New insights on blue pigments used in 15th century paintings by synchrotron radiation-based micro-FTIR and XRD

AnalyticalMethods

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aDpt. d'Enginyeria Quımica. EPSEVG, Unive

Balaguer s/n, 08800 Vilanova i la Geltru, Bar

upc.edu; Fax: +34 938967700; Tel: +34 938bALBA-CELLS synchrotron, Carretera BP

Barcelona, SpaincDpt. Fısica i Enginyeria Nuclear. ESAB, Univ

del Baix Llobregat, c/Esteva Terrades 8, 088

Cite this: Anal. Methods, 2014, 6, 3610

Received 20th February 2014Accepted 11th April 2014

DOI: 10.1039/c4ay00424h

www.rsc.org/methods

3610 | Anal. Methods, 2014, 6, 3610–36

New insights on blue pigments used in 15th centurypaintings by synchrotron radiation-based micro-FTIR and XRD

Nati Salvado,*a Salvador Butı,a Miguel A. G. Arandab and Trinitat Pradellc

The blue pigments used on altarpieces in the 15th century in Catalonia and Crown of Aragon are principally

composed of the azurite mineral. To a lesser extent, lapis lazuli, also of mineral origin, was used and

occasionally in the background areas and outlining the principal figures, indigo (of vegetal origin) was

used for the chromatic preparation layer. Data from several altarpieces belonging to well-known artists

of that time are analysed by synchrotron radiation X-ray diffraction (SR-XRD), micro-infrared

spectroscopy (m-FTIR), synchrotron radiation micro-infrared spectroscopy (mSR-FTIR), Raman

spectroscopy, and scanning electron microscopy with X-ray microanalysis (SEM-EDS). X-ray diffraction

and infrared spectroscopy in association with synchrotron radiation have proven to be especially useful

due to the micron-sized spot size, high brilliance and energy tunability, which help to obtain good

separation of signals coming from different phases/substances and determine their localization in the

various paint layers. The examples presented illustrate the potential of each analytical technique for the

identification of the type of material present in the 15th century paintings. Moreover, the natural origin

and composition of the pigments and their distribution in the paint layers are determined and some

correlations with other contemporary paintings are proposed. Finally, the alteration compounds related

to blue pigments are determined in each case.

Introduction

The study of ancient paintings, and in particular Gothic altar-pieces, is a challenge for analytical chemists owing to the smallsize of the samples, the micrometre-sized layered paint struc-ture (typical layer thicknesses vary between 10 and 100 mm), thehigh number (ve or more) of different substances, the lowamount of material (in some cases a few particles dispersed inthe layer) and the diverse nature of compounds present(pigments, binders, impurities and aging and reactioncompounds). There is no single optimal analytical techniquethat is useful for the determination of all these compounds.Moreover, the determination of the spatial distribution andmorphology of the compounds is oen as important as thechemistry and speciation of the pigment particles.

Different levels of sensitivity are required to determine themain inorganic and organic fractions and impurities. Theidentication of the impurities provides information not only

rsitat Politecnica de Catalunya, Av. Vıctorcelona, Spain. E-mail: nativitat.salvado@

967717

1413, Km. 3.3, E-08290 Cerdanyola,

ersitat Politecnica de Catalunya, Campus

60 Castelldefels, Barcelona, Spain

21

about the pigment source and/or synthesis, but also about thereaction and aging compounds present, which are essential forverifying the reactivity and stability of the paint layers.

As a result, a variety of analytical techniques are being usedfor the study of ancient paintings, such as separation tech-niques, X-ray uorescence spectroscopy (XRF), scanningelectron microscopy with an energy dispersive spectrometer(SEM-EDS), Raman spectroscopy, Fourier transform infraredspectroscopy (FTIR), and X-ray diffraction (XRD).1–6

This paper illustrates how a specic sample preparation anda tailored-combination of analytical techniques can solve mostof the analytical problems encountered in the study of the bluepaints in altarpieces from the 15th century. Information isobtained with regard to the painting techniques and thematerials present, including pigments, binders, reactioncompounds, impurities and reagents remaining in the originalpigments, as well as on their morphology and spatialdistribution.

The usefulness of synchrotron radiation (SR) in the charac-terization of materials in ancient paintings has already beendemonstrated in a number of studies.7–9 SR allows to focus onareas as small as a few micrometers, which helps in obtainingspectra of very high signal-to-noise ratio due to the high bril-liance, collimation and monochromacity of the beam. Thecombination of FTIR and XRD is particularly useful for thestudy of ancient paint layers.

