The Matter from Which an Orange Colour Is Made: On the Arsenic Pigment Used in a Portuguese Mannerist Painting
Abstract
The painting The Descent from the Cross, painted in 1620 by Pedro Nunes (1586–1637), presents two large figures with orange-coloured fabrics with conservation problems. Through the analysis of two samples with several analytical techniques, especially scanning electron microscopy combined with X-ray spectroscopy and Raman microscopy, it was possible to conclude that the orange colour is due to a complex artificial pigment made of amorphous arsenic sulphide. It essentially consists of spherical particles obtained by sublimation and condensation, possibly from orpiment, which ended up being joined with irregularly shaped particles resulting from crushing of the residual fraction obtained by solidification and fusion. This is a rare documented case of the extensive use of artificial arsenic sulphides in European easel painting, especially outside Italy. The conservation problems can be explained by the great sensitivity of the arsenic sulphides to photodegradation and the formation of powdery compounds.
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Citation: Cruz, A.J.; Melo, H.P.; Valadas, S.; Miguel, C.; Candeias, A. The Matter from Which an Orange Colour Is Made: On the Arsenic Pigment Used in a Portuguese Mannerist Painting. Heritage 2022,5, 2646–2660. https://doi.org/10.3390/ heritage5030138 Academic Editors: Valeria Di Tullio and Brenda Doherty Received: 21 July 2022 Accepted: 9 September 2022 Published: 13 September 2022 Publisher’s Note: MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affiliations. Copyright: © 2022 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/). heritage Article The Matter from Which an Orange Colour Is Made: On the Arsenic Pigment Used in a Portuguese Mannerist Painting António João Cruz 1,2,* , Helena P. Melo 1,3 , Sara Valadas 1,3 , Catarina Miguel 1,3 and António Candeias 1,3 1HERCULES Laboratory, University of Évora, Palácio do Vimioso, Largo Marquês de Marialva 8, 7000-809 Évora, Portugal 2Instituto Politécnico de Tomar, Estrada da Serra, 2300-313 Tomar, Portugal 3City University of Macau Chair in Sustainable Heritage, University of Évora, Casa Cordovil, 7000-651 Évora, Portugal *Correspondence: ajcr[email protected] Abstract: The painting The Descent from the Cross, painted in 1620 by Pedro Nunes (1586–1637), presents two large figures with orange-coloured fabrics with conservation problems. Through the analysis of two samples with several analytical techniques, especially scanning electron microscopy combined with X-ray spectroscopy and Raman microscopy, it was possible to conclude that the orange colour is due to a complex artificial pigment made of amorphous arsenic sulphide. It essentially consists of spherical particles obtained by sublimation and condensation, possibly from orpiment, which ended up being joined with irregularly shaped particles resulting from crushing of the residual fraction obtained by solidification and fusion. This is a rare documented case of the extensive use of artificial arsenic sulphides in European easel painting, especially outside Italy. The conservation problems can be explained by the great sensitivity of the arsenic sulphides to photodegradation and the formation of powdery compounds. Keywords: pigments; artificial arsenic sulphide; amorphous arsenic sulphide; historic treatises; Mannerism; Portugal 1. Introduction A large-scale application of an orange paint with an unusual surface appearance was found on The Descent from the Cross, a large panel (460 cm × 304 cm) painted in 1620 by Pedro Nunes (1586–1637) for the chapel of Esporão in Évora’s cathedral, Portugal [ 1 ] (Figure 1). This orange paint was found in the draperies of two apostles, the one standing tall to the right of the composition being more than two meters high. These areas raised interest, firstly, by the conservation problem that they exhibited, and secondly, by the material responsible for the orange colour. One of the initial hypotheses was that it was realgar, an orange-coloured pigment little used in European easel painting [ 2 ], particularly in Portugal [ 3 ], even though it fits well with the Mannerist palette, characterised by contrasts between