Birch bark tar and jewellery: The case study of a necklace from the Iron Age (Eckwersheim, NE France)

Journal of Archaeological Science: Reports 20 (2018) 72–79 Contents lists available at ScienceDirect Journal of Archaeological Science: Reports journal homepage: www.elsevier.com/locate/jasrep Birch bark tar and jewellery: The case study of a necklace from the Iron Age (Eckwersheim, NE France) T Blandine Courela, , Philippe Schaeffera, Clément Féliub,c, Yohann Thomasb, Pierre Adama ⁎ a Université de Strasbourg, CNRS, CHIMIE, UMR 7177, F-67000 Strasbourg, France INRAP Grand-Est, F-67100 Strasbourg, France c Université de Strasbourg, Université de Haute Alsace, CNRS, ArchIMèdE UMR 7044, F-67000 Strasbourg, France b A R T I C LE I N FO A B S T R A C T Keywords: Birch bark tar Conifer resin Early Iron Age Necklace Jewellery Lupane-related triterpenoids Organic residue analysis has been undertaken on an organic material found on a necklace with a pendant unearthed from a necropolis dated to the Early Iron Age (800–475 BCE) and located in Eckwersheim (NE France). The molecular composition of the substance, which was investigated using gas chromatography coupled to mass spectrometry, points towards an adhesive used to stick two bronze half-spheres to form a pendant. The predominance of triterpenoids from the lupane series led to the identification of the adhesive as a birch bark tar and constitutes a rare example of the use of such a material in jewellery in the past. 1. Introduction Archaeological evidence of birch bark tar, an adhesive substance obtained by dry pyrolysis of birch bark (e.g. Regert et al., 1998; Koller et al., 2001; Rageot et al., 2015), is abundantly documented in the literature. Considered as one of the first man-made organic material, birch bark tar is frequently present among archaeological finds dated from the Neolithic period in Europe and its first use by Neanderthalians is dated back to the Palaeolithic (Koller et al., 2001). This sticky and hydrophobic material has been used for many purposes such as the hafting of lithic or bone tools (e.g. Regert et al., 1998; Koller et al., 2001), the reparation and waterproofing of pottery (e.g. Binder et al., 1990; Charters et al., 1993; Connan et al., 2000; Urem-Kotsou et al., 2002; Rageot et al., 2015), the decoration of ceramics (e.g. Vogt, 1949; Trąbska et al., 2011; Rageot et al., 2015) and, more surprisingly, as chewing-gum (e.g. Aveling and Heron, 1999; Van Gijn and Boon, 2006; Karg et al., 2014). The identification of birch bark tar mainly relies on the detection of lupane-related triterpenoids as diagnostic molecular biomarkers. These compounds possess indeed ideal features since lupane-related triterpenoids are abundant in birch tar and their assemblage can be considered as highly specific (Hayek et al., 1989; Schnell et al., 2014). In addition, these compounds are relatively resistant to various alteration processes such as biodegradation which probably accounts for their good preservation even in ancient archaeological samples dating back to the Palaeolithic and Mesolithic Periods (Koller et al., 2001; Aveling and Heron, 1998). We report here the investigation by gas chromatography–mass spectrometry (GC–MS) of the lipid content extracted from the adhesive having served to assemble two bronze half-spheres in order to form the pendant of a necklace (Fig. 1) dating to the Hallstatt D1 period (625–550 BCE). In the light of the molecular investigation of reference samples of birch bark and birch bark tars, the origin of the lupanerelated triterpenoids detected within the lipid extract is discussed as well as its significance in an archaeological context. 