An integrated spectroscopic approach to investigate pigments and engobes on pre-Roman pottery

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Research ArticleReceived: 29 July 2010 Accepted: 15 October 2010 Published online in Wiley Online Library: 7 December 2010

(wileyonlinelibrary.com) DOI 10.1002/jrs.2845

An integrated spectroscopic approachto investigate pigments and engobeson pre-Roman potteryGiuseppe E. De Benedetto,a∗ Silvia Nicoli,a Antonio Pennetta,a

Daniela Rizzo,a Luigia Sabbatinib and Annarosa Mangoneb

Painted Canosa ceramics were examined to identify the nature of the pigments employed and their manufacturing technology.A multi-technique approach was used, comprising Raman microspectroscopy and laser ablation hyphenated to inductivelycoupled plasma-mass spectrometry (LA-ICP-MS). The analysed samples were mainly produced for burial in tombs and were notintended for everyday use. They belong to the period between the end of the mid-7th century and the first half of the 4th centuryBC, and were excavated from the Toppicelli archaeological district near the suburbs of Canosa (Puglia, Italy). Forty-eight potteryfragments were available for this study. No handling of the samples was required for the Raman study, and it was possible toexcise the pigmented layer in such a way that the lacunae were not distinguishable to the naked eye due to the micrometricsize of the laser spot as far as LA-ICP-MS is concerned. Their combination turned out to be quite useful for the investigationof these archaeological materials: the chemical nature of the white, red, brown and black pigments employed in the potterymanufacture was investigated. Iron and manganese compounds were identified as the red and brown/black main colouringsubstances, respectively; on the other hand, whites and engobes (whitish slips) were based on kaolinite. This set of colouringsubstances is of importance, as it enabled the artisan to obtain in one oxidising firing cycle brown, black and red paints. Finally,the finding of manganese black in these Canosa potsherds confirms that Canosa was an important centre connecting the nearEast to central Italy and Europe since the pre-Roman age. Copyright c© 2010 John Wiley & Sons, Ltd.

Supporting information may be found in the online version of this article.

Keywords: micro-Raman; LA-ICP-MS; manganese black; pre-Roman pottery; kaolinite

Introduction

Canosa, and more generally the basin of the river Ofanto in Puglia(southern Italy), was extensively inhabited from around the BronzeAge until the late Middle Ages, and hence several different areas ofarchaeological interest lie in this region. In one of these areas, theToppicelli district in the neighbourhood of Canosa, excavationsstarted in 1975 by the Sopraintendenza ai Beni Archeologici dellaPuglia. The finding of tanks for clay sedimentation surroundedby stone walls and of large holes full of kiln wastes suggestedthe presence of an extensive potters’ quarter, whose activitylasted at least for three centuries.[1] The samples of the presentstudy belong to this archaeological site and are decorated withgeometric patterns made of white, red, brown and black pigments,some of which have a whitish slip or engobe and have been datedbetween the 4th and 7th century BC.[1,2]

Pigment, glaze and body analysis of ceramics can be undertakenfor different purposes like classifying the artefacts, knowing thetechnological features such as firing temperature as well as reveal-ing ancient trade routes or sources of raw materials. A few studieson the pigment identification in pottery excavated in similararchaeological contexts have already been published: the red-pigmented layer used in a polychrome image of a woman’s headfound in a hypogeum excavated in the zone of Canosa was identi-fied as haematite,[3,4] while a mixture of haematite and ochre[5] wasused on pottery found in Arpi (Puglia, Italy). Carbon of either veg-etal or animal origin was identified in black-pigmented layers on

pottery excavated in the same archaeological site.[6] X-ray photo-electron spectroscopy (XPS) and Fourier transform infrared (FTIR)spectroscopy have also been employed to investigate the pig-ments used to decorate pottery unearthed in Canosa:[7 – 9] the ma-terials used were found to be based on kaolinite, feldspars, carbon,manganese and iron compounds. Finally, rose madder, goethite,Egyptian blue, burnt Sienna and a carbonaceous black were identi-fied in the later findings (3rd–4th century BC) of Magenta ware.[10]

Raman spectroscopy has been applied with success to the studyof pigments and slips on ceramics.[11 – 15] The wide wavenumbercoverage of the Raman spectra, typically 100–3200 cm−1, providesunequalled means of analysis of inorganic minerals (especiallyheavy metal salts, oxides or sulfides). Raman spectroscopy alsopossesses several advantages, the foremost being the non-destructiveness and the requirement of little or no samplepreparation.

