Characterization of Lapis Lazuli Pigments Using a Multitechnique

Sep 17, 2009 - Many of the Raman spectra obtained from areas painted with ultramarine pigments in illuminated manuscript leaves from the 14th century ...
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Anal. Chem. 2009, 81, 8513–8518

Characterization of Lapis Lazuli Pigments Using a Multitechnique Analytical Approach: Implications for Identification and Geological Provenancing Catherine M. Schmidt,* Marc S. Walton, and Karen Trentelman The Getty Conservation Institute, 1200 Getty Center Drive, Suite 700 Los Angeles, California 90049 Many of the Raman spectra obtained from areas painted with ultramarine pigments in illuminated manuscript leaves from the 14th century Italian manuscript the Laudario of Sant’Agnese, in the collection of the J. Paul Getty Museum, also contain strong bands not typically associated with this pigment. The source of these features was investigated using a multitechnique analytical approach. Techniques employed include Raman microspectroscopy, scanning electron microscopy with energydispersive spectroscopy, electron probe microanalysis, and laser ablation inductively coupled plasma mass spectrometry. The results indicate the presence of diopside (CaMgSi2O6), a mineral commonly associated with lapis lazuli in nature, and suggest that transition metal dopants in the diopside may be responsible for the Raman features, likely the result of fluorescence with vibronic coupling. The implication of this result with respect to using Raman spectroscopy as a fast, noninvasive, and nondestructive method for determining the geological provenance of natural lapis lazuli pigments used in art is discussed. Lapis lazuli is a semiprecious bright blue stone found primarily in the mountains of Afghanistan.1 Long associated with artistic endeavor, lapis lazuli has been described in ancient texts, but was used in antiquity almost exclusively for carved objects rather than as a pigment.1-3 The earliest known sustained uses of lapis lazuli as a painting pigment are in Chinese Turkestan between the fifth and eighth centuries C.E.4 and in Afghan cave paintings in the sixth-seventh centuries C.E.2,4-6 However, the recent discovery * To whom correspondence should be addressed. E-mail: [email protected]. Fax: 310-440-7711. (1) von Rosen, L. Lapis Lazuli in Geological Contexts and in Ancient Written Sources; Paul Åströms forlag: Partille, Sweden, 1988. (2) Eastaugh, N.; Walsh, V.; Chaplin, T.; Siddall, R. The Pigment Compendium: A Dictionary of Historical Pigments; Elsevier: Amsterdam The Netherlands, 2004. (3) von Rosen, L. Lapis Lazuli in Archaeological Contexts; Paul Astroms Forlag: Jonsered, Sweden, 1990. (4) Gaetani, M. C.; Santamaria, U.; Seccaroni, C. Stud. Conserv. 2004, 49, 13– 22. (5) Plesters, J. In Artist’s Pigments: A Handbook of Their History and Characteristics; Roy, A., Ed.; National Gallery of Art: Washington, DC, 1993; Vol. 2, pp 37-65. (6) Gettens, R. J.; Stout, G. L. Painting Materials: A Short Encyclopaedia; D. Dover Publications: New York, 1966. 10.1021/ac901436g CCC: $40.75  2009 American Chemical Society Published on Web 09/17/2009