2. Necropolis of the Forest of Brumath and funerary ornaments of tomb 6008 In 2010, archaeological excavations were undertaken in Eckwersheim (Alsace, NE France) by a team from INRAP (Institut National de Recherches Archéologiques Préventives). These excavations completed previous excavations of an important necropolis composed of more than thirty tumuli in the Forest of Brumath (Thomas and Féliu, 2012; Michler et al., 2017). Among them, a 42 m diameter circular pit delineated Tumulus 2 (Fig. 2) was shown to be composed of a primary mound measuring before erosion 25 m diameter and probably up to 4 m high, in which a central burial (Tomb 6011, Hallstatt C-D1 period) was placed. Surrounding the central tomb, 17 further burials dating back to Hallstatt D1 and D2 were disposed concentrically. Tomb 6008 was one of the burial located in the outlying area of Tumulus 2, at a distance of 15 m from the tumulus centre. The burial was a quadrangular pit (2.7 m long × 1.75 m wide; Fig. S1). The Corresponding author at: The British Museum, Great Russell Street, London WC1B 3DG, United Kingdom. E-mail addresses: BCourel@britishmuseum.org (B. Courel), p.schaef@unistra.fr (P. Schaeffer), clement.feliu@inrap.fr (C. Féliu), yohann.thomas@inrap.fr (Y. Thomas), padam@unistra.fr (P. Adam). ⁎ https://doi.org/10.1016/j.jasrep.2018.04.016 Received 9 January 2018; Received in revised form 9 April 2018; Accepted 20 April 2018 2352-409X/ © 2018 Published by Elsevier Ltd. Journal of Archaeological Science: Reports 20 (2018) 72–79 B. Courel et al. Fig. 1. Photo of the upper part of the Tomb 6008 with its furniture and artefacts dated to the Hallstatt D1 period (625–550 BCE) (a); necklace (b and c.1) with the pendant (b and c.2); Picture of the organic substance found between the two bronze halfspheres sampled for the present study (d). Photos by ©Y. Thomas, INRAP and drawing by ©J. Gelot, INRAP. based core (Fig. 1.d). In view of the importance of this vegetal-based core and the design of the pendant, archaeologists have assumed that the organic substance was part of the original design rather than a subsequent repair. Two other pendants were found in Tombs 6005 and 6026 (Fig. 4). 3. Materials and method 3.1. Archaeological and reference samples 0.7 mg of the archaeological organic core (1.31 g, diameter of ca. 16 mm) located inside the pendant of tomb 6008 has been sampled for organic residue analysis (sample A, INRAP - reference number: PRL BG 6008–04). The reference samples comprise: (1) the lipid extract of pieces of a rotten birch wood (Betula pendula; sample B); (2) the lipid extract of pieces of the same birch bark sample pyrolysed with a heat gun during 10 min under N2 atmosphere (sample C); (3) a birch bark tar from Betula pendula prepared by descending distillation (sample D, reference number 162/5 20 18 00, Dr. Andreas Kurzweil, Museumsdorf Düppel, Berlin, Germany; see Thomas and Claude, 2011 and Pietrzak, 2012, p.41–42 for detailed information on the technical aspects of the descending distillation method). 3.2. Lipid analysis Samples A - D were extracted by sonication using a mixture of dichloromethane/methanol (CH2Cl2/CH3OH, 1:1, v/v) followed by filtration of the supernatant through celite and removal of the solvent under reduced pressure. An aliquot of the extract in CH2Cl2 was acetylated (Ac2O, N-methylimidazole, 30 min, ambient temperature) and, after removal of the solvents and excess reagents, treated with a solution of diazomethane in diethylether to methylate the carboxylic acids. The derivatised crude extract was fractionated on a silica gel column into an apolar fraction eluted with CH2Cl2/EtOAc (8:2, v/v) which was analyzed by GC–MS and a more polar fraction eluted with CH2Cl2/CH3OH (1:1, v/v) which was not further investigated. Fig. 2. Tumulus 2 unearthed in Eckwersheim (NE France). Drawing by ©Y. Thomas, INRAP. presence of a wood coffin, approximately 0.86 m wide, was suggested based on the identification of decomposed woody material remains. Additional evidences supporting this identification included black organic matter observed within the archaeological soil, which was in contact with the coffin, and the difference between the soil having filled stratigraphic units 3 and 4 (Fig. S1). Adnorments including a pair of fibulae, a necklace and its pendant, bracelets and belt components were unearthed from Tomb 6008 (Fig. 3). The navicella-type fibula is typical of the Hallstatt D1 period (625–550 BCE). The pendant, attached to the torc by means of a small ring (Fig. 1.b), is composed of two small halfspheres made of copper alloy which