ARCHAEOMETRICAL INVESTIGATION OF SICILIAN EARLY BYZANTINE GLASS: CHEMICAL AND SPECTROSCOPIC DATA*

ARCHAEOMETRICAL INVESTIGATION OF SICILIANEARLY BYZANTINE GLASS: CHEMICAL AND

SPECTROSCOPIC DATA*

R. ARLETTI,1† C. GIACOBBE,2 S. QUARTIERI,2 G. SABATINO,2

G. TIGANO,3 M. TRISCARI2 and G. VEZZALINI1

1Department of Earth Sciences, University of Modena and Reggio Emilia, L.go S. Eufemia 19,I-41100 Modena, Italy

2Department of Earth Sciences, University of Messina, Salita Sperone, 31, I-98166 Messina S.Agata, Italy3Soprintendenza ai BB.CC.AA. di Messina, Sezione Archeologica, Viale Boccetta, 38,

I-98100 Messina, Italy

A series of early Byzantine glasses, recovered in Ganzirri (Sicily, Italy), was analysed formajor, minor and trace elements. All the analysed fragments were found to be natron-basedsilica–lime glass. Concerning minor and trace elements, the samples can be divided into twogroups: glass with high Fe, Ti and Mn contents (HIMT glass) and glass with low levels ofFe, Ti and Mn. These results, strictly in agreement with literature data for glass of theMediterranean region, can be interpreted as a consequence of the wide trade networkestablished in this region and of the intense circulation of raw glass and artefacts fromdifferent Mediterranean areas. X-ray absorption spectroscopy studies at Fe and Mn K-edges,performed on HIMT glass, indicate that Fe is in the oxidized form while Mn is in the reducedform.

KEYWORDS: GLASS, EARLY BYZANTINE, ITALY, CHEMICAL ANALYSES, EMPA,LA–ICP–MS, XAS

INTRODUCTION

In the late 1980s, during excavations aimed at the construction of residential buildings inGanzirri village (10 km north of Messina, Sicily, Italy), the ruins of a Late Roman–Byzantinesettlement came to light (Tigano 2002). The site is located at the foot of the hills near the saltlake called ‘Pantano Grande’, at the edge of the Stretto di Messina. Due to its position, thearea played an important role from the prehistoric era down to the Late Roman period. This isnot surprising, since this site contains almost the only loam in the whole area.

Archaeological studies of the site and of the ceramic artefacts recovered suggest that theGanzirri settlement was inhabited from the middle of the fourth century to the late seventhcentury ad. The reasons for abandonment of the site are still under discussion, but it is wellknown that this period was a time of demographic and economic crisis for the whole of theisland of Sicily. The large quantity of ceramic finds indicate the integration of Ganzirri into awell-developed trade system, receiving goods from both the North African and EasternMediterranean regions, as already attested for other Sicilian and Calabrian sites.

Along with the ceramic artefacts, some glass fragments were recovered during the excavations.The study of these glass finds represents an important opportunity to increase our knowledge

*Received 14 January 2008; accepted 20 October 2008†Corresponding author: email [email protected]

Archaeometry 52, 1 (2010) 99–114 doi: 10.1111/j.1475-4754.2009.00458.x

© University of Oxford, 2009

of glass manufacturing in the Late Roman–Byzantine age. In this period (from the fourth tothe seventh centuries ad) the glass composition, which was almost constant for a long periodof time in different and disparate regions (i.e., silica–soda–lime glass typical of the RomanImperial Age), starts to change and begins to show some differences, particularly linked tothe minor components. In particular, the so-called HIMT glass, characterized by high levelsof iron, titanium and manganese (Freestone 1994), begins to appear in western Europeanregions.

