Unravelling provenance and recycling of Late Antique glass with trace elements

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Unravelling provenance and recycling of Late Antique glass with trace elements

Andrea Ceglia, Peter Cosyns, Nadine Schibille, Wendy Meulebroeck

To cite this version:

Andrea Ceglia, Peter Cosyns, Nadine Schibille, Wendy Meulebroeck. Unravelling provenance and

recycling of Late Antique glass with trace elements. Archaeological and Anthropological Sciences,

Springer, 2017. �hal-01844088�

(will be inserted by the editor)

Unravelling provenance and recycling of Late Antique glass with trace elements

Andrea Ceglia · Peter Cosyns · Nadine Schibille · Wendy Meulebroeck

Received: date / Accepted: date

Abstract Earlier research has shown that several common late antique glass types circulate in Cyprus between the 5th and the 7th century AD, specifically Levantine 1, HLIMT, HIMTa, HIMTb and Egypt 1, HIT, Roman and a plant ash glass. By investigating the glass material from Yeroskipou-Agioi Pente, Maroni-Petrera and Kalavasos-Kopetra we aimed to refine the chemical groups present within three late antique Cypriot sites and define the relations between trace elements obtained from LA-ICP-MS. Our data demonstrate compositional patterns that can be exploited to provenance late antique glass by investigating the REE-bearing mineral fractions, the amount of zircon and the carbonaceous fraction of the sand. In addition Nb and Ti display a strong linear relation which depends on the glass type. Finally the paper discusses the occurrence of glass recycling on the island and how this activity influenced the concentration levels of specific trace elements. Our study thus sets out an analytical framework to identify recycling events tailored on each compositional type.

Keywords Archaeological glass · Cyrpus · Late-Antiquity · LA-ICP-MS

A. Ceglia

Department of Applied Physics and Photonics B-PHOT group, Vrije Universiteit Brussel Pleinlaan 2, B-1050 Brussels, Belgium E-mail: aceglia@b-phot.org

P. Cosyns

Department of Art Sciences and Archaeology MARI research group, Vrije Universiteit Brussel Pleinlaan 2, B-1050 Brussels, Belgium

N. Schibille

IRAMAT-CEB, UMR 5060, CNRS

3D rue de la F´ erollerie, 45071 Orl´ eans Cedex 2, France W. Meulebroeck

Department of Applied Physics and Photonics

B-PHOT group, Vrije Universiteit Brussel

Pleinlaan 2, B-1050 Brussels, Belgium

1 Introduction

1

The last decades have seen an evolution of the analytical tools used to characterize

2

and provenance glass, highlighting the every growing need of trace element analyses

3

to identify meaningful groups (Dussubieux et al., 2016). With the contribution

4

of archaeometry, it is now widely accepted that in the first millennium AD, the

5

glass industry was mostly organized into large primary factories located in Egypt

6

and the Levant and secondary workshops where raw glass chunks were remelted

7

and shaped into objects (Freestone et al., 2002). Nevertheless, we are still far from

8

having a clear understanding of the first millennium glass industry. There are

9

many geographical gaps that would give us useful insights into the connections

10

between areas and even if the number of publications on the topic keeps increasing

11

(Bugoi et al., 2016; Cholakova et al., 2016; Gliozzo et al., 2016; Maltoni et al., 2016;

12

Maltoni and Silvestri, 2016; Phelps et al., 2016; Schibille et al., 2016b,a; Silvestri

13

et al., 2017) much work still needs to be carried out. We therefore started working

14

on Cypriot glass material from the early Christian period, because Cyprus occupies

15

a central hub of the commercial routes between the Near East and the rest of the

16

Roman Empire. The study of the archaeological glass on the island can thus offer

17

a way to reach a wider understanding of the glass industry as a whole and add

18

another piece of information to the puzzle of ancient glass distribution (Freestone

19

et al., 2002; Ceglia, 2014; Ceglia et al., 2015a, 2016; Bonnerot et al., 2016).

20

Previous research concentrated on the glass finds from three ecclesiastical sites:

21

Agioi Pente at Yeroskipou, Maroni-Petrera and Kalavasos-Kopetra all located on

22

the south-west to central-south coastline of Cyprus (Ceglia et al., 2015a, 2016).

23

(Ceglia et al., 2015a, 2016) have carried out chemical and spectroscopic analysis

24

by means of EPMA and optical absorption spectroscopy in order to characterise

25

the material and to study the redox state of iron in naturally coloured glasses.

26

These studies provided new evidences on the glass compositions found in three

27

Late Antique sites in Cyprus. By comparing them to the known primary glass

28

groups produced in Syria, Palestine and Egypt we were able to identify glasses of

29

the following chemical compositions: Levantine 1, Egypt 1, HIMTa and HIMTb,

30

HLIMT

, HIT, two Roman fragments and a plant ash object (Ceglia et al., 2015a,

31

2016, and references therein).

32

In this work we re-analysed the material presented in Ceglia et al. (2015a)

33

using LA-ICP-MS in order to implement the complementary trace element data

34

of these objects to further elucidate earlier research results. We discuss trace

35

element patterns to refine the chemical characterisation of the primary production

36

groups represented among the three 5th to 7th century AD Cypriot assemblages.

37

Furthermore, the trace elements enable us to ascertain the evidence of recycling

38

within the dataset. We propose here new trace element thresholds to detect recycling

39

tailored on the different glass groups.

40

1

Hereafter we will refer to this group as Foy 2 in order to be consistent with the latest

accepted nomenclature. For more information about HIMTa and HIMTb see (Ceglia et al.,

2015a). HIT is described in (Rehren and Cholakova, 2010)

2 Experimental

41

The objects studied total 179 glass fragments, mostly naturally coloured, from

42

the three Cypriot sites of Yeroskipou-Agioi Pente, Maroni-Petrera and Kalavasos-

43

Kopetra. In Appendix A a more detailed description can be found. In this paper

44

58 elements were determined by Laser Ablation Inductively Coupled Plasma

45

Mass Spectrometry (ICP-MS) at the Centre Ernest-Babelon of the IRAMAT

46

(Orl´ eans)(Gratuze, 2016; Schibille et al., 2016a). The operating conditions of the

47

193 nm laser were set at an energy of 4 to 6 mJ, with a repetition rate of 10 Hz and

48

a spot size diameter of 100 µm allowing a micro sampling invisible to the naked eye.

