Vibrational spectroscopy correlated with elemental analysis for the investigation of smalt pigment and its alteration in paintings

AnalyticalMethods

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aIPANEMA, USR3461 CNRS/MCC, Synchrot

Gif-sur-Yvette Cedex, France. E-mail: lauria

12; Tel: +33 40 79 54 03bCentre de Recherche et de Restauration d

F-75001 Paris, FrancecScientic Department, National Gallery, Tr

† Present address: Centre de Recherche(CRCC), MNHN, 36 rue Geoffroy Saint Hil

Cite this: Anal. Methods, 2013, 5, 4628

Received 29th May 2013Accepted 31st May 2013

DOI: 10.1039/c3ay40906f

www.rsc.org/methods

4628 | Anal. Methods, 2013, 5, 4628

Vibrational spectroscopy correlated with elementalanalysis for the investigation of smalt pigment and itsalteration in paintings

Laurianne Robinet,†*ab Marika Springc and Sandrine Pages-Camagnab

The blue pigment smalt, a potash silicate glass coloured with cobalt, was commonly used between the

sixteenth and eighteenth centuries. The composition is complex and can vary considerably depending

on the manufacturing process and the elements that are present in the raw materials in addition to the

essential silica, potassium and cobalt. It also often degrades through leaching of potassium, leading to a

change in cobalt coordination from tetrahedral to octahedral, and a loss of the blue colour. In this study

smalt pigment (both well preserved and altered) in samples from paintings in French collections (mainly

Musee du Louvre) and in the National Gallery, London, as well as modern references of more simple

composition, was analysed by the complementary vibrational techniques Raman micro-spectroscopy and

synchrotron Fourier transform infrared micro-spectroscopy. Comparison of the spectra with those from

modern smalt, together with spectral decomposition and correlation with quantitative SEM-EDX

elemental analysis, sheds new light on the role of the various cations in the silicate structure. Important

modifications in the structure of the pigment on alteration are also revealed, in particular leaching of

alkali and the formation of silanols, which subsequently condense to create new bridging Si–O–Si bonds

and molecular water in the glass. The sensitivity of the glass to dry conditions creates a tendency

towards shrinking and cracking, which is very likely a contributing factor in the loss of cohesion,

breakdown and blanching often observed in paint containing degraded smalt.

1 Introduction

The blue pigment smalt, which was widely used between thesixteenth and eighteenth centuries, is a cobalt-containing potashglass. It wasmade from a source of silica such as pebbles or sand,potash as the ux and calcined or roasted ‘zaffre’ (a mixture ofroasted cobalt ore and varying amounts of quartz or sand). Smaltis far higher in cobalt content than blue vessel or window glass (inpaintings it typically contains 3–7 weight% CoO), as this isnecessary for it to still retain a blue colour when ground to apowder. Historic documentary sources indicate that a number ofgrades were sold which differed in the intensity of colour, eitherdue to variation in the particle size (more nely ground smalt ispaler in colour) or the cobalt content. It was employed not only asan artists' pigment but also for processes such as laundry blueingand tinting of paper, for which lower grades that contained lesscobaltwereprobablyused (smaltwitharound1wt%CoOhasbeen

ron SOLEIL, Saint Aubin, BP48 F-91192

[email protected]; Fax: +33 40 79 53

es Musees de France, Palais du Louvre,

afalgar Square, London WC2N 5DN, UK

sur la Conservation des Collectionsaire, 75005 Paris, France.

–4638

reported on tinted paper).1 In addition to silica, potassium andcobalt, historic smalt always contains other elements that origi-nate from the silica source, such as aluminium and iron, or fromthe cobalt ore, such as nickel, bismuth and arsenic. The last ofthese is oen present at levels equal to or considerably greaterthan cobalt. The composition can vary signicantly, perhaps dueto theuseofdifferent recipesbut alsoaccording to theoriginof therawmaterials and theway inwhich theyhavebeenprocessedpriorto manufacture of the pigment. The arsenic content in particularcan vary greatly depending not only on the particular cobalt orethat was used but also on the degree to which it was roasted.

Smalt is not very stable and has almost always discoloured atleast to some extent. The analyses of smalt in paintings fromFrench museums (mainly from the Musee du Louvre, Paris) andthe National Gallery, London, discussed here were part of abroader study investigating the processes occurring on alter-ation of the pigment. As with other alkali glasses, leaching of thealkali occurs on deterioration and it had already been noted thatin smalt this was accompanied by a loss of the blue colour.2–4TheX-ray absorption spectroscopy at the CoK-edge carried out in therst part of this study, published elsewhere, demonstrated thatthe origin of the loss of colour was a change of Co2+ from atetrahedral towards an octahedral coordination. A quantitativecorrelation wasmade with the loss of potassium, indicating thatit was directly responsible for this change in the cobalt

This journal is ª The Royal Society of Chemistry 2013

http://dx.doi.org/10.1039/c3ay40906f

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

http://pubs.rsc.org/en/journals/journal/AY?issueid=AY005018

Paper Analytical Methods

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environment.5 Further XAS measurements were made subse-quently using an alternative approach on articially leachedreference samples of modern smalt of more simple compositionpresented in this paper. The variations in the intensity of the pre-edge peak associated with the coordination change of Co2+ weremapped across individual particles of smalt by collecting theX-ray uorescence response at the energy of this peak. Thecorrelation between the colour as seen in the visible light image,the elemental composition and the modication in cobaltcoordination is thus directly visible (Fig. 1).