This journal is © The Royal Society of Chemistry 2014

http://dx.doi.org/10.1039/c4ay00424h

http://pubs.rsc.org/en/journals/journal/AY

http://pubs.rsc.org/en/journals/journal/AY?issueid=AY006011

Paper Analytical Methods

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A typical 12–15 mm spot size may be used with mid mSR-FTIR(650–4000 cm�1). The small spot and specic sample prepara-tion (pressing small fragments extracted from each layer in adiamond cell) permits the separation and identication ofcompounds even when present in small amounts. The highselectivity of the small spot that is analyzed compensates for therelatively low detection limit of FTIR. Commercially availableFTIR microscopes are only equipped with mid-IR detectors.However, many substances of interest such as oxides,sulphides, chlorides show IR absorption bands below 700 cm�1.Other substances show their absorption bands overlapping withthose of other compounds in the mid-infrared region but not inthe far-infrared region. Note that the far infrared region (700–200 cm�1) can be reached by using a bolometer; however, in thisregion, diffraction limits the lowest spot size to about 50 mm.

A square or rectangular spot size as small as 20 mm by 50mm and transmission geometry is the optimal set-up for thestudy of the layered structure of paint layers with SR-XRD. Crosssections of paint samples are prepared with typical thicknessesthat vary between 100 mm and 200 mm, depending on thestability of the material, for the Gothic paintings being exam-ined in this study. The small spot size results in high selectivity,and although the sensitivity of XRD is not very high (between 1–5% depending on peak overlap and the crystallinity of thesubstance), the use of a small spot reduces the detection limit tolower values.

High resolution SR-XRD by means of a multi-crystal analyzerdetector system working in transmission geometry is chieycharacterized by its high angular resolution. High angularresolution is very important to separate overlapping XRDdiffraction peaks, which usually happens in paint layers due tothe large number of contributing phases. A wide angular rangeis fundamental for the quantication of the various compoundsby performing a Rietveld renement of the data. In this case, wedo not have the same high focusing capability, but the highangular resolution and large angular range can reduce thedetection limits down to 0.1 wt% for the compounds charac-terized using this technique.

A combination of mid and far mSR-FTIR, mSR-XRD and highresolution SR-XRD is able to identify most of the compounds ofinterest. These tools and SEM-EDS, which provides elementalcomposition, spatial distribution and morphology of the parti-cles in the paint layers, are used in the present study for theinvestigation of the blue paints in the altarpieces. Ramanspectroscopy has also been used to complement some of theanalysis. The examples presented illustrate the potential of eachanalytical technique for the analysis of this type of material.

The present study focuses on the altarpiece paintingsproduced in Catalonia and Crown of Aragon during the 15th

century. The blue samples belong to a set of artworks selected toinclude both tempera and oil techniques, and the differentschools from the geographical area. According to these criteria,the following altarpieces have been selected: “Saint John andSaint Stephen” (1455–1453) by Honorat Borrassa† and “The

† Currently, the MNAC attributes this work to the master of Sant Joan i SantEsteve.


Virgin” by Joan Antigo, both from the Girona School, are pain-ted with oil and egg tempera, respectively; “Constable” (1465) byJaume Huguet and “Saint Vincent” (1455–1460) by BernatMartorell, belonging to the school of Barcelona, both arepainted mainly with egg tempera; “Our Lady of the Counsellors”by Lluıs Dalmau from Barcelona painted with oil; “The Virgin”by Pasqual Ortoneda (1459) from the school of Tarragonapainted with a glue tempera technique; and “The apparition ofthe Virgin to Saint Francis in the Porciuncula” from theValencian school painted with a mixed technique. The lastaltarpiece and those by Honorat Borrassa and Bernat Martorellare on display at the MNAC (Museu Nacional d'Art de Cata-lunya) in Barcelona;10 Jaume Huguet's altarpiece is displayed atits original location in the chapel of “Santa Agata” in the RoyalPalace in Barcelona; and Pasqual Ortoneda's altarpiece is dis-played in the Vinseum museum, in Vilafranca del Penedes,Barcelona. Finally, the altarpiece by Joan Antigo is displayed atits original location in the monastery of Saint Stephen inBanyoles, Girona.

Taking into account the analysis carried out and the docu-ments studied, the blue pigment used in this period and in thisgeographical area is mainly the mineral azurite. To a lesserextent, lapis lazuli, also of mineral origin, was used and occa-sionally in the background areas and outlining the principalgures, indigo (of vegetal origin) was used for the chromaticpreparation layer.

All the data presented in this paper have been compiled fromthe analysis performed over the course of several years of study.