have been placed around a vegetal- 3.3. GC–MS GC–MS analyses were carried out using a Thermo Trace gas chromatograph (Thermo Scientific) equipped with an autosampler Tri Plus, a programmed temperature vaporizing (PTV) injector and a HP5-MS 73 Journal of Archaeological Science: Reports 20 (2018) 72–79 B. Courel et al. Fig. 3. Funerary artefacts found in Tomb 6008: fibulae (1 and 2), torc (3.1) with a pendant and a small ring (3.2), bracelets (4.1, 4.2 and 5), belt components (6) and a ring (7). Drawings by ©J. Gelot and Y. Thomas, INRAP. angiosperm source. These triterpenoids comprise, notably, lupeol 1 (numbers refer to structures presented in the Appendix), betulin 2, lupenone 3, betulone 4, lupanone 5 and lupan-3,28-diol 6 (Fig. 5) which are important triterpenes of birch bark (Hayek et al., 1990; Krasutsky, 2006; Schnell et al., 2014; Orsini et al., 2015) and, consequently, to birch bark tar (Modugno et al., 2006; Regert et al., 2006). The predominance of the same compounds among the lipids of the reference birch bark samples B-D (Fig. 6) was thus not unexpected. Betulin 2, in particular, largely predominates the gas chromatogram of the lipids from the birch bark sample B as previously reported (Orsini et al., 2015), representing in our case ca. 60% of the apolar lipids. The identification of triterpenoids from the lupane series in the archaeological sample A points thus clearly to birch bark tar which may have been used as an adhesive material. It can be proposed that bark of either Betula pendula (silver bitch) or Betula pubescens (downy birch) served as column (30 m × 0.25 mm i.d. × 0.25 μm film thickness) using He as carrier gas (constant flow rate at 1.1 ml/min). Temperature program: 70 °C–200 °C (10 °C/min), 200 °C–300 °C (4 °C/min), isothermal at 300 °C (40 min). The mass spectrometer was operating in the electron ionization (EI) mode at 70 eV with a scan range of 50 to 700 m/z. The data were investigated using Xcalibur Software and mass spectra were compared with the NIST library and literature data. 4. Results and discussion 4.1. Triterpenoids of the lupane series as main components in birch bark GC–MS analysis of the organic extract of the substance from the pendant (sample A) led to the identification of specific triterpenoids from the lupane series, indicating a predominant contribution from an 74 Journal of Archaeological Science: Reports 20 (2018) 72–79 B. Courel et al. significantly different from that observed with fresh birch bark (Fig. 6). This can likely be explained by the various alteration processes affecting triterpenoids upon the thermal treatment used for the preparation of the substance, in addition to other processes induced by oxidative ageing and/or by diagenetic alteration during burial in the soils (see discussion below). Such processes may notably account for the lower proportion of betulin 2 in the archaeological sample as compared to the large predominance of 2 in fresh birch bark (Fig. 6; Hayek et al., 1989), as well as for the absence of betulonal 7 and betulinic acid 8 - two native triterpenoids from birch bark (cf. sample B; Fig. 6) - in the archaeological material (Fig. 5). In parallel with the absence of some genuine lupane-related triterpenoids from birch bark, three families of triterpenes likely resulting from alteration processes were detected in sample A. They comprise Aneo‐triterpenoids (e.g., 9, 10), Δ2 triterpenoids (e.g., 12, 13) and allobetulane derivatives (e.g., 14–16). Δ2 lupane-derivatives 12 and 13 detected in samples A, C and D most likely originate from the thermallyinduced dehydration of lupeol 1 and betulin 2 and can be considered as typical pyrolytic compounds. Their formation was observed, notably, during a controlled pyrolysis experiment of birch bark by Regert et al. (2006) and they generally occur in archaeological birch bark tars (e.g. Binder et al., 1990; Charters et al., 1993; Aveling and Heron, 1998; Regert et al., 2003; Rageot et al., 2015). Allobetulane derivatives (11, 14–16; mass spectra in Fig. S2) were also present in samples A, C and D. These compounds are postulated to be formed by an acid-catalysed