As well as the loss of colour in the pigment itself, in oil paintcontaining smalt there are concurrent processes that occur,resulting in the paint as a whole not simply fading butbecoming greyish or brown, sometimes also developing ablanched appearance. FTIR microscopy has already been usedto investigate these changes, showing that the leached potas-sium reacts with fatty acids in the oil to form potassium soapswhich can migrate to the paint surface and subsequently reactwith dirt and other environmental pollutants resulting in adisguring crust.4

This study concentrates instead on using vibrational spectro-scopic techniques to investigate changes on degradation in theglass itself. The research has also included detailed considerationof spectra from well-preserved smalt, with the aim of gaininginsight into the role that the various cations play in the glassstructure. This is of considerable interest as it might ultimatelyallow some hypotheses to be drawn about how and why thephysical properties of smalt vary according to the composition; ithas been noted, for example, that there are differences in thedegree of degradation of the smalt in two separate areas of apainting by Anthony van Dyck that seem to be related to differ-ences in composition.6 This subject has not been explored indetail, even though FTIR and Raman spectroscopy are oen usedon samples from historic paintings. A better appreciation of theinuence of the composition of smalt on the infrared and Ramanspectra is also important for interpretation of routine analyses ofpaint containing this very commonpigment. Inmost infrared andRaman databases modern reference samples have been used forthe smalt spectra, with no indication of the composition. Theseare not very representative of what is found in smalt fromhistoricobjects and paintings.

Fig. 1 Artificially leached smalt particles: (a) an optical microscope image; (b) backsof the SOLEIL synchrotron (technical conditions can be found in ref. 5).


2 Background and methodology

FTIR and Raman spectroscopy are complementary as they areable to probe different structural changes in glass since theyobey different selection rules. They have largely been applied tobulk samples of glass to investigate the chemical structure,specically to examine the different silicate units present, theirproportions and their arrangement within the network.

The region ofmost interest in the Raman and infrared spectraof glasses is between 300 cm�1 and 1200 cm�1, corresponding tovibrations of the silicate network, withmainly bending vibrations(d) in the region 300–600 cm�1 and stretching vibrations (n) in theregion 900–1200 cm�1.7,8 The vibrations of the Si–O bonds aremodied by their environment, that is, the type and concentra-tion of the neighbouring cations. The cations in the smalt glass,whether added intentionally or not, can therefore affect the sili-cate network, and thus the vibrational spectra. All the cationsinduce depolymerisation of the silicate network by breaking theSi–O connectivity and creating an ionic bond between the oxygenand the cation, which is weaker than the Si–O bond. The oxygenscoordinated to cations in the silicate structure are called non-bridging oxygens (NBOs), as opposed to the bridging oxygens(BOs) linking two silicates. Depending on the glass compositiondifferent silicate species containing between zero and four NBOs,also called Qn species with n equal to the number of BOs, can beformed and coexist in equilibrium in the structure. The cationspresent in the structure will be preferentially coordinated to aparticular silicate species depending on their type and theirconcentration in the glass. In general, Q3 (1 NBO) species aremostly coordinated to mono-charged cations (such as alkali) andQ2 (2 NBOs) species to doubly charged cations. However, whenpresent in high concentration in the glass,mono-charged cationswill coordinate to Q2 species as well as Q3.

To carry out analogous studies onmicroscopic smalt particles,reectance FTIRmicroscopy with a synchrotron source providinga high brilliance and allowing a high spatial resolution (around10mmfor the full spectral range) was used here, so that individualparticles of smalt could be analysed in paint samples prepared ascross-sections. The shiny surface of the glass allows good qualityspecular reectance spectra to be collected with FTIR, and struc-tural information can be gained directly from these spectra

cattered electron SEM image; (c) XRF mapping at 7711 eV on the LUCIA beamline

Anal. Methods, 2013, 5, 4628–4638 | 4629


Analytical Methods Paper

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without any further treatment. Raman microscopy has an evenhigher spatial resolution of around 2 mm, making it possible tocollect spectra from much smaller smalt particles, or even fromthe unaltered core and the degraded rim of a single particle.Additional information can be gained from the Raman spectra ofglasses, by using spectral decomposition to separate the differentvibrational components within the broad bands and by corre-lating these components with the elemental composition. Thecorrelations established help assign the vibrational components,understand the role of the various ions in the glass and allowqualitative and quantitative information to be extracted from theRaman spectra.9,10 The application of spectral decomposition inthe case of infrared spectra is more complex as it requires math-ematical modication of the spectra or of the model of decom-position. A few tting approaches have been presented in theliterature; however, these are always based on simple binary glasscompositions.11,12 For this reason, structural interpretations havebeenmostly based on reectance spectra, andhavegenerally beenmore qualitative than quantitative,13–15which is also the approachthat has been taken in this study.

3 Experimental3.1 Samples

The laboratory of the Centre de Recherche et de Restaurationdes Musees de France (C2RMF) and the Scientic Departmentof the National Gallery London both own a large bank of smalt-containing paint samples which have been collected over many

Table 1 Details of paint samples examined [reference numbers starting with ‘L’ ar

Samplereference Painting details

L6902 Jacopo Tintoretto, The Virgin and Child with Saint CathSaint Mark and Saint John the Baptist (A122), around 1canvas, Musee des Beaux-Arts, Lyon

L4811 Girolamo Mazzola-Bedoli, The Nativity (inv.389), arouncanvas, Musee du Louvre, Paris

L2925 Paolo Veronese, The Gods of Olympus: Jupiter, Apollo, D(RF2183), around 1557, fresco transferred to canvas, M

NG26S6 Paolo Veronese, The Consecration of Saint Nicholas (NGcanvas, National Gallery, London