of an isolated occurrence in Mycenaean Greece (13th century B.C.E.) may push the date back even earlier.7 In Europe during the 14th and 15th centuries C.E. pigment from lapis lazuli, referred to as ultramarine, was widely used in Italian panel paintings and manuscript illuminations.2 Due to its expense, ultramarine was often reserved for iconographically significant figures such as the Virgin Mary or Christ.2,5 To reduce the amount of pigment used, and thus the cost, ultramarine was often underpainted with carbon black, azurite, indigo, or green pigments such as green earth.2,5,8 The less expensive ultramarine ash (see below) also found use as a pigment.5,9 Because the term “lapis lazuli” can refer to both the semiprecious stone and the pigment derived from it, in this paper we will refer to the pigment form as either ultramarine or lapis lazuli. Lapis lazuli contains the mineral lazurite (Na8[Al6Si6O24]Sn), which is responsible for its bright blue color,5,10 along with a variety of accessory minerals including: calcite, pyrite, amphibole, colorless pyroxene (often diopside),2,5,10-16 hau¨yne,5,12,17 sodalite,5,9,12,13,17 forsterite,2,5,14,16 muscovite,2,5,14 nosean,17 phlogopite,10,12,15,16 and wollastonite.2,5,14 In the traditional method of producing pigment from lapis lazuli, as outlined by Cennini,8 these minerals are removed by a lengthy purification process which, when repeated several times, results in several grades of pigment, each less saturated in color than the one before. The last, crudest, grade is typically referred to as ultramarine ash.2,5 Calcite generally remains present, even following extraction by the Cennini method.5 Though this is the most well-known method of preparing pigment from lapis lazuli, there may have been other purification methods that were guarded as alchemical secrets,18 the efficacy of which are therefore unknown. Lazurite is easily identified by Raman spectroscopy by the presence of a strong band centered near 549 cm-1 (the S3(7) Brysbaert, A. Stud. Conserv. 2006, 51, 252–266. (8) Cennini, C. d. A. The Craftsman’s Handbook (Il Libro dell’Arte) (translated by D. V. Thompson); Dover Publications: New York, 1954. (9) Laurie, A. P. The Pigments and Mediums of the Old Masters; MacMillan and Co. Ltd: London, 1914. (10) Ballirano, P.; Maras, A. Am. Mineral. 2006, 91, 997–1005. (11) Encyclopedia of Minerals; Roberts, W. L., Rapp, G. R. J., Weber, J., Eds.; Van Nostrand Reinhold: New York, 1974; p 348. (12) Aleksandrov, S. M.; Senin, V. G. Geochem. Int. 2006, 44, 976–988. (13) Grice, J. D.; Gault, R. A. Rocks Miner. 1983, 58, 12–19. (14) Catalano, I. M.; Genga, A.; Laganara, C.; Laviano, R.; Mangone, A.; Marano, D.; Traini, A. J. Archaeol. Sci. 2007, 34, 503–511. (15) Hogarth, D. D.; Griffin, W. L. Lithos 1978, 11, 37–60. (16) Wyart, J.; Bariand, P.; Filippi, J. Gems Gemol. 1981, 17, 184–190. (17) Jaeger, F. M. Trans. Faraday Soc. 1929, 25, 320–345. (18) Berke, H. Chem. Soc. Rev. 2007, 36, 15–30.

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Table 1. Ultramarine (Lapis Lazuli) Pigments Examined for This Work

Figure 1. Raman spectrum acquired from a 14th century Italian illuminated manuscript, from the Laudario of Sant’Agnese by Pacino di Bonaguida; 785 nm excitation, 1.25 mW/µm2, 10 s acquisition. The starred peak is the main diagnostic feature for ultramarine pigments, and the band at 1049 cm-1 is likely due to the presence of the pigment lead white (2PbCO3 · Pb(OH)2) (ref 20) mixed with the ultramarine.

symmetric stretching mode) and a weaker band centered between 582 and 586 cm-1 (the S2- symmetric stretching mode).19-22 However, in illuminations from a 14th century Italian manuscript in the collection of the J. Paul Getty Museum, the Laudario of Sant’Agnese, many of the Raman spectra collected from areas painted with ultramarine, an example of which is shown in Figure 1, contained a number of very strong bands in addition to the characteristic lazurite bands. The relative intensity of these bands was observed to remain constant between spectra taken from different particles, but the overall intensity with respect to the diagnostic lazurite peak varied, suggesting these features originate from a single source other than lazurite. Interestingly, a similar, but slightly shifted, pattern of features was also observed in Raman spectra obtained from ultramarine containing areas in a leaf from the 15th century French illuminated manuscript the Hours of Louis XII by Jean Bourdichon,23,24 also in the collection of the J. Paul Getty Museum. Motivated by the in situ observation of these unusual spectral features in multiple works of art, a multitechnique analytical investigation was carried out in order to understand their source. Scanning electron microscopy with energy-dispersive spectroscopy (SEM-EDS), electron probe microanalysis (EPMA), and laser ablation inductively coupled plasma mass spectrometry (LAICPMS) were used in conjunction with Raman microspectroscopy to (a) identify associated minerals in ultramarine (lapis lazuli) pigments which may give rise to these spectral features, (b) explore the mechanism by which they are produced, and (c) probe the utility of these features to serve as a means to provenance pigment prepared from lapis lazuli. (19) Holtzer, W.; Murphy, W. F.; Bernstein, H. J. J. Mol. Spectrosc. 1969, 32, 13–23. (20) Burgio, L.; Clark, R. J. H. Spectrochim. Acta, Part A 2001, 57, 1491–1521. (21) del Federico, E.; Schoefberger, W.; Kumar, R.; Ling, W.; Kapetanaki, S.; Schelvis, J.; Jerschow, A. Mater. Res. Soc. Symp. Proc. 2005, 852, 247– 254. (22) Clark, R. J. H.; Franks, M. L. Chem. Phys. Lett. 1975, 34, 69–72. (23) Trentelman, K.; Turner, N. J. Raman Spectrosc. 2009, 40, 577–584. (24) Trentelman, K. J. Raman Spectrosc. 2009, 40, 585–589.

