Investigation of the adhesive residue on the flint insert and the adhesive lump found from the Pulli Early Mesolithic settlement site (Estonia) by micro-ATR-FT-IR spectroscopy/Pulli asulakohast leitud tulekivist pistiktera kiti ja kitikamaka uuringud mikro-atr- ft-ir-spektroskoopia meetodil.

Vahur, Signe ; Kriiska, Aivar ; Leito, Ivo 等

Introduction

Combined tools made of organic and mineral materials were used across extensive regions in Eurasia during the Stone Age (e.g. Rimantene 1971, 164 ff.; Matyushin 1989, 130; Oshibkina 1997, 69; Vankina 1999, fig. XXIX). In functional sense we are mostly dealing with either arrow- or spearheads, in less frequent cases daggers or other tools. In the Baltic Sea region mostly combined tools were used during the Mesolithic, especially in its earlier phase. These tools were made by attaching inserts of flint (rarely quartz) to grooves carved into bone (or wooden?) points (e.g. Apals et al. 1974, plate 1; Jaanits et al. 1982, 30). Although such combined tools are rarely found, the inserts are discovered frequently. Adhesives were used to attach the inserts to the points, in few cases the remains of the adhesive have also been preserved. From some Stone Age sites in Scandinavia, Finland, Russia, Germany and Switzerland, adhesive lumps were found, sometimes with chewing marks (e.g. Vilkuna 1963; Apals et al. 1974 and references therein; Mikhail C. Zhilin pers. com. Aprill 22, 2009).

It is generally assumed that the adhesive was made of mixtures of (fir or pine) resin, birch bark tar, beeswax, fat, etc. (e.g. Jaanits et al. 1982, 30; Regert et al. 2003, 1622). In particular the use of birch bark tar has been emphasized, sometimes in combination with other materials (Stacey 2004, 1 and references therein). Nevertheless, there is no sufficient data available to make broad material usage generalizations across different areas of Europe. Therefore, the remains of adhesive found from Pulli, two cases of which will be discussed as follows, are definitely interesting for the wider public.

A flint insert with adhesive remains on it has been found from the Pulli Early Mesolithic settlement site (Fig. 1) in south-western Estonia, radiocarbon dated to 8700-8550 cal BC (Veski et al. 2005, table 2). This small (18 x 8 x 2 mm) microlith (AI 4476: 1042) is made of blackish brown flint blade knapped from creataceous flint. One of its edges is retouched and the end can be distinguished as a barb. There are remains of black adhesive on the surface of the artefact. From the same settlement site some small black lumps have also been found, which are probably pieces of adhesive. One of the lumps (1) (AI 4476: 662) is of irregular oval shape (49 x 16 x 13 mm, 5.2 g.) (Fig. 2) and has smooth surface.

The goal of the analyses was to determine the chemical composition of the flint insert and the black adhesive layer on it and of the adhesive lump, and also to determine whether the composition of the adhesive on the insert is similar to the lump. The samples were analysed by attenuated total reflectance Fourier transform infrared spectroscopy (ATR FT-IR spectroscopy).

Infrared spectroscopy is an effective analytical method to study both organic and inorganic compounds. The vast majority of materials (e.g. oils, waxes, resins, proteins, inorganic additives etc.) absorb infrared radiation and have rather characteristic infrared spectra (Regert et al. 2003; Vahur et al. 2005). Thus, classes of compounds can almost always be identified, in most cases even in mixtures. Identification of individual compounds in their pure forms is in many cases also possible. However, identification of individual compounds in mixtures is often difficult (depending on the complexity of the mixture).

[FIGURE 1 OMITTED]

[FIGURE 2 OMITTED]

There are various techniques of sample handling used in IR spectroscopy. The reflectance techniques such as ATR (Attenuated Total Reflectance) are increasingly used. ATR FT-IR spectroscopy is a non-destructive, fast and convenient method often requiring minimal sample preparation. With micro-ATR accessory it is possible to use very small sample size and the IR spectra can be measured directly from the sample surface. These are the reasons why micro-ATR sampling method was chosen for the present investigation.

