals.

It has been argued above and in detail in Freestone and Gorin-Rosen (1999) that plant-ashglasses such as those from Banias are likely to have been produced from a mixture of low-limesand or crushed quartz pebbles with plant ash. The strontium, along with the calcium, is thereforelikely to have derived from the ash itself in such cases.

The glass-making factory at Bet Eli‘ezer is situated a few kilometres from the coast and willalmost certainly have used coastal sand, most probably from the sand dunes located behind thesite, although possibly from the beach. The sands of the region contain lime in the form of shellfragments and also may be expected to contain fragments of kurkar (calcareous cemented dune

28 I. C. Freestone et al.

sand) from the cliffs. Indeed, the kurkar cliffs themselves, located close to the site, are potentialsources of raw material for glass-making, but their indurated form would have necessitatedgrinding before use, so that in practice they are not likely to have been favoured over suitabledune or beach sand. Holocene shell has an isotopic composition equivalent to that of modernseawater, at around 87Sr/86Sr = 0.70917 (DePaulo and Ingram 1985; Hess et al. 1986; adjustedin line with our mean value for NBS standard SRM987—see above). The Bet Eli‘ezer sampleshave 87Sr/86Sr ratios slightly below this value, at 0.708979–0.709053, fully consistent with aderivation of the lime from a sand in which the carbonates consist mainly of modern marineshell, with minor limestone or older shelly material (Fig. 3).

The Bet She‘an glasses are from a workshop for fabricating glass vessels and the glass wasprobably made elsewhere. As discussed above, they represent the Levantine I glass group,which includes late Roman and Byzantine glass made from Belus-type sand. The Bet She‘anglasses have 87Sr/86Sr of 0.708912–0.708956, slightly lower than the glasses from the tankfurnaces at Bet Eli‘ezer. The similar but differing Sr isotope signature suggests that the BetShe‘an glass was made from a different sand source, containing less shell and more limestone.This is consistent with the two groups having been made using sands from different locationson the Levantine coast, as might be expected.

Although they are also natron-based glasses, the Tel el Ashmunein samples have signific-antly lower isotopic ratios than those from Israel (Fig. 3). Clearly, they were made using adifferent sand source, as indicated by their distinctive elemental compositions (Freestone et al.2000, 2002a). The low isotope ratios indicate that modern marine shell was not the majorsource of the lime, which is most likely to have been limestone. This is supported by the datafor the total strontium contents of the glasses, which are significantly lower than those from BetEli‘ezer and Bet She‘an, suggesting that whereas the sands used to make the glasses from Israelwere beach sands, with high shell contents, the glasses from Ashmunein were made from aninland sand, containing limestone. It is also observed that the lower strontium contents of theAshmunein glasses are consistent with the assumption that the contribution of natron to theoverall Sr content is likely to have been low relative to that of the calcium carbonate. The low87Sr/86Sr ratios of the Ashmunein glasses would appear to correspond to those expected forTertiary limestone, probably of Oligocene or possibly late Eocene date. It is inferred that thelime-rich particles in the sand used to make the Ashmunein glasses were derived from geo-logical outcrops of Oligocene or late Eocene limestone. Outcrops of Oligocene limestone donot occur in the immediate region of Ashmunein, but further to the north, around the latitudeof modern Cairo. It therefore seems likely that the raw glass was not made at Ashmunein itself,which supports the interpretation of this site as a secondary workshop that specialized infabricating vessels using glass made elsewhere and traded in chunk form.

The Banias samples have substantially lower isotope ratios than the glasses from Bet Eli‘ezerand Bet She‘an, although they are similar to the Ashmunein values (Table 2, Fig. 3). In the firstinstance, it is assumed that the isotope ratio of the ash reflects the bio-available strontium of thesoil on which the plant grew, and that the soil in turn reflected the isotopic composition of theunderlying parent rock material. Then the ash used to produce the Banias glass could, inprinciple, have been produced from plants that grew on limestones of a number of geologicalages (Fig. 1). These include Jurassic limestones, which outcrop around Banias. Analysis ofgroundwater in the Banias area has given 87Sr/86Sr values of around 0.7072 (Starinsky et al.1980), significantly lower than the 0.7077 determined here for the glasses. This may mean thatthe Banias glass was made locally, using the ash of local plants, but that the composition of thesoil was not a precise reflection of the bedrock (e.g., Hamidi et al. 1999; Price et al. 2002), or

Strontium isotopes in the investigation of early glass production 29

that the glass was made locally using imported plant ash. It is more likely, however, that rawglass was not made from its raw materials at Banias, but that the glass chunks analysed wereimported to the workshop there, to be used in the fabrication of vessels.

It is of interest to note here that the Banias glasses, which are believed to have been madeusing plant ashes, have high strontium concentrations, of the same order as those of glassesmade from natron and coastal sand. Clearly, the Sr-bearing phases in plants (for example,calcium oxalates: Scurfield et al. 1973; Webb 1999) have capacities for strontium closer toaragonite than calcite. Contamination by wood ash, in firing or melting, might therefore insome circumstances significantly increase the strontium concentration of a natron-based glassand affect its 87Sr/86Sr ratio.

The overall pattern summarized in Figure 3, which shows the strontium isotope ratio againstelemental strontium, indicates that glasses based upon plant ash are distinguished from thosebased upon natron, and that natron glasses made using shell-bearing coastal sand may bedistinguished from those made from inland sands containing limestone fragments. It should benoted, however, that many geologically old rocks have high 87Sr/86Sr values, so that plantsgrowing on soils derived from such rocks would have 87Sr/86Sr values as high, if not higherthan, that of modern seawater. Such a glass made from high-87Sr/86Sr plant ash might showSr isotope characteristics and elemental Sr contents similar to those of natron glass made frombeach sand, or using shell as a source of lime, but would, of course, be readily distinguished byits minor element composition (K2O, MgO and P2O5 contents).