intramolecular rearrangement of ring E (Green et al., 2007; Salvador et al., 2009). Since these compounds exclusively occur in tars (sample D) and not in birch bark (sample B), they are thus considered, like Δ2 derivatives, to be closely associated to the thermal alteration undergone by birch bark during tar preparation (cf. Rageot et al., 2015). The acid catalysis necessary for the intramolecular rearrangement of ring E might have been provided by phenols formed by the pyrolytic degradation of lignin (Faix et al., 1990). The A-neo‐triterpenoids found in sample A (9 and 10; MS data shown in Fig. S3 in Supplementary data) and in the reference thermallytreated samples C and D (9–11) derive, respectively, from lupeol 1, betulin 2 and allobetulin 16 by contraction of ring A. Their formation most likely involves the acid-catalysed loss of the C-3 alcohol moiety followed by a Wagner-Meerwein rearrangement (Salvador et al., 2009 and references therein). To our knowledge, these compounds have not been reported from fresh birch bark and might have two distinct origins. They can be formed during the preparation of the tar as shown by Rageot (2015) in the case of birch bark tars prepared in the laboratory, the acid catalysis necessary for their formation being possibly induced, as mentioned above, by phenols formed upon lignin pyrolysis (Faix Fig. 4. (a) Upper part of the bronze sphere attached to a ring by means of a thin and flat pin found in Tomb 6005; (b) Pendant of the Tomb 6026, on the left: organic core, on the right: one of the damaged half-sphere (bronze sheet remains maintained in sandy sediment). Eckwersheim, NE France. Photos by ©Y. Thomas, INRAP, drawing by ©J. Gelot, INRAP. raw material for the preparation of the adhesive since they are the most common Betula species in Europe (Beck et al., 2016). 4.2. Altered triterpenoids as markers for ageing and thermal treatment Even if lupane-related triterpenoids native of birch bark still constitute a substantial amount of the birch bark tar samples - including mainly lupeol 1 and betulin 2 in accordance with the literature (Modugno et al., 2006; Regert et al., 2006) -, the distribution of the triterpenoids in the archaeological sample (Fig. 5) is, however, Fig. 5. Gas chromatogram of the lipid extract of the adhesive of the necklace pendant (sample A). Bold numbers refer to the structures shown in Appendix. Alcohols are analyzed as acetates and carboxylic acids as methyl esters. 75 Journal of Archaeological Science: Reports 20 (2018) 72–79 B. Courel et al. Fig. 6. Gas chromatograms of the organic extract of (a) altered birch bark sample (sample B); (b) altered birch bark sample after pyrolysis (sample C); (c) reference birch bark tar prepared by descending distillation (sample D). Bold numbers refer to the structures shown in Appendix. Alcohols are analyzed as acetates and carboxylic acids as methyl esters. They comprise di-dehydroabietic acid 17, dehydroabietic acid 18, 7oxodehydroabietic acid 19 along with the related compounds 20 and 21 bearing a hydroxyl function at C-15. All of them derive from abietic acid 22, a diterpenic acid ubiquitously found in conifer resins (Otto and Wilde, 2001) and especially abundant in resins from Pinaceae (Otto et al., 2007). The contribution of these diterpenoids revealed thus the presence of conifer material in the archaeological substance which could either correspond to a raw resin or a tar made by dry distillation of conifer wood (e.g. Evershed et al., 1985; Connan and Nissenbaum, 2003; Bailly, 2015). However, in the case of conifer tars, aromatic diterpenoids like retene 23 resulting from the thermal transformation of resinic acids upon pyrolysis are generally abundant. In the present case, such compounds could not be detected, suggesting that the use of a conifer tar can be ruled out. Thus, the ingredient corresponds most likely to a conifer resin. In addition, the identification of oxidized et al., 1990). In addition, Rageot (2015) has shown that their relative abundance is correlated with the experimental conditions, higher temperatures or longer heating periods likely favoring their formation. Nevertheless, the detection of A-neo‐triterpenoids, even in small amounts, in the altered birch bark sample B indicates that natural alteration processes may also account to some extent for their formation, possibly induced by enzymes or acids released from decaying wood or from (micro)organisms involved in wood degradation. 