Experimental

Samples

The following samples were analysed: the flint insert (Fig. 1), black adhesive layer on the flint insert and a lump (presumably adhesive, Fig. 2). The main adhesives used in the prehistoric time were different wood tars (e.g. birch bark tar), different resins (e.g. pine resin) and their mixtures (Jaanits et al. 1982, 30; Regert et al. 2003, 1622). As stated above, these materials were probably used as adhesives for combined tools also in the Baltic region. In order to identify the adhesive materials, the spectra of birch bark tar and pine and fir resin (as the most probable ones in the Baltic geographic area) had to be registered. The pine (Pinus sylvestris) and fir (Picea abies) resin samples were of contemporary Estonian origin. The birch bark tar sample was prepared in Finland by methods of experimental archaeology carried out by Johanna Seppa.

The birch bark tar was obtained by thermal treatment of birch bark (Betula alba). The main constituents of birch bark are cellulose, tannins and oils. Birch bark tar has complex composition (Regert et al. 2006, 247 f.), containing different oils, phenolic compounds, sesquiterpenoids, partially decomposed cellulose, etc.

The samples did not need any special preparation. The spectrum of the insert was registered directly from the insert. Two pieces of the black thin adhesive layer were taken from the insert with a blunt scalpel and tweezers. The pieces of the black layer were easily removable. For the adhesive lump measurements, very small pieces of black sample were gently scraped from the adhesive lump. At first, the surface layer was scraped away and for the measurements of the IR spectrum, sample pieces from the inner part of the adhesive lump were scrabbled. Sample pieces were placed on the ATR crystal and force equivalent to 2.5 kg was used for pressing the sample pieces against the ATR crystal. After that the ATR-FT-IR spectrum was scanned.

Samples were taken from four different locations of the lump and all the IR spectra were very similar. The spectrum of the best quality is discussed in this paper.

Method

IR spectra were registered using a Nicolet 6700 FT-IR spectrometer equipped with the "Smart SplitPea" micro-ATR accessory. Thermo Electron's OMNIC Software was used for treatment of spectral data. The used FT-IR spectrometer has DLaTGS Detector, Vectra Aluminum Interferometer and sealed and desiccated optical bench with CsI optics.

The Smart SplitPea is a horizontal attenuated total reflectance (ATR) microsampling accessory for Thermo Electron's Nicolet FT-IR spectrometers. The SplitPea is designed for fast analysis of very small samples of bulk solids, powders and liquids using the ATR technique. The sampling area of the accessory has the diameter of 500 um. The accessory has a diamond ATR crystal and calibrated pressure applicator with a 50x ViewThruPress. The following spectrometer parameters were used: resolution 4 [cm.sup.-1], spectral range 225-4000 [cm.sup.-1], number of scans: 128, Level of Zero filling: 0, Apodization: Happ-Genzel.

Results

IR spectrum of the flint insert

The IR spectrum of the flint insert is presented in Figure 3. In the IR spectrum it is possible to identify Si-O stretch vibrations characteristic of various forms of silica (SiO2): at 1074 [cm.sup.-1] there is the Si-O stretching band and the bending bands for Si-O occur at 797 [cm.sup.-1], 781 [cm.sup.-1] and 462 [cm.sup.-1] (Derrick et al. 1999, 114 ff.). Crystalline silica mineral quartz has an absorption band near 1100 [cm.sup.-1] (the exact wave number depends on the type of silica), it also has a unique doublet band near 790 [cm.sup.-1], which is very characteristic (Derrick et al. 1999, 114 ff.) (Table 1).

[FIGURE 3 OMITTED]

IR spectrum of the black adhesive layer on flint insert

Two spectra collected from different parts of the black adhesive layer on the flint insert are presented in Figure 4. A number of characteristic spectral features can be seen in the FTIR spectra of the black layer and these are presented in Table 2. The spectra are somewhat different but the main features are the same.