Sample preparation and analyticalmethods

Optical microscopy (OM) and Scanning electron microscopy(SEM; JEOL-5600) with elemental analysis (PCXA LINK EDS) areused in order to obtain information on the composition, sizedistribution and homogeneity of the particles in the paintsamples. Small fragments or cross sections were carbon coatedto ensure good electrical conductivity necessary to perform theSEM-EDS analysis (20 kV or 25 kV of voltage and 1 nA ofcurrent). In order to obtain cross sections, small fragments wereembedded in a polyester resin polymerized by a peroxo organiccatalyser in conditions of low humidity. Aer polymerization, acut was made using a precision low speed saw with a diamondwafering blade (150 micron thickness), which allows precisecutting. The sample was then thinly polished with a 1 mm sizediamond paste. Water was the only uid used for lubricationand was used to the minimum possible extent to avoiddamaging the sample. For sample embedding, a polyester resinwas chosen because it shows the most adequate set of proper-ties considering the characteristics of the samples: thetemperature does not increase during the polymerization,the resin is adequately hard for cutting and polishing, and thepolishing time is minimised by avoiding water. Thin cuts of 100to 200 mm thickness were also performed by shiing the dia-mond blade, which were also used to perform transmissionmSR-XRD measurements.

Anal. Methods, 2014, 6, 3610–3621 | 3611


Analytical Methods Paper

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Synchrotron-based micro X-ray diffraction (mSR-XRD) datawere obtained at beamline BM16 of the European SynchrotronRadiation Facility (ESRF, Grenoble). Small fragments cut fromthe samples were previously selected with the help of an opticalmicroscope. These fragments were placed on an adhesivesupport from where measurements were taken in transmissiongeometry using a beam footprint of 30� 30 microns. Thin crosssections were also prepared from some select samples of 150microns in thickness. A beam footprint of about 20 � 50 mm2

was found to be the most adequate for distinguishing thecompounds present in the different layers. A smaller beamfootprint may lead to a spotty single crystal-like 2D X-raydiffraction pattern, dominated by only some of the big crystal-lites, which precludes the identication of other compounds. Alarger footprint hinders the separation of the compoundspresent in the different layers. The setup included a CCD ADSCQ210r detector and 10 keV (l¼ 1.24 A) or 12.7 keV (l¼ 0.98 A) X-rays. This setup provides high sensitivity to identify compoundspresent in very low amounts and a low angular limit adequatefor some of the organic compounds present; however, it hassome limitations in the angular resolution for the highly over-lapping diffraction patterns.

High angular resolution SR-XRD measurements were alsoperformed at beamline BM01 (ESRF). For these measure-ments, small fragments of the sample were placed in a glasscapillary to allow for spinning, which improves the particlestatistics. A two circle diffractometer with three line detectorsmeasuring an angle of 5� was used. Measurements were takenin the Debye–Scherrer transmission geometry at 24.9 keV(0.50 A). A 2q range of 25� was recorded for a period of 15hours. Aer the measurements, the paint fragments weretaken out of the glass capillary and prepared for otheranalytical techniques.

Synchrotron-based micro infrared spectroscopy (mSR-FTIR)4000–650 cm�1 range measurements were obtained at variousbeamlines. The MIRIAM beamline of the Diamond Light Sourcesetup consists of a Bruker 80 V Fourier transform IR interfer-ometer coupled with a Hyperion 3000 microscope, equippedwith a broad band 100 � 100 mm2 area and an MCT detector.The SMIS beamline of Soleil Synchrotron setup includes aThermo Nicolet Continuum XL FT-IR imaging microscope withan MCT detector. Far mSR-FTIR range measurements wereperformed at the IR 1 beamline of the ANKA synchrotron lightsource, Forschungszentrum, Karlsruhe, Germany. The BrukerIFS 66v/s spectrometer (operated at 4.2 K) was equipped with aBruker IRScope II microscope and a bolometer for the far-infrared (700–200 cm�1) range. In order to perform trans-mission measurements, at samples of adequate thickness(about 2 mm thick) were obtained by squeezing the samplesbetween the two diamond windows of an anvil cell using frag-ments previously cut and selected under an optical microscope.The spectra were obtained using only one of the windows intransmission mode, and from different areas using a micro-beam of either 12 � 12 mm or 15 � 15 mm for the mid-FTIRand 50 � 50 mm for the far-FTIR as dened by the slits. Foreach measurement, 128 scans were recorded with a resolutionof 4 cm�1.

3612 | Anal. Methods, 2014, 6, 3610–3621

Raman spectra were recorded using a Thermo Scientic DXRRaman microscope using 633 nm excitation, 100� objectivelens and#1 mWpower. This equipment is available at the SMISbeamline of the Soleil Synchrotron.