In this work we analysed eight fragments of common transparent glass dated from thefourth to the seventh centuries ad, coming from ‘casa 7’ of the Ganzirri site (Tigano 2002).They are characterized by different colours, ranging from completely colourless to light green,yellow–green olive and shades of light brown (Table 1). The chronological assignment of thesamples is based on a precise study of the stratigraphic context and on the form determinationof some of the glass finds, most of which were classified as Isings 111 (Isings 1957).

The interest of this study is based on the analysis of the complete set of trace elements andrare earth elements (REE). As will be demonstrated, the trace element contents are efficientindicators for a clear identification of HIMT glass, as suggested by Freestone et al. (2002b).The aims of the present work are as follows:(1) to provide a chemical characterization of the glass samples coming from the Ganzirri site;(2) to compare the data with those from literature concerning glass of the same period, fromdifferent geographical areas;(3) to demonstrate the utility of trace element composition in providing useful information toassist glass discrimination over and above the compositional groups adopted in the literature;and(4) to provide information on Fe and Mn speciation on selected samples, and to establishwhether Mn was intentionally added to control the colour of the glass or was present as animpurity in the sand.

EXPERIMENTAL

Electron microprobe analysis (EMPA)

Wavelength-dispersive electron microprobe analysis was used to determine the chemicalcomposition of major and minor elements. The analyses were carried out on polished samplesusing an ARL-SEMQ electron microprobe. The elements analysed were Si, Ti, Al, Mn, Mg,Fe, Ca, K, Na, Cr, Co, Sb, Cu and Sn. The following natural standards were employed:microcline (K, Al), albite (Na), spessartine (Mn), ilmenite (Fe, Ti), clinopyroxene (Si, Ca),olivine (Mg) and chromite (Cr). Metallic cobalt and metallic antimony were used for Co andSb calibration, while synthetic cassiterite and a Cu94Sn6 alloy were used for the calibration ofSn and Cu, respectively. The analyses were performed at 15 kV and 20 nA, with a spot size of30 mm—to prevent the loss of light elements under the electron beam—and using countingtimes of 5, 10 and 5 s on background, peak and background, respectively. Several points wereanalysed on each sample to test the homogeneity and the mean value of all the measurementswas calculated. The results were processed for matrix effects using the PHI(rZ) absorptioncorrection of the Probe program (Donovan and Rivers 1990). The measurement accuracyfor the elements analysed is better than 3%. The results are reported in Figures 1–4 andin Table 1, where an idea of the precision of the data is given by the standard deviationvalues.

100 R. Arletti et al.

© University of Oxford, 2009, Archaeometry 52, 1 (2010) 99–114

Tabl

e1

Che

mic

alda

tafo

rm

ajor

and

min

orel

emen

tsob

tain

edby

EM

PA(o

xide

wt%

).C

ran

dSn

wer

eno

tre

port

ed,s

ince

they

wer

ebe

low

the

dete

ctio

nli

mit

;n.

d.,n

otde

tect

ed

Sam

ple

Col

our

Num

ber

ofsp

ots

GN

Z1

Oli

ve-g

reen

6

GN

Z3

Yell

ow-g

reen

7

GN

Z4

Lig

htgr

een

6

GN

Z5

Oli

ve-g

reen

7

GN

Z6

Oli

ve-g

reen

6

GN

Z7

Col

ourl

ess

7

GN

Z8

Oli

ve-g

reen

6

GN

Z9

Lig

ht-b

row

n6

Wt%

Std.

Dev

.W

t%St

d.D

ev.

Wt%

Std.

Dev

.W

t%St

d.D

ev.

Wt%

Std.

Dev

.W

t%St

d.D

ev.

Wt%

Std.

Dev

.W

t%St

d.D

ev.

SiO

267

.10.

467

.30.

370

.50.

565

.10.

265

.10.

271

.90.

565

.70.

368

.50.

5A

l 2O

33.

870.

012.

770.

093.

110.

063.

390.

083.

50.

13.

620.

053.

10.

22.

740.