∗ Correspondence to: Giuseppe E. De Benedetto, Laboratorio di Chimica Analiticaper l’Ambiente e i Beni Culturali, Dipartimento dei Beni delle Arti e della Storia,Universita del Salento, viale San Nicola, I-73100 Lecce, Italy.E-mail: [email protected]

a Laboratorio di Chimica Analitica per l’Ambiente e i Beni Culturali, Dipartimentodei Beni delle Arti e della Storia, Universita del Salento, viale San Nicola, I-73100Lecce, Italy

b Dipartimento di Chimica, Universita di Bari, via Orabona, 4, I-70106 Bari, Italy

J. Raman Spectrosc. 2011, 42, 1317–1323 Copyright c© 2010 John Wiley & Sons, Ltd.

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G. E. De Benedetto et al.

Laser ablation inductively coupled plasma mass spectrometry(LA-ICP-MS) is one of the most rapidly growing techniques tocharacterise the distribution of trace elements in archaeological,environmental and geological materials. A review of LA-ICP-MStechniques to characterise archaeological materials is detailedelsewhere.[16] LA-ICP-MS offers unique advantages as an elementalanalytical technique, as it gives the chemical composition at tracelevel and the isotopic pattern and requires only micrometricsample.[17,18] Pigments,[19] glazes[20] and ceramics[21] have beenstudied by employing this technique.

The aim of the present work was to perform Raman and LA-ICP-MS analyses of several fragmentary Canosa ceramics to identifythe nature of the pigments employed and to get informationon the manufacturing techniques used to produce them. Thiscase study permits also the assessment of the potential of Ramanand LA-ICP-MS techniques as complementary tools for valuablearchaeological or artistic objects, as, indeed, no preparation of thesample is required and the required microsampling is generallyacceptable to archaeologists.

Materials and Methods

Samples

The fragments chosen for analysis (Table S1) were excavated from13 tombs and two kilns unearthed in the Toppicelli archaeologicaldistrict. The tombs were spread over a large area, not far fromthe potters’ quarter. The richer and larger tomb (presumably ofa prince) was dated to the second half of the 7th century BC,[22]

whereas the other 12 smaller and less elaborate ones, about 200 maway from the prince’s tomb, were dated to a period from themid-6th to the end of the 5th century BC.[23] Most of the potteryfindings are characterised by the presence of geometric drawingsbased on white, red, brown and black colours (Fig. S1).

The decorated ceramic fragments, chosen to represent theconsiderable amount of archaeological finds, were labelledaccording to the sites they were excavated from. The shardslabelled A are those found in the prince’s tomb. The samplesB were chosen among the pottery shards found in one of theclay sedimentation tanks of kiln 1, whereas the samples C camefrom one of the refuse pits of kiln 2, not far from kiln 1. Theseshards were attributed to the sub-geometric south-Daunio period,ranging from the 7th to the end of the 6th century BC. Sampleslabeled D and E were chosen from the numerous small brokenpieces of pottery found in 2 (tomb number 2 and 4, respectively)of the 12 minor tombs, and were dated to the period ranging fromthe second half of the 6th century to the mid-5th century BC.

Instrumentation

Micro-Raman spectra were recorded with a Renishaw Inviainstrument equipped with both diode (785 nm) and He–Cd(325 and 442 nm) lasers. A low incident laser power on thesamples, up to 2.0 mW, avoided alterations during analyses.Two objectives of 20× and 50× of a Leica DMLM microscopewere used to focus the laser beam on the samples and topick up the backscattered Raman signal. The former objective,having a larger focal length, was used with the coarser samples.The exposure time and the accumulations were selected to getsufficiently informative spectra; five scans of 20 s each weregenerally used. The wavelength scale was calibrated using a Si(111)standard (520.5 cm−1). The pigments were identified through a

comparison with the library of standard spectra collected withthe same apparatus[24] or with the reference spectra in theliterature.[25 – 28] The recorded spectra were sometimes treatedby baseline correction, and no other tool, such as smoothing, wasused.