49

Take-up time was 50 seconds and the measurements were carried out on a list of

50

pre-selected isotopes. For silicon, the

Si isotope was employed as internal standard.

51

The analyses were carried out on a single spot on the polished sections previously

52

used for EPMA avoiding any surface contamination or corrosion interference. The

53

so-obtained signal intensities were converted by means of an average response

54

factor KY, determined using a combination of five different standard reference

55

materials (SRM). Detection limits vary according to the ablation parameters (spot

56

size diameter and laser repetition rate) and to the optimisation parameter of the

57

mass spectrometer. Typical detection limits for soda-lime glasses are given in

58

(Gratuze, 2014, Table 13.4). Corning A and NIST SRM612 were regularly analysed

59

as unknown samples to determine the accuracy and precision of the data, which

60

were determined by comparing the obtained results against reported values for

61

Corning A (Brill, 1999; Wagner et al., 2012; Vicenzi and Logan, 2002) and NIST612

62

(Jochum et al., 2011). The data are presented in the supplementary material

63

(Appendix B). Accuracy is within 4% relative for all major elements in NIST612

64

and Corning A, with the exception of alumina (8%) and lime (15%) in the latter

65

reference glass. The accuracy for most trace elements in NIST 612 and Corning A

66

are within 10% with the majority within 5%.

67

3 Results

68

Although EPMA is generally considered more accurate for major elements over

69

LA-ICP-MS, the quantification presented in this work, obtained with the latter

70

technique, is in very good agreement with the previous EPMA data (Ceglia et al.,

71

2015a). In Figure 1 we show the relation for major and minor elements analysed

72

by the two methods. Pearson’s linear correlation coefficient varies from 0.97 to 1

73

indicating a strong positive correlation. The angular coefficient are very close to

74

one - ranging between 0.96 for Fe

O

and 1.07 for TiO

- suggesting a very good

75

agreement between the two techniques. Two samples SF34, KK293 have contrasting

76

values of MnO and CaO between the two techniques. We believe that this is due

77

to inhomogeneity of these samples rather than a problem with the measurements.

78

As already seen in (Ceglia et al., 2015a) all glasses, with the exception of a

79

plant ash fragment, are low magnesia soda-lime-silica glass typical of the Roman

80

and Early Byzantine period. In (Ceglia et al., 2015a) 6 groups were defined:

81

Levantine 1, Foy 2, HIMTa, HIMTb, Egypt 1 and HIT. The major, minor and

82

trace elements composition obtained by LA-ICP-MS analyses for all samples is

83

reported in the supplementary material (Appendix C). Please note that in this

84

work HLIMT is referred to as Foy 2. In this paper a further refinement is presented

85

as two more groups have been recognized: High Mn Levantine 1 and High Fe Foy

86

2. Furthermore, thanks to LA-ICP-MS, some glasses with doubtful assignments

87

have been confirmed or reassigned to another group (for more detail please see

88

section 4.2). The reassignment of samples to specific glass types was achieved by a

89

reiterative analysis of the full data - major, minor and trace elements - together

90

with a multidimensional principal component analysis (PCA) carried out using

91

Matlab. To perform the statistical analysis we selected 14 elements that are believed

92

to be representative of the two main components of glass sand and natron (Si,

93

Na, Mg, K, Ca, Al, Fe, Ti, Li, B, Zr, Sr, La and Ce) deliberately not including

94

colouring / decolouring elements. This allows us to discriminate between glass types

95

using different sand sources and/or recipes without interferences due to recycling

96

or additives. The principal component scores are calculated on the mean-scaled

97

LA-ICP-MS data. Principal components 1 and 2 account for more than 70% of

98

the variability and separate the data into distinct primary production groups

99

(Figure 2). The vectors associated with the two principal components are shown on

100

the top-right of Figure 2. They point at how samples are separated, i.e. Levantine

101

glass is richer in SiO

, Egypt 1 in Al

O

, Foy 2 in Sr, while HIMT in La, Ce, Zr,

102

TiO

and Fe

O

.

103

74 72 70 68 66 64 62 60

SiO2 EPMA

74 72 70 68 66 64 62 60

SiO2 LA-ICP-MS y = 1.0034 x r = 0.968

5.0 4.5 4.0 3.5 3.0 2.5 2.0 1.5

Al2O3 EPMA

5.0 4.5 4.0 3.5 3.0 2.5 2.0 1.5

Al2O3 LA-ICP-MS y = 1.0103 x r = 0.972

4.0 3.5 3.0 2.5 2.0 1.5 1.0 0.5 0.0

Fe2O3 EPMA

5 4 3 2 1 0

Fe2O3 LA-ICP-MS y= 0.958 x r = 0.996

0.8 0.7 0.6 0.5 0.4 0.3 0.2 0.1 0.0

TiO2 EPMA

0.7 0.6 0.5 0.4 0.3 0.2 0.1 0.0

TiO2 LA-ICP-MS y = 1.0665 x r = 0.998

3.5 3.0 2.5 2.0 1.5 1.0 0.5 0.0

MnO EPMA

3.5 3.0 2.5 2.0 1.5 1.0 0.5 0.0

MnO LA-ICP-MS KK293

SF34

y = 1.0149 x r = 0.996

22 20 18 16 14 12 10

Na2O EPMA

22 20 18 16 14 12 10

Na2O LA-ICP-MS y = 1.0193 x r = 0.975

3.0 2.5 2.0 1.5 1.0 0.5 0.0

K2O EPMA

3.0 2.5 2.0 1.5 1.0 0.5 0.0

K2O LA-ICP-MS y= 1.0045 x r = 0.994

3.5 3.0 2.5 2.0 1.5 1.0 0.5 0.0

MgO EPMA

3.5 3.0 2.5 2.0 1.5 1.0 0.5 0.0

MgO LA-ICP-MS y = 1.0127 x r = 0.989

12 10 8 6 4 2

CaO EPMA

12 10 8 6 4 2

CaO LA-ICP-MS SF34

KK293 y = 0.97936 x r = 0.979

Fig. 1: Relation between the EPMA and LA-ICP-MS data. The Parson r coefficient are better than 0.97 and the angular coefficient ranges between 0.96 and 1.07.

