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J. Cult. Heritage 1 (2000) S269 S272 2000 Editions scientiques et medicales Elsevier SAS. All rights reserved

S1296-2074(00)00176-X /FLA

Micro-Raman spectroscopy for standard and in situcharacterisation of painting materials

Alessandra Perardi, Angela Zoppi, Emilio Castellucci*

European Laboratory for non-Linear Spectroscopy (LENS), University of Florence, Largo E. Fermi 2,50125 Florence, Italy

Abstract Micro-Raman is a spectroscopic technique that allows the identication of painting materials even if nelygrained and mixed with others, dispersed in a binder or layered on surfaces. It is used for non-destructive, in situmeasurements and it is suitable for selective studies on inhomogeneous materials or surface investigations. Some examplesare described of the use of this technique in the eld of art conservation and diagnostics, with regard to pigments, dyesand some products of metal alteration. Raman spectra obtained on standard painting materials were arranged in a

database and published on the Web. 2000 E

ditions scientiques et medicales Elsevier SASKeywords: micro-Raman / pigments / dyes / corrosion / non-destructivity

1. Introduction:the micro-Raman technique

Raman spectroscopy allows the identication of homogeneous materials on the basis of their molecu-lar vibrational spectra, obtained by excitation with

visible laser light. In micro-Raman spectroscopy (orRaman microscopy) the laser beam is focused bymeans of a microscope objective, employing abackscattering conguration; thus, the Raman scat-tered light is collected within the cone dened by thesame objective [1]. Consequently, a much smallerarea needs to be irradiated for measurements than incommon Raman spectroscopy: the minimum theoret-ical diameter of the laser spot on the surface of thesample is represented by the diffraction limit. For a100 magnication objective, the calculated diffrac-tion limit is 0.63 mm with a 488 nm laser radiation,0.66 mm for 514.5 nm, 0.83 mm for 647.1 nm.

Due to the low depth of eld, only a very thin layer

of the sample is irradiated, so that the resultinginformation comes mainly from the surface of theobject, without interference from layers underneath.The technique is non-destructive and the instrumenta-tion efciency in collecting the scattered light is suchthat no more than a hundred micro-watts of laserexcitation power on the sample are usually required,a fact that accounts for a safe use in direct measure-ments on works of art. Another advantage is thescarce or lack of need for sample preparation; thus,the technique can be applied in situ, that is withoutany sampling of the art object to be analysed, pro-vided it is possible to put it under the microscopeobjective ( gure 1 ).

2. Applications

Raman microscopy has been extensively used forthe characterization of organic and inorganic paintingmaterials, such as pigments, lakes and dyes on differ-

* Correspondence and reprints: castel@lens.uni.it (E. Castellucci).

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Figure 1. Micro-Raman instrumental set-up for the studyof works of art.

data acquired from standard samples of knowncomposition. As a helpful tool in this comparison,our research group has developed a database,recording Raman spectra of standard pigments andlakes, also accessible on-line at the web-sitewww.chim.uni.it /raman.

In the following, some examples are reported of possible uses of the micro-Raman technique in theeld of art conservation and diagnostics, with regard

to pigments, dyes and products of metal alteration.

2.1. Metal corrosion products

An ancient iron nail from the archaeological siteof Tharros, Sardinia, was examined by micro-Ra-man spectroscopy. This site was founded by thePhoenicians around the VIII century BC and becamean important Punic harbour and one of the maintrade centres in the Mediterranean; it passed laterunder the Roman inuence and domination. Thenail probably belongs to the latter period. It under-went a severe decay during burial and the most partof the metal transformed into thin-layered crusts of

iron oxides and hydroxides. The corrosion productswere identied on the basis of their different crystalstructure: black magnetite (Fe 3 O 4 ) and maghemite(g -Fe2 O 3 ), orange lepidocrocite ( g -FeOOH) andgoethite ( a -FeOOH). The Raman spectrum of agoethite layer is shown in gure 2 . Traces of he-matite ( a -Fe2 O 3 ) found on the external surface of

ent supports: canvas, parchment, paper, wood, ce-ramic, stone, glass, wall and so on [2]. It is alsoemployed in the study of other materials of anartistic and historic interest, also including alterationproducts. For a chemical structural identication of compounds, Raman spectra recorded in the analysisof art objects are usually compared with reference

Figure 2. Raman spectrum of goethite corrosion layer on Tharros nail. Laser excitation line 647.1 nm, objectivemagnication 100 , monochromator entrance slit 0.1 mm, power at the sample 120 mW, acquisition time 100 s.

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Figure 3. Raman spectrum of blue silk thread (indigo).Laser excitation line 647.1 nm, objective magnication100 , monochromator entrance slit 0.01 mm, power atthe sample 280 mW, acquisition time 300 s, baselinesubtraction.