The spectral features can be rationalized as follows. A very broad OH stretch band appears in the region 3600-3000 [cm.sup.-1]. OH groups are due to the phenolic compounds and alcohols and carboxylic acids formed via partial hydrolysis (and maybe oxidation) of the oils during the ageing. The intense C-H stretching vibrations appear at the region of 2919-2849 [cm.sup.-1] and bending bands at 1456-1374 and 720-727 [cm.sup.-1] (cis-C-H out-of-plane deformation). These bands are characteristic of alkyl fragments. In particular, the 720-727 [cm.sup.-1] band is characteristic of compounds containing long aliphatic chains (typical in oils). Intense carbonyl (C=O) bands can be seen at around 1730 and 1700 [cm.sup.-1]. Esters, ketones, aldehydes and carboxylic acids have C=O stretch bands in the region of 1750-1700 [cm.sup.-1] (Meilunas et al. 1990, 38; Ahola 1993, 28 ff.). A carbonyl in an ester occurs usually at 1750-1730 [cm.sup.-1], in a ketone usually at around 1720-1690 [cm.sup.-1], the carbonyl groups in aldehydes absorb in the range of 1740-1720 [cm.sup.-1] (Derrick et al. 1999, 100 ff.) and carboxylic acids in the range of 1730-1700 [cm.sup.-1] (Smith 1999, 245). Most likely the maxima at 1730 and 1700 [cm.sup.-1] are mainly due to esters and carboxylic acids, respectively. The presence of esters also explains the C-O and C-O-C stretching bands appearing at 1260-1000 [cm.sup.-1]. There is an interesting region in the range of 1600-1540 [cm.sup.-1]. Skeletal vibrations from the C-C interactions in the ring absorb in the region 1600-1585 [cm.sup.-1] and the asymmetric stretch of carboxylates appears at 1650-1540 [cm.sup.-1] (Derrick et al. 1999, 100 ff.; Smith 1999, 245). Little evidence of the presence of significant amounts of aromatic compounds can be seen in the spectrum: there is very weak aromatic C-H stretch (3100-3000 [cm.sup.-1]), and there are also no intense bands in the region of 700-800 [cm.sup.-1], which are very characteristic of aromatic compounds. This leads us to suggest that the broad band at 1650-1540 [cm.sup.-1] is mostly due to carboxylate anions. The anions of the carboxylic acids were probably formed during long-time interaction of the material with the carbonaceous material in soil buffered at pH range above the pKa values of carboxylic acids. There is a broad band between 1050 and 1000 [cm.sup.-1], which is present in one spectrum and absent in the other. This band is very characteristic of silicates and carbohydrates (incl. cellulose). In our case this may be due to either particles of sand or residues of plant material.

[FIGURE 4 OMITTED]

The appearance of the spectra suggests that the adhesive is a complex mixture containing different alkyl (and possibly aryl) compounds containing also hydroxyl and carbonyl groups.

IR spectrum of the adhesive lump

The spectrum of the sample piece of adhesive is presented in Figure 5. The spectrum matches exactly one of the arrowhead's black adhesive spectra.

IR spectrum of the reference birch bark tar

The reference spectrum of birch bark tar is presented in Figure 6. It is similar to the spectrum of the sample piece of adhesive (Fig. 5). Nevertheless, there are two distinct differences: the broad band in the region of 3600-3000 [cm.sup.-1] and the band in the region of 1650-1540 [cm.sup.-1] are very weak in the reference spectrum. As explained in the previous section, both these bands in the adhesive spectra may be due to carboxylic acids formed during decomposition of the original material on ageing. A more subtle, but possibly important difference, is the level of separation of the C-H stretch bands at ca 2920 and 2850 [cm.sup.-1]. They are more clearly separated in the spectrum of the adhesive sample. This indicates higher content of compounds with long un-branched alkyl chains in the adhesive sample.

[FIGURE 5 OMITTED]

[FIGURE 6 OMITTED]

IR spectrum of the fir and pine resin

For reference, fir and pine resin spectra are presented in Figures 7 and 8. The spectra differ somewhat from each other and the following features distinguish them from the arrowhead's adhesive spectrum:

1. The two bands of the aliphatic C-H stretch group (with maxima between 2840 and 2930 [cm.sup.-1]) are significantly less separated in resin spectra. This shape of the C-H stretch group is characteristic of highly branched and cycle-containing alkyl groups. Such alkyl fragments are common to terpenoid compounds that are the main constituent of fir and pine resins. In contrast to this, long straight alkyl chains display well separated maxima.

