Contribution to the identification of α-, β- and ε-copper phthalocyanine blue pigments in modern artists' paints by X-ray powder diffraction, attenuated total reflectance micro-fourier transform infrared spectroscopy and micro-Raman spectrosc

Contribution to the identification of α-, β- and ε-copper phthalocyanine blue pigments in modern artists' paints by X-ray powder diffraction, attenuated total reflectance micro-fourier transform infrared spectroscopy and micro-Raman spectroscopy

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  Contribution to the identi fi cation of  a -, b - and « -copper phthalocyanine blue pigments inmodern artists ’  paints by X-ray powderdiffraction, attenuated total re fl ectancemicro-fourier transform infrared spectroscopyand micro-Raman spectroscopy † C. Defeyt, a * P. Vandenabeele, b B. Gilbert, c J. Van Pevenage, d R. Cloots e and D. Strivay a Sincetheendofthe20thcentury,the a -, b -and « -copperphthalocyanine(CuPc)bluepigmentsarewidelyusedinmodernartists ’ paints.Theidenti fi cationoftheCuPccrystallinestructurecanprovideusefultechnicalandchronologicalinformationforthestudyof works of art. Although when a CuPc blue pigment is identi fi ed, its crystalline structure often remains unspeci fi ed despite theinterest for conservation science. In this study, X-ray powder diffraction, attenuated total re fl ectance micro-Fourier transforminfrared spectroscopy and micro-Raman spectroscopy analyses have been carried out on 15 dry pigment samples of CuPc andacrylic, vinylic, alkyd, arabic gum and oil-based artists ’  paints. By using the polymorphic markers underlined for dry pigments,the CuPc crystalline structure has been successfully identi fi ed for most of the analysed artists ’  paints. However, according totheanalyticaltechniqueusedandthe investigatedpaintsample,the obtainedresults largelydiffer.Copyright© 2012 John Wiley& Sons, Ltd. Supporting information may be found in the online version of this article. Keywords:  modern artists ’  paints; copper phthalocyanine blue pigments; conservation science; nondestructive methods; crystallinestructure Introduction Nowadays, copper phthalocyanine (CuPc), blue and green pig-ments, are among the most important modern synthetic organicpigments used in artists ’  paint formulations. [1] CuPc pigmentshave beenadopted by manyfamous20th century artists,includingR. Lichtenstein, P. Delvaux, W. Kandinsky, Y. Klein, S. Francis, E. KellyandB.Newman. [2 – 8] The fi rstCuPcpigment(C 32 H 16 CuN 8 ),patentedin 1928, is a blue organometallic polymorphic compound belong-ing to the class of polycyclic pigments. [9,10] This study focuses onthe CuPc polymorphs commonly used by artists ’  colour makersand corresponding to the stabilised and unstabilised  a -,  b - and e -polymorphic modi fi cations. The Colour Index generic name usedfor the CuPc blue pigments is Pigment Blue 15 (PB15). Morespeci fi cally, PB15:0 is used for the unstabilised  a -CuPc, PB15:1 forthe noncrystallising  a -CuPc, PB15:3 for the unstabilised  b -CuPc,PB15:4 for the non fl occulating  b -CuPc and PB15:6 for the unstabi-lised  e -CuPc. [11] PB15:2, corresponding to the noncrystallising and non fl occulat-ing a -CuPc, is not investigated in this study because of its marginaluse in artists ’  paints (no mention of PB15:2 among the 35 colourcharts consulted). The  a -,  b - and  e - CuPcs are characterised bydifferent arrangements in the crystal lattice and differ in stability,solubility and hue. [12] CuPc blue pigments are partially soluble inaromatic hydrocarbons and one of them, PB15:0, can fade in pres-ence of these solvents. [13,14] Although the use of aromatic solventsas part of conservation treatments of easel paintings is relativelycommon. [15] In addition, the  a -CuPc pigment was introduced on * Correspondence to: C. Defeyt, Centre Européen d  ’    Archéométrie and Institut dePhysique Nucléaire, Atomique et de Spectroscopie, Université de Liège, B-4000Liège, Belgium. E-mail: † This article is part of the Journal of Raman Spectroscopy special issue entitled  “   Raman spectroscopy in art and archeology  ”    edited by Juan Manuel Madariagaand Danilo Bersani. a  Centre Européen d  ’    Archéométrie and Institut de Physique Nucléaire, Atomiqueet de Spectroscopie, Université de Liège, B-4000 Liège, Belgium b  Department of Archaeology, Ghent University, B-9000 Ghent, Belgium c  Chimie Analytique et Electrochimie, Université de Liège, B-4000 Liège, Belgium d  Raman spectroscopy research group, Ghent University, B-9000 Ghent, Belgium e  Laboratoire de Chimie Inorganique Structurale, Université de Liège, B-4000Liège, Belgium  J. Raman Spectrosc.  2012 ,  43 , 1772 – 1780 Copyright © 2012 John Wiley & Sons, Ltd. Research article Received: 21 December 2011 Revised: 27 April 2012 Accepted: 7 May 2012 Published online in Wiley Online Library: 15 July 2012 ( DOI 10.1002/jrs.4125 1 7 7 2    the European market in 1935, the  fi rst  b -CuPc pigment waspatented in 1953 and  e -CuPc one was not available before the1970s. [9,11] Of course, these chronological details, supported bypatent literature, provide objective criteria for dating and theauthenticating artworks.In conservation science, it is important to be able to identify thecrystalline structure of the CuPc pigment detected in paintings. Onthe one hand, the identi fi cation of the polymorphic modi fi cationprovides chronological precisions, which could be used for exam-ple, to highlight anachronisms. On the other hand, in the case of restoration campaigns, the conservation treatments could be moreadapted according to the CuPc pigment contained in paint layers.However, most of times when CuPc blue pigments are identi fi edin artworks, mainly by Fourier transform infrared (FTIR) spectros-copy and Raman spectroscopy, the crystalline structure remainsunspeci fi ed. [3,8,16] On the basis of conservation science literature,the crystalline structure is mainly speci fi ed when destructive meth-ods, such as pyrolysis – gas chromatography/mass spectrometry,have been performed. [2,5] The polymorphism of phthalocyanine compounds has beenwidelyinvestigatedwithaspecialinterestinthe a – b  transformation.Heutz  et al  . [17] studied the  a – b  transition of metal-freephthalocyanines (H 2 Pcs) by comparing the morphological,spectroscopic and structural properties before and after anneal-ing. Also, Gaffo  et al  . [18] observed the  a – b  transformation of zincphthalocyanines(ZnPcs)byusingvibrationalspectroscopies.Aboutthe polymorphism of the actual CuPc blue pigments, Raman datahave been reported by Lutzenberger [7,19] and Scherrer  et al  . [19] The X-ray powder diffraction (XRD) data obtained for dry pigmentsofCuPchave beenreportedby Lomax. [20] Lomax  et al  . [21] also com-pared FTIR spectroscopy and direct temperature-resolved massspectrometry results to investigate the CuPc polymorphs. Experimental This study is carried on 15 dry pigment samples of CuPc blue and 9commercial artists ’  paints for which CuPc blue isused inthe formu-lation.DifferentbindingmediaandCuPcconcentrationshavebeeninvestigated. Dry pigments and artists ’  paints were supplied by thewell-known artists ’  paint manufacturers Winsor & Newton, Lefranc& Bourgeois, Royal Talens, BLOCKX, M. GRAHAM & Co. and by thepigment manufacturers Kremer and Europhtal. Details on thestudied samples are given in Tables 1 and 2. All the samples havebeen analysed using XRD, attenuated total re fl ectance micro-Fourier transform infrared spectroscopy ( m -FTIR-ATR) and micro-Raman spectroscopy ( m -RS). Dry pigments and artists ’  paints weremeasured under the same measurement conditions. Table 1.  