In Figure 3 we show the trace elements patterns of the main compositional groups

104

normalized to the upper continental crust composition. This type of representation

105

has been largely used since its first use in (Freestone and Hughes, 2000) as it allows

106

a quick overview of several elements. The distributions are all very similar with

107

exception of Sr, Zr, Ce and La. Barium is also showing some differences but it

108

should be noted that this element is highly variable in group HIMTa. In general, it

109

is possible to see an increase of the trace levels from Levantine to HIMTb glass.

110

-8 -6 -4 -2 0 2 4 6 8

PC2

10 8

6 4

2 0

-2 -4

PC1

Al₂O₃

Fe2O3

TiOCe2Zr La Na₂O Li Sr CaO K₂O

B MgO High Mn Lev 1

Lev 1 Foy 2 High Fe Foy 2 HIMTa HIMTb Egypt 1 HIT

Fig. 2: Principal component 1 and 2 explain most of the variance of the dataset.

The groups are separated, although some overlap remains. On the top-right the vectors of the first two principal components are shown.

6

0.18 2 4 6 8

1

2 4

Normalized concentrations

Li Ga Rb Sr Y Zr Nb Ba La Ce Pr Nd Sm Eu Gd Tb Dy Ho Er Tm Yb Lu Hf

High Mn Lev 1 Lev 1 Foy 2 High Fe Foy 2 HIMT a HIMT b Egypt 1

Fig. 3: Concentration of trace elements normalized to the upper continental crust (Kamber et al., 2005) for a selection of elements.

Table 1 summarizes the compositions of what we considered “pristine” or

111

not recycled glasses presented in this work. It reports also the average composi-

112

tion of the raw chunks analysed in Foy et al. (2003) as a source of comparison.

113

How we distinguished between recycled and unrecycled glass is detailed in sec-

114

tion 4.3. In the discussion on recycling we have not considered high Fe Foy 2 and

115

high Mn Levantine 1 because the former consists of too few samples and the latter

116

is not an homogeneous group.

117

Table 1: Average concentrations (µ) and standard deviation (σ) of the pristine glass for each group. The last three groups are taken from (Foy et al., 2003).

In parenthesis there is the number of samples selected. The criteria to choose non-recycled glass are explained in the text. In Appendix D the average data are reported for all the 58 elements analysed. In addition, in Appendix E all samples belonging to each group are reported along with their chemical composition.

SiO2 Al2O3 Fe2O3 TiO2 MnO Na2O K2O MgO CaO Cl P2O5 Zr Sr Ba Cr Co Cu Zn Pb Sb Ce

Lev 1 (17) µ 70.9 3.10 0.43 0.08 0.02 15.1 0.50 0.70 8.1 0.87 0.044 43.7 402 202 17.14 1.32 2.30 6.07 3.55 0.02 11.22

σ 1.3 0.15 0.05 0.01 0.00 1.1 0.09 0.18 0.9 0.08 0.006 6.0 50 15 4.93 0.14 0.51 0.68 2.51 0.03 0.81

Foy 2 (21) µ 65.6 2.45 0.97 0.14 1.41 18.2 0.65 1.02 8.4 0.84 0.123 76.3 677 265 16.49 6.78 42.55 22.62 59.11 151.19 12.74

σ 1.1 0.17 0.15 0.02 0.19 1.6 0.09 0.11 0.7 0.08 0.044 7.7 61 34 2.53 1.42 8.81 3.98 22.57 76.22 0.94

HIMTa (9) µ 65.1 3.0 1.89 0.47 2.18 18.48 0.39 1.12 6.0 0.99 0.046 219.5 400 874 61.53 10.86 49.72 28.47 9.51 0.77 15.99

σ 1.7 0.3 0.40 0.16 0.39 1.35 0.05 0.18 0.9 0.09 0.012 55.1 38 717 20.73 2.53 7.92 6.97 4.51 0.77 3.02

HIMTb (5) µ 63.8 3.31 3.81 0.54 1.69 18.2 0.40 1.17 5.7 0.92 0.129 249.9 410 269 71.23 12.30 74.44 84.81 17.41 0.49 17.55

σ 0.5 0.25 0.22 0.07 0.16 0.1 0.03 0.12 0.2 0.07 0.018 32.8 12 101 11.15 1.18 9.23 32.09 10.96 0.31 0.82

Egypt 1 (3) µ 70.7 4.13 1.54 0.44 0.04 17.8 0.49 0.73 3.0 0.96 0.073 148.4 190 186 62.92 4.93 3.27 21.89 3.26 bdl 15.74

σ 0.9 0.43 0.19 0.06 0.01 1.3 0.04 0.09 0.1 0.23 0.006 13.5 14 22 11.02 0.60 0.42 1.32 1.32 bdl 1.83

Serie 2 (10) µ 64.52 2.51 1.05 0.16 1.74 18.70 0.77 1.19 8.01 0.16 87 700 348 20 31 52 70 127 13

σ 1.04 0.15 0.12 0.02 0.16 1.48 0.19 0.14 0.35 0.04 8 49 56 2 18 9 19 72 1

Serie 1 (9) µ 63.63 2.89 1.88 0.49 2.23 20.48 0.35 1.30 6.37 0.12 229 503 1006 63 29 73 26 3 18

low iron σ 0.85 0.28 0.29 0.06 0.42 1.37 0.08 0.26 0.93 0.02 27 78 921 9 5 29 30 7 2

Serie 1 (3) µ 62.90 3.01 3.90 0.51 2.35 17.68 0.57 1.39 7.21 0.24 249 632 433 68 36 135 209 8 20

high iron σ 0.44 0.39 0.40 0.11 0.65 0.29 0.07 0.22 0.74 0.01 38 77 80 12 2 58 212 7 2

4 Discussion

118

In order to ensure a smooth flow this section has been divided into three parts,

119

each tackling one of the three main points of this paper. First, we will use the trace

120

elements to discuss the differences among the glass types identified and to compare

121

them with data available in the literature. Then, we will examine in detail which

122

glasses were reassigned with respect to (Ceglia et al., 2015b). Finally we present a

123

thorough analysis of recycling tailored to the different compositional groups.