LA-ICP-MS analyses of selected ceramic shards were performedusing a New Wave Research UP213 aperture-imaged laserablation accessory coupled to a Thermo Elemental X7 ICP-MSwith an enhanced-sensitivity XS-option interface. The operatingconditions for the laser and the ICP-MS are given in Table S2.The laser fluence was kept low to reduce the contribution fromthe underlying ceramic. Nonetheless, this contribution cannot beruled out.

Sample ablation was conducted under a He atmosphere andthe ablated material was carried to the ICP-MS in a mixed streamof Ar and He. Calibration was done using the silicate glasses NIST610 and 614 measured twice at both the beginning and end ofeach experiment. In-run analysis of the NIST 612 glass checkedthe accuracy, which was better than 15%. The quadruple massanalyser allows very fast peak jumping across the analyte massrange. Detection limits for heavy elements (>80 amu) are typicallyin the 10–50 ppb range. However, high backgrounds causedby molecular interferences produced by atmospheric elementsmeans that detection limits are typically in the range of 500 ppbto 10 ppm for elements such as Al, Si, Fe, Mg, etc.

Results

White-pigmented layers and whitish slips

Several fragmentary ceramic shards displayed white-pigmentedsurfaces sometimes poorly adherent. Raman measurements wereperformed at different points of the white-pigmented layer bymeans of the diode laser: a simple pattern of Raman lines at 398, 514and 640 cm−1 was generally obtained when the region between100 and 3000 cm−1 was investigated. This pattern suggested thepresence of anatase[29,30] as the white pigment. However, previousanalyses carried out by means of infrared spectroscopy had shownthe presence of kaolinite.[7] This mineral has a characteristic patternof peaks in the region 3500–3700 cm−1. As the CCD detector ofour Raman instruments is not efficient at wavelengths longer than1 µm, the 442-nm line of the He–Cd laser was used, permitting thespectral recording of the white-pigmented layer up to 4000 cm−1,which gave the most effective results. Figure 1 shows, as anexample, the spectrum recorded from the white-pigmented layerof the sample A7. The main peaks at 398, 515, 639, 3617, 3660 and3685 cm−1 are present, and this pattern confirmed the presenceof both kaolinite[25] and anatase.[29,30] Actually, all the analysedsamples (the shards labelled A1, A7, A8, A11, A17, A21, B1, B2, B10,C1, D5 and E2) contained kaolinite, which is to be considered theprincipal material used in the preparation of the white-pigmentedlayers and whitish slips. Band component analysis of the Ramanspectrum of the hydroxyl stretching region showed that the3685 cm−1 peak is the result of a superposition of two bands at3679 and 3688 cm−1: the recorded pattern and the absence of apeak at about 3597 cm−1 suggest that low-defect kaolinites hadbeen used.[25] Measurements performed at different points alsorevealed the presence of quartz (464 cm−1) on samples A1, A8,A17, A21, B10 and D5; feldspars (510 cm−1) on samples A11 andA21; calcite (1087 cm−1) on samples A11, B1, B10, E2 and rutile onsample B10 (447, 609 cm−1).

wileyonlinelibrary.com/journal/jrs Copyright c© 2010 John Wiley & Sons, Ltd. J. Raman Spectrosc. 2011, 42, 1317–1323

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Spectroscopic approach to investigate pigments and engobes

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Figure 1. Micro-Raman spectrum recorded on the white-pigmented layer present on sample A7. Operating conditions: He–Cd laser operating at 442 nm,2 mW at the sample, 50× objective, integration time 20 s, 3 scans. The spectrum is baseline-corrected.

Figure 2. Box plot of the oxide concentrations of selected elements obtained by LA-ICP-MS. Boxes are drawn between the quartiles. The whiskers extendto the minimum and maximum values.

The elemental composition as determined by LA-ICP-MS data(Fig. 2) confirmed that kaolinite was the pigment used for thewhite decorations, due to the high Al/Si ratio recorded. Ramanspectroscopy demonstrated the presence of kaolinite and anatasein all the whitish slips; however, as the elemental analysisclearly indicated, it was mixed with other aluminosilicates ofthe magnesium–calcium and sodium–potassium series beforeits use.

Red-pigmented layers

Microscopic examination of the red-pigmented layers foundon the investigated potsherds revealed that they were morehomogeneous and often well preserved. These layers gave moreuseful Raman spectra when probed with the near-IR laser.