Supplier, commercial reference, Colour Index name and polymorphic modi fi cation of dry pigments of CuPc samples studied in this work Supplier Commercial reference Colour Index name Date Polymorphic modi fi cationEurophtal Europhtal Blue 2103 PB15:0 2009 Unstabilised  a -formKremer Heliogen Blue 23050 PB15:1 2009 Noncrystallising  a -formBASF Heliogen Blue L6920 PB15:1 2008 Noncrystallising  a -formWinsor & Newton Winsor blue L2114M PB15:1 2010 Noncrystallising  a -formEurophtal Europhtal Blue 2237 PB15:1 2009 Noncrystallising  a -formAldrich product Copper (II) phthalocyanine 546682 PB15:3 2009 Unstabilised  b -formKremer Heliogen Blue 23060 PB15:3 2009 Unstabilised  b -formCiba Irgalite Blue PG PB15:3 2008 Unstabilised  b -formWinsor & Newton Winsor Blue L21707 PB15:3 2010 Unstabilised  b -formEurophtal Europhtal Blue 2278 PB15:3 2009 Unstabilised  b -formRoyal Talens Phthalo Blue (green shade) PB15:4 2002 Non fl occulating  b -formEurophtal Europhtal Blue 2090 PB15:4 2009 Non fl occulating  b -formKremer Heliogen Blue 23070 PB15:6 2009 Unstabilised  e -formKremer Heliogen Blue 23070 PB15:6 2002 Unstabilised  e -formRoyal Talens Phthalo Blue (red shade) PB15:6 2002 Unstabilised  e -form Table 2.  Overview of the artists ’  paints investigated in this work Sample code a Supplier Declared pigment Date Binding mediaWinton 516 Winsor & Newton PB15:1 2009 Modi fi ed oilGrif  fi n 514 Winsor & Newton PB15:1 2009 AlkydRembrandt 576 Royal Talens PB15:4 2009 Linseed oilRembrandt 583 Royal Talens PB15:6 2009 Linseed oilLiquitex 316 Lefranc & Bourgeois PB15:3 2010 AcrylicLiquitex 314 Lefranc & Bourgeois PB15:6 2011 AcrylicGraham 141 M. GRAHAM & Co. PB15:0 2010 Arabic gumBlockx 451 BLOCKX PB15:1 2009 Poppy oilFlashe 036 Lefranc & Bourgeois PB15:0 2009 VinylicThe CuPc pigments and the binding media reported are based on the composition given by the artists ’  paint suppliers. a The name indicates the sample colour range and the number indicates the colour reference given by the supplier. Identi fi cation of   a -,  b - and  e -CuPc in artists ’  paints  J. Raman Spectrosc.  2012 ,  43 , 1772 – 1780 Copyright © 2012 John Wiley & Sons, Ltd. 1 7 7  3    For the characterisation of   a -,  b - and  e -CuPcs by nondestructivemethods, we start to bestow our attention on the results obtainedfor dry pigments. For PB15:1, PB15:3, PB15:4 and PB15:6 samples,the XRD, FTIR and Raman measurements have been averaged tohighlight the signi fi cant similarities and differences between theinvestigated pigments. For the PB15:0 type, the reported resultscorrespond to the measurements performed on the only PB15:0sample. By this way, we attempted to  fi nd polymorphic markersfor each analytical technique used. The polymorphic markersunderlined for dry pigments have been used to identify thepolymorphic modi fi cation of CuPc blue pigment contained in nineartists ’  paints. Analyses were carried on dried  fi lms, which havebeen applied on glass slide.XRD measurements were performed with a Bruker D8 Focusdiffractometer equipped with a Co tube. For all the samples, a steptime of 1s and an angular step of 0.02  was used; 2 θ  rangedbetween 3  and 70  . The IR spectra of the samples were recordedin the 4000- to 650-cm  1 range, but only the 1600- to 700-cm  1 characteristicregionwillbediscussed.ThespectrometerisaNicoletNexus FTIR provided with a Continuum microscope and an ATR Gecrystal. The spectral resolution is 4cm  1 , and the spectra wererecorded with the automatic suppression of CO 2  and H 2 O vapourbands. Raman analyses were performed with a Bruker Senterraspectrometer by using a diode laser with a wavelength of 785nm.The laser power was reduced at 10% (~4.22mW at the sample).For dry pigments and artists ’  paints,  fi ve spectra from differentpigment grains were recorded in the spectral range of 80 – 2660cm  1 by using a 50   objective. Raman spectra wererecorded