2. All resin spectra contain a very strong absorption at 1690 [cm.sup.-1] (due to the carboxylic groups of resin acids), which is missing in the adhesive spectra. At the same time the adhesive samples have a prominent band at 1700-1720 [cm.sup.-1] (due to esters), which is missing in the resin spectra.

3. The large group of bands centered around 1100-1000 [cm.sup.-1], which is a prominent feature in the adhesive spectra, is missing in the resin spectra.

In contrast to the adhesives, the resins used for recording the spectra are not of ancient origin. We therefore need to consider possible changes that could have taken place during ageing. First of all these are related to the group of bands centered around 1100-1000 [cm.sup.-1], which are due to C-O and C-O-C stretch vibrations. These bands could emerge also in resin spectra upon oxidation of the resins. However, the two maxima of the aliphatic C-H stretch group cannot become more separated with time, which would be necessary for the adhesive to be a resin.

[FIGURE 7 OMITTED]

[FIGURE 8 OMITTED]

Discussion and conclusions

The ATR-FTIR spectroscopic analyses show that the spectrum of the adhesive lump exactly matches one of the insert's black adhesive spectra. Both adhesive spectra contain bands due to alkyl groups at 2918-2849 [cm.sup.-1] (aliphatic C-H stretch) and 1461-1374 [cm.sup.-1] (C-H deformation), carbonyl groups at 1732-1703 [cm.sup.-1] (C=O stretch) and C-O stretches (and also C-O-C stretches) at 1264-1037 [cm.sup.-1].

The adhesive spectra differed considerably from the tree resin spectra, although the presence of some tree resin in the adhesive cannot be ruled out. At the same time the adhesive spectra correlated rather well with the birch bark tar spectrum. Comparing the IR spectrum of the birch bark tar to the adhesive spectra there are three differences. (1) The broad band in the region of 3600-3000 [cm.sup.-1] and (2) the band in the region of 1650-1540 [cm.sup.-1] are very weak in the reference spectrum and (3) the C-H stretch bands at 2920 and 2850 [cm.sup.-1] are better separated in the spectrum of the adhesive sample.

The broad bands 3600-3000 [cm.sup.-1] and 1650-1540 [cm.sup.-1] in the adhesive spectra may be due to carboxylic acids formed during decomposition of the original material on ageing. The better separation of the bands at 2920 and 2850 [cm.sup.-1] indicates higher content of compounds with long un-branched alkyl chains in the adhesive sample. The most obvious material with long alkyl chains (and also ester groups contributing to the bands of C=O and C-O-C) is fat.

We can thus conclude that the black adhesive lump and layer in the flint insert is birch bark tar, probably mixed with some fat and possibly containing some pine or fir resin. If adhesive was made in what is now Estonia, resin can be only from pine, because in Early Mesolithic period fir did not grow in this area (e.g. Poska 2001, 13).

There are several more or less distant analogues to the results of the analyses from Pulli. While ethnographic parallels were previously used to interpret the finds of adhesives, then predominantly since the 1990s analytical methods have been applied to study various amorphous substances (Regert et al. 2003, 1620 and the references therein).

Because organic material is poorly preserved in soil, this kind of finds, especially in case of the Stone Age, are rare. We are usually dealing with bone or antler, more seldom wooden, artefacts. Only in exceptional cases organic substance can be found, and in these cases it is possible to speculate that it might have been used as an adhesive. However, this kind of unique substances have been documented on European prehistoric artefacts already for more than hundred years (e.g. Regert et al. 2003, 1621 and the references therein). In our neighbouring countries more attention to the topic has been paid in Finland and in Scandinavia, where lumps of organic substance have been found from several settlement sites. While bite marks can often be observed on these pieces, the bits have also been interpreted as chewing resin (e.g. Palsi 1932; Kopisto 1963; Vilkuna 1963; Edgren 1984, 761). Middle and Late Neolithic pottery sherds repaired with adhesive have also been found in Finland. The adhesive has been identified as birch bark tar (Pesonen 1994; 1999 and references therein; Reunanen & Holmbom 1999). Birch bark tar is identified also in a number of Iron Age artefacts in Europe (e.g. Stacey 2004 and the references therein).