The samples A2, A23, A30, A39, A41, C8, C10, C12 and C14are characterised by the presence of haematite alone. Figure 3(A)

J. Raman Spectrosc. 2011, 42, 1317–1323 Copyright c© 2010 John Wiley & Sons, Ltd. wileyonlinelibrary.com/journal/jrs

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Figure 3. Micro-Raman spectrum recorded on the red-pigmented layers present on sample C14 (A), D4 (B) and A36 (C). Operating conditions: diode laseroperating at 785 nm, 1 mW at the sample, 50× objective, integration time 10 s, 3 scans.

shows a typical spectrum obtained by analysing a red-pigmentedlayer belonging to the R1 group of shards. As can be seen, onlythe expected modes relevant to α-Fe2O3 are present.[31]

The samples A3, D4 and D5 also contain haematite but possessa simple luminescence pattern (Fig. 3(B)). It is well known thatRaman analysis can be complicated by the presence of naturallyfluorescent organic materials, by atomic fluorescence in somematerials or by fluorophores that have become incorporated intoartefacts from burial. However, due to the lack of any other redchromophore and because elemental analyses did not show anydistinctive pattern, these samples have been grouped into thepreviously described R1 group as well.

The red layers on potsherds A0, A6, A10, A14, A20, A24, A33and A36 appeared homogeneous under the microscope andwere thicker than those of the R1 group. The Raman spectrawere collected in different points of these samples, and Fig. 3(C)represents a typical spectrum. It shows peculiar features and, inparticular, a broadening of the haematite peaks along with otherpeaks at 350, 645, 1225, 1327, 1398, 1439, 1496 and 1640 cm−1. Ithas already been shown that the substitution of iron in haematitewith other cations, aluminum[15,32 – 33] or titanium[15] for instancecauses the broadening and the upshifting of some haematitepeaks. The simultaneous presence of an additional band atabout 670 cm−1 in the Raman spectra of aluminium-containinghaematite[32] or at 680 cm−1 when titanium substitution occurs[15]

is another feature of those haematite spectra. In the present case,both the band broadening and the small upshifts suggest thesubstitution of iron with some cation in the structure; however,the additional peak is at about 645 cm−1, and thus titanium andaluminium substitutions are not expected. The elemental analysisof these samples shows a slightly higher concentration of copperand zinc; however, to our knowledge there are neither iron-copperor iron-zinc oxides showing their most intense Raman band around

645 cm−1. As a result, a substitution-induced activation of themode can be hypothesised, but which substitution has occurredis not clear. The other broad bands at higher wavenumbers (1225,1398, 1439, 1496, 1640 and 1750 cm−1) are most probably afluorescence pattern; in fact, when the samples were probedwith the 442-nm laser line, they were not recorded. As for theminor components, samples A14 and A20 showed the presenceof gypsum (1009 cm−1).

Brown–black-pigmented layers

The pottery also presented pigmented layers whose coloursranged from brown to black. The diode laser, operating at785 nm, permitted the comprehensive investigation of thechemical/mineralogical nature of these pigmented layers. In allthe samples, manganese oxides were found and the relevantRaman spectra permitted separating the shards in two groups.

The first group, labelled Br1, comprises only the samples A3,A23, C9 and D2. Light microscopy revealed small particles with adark brown coloration, which were homogeneously distributed.These particles gave a Raman spectrum with a series of broadbands as shown in Fig. 4(A). The deconvolution of the regionbetween 200 and 800 cm−1 permitted the identification of thepeaks at 373, 487, 552, 588 and 621 cm−1, which suggests thepresence of a manganese dioxide, possibly having an hollandite-type structure,[26,34] the differences being due to the peculiaritiesof the manganese dioxides whose network of [MnO6] octahedracan have tunnels of different dimensions with different cations.[34]

The Raman spectra of such materials are thus likely to be slightlyvariable. These pigmented layers also exhibited thermal alterationswhen exposed to higher laser fluences, as already observed forsamples from medieval Canosa glazed ceramics:[35] the samespot irradiated with about 2 mW gives a Raman spectrum with


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200 300 400 500 600 700 800

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487552

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Figure 4. Micro-Raman spectra obtained on the black-pigmented layer on sample A23 at different power values of the diode laser on the sample. (A) 0.2mW at the sample, 50× objective, integration time 30 s, 5 scans; (B) 2 mW at the sample, 50× objective, integration time 10 s, 5 scans.