124

4.1 Provenancing with trace elements

125

LA-ICP-MS gave us the possibility to further characterise the glasses previously

126

analysed by EPMA (Ceglia et al., 2015a). The trace elements can be related to

127

accessory minerals present in the sand such as feldspar, pyroxene, amphibole,

128

zircon, monazite and more. Many of the trace and Rare Earth Elements (REE)

129

are not influenced by the use of decolourants or by recycling. They can thus serve

130

as reliable indicators to distinguish sand sources. Other researchers have already

131

discussed interesting relations based on elements such as Zr and Ti (Aerts et al.,

132

2003), Zr, Ti, Cr and La (Shortland et al., 2007; Walton et al., 2009) and Zr, Sr

133

and Ba (Freestone and Hughes, 2000; Paynter, 2006; Silvestri, 2008; Silvestri et al.,

134

2008). According to (Brems and Degryse, 2014) the most diagnostic trace elements

135

are Ti, Cr, Sr, Zr, and Ba.

136

The concentration of most trace elements increases from Levantine 1 to HIMTb

137

(Figure 3). This is a well-known phenomenon, associated with the maturer sand

138

source used for Levantine 1 compared to Egyptian productions such as HIMT glass.

139

It is possible to identify certain elements and ratios that can be very effective

140

to differentiate the different glass types. We focussed on three components: the

141

REE-bearing mineral fraction, the amount of zircon and the amount of Sr in the

142

carbonaceous fraction. The most abundant REEs are Ce, La and Nd. In our dataset

143

Nd is strongly correlated with La. Therefore to characterize the sand the ratio

144

Ce/La seems to be very promising. The content of Zr appears to be a reliable

145

marker for differences in the silica source, usually introduced in the form of zircon, a

146

zirconium silicate (ZrSiO

). To compare the zircon content in different glass types,

147

we normalized the Zr content to the silica concentration. The third parameter is

148

the ratio of Sr to CaO which is indicative of the carbonaceous fraction of the sand.

149

However, it must be noted that strontium can also be incorporated into the glass

150

batch as constituent of clay minerals, feldspars or the manganese bearing minerals

151

(Ganio et al., 2012; Cholakova et al., 2016). In Figures 4a and 4b we report the

152

relations between these three markers that can separate different types of glass

153

very well.

154

The sand of the Syro-Palestinian coast has a ratio of Ce to La of 1.8-1.9, only

155

40-55 ppm of Sr per wt% of CaO and the lowest zircon-to-silica ratios. Group

156

Foy 2 has lower Ce/La (roughly 1.6-1.7), a higher zircon fraction and the highest

157

Sr/Ca ratio. However, this does not apply to the high-Fe Foy 2 group that should

158

be considered a contemporary yet different production to Foy 2 as already pointed

159

out by Schibille et al. (2016a). Egypt 1, HIMTa and HIMTb have a high Zr/Si

160

ratio with HIMTa having a relatively large spread. Conversely the ratio between

161

strontium and calcium is constant across these groups (about 70 ppm per wt% of

162

CaO). The pattern of REEs minerals, however, differ notably between the three

163

groups with HIMTb having the highest contribution of La and Egypt 1 the lowest.

164

One of the two HIT fragments (ID821) plots together with the blurred group of

165

HIMTa, while the other HIT sample (NSF1065) has a much lower Sr/Ca ratio

166

compared to any of the other glass types, which prompts questions about its origin

167

which are at the moment unresolved. In Figure 4c and 4d Foy 2, High Fe Foy 2

168

and Levantine 1 glass from Schibille et al. (2016a) are overlapped to our data. The

169

same trends are clearly evident confirming the definition of the glass groups.

170

Occasionally the Sr to CaO ratio increases in groups with high Mn due to the

171

addition of strontiomelane as decolouring agent (Ganio et al., 2012; Cholakova et al.,

172

2016). Indeed, the biplot of Sr/CaO and MnO shows that there is a gradual increase

173

from Levantine 1 over High Mn Levantine 1 to HIMT glass (Figure 5). Given that

174

Levantine 1 represents the natural levels of Sr/CaO due to inclusions of seashells,

175

the higher content of Sr/CaO in HIMT glass could be explained by the addition of

176

the decolouring minerals. Therefore the Sr/CaO ratio in HIMT prior to the addition

177

of MnO would be similar to Levantine 1 and due to the inclusion of seashells, which

178

would support Nenna’s proposition of the Mediterranean coastline of the Sinai

179

as region of provenance of HIMT glass Nenna (2014). Nevertheless, it cannot be

180

stated conclusively whether the source of Mn in HIMT is strontiomelane or not

181

for several reasons. First of all one of the HIT glasses (ID821) has similar Sr/CaO

182

and Zr/SiO

to HIMTa although it has no manganese. Similarly, Egypt 1 glass

183

has higher Sr/CaO than Levantine 1 glass even if it contains no manganese.The

184

higher amount of Sr/CaO in the Egyptian glass could be explained by diagenetic

185

modification of calcite into aragonite and/or by the presence of other Sr-bearing

186

minerals. The use of strontiomelane was suggested to explain the high content of Sr

187

in Foy 2 glass based on the correlation between this element and MnO (Cholakova

188

et al., 2016). In our case, there is no evidence of a correlation with Mn. The high

189

Sr/CaO can be explained either by the use of sand/raw materials richer in Sr

190

and/or by the use of a different source of strontiomelane since this mineral is highly

191

variable in Sr content (Meisser et al., 1999).