complementary acid colours, namely, orange and blue, on the one hand, and rose and green, on the other. Pedro Nunes was a painter who, for some years, learnt the art in Rome [ 1 ], and, therefore, associated with this hypothesis, there was an additional reason of interest: to know whether or not the evident Italian artistic influences in the painting were accompanied by the same influences regarding the materials used, in particular, the realgar, which, at the time, seems to have mainly be used in Italy [2,4] p. 128. In order to investigate these questions, microsamples were collected from the orange areas and analysed with microscopic and spectroscopic techniques. The results obtained, which are presented below, turned out to be much more interesting than initially anticipated, as they showed the use, in an extensive area, of a material still little documented in European easel painting—an artificial arsenic sulphide. Heritage 2022,5, 2646–2660. https://doi.org/10.3390/heritage5030138 https://www.mdpi.com/journal/heritage
Heritage 2022,52647 Heritage2022,5,FORPEERREVIEW 2 anticipated,astheyshowedtheuse,inanextensivearea,ofamaterialstilllittledocu‐ mentedinEuropeaneaselpainting—anartificialarsenicsulphide. Figure1.TheDescentfromtheCrossbyPedroNunes,1620,EsporãoChapel,Évora’sCathedral, Portugal,(a)andasurfacedetailoftheorangecloak(markedwithawhiterectangle)underincident (b)andrakinglight(c). 2.Methods Thepaintingwassubjecttoavisualinspectionofthesurfaceinsituunderincident andrakinglight.DigitalphotographsweretakenwithaCanonG15camera.Twosamples werecollectedfromthemedium(#01)andlight(#02)tintsofthecloakoftheapostlestand‐ ingtotherightofthecomposition,inareasexhibitinganirregulargrainysurface. Partofeachsamplewassandwichedbetweentworigidtransparentpolymethyl methacrylate(PMMA)cubeswiththeself‐curingresinSpofacrylfromSpofaDentaland polishedasacross‐section.Thecross‐sectionswereexaminedwithopticalmicroscopyin reflectionmode(OM),underincidentvisible(OM‐Vis)andultravioletradiation(OM‐ UV).ALeicaDM2500microscopewasusedwithanexcitationfilterBP340–380,adichro‐ maticmirror,andasuppressionfilterLP425.DigitalimagesweretakenwithaLeicadig‐ italcameraDFC290HD.Theopticalpropertiesoftheartificialarsenicsulphidewerefur‐ therinvestigatedusingfragmentsofsample#02examinedinpetrographicmicroscope LeicaDM2700PcoupledwithaFlexacamC3camera. Subsequently,theuncoatedcross‐sectionsaswellasloosefragmentsofsample#02 wereanalysedwithscanningelectronmicroscopywithenergydispersiveX‐rayspectrom‐ etryonascanningelectronmicroscopePhenomPro‐X,operatedat15kV(SEM‐EDS).The Figure 1. The Descent from the Cross by Pedro Nunes, 1620, Esporão Chapel, Évora’s Cathedral, Portugal, ( a ) and a surface detail of the orange cloak (marked with a white rectangle) under incident (b) and raking light (c). 2. Methods The painting was subject to a visual inspection of the surface in situ under incident and raking light. Digital photographs were taken with a Canon G15 camera. Two samples were collected from the medium (#01) and light (#02) tints of the cloak of the apostle standing to the right of the composition, in areas exhibiting an irregular grainy surface. Part of each sample was sandwiched between two rigid transparent polymethyl methacrylate (PMMA) cubes with the self-curing resin Spofacryl from SpofaDental and polished as a cross-section. The cross-sections were examined with optical microscopy in reflection mode (OM), under incident visible (OM-Vis) and ultraviolet radiation (OM-UV). A Leica DM2500 microscope was used with an excitation filter BP 340–380, a dichromatic mirror, and a suppression filter LP 425. Digital images were taken with a Leica digital camera DFC290HD. The optical properties of the artificial arsenic sulphide were further investigated using fragments of sample #02 examined in petrographic microscope Leica DM2700P coupled with a Flexacam C3 camera. Subsequently, the uncoated cross-sections as well as loose fragments of sample #02 were analysed with scanning electron microscopy with energy dispersive X-ray spectrometry on a scanning electron microscope Phenom Pro-X, operated at 15 kV (SEM-EDS). The SEM-EDS microscope uses a charge reduction mode via a low vacuum sample holder. In addition to X-ray data, backscattered electron images (SEM-BSE) were used.