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sample

pigment name

manufacturer

catalog no.

1 2 3 4 5 6 7 8 9

ultramarine ash lapis lazuli lapis lazuli, pure lapis lazuli, crystalline lapis lazuli lapis lazuli lapis lazuli lapis lazuli, sky-blue ultramarine blue dark

Kremer Kremer Kremer Kremer Kremer Verfmoolen “De Kat,” NL Kremer Kremer Kremer

10580 10500 10530 10540 10510 n.a. 10560 10562 45010

EXPERIMENTAL SECTION Since samples typically cannot be removed from illuminated manuscripts to enable a detailed analysis of their composite materials, for this study prepared pigments were used as surrogates. Nine commercially produced ultramarine pigments (Table 1, and Supporting Information) available in the Getty Reference Collection, an in-house assemblage of reference artists’ materials, were selected for study. Sample nos. 1-8 are from natural lapis lazuli sources, whereas sample no. 9 is a synthetically produced ultramarine. Sample no. 8 is specifically identified by Kremer as having a Chilean source; the sources for other samples are less clear. The synthetic ultramarine consists of finely ground, uniformly blue particles. The natural materials, in contrast, are mixtures of coarsely ground colorless or white and blue particles, lending several of the pigments a gray-blue hue. All analyses were conducted on as-purchased powdered pigment samples; except where noted, no additional sample preparation was performed. Raman spectra were collected using Renishaw inVia Raman microscopes (Renishaw, Inc. Hoffman Estates, IL) using 785, 633, 514, or 488 nm laser excitation. All spectra were calibrated using the 520.5 cm-1 silicon Raman band. Spectra were collected using an L-50X microscope objective (8 mm working distance, N.A. ) 0.5). Laser power and collection times were varied for each particle examined to optimize the signal while avoiding sample degradation; specific collection conditions are indicated in figure captions for all presented spectra. Qualitative elemental analysis was performed using a variable pressure Philips XL30 SEM-FEG with the Oxford INCA EDS system, operated at 1 Torr in H2O mode; EDS analysis used a 30 s accumulation. Major element chemistry was quantified using a Cameca SX 100 electron probe microanalyzer (15 kV, 10 nA, 10 µm spot size). Individual particles were analyzed in triplicate when possible, and analyses with an analytical total of less than 90% were discarded. Results were calibrated using a set of standard minerals (Minm 25-53, Astimex Scientific Ltd.), and an obsidian glass reference material was employed as a secondary standard. SEM-EDS and EPMA data were collected from particles of Kremer lapis lazuli (sample no. 2) in a thin section using Specifix 20 epoxy resin on a petrographic slide polished using 6 and 2 µm water-based diamond suspensions to expose the particles. Trace element chemistry was quantified using LA-ICPMS using a GBC Optimass 9500 inductively coupled plasma time of flight mass spectrometer (GBC Scientific Equipment, Dandenong, Victoria, Australia) coupled to a New Wave Laser UP213 laser ablation system (Q-switched frequency-quintupled 213 nm Nd:

Figure 2. Raman spectra acquired from (a) synthetic ultramarine (sample no. 9), (b) a natural lapis lazuli pigment (sample no. 2), and (c) a second natural lapis lazuli pigment (sample no. 8), intensity ×2. All spectra were acquired using 785 nm excitation (spectra a and c using ∼0.3 mW/µm2, spectrum b using ∼1 mW/µm2). The starred peak is the main diagnostic feature for ultramarine pigments at ∼549 cm-1. Spectra are offset for clarity.