192

Another interesting pattern that can be noted is the relationship between

193

titanium and niobium. This is partially related to the Zr-Ti relation often used

194

in glass studies (see for examples (Brems and Degryse, 2014)), but it provides

195

2.2

2.0

1.8

1.6

1.4

1.2

1.0

Ce/La (ppm/ppm)

6 5 4 3 2 1 0

Zr/SiO2 (ppm/wt%)

High Mn Lev 1 Lev 1 Foy 2 High Fe Foy 2 HIMTa HIMTb Egypt 1 HIT

100

80

60

40

20

0

Sr/CaO (ppm/wt%)

6 5 4 3 2 1 0

Zr/SiO2 (ppm/wt%)

High Mn Lev 1 Lev 1 Foy 2 High Fe Foy 2 HIMTa HIMTb Egypt 1 HIT

a b

2.2

2.0

1.8

1.6

1.4

1.2

1.0

Ce/La (ppm/ppm)

6 5 4 3 2 1 0

Zr/SiO2 (ppm/wt%) Egypt 1

HIMTa

HIMTb

High Fe Foy 2 Foy 2

Lev 1

Lev 1 - Schibille et al 2016 Foy 2 - Schibille et al 2016 High Fe Foy 2 - Schibille et al 2016

100

80

60

40

20

0

Sr/CaO (ppm/wt%)

6 5 4 3 2 1 0

Zr/SiO2 (ppm/wt%) Egypt1

HIMTa HIMTb High Fe Foy 2 Foy 2

Lev 1

Lev 1 - Schibille et al 2016 Foy 2 - Schibille et al 2016 High Fe Foy 2 - Schibille et al 2016

c d

Fig. 4: REEs minerals are characterised by the relative amounts of Ce, and La.

Investigating these elements helps define the geochemistry of the sand source of the glass types. In c and d the data from (Schibille et al., 2016a) is overlapped to confirm the discrimination power of these ratios.

100 90 80 70 60 50 40 30 20 10

Sr/CaO (ppm/wt%)

3.5 3.0

2.5 2.0

1.5 1.0

0.5 0.0

MnO (wt%)

HighMnLev1 Lev1 Foy2 HighFeFoy2 HIMTa HIMTb Egypt1 HIT

Fig. 5: Relation between Sr/CaO and MnO for the Cypriot glass.

a different view because Nb seems to be added to the batch almost exclusively

196

with Ti. The geochemical association of Nb with Ti-bearing minerals as rutile

197

and ilmenite has long been recognised. It is known that its content varies with

198

the type of rock in which the titanium minerals occur (Fleischer et al., 1952).

199

Therefore, Nb could be an interesting element for provenancing ancient glass since

200

its concentration depends on the quantity, type and origin of Ti minerals that

201

are added to the glass batch as sand impurities. Variations in the Nb/Ti ratio

202

imply different glass types (Figure 6a). Foy 2 and Levantine 1 have the same ratio

203

between Nb and Ti although remaining very distinctive in the absolute amount

204

of elements. Conversely, Egypt 1, HIMT and HIT show a lower Nb/Ti ratio. The

205

calculated correlation coefficients of about 0.97 for both groups, indicating positive

206

linear relationships. Unfortunately niobium is not often reported in the literature

207

and, if so, it is generally rounded to the ppm. Exceptions are the data published

208

by Conte et al. (2014) on the Late Antique and Early Medieval glass finds from

209

Butrint (Albania), Gliozzo et al. (2015) on the colourless glass from the Palatine

210

and Esquiline hills in Rome (Italy) and Schibille et al. (2016b) on Byzantine glass

211

weights. By plotting the Nb and Ti values from these papers for the glasses with

212

Levantine, HIMT and Foy 2 compositions, we notice that their data confirms the

213

existence of two linear correlations between Nb and Ti (Figure 6b). Having the

214

same Nb/Ti ratio, of course, does not mean that the glass origin was the same,

215

but it implies that the minerals introducing these elements to the glass batch are

216

the same, probably derived from the same type of rock. Therefore it is interesting

217

to note that Foy 2, high-Fe Foy 2 and Levantine 1 cluster on the same correlation

218

lines. This suggests that these three types of glass productions exploited sand

219

sources that have the same Nb-Ti bearing minerals, which differ from the minerals

220

occurring in the sands used for Egypt 1 and HIMT.

221

4.2 Reassignment of glass

222

One of the aims of this paper was to refine the classification of the glasses analysed

223

by Ceglia et al. (2015a). In that paper the material was clustered on the basis

224

of the major and minor element composition but in some cases it was difficult

225

to determine the group because of a mixed glass chemistry. Eight glasses were

226

assigned to Foy 2 with a question mark - two from Yeroskipou-Agioi Pente (ID817e

227

and ID828), four from Maroni-Petrera (NSF1007a, SF42, SF109a and SF113) and

228

two from Kalavasos-Kopetra (KK293 and KK303) - based on the iron, titanium,

229

manganese and calcium contents. With the complete chemical data we can now

230

reconsider the assignments of these glasses.

231

The major and minor oxides composition of samples ID817e and ID828 from

232

Yeroskipou-Agioi Pente are in good agreement with the profile of Foy 2 glass except

233

for the high Fe

O

(2.35 and 3.47 wt% respectively). The trace elements confirmed

234

their similarity to this group having an elevated content of Sr (613 and 560 ppm),

235

Zr (84 and 83 ppm), Li (higher than 5 ppm) and being impure with Sb (77 and 62

236

ppm). In view of their iron concentration they have to be attributed to the High-Fe

237

Foy 2 group as already reported in (Cholakova et al., 2016) and (Schibille et al.,

238

2016b). These glasses have a more intense colour than Foy 2 glasses due to the

239

higher concentration of Fe. In (Ceglia et al., 2016) we have reported the optical

240

and colour analysis of these glasses. Sample ID817e has a colour similar to HIMTa

241

8 7 6 5 4 3 2 1 0

Nb (ppm)