Heritage 2022,52648 Micro-Raman spectroscopy ( µ -RS) was used in the paint cross-sections using a Raman spectrometer Horiba XPlora equipped with a diode laser of 10.3 mW operating at 785 nm, coupled to an Olympus microscope. Raman spectra were acquired in extended mode in the 100–1500 cm –1 region. The laser was focused with an Olympus 50 × lens, 10% of the laser power on the sample surface (5 s exposure, 5 cycles of accumulation). The spectra were analysed with the equipment software (LabSPEC 5 from Horiba Jobin Yvon, France) and the Spectragryph (v. 2.15, Dr. Friedrich Menges, Oberstdorf, Germany) application. Micro X-ray diffraction was performed using a Bruker AXS D8 Discover diffractometer with a Cu K α radiation source and a Bruker LynxEye energy dispersive one-dimensional detector ( µ -XRD). The top layer of the orange paint was separated from the sample, and the diffractogram was acquired in the interval 5–60 ◦ (2 Θ ) with a step of 0.05 ◦ and 4 s per step. EVA software (with ICDD PDF X-ray patterns database) was used for the identification. 3. Results 3.1. Colour and Morphology To the naked eye, the medium and light tints of the orangey garments of the apostle bending over the cross and the one standing to the right of the composition exhibited an irregular surface with small spherical nodules protruding from within the paint layer and leaving an open crater whenever lost (Figure 1). Areas of the garments with this specific surface texture had a muted orange to ochre yellowish colour and absorbed the varnish coating that protects the painting, creating matt areas perceived as stains over the surface. The two cross-sections examined under the optical microscope showed a similar structure with a first underlayer of ochre colour with a thickness of 25–30 µ m, covered by a bright orange paint, of around the same thickness, with bright orange particles without UV fluorescence (Figure 2). Among these particles, it is possible to distinguish spherical particles and particles of irregular or undefined shapes (OM). The former have a diameter of up to about 15 µ m, while irregularly shaped particles can, in some cases, reach 40 µ m in their largest dimension. Some of the spherical particles show a brighter outer shell than the core (Figure 2). Examination of the top bright orange layer under the petrographic microscope reveals that some particles show intense green (Figure 3a) or blue (Figure 3b) interference colours. These interference colours, the particles’ orange colour, and the absence of UV fluorescence are compatible with the minerals realgar or pararealgar, both of natural origin [ 5 ]. However, the spherical shape of some of them is a clear indication of artificial arsenic sulphide particles obtained by sublimation [6]. Focussing on the upper orange bright paint layer, SEM-BSE images show a large number of spherical particles with widely varying sizes ranging from, at least, 3 to 15 µ m (Figure 4). Some of them are clearly crushed as a result of the grinding of the pigment (Figure 4a,b). In addition to these, particles with other shapes and also with a wide range of sizes are observed. The latter often reveal cracks of variable thickness. In a significant number of cases, the separation of the fragments resulting from the cracking seems to have occurred after the formation of the