CHARACTERISATION OF FINE WALL AND EGGSHELL ROMAN POTTERY BY RAMAN SPECTROSCOPY more

Research Article Received: 4 December 2009 Accepted: 4 June 2010 Published online in Wiley Online Library: 21 July 2010 (wileyonlinelibrary.com) DOI 10.1002/jrs.2748 Characterisation of fine wall and eggshell Roman pottery by Raman spectroscopy† M. Olivares,a∗ M. C. Zuluaga,b L. A. Ortega,b X. Murelaga,c A. Alonso-Olazabal,b M. Urteaga,d L. Amundaray,d I. Alonso-Martina and N. Etxebarriaa Roman pottery from the Oiasso harbour (nowadays Irun, Basque Country) was thoroughly studied by micro-Raman spectroscopy and X-ray diffraction (XRD) analysis in order to identify and characterise the mineralogical composition of those samples and to get a deeper insight into the technologies involved in the elaboration of the ceramic artefacts. The diffraction pattern of the ceramic body shows the presence of mullite and trydimite, which suggests firing temperatures above 1100 ◦ C, and alumina-rich raw materials. Additionally, the presence of pseudowollastonite and diopside observed by the XRD and Raman spectroscopy is explained by the high firing temperatures of lime-rich raw materials. Raman microscopy has also provided further information about the heating temperature and composition of the raw materials. The presence of rutile instead of anatase also suggests a strong heating process. Moreover, the presence of hematite (α-Fe2 O3 ) and maghemite (γ -Fe2 O3 ) instead of magnetite (Fe3 O4 ) suggests the oxidising conditions during ceramics firing. The comparison of the minerals found in the different Roman potteries with the characteristic mineralogy of the archaeological site suggests the use of raw material coming from different source areas, opening up an interesting discussion about the commercial networks. Copyright c 2010 John Wiley & Sons, Ltd. Keywords: Raman spectroscopy; archaeological materials; Roman pottery; firing temperature Introduction Scientific studies in art and archaeology are gaining interest owing to the increasing technical requirements in cultural heritage studies. A wide range of archaeological and arthistorical objects are commonly analysed, including ceramics,[1] artworks[2] and historical paintings,[3,4] pigments[5,6] and historical buildings,[7] among others. The interest in the chemical analyses of archaeological samples lies in the knowledge of their material composition in order to characterise and understand the context in which the artefact was created, to avoid fakes,[8] or to determine their real historical value.[9] Besides, the co-operation of scientists and experts in archaeological science is required in order to grasp the historical, archaeological and compositional information of art-historical objects. In the context of Roman archaeology in Hispania, the northern area and particularly the Cantabrian area were considered less significant. However, the archaeological activities carried out since 1992 have led to the discovery of the Roman harbour of Irun (Basque Country).[10] The works in this site led to the identification of the location of the Oiasso village located near the Bidasoa estuary on the boundary between Aquitaine and Iberia.[11] The urban character of the settlement of Oiasso is confirmed by the archaeological indicators, which highlight the presence of public toilets and baths and a number of evidences of everyday activity, among others natural tissues, footwear, ornaments and jewellery and gambling. The harbour foundation has been dated between 70 and 120 AD, maintaining its activity at least until the end of the 2nd century. The Oiasso harbour played an important commercial route in the Atlantic area. In fact, the harbour activity was influenced by the mining of argentiferous galena from Pe˜ as n de Aya (Basque Country) to obtain silver. Additionally, among the imported products, Terra sigillata tableware pottery and other fine wall ceramics were of notable interest. The aim of this work was to identify and characterise the mineralogical composition of Roman pottery fragments from the Oiasso harbour. This characterisation can contribute to the understanding of the nature of raw materials and the technology implemented for the elaboration of the ceramic artefacts, such as firing temperature and firing atmosphere under which these vessels were produced. Moreover, going deeper into the data interpretation, the characterisation of the raw material can give us the clues to interpret diverse issues such as the provenance of the source of the archaeological artefacts and consequently the commercial routes and the historic migrations or mobility patterns. Since the use of destructive and invasive techniques is severely limited, vibrational