5000 4000

3000 2000

1000 0

Ti (ppm)

r

=0.965 r

=0.969

High Mn Lev1 Lev 1 Foy 2 High Fe Foy 2 HIMTa HIMTb Egypt1 HIT

a 8

6

4

2

0

Nb (ppm)

5000 4000

3000 2000

1000 0

Ti (ppm)

Lev 1 - Schibille et al 2016 Foy 2 - Schibille et al 2016 High Fe Foy 2 - Schibille et al 2016 Gliozzo 2015

Conte 2014

b

Fig. 6: Niobium and titanium are strongly correlated in Ti-bearing minerals as rutile and ilmenite. Our dataset shows the existence of two different Nb/Ti ratios which is confirmed in the glass analysed by (Gliozzo et al., 2015), (Schibille et al., 2016a) and (Conte et al., 2014). The dashed lines are calculated with the least square method on the data presented in this paper.

glasses while ID828 had all optical parameters in the range of the HIMTb glasses.

242

Similarly (Schibille et al., 2016a) reports that the high Fe Foy 2 glass has olive to

243

yellow-green colours, typical of HIMT glass.

244

The association of the three Maroni-Petrera samples NSF1007a, SF42 and

245

SF109a to Foy 2 had been doubtful due to their lower K

O and CaO contents,

246

while SF113 shows higher TiO

and Al

O

. The trace element patterns substantiate

247

the attribution of samples NSF1007a, SF42, SF109a to Foy 2, even if the content of

248

Sr (about 430 ppm) and Li (less than 5 ppm) are lower than for most Foy 2 glasses.

249

However, the ratios of Sr/Ca and Ce/La fit well with the rest of the group. Sample

250

SF113 is assigned to HIMTa because of its Sr/Ca and Nb/Ti ratios, despite its low

251

Zr/Si ratio.

252

One of the two glasses from Kalavasos-Kopetra originally categorised as Foy 2

253

(sample KK293) shows a discrepancy in the CaO content between EPMA (5 wt%)

254

and LA-ICP-MS (7.45 wt%). Nevertheless both glasses have Sr/Ca, Zr/Si and

255

Ce/La ratios in line with the values of the Foy 2 group and can therefore be

256

assigned to this group.

257

One of the glasses originally attributed to Levantine 1 (sample SF34) has higher

258

amounts of Ti (0.12 wt%) with respect to the typical values for this glass type.

259

Trace elements confirm that this glass falls into the Foy 2 group instead as the

260

ratios of Ce/La and Zr/Si are similar to this group. On the other hand the amount

261

of strontium normalized to calcium is low, leaving a certain degree of uncertainty to

262

this assignment. Six Levantine 1 samples (ID108, NSF1020a, NSF1020b, SF1, SF17,

263

SF103) have MnO higher than 0.5 wt% and they are therefore here sub-grouped in

264

the high Mn Levantine 1 glass type.

265

4.3 Recycling

266

Glass recycling appears to be a common activity during the Roman and Late

267

Antique period. Early Christian Cyprus is no exception, even if the island is located

268

very close to the primary production regions. Nevertheless, no clear recycling indica-

269

tors have to date been identified (Freestone, 2015, and references therein). Usually,

270

concentrations of elements related to (de)colouring activities above background

271

levels are considered markers of recycling. These background levels have yet to be

272

defined. It is generally accepted that the deliberate use of (de)colourants would

273

induce an increase of these elements, above 1000 ppm. Unintentional additions of

274

coloured fragments in a colourless (or naturally coloured) batch provokes a rise

275

of the concentrations of elements associated with colours with respect to natural

276

impurities in the raw materials. Therefore, when the concentrations of certain

277

elements (normally Co, Zn, Sn, Cu and Pb) ranges between 100 ppm and 1000

278

ppm, it is generally interpreted as an indication of glass recycling (Freestone et al.,

279

2002; Wedepohl and Baumann, 2000; Degryse et al., 2010; Foster and Jackson,

280

2010). If glass objects present lower concentrations of these elements, i.e. between 1

281

ppm and 100 ppm, either glassmakers used “fresh” glass from primary workshops,

282

or limited recycling occurred, or much attention was paid to the cullet selection in

283

order to avoid contamination (Silvestri, 2008; Silvestri et al., 2008).

284

These guidelines are very general and do not address the intrinsic variability of

285

glass groups, or, more appropriately, of sand sources. Hence, we have examined

286

in detail the trace elements in the glasses of the Cypriot assemblage to propose

287

a definition of recycling based on our large dataset. We defined thresholds for

288

transition metals according to the glass type. When all the trace elements are below

289

these thresholds, the sample can be considered to represent a pristine raw glass.

290

Levels of trace elements in excess of these thresholds are indicative of contamination

291

through recycled cullet. Besides the elements normally considered (Co, Zn, Sn,

292

Cu and Pb), Sb is another good marker for detecting recycling as its relative

293

abundance in the upper continental crust is 0.2-0.45 ppm (Jackson, 1996; Rudnick

294

and Gao, 2003). Additionally, we take into account also K

O and P

O

. The

295

relation between melting cycles and ashes was demonstrated by the experiment

296

undertaken by Paynter (2008). The furnace atmosphere is rich in potash vapour

297

and particulate phosphate, which can partly be absorbed by the glass melt. As

298

a result the content of both oxides increases accordingly to the exposure time of

299

the melt to the fumes. The higher the number of re-melting events, the higher the

300

concentration of the ash impurities in the glass.

