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Proceedings of the 39th International Symposium for Archaeometry, Leuven (2012) 164-170
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Byzantine glass bracelets (10th-13th century A.D.) found on Romanian territory investigated using external IBA methods
R. Bugoi1, I. Poll2, Gh. M!nucu-Adame"teanu2, T. Calligaro3 and L. Pichon3
1. Horia Hulubei National Institute for Nuclear Physics and Engineering, 30 Reactorului Street, M!gurele 077125,
Romania, [email protected]
2. The Museum of the City of Bucharest, 2 I.C. Br!tianu Avenue, Bucharest 030174, Romania, [email protected], [email protected]
3. Centre de Recherche et de Restauration des Musées de France, CNRS UMR 171, Palais du Louvre, Paris CEDEX 01, France,
[email protected], [email protected] ABSTRACT This paper reports the chemical composition of twelve glass bracelet fragments discovered in the Byzantine site of Nuf!ru, Romania and dated from the 10th to the 13th century A. D. The experimental data were obtained using Ion Beam Analysis (IBA) techniques. The measurements were made using the external 3MeV proton micro-beam of the AGLAE accelerator facility of the Centre de Recherche et de Restauration des Musées de France (C2RMF), located in the basement of the Louvre museum in Paris and exclusively dedicated to archaeometric research. The chemical composition of the glass bracelets was determined by simultaneously using two IBA techniques, namely Particle-Induced X-ray Emission (PIXE) and Particle Induced Gamma-ray Emission (PIGE). PIXE-PIGE measurements provided the chemical composition of the bulk glass, the main aim of the investigation being to acquire some information about the glass recipes employed and the raw materials used by the Byzantine glassmakers. The external IBA data indicate that the analyzed bracelets are soda-lime-silica glasses, most of them pertaining to the ‘mixed natron-plant ash’ category. This type of ancient glass was previously identified in artefacts dated to the end of the 1st millennium A.D. The chemical compositions suggest that the craftsmen who manufactured the bracelets practiced glass recycling. The PIXE-PIGE compositional data reported in this paper also helped in the identification of the chromophores used to provide the colour of the glass bracelets – cobalt, manganese and iron ions. KEYWORDS Byzantine glass, glass bracelets, cromophores, PIGE, PIXE.
Introduction Samples and archaeological background Archaeological excavations from the last four decades revealed the existence of a Byzantine site buried under Nuf!ru village located on St. George’s arm of the Danube - see figure 1. The archaeological investigations brought to light numerous finds (ceramics, coins, weapons, glass items, etc.) showing that an important urban settlement thrived in this very place during the 10th-13th centuries A.D. The settlement was protected by a fortification raised a fundamentis by the Byzantines at the end of the 10th century - beginning of the 11th century, during the reign of the emperor Basil II (976–1025) (Damian et al. 2007-2008, M!nucu-Adame"teanu 1998, 2001).
Fig. 1. Map showing the location of Nuf!ru, the archaeological site where the Byzantine glass bracelets analyzed in this study were discovered.
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The large dimensions of the inhabited area and the high number of tombs from Nuf!ru indicated that the apex in the evolution of Nuf!ru Byzantine settlement was during the 11th century A.D. Among the most frequently discovered adornments were glass bracelets, mostly in a fragmentary state. Such artefacts were encountered both in the inhabited levels and the graves (Damian et al. 2007-2008, M!nucu-Adame"teanu et al. 1997). Similar bracelets were also discovered in other nearby Byzantine settlements located on the Danube arms, in archaeological sites such as P!cuiul lui Soare, Dinogetia-Garv!n and Isaccea-Noviodunum-Vicina – see figure 2 (Diaconu 1965, Diaconu et al. 1972, M!nucu-Adame"teanu et al. 2012, #tefan et al. 1967).