08T

iO2

0.49

0.01

0.15

0.02

0.06

0.01

0.56

0.04

0.50

0.02

0.11

0.04

0.59

0.01

0.14

0.03

MgO

1.41

0.03

1.3

0.1

0.41

0.02

1.26

0.09

1.3

0.1

1.03

0.04

1.07

0.05

1.25

0.04

FeO

1.69

0.00

0.99

0.04

0.27

0.03

1.91

0.07

3.69

0.08

0.56

0.02

1.7

0.1

0.74

0.02

MnO

1.77

0.06

1.94

0.06

0.01

0.01

2.36

0.08

1.8

0.1

0.03

0.05

2.2

0.1

0.07

0.03

CaO

5.67

0.01

7.8

0.2

7.7

0.3

6.0

0.3

6.7

0.2

8.4

0.5

6.1

0.3

8.8

0.2

Na 2

O17

.00.

117

.10.

416

.30.

318

.10.

515

.90.

514

.50.

718

.60.

515

.90.

7K

2O0.

780.

020.

910.

010.

930.

030.

570.

020.

740.

010.

790.

020.

410.

010.

980.

01C

oOn.

d.–

0.01

0.01

n.d.

–n.

d.–

n.d.

–n.

d.–

0.01

0.01

n.d.

–C

u 2O

0.01

0.01

0.02

0.02

0.01

0.01

0.01

0.02

n.d.

–n.

d.–

0.01

0.01

0.01

0.01

Sb2O

30.

040.

040.

090.

020.

090.

030.

090.

030.

070.

050.

090.

030.

040.

040.

080.

03To

tals

99.8

410

0.25

99.4

299

.30

99.4

010

0.93

99.5

299

.22

Archaeometrical investigation of Sicilian early Byzantine glass 101


Laser ablation – inductively coupled plasma – mass spectroscopy (LA–ICP–MS)

LA–ICP–MS was used to determine the chemical composition of trace elements. The analyseswere performed with a Thermo Electron X7 quadrupole based ICP–MS coupled with a fre-quency quintupled (l = 213 nm) Nd : YAG laser, installed at the Department of Earth Science,University of Perugia. The laser repetition rate and the laser energy density on sample surfacewere fixed to 10 Hz and ~10 J cm-2, respectively. The analyses were performed using a laserspot diameter of 70 mm on the same polished samples used for EMPA analyses. Externalcalibration was performed utilizing the NIST SRM 612 glass as external standard and 29Si,previously determined by EMPA, as internal standard following the method proposed by

Figure 1 K2O versus MgO (wt%) for the analysed samples.

Figure 2 CaO versus Al2O3 (wt%) for the analysed samples.



Figure 3 TiO2 versus FeO (wt%) for the analysed samples.

Figure 4 TiO2 versus FeO (wt%) (a) and TiO2 versus MnO (wt%) (b) for the Ganzirri samples and for glass of the sameperiod from Cyprus (Freestone 2002b), Rome and Carthage (Verità 1995) and northern Sinai (Freestone 2002a).



Longerich et al. (1996) and the analytical protocol described in Petrelli et al. (2008). Thereference material USGS BCR2G was used as a quality control. Precision and accuracy werebetter than 7% and 8%, respectively (Petrelli et al. 2008). The results are reported in Table 2and Figures 5 and 6.

X-ray absorption spectroscopy

Fe and Mn K-edge XANES (X-ray Absorption Near Edge Spectroscopy) spectra were collecteddirectly on the GNZ 5 and GNZ 6 glass fragments in fluorescence mode at the GILDA-CRGbeamline (ESRF, Grenoble, France). A dynamically and sagittally focusing monochromatorwith Si (311) crystals (Pascarelli et al. 1996) was used. Energy calibrations were achievedusing Fe and Mn foils as references, and the position of the first inflection point was taken at7112.0 and 6539.1 eV, respectively. A synthetic standard glass (ST1) with a chemical composi-tion of SiO2 = 72.0 wt%, Al2O3 = 1.6 wt%, Na2O = 13.2 wt%, K2O = 0.7 wt%, CaO = 9.7 wt%,MgO = 2.6 wt%, Fe2O3 = 0.15 wt% and FeO = 0.076 wt% (Quartieri et al. 2005 and referencescited therein) was used as reference for iron. The relative percentages of Fe2+ and Fe3+ weredetermined by EPR (Orsega and Geotti-Bianchini 2000). Pyrolusite (Mn4+O2) and tephroite