2.2. Textile dyes

Micro-Raman spectroscopy was used to identifydyes in a late 1400 Medici silk stole from Volterra.Florentine heraldry had a great inuence on textiledesign in the XV century, and the Medici familyamong others made extensive use of their coat of arms in architectonic decoration and art objects.The fabric under study is attributed to a factory

working in Florence in the second half of 1400; justin those years, Volterra was entering the sphere of inuence of Florentine policy. The cloth was origi-nally a vestment, turned inside out and reused tomake a stole at the end of the XIX century. Thepattern, gold and blue on red satin, has two borderswith fruits, pearls and the Medici shield; the centreis decorated with leaves and a composition of cherubs surrounding the family coat of arms.

The analysis of red warp and blue woof silkthreads gave the spectra of kermesic acid and indigo[4], obtained under resonance conditions; the reso-nance effect can result in a strong enhancement of some Raman bands, which helps in identifying even

tiny amounts of material [5]. Figure 3 reports thespectrum of a blue indigo thread, with three strongRaman bands assigned to total-symmetricvibrations.

2.3. Frescoes

An extensive study was performed on XV centuryearly Renaissance frescoes painted by Michele diMatteo, Lorenzo di Pietro (Vecchietta) and Ben-venuto di Giovanni in the baptistery of the Cathe-dral in Siena. They are located inside the apse andrepresent scenes from the Passion of Christ andfrom the life of Saint Anthony [6]. Cross-sections of the frescoes were analysed and the following pig-ments identied: red ochre, lead tin yellow (type I),malachite, azurite, red lead. Figure 4 shows thespectrum obtained for azurite; this compound wasidentied by comparison with a standard. Azurite, acopper carbonate with the formula2CuCO 3 .Cu(OH) 2 , has the characteristic carbonateband at 1 099 cm 1 , though this is not the mainpeak, which is instead observed at 406 cm 1 andcorresponds to a stretching of the Cu O bond [7].

3. Conclusions

The above examples clearly illustrate the possibleapplications of Raman microscopy in the eld of cultural heritage and how this versatile technique

Figure 4. Identication of azurite in fresco cross-section.Laser excitation line 514.5 nm, objective magnication100 , monochromator entrance slit 0.1 mm, power atthe sample 250 mW, acquisition time 60 s 5 accumula-tions, baseline subtraction.

the nail have been recognized as coming from theiron-rich soil of the site.

It is interesting to note that the black compoundsare bad scatterers and their Raman spectra arerather weak, while, on the other hand, goethite,lepidocrocite and hematite give rise to strong andsharp bands. For these analyses a low laser powerwas used, in order to avoid the danger of thermaldegradation of the sample pointed out by manyauthors [3]: at high temperature, transformation of

magnetite and maghemite into hematite occurs, anddehydration of goethite and lepidocrocite leads tohematite and maghemite.

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can be adapted to the study of many differentobjects and materials. The microscope objective fo-cuses analysis on particles of a few micrometres insize: pigment grains in painting layers, thin corro-sion layers, the surface of a single thread. Thus, theycan be better isolated from interference from thesurrounding environment.

The main problems in the use of this techniquearise from possible local burning (or thermal decom-

position, as mentioned before) due to the focaliza-tion of the laser light and from the excitation of uorescence emission, which can conceal the Ramanspectrum. To avoid excessive heating, low laserpowers are usually employed in sample excitation;as to uorescence, when it is not possible to avoid itby focusing the laser on isolated non-luminescentparticles, it may be of help choosing a laser excita-tion wavelength outside the absorption spectralrange. In some cases, anyway, a moderate absorp-tion of radiation can enhance the Raman response,with a resonance effect: in fact, we took advantageof this enhancement in the analysis of the textiledyes.

Acknowledgements . The authors would like tothank persons and institutions who provided thesamples: Dr L. Ruatta, Politecnico di Torino, for the

nail; Dr S. Di Blasi and Dr L. Ciampini for the silkstole; Dr M. Matteini, OPD Firenze, for the frescocross-sections.

References

[1] Turrell G., Dhamelincourt P., Micro-Raman spec-troscopy, in: Laserna J.J. (Ed.), Modern Techniques inRaman Spectroscopy, Wiley, New York, 1996, pp. 109142.[2] Coupry C., Lautie A., Revault M., Dulho J., Contri-bution of Raman spectroscopy to art and history, J.Raman Spectrosc. 25 (1994) 8994.[3] De Faria D.L.A., Venancio Silva S., de Oliveira M.T.,Raman microspectroscopy of some iron oxides and oxy-hydroxides, J. Raman Spectrosc. 28 (1997) 873878.[4] Coupry C., Sagon G., Gorguet-Ballesteros P., Ramanspectroscopic investigation of blue contemporary textiles, J. Raman Spectrosc. 28 (1997) 8589.[5] Bussotti L., Castellucci E., Matteini M., The micro-Raman technique in the studies for the conservation of artworks: identication of lakes in paints, Sci. Technol. Cult.Heritage 5 (1996) 1319.

[6] Alessi C. et al., Le pitture murali della zona presbiteri-ale del battistero di Siena: Storia, studi e restauri, OPDRestauro 4 (1992) 927.[7] Guineau B., Analyse non destructive des pigments parmicrosonde Raman laser : exemples de lazurite et de lamalachite, Stud. Conserv. 29 (1984) 3541.

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