L2281 Frans Floris I, Dieu rassemblant et protegeant son peupCrucie (inv.20746), 1562, oil on panel, Musee du Lou

L2720 Giorgio Vasari, The Annunciation (inv.732), 1563–1572,du Louvre, Paris

NG5467 Paolo Fiammingo, Landscape with the Expulsion of thearound 1590, oil on canvas, National Gallery, London

NG13S8 Bartolome EstebanMurillo, The Heavenly and Earthly T1682, oil on canvas, National Gallery, London

L3444 Jacob Jordaens, The miracle of the shes, 17th century,des Beaux-Arts, Strasbourg

L2177 Jean-Baptiste Weenix, Depart d'une troupe orientale, prcorsaires repousses (inv.1935), third quarter of the 17thcanvas, Musee du Louvre, Paris

L6801 Gabriel Blanchard, The Triumph of Diana (822.1.22), acanvas, Beaux Arts Museum, Rouen

L6064 Luca Giordano, The Death of Seneca (MI 871), aroundMusee du Louvre, Paris

L4346 François Lemoyne, Hercule killing Cacus, 1718, oil onLouvre, Paris

4630 | Anal. Methods, 2013, 5, 4628–4638

years. Among these, thirteen samples from paintings datingbetween the sixteenth and eighteenth centuries were selectedfor this study, as they contained smalt particles large enough tobe analysed individually by FTIR or did not generate stronguorescence by Raman spectroscopy. Details of the paintingsstudied are listed in Table 1. All the samples contained somewell-preserved particles that retained a blue colour as well asaltered pale or colourless particles, making it possible tocompare the vibrational spectra from each. Four modern smaltsfrom the National Gallery reference collection with a moresimple composition consisting of only Si, Co, K and in one caseAs were used for comparison to assist interpretation (Table 2).Two of these (labelled Ex3f and 4fe10 signifying different gradesof the pigment) were obtained from Reckitts (UK) in the 1950s.The Mason strewing smalt also dates from the mid-twentiethcentury, while the Cornelissen smalt was purchased veryrecently. Articially altered smalt was also created by leachingthe smalt reference from Cornelissen in a diluted acetic acidsolution under static conditions at 60 �C for four months. Thepaint microsamples and the modern smalt particles wereprepared as cross-sections by mounting in polyester resin, fol-lowed by cutting with a diamond saw or grinding on siliconcarbide (SiC) paper. Polishing was carried out with ne gradesof SiC paper or Micromesh� polishing cloth.

3.2 Analytical techniques

The paint cross-sections were examined and photographed witha light microscope (Leica DM4000M or Nikon Eclipse LV100 D)

e from the C2RMF and with ‘NG’ from the archives of the National Gallery]

Sampling area

erine, Saint Augustine,546–1549, oil on

Virgin's cloak, on the shoulder

d 1552–1554, oil on Virgin's cloak, on her le knee

iana and Marsusee du Louvre, Paris

Light blue sky at the right

26), 1562, oil on Brown discolored shadow of the bishop'sblue drapery

le par la grace duvre, Paris

Bottom of mauve dress at the le

oil on panel, Musee Blanched blue of the Virgin's cloak

Harpies (NG5467), Light blue area of the sky

rinities (NG13), 1675– Greyish-blue sky

oil on panel, Musee Sky at the le

eviously named Lescentury, oil on

Blue from the sky

round 1680, oil on Grey sky in the centre

1684, oil on canvas, Light blue of Seneca's drapery

canvas, Musee du Drapery of Cacus




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under normal and ultraviolet light (lter block A; excitation BP,340–380 nm; emission LP, 430 nm).

Quantitative elemental analysis and elemental mapping werecarried out using a scanning electronmicroscope (SEM) coupledto an energy dispersive X-ray spectrometer (EDX). Paint cross-sections from the National Gallery London were analysed in aCarl Zeiss EVO� MA10 SEM with an Oxford Instruments INCA350 EDX system and an X-max 80 mm2 EDX detector. Variablepressure conditions allowed charge compensation on the non-conducting samples, avoiding the need for carbon coating andallowing further analysis by other techniques. The operatingparameters were 20 kV, 200 pA beam current, 30 Pa chamberpressure (the minimum necessary to limit charging) and watervapour as the chamber gas. The accuracy of the standardlessquantitative analysis was evaluated by analysis of CorningMuseumofGlass reference glassDand themeasured valueswerefound to be well within 0.2 weight% oxide of the reference valuesfor the majority of the elements. Paint cross-sections from theC2RMF were analysed on a JEOL JSM840 SEM with an OxfordLink ISISEDXsystemandaSi(Li) 10mm2detector. Theoperatingparameters were 20 kV with a working distance of 15 mm. Thesamples were carbon coated and analysed under high vacuumconditions. A cobalt pellet was used for calibration at high-magnication (around 15 000�) and the same acquisitionparameters were used to analyse the smalt samples. All thecompositional values presented in Table 2 were corrected forcalciumand lead, since these elements originatemostly from thepigments mixed with the smalt or, in the case of one of the

Table 2 Quantitative SEM-EDX analysis of the elemental composition of smalt in pafrom the C2RMF and with ‘NG’ from the archives of the National Gallery]

Mol% SiO2 Al2O3

Paint samplesTintoretto – L6902 Well preserved 81.39 0.65Mazzola-Bedoli – L4811 Well preserved 78.08 0.45Veronese – L2925 Well preserved 79.08 1.41

Altered 87.71 0.47Veronese – NG26S6 Well preserved 73.40 0.29

Altered 79.18 0.32Altered 80.59 0.36

Floris – L2281 Well preserved 73.23 0.28Vasari – L2720 Well preserved 73.99 0.27Fiammingo – NG5467 Well preserved 79.23 0.03