In Estonia organic adhesive on bone artefacts has been already described in the 19th century (Grewingk 1882, plate III). More recently organic adhesive has been observed on pottery sherds. However, the analyses of the pieces of the adhesive are unique in Estonia. Because no analyses of adhesives used to attach flint inserts have been conducted in our neighbouring areas, then the results we have gained are very difficult to place into a broader context. Whereas birch bark tar was a very common component in adhesives used for several purposes already from the Middle Paleolithic (Koller et al. 2001, 329) and was used even later (e.g. Nykanen & Seppa 1997, 25; Stacey 2004), it can be assumed that it was one of the main adhesives used to complete combined artefacts during the Stone Age in Europe.

doi: 10.3176/arch.2011.1.01

Acknowledgements

The authors are grateful to the Finnish archaeologist Johanna Seppa who made birch bark tar, and to the Finnish archaeologist Petro Pesonen who sent a piece of the above-mentioned tar to Estonia as comparative material. The authors also thank Kristel Kulljastinen for photographing the findings and the Russian archaeologist Mikhail C. Zhilin for personal comments. The research was supported by the European Union through the European Regional Development Fund (Centre of Excellence in Cultural Theory).

References

Ahola, P. 1993. Chemical and Physical Changes in Paints or Pained Wood Due to Ageing. VTT Technical Research Centre, Espoo.

Apals, J., Atgazis, M., Daiga, J., Denisova, R., Graudonis, J., Loze, I., Mugurevics, E.,

Aveling, E. M. & Heron, C. 1974. Chewing tar in the early Holocene: an archaeological and ethnographic evaluation.--Antiquity, 73, 579-584.

Derrick, M. R., Stulik, D. & Landry, J. M. 1999. Infrared Spectroscopy in Conservation Science. The Getty Conservation Institute, Los Angeles.

Edgren, T. 1984. Kivikausi. (Suomen historia, 1.) Weilin & Goos, Espoo, 18-95.

Grewingk, C. 1882. Geologie und Archaeologie des Mergellagers von Kunda in Estland. (Archiv fur die Naturkunde Liv-, Est- und Kurlands, I.) Dorpat.

Jaanits, L., Laul, S., Lougas, V. & Tonisson, E. 1982. Eesti esiajalugu. Eesti Raamat, Tallinn.

Kivimae, J., Kriiska, A., Poltsam, I. & Vunk, A. 1998. Merelinn Parnu. Parnu Linnavalitsus, Parnu.

Koller, J., Baumer, U. & Mania, D. 2001. High-tech in the middle Palaeolithic: neandertal-manufactured pitch identified.--European Journal of Archaeology, 4, 385-397.

Kopisto, A. 1963. Vahan lisaa purupihkasta.--Kotiseutu, 6, 206.

Matyushin, G. N. 1989. = [TEXT NOT REPRODUCIBLE IN ASCII] CCCP.

Meilunas, R. J., Bentsen, J. G. & Steinberg, A. 1990. Analysis of aged paint binders by FTIR spectroscopy.--Studies in Conservation, 35, 33-51.

Nykanen, P. & Seppa, J. 1997. Koivutervan valmistus ja kaytto.--Muinaistutkija, 1, 25-32.

Oshibkina, S. V. 1997. = OiiiHOKiiHa C. B. [TEXT NOT REPRODUCIBLE IN ASCII].

Palsi, S. 1932. Kivikautista purupihkaa.--Suomen Museo, 1931-1932, 64.

Pesonen, P. 1994. Tervanpolton juurilla--koivutervan kaytto saviastian korjauksessa kivikaudella. Tekniikan waiheita, 1, 4-7.

Pesonen, P. 1999. Radiocarbon dating of birch bark pitches in typical Comb Ware in Finland. Dig it All. Papers Dedicated to Ari Siiriainen. Ed. M. Huurre. The Finnish Antiquarian Society, The Archaeological Society of Finland, Helsinki, 191-197.

Poska, A. 2001. Human impact on vegetation of coastal Estonia during the Stone Age. (Acta Universitatis Upsaliensis. Comprehensive Summaries of Uppsala Dissertations from the Faculty of Science and Technology, 652.) Uppsala.