Results and discussionLapis lazuli

Lluıs Dalmau used lapis lazuli in the blue paint layers in thealtarpiece “The Virgin of the Councillors”.11 Lapis lazuli, asemiprecious stone, was an expensive pigment that was rarelyused in 15th century paintings from the Crown of Aragon. Itspresence must be associated with the importance of the artworkthat was going to be exhibited in front of the Chapel of City Hall.In fact, Lluıs Dalmau was required to use blau d'Acre (lapislazuli) according to the terms of the contract dated on October29th, 1443. Fig. 1a and b shows the sample extracted from thecloak of one of the choir angels. The paint is formed by asequence of layers containing particles of increasing particlesize from the surface to the inner layers. The rst two layers (25mm and 65 mm thick, respectively) contain lapis lazuli particlesand drying oil, whereas the third layer contains coarser lapislazuli particles mixed with some particles of white lead, dryingoil and lead carboxylates. Below these three layers, a fourth layer(50 mm thick) containing coarse angular azurite(Cu3(CO3)2(OH)2) particles, white lead, drying oil and some leadcarboxylates is present. The sequence of lapis lazuli/azuritelayers and the increasing particle size is the same as thatdescribed for the blue clothes of the “Adoration of the MysticLamb” (exhibited in the Cathedral of Gent, Belgium) by Jan VanEyck,12 which is a Flemish panel painting with which “Our Ladyof the Counsellors” shares clear stylistic similarities. Accordingto historical documents, Dalmau lived for about ve years inFlanders supported by the King Alfons el Magnanim, to learnabout the Flemish ARS NOVA.11 The data obtained herein forthe rst time provides evidence of a direct link between LluısDalmau and Van Eyck's workshop.

mSR-XRD analysis of different areas of the lapis lazuli layersshow the presence of lazurite ((Na,Ca)4–8[Al6Si6O24](SO4,S)1–2),nepheline (Na3(Na,K)][Al4Si4O16]), sodalite (Na8[Al6Si6O24]Cl2),quartz (SiO2), sanidine/albite ((Na,K)(Si3Al)O8), and at somelocations phlogopite (K2(MgFe)6[Si6Al2O20](OH)4) is also detec-ted (see Fig. 1c). Lazurite and sodalite are the mineralsresponsible for the blue colour, whereas the other minerals areassociated with the original deposits. A high resolution SR-XRDpowder pattern was obtained from one of the fragments. Thispattern was analysed by the Rietveld method to obtain quanti-tative phase analysis. The t between the data and the model isvery good as shown in Fig. 2 (the difference curve is very at),and the results of the analysis are given in Table 1. The entireblue painting layer contains 24.6 wt% of azurite and 75.4% ofminerals related to the lapis lazuli (see Table 1). From thisanalysis, it is possible to determine the mineral content of thelapis lazuli layer, which contains 62.4% lazurite, 23.6% neph-eline, 6.9% quartz, 6.1% sodalite and 1.0% sanidine, byrenormalization to 100 wt%. Other minerals, if present at all,must be below 0.1% of the mixture. The unit cell values are also



Fig. 1 (a) Detail of “Our Lady of the Councillors” altarpiece by Lluıs Dalmau. Dalmau (© MNAC-Museu Nacional d'Art de Catalunya, Barcelona.Photo: Calveras/Merida/Sagrista). (b) Backscattered SEM image from a polished cross section of the blue paint. The red arrows indicate Ba and Sparticles. (c) mSR-XRD pattern from (I) the azurite paint layer and (II) the lapis lazuli paint layer.


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reported in Table 1 as their values could give insight into thelapis lazuli provenance by comparison with data reported inother studies. Some of the reported phases were also identied


by FTIR and the data are shown in Fig. 3. The occurrence ofassociated minerals indicates the natural origin of the lapislazuli. Articial ultramarine has a composition similar to

Anal. Methods, 2014, 6, 3610–3621 | 3613


Fig. 2 Observed (crosses), calculated (full line) and the difference(bottom) data for the Rietveld refinement fit of the high angularresolution synchrotron X-ray powder diffraction pattern of the samesample shown in Fig. 1 (lapis lazuli paint layer). The bars indicate thepositions of Bragg peaks of different phases.