Altered 87.88 0.72Murillo – NG13S8 Well preserved 82.89 0.60

Altered 88.74 0.58Altered 95.75 0.30

Jordaens – L3444 Well preserved 81.51 0.67Weenix – L2177 Well preserved 81.62 1.21Blanchard – L6801 Well preserved 81.87 0.77Giordano – L6064 Well preserved 82.27 0.73Lemoyne – L4346 Well preserved 83.80 0.64

ReferencesCornelissen – XS10 Well preserved 77.33 0.32Cornelissen – XS6A Altered 91.23 0.36Reckitts – NGEx3f Well preserved 83.47 —Reckitts – NG4fe10 Well preserved 84.75 —Mason strewing Well preserved 86.79 0.33


paintings by Veronese (L2925), which is the only example of afresco included in the study, from the binder.When consideringthe results, however, it should be borne inmind that the samplesimpose some limits on the accuracy, since in addition to thecontribution from the pigments around the smalt particles, it isnot possible to know their thickness, and there are someinconvenient peak overlaps for the elements present.

Synchrotron Fourier-transform infrared microspectroscopyspot analysis (and raster scanning) was performed at the SMISbeamline of the SOLEIL synchrotron source (Saint-Aubin,France). The beamline is coupled to a NEXUS FTIR Nicolet 5700spectrometer attached to a CONTINUUM XL microscope(Thermo Scientic). The detector of the infrared microscope is amedium range mercury cadmium telluride detector (MCT-A,50� 50 mm2). Themicroscope was operated using a�32 innitycorrected Schwarzschild objective with a numerical aperture(NA) of 0.65. Spectra were obtained using a double path singlemasking aperture (confocal arrangement) 11 � 11 mm2 in size,so that the full spectral range from 650–4000 cm�1 could becollected. Mapping measurements were also carried out withthe intention of investigating interactions between the smaltpigment and the medium around it; however, very poor spectrawere obtained from the matrix due to the low reectance signalin this area of the sample and the difference in the level of theglass particles and the surrounding binder (Fig. 2). The spectradiscussed here were therefore only collected from the glass.

The smalt references, having a larger particle size (around70 mm), could be analysed on a lab-based infrared instrument

int cross-sections and reference samples. [Reference numbers starting with ‘L’ are

Na2O K2O FeO CoO As2O3 NiO Bi2O3

0.80 9.07 2.51 3.70 0.34 0.73 —0.24 7.66 1.93 5.56 5.37 0.71 —0.86 10.39 2.64 2.20 1.91 0.12 —0.45 1.32 2.12 3.79 2.45 0.83 —0.20 10.65 3.21 5.78 5.14 1.08 0.250.21 3.79 3.60 6.29 5.40 1.03 0.180.12 2.07 3.83 6.48 5.39 0.93 0.220.45 12.13 3.37 5.29 0.88 1.81 1.580.49 11.27 2.22 6.11 4.97 0.68 —0.29 10.64 2.68 3.76 2.92 0.46 —0.17 1.73 3.22 2.73 3.48 0.06 —0.44 8.21 3.02 3.08 1.13 0.55 0.070.11 2.46 2.85 3.26 1.20 0.71 0.090.09 0.92 1.06 0.75 0.22 0.65 0.010.89 8.42 3.09 1.75 2.58 — —0.50 8.18 3.53 2.56 1.33 0.38 —0.40 7.74 3.06 3.19 2.17 0.55 —1.26 6.93 3.61 3.15 0.48 1.15 —0.03 7.83 2.95 3.08 0.17 1.32 —

0.43 14.76 — 7.16 — — —— 0.59 — 7.81 — — —— 10.65 — 5.88 — — —— 11.86 — 3.38 — — —— 10.37 0.37 0.80 1.34 — —

Anal. Methods, 2013, 5, 4628–4638 | 4631


Fig. 2 Paint cross-section from the painting by Murillo (NG13S8): optical microscope image and topographic backscattered electron SEM image showing thesignificant difference in the level of the smalt particles and the surrounding binding medium.


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with a Hyperion 3000 microscope coupled to a Vertex 70spectrometer (Bruker). The spectra were collected using abroad rangemercury cadmium telluride detector (MCT-B, 100�100 mm2) from 420 to 4000 cm�1 and a �15 objective with anumerical aperture (NA) of 0.4. All spectra were collected inreectance mode at a spectral resolution of 8 cm�1 and aver-aging either 128 or 512 scans. The background was measuredon a gold mirror.

Raman micro-spectroscopy measurements were undertakeneither on a LabRam Innity (Jobin-Yvon-Horiba) at the C2RMFor a DXR micro-spectrometer (Thermo scientic) on the SMISbeamline, SOLEIL. The 532 nm line of a doubled diode laser wasused as the exciting radiation on both instruments and thepower of illumination at the sample surface ranged between 5and 10 mW. The backscattered light was collected throughthe �100 microscope objective and the Rayleigh scattering wasltered either by a notch or an edge lter. The spectra werecollected between 150 and 1650 cm�1 and between 2800 and4000 cm�1 and the approximate acquisition time was 60 s or 90s repeated three to ve times. The strong uorescence preventedthe collection of Raman spectra in a large number of paintsamples, so only a fraction of the samples analysed are pre-sented in this paper.

3.3 Data treatment

Infrared and Raman spectra were manually baseline correctedusing multiple linear segments, with nodes placed at similarpositions.10 Spectral decomposition of the Raman spectra wascarried out using Gaussian functions in OriginPro 8.0. Thedecomposition model previously developed for the spectra ofhistoric alkali silicate vessel and window glasses9 was applied tosmalt glass.