4.3. Input of conifer resin attested by diterpenoids Besides the predominant lupane-related triterpenoids reported above, GC–MS analysis of the organic extract of the substance from the pendant (sample A) revealed an additional contribution of small amounts of early eluted compounds corresponding to diterpenoids. 76 Journal of Archaeological Science: Reports 20 (2018) 72–79 B. Courel et al. 2015). The identification of birch bark tar on the Iron-Age pendant thus represents a unique case of the use of this plant product in Alsace. Moreover, in addition to the investigated pendant from Tomb 6008, two other artefacts found in Eckwersheim in Tombs 6005 and 6026 were assigned as small pendants made by joining together two bronze hollow-domes (Fig. 4). This feature is characteristic of elements described as Zweischalenhänger-type (Warneke, 1999), when the pendant is associated to a torc, or as Bommel-Ohranhänger-type when it is worn as earrings (exemples can be found within the funeral assemblages of Tombs 16, 26 and 44 found in the Magdalenenberg tumulus, VillingenSchwennigen, Germany; Spindler, 1971, 1972). Other specimens are known in Hallstatt tombs and reflect a common usage of such ornaments in Hallsttatt D1 (625–550 BCE), especially in the Upper Rhine region, from the Rhin-Main confluence area (in the North) to the Swabian Jura region (in the South; Warneke, 1999). In Alsace, such jewellery types are attested among the funerary ensemble of the Forest of Haguenau, but were not further studied (Schaeffer, 1930). In Eckwersheim, a nice specimen of Hallstatt pendant was found in Tomb 6005 (Fig. 4.a). It is composed of the upper part of a bronze sphere attached to a ring with a thin and flattened pin. This design suggested a pendant of Zweischalenhänger type. The current assumption is that this pendant was used as earrings. An organic core was observed within the bronze sphere of the pendants found in Tombs 6008 and 6026. However, only the organic core of the pendant of the Tomb 6008 was sampled for molecular analysis. Little is known of such pendants. Warneke suggested that most of them were made with an interior core, referring to the example of one pendant's core made of clay (found in Urbach 3) and another made of jet (in Hemishofen-Sankert SH; Warneke, 1999). Spindler (1973) suspected the use of bone within the specimen found in Tomb 71 of Magdalenenberg tumulus. However, no chemical analyses have been carried out on this specific type of artefacts to date. Therefore, our study brings new insights on the method employed to create Zweischalenhänger pendants and may suggest that less-known and/or locally produced variants exist, such as the use of an organic core made of birch bark tar. Further chemical investigations on Zweischalenhänger and Bommel-Ohranhänger pendants, at a regional and extra-regional scales, need to be undertaken to improve our understanding on their manufacturing process. derivatives of 18, comprising compounds with a ketone at C-7 (19 and 21) or a hydroxyl group at C-15 (20 and 21), suggests that the resin was severely altered by oxidative processes during the ageing of the adhesive substance (Colombini et al., 2005; Osete-Cortina and DoménechCarbó, 2005; Bailly, 2015). 