paint layer, due to the relative position that the various fragments from the same particle maintain (Figure 4b–d). Contrary to what could be expected from the observations with the petrographic microscope, namely, the interference colours, no marks were detected on the orange particles that could clearly be related to crystalline structures. An exception is a particle observed on the surface of sample #02, which appears to have a foliated structure (Figure 5a). The SEM-EDS results (see below) show that the orange particles have a high arsenic and sulphur content, as with the apparently amorphous phase seen in Figure 5c. Regardless of their shape, these particles are homogenous, and have well-defined borders and a similar average atomic number in the SEM-BSE images (Figure 4). In sample #02, some particles have brighter areas in the outer layer (Figures 5b and 6), but this should not imply a mean atomic number superior to that of the interior since, as suggested by the SEM-EDS results (see below), this may simply be an artefact of the particles morphology. Moreover, some
Heritage 2022,52649 of these loose sample particles have a heterogeneous structure coated on the outside by a homogeneous and compact material without crystalline forms, which in many particles appears to be in a degraded state (Figure 5a). In the upper part of the stratigraphic sections, it is possible to observe spherical particles with the rim (outer layer) degraded on the side facing the painting surface (Figure 6). They appear to have a narrow surface rim with a higher atomic number than the core, but the As and S concentration profiles obtained for some particles show no variation inside them (see below). Heritage2022,5,FORPEERREVIEW 4 Regardlessoftheirshape,theseparticlesarehomogenous,andhavewell‐definedborders andasimilaraverageatomicnumberintheSEM‐BSEimages(Figure4).Insample#02, someparticleshavebrighterareasintheouterlayer(Figures5band6),butthisshould notimplyameanatomicnumbersuperiortothatoftheinteriorsince,assuggestedbythe SEM‐EDSresults(seebelow),thismaysimplybeanartefactoftheparticlesmorphology. Moreover,someoftheseloosesampleparticleshaveaheterogeneousstructurecoatedon theoutsidebyahomogeneousandcompactmaterialwithoutcrystallineforms,whichin manyparticlesappearstobeinadegradedstate(Figure5a).Intheupperpartofthestrat‐ igraphicsections,itispossibletoobservesphericalparticleswiththerim(outerlayer) degradedonthesidefacingthepaintingsurface(Figure6).Theyappeartohaveanarrow surfacerimwithahigheratomicnumberthanthecore,buttheAsandSconcentration profilesobtainedforsomeparticlesshownovariationinsidethem(seebelow). Finally,SEM‐BSEimagesofthetoporangelayerexposeafewirregularlyshaped particlesdisplayingaloweratomicnumberthantheaforementionedparticles(Figure 4a,e). Figure2.Cross‐sectionofsample#01;OMunderincidentlight(a)andUVradiation(b). Figure 2. Cross-section of sample #01; OM under incident light (a) and UV radiation (b). Heritage2022,5,FORPEERREVIEW 5 Figure3.Exampleofparticleswithorangecolourshowinginterferencecoloursinthepetrographic microscope. Figure4.SEM‐BSEimagesfromcross‐sectionsofsample#01(a)andsample#02(b–e).Thegreen arrowshighlightparticlesfromtheorangeupperlayerexhibitingalowermeanatomicnumber. Figure 3. Example of particles with orange colour showing interference colours in the petrographic microscope.