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lazurite with uniform, small and round particles.13 Moreover,the presence of a sharp infrared band at 2340 cm�1 and a smallsatellite peak at 2274 cm�1 (asymmetric stretching n3 of

12CO2

and 13CO2, respectively) are related to the CO2 adsorbed ontothe structure of sodalite, which also conrms the natural originof the lapis lazuli14–16 (Fig. 3b). The band at 2040 cm�1 may beassigned to the n(CO) (absorbed carbon monoxide), which ispossibly linked to the purication process followed by theartist.17 The amount of sodalite (sodalite/lazurite ratio ¼ 10%)compares very well with the lapis lazuli used by Michelangelo inthe fresco of the Last Judgment (12%),18 and could indicate thesame geographical origin.13 However, the relatively higheramounts of nepheline and quartz (nepheline/lazurite ratio ¼38%, quartz/lazurite ratio ¼ 7%) compared to the pigment usedby Michelangelo (17% and 4%, respectively) may also indicatethe use of a less pure pigment. Puricationmethods for removalof greyish particles from the blue pigment are described incontemporary paint treatises.17,19

Small particles of white lead (1 mm) present in both theazurite and lapis lazuli layers must have been added to improvethe oil drying capability. The formation of lead carboxylate saltsfrom the reaction of free fatty acids present in the oil with whitelead is known to trigger the drying process. The amount of whitelead particles is very low, actually lower than 0.1%, because theywere not determined by Rietveld renement.

Table 1 Rietveld quantitative phase analysis results (phase contents andby Lluıs Dalmau

wt% Phase wt% a/A

lapis lazuli-layer 75.4% 62.5 Lazurite 47.1(2) 9.0800(2)23.6 Nepheline 17.8(1) 10.0001(3)6.9 Quartz 5.2(1) 4.9114(3)6.1 Sodalite 4.6(2) 8.8712(7)1.0 Sanidine 0.8(2) 8.552(5)

Azurite-layer 24.6% Azurite 24.6(1) 5.0071(1)

3614 | Anal. Methods, 2014, 6, 3610–3621

Finally, Lluıs Dalmau mixed the lapis lazuli with drying oil,which is not the most appropriate method considering that therefraction index of both lapis lazuli and oil is similar (about 1.5);consequently, the amount of light that scattered is low and theopacity of the paint is low.

Azurite

Azurite is a mineral pigment20 that was widely used in 15th

century wood paintings from the Crown of Aragon. Relevantareas of the altarpieces were painted with this pigment, e.g., themantel of the Virgins. Currently, these areas appear almostblack or very dark.

Fig. 4a and b shows OM and SEM images corresponding to across section of a sample taken from the mantel of the virgin inthe “Constable” altarpiece. The azurite particles and the envi-ronmental supercial layer deposition, which is perfectly inte-grated into the paint layer, can be observed.

The causes for this darkening can be due to the large particlesize, between 2 and 30 mm, of the pigment that was used. Theseparticle sizes were necessary to obtain saturated colours as thesmaller size particles produce less saturated blues. The use oflarge particles creates larger interstitial gaps between theparticles, which are oen not completely lled by the bindingmedia. However, the amount of binding media is greater thanthat found in the painting layers obtained with the lowerparticle size pigments, which also produced more darkenedareas. The darkening of the binding media is due to the aging ofthe organic compounds such as the proteins from the animalglue (Fig. 4). The fact that the darkening is more evidenttowards the surface (Fig. 4a) suggests that this is not the solecause of the colour alteration. The particle sizes at the surfaceare more irregular, and they favour the retention of dust andpollution deposited from the environment such as carbon fromthe candles and oil lamps used in the lighting, as well asgypsum, oxalates and silicates. These materials then becomeintegrated into the pictorial layers and in some cases, evenbecome consolidated as a consequence of the subsequentapplication of varnish layers. This makes the restoration of theoriginal blue hues unviable. The elimination of the upper layerswould result in the concomitant loss of the pigment, and thusthe nuances and motifs of the clothing.

Fig. 4b shows the presence of a white contrasted particlecomposed of S and Ba. These particles can be detected using thebackscattering electrons as they have a larger average atomicweight than the surrounding particles and the organic matrix

unit cell data) for a lapis lazuli sample of “The Virgin of the Councillors”

b/A c/A a/� b/� g/� V/A

9.0800(1) 9.0800(2) 90 90 90 748.61(4)10.0001(3) 10.0001(3) 90 90 120 726.22(4)4.9114(3) 5.4033(6) 90 90 120 112.88(1)8.8712(7) 8.8712(7) 90 90 90 698.1(2)12.997(6) 7.188(3) 90 115.99 90 718.1(4)5.8440(1) 10.3420(2) 90 92.42 90 302.35(1)



Fig. 3 mSR-FTIR spectra corresponding to: (a) lapis lazuli blue layer (layer II in Fig. 1) and (b) region where the absorption bands related toadsorbed CO2 are observed.


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(Fig. 1b and 4b). The sporadic presence of these particles iscommonly observed in samples containing azurite of mineralorigin. These particles are identied as barium sulphate, amaterial of paragenesis (mineral association) with azurite. Thisresult is an additional indicator of the mineral origin of thepigment.