spectroscopic techniques, and specifically Raman spectroscopy, has been strongly considered as ∗ Correspondence to: M. Olivares, Department of Analytical Chemistry, University of the Basque Country, Barrio Sarriena s/n, P.O. Box 644, E-48080 Bilbao, Spain. E-mail: maitane.olivares@ehu.es Paper published as part of the Art and Archaeology 2009 special issue. † a Department of Analytical Chemistry, University of the Basque Country, E-48080 Bilbao, Spain b Department of Mineralogy and Petrology, University of the Basque Country, E-48080 Bilbao, Spain c Department of Stratigraphy and Paleontology, University of the Basque Country, E.48080 Bilbao, Spain 1253 d Centro de Estudios e Investigaciones Hist´ rico-Arqueol´ gicas ARKEOLAN, o o E-20340 Irun, Spain J. Raman Spectrosc. 2010, 41, 1253–1259 Copyright c 2010 John Wiley & Sons, Ltd. M. Olivares et al. Figure 1. Archaeological pottery samples collected from Oiasso harbour: (a) fine-ware samples photographs; (b) drawing of fine-ware small cups and (c) drawing of eggshell ware mould-made hemispherical cup. demonstrated by the applications in the literature.[12 – 15] As a matter of fact, this non-invasive and non-destructive analytical method has proved particularly useful for the characterisation of ceramics.[16 – 20] Micro-Raman spectroscopy gives the possibility to obtain direct information about the structural characteristics and the minerals constituting the pottery without any sample pre-treatment. Additionally, Raman spectroscopy provides information on the molecular composition, and this allows us to distinguish between different mineral polymorphs [e.g. anatase and rutile (TiO2 ) or calcite and aragonite (CaCO3 )], which, in several cases, is the key to understand the context in which the artefact was created or the damage suffered by the materials. In addition to Raman spectroscopy, in this work we have used X-ray diffraction (XRD) to study the specific phases of crystalline minerals found in the ceramic pieces collected and to complement the information obtained from the Raman spectra. Taking into account that XRD is a technique focussed on the crystal structure of the mineral phases, the combination of both approaches can provide additional information. and fine-ware vessels were used to hold liquids and mainly to drink, although some types might have also had other uses, such as gauges, containers of ointments or cosmetics or in ritual ceremonies.[21 – 22] The studied potteries correspond to small cups, with a lightly carinated profile, and vary considerably in their details. Some have grooves below the rim, or a small foot ring. Mould-made hemispherical cups are the most common eggshell ware (Fig. 1). Most samples were imported, and were produced in the north of Italy and widely distributed across North–East Gaul, Britain and Germania. Techniques Micro-Raman spectroscopy Mineralogical and compositional analyses of archaeological pottery samples were carried out directly, without any pretreatment of the sample, by means of micro-Raman spectroscopy. Raman spectra were recorded with a Renishaw InVia Raman spectrometer coupled to a Leica DMLM microscope having a spatial resolution of 2 µm with 50× objectives. The system has two lasers, emitting at 514 nm (ion-argon laser, Modu-Laser) and 785 nm (diode laser, Torsana). In this work, the laser of 514 nm with a holographic grating of 1800 lines/mm was used. The laser of 514 nm has a nominal power at the source of 50 mW, the maximum power at the sample being 20 mW. In all measurements, the power of the laser was reduced by using neutral density filters in order to avoid the photo-decomposition of the samples (burning) or phase transitions. Each spectrum took 10 s to record, and 10 scans were accumulated in the spectral window from 100 to 3000 cm−1 with 10% of the maximum power of the 514-nm laser. The size of confocal hole, magnification of the objectives and laser power were varied to obtain the optimum measurement conditions at each spot. Although in some recorded spots a fluorescence background was observed, the Raman spectra J. Raman Spectrosc. 2010, 41, 1253–1259 Experimental Studied samples Thousands of potsherds were recovered (over 50 000) and recognised as belonging to more than 100 types of forms and to at least 60 different ways of pottery making.