Fig. 3 FTIR reflectance spectra of well-preserved smalt from the paint samples(a) and references (b).

4 Results and discussion4.1 Effect of composition on the spectra of well-preservedsmalt particles

The elemental composition of the smalt in the paintings andthe references is presented in Table 2. Historic and modernsmalts are both potash silicate glasses containing cobalt.Silicon together with oxygen is present as SiO4 tetrahedral unitsforming the backbone of the glass network. By creating NBOs

4632 | Anal. Methods, 2013, 5, 4628–4638 This journal is ª The Royal Society of Chemistry 2013


Fig. 4 Raman spectra of well-preserved smalt from the paint samples (a) and the references (b).


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the alkali, here potassium, lowers the temperature needed tomelt the components, decreasing the viscosity and thereforeimproving the workability of the glass. Cobalt is the colouringion, and the deep blue hue originates from the presence of Co2+

ions mostly in tetrahedral coordination. The other elementspresent may also have an inuence on the properties of theglass, but were generally not added intentionally. They insteadcame from the raw materials: aluminium and iron from thesand or pebbles used as the silica source; arsenic, nickel andbismuth from the cobalt ore. Arsenic compounds are some-times deliberately added to glass, as they can make it moreeasily fusible or can modify the optical properties (acting as adecolouriser, for example).16 However, arsenic-containingcobalt ores such as those from the Erzgebirge (Schneeberg,Siegen and Annaberg) were commonly used to manufacturesmalt, and it seems most probable that in general this is thereason for the presence of arsenic in this pigment. Even if theores were oen prepared by roasting, this is likely to have onlypartially removed this element.1

Fig. 3 and 4 present respectively infrared and Raman spectrafrom well-preserved smalt particles in the paint and from thereference samples. In the infrared spectra only the stretchingvibrations of the silicate network are observed, whilst bothstretching and bending vibrations are accessible in the Ramanspectra. The proles of the spectra and the composition areclose to that of an alkali disilicate glass, which implies that


three types of silicate species are expected to coexist in thestructure; Q3 in major concentration, as well Q2 and Q4. Thereare noticeable differences, however, between the historicalsmalt from paintings and the references, mostly in the regionbelow 1000 cm�1 in the infrared spectra and between 900 and1200 cm�1 in the Raman spectra. These spectral variations,linked to the compositional differences between the glasses,can provide some insight into the role and interaction of thecations with the silicate network. Using the spectral decompo-sition model, the effect of the elemental composition on eachdifferent region or component of the Raman spectra wasexamined in detail. The decomposition model applied togetherwith the proposed assignment and an example of spectraldecomposition is presented in Table 3 and Fig. 5.

The alkali ions (predominantly K+) are the main cations inthe well-preserved smalt glass and are expected to play two rolesin the structure, either interacting with the silicate network, oracting as charge compensating ions for As3+, Al3+ and Co2+ andpossibly Fe3+ and Ni2+. At the potassium content found in thehistoric smalt glass (�7 to 14 mol%), the alkali cations areexpected to create only Q3 and no Q2 species in the structure.17

The stretching vibration of Si–O� bonds in Q3 species appears at950–990 cm�1 in the infrared spectra and around 1100 cm�1 inthe Raman spectra. Due to the different selection rules, theRaman technique is more sensitive to the stretching vibrationof non-bridging bonds; as a result the Q3 component is very

Anal. Methods, 2013, 5, 4628–4638 | 4633


Fig. 6 Plot of the area of the component at 1100 cm�1 (AQ3) over the area ofthe bending region (300–700 cm�1) with the total alkali content in mol%.

Table 3 Parameters for spectral decomposition of Raman spectra of smalt(italicised values ¼ fixed parameter)

Frequency(cm�1)

Bandwidth(cm�1)

Suggestedassignment

280–370 dSi–O–Si Q4

380–470 dSi–O–Si Q4

500–570 dSi–O–Si Q3

580–630 dSi–O–Si Q2

770–800 Si–O–Si network830–850 nSi–O� (As)990 nSi–O Q2

1040 701090–1100 nSi–O Q3

1150 nSi–O Q3 + nSi–OQ4


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intense in the Raman spectra. FTIR is instead more sensitive tothe vibration of the antisymmetric modes; for this reason theintensity of the region between 1065 and 1095 cm�1 corre-sponding to the asymmetric stretching vibration of the bridgingnetwork in an alkali environment is very strong.13 Interestingly,the concentration of Q3 species in the Raman spectra seems tobe proportional to the alkali content, contrary to alkali–limeglass where the calcium present in the structure partially affectsthe vibration of Q3 species. The correlation was established byplotting the ratio of the areas of the component at 1100 cm�1

and the bending region (300–700 cm�1) against the total alkali

Fig. 5 Spectral decomposition of Raman spectra of smalt in paint sampleNG13S8, prepared as a cross-section.

4634 | Anal. Methods, 2013, 5, 4628–4638

content (Fig. 6). However, the linear trend observed issurprising, as it implies that the alkali ions that are acting ascharge compensators are still interacting with the silicatenetwork.