YAG laser, New Wave Research, Inc., Fremont, CA). External calibration employed NIST SRM glasses 610, 612, and 614, and accuracy was reassessed using NIST SRM glass 612 as a secondary standard throughout the experiment. Borate fusions of each pigment sample listed in Table 1 were prepared. Aliquots of pigment (between 5 and 10 mg) and approximately equal amounts of lithium borates-lithium bromide (66.67% Li2B4O7, 32.83% LiBO2, 0.50% LiBr, Claisse) were ground together, transferred to a graphite crucible, and heated for ∼3 min at 1000 °C. The resulting glass beads were mounted in Specifix 20 epoxy resin, polished to expose the bead interiors, and situated in a holder containing the NIST SRM glass standards. Ablation was performed with a 125 µm diameter laser spot (laser power 2.87 mJ/pulse at 10 Hz). Individual spots were ablated for 45 s with a dwell time of 25 s between repeated measurements. Analyses were run in triplicate on individual samples, and 3-4 samples of each pigment were analyzed. Data were reduced following the standard procedures of Longerich et al.25 for laser ablation generated aerosols using Si as an internal normalizing factor. RESULTS AND DISCUSSION Raman Microspectroscopy. All nine pigment samples listed in Table 1 were examined initially using Raman spectroscopy with 785 nm excitation, and the results are summarized in Figure 2. Spectra obtained from synthetic ultramarine (sample no. 9), shown in Figure 2a, contain bands at 375 (w, br), 548 (vs), 583 (sh), and 1093 (w) cm-1, in agreement with reported ultramarine spectra and assigned to vibrations in the lazurite lattice.10,20,22,26 By contrast, spectra collected from the natural ultramarine samples, in addition to bands attributable to lazurite, contain strong spectral features above 1100 cm-1. In this study, two distinct spectral patterns were identified. The more common of the two patterns, (25) Longerich, H.; Jackson, S.; Gunther, D. J. Anal. At. Spectrom. 1996, 11, 899–904. (26) Bell, I. M.; Clark, R. J. H.; Gibbs, P. J. Spectrochim. Acta, Part A 1997, 53, 2159–2179.

Figure 3. Raman spectra acquired from a nonpolished blue particle from lapis lazuli sample no. 2: (a) 785 nm excitation (∼0.3 mW/µm2), showing both the Stokes and anti-Stokes regions; (b) 633 nm excitation (∼0.1 mW/µm2); (c) 514 nm excitation (∼0.4 mW/µm2) intensity ×2; (a) 488 nm excitation (∼0.5 mW/µm2), intensity ×3. All spectra represent a single 10 s scan. The starred peak is the main diagnostic feature for ultramarine pigments at ∼549 cm-1. Spectra have been offset for clarity.

an example of which is shown in Figure 2b, was obtained from sample nos. 1-6 and contains bands centered at 1046 (m, br), 1238 (m, br), 1314 (s), 1364 (m, br), 1412 (w, br), 1514 (m), 1564 (sh), 1831 (m, br), 1985 (w), and 3272 (w) cm-1 in addition to the diagnostic lazurite band at ∼549 cm-1. These band positions are in excellent agreement with those observed from the Laudario of Sant’Agnese (Figure 1), and therefore sample no. 2 (Kremer lapis lazuli no. 10500) was selected for use in the indepth studies described below. The second spectral pattern, shown in Figure 2c, was identified in sample nos. 7 and 8 and contains features centered at 339 (w), 637 (w), 969 (m), 1185 (m, br), 1240 (m), 1298 (m), 1318 (m), 1353 (s), 1417 (s), 1482 (m), ∼1620 (m, br), ∼1800 (m, br), 1995 (m), 3234 (vw), 3267 (vw), 3285 (vw), 3329 (vw) cm-1 in addition to the lazurite band. To our knowledge this pattern has not yet been observed in a work of art, and thus our investigation focused on the more commonly encountered spectrum shown in Figure 2b. However, as will be discussed below, this second pattern may have implications for determining the geological provenance of different lapis lazuli pigments. The Raman spectral response of a particle from lapis lazuli sample no. 2 as a function of excitation wavelength was investigated using 488, 514, 633, and 785 nm excitation. As shown in Figure 3, the additional (nonlazurite) spectral features appear only in spectra acquired using 785 nm excitation; spectra acquired using 633, 514, or 488 nm excitation contain only the characteristic lazurite bands. This apparently strong wavelength dependence, and the broad shape of the additional spectral features above 1100 cm-1, raised speculation as to whether they might be fluorescence bands. Indeed, as shown in Figure 3a, whereas the diagnostic lazurite band at 549 cm-1 appears in both the Stokes and anti-Stokes regions of the spectrum acquired using 785 nm excitation, the strong features above 1100 cm-1 do not and, therefore, are not the result of Raman scattering. They instead are likely fluorescence emission bands resulting from an Analytical Chemistry, Vol. 81, No. 20, October 15, 2009