It is also worth noting that the analysis of several samplesfrom altarpieces of this period reveals the presence of a smallamount of malachite CuCO3$Cu(OH)2. Malachite could be oneof the natural impurities present in the original mineral or itcould be a weathering product of azurite. This last hypothesis isthe most probable considering that we have been able todetermine the presence of intermediate compounds betweenthe structure of azurite and those of malachite. This can beobserved in the sequence of mSR-FTIR spectra shown inFig. 5.21,22 Malachite cannot be considered a compound addedvoluntarily to modify the shade of the blue pigment as the greenpigment used on these altarpieces was a type of copper acetate


([Cu(CH3COO)2]x[Cu(OH)2]ynH2O) incorporating in some casesa basic copper chloride ([CuCl2]x[Cu(OH)2]y$nH2O). Fig. 6 showsthe sequence of painting layers (blue over yellow and ochrelayers) from a sample taken from the shawl of the virgin in “Theapparition of the Virgin to Saint Francis in the Porciuncula”.The blue paint layer of the shawl (90 mm thick) is formed bycoarse azurite particles (2CuCO3$Cu(OH)2; 10–20 mm) and asmall amount of malachite CuCO3$Cu(OH)2, as detected usingmSR-XRD (Fig. 7).

Moreover, when mixed with the copper green pigments ingreen paints, small quantities of azurite were oen added toreinforce the blue shade in background landscapes.3 Fig. 8shows a series of infrared spectra from one of the green layersand also the corresponding X-ray diffraction pattern.

In order to obtain less saturated blues, the pigment was alsodispersed in a white matrix of white lead (2PbCO3$Pb(OH)2 andPbCO3) and either egg yolk or a drying oil binder; therefore,different shades of light blues are obtained by dilution with a

Anal. Methods, 2014, 6, 3610–3621 | 3615


Fig. 4 Blue sample from “Constable” altarpiece by Jaume Huguet. (a)Bright field OM image from a cross section of the sample. (b) Back-scattered SEM image from a cross section of the sample. The red arrowindicates Ba and S particles.

Fig. 5 mSR-FTIR spectra corresponding to the purple layer of a samplefrom “The Virgin” altarpiece by Joan Antigo. The purple colour is amixture of blue, red and white pigments. In the figure, a sequence ofspectra related to the compounds identified in the blue pigment isshown.

Fig. 6 (a) Detail of “The apparition of the Virgin to Saint Francis in thePorciuncula” altarpiece (© MNAC-Museu Nacional d'Art de Catalunya,Barcelona. Photo: Calveras/Merida/Sagrista). (b) Detail of the locationwhere the sample was obtained. (c) Polarized light OM image from apolished cross section. The first paint layer corresponds to a yellowsurface decoration of the sleeve cuff and contains the yellow leadpigment, Pb2SnO4 and cassiterite SnO2, lead carboxylates, leadoxalates and drying oil (50 mm thick); the second is an ochre paint layer(30 mm thick) and contains illite (KAl2Si3AlO10(OH)2), quartz, colloidaliron hydroxides and drying oil. Below these first two layers, the bluepaint layer of the shawl (90 mm thick) is formed by coarse azurite(2CuCO3$Cu(OH)2) particles (10 to 20 mm). These layers are appliedover a gypsum ground layer.

3616 | Anal. Methods, 2014, 6, 3610–3621


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white pigment. Lead carboxylates resulting from the reaction offree fatty acids in the binder and the lead white are also formed,as shown in Fig. 9. Azurite does not typically produce Cucarboxylates, which is in contrast to what happens with copperacetates/basic copper acetates and basic copper chlorides thatare used to obtain the green pigments.1,3

Due to the transparency of azurite, painters frequentlyprepared lighter or darker blue shades over the ground as achromatic preparation layer. In the context of 15th centurypaintings from the Crown of Aragon, the ground layers werecomposed of gypsum and animal glue. The use of a chromaticpreparation layer provides a colour background that highlightsthe blue colour of the pigment. In most of the samples, this layercontained a few black carbon particles mixed with lead white,which exhibit a blue shade. Ramanmicrospectroscopy allowed usto determine the presence of these black carbon particles. Fig. 10shows a series of Raman spectra corresponding to the differentlayers of the blue painting.21 The presence of carbon black isdetected only in the chromatic preparation layer. Finally, groundazurite or indigo (in some cases) mixed with white lead were alsoused to obtain a chromatic preparation layer.

As shown in Fig. 5, we also found azurite mixed with red andwhite pigments to obtain the violet colour and also with yellowto obtain purple colours.



Fig. 7 mSR-XRD patterns from the sample shown in Fig. 6, (A) corre-sponding to layer 1 and (B) corresponding to layers 2/3.