4.4. Mode of preparation of the adhesive - origin of the conifer resin component The presence of lupane-related biomarkers which have obviously undergone thermal alteration (formation of Δ2-triterpenoids and allobetulane derivatives, notably; see above) in sample A clearly indicates that the organic substance corresponds to an adhesive material (birch bark tar) used to assemble the pendant. However, the contribution of small proportions of conifer resin raises the question whether the addition of the latter was intentional or was the result of a “contamination” during birch bark tar preparation. Indeed, a few examples of mixtures of archaeological birch bark tar containing additional substances have been reported by several authors. These substances comprise animal fat, beeswax, plant oil (Dudd and Evershed, 1999; Regert et al., 2003; Van Gijn and Boon, 2006; Rageot et al., 2015) and, in some rare cases, conifer resin/tar (Stacey, 2004; Rageot et al., 2015). The input of additional substances can sometimes be intentional in order to improve the properties of the material. For instance, the addition of beeswax to birch bark tar leads to a substance which is less brittle than pure birch bark tar (Regert et al., 2003; Van Gijn and Boon, 2006). It is interesting also to note that Stacey (2004) and Rageot et al. (2015) reported the identification of adhesives very similar in molecular composition to sample A, and which consisted of small amounts of conifer resin mixed with birch bark tar. They were used, respectively, for gluing coral studs on strap unions from harness fittings (Stacey, 2004) and for the reparation of pottery (Rageot et al., 2015). This supports the hypothesis that the admixture of small amounts of conifer resin to birch bark tar might possibly improve its performances as an adhesive and that such mixtures could have been prepared on purpose for this specific application. However, based on the relatively small contribution of the conifer resin as compared to the birch bark tar in the case of sample A, it cannot be excluded that the container used for the storage or the production of birch bark tar may have served previously to collect or store a conifer resin and might have thus contaminated the birch bark tar. It should however be mentioned that the low proportion of diterpenes relative to triterpenes observed in sample A does not necessarily reflect the initial proportions of conifer resin and birch bark tar making up the adhesive. Indeed, resinic acids might be significantly more sensitive to oxidative alteration given the presence of reactive benzylic positions on dehydroabietic acid 18 and related structures as illustrated by the detection of several oxidized dehydroabietic acid derivatives (19–21) in sample A. The presence of several oxygenated functionalities on these structures might thus be responsible for an enhanced water solubility as compared to lupane-related triterpenoids, thus favoring their progressive removal by leaching, notably after burial. 5. Conclusion Identification of triterpenoids from the lupane series in the organic material found on an archaeological necklace pendant clearly indicates that this substance was made predominantly of birch bark tar, together with a small contribution of conifer resin. This material likely corresponds to an adhesive that could have served to fix a decorative item on the pendant. Except the identification of birch bark tar serving to join two coral studs on horse rein rings (Stacey, 2004), no other evidence of birch bar tar adhesives was reported so far for making ornaments. Thus, our finding represents a unique example of birch bark tar used in jewellery. The question remains regarding the role of the conifer resin which could either be part of the ingredients added intentionally to improve the quality of the adhesive material, or correspond to a “contamination” introduced during birch bark tar preparation. 4.5. New insights on the manufacturing of Hallstatt jewellery Production and use of birch bark tar are attested in Western and Central Europe during Pre- and Protohistoric times (e.g., Binder et al., 1990; Sauter et al., 2000, 2002; Rageot et al., 2015; Sicard, 2017). However, no evidence for the use of this organic material has been recorded in Alsace, despite the presence of birch as a minor tree species, based on palynological and charcoal studies, in the region during the Hallstatt period (Vigreux et al., 2012; Ertlen et al., 2014; Croutsch et al., Acknowledgement We thank A. Kurzweil for the birch bark tar reference sample produced according to traditional techniques (Museumsdorf Düppel, Berlin). 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