Heritage 2022,52650 Heritage2022,5,FORPEERREVIEW 5 Figure3.Exampleofparticleswithorangecolourshowinginterferencecoloursinthepetrographic microscope. Figure4.SEM‐BSEimagesfromcross‐sectionsofsample#01(a)andsample#02(b–e).Thegreen arrowshighlightparticlesfromtheorangeupperlayerexhibitingalowermeanatomicnumber. Figure 4. SEM-BSE images from cross-sections of sample #01 ( a ) and sample #02 ( b – e ). The green arrows highlight particles from the orange upper layer exhibiting a lower mean atomic number. Heritage2022,5,FORPEERREVIEW 5 Figure3.Exampleofparticleswithorangecolourshowinginterferencecoloursinthepetrographic microscope. Figure4.SEM‐BSEimagesfromcross‐sectionsofsample#01(a)andsample#02(b–e).Thegreen arrowshighlightparticlesfromtheorangeupperlayerexhibitingalowermeanatomicnumber. Figure 5. SEM-EDS images of sample #02 showing degraded particles ( a , b ), a crystalline particle ((a), green arrow) and an amorphous phase (c). Finally, SEM-BSE images of the top orange layer expose a few irregularly shaped particles displaying a lower atomic number than the aforementioned particles (Figure 4a,e). 3.2. Elemental Composition The SEM-EDS results show that the bright orange upper paint layer is mainly composed of arsenic-rich components. It is over an underpaint formed essentially by compounds of Fe, Si, Al, Ca, and Pb, that is, a clay earth pigment with some ochre-iron-rich particles and, probably, some white lead. A total of 115 point analyses were performed by SEM-EDS in the orange layer (the vast majority) and in the ochre layer of the two samples, and in general, the elements As, S, C,
Heritage 2022,52651 and O correspond to almost all the detectable and quantified elements (Figure 7). In these points, the molar ratio As:S is, on average, 59:41, with an excess of arsenic in relation to the theoretical ratio of realgar or pararealgar (50:50), the ratio actually determined in realgar samples (48:52) [ 7 ], the theoretical ratio of orpiment (40:60), or for other arsenic sulphide compounds. The repeated analyses at different points of some particles, in general, did not show significant random errors, but no results were obtained to allow for an estimation of the systematic errors. However, the results obtained for a particle (Figure 4d) of pararealgar (identified by Raman spectroscopy) suggest that the actual ratio is lower than the values obtained by SEM-EDS (see below). This problem can be explained by the semi-quantitative nature of the SEM-EDS results. In this situation, the corrected molar ratio between As and S will in fact be around 47:53. Heritage2022,5,FORPEERREVIEW 6 Figure5.SEM‐EDSimagesofsample#02showingdegradedparticles(a,b),acrystallineparticle ((a),greenarrow)andanamorphousphase(c). Figure6.Sphericalparticles,insample#01,withtherimfacingtowardsthesurfaceofthesample showingsignsofcorrosion(notedwithgreenarrows). 3.2.ElementalComposition TheSEM‐EDSresultsshowthatthebrightorangeupperpaintlayerismainlycom‐ posedofarsenic‐richcomponents.Itisoveranunderpaintformedessentiallybycom‐ poundsofFe,Si,Al,Ca,andPb,thatis,aclayearthpigmentwithsomeochre‐iron‐rich particlesand,probably,somewhitelead. Atotalof115pointanalyseswereperformedbySEM‐EDSintheorangelayer(the vastmajority)andintheochrelayerofthetwosamples,andingeneral,theelementsAs, S,C,andOcorrespondtoalmostallthedetectableandquantifiedelements(Figure7).In thesepoints,themolarratioAs:Sis,onaverage,59:41,withanexcessofarsenicinrelation tothetheoreticalratioofrealgarorpararealgar(50:50),theratioactuallydeterminedin realgarsamples(48:52)[7],thetheoreticalratiooforpiment(40:60),orforotherarsenic sulphidecompounds.Therepeatedanalysesatdifferentpointsofsomeparticles,ingen‐ eral,didnotshowsignificantrandomerrors,butnoresultswereobtainedtoallowforan estimationofthesystematicerrors.However,theresultsobtainedforaparticle(Figure 4d)ofpararealgar(identifiedbyRamanspectroscopy)suggestthattheactualratiois lowerthanthevaluesobtainedbySEM‐EDS(seebelow).Thisproblemcanbeexplained bythesemi‐quantitativenatureoftheSEM‐EDSresults.Inthissituation,thecorrected molarratiobetweenAsandSwillinfactbearound47:53. Figure 6. Spherical particles, in sample #01, with the rim facing towards the surface of the sample showing signs of corrosion (noted with green arrows). For the spherical particles, the average value of the molar fraction of As is slightly lower than in the other particles and, above all, has a much smaller variation. The average ratio obtained for these particles (54:46) is similar to that of a glass used as a pigment in an 18th century sculpture, prepared from pararealgar [ 8 ], but this does not appear to be a common value. However, if we consider that the actual As values are lower than the values obtained by SEM-EDS (as described above), to those spherical particles corresponds a corrected As:S molar fraction of about 42:58, which is already comparable to usual glasses [9]. In the spherical particles, no significant difference was found between the inner and outer surface (interior and the exterior surface), regardless of its state. The situation was confirmed both through point analysis and concentration profiles (Figure 8).