Table 2 Chemical data for trace elements obtained by LA–ICP–MS (ppm)

Sample GNZ 1 GNZ 3 GNZ 4 GNZ 5 GNZ 6 GNZ 7 GNZ 8 GNZ 9

Ga 4.4 3.1 3.4 4.2 4.6 3.4 3.5 2.7Rb 5.4 7.8 7.9 6.9 7.5 10.8 4.3 13.8Sr 450 904 643 521 640 495 489 461Y 12.3 9.7 8.3 12.3 18.6 7.0 12.1 7.3Zr 293 91 67 269 246 45 296 37Nb 7.2 3.0 2.5 6.7 6.4 2.0 6.4 1.3Cs n.d. 0.2 0.2 0.1 0.1 0.1 n.d. 0.2Ba 446 477 272 754 287 258 908 238La 10.7 8.8 7.6 11.5 18 7.0 10.3 6.5Ce 20 14 13.8 21 21 13.6 19 11.8Pr 2.6 2.0 1.8 2.7 4.3 1.7 2.5 1.6Nd 10.9 8.4 7.5 11.3 18 7.2 10.5 6.6Sm 2.3 1.8 1.5 2.4 3.8 1.5 2.2 1.3Eu 0.6 0.4 0.4 0.6 0.9 0.4 0.5 0.4Gd 2.3 1.8 1.3 2.3 3.7 1.4 2.1 1.2Tb 0.3 0.3 0.2 0.3 0.5 0.2 0.3 0.2Dy 2.2 1.7 1.4 2.1 3.4 1.2 2.1 1.2Ho 0.4 0.3 0.3 0.4 0.7 0.3 0.4 0.2Er 1.3 1.0 0.8 1.3 1.9 0.7 1.2 0.7Tm 0.2 0.1 0.1 0.2 0.3 0.1 0.2 0.1Yb 1.4 1.0 0.8 1.3 1.9 0.7 1.5 0.7Lu 0.2 0.1 0.1 0.2 0.3 0.1 0.2 0.1Hf 7.0 2.2 1.7 6.6 5.8 1.2 7.0 1.0Ta 0.5 0.2 0.2 0.5 0.5 0.1 0.5 0.1Pb 31 455 86 136 262 7.5 15 6.7Th 2.6 1.5 1.3 2.5 2.5 1.0 2.5 0.8U 1.2 1.2 0.7 1.4 1.6 1.0 1.6 0.5



(Mn2+SiO4) were used for manganese. The latter two standards were chosen as representativeof the two extreme oxidation states of Mn. As shown by McKeown et al. (2003) and Farges(2005), Mn3+ edge and pre-edge energies fall between those of Mn2+ and Mn4+, with energyshifts of about 3 and 0.5 eV, respectively. These differences in the XANES energy positionsguarantee the possibility of clearly distinguishing the Mn oxidation state in the samples understudy.

All the XANES spectra were collected at room temperature, with energy steps of 0.1 eV.The pre-edge background was subtracted from the spectra of samples and reference compoundsand then the spectra were normalized on the high-energy side of the curve. The analysis of thepre-edge region was then performed by least-square fitting of three pseudo-Voight functions tothe pre-edge spectral envelope, using the program PeakFit4. The average pre-edge informationwas derived by calculating its centroid (intensity-weighted average of the components positions).Fe and Mn K-edge spectra of samples and reference compounds are reported in Figure 7 andthe energy position of the main features in Table 3. The results of the detailed study of thepre-edge peaks and of their fits are reported in Figures 8 and 9.