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Figure 5. Micro-Raman spectrum recorded onto the brown-black pigmented layer present on samples A14 (A) and D5 (B). Operating conditions: diodelaser operating at 785 nm, 1 mW at the sample, 50× objective, integration time 10 s, 5 scans.

broader bands (Fig. 4(B)). The pigments on all the other brownand black layers, all assigned to the group Br2 (Table S2 forthe relevant list), appeared at the microscope powdery, weredifficult to focus and showed the presence of a heterogeneouscoating consisting of a mixture of light and dark brown areas.These regions were analysed selectively, and the Raman spectraobtained are shown in Fig. 5. The darker particles showed anintense peak in the region 640–660 cm−1 superimposed on thespectrum of haematite (Fig. 5(A)). The Raman spectra recorded on

the light brown areas generally evidenced a lower intensity of themanganese peak with respect to those of haematite (Fig. 5(B)).These Raman spectra whose patterns were not sensitive to higherlaser fluence were collected till 1800 cm−1; however, neither thecarbon peaks at about 1320 and 1590 cm−1 nor the peak at960 cm−1 was recorded. These findings permitted the exclusion ofthe presence in our samples of carbon black, of both animal andvegetable origin. Magnetite has a Raman peak at about 667 cm−1;however the black-pigmented shards analysed did not show this


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peak in the Raman pattern. In the literature,[31,35] the difficulty inobtaining magnetite Raman spectra has been pointed out as dueto the laser-induced oxidation of the black iron spinel; nonetheless,we are confident that magnetite is not present as we acquired thespectra with both the laser lines available and at low laser power(0.2–0.5 mW on the samples). Moreover, elemental analysis wasperformed on the red, brown and black paints as well. In Fig. 2,the box and whiskers plot relevant to these layers are collected:all the brown and black paints are high in Mn, confirming thatmanganese compounds are responsible of the brown to blackcolours. A peculiar result is that the elemental profile of the brownand black layers is quite similar but differs from that of the redones mainly with respect to the manganese oxide concentration,which is at least one order greater.

It is quite difficult to distinguish the manganese oxide or oxidesresponsible along with haematite on these spectra: manganeseforms a large number of compounds also in non-stoichiometricand disordered systems, and only one Raman band was clearlyvisible in the collected spectra. Moreover, different manganesecompounds have been claimed to have intense Raman bands inthe 640–660 cm−1 window as well: bixbyite, Mn2O3

[26]; jacobsite,MnFe2O4

[36]; and hausmannite, Mn3O4[37] are some examples. As a

result, there is much uncertainty on this topic and any assignmentis questionable if not corroborated by a suitable complementaryanalysis such as X-ray diffraction (XRD). In the present case,we attempted to use elemental analyses. The elemental profileshowed that iron and divalent cations like calcium, strontium andbarium are overall in higher concentrations in the brown and blackpigmented layer than in the red ones. The fact that iron is moreconcentrated in brown and black pigments suggests the presenceof jacobsite as the manganese compound, whereas the divalentcations indicate that manganese dioxides having hollandite ortodorokite-like structures, i.e. with large opening channels, couldalso be present. Considering that a hollandite-type structure like(K,Pb)Mn8O16 has a Raman spectrum with intense peaks at 628 and586 cm−1, whereas jacobsite[36] and todorokite type structures[37]

give an intense peak at about 640 cm−1, these latter manganesecompounds are possibly present.

Discussion

As to the white shards, the presence of kaolinite and anatasewas revealed by Raman spectroscopy in all the investigated whitepaints or slips, possibly because the analysis could be focussedselectively through a microscope on individual pigment grains.Anatase is often a minor component of kaolins[38] and its presenceis confirmed by elemental analyses that showed a titanium dioxideconcentration of about 1%. This low amount can also explain themasking in the low wavenumber region of the Raman bands ofkaolinite and clay minerals present in our samples, anatase beinga very strong Raman scatterer.[38 – 40] The elemental analyses,indeed, showed that other minerals both in the decorations and,to a greater extent, in the engobes are present. However, kaoliniteis definitely the main white pigment used both for decorationof the artefacts and for the engobes production. Interestingly,the presence of anatase and the use of a low-defect kaoliniteevidenced by Raman spectroscopy, as well as the trace elementsrecorded by the elemental analyses, e.g. the peculiar Sr and Pbconcentrations, could be used to trace the provenance of therelevant mineral ores.