When coordinated with the silicate network, the doublycharged cations are expected to interact mostly with Q2 species.The stretching vibration of Si–O� bonds in Q2 species isbetween 860 and 940 cm�1 in the infrared spectra. Thedecomposition model developed for the Raman spectra ofhistoric alkali silicate glasses gave two components associatedwith two different Q2 species, at 950 and 990 cm�1.9 In thedecomposition of the Raman spectra of smalt, only the Q2

components at 990 cm�1 were present; the component at950 cm�1, mainly linked to Q2 species coordinated with alkalineearth cations, was absent. The absence of this band at 950 cm�1

is consistent with the composition of smalt glass, in which thealkaline-earth cations are either not present at all or only at lowlevels. In smalt the doubly charged cations are cobalt, nickeland, depending on its oxidation degree, iron. From the spectraof the modern smalt samples of simple composition, we noticethat the Q2 species at 990 cm�1 are very weak or non-existent,despite the high levels of Co2+, therefore suggesting that thecobalt cations do not interact with the Q2 species, or any part ofthe silicate network. Instead, the Raman spectra of the historicglass suggest that the intensity of the component at 990 cm�1 islinked to the concentration of iron with possibly nickel. A clearcorrelation is, however, difficult to establish due to the complexcomposition and the lack of knowledge about the degree ofoxidation of some of the cations in the glass, which in the caseof iron would strongly affect this region.18,19

Aer potassium, the cation that is present at most signicantconcentrations in smalt glass is arsenic, with the contentsometimes being as high as 5 mol%. Arsenic has been said tohave no interaction with the silicate network and to create itsown lattice in the glass, with formation of arsenate (As5+) andarsenite (As3+) chains of varying lengths.20–22 In the Ramanspectra, the vibration of nAs–O� in these arsenite and arsenatechains would create bands at 830 and 880 cm�1 respectively. In



Fig. 7 Correlation between the ratio of the area of the band at 800 cm�1 overthe area of the bending region (A800/Abending) in the Raman spectra and thearsenic content in the glass (+Bi2O3 for L2281).

Fig. 8 Comparison of FTIR spectra of well-preserved (dashed, black) and altered(plain, red) smalt in paint sample NG5467 (a) and in the Cornelissen reference (b).


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the Raman spectra of smalt, the presence of arsenic is visible inthe region around 800–900 cm�1, as evident from the spectrumof the only arsenic-containing reference Mason strewing smalt(Fig. 4b), although the band overlaps with the weak bandsassociated with the motion of silicon against its tetrahedraloxygen cage.8 The type of arsenic species present howeverdepends on the alkali content. As the alkali content decreasesthe As3+/As5+ ratio increases and more condensed arsenate andarsenite structures are expected needing fewer chargecompensating ions.21 In smalt, the composition approachesthat of the low alkali-containing glasses presented by Verweij(10 and 20 mol% K2O), therefore long arsenite chains (As3+) areexpected to dominate in the structure. Despite the overlap, theintensity of the region between 700 and 900 cm�1 can be seen toincrease proportionally with the concentration of arsenic in thesmalt glass (Fig. 7). An exception was the smalt in sample L2281,where the arsenic content is low, but a band of strong intensityin this region between 800 and 900 cm�1 was still observed(Fig. 4a). Unlike the other samples, however, it has a highbismuth content (almost 1.6 mol%). When both Bi2O3 andAs2O3 were included in the calculation for this sample in theplot in Fig. 7 the correlation was improved, which seems tosuggest that the vibration of bismuth species also affects thisregion of the spectra. Although it is difficult to predict the effectof arsenic on the properties of a glass as complex as smalt, thisinteraction is likely to modify the number of NBOs and mightperhaps have an inuence on its stability.

The lower frequency region of the spectra is associatedmostly with the vibration of the bridging network. The regionbetween 300 and 700 cm�1 of the Raman spectra, assigned tothe bending vibration of Si–O–Si bonds, is little affected by thecompositional variation between glasses (Fig. 4). In contrast,modications are visible in the region below 850 cm�1 of theinfrared spectra corresponding to the symmetric stretching ofthe bridging network (Fig. 3). In the spectra from historic smaltthe number of bands varies within this region, there beingeither one or two present, and they are more intense than in the


spectra of the reference samples. These observations suggestthat these differences are related to the additional cations in thecomposition.

4.2 Effect of alteration on the structure of smalt

When it degrades smalt pigment turns from a deep blue to acolourless or very pale tint (Fig. 1 and 2). As is evident from theelemental composition, the alkali ions in the glass are leachedout of the structure as a result of an ion-exchange reaction butthe other elements are generally retained (Table 2), although acertain degree of leaching of cobalt and nickel has sometimebeen observed where degradation is severe.4

The alteration induces strong modications in the vibra-tional spectra as shown in Fig. 8 for the infrared and Fig. 9 forthe Raman spectra. The similarity of themodications observedin the spectra for the smalt in the paint and the articiallyleached smalt conrms that an alkali leaching reaction domi-nates the alteration process of the glass. The decompositionmodel used for the Raman spectra of the well-preserved smaltwas also applied to the spectra of the altered smalt (Fig. 5).

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Fig. 9 Raman spectra of well-preserved and altered smalt in the paint sampleNG5467 (top) and the Cornelissen reference (bottom).

Fig. 10 Raman spectra over the region between 2800 and 4000 cm�1 of alteredsmalt in paint samples L2925 and L4346.

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As a result of the leaching of the alkali ions from the glass,the intensity of the Q3 component decreases in the Raman andinfrared spectra. This modication is particularly strong in theRaman spectra due to the high sensitivity of the technique tothe vibration of non-bridging oxygen. Because of the ionexchange, the leaching of potassium would be expected to beaccompanied by the formation of silanol species in the glass. Inthe infrared spectrum of the leached Cornelissen smalt, a shiand a slight increase of the components around 940 cm�1 mayindicate the formation of silanols (Fig. 8).15 In the Ramanspectra, evidence for hydration (silanols and/or molecularwater) of the silicate network was observed through the pres-ence of a broad band in the region between 3300 and 3700 cm�1

resulting from the stretching vibration of the O–H species in theglass (Fig. 10).