[11] The studied samples correspond to fine-ware pottery, known as eggshell pottery, and represent less than 3% of the found potsherds. These ceramics correspond to minute and scarce potsherds in most Roman archaeological findings due to the fragility and small size of the pieces. These fine-ware and eggshell pottery are dated from the 1st and 2nd centuries AD. Fine-ware ceramics correspond to tableware in contrast to cooking, storage or transport potteries. Eggshell 1254 wileyonlinelibrary.com/journal/jrs Copyright c 2010 John Wiley & Sons, Ltd. Characterisation of fine wall and eggshell Roman pottery by Raman spectroscopy are presented as acquired, i.e. no further transformations were carried out. The WIRE software (Renishaw, UK) was used to collect Raman spectra and the collected spectra were processed using the Omnic software (Nicolet, Madison, WI, USA). X-ray diffraction The mineralogical structure of the potsherds was determined by means of XRD. XRD data were collected from the archaeological pottery using a Philips PW1710 diffractrometer with Cu Kα radiation, automatic divergence slit and a diffracted-beam graphite monochromator. The XRD operating conditions were 40 kV and 20 mA, suitable for routine measurement of powder samples. Random powder samples were prepared for mineralogical analyses. X-ray diffraction XRD measurements provide information about the relative quantities of the different mineral phases present within a material and, thus, give compositional information about the raw material. In the first attempts of the potteries’ characterisation, promising indicators of the firing conditions were obtained by means of XRD. The diffraction pattern of the ceramic body is illustrated in Fig. 2. As can be seen, the presence of some minerals (i.e. mullite and trydimite) suggests the use of high firing temperatures. Moreover, the presence of mullite in the ceramics confirms also a firing temperature of over 1100 ◦ C of the kaolinite-rich material.[24] Furthermore, a high percentage of quartz and mullite supports the fact that the raw material was rich in aluminium and silicon.[28] Traces of pseudowollastonite (CaSiO3 ), diopside (CaMgSi2 O6 ) and plagioclase feldspar (anorthite) are likely to be present, as can be observed in the diffraction pattern illustrated in Fig. 3. Such traces can be indicative of the composition of the original clay or the raw material composition. In this sense, the presence of both pseudowollastonite and diopside minerals in the pottery indicates that the raw material contained calcite and dolomite, whereas the newly formed mineral phases indicate the firing temperature and conditions. Micro-Raman spectroscopy Based on the analysis of the diffraction patterns, we were able to identify the crystalline phases such as mullite, quartz, trydimite and several silicates. In addition to this, many of the facts already observed could be confirmed by Raman spectroscopy and additional information relating to traces and even glassy or molecular structures could be obtained by analysing directly the surface of the pieces. According to the XRD measurements, α-quartz was the more abundant mineral phase, and, in fact, the strong characteristic Raman band due to the symmetric bending vibration (Si–O–Si) of α-quartz at 464 cm−1 was detected (Fig. 4) in all of the analysed pottery samples. The Raman spectrum is also characterised by the medium bands due to the lattice modes at 198 and 262 cm−1 and the weak bands at 355 and 402 cm−1 due to asymmetric bending modes of the silica tetrahedra. Furthermore, the appearance of the Si–O asymmetric stretching mode at approximately 1130 cm−1 instead of at approximately 1000 cm−1 is in agreement with the high-temperature firing of the pottery.[29] It is known that α-quartz is commonly found in ancient ceramics, and this means that the raw material used to produce the pieces were rich in silica. This is also supported by the results of XRD measurements.[30] Additionally, the analysis of the traces may be the key when the determination of the ceramic production processes is required. This is the case with the presence of iron compounds. In Fig. 4, the Raman spectrum of ilmenite (FeTiO3 ) is illustrated, showing its strong characteristic Raman band at 681 cm−1 . The smaller characteristic Raman bands of ilmenite (i.e. 228 and 370 cm−1 ) have not been detected because of the fluorescence in this point of the sample. The presence of this mineral suggests that the composition of the raw material was rich in iron and titanium. Moreover, the study of the type of oxidation state of iron provides information about the atmosphere under which the clay was fired. According to this, the presence of black iron oxides, such as magnetite, would indicate a reducing atmosphere, and reddish iron oxides, such as hematite, would mean that an oxidising environment was used.