Turning to the glasses from Hambach and Gellep in Germany, analysed by Wedepohl andBaumann (2000), Figure 3 shows that they are close to Bet Eli‘ezer and Bet She‘an in terms oftheir 87Sr/86Sr ratios and Sr contents, although some of these glasses have lower 87Sr/86Sr valuesthan either of the sites in Israel. Wedepohl and Baumann used these strontium isotope data asone of their arguments that the glassmakers had added beach shell as a source of lime in theglass. The use of elemental and isotopic strontium data for this purpose was a very innovativeapproach, and it should be clear that the present paper was inspired by their study. However,the present work suggests that it is equally probable that the strontium in the German glasseswas the result of the use of beach sand, containing shell and some limestone, as a raw material,without the deliberate addition of shell.

CONCLUSIONS

The isotopic compositions of strontium in the glasses analysed here are fully consistent with theuse of raw materials previously inferred (Freestone and Gorin-Rosen 1999; Freestone et al. 2000).The glasses of the Levantine 1 group and those of the Bet Eli‘ezer furnaces were made fromLevantine coastal sands, while those from Ashmunein in Middle Egypt were made from aninland sand source, in which the source of lime was limestone rather than modern marine shell.This confirms the inference of Wedepohl and Baumann (2000) that there is a difference in thestrontium isotope compositions of glasses likely to have been made from materials containingmodern shell and those containing limestone. However, it is noted that isotope ratios close tothose of modern seawater need not indicate that shell was added to the glass batch intentionally,as a separate component. As suggested here, glasses made from shell-bearing coastal sands—for example, upon the coast of Israel—may impart similar isotope ratios to the glass. In termsof their strontium isotope compositions, the glasses analysed by Wedepohl and Baumann conformvery closely to glasses believed to have been made from coastal sands, although they differfrom the glasses from the Levantine coast in some other aspects of their compositions.

30 I. C. Freestone et al.

The isotopic compositions of glasses made using sands from different parts of the Levantinecoast will vary according to the relative quantities of limestone and shell in the sand. There isa potentially useful characterization tool available here for provenancing studies, althoughfurther work is needed to determine the variations that can be expected within and betweenglass batches. Gross differences in 87Sr/86Sr ratios are to be expected between glasses madefrom inland calcareous sands, where the isotope ratios will reflect those of the limestoneoutcrops contributing to the sediment, rather than marine shell. Some further work is requiredon the Sr content and isotopic ratio of natron and its contribution to that of the glass. However,the present results on glasses made from Israeli beach sands, compared with those of the natron-based glasses from Ashmunein, which were made with a limestone-bearing sand, suggest thatthe contribution of Sr from natron is not major.

Glasses based on plant ash appear to have high strontium contents, similar to those derivedfrom shell-rich sands. However, the Banias data suggest that the 87Sr/86Sr ratios are a reflectionof the parent material, and are very different from those of glasses with strontium derived frommodern marine shell. This characteristic should be useful in the determination of the origin ofthe lime in glasses where the starting materials are uncertain or controversial, such as the cobaltblue glasses of New Kingdom Egypt. Shortland and Tite (2000) have suggested, on the basis ofthe low potassium contents of these glasses, that their source of alkali was natron, as opposedto plant ash, which is considered to be the typical soda source in other Egyptian New Kingdomglass colours. This inference has been disputed by Rehren (2001). Strontium isotopes offer atool to resolve such issues. If natron was the sole alkali source, then the lime in the glasses,which would otherwise have been derived from plant ash, is likely to have been derived fromuse of calcareous (limestone-bearing) sand, from crushed limestone, or from crushed importedmarine shell. Determination of strontium contents and strontium isotope ratios of these glassesshould allow testing and elimination of a number of these possibilities.

For plant-ash glasses, and for glasses containing limestone-derived strontium, the Sr isotopedata would, once again, appear to offer a useful characterization tool, possibly in some casesproviding an indication of provenance. It is anticipated that the present approach may beextended to other materials of similar composition, such as faience and stonepaste ceramicbodies and alkaline or lead–alkaline glazes, and also to glasses of other types, such as the forestglasses of medieval Europe, which have high lime contents and are therefore likely to containsignificant strontium.

The present work confirms the potential of some of the possible applications for Sr isotopestudies in the investigation of glasses and other vitreous materials. New techniques for thedetermination of Sr isotopes, using laser ablation sampling and multi-collector inductivelycoupled plasma mass spectrometry, offer the potential for rapid measurements on small samples.As these techniques become widely available, Sr isotope measurements are likely to become animportant tool in the investigation of vitreous materials technology and provenance.

ACKNOWLEDGEMENTS

The idea for this project came following conversations with Professor K. H. Wedepohl, who wethank for his generosity and interest. The measurements were funded by the Renaissance Trust,as part of its generous support for the British Museum’s Glass After Rome programme. Wealso wish to thank A. J. Spencer and D. M. Bailey (British Museum) for the samples fromTel el Ashmunein, and Professor V. Tsaferis (Israel Antiquities Authority) for permission toanalyse the samples from Banias. G. Mazor and R. Bar-Nathan gave access to the material

Strontium isotopes in the investigation of early glass production 31

from Bet She‘an. Antony Simpson drew Figures 1 and 2. Dr Naomi Porat advised on the geo-logy of Israel and Caroline Cartwright advised on the mineralogy of plant materials. SheridanBowman, Michael Tite and two referees kindly commented on, and greatly improved, early draftsof this paper.

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