Two different red paints have been observed, all based onhaematite: Raman spectroscopy on the R2 group, however, evi-denced the broadening of some haematite peaks and the presenceof a new peak at about 645 cm−1, suggesting iron substitution. Theelemental analyses showed a higher concentration of copper andzinc in these samples; however, no evidence for the substitutionof these elements in the haematite structure from the observedspectra could be obtained. These elements could be responsibleof the peculiar fluorescence pattern, as suggested in the case oftransition-metal-containing diopside.[41]

Brown and black paints based on manganese are naturallyoccurring manganese- and iron-rich clays heated under oxidisingconditions. Manganese dioxide (MnO2), which can take differentstructures according to the channels and tunnels formed bythe MnO6 units,[37] is stable up to about 450 ◦C and then isconverted to bixbyite, and both bixbyite and haematite are stableup to 900–1000 ◦C, temperature at which they react and a newspinel phase form, jacobsite (MnFe2O4). The phase diagram isof limited validity due to the presence of clay minerals in thepaints; nonetheless, the fact that forms of manganese compoundsstable at different temperatures have been found suggests thatthe different hues and colours observed are the results of differentmanganese and iron ores and of different firing conditions of themixtures prior to the application on the ceramic body. The profileof the red and brown/black paints also suggests a difference in themineral ores used for the two colours, i.e. it was not the manganeseore added to the iron ore but a different one.

Phase analyses showed that these Canosa potsherds havebeen fired to a temperature ranging from 500 ◦C up to about1000 ◦C:[42] as kaolinite has been identified in all the white paints,and manganese dioxides in most of the brown–black paints, it ispossible to conclude that the decoration technique consisted inapplying pigments after a first firing of the pottery, and then theywere fired again at a temperature not exceeding 500–600 ◦C for ashort time.

The present samples share substantially homogeneous mate-rials and production technologies, and therefore it appears thatall the pottery had been locally produced and decorated, thusconfirming the result of a previous work based on trace elementalanalysis.[43]

Finally, the use of manganese ores as brown to black pigmentscould be archaeologically interesting, as their use has beenreported in the Near East, then in Cyprus, the Balkans and easternGreece. The Canosa findings turn out to be intermediate to thosefound in the Etruscan area, confirming once more that Canosacould have been an important centre of trade in pre-Roman Italy.

Conclusions

The substances responsible for the coloured geometric decorationfound on the Canosa wares were identified: haematite, asexpected, is the mineral identified in all the red pigmented layers.Iron and manganese oxides were identified on the brown-blackpigmented layers. The different manganese oxides identified onthe different groups are related to different ores; however, theabsence of carbon black and of magnetite in these Canosa waresis, from the technological point of view, peculiar and confirmsan oxidative atmosphere of the kiln during the firing. This set ofcolouring substances is of importance, as it enabled artisans toobtain in one oxidising firing cycle both black and red paints.

The great advantages related to the microscopic nature ofthe probes used are evident by comparing these results with


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those obtained by means of infrared spectroscopy in a previousinvestigation on samples coming from the same archaeologicalcontext:[7 – 9] the minute amounts of pigment dispersed in theKBr pellets did not permit in those analyses the identification ofhaematite in all the red-pigmented shards and kaolinite in thewhite ones.

Both micro-Raman spectroscopy and LA-ICP-MS proved effec-tive: the microscopic size of the laser probe and the possibility toanalyse the samples without any preparation are quite interestingcharacteristics whose importance in archaeological investigationhas been demonstrated once more. Also, microscopic shards couldbe excised from the whole artefact without any real damage,permitting reliable off-site analyses.

Acknowledgements

This work was supported by the Ministero dell’Istruzione,Universita e Ricerca (PRIN 2007). Dr Ph. Colomban and the refereesare thanked for useful suggestions.

Supporting information

Supporting information may be found in the online version of thisarticle.

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Documents

An integrated spectroscopic approach to investigate pigments and engobes on pre-Roman pottery