[26] In this work, iron oxides such as α-Fe2 O3 (hematite) and γ -Fe2 O3 (maghemite) were Results and Discussion Based on the good quality of XRD and Raman spectra, the characterisation of ceramics was accomplished and the keys to understand developing pyrotechnologies and raw material properties were achieved. It is known that some minerals decompose upon heating and, consequently, form new mineral phases. Based on the analysis of these minerals, the manufacturing conditions of the archaeological artefact can be arrived at. This is the case for illite/muscovite, which gradually decomposes between 850 and 1000 ◦ C. Due to the presence of titanium in these minerals (referenced traces up to 1 wt%), titanium oxides are formed during the breakdown of illite/muscovite.[23 – 26] This information can play an important role in the characterisation because the different polymorphs of titanium dioxide appear at different temperatures. According to Trindade et al., carbonate type minerals are involved in most of the mineralogical changes observed in potteries.[27] This is the case with wollastonite, which is formed at the carbonate–quartz interface through the following reaction: CaCO3 (calcite)+SiO2 (quartz) → CaSiO3 (wollastonite)+CO2 (1) Another example is the formation of diopside, which requires a dolomite–quartz interface and temperatures up to 900 ◦ C. In these conditions, diopside starts to form according to the following reaction: 2SiO2 (quartz) + CaMg(CO3 )2 (dolomite) → CaMgSi2 O6 (diopside) + 2CO (2) Moreover, new feldspars can be formed after firing and having the necessary minerals. This is the case of anorthite, which can be observed at 900 ◦ C if calcium carbonate, illite and quartz are present in the raw material: Illite + 2calcite + 4quartz → 2K-feldespar + 2anorthite + 2CO2 + H2 O (3) In alumina-rich raw materials, mullite formation occurs, which is the unique stable synthetic phase in the alumina–silica phase diagram with composition close to 3Al2 O3 · 2SiO2 . The detection of mullite plays an important role, as its crystallisation requires kaolinite-rich raw materials and high temperatures (over 1100 ◦ C), as can be seen in the following reaction[24] : Al2 Si2 O5 (OH)4 (kaolinite) → Al6 Si2 O13 (mullite) + 4SiO2 (amorphous) + 6H2 O (4) 1255 J. Raman Spectrosc. 2010, 41, 1253–1259 Copyright c 2010 John Wiley & Sons, Ltd. wileyonlinelibrary.com/journal/jrs M. Olivares et al. Figure 2. XRD pattern of Oiasso Roman pottery showing firing temperature indicator minerals: mullite, trydimite and rutile. Figure 3. XRD pattern of Oiasso Roman pottery showing firing temperature and compositional indicator minerals: pseudowollastonite, diopside and plagioclase. detected, as can be seen in the Raman spectra of Fig. 4. Together with the characteristic Raman bands of hematite [226 cm−1 (s), 296 cm−1 (vs), 410 cm−1 (s), 612 cm−1 (m) and 1320 cm−1 ], traces of maghemite [500 cm−1 (broad)] and magnetite [Fe3 O4 , 667 cm−1 (broad)] were also detected. Nevertheless, according to several authors, this last broad band (∼660 cm−1 ) can be attributed to a molecular distortion of hematite, so, a priori, both possibilities can be considered.[31] The presence of magnetite means that the items were fired under reducing conditions, which could have been induced by the presence of organic components in the raw clay mineral. However, as the presence of hematite is significantly higher, it can be suggested that the firing conditions were oxidising. This means that the presence of magnetite can be indicative of an incomplete phase transformation from magnetite to hematite, and so maghemite can be an intermediate of such an oxidation process.[32] Other minerals that appear in the ceramics at trace levels are titanium dioxides. The analysis of the crystalline form in which they appear in the vessels plays an important role, as they can provide constraints on the minimum firing temperature conditions. It is known that they appear during illite/muscovite breakdown between 500 and 600 ◦ C but, if the temperature is higher, the predominant TiO2 polymorph is different. Figure 5 shows the Raman spectra of the different TiO2 polymorphs found in the analysed vessels. Anatase represents one of three naturally grown polymorphs of titanium dioxide. The vibrational Raman modes of anatase are characterised by a strong band at 144 cm−1 together with 195 cm−1 (w), 395 cm−1 (m), 512 cm−1 (m) and 636 cm−1 (m) Raman bands (Fig. 5). It is a common component of soil J. Raman Spectrosc. 2010, 41, 1253–1259 1256 wileyonlinelibrary.com/journal/jrs Copyright c 2010 John Wiley & Sons, Ltd. Characterisation of fine wall and eggshell Roman pottery by Raman spectroscopy Figure 4. Raman spectra of the main component and different iron compounds found in the analysed ceramics: (a) α-quartz, (b) ilmenite (FeTiO3 ), (c) mixture of iron oxides: hematite (α-Fe2 O3 ), traces of maghemite (γ -Fe2 O3 ) and traces of magnetite (Fe3 O4 ). Figure 5. Raman spectra of the polymorphs of titanium dioxide present in Roman ceramics: (a) anatase, (b) both anatase and rutile minerals, (c) anatase to rutile transition phase and (d) rutile. minerals and is also present as ancillary constituent in kaolins, so most probably the anatase mineral was part of the composition of the used clay.