Fig. 2. Map indicating the location of all archaeological sites from Romania where Byzantine glass bracelets were discovered - for more details on this topic, see M!nucu-Adame"teanu et al. (2012). Glass bracelets were fashionable trinkets in the Byzantine Empire during the 10th-13th centuries A.D., similar items were frequently found in archaeological sites located in what is now the territory of Greece, Bulgaria, Serbia, Turkey and Russia (Antonaras 2005, Cangova 1961, Lauwers et al. 2010, Lightfoot 2005, Ne"eva 1979, Philippe 1970, Ristovska 2009, Shtereva 2000). The Byzantine glass bracelets from Nuf!ru have different cross-section shapes (e.g. circular, semi-circular, ellipsoidal, or square); some of them are straight, while others are twisted. They have various dimensions. Their external surface is some cases smooth, while grooves and protuberances are visible on the external side of other bracelets. Most of these glass adornments are coloured in different hues of blue, but green, black, violet, yellow or colourless bracelet fragments were also found. Some bracelets were painted with geometrical or vegetal decorations produced by thin strokes of paint (! several mm thick), while others were made by twisting glass canes of
different colours (M!nucu-Adame"teanu et al. 2012). The bracelets show variable degrees of transparency, a feature that was strongly influenced by the glass weathering phenomena that took place during the burial of these fragile objects in humid soil for centuries (Melcher et al. 2008). The external IBA investigations of the twelve Byzantine glass bracelets from Nuf!ru reported in this paper were initiated to determine the chemical composition of these finery items, with the aim of identifying the recipes and raw materials employed in their manufacturing. The interest in this subject was also triggered by the relative scarcity of archaeometric studies on Byzantine glass artefacts from the Middle Byzantine period (10th-13th centuries A.D.) (James 2012, Keller et al. 2010). Some considerations about the chemical composition of archaeological glass objects From ancient times, glass was made by mixing different basic components, each playing its role. The fundamental component of ancient glass is silica (SiO2), also known as the ‘vitrifying agent’ or ‘network former’, found in nature as sand or quartzite pebbles. Pure silica could hardly be used for glass production due to its high melting point (! 1700°C). Therefore fluxes, which lowered the melting point of the mixture, were introduced to the glass batch. The most common fluxes, employed throughout the history of glassmaking, have either vegetal (ashes of coastal plants or trees) or mineral origin (natron, also known as trona - i.e. a mix of sodium carbonates, sulphates, and chlorides). These raw materials are reflected in the chemical composition of glass by the presence alkali metals oxides, such Na2O and/or K2O. Glass made of silica and alkali oxides only would not be at all stable. Lime (CaO) plays the role of glass stabilizer, making this material more durable against weathering. Stabilizer was either already contained in the basic raw materials (calcareous sands or plant ashes) or it was purposely added to the glass batch, for example as crushed shells (Freestone 2006, Henderson 2000, Wedepohl et al. 2000). Glass raw materials also contain impurities such as aluminium, magnesium, phosphorus, chlorine, titanium, zirconium and iron compounds etc. (Fiori et al. 2004, Freestone 2005, 2006, Henderson 2000). In addition to the basic constituents, colorants, decolourants and/or opacifiers, such as manganese, tin, antimony, cobalt, and copper compounds, generally in low amounts, were intentionally introduced to the batch with the purpose of modifying the final appearance of the glass (Fiori et al. 2004, Freestone 2005, 2006, Henderson 2000). Various authors that studied the chemical composition of ancient glass objects proposed different classifications, the distinction between groups being made using the concentrations of the major and minor components. From ancient glass investigations published in the last decades, a relatively limited number of glass categories were identified from the Bronze Age until the Late Middle Ages (Freestone 2005, Gratuze et al. 1990, Sayre et al. 1961). The scientific studies on the chemical composition of vitreous artefacts
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provided information about the ancient glass production technologies and raw materials. During the Roman period, there was a prodigious expansion in the employment of glass items. Roman glass artefacts had a remarkably homogeneous chemical composition (Freestone 2005, Henderson, 2000). Thus, the dominant type of glass from the middle of the 1st millennium B.C. until the end of the 1st millennium A.D. in the Mediterranean zone and Europe was soda-lime-silica glass with low K2O and MgO contents, induced by the use of natron as a raw material (Foster et al. 2009, Freestone 2005, 2006, Fiori et al. 2004, Henderson, 2000). The end of the 1st millennium A. D. witnessed the abandonment of the Egyptian natron sources for glass manufacturing (Gratuze et al. 1990, Shortland et al. 2006). Plant ashes re-emerged as a flux for glass production in the Mediterranean