The Q2 vibrations are mostly unchanged in the infrared andRaman spectra of the historic samples, which supports thehypothesis that it is the iron cations (retained in the structureon deterioration) that interact with this vibrational species. Inthe Cornelissen smalt reference sample, which does not containiron, the Q2 species (at low levels) are coordinated with some ofthe potassium. As a result of the leaching, a decrease is observedin the intensity of the Q2 component in the infrared spectra(Fig. 8). The coordination change of the cobalt ions induced bythe leaching of alkali is not apparent in the vibrational spectra,supporting the supposition that there is an absence of inter-action of the cobalt ions with the silicate network.

The bridging network is also modied by the alteration.In the infrared spectra the main band between 1065 and1095 cm�1 becomes sharper and appears to move towards ahigher frequency. The position of this band has previously beencorrelated with the SiO2 content in glass, with a shi towardshigher frequencies occurring as the degree of polymerisation ofthe glass increases. This “shi” has sometimes been said tooriginate from an increase in the Si–O–Si bond angle, but it hasalso been proposed that it is not a real shi in the position of asingle band but is in fact due to a change in the relative intensityand bandwidth of several of the components making up theband.12,23,24 Such a correlation is observed for the smalt samples

Fig. 11 Plot of the FTIR peak summit against SiO2 content (calculated withoutarsenic) in smalt from paint and reference samples.



Fig. 12 Scheme of the alteration mechanism of smalt pigment.


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when the summit of this band in the well-preserved and alteredsmalt spectra is plotted as a function of the SiO2 content in theglass. This correlation was improved by using the value of theSiO2 content without taking into account the arsenic content,expected to have its own network in the glass (Fig. 11). There-fore, this correlation can be used to access the SiO2 content ofthe glass directly from the FTIR spectra and also supports thehypothesis of the absence of interaction between the arseniccations and the silicate network. In the Raman spectra, theA500/A1000 ratio can be calculated to examine the degree ofpolymerisation in the glass.25 In the smalt spectra, this ratioincreases from �1.3 in the well-preserved pigment to �3 in thedegraded pigment, which conrms an increase in the degree ofpolymerisation in the glass on alteration. In addition, in theinfrared spectra from altered glass particles the intensity of theband in the region between 1160 and 1220 cm�1, correspondingto the stretching vibration of the fully polymerised species Q4

(SiOSi), increases. In the region between 300 and 700 cm�1 ofthe Raman spectra of the altered pigment, the slight change inthe band prole with an apparent shi towards lower wave-numbers (Fig. 9) may result from a decrease in the Q3 speciesand formation of new Q4 species. All these spectral modica-tions reveal that new bridging bonds have been formed as aresult of condensation of the silanol species in the glass, thuscreating a more polymerised network in the altered pigment.Finally, the presence in some infrared spectra of weak bandsassociated with the vibration of molecular water around1620 cm�1 (not shown here) and the low silanol species contentobserved is consistent with a polymerisation reaction.15

These new data on the silicate structure can be combinedwith the results that have been obtained from earlier studies ofthe same samples using complementary analytical techniquesto gain a broad picture of the alteration of smalt pigment inpaintings. Fig. 12 attempts to summarise the various concur-rent or successive reactions taking place in the environmentaround the smalt particles as well as within them duringalteration of the pigment. Smalt behaves in a way that is typicalof alkali silicate glasses, being sensitive to humidity. Thus, the


rst reaction to take place is an ion exchange between hydro-genated species in its environment and the alkali in the glassstructure. In paintings, hydrogenated species are expected tooriginate from the atmosphere and/or the free fatty acidsformed during the ageing of the oil. The leaching of thepotassium ions induces reorganisation within the structure,with condensation following hydration of the silicate networkand a coordination change of the cobalt (the latter causing thecolour loss), as well as the formation of metallic soaps byreaction with the fatty acids in the binder causing browning ofthe oil medium.

5 Conclusion

Infrared and Raman spectroscopy of smalt pigment providesparallel as well as complementary information on the silicatenetwork, the role of the cations present in the glass and theeffect of the alteration on the structure. Information on thedepolymerised network was mostly gained from Raman spectra,whilst information on the polymerised network (Q4) was mostlyaccessed from the FTIR spectra. The two vibrational spectros-copy techniques can thus help to distinguish degraded smalt (inwhich there is a higher degree of polymerisation) from smaltthat is intentionally pale in colour and is not degraded, giving abasis for interpretation of the original appearance of a passagecontaining smalt in a painting. Raman spectroscopy allowsanalysis at a signicantly higher spatial resolution (around1 mm) and collection of the full spectral range, whilst infraredspectroscopy has the advantage of not being limited by uo-rescence and not causing beam damage. Infrared imagingtechniques combined with the use of a germanium ATR wouldfurther extend the possibilities by improving the spatialresolution.

The composition of the smalt in paintings is verycomplex, and can vary considerably depending on differentmanufacturing processes and raw materials. Nevertheless,comparison of the spectra with those from modern smalt ofmore simple composition, together with the spectral decom-position and the correlations established between the vibra-tional techniques and the quantitative elemental analysis,sheds new light on the role of the various cations in the silicatestructure. Several of the main cations present in the glasscomposition do not appear to interact with the silicate network,in particular arsenic and cobalt. Arsenic, like aluminium, is anintermediate ion, which can substitute silicon and take the roleof the former, using alkali and alkaline-earth metal ions ascharge-compensating ions to maintain a tetrahedral coordina-tion. It is suspected here that cobalt, like arsenic, creates its ownnetwork in the glass.