Fig. 8 (a) Detail of “Our Lady of the Counsellors” altarpiece by LluısDalmau (© MNAC-Museu Nacional d'Art de Catalunya, Barcelona.Photo: Calveras/Merida/Sagrista). (b) Polarized light OM image andbackscattered SEM image from a polished cross section of blue/greensample. (c) mSR-XRD pattern of a thin cross section from the blue/greenpaint layer. (d) mSR-FTIR spectra from the blue/green paint layer. (I)Azurite (2CuCO3$Cu(OH)2), white lead (PbCO3/2PbCO3$Pb(OH)2) anddrying oil, (II) azurite (2CuCO3$Cu(OH)2) and (III) basic copper acetates;drying oil, white lead (PbCO3/2PbCO3$Pb(OH)2) and oxalates.


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Indigo

Indigo has long been used as a colouring agent. It can beobtained from a great variety of plants whose leaves containindigo precursors such as indoxyl-b-D-glucoside (indican) andindoxyl-5-ketoglucomate (isatan B). These substances aresoluble in water, through a process of enzymatic fermentation(b-glucosidase) and later oxidation with atmospheric oxygen,and transformed into the blue substance we call indigo.23–29

This substance is still being used as a dye for fabrics,although the products currently produced are synthetic. Indigohas also been used as a pigment in paintings on illuminatedmanuscripts, boards, murals and canvases mixed with variousbinding media.

In the 15th century, in theWesternMediterranean area, indigocould be obtained from Indigofera intorea L., a natural plant fromIndia that was traded through Baghdad, Venice and Genoa (thenames that are sometimes associated with the pigment). It couldalso be obtained from other plants such as Isatis tintorea L.(named Pastel in French and woad in English) cultivated indifferent European areas, including some close to the Crown ofAragon, such as Languedoc, which is between Toulouse and Albi.

The pigment of Asiatic origin was mixed with lime andpressed into small bricks. This material was not cheap but itwas quite pure when separated from lime aer drying. However,the product most widely used by painters was that obtained

This journal is © The Royal Society of Chemistry 2014 Anal. Methods, 2014, 6, 3610–3621 | 3617


Fig. 9 Blue sample from “Saint Vincent” altarpiece by Bernat Martorell(a) mSR-FTIR spectra corresponding to the blue layer: (I) binding mediaidentified as egg yolk, (II) azurite particle and (III) lead carboxylates andbasic lead carbonate. (b) mSR-XRD pattern of a thin cross section fromthe blue paint layer.

Fig. 10 Blue sample from “Saint John and Saint Stephen” altarpiece byHonorat Borrassa. (a) Polarized light OM image from a polished crosssection of the blue sample. (b) Raman spectra from a polished crosssection of the blue sample. From bottom to top: gypsum, lead white,carbon black, iron oxides (goethite), and azurite.


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from autochthonous plants, which were more accessible,although it exhibited a lower purity.

Indigo from this period was rarely used in the surface layersof the paintings27 because azurite and lapis lazuli were chieyused. Indigo blue has been identied in the backgrounds of thealtarpieces and in the blue painting chromatic preparationlayers mixed with white lead.

Indigo can be determined by several analytical methods suchas chromatographic techniques, mass spectrometry, surface-

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enhanced Raman scattering (SERS), which requires pre-treat-ment of the sample. However, its determination is more difficultwhen it appears in thin inner layers (below 10 mm) mixed withother materials (such as white lead and binding media). Fig. 11shows its identication using m-FTIR in a sample taken from theshawl of the virgin in the altarpiece by Pascual Ortoneda. A goodseparation is needed to avoid the presence of overlapping bandsand to obtain an unambiguous determination.

Recent work by Amat et al.,29 on spectroscopic properties ofindigo dye, assigns the observed infrared bands to functionalgroups and compares them to a theoretical spectrum. In ourinfrared spectrum from the paint layer, a series of bands areidentied: 1626 cm�1 nCO, 1614 cm�1 shoulder nCC, 1585 cm�1

nCC dCH, 1483 cm�1 nCC, 1461 cm�1 nCC, �1400 cm�1 dNH,1319 cm�1 nCC, 1299 cm�1 dCH, 1196 cm�1 nCC, 1172 cm�1

dCC, 1126 cm�1 dCC, 1077 cm�1 nC–NH–C, (1037 cm�1), (876cm�1), 754 cm�1 dCH, (748 cm�1), 710 cm�1 g5-ring, and (696cm�1). We compare these bands with reference data from theliterature25,29 and with our own reference spectra of Isatis tinc-toria L. (Blue Pastel from Languedoc) and Indigofera tinctoria L.(from Mexico). We observe that the spectra of the Indigoferatinctoria L. dye shows sharper and clearer bands than those of