CHEMICAL INVESTIGATION

Results

The chemical analyses of the major elements reported in Table 1 indicate that all the Ganzirriglass finds have a silica–soda–lime composition (Table 1), typical of the Western Mediterra-nean from the Roman to the Byzantine age, produced with siliceous–calcareous sands (e.g.,Turner 1956; Sayre and Smith 1961; Henderson 1985; Verità 1995; Freestone et al. 2002a,b;Arletti et al. 2005). The data reported in Table 1 and Figure 1 clearly show that all the Ganzirrisamples were produced with natron as the source of flux; in fact, the levels of MgO and K2Onever exceed 1.5%, while the Na2O contents are quite high and range from 14.5 to 18.6%.Natron was widely used in glass production from the six and seventh centuries bc up to theend of the first millennium ad, when it was substituted by plant ashes (Liliquist and Brill

Figure 5 REE average composition for the two Ganzirri groups of samples, normalized to the upper continental crustcomposition (Wedepohl 1995).



1995). Figure 2 shows that the levels of CaO and Al2O3 range from 5.67 to 8.8% and from2.74 to 3.87%, respectively. These values are quite common in silica–soda glass and reflect theimpurities present as carbonates and feldspars in the sands. However, it is worth noting that allthe glass samples with an olive-green colour (GNZ 1, 5, 6 and 8) fall in the lower portion ofthe diagram, hence showing lower levels of CaO. On the contrary, on the basis of the Al2O3/CaO ratio all the other samples fall within the compositional field of Levantine I glass recognizedby Freestone et al. (2000) on the basis of their study on raw glass chunks from a Byzantineglass-working site in Israel. Levantine I glass typology was later recognized in several archaeo-logical sites from the second half of the first millennium ad onwards.

In addition to this difference in the CaO levels, other differences in the chemical data ofGanzirri glass allow us to split the samples into two distinct groups, notwithstanding thehomogeneous content of most of the major elements. On the basis of Ti and Fe levels, it is

Figure 6 Trace element composition of samples from Ganzirri Group I (a) and Group II (b), normalized to the uppercontinental crust composition (Wedepohl 1995).



Figure 7 Normalized Fe (a) and Mn (b) K-edge for the analysed samples and reference compounds.

Table 3 Fe and Mn K-edge XANES feature positions (eV) in the glass samples and in the reference compounds;the letters a, b, c and d refer to Figure 7

Fe K-edge

Sample Pre-edge component (a) Pre-edgecentroid (a)

Shoulder (b) Shoulder (c) Edge (d)

GNZ 5 7114.9 7115.5 7116.3 7115.3 7119.9 7123.8 7133.2GNZ 6 7114.3 7115.3 7116.1 7115.2 7119.9 7123.9 7133.6ST1 (70% Fe3+) 7113.6 7115.2 7116.1 7114.7 7119.9 7123.6 7132.7

Mn K-edge

Sample Pre-edge component (a) Pre-edgecentroid (a)

Shoulder (b) Shoulder (c) Edge (d)

GNZ 5 6539.4 6539.9 6540.6 6539.9 6545 6550.6 6552.2GNZ 6 6538.9 6539.9 6540.7 6539.8 6545.5 6550.5 6552.7Tephroite (Mn2+) 6539.3 6539.7 6540.4 6539.8 6545.5 6550 6551.5Pyrolusite (Mn4+) 6540.5 6542.3 6542.3 6542.1 6552.8 6558.3 6560.4



Figure 8 Normalized Fe K-pre-edge spectra (dotted) and the best model calculated for the Ganzirri samples and for theFe-reference compounds. (a) ST 1; (b) GNZ 5; (c) GNZ 6.