Vibrational spectroscopy also revealed important modica-tions taking place in the structure of the pigment followingalteration, with the leaching of alkali and the formation ofsilanols which subsequently condense to create new bridgingSi–O–Si bonds and molecular water in the glass. As a result ofthe multiple reactions taking place within and around thepigment, this alteration is not reversible. In addition, thehydrated and more polymerised structure formed would result

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in a reduction in volume of the altered pigment, as well asincreased sensitivity to changes in humidity. A dry environmentcould induce mechanical constraints and cracking in the glassthrough shrinkage. This process could affect not only theintegrity of the glass pigment particles but also of the wholepaint lm, through possible loss of contact between the particlesurface and the binder, perhaps contributing to the blanchedand broken up appearance of paint containing smalt, oen seenin areas where this pigment has been used in paintings.

Acknowledgements

The authors acknowledge SOLEIL for provision of synchrotronradiation facilities and the SMIS beamline scientists PaulDumasand Christophe Sandt for support and fruitful discussion duringthe experiments. We also thank Nicolas Trcera from the LUCIAbeamline for the XRFmapping of the articially leached samplesand Alain Duval from the C2RMF for carrying out the SEM-EDXanalyses on the C2RMF samples. The IPANEMA platform atSOLEIL is being jointly developed by CNRS, MNHN, and theFrench Ministry of Culture and Communication, and benetsfrom a CPER grant (MESR, Region Ile-de-France).

References

1 F. Delamare, in Bleus en Poudres. De l'Art a l'Industrie 5000ans d'innovations, Mines Paris Paritech les presses, Paris,2007, pp. 71–122.

2 H. Paschinger and H. Richard, in Beitrag derNaturwissenschaen zur Erforschung und Erhaltun unsereskulturelleb Erbes, ed. M. Schreiner, Bohlau Verlag, Wien,Austria, 1995, pp. 63–66.

3 J. J. Boon, K. Keune, J. van der Weerd, M. Geldof andJ. R. J. Van Asperen de Boer, Chimia, 2001, 55, 952–960.

4 M. Spring, C. Higgitt and D. Saunders, National GalleryTechnical Bulletin, 2005, 26, 56–70.

5 L. Robinet, M. Spring, S. Pages-Camagna, D. Vantelon andN. Trcera, Anal. Chem., 2011, 43, 5145–5152.

6 M. Spring, V. Kugler and S. Bean, in Historical Technology,Materials and Conservation: SEM and Microanalysis, ed.

4638 | Anal. Methods, 2013, 5, 4628–4638

N. Meeks, C. Cartwright, A. Meek and A. Mongiatti,Archetype Publications, London, 2012, pp. 114–122.

7 T. Furukawa, K. E. Fox and W. B. White, J. Chem. Phys., 1981,75, 3226–3237.

8 P. McMillan, Am. Mineral., 1984, 69, 622–644.9 L. Robinet, K. Eremin, C. Coupry and C. Hall, J. RamanSpectrosc., 2006, 37, 789.

10 L. Robinet, A. Bouquillon and J. Hartwig, J. Raman Spectrosc.,2008, 39, 618–626.

11 S. A. MacDonald, C. R. Schardt, D. J. Masiello andJ. H. Simmons, J. Non-Cryst. Solids, 2000, 275, 72–82.

12 K. N. Dalby and P. L. King, Am. Mineral., 2006, 91, 1783–1793.

13 T. A. Sidorov, J. Appl. Spectrosc., 1967, 7(3), 258–261.14 D. M. Sanders, W. B. Person and L. L. Hench, Appl. Spectrosc.,

1974, 28(3), 247–255.15 M. E. Lynch, D. C. Folz and D. E. Clark, J. Non-Cryst. Solids,

2007, 353, 2667–2674.16 J.-C. Leuchs, Traite complet des proprietes, de la preparation et

de l'emploi des matieres tinctoriales et des couleurs. 2e partie:fabrication des couleurs, Mahler and Co, Paris, 1829.

17 V. P. Zakaznova-Herzog, W. J. Malfait, F. Herzog andW. E. Halter, J. Non-Cryst. Solids, 2007, 353, 4015–4028.

18 V. Magnien, D. R. Neuville, L. Cormier, J. Roux,J.-L. Hazemann, O. Pinet and P. Richet, J. Nucl. Mater.,2006, 352, 190–195.

19 Z. Wang, T. F. Cooney and S. K. Sharma, Geochim.Cosmochim. Acta, 1995, 59(8), 1571–1577.

20 T. A. Sidorov and N. A. Prudnikova, J. Appl. Spectrosc., 1968,8(1), 105–108.

21 H. Verweij, J. Am. Ceram. Soc., 1981, 64(8), 493–498.22 H. Verweij, J. Non-Cryst. Solids, 1980, 38(9), 885–890.23 D. E. Clark, E. C. Ethridge, M. F. Dilmore and L. L. Hench,

Glass Technol., 1977, 18(4), 121–124.24 P. L. King, P. McMillan and G. Moore, in Infrared

Spectroscopy in Geochemistry, Exploration Geochemistry, andRemote Sensing, Short Course 33, ed. P. L. King, M. S.Ramsey and G. A. Swayze, Mineralogical association ofCanada, 2004, pp. 93–133.

25 P. Colomban, J. Non-Cryst. Solids, 2003, 323, 180–187.



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Vibrational spectroscopy correlated with elemental analysis for the investigation of smalt pigment and its alteration in paintings