301

For Levantine 1 initially we have set the thresholds between recycled and

302

unrecycled glass based on the data from Apollonia published in (Freestone et al.,

303

2002). The authors report the average trace elements contents of tank furnace

304

glass and vessels from Apollonia. Using this as reference material, we defined as

305

recycled the glass with more than 5 ppm of Co, 15 ppm of Cu and Zn, 35 ppm of

306

Pb and 3 ppm of Ag. These would leave us with 67 glass samples. The average

307

concentrations of recycling markers is very low, nevertheless several of these glasses

308

have still 0.1 or more P

O

and few hundreds of ppm of Mn. Therefore some

309

recycling is still present within this subgroup of fragments. Recently (Phelps et al.,

310

2016) have published LA-ICP-MS data of 5 samples tank furnace from Apollonia

311

with extremely low values of these additives (1.5 ppm of Co and 1.3 of Cu, 6.4 of

312

Zn, 3 of Pb and 0.03 of Ag). In view of this information we reduced the thresholds

313

to 3 ppm of Co and Cu, 10 ppm of Zn and Pb and 1 ppm of Ag obtaining a

314

group of 17 glasses. Even though these values could seem too stringent, the average

315

composition of this small group resembles very well the primary furnace from

316

Apollonia. Cypriot glass has 139 ppm of Mn 1.3 ppm of Co, 2.3 ppm of Cu, 6.1

317

ppm of Zn, 3.6 ppm of Pb, 0.04 ppm of Ag, 0.55 ppm of Sn, 0.02 ppm of Sb and

318

0.04 P

O

(see Table 1). These are probably the only glasses that did not undergo

319

more than one or very few remelting cycles. The other Levantine objects appear to

320

have been remelted several times with the addition of coloured cullet to the batch

321

more or less evident in the trace element make-up.

322

It is much more complicated to evaluate the degree of remelting/recycling in

323

the other groups, as no raw glass from primary factories has been recovered so

324

far. In addition, Foy 2, HIMTa, HIMTb and Egypt 1 are made with raw materials

325

containing more impurities increasing the natural levels of transition metal ions.

326

However, we can tentatively use the data of some raw glass chunks found in

327

secondary glass workshops analysed by Foy et al. (2003).

328

Within their data set, Foy et al. (2003) have published the chemical composition

329

of 23 glass chunks belonging to S´ erie 2.1 and report the trace elements of 15 of

330

those samples. The majority of these chunks have Pb and Cu lower than 100 ppm.

331

By removing the samples with more than 100 ppm of Cu and Pb from our Foy 2

332

group, we are left with a quite homogeneous group that could define threshold

333

values for recycling: 7 ppm of Co, 43 ppm of Cu, 23 ppm of Zn, 59 ppm of Pb, 0.11

334

ppm of Ag, 6.73 ppm of Sn, 151 ppm of Sb (Table 1). The average composition of

335

the Cypriot Foy 2 is very similar to that one published by Foy et al. (2003) as well

336

as to recently published Byzantine glass weights (Schibille et al., 2016a).

337

Nevertheless, in both cases there are still signs of recycling. The average content

338

of Sb is higher than 100 ppm, well above the natural levels, and is roughly linearly

339

correlated to lead, supporting the hypothesis that Pb is added to the batch together

340

with Sb. Moreover the average composition has more than 0.1 wt% of P

O

which

341

is proportional to K

O indicating that their content increased during the melting

342

process (Figure 7). Since this glass type has been found at several places over the

343

Mediterranean world as well as in continental Europe, the large amount of Sb in

344

these objects suggests that Foy 2 was produced in Egyptian primary workshops

345

0.35 0.30 0.25 0.20 0.15 0.10 0.05 0.00 P

O

(wt%)

1.2 1.0

0.8 0.6

0.4 0.2

K

O (wt%)

Foy 2 (Schibille et al. 2016) raw glass (Foy2003)

Fig. 7: Foy 2 glass exhibits a linear correlation between P

O

and K

O which it underwent to several remelting cycles. The dataset from Schibille et al. (2016a) and Foy et al. (2003) have been removed of the samples with Pb and Cu >100 ppm.

where abundant amount of Sb-decoloured cullet was available to ease the melting

346

process and reduce costs.

347

HIMTa and HIMTb correspond to S´ erie 1 in (Foy et al., 2003), where they

348

reported 12 glass chunks, three with more than 3 wt% of Fe

O

.The content of

349

Sb in these glasses is very low, therefore, we initially considered all glass with Sb

350

higher than 2 ppm as being recycled glass. Nine HIMTa glasses from Cyprus

351

could be considered unrecycled. On average they have 13 ppm of Co, 58 ppm of

352

Cu, 30 ppm of Zn, 13 ppm of Pb, 0.17 ppm of Ag and 1.74 ppm of Sn (Table 1).

353

These objects have very low P

O

and K

O (0.046 and 0.39 wt% respectively),

354

confirming that the glass did not undergo multiple or long remelting procedures.

355

Compared to the raw chunks (Table 1) the average of several markers is lower,

356

but the difference is biased by the presence of one glass chunk with high recycling

357

indicators (VRR173).

358

All the 5 samples of HIMTb have Sb lower than 1 ppm. However they clearly

359

belong to three different melting events: 1) ID94 and ID589, 2)ID476 and ID572a

360

and 3) ID496. Batch 1 has more than 100 ppm of Zn while the other two have about

361

60 ppm. Copper ranges between 64-89 ppm, lead between 7-35 ppm, cobalt between

362

11-13 ppm. Moreover all glasses have more than 0.1 P

O

. It is complicated to

363

establish whether these glasses are recycled and what the levels of recycling markers

364

in raw glass are. However, Foy et al. (2003) report three raw chunks of this glass

365

type (VRR49, VRR50 and VRR52 from S´ erie 1), which have even higher levels

366

of copper, lead and P

O

, paving the way to the hypothesis that the enrichment

367

in these elements is happening at the primary level. The variability might be due

368

either to the intrinsic inhomogeneity of the raw materials or, similarly to Foy 2,

369

to the addition of extra ingredients to the batch other than sand and natron, i.e.

370

recycled cullet. The addition of recycled material is a substantial hypothesis since

371

high levels of Sb, Pb and Cu were found in the raw chunks of HIMTb composition

372

(Foy et al., 2003).

373

Among the five Egypt 1 fragments one, sample ID792, is surely made of

374

recycled glass having 2224 ppm of Pb. Sample ID464 has 0.34 wt% of P

O

which

375

suggests that it could have been recycled. The other 3 samples have on average 5

376

ppm of Co, 3 ppm of Cu, 22 ppm of Zn, 3 ppm of Pb, 0.14 ppm of Ag, 0.46 ppm of

377

Sn, no Sb and low P

O

(Table 1). These three glasses may represent the natural

378

levels of these elements in the Egypt 1 glass type.