[33] If only anatase were found, it could be thought that the firing temperature was below the anatase–rutile phase transition, which takes place above 800–950 ◦ C.[34] However, in almost all the samples, Raman spectra of the transition process and rutile phase were acquired. It is well known that anatase converts directly or via brookite to rutile after heating. As no brookite Raman spectrum was found, it can be suggested that the transition occurs directly and can be monitored by Raman spectroscopy due to the characteristic strong band of anatase at 153 cm−1 instead of at 144 cm−1 . In the same way, the other bands are also shifted to 205, J. Raman Spectrosc. 2010, 41, 1253–1259 278, 399 and 610 cm−1 (Fig. 5). This transition depends on several factors such as the size of particles, the presence of other minerals like iron-rich clay minerals and, above all, the firing temperature.[35] Since the reactions occur in the solid state and that the heating rate is high, the co-existence of low- and high-temperature mineral phases is possible. In this sense, besides anatase, rutile was also widely detected among the samples, which is characterised by the following Raman bands: 240, 438 and 606 cm−1 (Fig. 5). Although anatase was not completely transformed into rutile, the presence of the high-temperature polymorph rutile was sufficiently high to suggest that rutile came from the firing of anatase-rich clay at high temperatures (>1200 ◦ C). 1257 Copyright c 2010 John Wiley & Sons, Ltd. wileyonlinelibrary.com/journal/jrs M. Olivares et al. Figure 6. Raman spectra of both polymorphs of CaSiO3 found in analysed vessels: (a) pottery sample, (b) natural wollastonite, (c) pseudowollastonite and (d) quartz. In the same way as for titanium dioxide minerals, the presence of wollastonite gives clear clues about the firing temperature. In Fig. 6, the Raman spectra of both polymorphs of CaSiO3 , pseudowollastonite and natural wollastonite are shown. The characteristic Raman signature of wollastonite is the strong band at 971 cm−1 together with those at 413 (w), 637 (w) and 1050 cm−1 (m), whereas the characteristic Raman bands of pseudowollastonite are the strong band at 988 cm−1 and those at 372 (w), 580 (w) and 1075 cm−1 (m) as has been reported in the literature.[36] If the firing temperature is higher than their decomposition temperature (950 ◦ C), feldspars with high concentrations of calcium can be obtained. In fact, the detection of wollastonite in pottery samples suggests once again that the firing temperature was high. Besides the information about the pyrotechnological conditions used to produce the analysed ceramics, the composition of the used raw material can be also suggested. In this sense, the presence of mullite indicates the use of alumina-rich clays for manufacturing ceramics with a high content of kaolinite, known as chine clay deposits. These types of deposits are associated with severe weathering of alkali feldspar granites. Neither chine clay deposits nor alkali feldspar granites occur in the studied geological region. This implies that the vessels rich in mullite found in the studied site have a foreign origin. The presence of fine wall potteries with different high-temperature mineralogy in the same archaeological site indicates the use of different raw materials coming from different source areas. All this may open up an interesting discussion about the commercial networks in the 1st and 2nd centuries AD in the Cantabrian coast. Acknowledgements This work was partially supported financially by UNESCO 07/01. MO is grateful to the University of the Basque Country for a Ph.D. Fellowship. Thanks are also due to the LASPEA service and SGIker service from the University of the Basque Country. Conclusions The combination of XRD and micro-Raman spectroscopy techniques was shown to be a powerful analytical method to characterise archaeological artefacts. In archaeological pottery analysis, the specific and non-destructive identification of trace minerals can provide archaeologists with valuable information about the manufacturing conditions such as the minimum firing temperature and the atmosphere under which the pottery was produced. In this work, we have obtained satisfactory results regarding the pyrotechnological conditions implemented for the production of the analysed Roman ceramics and also compositional information of the raw material. 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