Figure 9 Normalized Mn K-pre-edge spectra (dotted) and the best model calculated for the Ganzirri samples and for theMn-reference compounds. (a) GNZ 5; (b) GNZ 6; (c) tephroite; (d) pyrolusite.



possible to identify two well-separated groups of samples (Fig. 3): Group I (samples GNZ 3,4, 7 and 9), characterized by low Fe and Ti levels, and Group II (samples GNZ 1, 5, 6 and 8),characterized by high Fe and Ti levels. While the samples of the first group show variablecolours—from colourless to green or light brown—all the samples of the second group aredeep olive-green coloured. Table 1 clearly shows that, along with the high Fe and Ti contents,the samples pertaining to Group II also contain high levels of Mn. As a consequence, thechemical composition of Group II is strictly consistent with that reported for HIMT glass, firstrecognized and named by Freestone in 1994, after a study of raw glass chunks from Carthage.HIMT glass shows high levels of iron, manganese and titanium. Similar glass was lateridentified at many sites across Europe: Augusta Praetoria, Group E (Mirti et al. 1993); Rome(Verità 1995); Modena (Arletti et al. 2005); the Western Mediterranean area (Foy et al. 2000);Cyprus (Freestone et al. 2002b); northern Sinai (Freestone et al. 2002a); the United Kingdom(Freestone et al. 2005); and Germany and Belgium (Aerts et al. 2003). The issue of theprovenance and production of HIMT glass is still actively debated by the scientific community.It is certain that this material was not abundant in the Eastern region, whereas it was widelytraded in the Western Mediterranean area, which suggests that it was not produced on theLevantine coast (Freestone et al. 2002b).

Regarding the colouring elements, on the basis of the data reported in Table 1 it is evidentthat Ti, Fe and Mn are responsible for the colour of the samples. The levels of Co and Cu, infact, are very low and mostly lower than the detection limit, so they cannot play any role inthe final hue of the glass. The relatively low levels of Sb ensure that the presence of this elementis not associated with glass colour nuances. Since Sb was widely used as a decolourizer until thethird century ad, its occurrence in some of the Ganzirri samples, in amounts lower than 1000ppm, could be the sign of the recycling of older Sb-bearing glass. The colourless aspect ofsample GNZ 7, where no Mn and a low antimony level are observed, could have beenobtained by the oxidation of the low content of iron induced by the furnace atmosphere.

Figures 5 and 6 report the trace element contents of the analysed sample: since the REEcontents are almost the same for the glass of the same group, in Figure 5 the averaged values—normalized to the concentration of the upper crust (Wedepohl 1995)—are reported for eachgroup. Most of the samples are depleted in REE (Fig. 5), with a slight enrichment for heavyREE. The main difference between the two groups lies in the relative abundance of REE,which is higher in the Group II samples.

The distribution of the other trace elements (Fig. 6) is more complex. From Table 2, itappears that most of the trace elements are present in higher levels in the Group II samples(Ga, Y, Zr, Nb, Hf, Ta, Th and U), even if the major differences are linked to the amounts ofZr and Hf (on average, measured at levels of 60 and 1.5 ppm in Group I and at levels of 276and 7 ppm in Group II). Group I glass is depleted for most of the trace elements, with theexception of Sr; this last element, even if with a wide variability, is enriched in all the samples.This could be attributed to the presence of aragonite coming from shells (in which Sr largelysubstitutes for Ca) in the sands used as vitrifying raw materials (Freestone et al. 2003;Freestone 2006). The highest levels of Sr are present in sample GNZ 3, from Group I. Thissample also shows high levels of Ba although, on average, Group I has the lowest values. GNZ3, represents an outlier: it is an example of near-perfect glass decoloured with Mn (it is yellow–green in colour and it shows FeO = 0.99% and MnO = 1.94%), probably obtained by the recyclingof ancient glass with the addition of shells as stabilizer. The presence of lead in amountexceeding 100 ppm (455 ppm in sample GNZ 3) has been interpreted as a sign of the recyclingof earlier glass (Jackson 1996).