and Middle Eastern regions (Gratuze et al. 1990, Whitehouse 2002), wood ashes playing a similar role in the Central and Western Europe zones (Wedepohl et al. 2011). These changes in glass raw materials were reflected by the chemical composition of ancient vitreous artefacts. Of a particular interest for this paper is the identification of the intermediate type of ‘mixed natron-plant ash’, found in artefacts dated to end of 1st millennium A.D., excavated from various places in Europe and the circum-Mediterranean region (Andreescu-Treadgold et al. 2006, Arletti et al. 2010, Dussart et al. 2004, Henderson et al. 2004, Silvestri et al. 2011, Uboldi et al. 2003). Experimental The twelve bracelet fragments discovered in Nuf!ru were measured using the external IBA set-up of the AGLAE accelerator of C2RMF, located in the basement of the Louvre Museum in Paris (Pichon et al. 2010, Salomon et al. 2008). The experiments were performed using a 3MeV external proton beam focused to 50µm, with current intensities of hundreds of pA and acquisition times of around 2 minutes. The measuring zone was flushed with a 2.5 l/min stream of helium. For PIXE signals acquisition, two Si(Li) detectors were used, one for the quantitative determination of major elements, and the other optimized for the detection of trace-elements. This second Si(Li) detector was covered with a 50µm aluminium filter, in order to improve the detection limits for the trace-elements contained in the glass matrix. A HPGe detector was employed for the detection of prompt-induced gamma-rays, particularly for the 440keV gamma-ray emitted by the sodium atoms present in the glass samples. Since the analyzed glass fragments were buried for almost a millennium, relatively thick corrosion layers (tens or even hundreds of microns) were present on bracelets surfaces. In order to secure an accurate determination of the chemical composition, before the IBA experiment, the glass bracelet fragments were wet polished using SiC paper (1200 grit) on
their cross-sections. The actual measurements took place on 200!200µm2 areas on these freshly cleaned flat zones. BRILL A glass standard from the Corning Museum of Glass was used to obtain PIGE quantitative results, mainly the quantification of sodium using the 440keV proton-induced gamma-ray line. BRILL B glass standard from the Corning Museum of Glass was used to check the accuracy of the IBA data. PIXE-PIGE results on this reference glass are reported at the end of table 1, together with the corresponding recommended values. The Limits of Detection (LOD) for a soda-lime-silica glass matrix analyzed with the above-described experimental IBA set-up range from 10µg/g up to 1000µg/g, depending on the element. PIXE quantitative results were obtained by running TRAUPIXE software (Pichon et al. 2010) using GUPIXWIN engine (Campbell et al. 2010, Maxwell et al. 1989, Maxwell et al. 1995) with the assumption that targets were thick (>100µm) and homogeneous and that all elements were in present in oxide form. Prior to the IBA measurements, the bracelets were visually examined using an optical microscope. The chemical composition data for the analyzed bracelet fragments, given in the most stable oxides (except for Cl), and expressed as wt% and/or "g/g, and normalized to 100%, are listed in the table at the end of the paper (N.D. means Not Detected). The total combined uncertainty (relative values) of the reported concentrations is of the order of 5-10% for major and minor elements, and up to 20% for trace-elements. Results and discussions The quantitative results reported in table 1 show that the analyzed Byzantine glass bracelet fragments are soda-lime-silica glasses, with the following mean values for the main oxides: 12.2 (± 1.9) wt% Na2O, 7.3 (± 1.6) wt % CaO and 68.7 (± 4.6) wt% SiO2. The compositions are in good agreement with the ones reported by Lauwers (Lauwers et al. 2010) for some Byzantine glass bracelets discovered from Sagalassos, in present Turkey, also dated to the 10th -13th century A.D. Figure 3 shows that the majority of the MgO and K2O concentrations for the analyzed objects are situated between the two distinct fields of the typical natron and plant ash glasses indicated by Lilyquist (Lilyquist et al. 1993). Correlating the information provided by this plot and the data from other studies on ancient glass artefacts from the same period, most of the investigated Byzantine glass bracelet fragments (ten out of twelve samples) can be assigned to the intermediate type of ‘mixed natron-plant ash’ glass (Andreescu-Treadgold et al. 2006, Arletti et al. 2010, Dussart et al. 2004, Henderson et al. 2004, Silvestri et al. 2011, Uboldi et al. 2003).
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The exceptions are Nuf!ru 1978/1 and Nuf!ru 1980/5 fragments, with relatively high contents of MgO and K2O (both !2.0wt%), that can be positively identified as manufactured using plant ashes. One can speculate that the craftsmen manufacturing the Byzantine glass bracelets recycled natron glass – largely widespread during the Roman period, mixing it with a certain proportion of plant ash glass. However, they also used an alternative solution: glass made exclusively using plant ash, as testified by the coeval Nuf!ru 1978/1 and Nuf!ru 1980/5 samples.