Heritage 2022,52652 Heritage2022,5,FORPEERREVIEW 7 Figure7.SEMback‐scatteredelectronimageofthedetailsofsample#02(a);SEM‐EDSmapsofAs (b),Al(c),andS(d);andSEM‐EDSspectra(e,f)ofpointsP1andP2locatedin(a). Forthesphericalparticles,theaveragevalueofthemolarfractionofAsisslightly lowerthanintheotherparticlesand,aboveall,hasamuchsmallervariation.Theaverage ratioobtainedfortheseparticles(54:46)issimilartothatofaglassusedasapigmentin an18thcenturysculpture,preparedfrompararealgar[8],butthisdoesnotappeartobea commonvalue.However,ifweconsiderthattheactualAsvaluesarelowerthantheval‐ uesobtainedbySEM‐EDS(asdescribedabove),tothosesphericalparticlescorrespondsa correctedAs:Smolarfractionofabout42:58,whichisalreadycomparabletousualglasses [9]. Inthesphericalparticles,nosignificantdifferencewasfoundbetweentheinnerand outersurface(interiorandtheexteriorsurface),regardlessofitsstate.Thesituationwas confirmedboththroughpointanalysisandconcentrationprofiles(Figure8). Figure 7. SEM back-scattered electron image of the details of sample #02 ( a ); SEM-EDS maps of As (b), Al (c), and S (d); and SEM-EDS spectra (e,f) of points P1 and P2 located in (a). Heritage2022,5,FORPEERREVIEW 8 Figure8.Linescanofsphericalparticlesfromsample#01. Amongthenon‐sphericalparticles,thereisasignificantnumberwithacomposition comparabletothatofthesphericalparticles,butthereisalsoasignificantnumberwitha verydifferentcomposition,inparticularwithareducedproportionofsulphur,anelement thatisnotevendetectableinmanyofthesecases.Oneofthecasesistheirregularand fracturedparticleatthepaintsurface,withalowaverageatomicnumber,whichtheSEM‐ EDSresultssuggestcouldbearsenolite(As2O3)(Figure7e). Forthecrystalline‐flake‐likematerialobservedinFigure5a,themolarratioAs:Sis 51:49accordingtothetwoanalysesperformed.Inreality,theratioisabout39:61ifwe takeintoaccountthecorrectionmentionedabove,avaluethat,togetherwiththelamellar structureoftheparticle,suggeststhatitisorpiment. Where,besidesAs,S,CandO,otherelementsweredetectedwithsignificantconcen‐ tration,theseweregenerallyCa,Si,andAl,andlessfrequentlyK,P,Na,Fe,andPb.None ofthe115analysesdetectedelementsthatappearassociatedwithsomespecificarsenic minerals,suchasantimonyorselenium[10]. TheanalysesinwhichsignificantconcentrationsofPbweredetectedcorrespondto pointsintheunderlyingochrelayer,mainlyfromareaswithsmallparticles.Inthese points,regardlessofwhethertheycorrespondtotheseparticlesortothematrixsurround‐ ingthem,asignificantcorrelationwasfoundbetweenthePbandtheAsconcentration (Figure9),whichsuggeststhatthetwoelementsarecombinedinthesamecompound, namely,aleadarsenate. Figure 8. Line scan of spherical particles from sample #01. Among the non-spherical particles, there is a significant number with a composition comparable to that of the spherical particles, but there is also a significant number with a very different composition, in particular with a reduced proportion of sulphur, an element that is not even detectable in many of these cases. One of the cases is the irregular and fractured particle at the paint surface, with a low average atomic number, which the SEM-EDS results suggest could be arsenolite (As2O3) (Figure 7e).