Fig. 11 (a) Detail of “The Virgin” altarpiece by Pasqual Ortoneda (© CRBMC-Centre de Restauracio de Bens Mobles de Catalunya. Photo: CarlesAymerich). (b) OM image of blue sample. Sequence of layers (from bottom to top): gypsum background, dark blue layer, blue layer and dust/altered superficial layer. (c) mSR-FTIR spectra corresponding to (I) and (II) the dark blue layer, (III) and (IV) blue layer and (III) superficial layer. (d)2000–650 cm�1 region of mSR-FTIR spectrum II of Fig. 11c. The indigo from the paint layer (spectrum III) is compared to the indigo from Isatistintorea L. (spectrum II) and the indigo from Indigofera tintorea L. (spectrum I).

This journal is © The Royal Society of Chemistry 2014 Anal. Methods, 2014, 6, 3610–3621 | 3619


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Isatis tinctoria L. The IR spectrum of the paint compares betterwith that of Isatis tinctoria L. (Fig. 11d). This could be a reec-tion of the low purity of the material, which may be related tothe presence of carbohydrates in the raw material. The changein the shape of the spectrum is due to the contribution of thecarbonyl and carbohydrate absorption bands in the region of1200–900 cm�1. Therefore, these results demonstrate that theindigo used in the pigment of the altarpiece “The Virgin” byPasqual Ortoneda was extracted from nearby plants.

Conclusion

This article illustrates how specic sample preparation and aparticular combination of analytical techniques can solve mostproblems that may be encountered when studying blue paint inaltarpieces from the 15th century. To obtain good mSR-XRDmeasurements in transmission mode, thin sections with athickness of �150 microns were prepared. For high angularresolution SR-XRD measurements, small fragments of thesample were placed in a glass capillary to allow for spinning,which improved the particle statistics. With the aim of obtain-ing good mSR-FTIR spectra, measurements in transmissionmode were taken using a diamond anvil cell. The results arebased on analyses performed over a number of years on a largeselection of artworks from the 15th century.

From this study, we may conclude that lapis lazuli was usedonly in very precious artworks. In the case examined herein, itwas bound to the drying oil and applied only in the mostsupercial layers of the blue paint. High resolution SR-XRDcoupled with the Rietveld method was used to determine andquantify the minerals constituting the lapis lazuli used, whichmay come from the same source as those used by Michelangelo.The FTIR analysis has shown the presence of absorbed CO2,which highlights the natural origin of the material. A sequence ofseveral paint layers (coarse atzurite, ne atzurite and ne lapis-lazuli) is also related to Jan van Eyck's “Adoration of the MysticLamb” with which it also shares clear stylistic resemblances.

The darkening of the azurite blue painted areas is related tothe large size of the pigment particles that increase the porosityof the paint where dust from the environment is trapped. Thisdirt is difficult to remove and has been oen consolidated withsubsequent varnish layers. In addition to the azurite particles, afew small BaSO4 particles were also detected as an associatedmineral. Due to the weathering of azurite, small quantities ofmalachite are also formed. Aged azurite does not tend to formcopper carboxylates in contrast with other copper pigmentssuch as acetates.

The indigo blue was used in the large background andchromatic preparation areas. The pigment used in this area andduring this time period was principally obtained from theplants found in nearby areas (Isatis tintorea L.).

Acknowledgements

We acknowledge the following synchrotron radiation facilities:ESRF under proposals EC-69 (beamline BM01) and CRG 16-01-709/16-01-733 (beamline BM16); ANKA, Germany under

3620 | Anal. Methods, 2014, 6, 3610–3621

proposal EU IA-SFS MS-97 (beamline IR1); Diamond LightSource, UK under proposal EU SM6521 (MIRIAM beamline);and SOLEIL Synchrotron, France under proposal EU 20090887(beamline SMIS). N.Salvado and S. Butı received nancialsupport under MICINN (Spain), grant HAR2009-10790 andunder Generalitat de Catalunya, grant 2009SGR01251. T. Pradellreceived nancial support under MICINN (Spain), grantMAT2010-20129-C02-01 and under Generalitat de Catalunya,grant 2009SGR01225. Part of this work was carried out withinthe framework of agreements of collaboration between theUniversitat Politecnica de Catalunya (UPC) and the “MuseuNacional d'Art de Catalunya” (MNAC) and the “Centre de Res-tauracio de Bens Mobles de Catalunya” (CRBMC). We wish tothank the VINSEUM Museum for their collaboration in thestudy and for the access given to sampling of the altarpiece.

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New insights on blue pigments used in 15th century paintings by synchrotron radiation-based micro-FTIR and XRD