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Figure 4. (A) Representative colorless (a) and blue (b) particles from Kremer lapis lazuli pigment (sample no. 2), (B) Raman spectra, and (C) elemental analysis (SEM-EDS) of particles marked in panel A. In both panels B and C, the gray upper trace corresponds to colorless particle (a), and the black lower trace corresponds to blue particle (b). Raman spectra were collected using 785 nm excitation, ∼0.3 mW/µm2, and a 10 s acquisition.

electronic mechanism activated by exposure of one of the associated minerals in this lapis lazuli sample to 785 nm radiation. Identification of Associated Minerals: SEM-EDS and EPMA. Particle-specific SEM-EDS and Raman spectra were collected on individual blue and colorless particles (outlined in Figure 4A) on a polished thin section slide prepared from sample no. 2. Overall, the nonlazurite spectral features were found to correlate with colorless particles, which in turn were found to contain elevated levels of magnesium and calcium with respect to dark blue particles. For example, as shown in Figure 4B, the Raman spectrum collected from the large colorless particle (trace a) was found to contain only the nonlazurite spectral features, whereas the spectrum from the dark blue particle (trace b) contained only the diagnostic lazurite Raman bands. As shown in Figure 4C, SEM-EDS analysis revealed the colorless particle contains more magnesium, silicon, and calcium (trace a) and less sodium, aluminum, and sulfur than the dark blue particle, which was found to contain oxygen, sodium, aluminum, silicon, sulfur, calcium, and potassium (trace b), as might be expected for lapis lazuli pigments.27 Since lazurite nominally contains only sodium, aluminum, silicon, sulfur, and oxygen, these data suggest that magnesium and calcium (and possibly silicon) must be important components of the material producing the nonlazurite features in Raman spectra taken using 785 nm excitation. Electron probe microanalysis was performed to quantitatively measure the major element chemistry of the blue and colorless particles in sample no. 2. Elements quantified include sodium, magnesium, aluminum, silicon, potassium, calcium, titanium, iron, phosphorus, sulfur, chlorine, and nickel, reported as weight percent of Na2O, MgO, Al2O3, SiO2, K2O, CaO, TiO2, FeO, P2O5, S2-, Cl-, and NiO, respectively. A clear difference in the chemistry of the blue versus colorless particles can be seen in these data, summarized in Table 2. As expected, the chemistry of the blue particles (∼36% SiO2, 30% Al2O3, 19% Na2O, and 8% S2-) was well-matched to lazurite (31-36% SiO2, 24-28% Al2O3, (27) da Cunha, C. Le Lapis Lazuli: Son Histoire, ses Gisements, ses Imitations; Editions du Rocher, France, 1984.

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Table 2. Summary of Major Chemistry Data (Given in Weight Percent) from Three Blue and Three Colorless Particles from Lapis Lazuli (Sample No. 2)a species

av of blue particles

av of colorless particles

>Na2O MgO Al2O3 SiO2 K2O CaO TiO2 FeO P2O5 S2– Cl– NiO total

19 ± 1 0.01 ± 0.02 30.2 ± 0.4 36 ± 1 1.5 ± 0.4 3±2 – 0.03 ± 0.03 0.01 ± 0.04 8±1 0.5 ± 0.2 – 98 ± 1

1.0 ± 0.4 17.6 ± 0.4 1.6 ± 0.6 56.2 ± 0.2 0.01 ± 0.02 24.4 ± 0.6 0.07 ± 0.06 0.02 ± 0.02 0.003 ± 0.02 – 0.011 ± 0.004 – 100.9 ± 0.2

lazuriteb

diopsidec

15–20 24–28 31–36

14–19 0–8 54–57 18–26

2–7

a “–” indicates that the average is e0. b Refs 10, 12, 28, 29. c Refs 10 and 28.