0 1 2 3 4 5 6 7 8 90
1
2
3
4
5
1978
/1
1979
/7 19
79/9
1980
/2
1980
/3
1980
/4
1980
/5
1979
/8
1981
/6
1981
/16
1981
/27
1981
/52
K2O
(wt%
)
MgO(wt%)
natron
plant ash
Fig. 3. K2O (wt%) versus MgO (wt%) concentrations for the Byzantine glass bracelets. The general pattern of chemical compositions presented in table 1 suggests a somehow careless recycling of coloured glass chunks/glass debris made to obtain the rods from which the Byzantine bracelets were fashioned. The practice of melting coloured glass mosaic tesserae for obtaining coloured glass objects during the early medieval period was mentioned by Theophilus in his treatise De diversis artibus (Theophilus). Blue was the most often encountered colour in these glass artefacts. Thus, five out of the twelve analyzed bracelet fragments (Nuf!ru 1978/1, Nuf!ru 1979/7, Nuf!ru 1980/4, Nuf!ru 1981/6 and Nuf!ru 1981/27) are dark blue. Correlating their colour with the chemical composition, the responsible chromophore was identified as cobalt (CoO " 500µg/g mean value in these blue bracelets) - see table 1. In these samples, cobalt is accompanied by relatively high levels of iron (" 1.5wt%) and zinc (" 600µg/g), which are associated elements in some cobalt ores (Fiori et al. 2004, Henderson 2000). Three bracelet fragments (Nuf!ru 1980/2, Nuf!ru 1981/16 and Nuf!ru 1981/52) are green. Their intense colouration could be correlated with the high amounts of iron present in their composition (3.0, 2.2 and 5.0wt% Fe2O3, respectively). In this case, iron is not present as a contaminant of the raw materials (sand or chromophoric compounds), but rather an illustration of iron minerals purposely introduced to the glass batch for colouring purposes (Fiori et al. 2004, Henderson 2000, Van der Linden et al. 2009).
One of the analyzed samples is violet, Nuf!ru 1979/8, a fragment whose composition shows a rather high content of manganese (MnO=1.8wt%), indicating the intentional use of pyrolusite (MnO2) for its colouring (Fiori et al. 2004, Henderson 2000). A similar situation was encountered for the beige bracelet, Nuf!ru 1979/9, for which 2.6wt% MnO was found in its composition. In both these cases, the MnO amount is roughly double than the one of Fe2O3. Nuf!ru 1980/3 fragment owes its slight greenish colouration to its rather high iron content (Fe2O3=1.6wt%) that was not fully compensated for by the manganese present in it (MnO"7400µg/g). A special case is the Nuf!ru 1980/5 bracelet, a transparent colourless glass fragment. This is one of the objects identified as having relatively high contents of both MgO and K2O, compared to the majority of samples analyzed in this experiment. Both MnO and Fe2O3 concentrations are not only very low (both " thousands of µg/g), but also rather well-balanced (6178µg/g MnO versus 4319µg/g Fe2O3) and all other chromophoric trace-elements and contaminants are either in very small amounts, or below the LODs. All these findings that can be correlated with the fact that this is practically a colourless glass fragment. All in all, one might conclude that this glass fragment did not result from recycling other glass chunks, but it was the outcome of primary manufacturing procedures based on the use of plant ashes as a raw material. Conclusions This paper reports the chemical composition of twelve Byzantine glass bracelets found in Nuf!ru, Romania, obtained using external PIXE-PIGE techniques at AGLAE accelerator of C2RMF. Most of the analyzed glass bracelets were assigned to ‘mixed natron-plant ash’ type, a transitional category of glass previously encountered in vitreous objects dated to the end of the 1st millennium A.D. The chemical composition of these glass artefacts reflects the abandonment of natron, practically the only kind of flux used for glassmaking during the Roman period, and the reintroduction of plant ash glass. The experimental data helped in the identification of glass chromophores (cobalt, manganese and iron ions), and also suggested the practice of coloured glass recycling procedures. Acknowledgements Financial support by the Access to Research Infrastructures activity in the 7th Framework Programme of the EU CHARISMA Grant Agreement no. 228330 (http://www.charismaproject.eu) is gratefully acknowledged. Brice Moignard from C2RMF is thanked for the technical support provided during the experimental campaign at AGLAE accelerator from June 2010. The authors are grateful to Isabelle Biron and Patrice Lehuédé from C2RMF for the exciting and fruitful discussions about ancient glass. Mihai Florea from Muzeul Na"ional de Istorie a României, Bucure#ti is acknowledged for drawing the maps published in this paper.
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Appendix: Table 1.