Heritage 2022,52653 For the crystalline-flake-like material observed in Figure 5a, the molar ratio As:S is 51:49 according to the two analyses performed. In reality, the ratio is about 39:61 if we take into account the correction mentioned above, a value that, together with the lamellar structure of the particle, suggests that it is orpiment. Where, besides As, S, C and O, other elements were detected with significant concentration, these were generally Ca, Si, and Al, and less frequently K, P, Na, Fe, and Pb. None of the 115 analyses detected elements that appear associated with some specific arsenic minerals, such as antimony or selenium [10]. The analyses in which significant concentrations of Pb were detected correspond to points in the underlying ochre layer, mainly from areas with small particles. In these points, regardless of whether they correspond to these particles or to the matrix surrounding them, a significant correlation was found between the Pb and the As concentration (Figure 9), which suggests that the two elements are combined in the same compound, namely, a lead arsenate. Heritage2022,5,FORPEERREVIEW 9 Figure9.Relationshipbetweenatomicconcentrationofleadandarsenicatthepointsanalysedby SEM‐EDSintheunderlyingochrelayer(n=12). 3.3.StructuralData IntheX‐raydiffractionspectraobtainedforthepaintsurface,onlyanhydriteand quartzaredetectable,andnopeaksduetoarseniccompoundsareevident,whichisan indicationoftheessentiallyamorphousnatureofthesecompounds.Thisresultisinagree‐ mentwiththegeneralityofthenumerousRamanspectraobtained,whereaverybroad bandisobservedatabout345cm–1,characteristicofarsenicsulphidesinaglassystate[11]. Thisbandatabout345cm–1isthedominantbandinmostoftheRamanspectra(Fig‐ ure10),inwhich,insomeofthem,wealsoobserveweakandequallybroadbandsat about190,221,236,277,298,318,350,and360cm–1.Thespectra(a)and(c)inFigure10are examples,respectively,ofthecasesoflowestandhighestvisibilityoftheseweakbands, whichoftenappearinglassyarsenicsulphides[11]andcorrespondtocrystallineAs4S4 (~190,~221,~236,~277,~350and~360cm–1)andAs2S3(~297and~318cm–1)nanophases thatformintheglassymatrix[12].Thesespectrawereobtainedbothforsphericalparticles andforfragmentswithothershapes.Inthecaseofthespheresforwhichspectrawere obtainedfortheinteriorandforthesurfacering,nosignificantdifferencesweredetected betweenthesezones(Figure10a,b),except,insomecases,aslightdecreaseinamorphous featuresinside.Accordingtotheliterature,someofthementionedbands,namely,those appearingatabout190and360cm–1,haveanintensitythatincreaseswiththemolarfrac‐ tionofAs[12]. Figure 9. Relationship between atomic concentration of lead and arsenic at the points analysed by SEM-EDS in the underlying ochre layer (n= 12). 3.3. Structural Data In the X-ray diffraction spectra obtained for the paint surface, only anhydrite and quartz are detectable, and no peaks due to arsenic compounds are evident, which is an indication of the essentially amorphous nature of these compounds. This result is in agreement with the generality of the numerous Raman spectra obtained, where a very broad band is observed at about 345 cm –1 , characteristic of arsenic sulphides in a glassy state [11]. This band at about 345 cm –1 is the dominant band in most of the Raman spectra (Figure 10), in which, in some of them, we also observe weak and equally broad bands at about 190, 221, 236, 277, 298, 318, 350, and 360 cm –1 . The spectra (a) and (c) in Figure 10 are examples, respectively, of the cases of lowest and highest visibility of these weak bands, which often appear in glassy arsenic sulphides [ 11 ] and correspond to crystalline As 4 S 4 (~190, ~221, ~236, ~277, ~350 and ~360 cm –1 ) and As 2 S 3 (~297 and ~318 cm –1 ) nanophases that form in the glassy matrix [ 12 ]. These spectra were obtained both for spherical particles and for fragments with other shapes. In the case of the spheres for which spectra were obtained for the interior and for the surface ring, no significant differences were detected between these zones (Figure 10a,b), except, in some cases, a slight decrease in amorphous