15-20% Na2O, and 2-7% S2- depending on the sample).10,12,15,28,29 The chemistry of the colorless particles (∼56% SiO2, 24% CaO, 18% MgO, and 2% Al2O3) was well-matched to diopside (54-57% SiO2, 18-26% CaO, 14-19% MgO, and 0-8% Al2O3 depending on the sample).10,28 These results, taken in combination with the Raman and EDS data previously obtained on these particles, suggest that diopside is the source of the nonlazurite features in the 785 nm Raman spectra of natural lapis lazuli, both in the reference pigment samples and, by extension, in the ultramarine pigment used in the Laudario of Sant’Agnese. However, the observed features above 1100 cm-1 do not correspond to the known Raman spectrum of diopside, which typically consists of sharp bands at 323 (m), 356 (w), 386 (m), 559 (w), 666 (m-s), and 1013 (s) cm-1.30-33 (28) Anthony, J. W.; Bideaux, R. A.; Bladh, K. W.; Nichols, M. C. Handbook of Mineralogy Mineral Data Publishing; Mineralogical Society of America: Tucson, AZ, 1990. (29) Hogarth, D. D.; Griffin, W. L. Lithos 1976, 9, 39–54. (30) Richet, P.; Mysen, B. O.; Ingrin, J. Phys. Chem. Miner. 1998, 25, 401–414. (31) Chopelas, A.; Serghiou, G. Phys. Chem. Miner. 2002, 29, 403–408.

Figure 5. Raman spectra acquired from (A) colorless particle from Kremer lapis lazuli pigment (sample no. 2): (a) 785 nm (∼0.3 mW/ µm2, 10 s scan); (b) 514 nm (∼2 mW/µm2, 20 s scan); (c) 488 nm (∼3 mW/µm2, 10 s scan); and (B) a Ward’s Mineral diopside standard: (a) 785 nm (∼1 mW/µm2, 10 s scan); (b) 514 nm (∼5 mW/µm2, 30 s scan); (c) 488 nm (∼6 mW/µm2, 30 s scan). Spectra are shown on the same ordinate scale, but offset for clarity.

To investigate the reason for this apparent anomaly, and verify the presence of diopside, Raman spectra from the colorless particle outlined in Figure 4 were collected as a function of excitation wavelength. The results, presented in Figure 5A, confirm that diopside is present, as evidenced by the characteristic Raman bands clearly visible in spectra acquired using 488 and 514 nm excitation. However, the spectrum acquired using 785 nm excitation is dominated by strong fluorescence bands. It should be noted that diopside bands are similarly not visible in the 488 and 514 nm Raman spectra shown in Figure 3, as these spectra were acquired from a nonpolished blue particle, and consequently are dominated by scattering from the lazurite component of the sample. Colorless particles found either in the crevices of or near the blue particle probably contribute to the fluorescence bands in the 785 nm spectrum shown in Figure 3, but as is the case in Figure 5A, the intensity of the fluorescence masks the presence of diopside bands in this spectrum. (32) Zoppi, A.; Lofrumento, C.; Castellucci, E. M.; Migliorini, M. G. Spectrosc. Eur. 2002, 14, 16–20. (33) Boschetti, C.; Corradi, A.; Baraldi, P. J. Raman Spectrosc. 2008, 39, 1085– 1090.