Regert, M., Alexandre, V., Thomas, N. & Lattuati-Derieux, A. 2006. First results on headspace-solid phase microextraction-gas chromatography/mass spectrometry of volatile organic compounds emitted by wax objects in museums.--Journal of Chromatography A, 1101: 1-2, 245-253.

Regert, M., Garnier, N., Decavallas, O., Cren-Olive, C. & Rolando, C. 2003. Structural characterization of lipid constituents from natural substances preserved in archaeological environments. Measurement Science and Technology, 14, 1620-1630.

Reunanen, M. & Holmbom, B. 1999. Analysis of the pitch sample.--Dig it All. Papers Dedicated to Ari Siiriainen. Ed. M. Huurre. The Finnish Antiquarian Society, The Archaeological Society of Finland, Helsinki, 197-199.

Rimantene, R. K 1971. = [TEXT NOT REPRODUCIBLE IN ASCII].

Smith, B. 1999. Infrared Spectral Interpretation. Asystematic Approach. CRC Press, London.

Stacey, R. 2004. Evidence for the use of birch-bark tar from Iron Age Britain.--Past. The Newsletter of the Prehistoric Society, 14, 1-2. http://www.ucl.ac.uk/prehistoric/past/past47.html 09.03.2009.

Vahur, S., Virro, K. & Leito, I. 2005. Web-based infrared spectral databases relevant to conservation.--CAC, Journal, 10-17.

Vankina, L. 1999. The collection of Stone Age bone and antler artifacts from Lake Lubana. Catalogue. (Latvijas Vestures Muzeja raksti, 4. Arheologija.) Latvijas Vestures Muzeja, Riga.

Veski, S., Heinsalu, A., Klassen, V., Kriiska, A., Lougas, L., Poska, A. & Saluaar, U. 2005. Early Holocene coastal settlements and palaeoenvironment on the shore of the Baltic Sea at Parnu, southwestern Estonia.--Quaternary International, 130, 75-85.

Vilkuna, K. 1963. Purupihka, vanhin nautintoaineemme.--Kotiseutu, 4, 76-84.

(1) This has erroneously been taken for a schematic animal figurine of bone (e.g. Jaanits et al. 1982, 30; Kivimae et al. 1998, 28).

Table 1. Bands wavenumbers in IR spectrum of flint insert

Band ([cm.sup.-1])    Intensity   Assignment

2922, 2851            Weak        Aliphatic C-H stretch
1165, 1074            Strong      Si-O stretch
795, 781, 695, 462    Medium      Si-O bending

IR spectrum of the black adhesive layer on flint insert

Table 2. Bands wavenumbers in the IR spectra of black layer on insert

Band            Intensity, shape     Assignment (possible compound
([cm.sup.-1])                        classes are given in brackets)

3600-3000       Medium, very broad   O-H stretch, hydrogen-bonded
                                       (alcohols, carboxylic acids,
                                       water)
2919-2849       Strong peak with     Aliphatic C-H stretch
                  two maxima           (alkyl groups)
1725-1700       Strong               C=O stretch (esters, ketones,
                                       aldehydes)
1600-1540       Medium, broad        Near ~1597 [cm.sup.-1] aromatic
                                       ring (C-C) stretch; near ~1540
                                       [cm.sup.-1] asymmetric C-O
                                       stretch (carboxylates)
1456-1374       Medium               C-H deformation (alkyl groups)
1248            Weak                 C-O stretch (esters, alcohols)
1168-1088       Medium               C-O-C stretch (esters)
1035-1039       Variable             C-O stretch or Si-O stretch
                                       (alcohols, silicates)
720-727         Weak                 C-H wagging (alkyl groups)

Signe Vahur, Chair of Analytical Chemistry, Institute of Chemistry, University of Tartu, 14a Ravila St., 50411 Tartu, Estonia; signe.vahur@ut.ee

Aivar Kriiska, Chair of Archaeology, Institute of History and Archaeology, University of Tartu, 3 Lossi St., 51003 Tartu, Estonia; aivar.kriiska@ut.ee

Ivo Leito, Chair of Analytical Chemistry, Institute of Chemistry, University of Tartu, 14a Ravila St., 50411 Tartu, Estonia; ivo.leito@ut.ee