Heritage 2022,52654 features inside. According to the literature, some of the mentioned bands, namely, those appearing at about 190 and 360 cm –1 , have an intensity that increases with the molar fraction of As [12]. Heritage2022,5,FORPEERREVIEW 10 Figure10.Ramanspectrarestrictedtothe100–500cm−1regionoftheacquiredextendedrange:(a) centreofthesphericalparticle,ofsample#01,visibleintheupper‐right‐handcornerofFigure4a (analysisAs6i);(b)surfaceringofthesameparticle(analysisAs6e);(c)irregularlyshapedparticle fromsample#02obtainedbyfragmentation(analysisAs15);(d)particleofsample#02,visiblein Figure4d(analysisAs11);(e)centreofasphericalparticleofsample#02(analysisAs13i).Spectra offsetforclarity. Themainexceptiontothespectrawithamorphouscharacteristicsistheoneobtained foralarge,non‐sphericalparticleonthesurfaceofthestratum,exposedtotheoutsideand withalargenumberofcracks(Figure4d):averyintenseandthinbandat234cm–1and others,alsowelldefined,at142,154,174,189,200,276,326,349and366cm–1,correspond tothespectrumofthepararealgar(Figure10d)[8,13].Suchparticles,whichareaminority, explaintheobservationsmadewiththepetrographicmicroscope.Forthementionedpar‐ ticleofpararealgar,accordingtotheSEM‐EDSanalyses,themolarratiobetweenAsand Sis62:38(averageoffourpointanalyses),whichcorrespondstoanAsvalueclearlyhigher thanexpectedforthismineral(50:50).Ifweconsiderthatthisisarelativesystematicerror, thismeansthattheotherratiosobtainedbySEM‐EDSarealsohigherthantherealvalues andshouldbecorrectedaccordingly(theratioiscorrectedbyfirstconsideringthataquo‐ tientof1.63correspondstoaquotientof1andthennormalisingthetwotermsofthe quotientsothattheirsumis100). AnotherRamanspectrumdifferentfrommostspectrawasobtainedforaspherical particleapparentlyequaltoothersthatgaverisetotypicalspectraofglassymaterials.In thiscase,themostintensebandsoforpiment,namely,at297,315and359cm–1[11](Figure 10e),aresuperimposedonthespectrumwithamorphouscharacteristics.Thissituationis Figure 10. Raman spectra restricted to the 100–500 cm −1 region of the acquired extended range: (a) centre of the spherical particle, of sample #01, visible in the upper-right-hand corner of Figure 4a (analysis As6i); ( b ) surface ring of the same particle (analysis As6e); ( c ) irregularly shaped particle from sample #02 obtained by fragmentation (analysis As15); ( d ) particle of sample #02, visible in Figure 4d (analysis As11); ( e ) centre of a spherical particle of sample #02 (analysis As13i). Spectra offset for clarity. The main exception to the spectra with amorphous characteristics is the one obtained for a large, non-spherical particle on the surface of the stratum, exposed to the outside and with a large number of cracks (Figure 4d): a very intense and thin band at 234 cm –1 and others, also well defined, at 142, 154, 174, 189, 200, 276, 326, 349 and 366 cm–1, correspond to the spectrum of the pararealgar (Figure 10d) [ 8 , 13 ]. Such particles, which are a minority, explain the observations made with the petrographic microscope. For the mentioned particle of pararealgar, according to the SEM-EDS analyses, the molar ratio between As and S is 62:38 (average of four point analyses), which corresponds to an As value clearly higher than expected for this mineral (50:50). If we consider that this is a relative systematic error, this means that the other ratios obtained by SEM-EDS are also higher than the real values and should be corrected accordingly (the ratio is corrected by first considering that a