Interestingly, not all diopsides produce these strong fluorescence bands. Representative Raman spectra collected from a sample of a Ward’s diopside standard (Ward’s Natural Science Establishment, 49 E 5870; source: Magog, QC, Canada), shown in Figure 5B, do not show these fluorescence bands under any excitation wavelength. This suggests that the diopside found in association with lapis lazuli is not pure but likely contains a trace contaminant which enables an electronic mechanism to be activated by exposure to 785 nm radiation. LA-ICPMS: Trace Element Analysis. LA-ICPMS was used to determine the identity and quantity of any trace elements present in the natural lapis lazuli samples. The results, shown in Figure 6, were normalized to synthetic ultramarine to eliminate contributions from the lazurite component. The data can be separated into two groups based on their magnesium content, which in turn correlates to the two patterns observed during Raman analysis: sample nos. 1-6 have high magnesium content (gray bars in Figure 6), consistent with the identification of diopside as an associated mineral, and correlate to the Raman pattern investigated in this work (shown in Figure 2b). By contrast, sample nos. 7 and 8, with much lower magnesium content (white bars in Figure 6), correlate with the spectrum shown in Figure 2c; the mineralogy of these samples has not yet been examined since this pattern has not been identified on works of art. The trace element signatures of these two groups show small but significant differences, possibly reflecting local geological differences. Several transition metals were found to be present in greater proportion in the natural materials than in the synthetic. Most notable is vanadium, which is present at ∼20-50 times the concentration found in the synthetic pigment. However, chromium, nickel, manganese, iron, arsenic, and titanium are also found in elevated concentrations. The absorption properties of diopside are known to be strongly influenced by the presence of trace quantities of transition metals.34,35 The transition metals identified in these samples therefore, either individually or in combination, may create accessible states in the lapis lazuliassociated diopside through excitation of their d-orbitals, with vibronic coupling to the ground states resulting in the observed fluorescence band structure.36 SUMMARY AND CONCLUSIONS The study of ultramarine (lapis lazuli) pigments presented in this work, employing Raman microspectroscopy, SEM-EDS, EPMA, and LA-ICPMS, confirmed the presence of diopside as a naturally occurring mineral contaminant in lapis lazuli. Trace element analysis further showed that trace transition metals, in particular vanadium, but also chromium, nickel, manganese, iron, arsenic, and titanium, are present within the diopside inclusions. Transition metal containing diopside was shown to correlate with the nonlazurite features observed between ∼1100 and 3300 cm-1 in Raman spectra collected using 785 nm excitation. It is speculated that absorption through the transition metal d-orbital states results in strong fluorescence with vibronic coupling. (34) Cloutis, E. A. J. Geophys. Res. 2002, 107, 5039–5050. (35) Fritz, E. A.; Laurs, B. M.; Downs, R. T.; Costin, G. Gems Gemol. 2007, 43, 146–148. (36) Bormett, R. Renishaw, Inc. Personal communication, 2008.

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Figure 6. Elemental signature of natural lapis lazuli pigment samples referenced to that of synthetic ultramarine dark (ppm/ppm): filled gray bars represent the average from sample nos. 1-6, which share the non-Raman features found in the Laudario of Sant’Agnese (see Figure 2b), unfilled white bars represent the average of sample nos. 7 and 8, which share a distinguishable non-Raman pattern (see Figure 2c). Only elements which show a ratio >1 are shown.

These trace elements would not be expected to be identifiable in routine analysis by other elemental analysis techniques commonly employed in the examination of cultural heritage objects, such as X-ray fluorescence spectroscopy. In this analysis, observation of trace metals required destructive analysis, a method that cannot be used for the illuminated manuscripts that initiated the investigation. Therefore, the fact that the nonlazurite features stemming from these impurities are observable in nondestructive Raman analysis is particularly noteworthy. The presence of these characteristic nonlazurite fluorescence/ vibronic features not only indicates that a particular ultramarine (lapis lazuli) pigment is derived from a natural source but, more significantly, may be indicative of its particular geological origin. For example, differences in the local geological provenance of the lapis lazuli pigment may explain the slightly shifted fluorescence pattern observed in works by Jean Bourdichon compared to that observed in the Laudario of Sant’Agnese. Similarly, the second fluorescence pattern observed in the natural lapis lazuli samples may reflect a more significant geological difference: sample no. 8 is identified by Kremer as having a Chilean source, whereas lapis used in medieval manuscripts is more likely to have come from Afghanistan. As analyses of additional lapis lazuli and/or diopside reference materials of known geological origin are acquired, and a database of their fluorescence signatures developed, Raman

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analysis alone may eventually provide a sufficiently unique identification to determine geological provenance of these materials. Such identifications would, of course, be strengthened by the addition of EPMA and/or LA-ICPMS analysis, when possible. In the meantime, in situ Raman microscopy may be used to identify objects as having a common origin based on their characteristic nonlazurite fluorescence pattern. ACKNOWLEDGMENT Funding for this work is provided by the Getty Conservation Institute. Funding for the 785 nm Raman system employed in this work is provided by the National Science Foundation (DMR IMR Grant 0506683). The authors thank Richard Bormett (Renishaw Incorporated) for useful discussions. C.M.S. gratefully acknowledges a Getty Conservation Institute Postdoctoral Fellowship. SUPPORTING INFORMATION AVAILABLE Additional information as noted in text. This material is available free of charge via the Internet at http://pubs.acs.org.

Received for review June 30, 2009. Accepted August 22, 2009. AC901436G