Concerning the low-Fe samples (Group I)—with the exception of sample GNZ 3—the lowconcentrations of trace elements suggest the use of mature sands, rich in quartz and poor inclay and heavy minerals, expected to preferentially host REE and heavy elements (such as Zr,Hf and Ti). On average, this trace element distribution is consistent with that recognized in theLevantine I glass from Cyprus studied by Freestone et al. (2002b).

The chemical results for the Ganzirri samples are strictly consistent with those reported inthe literature for glass of the same period from different regions. Figure 4 reports the TiO2 versusFeO and TiO2 versus MnO plots for the samples analysed here, and for those dated betweenthe fourth and the seventh centuries ad from Cyprus (Freestone et al. 2002b), Rome andChartage (Verità 1995) and northern Sinai (Freestone et al. 2002a). In these plots, it canclearly be seen that all the sample sets split into two different groups, characterized by lowlevels of Fe, Mn and Ti (open symbols) and high levels of Fe, Mn and Ti (HIMT glass, solidsymbols), respectively.

The trace element distributions of the Ganzirri finds (Figs 5 and 6) further confirm theresults obtained for major and minor elements, and highlight the differences between theGroup I and Group II samples. Our data closely agree with the results obtained by Freestone(2002b) on glass from Cyprus: in that study, the analysed samples also split into two differentcompositional groups, characterized by different trace element patterns, with the highest levelsof REE, Zr, Ba and Hf associated with the HIMT group samples.

The results of our work are also strictly consistent with those of a very recent studyperformed by Santagostino Barbone (2007) on vitreous finds (dated between the fourth and thesixth centuries ad) and recovered in Herdonia (Foggia, Italy). The co-presence of two distincttypologies of glass in several localities of the Mediterranean basins clearly indicates that atleast two distinct glass factories provided the glass supply during those centuries.

XANES INVESTIGATION

Fe K-edge

Several XANES studies are reported in the literature on Fe-bearing glass of geological orarchaeological interest (e.g., Calas and Petiau 1983; Waychunas et al. 1983; Delaney et al.1996; Wu et al. 1999; Galoisy et al. 2001; Giuli et al. 2002; Quartieri et al. 2002, 2005;Farges et al. 2004, 2005a,b; Wilke et al. 2004, 2006, 2007). The Fe K-edge XANES spectradisplay a number of features that may be attributed to transitions between bound electronicstates and that shift to higher energies with increasing oxidation state (Berry et al. 2003 andreferences cited therein). In particular, in addition to the bond distances, the energy position ofthe pre-edge peak is also strongly influenced by the oxidation state (the centroids of the pre-edgepeaks of the Fe3+ rich minerals and glass varieties are shifted towards higher energy), while itsintensity varies considerably as a function of the site symmetry (Tossel et al. 1974; Waychunaset al. 1983; Wu et al. 1999; Wilke et al. 2007).

The Fe K-edge XANES spectra of two HIMT Ganzirri glass samples (GNZ 5, GNZ 6)—withdifferent ratios of Fe and Mn—and of the reference silicatic glass ST1 are reported in Figure 7 (a),while the pre-edge peaks and their fits are shown in Figure 8. The energy position features aresummarized in Table 3. Figure 7 (a) and Table 3 show that the Fe-XANES spectra of the twofragments and of the reference glass are rather similar, both in their general shape and in theenergy positions of the various features (labelled from (a) to (d) in Fig. 7 (a)). This indicatesthat most of the iron present in both of these samples in different concentrations (GNZ 5,



FeO = 1.91 wt%; GNZ 6, FeO = 3.69 wt%) is in the Fe3+ form. The analysis of the pre-edgepeaks confirms this situation; in fact, the positions of the centroids for the two samples (Table 3)are 0.6 eV shifted towards higher energy with respect to the standard glass.