379

The values reported in Table 1 could be taken as new recycling thresholds

380

tailored on each compositional group. Nevertheless, one should remain cautious as

381

the only glass that can be surely recognized as clean raw glass is Levantine 1 since

382

Phelps et al. (2016) provided up to date data of samples from primary furnaces.

383

As discussed above Foy 2 glass is almost surely made of recycled cullet already

384

at the primary production center which makes it difficult to identify recycling

385

at the secondary level. The thresholds for the other types could only be inferred

386

from the presence of recycling markers or at best comparing raw chunks found in

387

secondary workshops. The content of elements such as Co, Cr, Ni, Zn increases

388

from HIMTa to HIMTb. The high amount of P

O

possibly suggests long and/or

389

repeated melting cycles. However, the concentration of Pb is much lower than the

390

100 ppm usually taken as recycling threshold.

391

In view of our analysis it appears that even though Cyprus was close to primary

392

centers a large amount of glass was recycled. A source of recycling material would

393

be mosaic tesserae of which the island is very rich. Contemporary Cypriot tesserae

394

were opacified using tin based opacifiers: lead stannate for yellow and green (with

395

the addition of copper) and SnO for white and blue (with the addition of cobalt)

396

tesserae (Bonnerot et al., 2016). Therefore recycling can be demonstrated by

397

plotting Pb against Sn and Cu (Figure 8). We should note that an increase of Pb,

398

Cu and Sn has been related also to the use of arsenical bronze (see (De Francesco

399

et al., 2010) and (Cagno et al., 2012)), but in view of the abundance of glass

400

tesserae on the island the latter material was much more likely used. The positive

401

correlation between Pb and Sn is rather evident, while between Pb and Cu it

402

is clear only for Levantine 1. Lead and copper are less correlated in the other

403

groups, probably because of the natural variations of these elements in the heavy

404

minerals present in the raw materials. Most of the Cypriot Levantine 1 glass has

405

been diluted with recycled mosaic tesserae. This practice is less easy to unravel for

406

Foy 2 glass as the raw product has already high content of recycling indicators,

407

however the common increase of Sn and Pb hints at a certain amount of tesserae

408

recycling. Five out of 14 HIMTa samples and two out of five Egypt 1 glasses are

409

made with the addition of recycled material, while all HIMTb from Cyprus contain

410

natural (or lower) levels of colourant if compared to the raw glass chunks found

411

in France. All results point to local Cypriot vessel glass production and thus the

412

existence of secondary glass workshops in Cyprus. Nevertheless, analytical results

413

need corroboration with the results of an ongoing typological study.

414

5 Conclusions

415

This work provides complementary trace elements data to the ongoing research on

416

the 5th-7th century AD glass assemblages from Cypriot early Christian ecclesiastic

417

buildings. The analysis of the trace elements allowed us to underline relationships

418

between three different components of the sand used in primary production such

419

10

10

10

10

10

10

10

Pb (ppm)

0.1

1

10

100

Sn (ppm)

High Mn Lev1 Lev 1 Foy 2 High Fe Foy 2 HIMTa HIMTb Egypt1 HIT

10

10

10

10

10

10

10

Pb (ppm)

1

2 3 4 5 6 7

10

2 3 4 5 6 7

100

2 3 4 5 6 7

1000 Cu (ppm)

High Mn Lev1 Lev 1 Foy 2 High Fe Foy 2 HIMTa HIMTb Egypt1 HIT

Fig. 8: There is a linear relation between lead and tin possibly because of the recycling of tesserae opacified with lead stannate. Lead is also weakly correlated with copper. Note the logarithmic scale.

as REEs, zircon and the carbonaceous fraction. Using the Nb-Ti biplot and the

420

Ce/La, Sr/Ca and Zr/Si ratios we were able to further discriminate the major

421

groups.

422

In addition, we wanted to evaluate and eventually refine the group assignments

423

made by EPMA. Very interestingly most was confirmed by LA-ICP-MS analysis,

424

but the latter technique helped assigning some glasses for which the major and

425

minor elements gave doubtful information. Trace elements provide so much more

426

information that should be preferred wherever possible as it is equally good for

427

major, minor and trace elements analysis.

428

Finally the trace element analysis allowed us to investigate glass recycling. By

429

excluding the samples which showed higher contents of recycling markers (Pb, Zn,

430

Cu, Sb and Co) and, when available, by comparing our data with published raw

431

glass, we were able to determine the composition of pristine glass of the main

432

groups reported in Table 1. We have shown that previous thresholds are very

433

high, e.g. 30 ppm of Sb (Degryse, 2014) or 10-20 ppm of Sb Rehren et al. (2015).

434

Pristine glass, with exception of Foy 2, has no Sb. Sb levels in excess of 1-2 ppm

435

are indicative of glass made with the addition of Sb-bearing cullet. With these

436

new thresholds we have shown that it is almost impossible to find unrecycled glass

437

during the Late Antique period even in a region in close proximity to primary

438

production centers. The practice of recycling would likely become more important

439

the farther we move from these primary production locations. Furthermore, the

440

common increase of Pb and Sn in the Cypriot glass suggests that glassmakers used

441

locally available glass mosaic tesserae as glass cullets.

442

Acknowledgements The authors are thankful to the Department of Antiquities for providing

443

access to the material. In particular we wish to express our gratitude to Prof. Demetrios

444

Michealides, Prof. Marcus Rautman and Prof. Sturt Manning for having granted us permission

445

to study the material they excavated. We would like to thank also Bernard Gratuze for helping

446

with the LA-ICP-MS measurements. This project has received funding from the European

447

Research Council (ERC) under the European Unions Horizon 2020 research and innovation

448

programme (grant agreement No. 647315 to NS). The funding organisations had no influence

449

in the study design, data collection and analysis, decision to publish, or preparation of the

450

manuscript.

451

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