Mn K-edge

The study of the oxidation state of manganese in ancient glass has been used as a tool forunderstanding whether this element was present as an impurity in the raw materials orintentionally added to the batch as a decolouring agent (see, for example, Quartieri et al.2005). It is, in fact, well known that in ancient times Mn was added to the batch to controlthe final colour of the glass—by means of a redox reaction—neutralizing the effect of theFe2+ of heavy minerals present as impurities in the sand and imparting the typical blue–greencolour to the artefacts.

The Mn K-edge XANES spectra and the pre-edge fits of the two HIMT samples and of tworeference compounds are shown in Figures 7 (b) and 9, respectively. The energy positions ofthe main XANES features, of the pre-edge fit components and of its centroid are reported inTable 3. There is similarity between the spectra of the Ganzirri samples and that of tephroite(Mn2+) (Table 3). In particular, the position of the centroid obtained by the pre-edge fit isalmost the same. On the contrary, all the XANES features of pyrolusite are shifted by a feweV towards higher energy. These data indicate that the Mn present in the two glass samples ismainly in its reduced form. This result, along with the prevalent presence of Fe3+, suggests theoccurrence of a redox interaction between iron and manganese, leading to the oxidation ofiron to Fe3+ and the reduction of manganese to Mn2+. The hypothesis of an intentional additionof manganese compounds is also suggested by the fact that the common levels of manganesepresent in rocks and sand rarely exceed 0.1%, while the Mn oxide content in these HIMT glassis much higher (see Table 1). It is well known that pyrolusite was commonly used as adecolourant during the Roman age, but we can rule out that in our samples Mn was added viathis mineral. In fact, since in sample GNZ 5 the Mn content largely exceeds the quantityrequired for the stoichiometric oxidation of Fe2+ to Fe3+, significant residual Mn4+ should befound in this sample. It should also be noted, from Table 2, that the samples richest in Mn arein general also the richest in Ba. This amount of Ba could suggest the presence of small quan-tities of psilomelane [(Ba,H2O)Mn5O10] in the raw materials. This phase is documented inmanganese oxides/hydroxides deposits (Peacor and Wedepohl 1969). On the other hand, Mncould have been added mainly in the 3+ oxidation state as hydroxides, and then reduced to 2+upon redox interaction with Fe2+. However, in this case again some residual Mn3+ should bepresent, at least in sample GNZ 5, since, in this sample, its content exceeds the stoichiometricrequirement. This discussion, together with the olive-green nuance of the GNZ 5 and GNZ 6samples, requires further investigations to understand in detail the relationship between thechemistry and the origin of the normally dark colour of HIMT glass.

CONCLUSIONS

The Ganzirri glass finds can be classified, on the basis of the CaO/Al2O3 ratio, as Levantine Iglass. Moreover, the relative contents of both major and minor elements allow discriminationinto two distinct groups, one of which is composed of HIMT glass. The utility of trace elementanalysis in providing an even clearer discrimination is confirmed, since the relative abundances ofthese elements are systematically higher in HIMT glass compared to the other group.



The co-presence of two distinct glass types is reported for several locations in the Mediter-ranean area during Byzantine times and, in particular, the REE and trace elements patterns ofthe Ganzirri samples are extremely similar to those found for both groups of Cyprus finds. Thepresent results, and in particular those for the trace elements, confirm that the glass artefactsfound in both localities were produced with two different glass batches, made with distinctcoastal sands. This conclusion is not surprising, considering the strategic positions of bothsites for trading in the Mediterranean region.

ACKNOWLEDGEMENTS

The BM08 GILDA beamline staff (ESRF, Grenoble) are acknowledged for their assistanceduring the XAS experiments. Two anonymous referees are acknowledged for their usefulsuggestions, which greatly improved the manuscript.

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ARCHAEOMETRICAL INVESTIGATION OF SICILIAN EARLY BYZANTINE GLASS: CHEMICAL AND SPECTROSCOPIC DATA*