with an accumulation time of 30s. All the Raman datapresented in this article are derived from the baseline-correctedspectra. The baseline correction was performed on the averagedspectra by working under Grams. Results and discussion X-ray powder diffraction Dry pigments As expected, the diffractogram patterns obtained for the CuPcpolymorphs allow to discriminate the  a -,  b - and  e -CuPc crystallinestructure (Fig. 1). The crystalline structure of the investigated CuPcdry pigment has been identi fi ed by comparing our own data withXRD data reported in literature. [22 – 24] The crystalline structuresidenti fi edfordrypigmentsareintotalaccordancewiththesupplierdeclarations. For the PB15:1, PB15:3, PB15:4 and PB15:6 samples,the average of the experimental  d   values recorded for every peak has been calculated (Table 3). If we compare the averaged  d   valuesobtained for  a -,  b - and  e -CuPc samples, we can see that most of them differ signi fi cantly. Moreover, several peaks are present orabsent according the crystalline structure. For example, one of thethree main peaks ( d  =9.6) observed for the  b - and  e -CuPc samplesis not observed for the a -CuPc samples. Heutz  et al  . [17] investigatedthediffractionpeaksduetothere fl ectionplanesofthe a phaseandthe  b  phase to identify the  a – b  transformation of H 2 Pc thin  fi lmsafter annealing.However, diffractograms of CuPc samples having the samecrystalline structure do not differ much from each others. Theaveraged  d   values and diffractogram patterns are very close andcannot be considered as reliable markers to discriminate PB15:0from PB15:1 and PB15:3 from PB15:4. The similarity of the  d  spacings is con fi rmed by the XRD results reported by Lomax. [20] Figure 1.  Examples of diffractograms obtained for dry pigments of PB15:0, PB15:3 and PB15:6. Table 3.  Average of experimental interplanar distances  d  (hkl) /Årecorded for dry pigments of PB15:0, PB15:1, PB15:3, PB15:4 andPB15:6 d  (hkl) /ÅPB15:0 PB15:1 PB15:3 PB15:4 PB15:6 a -CuPc  a -CuPc  b -CuPc  b -CuPc  e -CuPc13.02 13.19  — —  13.3512.07 12.24 12.62 12.64  —— — — —  11.67 — —  9.68 9.68 9.658.84 8.91 8.41 8.43  —— —  7.10 7.10 7.75 — —  6.30 6.31 6.235.69 5.71 5.75 5.77 5.725.48 5.49  — —  5.27 — —  4.90 4.90 5.07 — —  4.79 4.81 4.84 — — — —  4.34 — —  4.16 4.16 4.17 — —  4.00 4.00 4.05 — —  3.90 3.90  — 3.71 3.71 3.75 3.75 3.773.56 3.56  — — — 3.35 3.35 3.41 3.41 3.293.24 3.24 3.19 3.19 3.142.93 2.95 2.94 2.94 2.95 C. Defeyt  et al  .  Copyright © 2012 John Wiley & Sons, Ltd.  J. Raman Spectrosc.  2012 ,  43 , 1772 – 1780 1 7 7 4   Nevertheless, it is important to note that compared with theunstabilised a -and b -CuPc,theintensitiesrecordeddecreasesignif-icantly for samples corresponding to the stabilised  a - and  b -CuPcsamples.  Artists ’    paints Concerning the diffractograms recorded for the commercial artists ’ paints, several peaks characteristic of the CuPc blue can beobserved. These peaks are reported in Table 4. Despite the rela-tively low concentration of CuPc pigments in paint systems, dueto their high tinting strength, phthalo blue was detected in everyinvestigated sample. The number of CuPc diffraction peaksobserved varies from 2 to 10 depending on the sample. Obviously,the XRD measurements have also highlighted the presence of some other compounds, as  fi llers, extenders and white inorganicpigments. In the cases of the vinylic and oil-based paints, veryfew re fl ections can be attributed to the phthalo blue. On thecontrary, the blue pigment is clearly visible in the diffractogramsobtained for the alkyd, acrylic and gum arabic paints samples.Thesedifferencescouldbeexplainedbythenatureandthepropor-tion of the other compounds used in the paint formulations. [25] Experimental  d   values and position peaks recorded for artists ’ paintsandfordrypigmentsareverysimilar.The d  valuesattributedto the phthaloblue present have beenisolatedandcomparedwith d   values recorded for the dry pigments to identify its crystallinestructure. Most of the diffraction peaks observed for the drypigment samples of   a -CuPc appear in the diffractogram obtainedfor the Graham 141 sample. Concerning the Winton 516, Grif  fi n514 and Blockx 451 samples, the  d   values recorded for diffractionpeaks induced by CuPc are also close to the  d   values recorded forthe dry pigments of   a -CuPc. This is why, for these samples, wecanconcludethattheCuPcbluepigmentusedismainlycomposedof   a -form crystallites.For the Liquitex 316 sample, the recorded  d   values are unambig-uously attributed to a  b -CuPc pigment. Most of the diffractionpeaks observed for the dry pigment samples of  b -CuPc are presentin the Liquitex 316 diffractogram. The diffractograms obtained forthe Flashe 036 and Rembrandt 576 samples suggest a lowconcentrationofCuPc blueinthemixture, bothofthemcontaininganimportantproportionof  fi llers(PW18forFlashe 036 andPW5forRembrandt 576). The identi fi cation of a  b -CuPc pigment is basedon the  d   values and positions recorded for two small peaks,corresponding to the two main peaks reported for the  b -CuPc drypigment samples. The  b -CuPc identi fi cation for the Flashe 036sample is not in agreement with the PB15:0 marked on the painttube. The recorded peaks at  d  =13.39 and  d  =9.67 for Liquitex314andtherecordedpeaksat d  =13.32and d  =9.80forRembrandt583 do not match with  b - or  a -CuPc. In fact, the experimental  d  valuesarisingfromthephthalobluearemuchclosertothe d  valuesrecorded for the dry pigments of   e -CuPc. IR spectroscopy Dry pigments Forthe15drypigmentsofCuPcanalysedusing m -FTIR-ATR,mostof the recorded band positions are highly similar to each other(Table 5). For the samples having the same crystalline structure,shifts are — if observed — less than 2cm  1 . However, as shown inFig. 2(a), three band positions differ slightly depending to the CuPccrystalline structure. One of the three shifts concerns the mostintense band observed for the CuPc polymorphs. On the basis of the FTIR results, we identi fi ed  fi ve polymorphic markers.Ononehand,ifwecomparetheaveragedbandpositions,threerelevant shifts are observed between the polymorphs. The bandpositions recorded at 877 – 878, 781 and 729 – 731cm  1 for the b -CuPc samples have been recorded at 864 – 866, 770 – 771 and719 – 721cm  1 for the  a -CuPc samples and at 880, 774 and726cm  1 for the  e -CuPc samples. These differences are inagreement with the literature because the IR bands in the700 – 800cm  1 range are often used to discriminate between the a  and the  b  modi fi cations. [26] In Gaffo  et al  ., [18] a shift observedin this region indicated the  a  to  b  phase transition of ZnPcthin  fi lms after annealing. On the other hand, the shoulderobserved at 1174 – 1175cm  1 and the small band present at1100 – 1101cm  1 for the  b -CuPc samples (PB15:3 and PB15:4) aremissing from the FTIR spectra obtained for the  a - and  e -CuPc Table 4.  Experimental interplanar distances  d  (hkl) /Å attributed to the CuPc blue pigment present in the investigated artists ’  paints d  (hkl) /ÅBlockx 451 Winton 516 Graham 141 Grif  fi n 514 Flashe 036 Liquitex 316 Rembrandt 576 Liquitex 314 Rembrandt 583 a -CuPc  a -CuPc  a -CuPc  a -CuPc  b -CuPc  b -CuPc  b -CuPc  e -CuPc  e -CuPc13.22 13.22  —  13.21  — — —  13.39 13.32 —  12.51 12.87 12.31 12.59 12.55 12.62 11.69 11.89 — —  11.96  — — — — — —— — —  9.03 9.61 9.62 9.71 9.67 9.808.94 8.94 8.81  — —  8.38  — — —— — — — —  7.07  — —  7.68 — —  5.47 5.46  — — — — —— — — — —  4.88  — — —— — — — —  4.78  — — —— —  3.70  — —  3.74  — — —— —  3.56  — — — — — —— —  3.34  — —  3.40  — — —— —  3.22  — —  3.18  — — —— — — — —  2.94  — — — Identi fi cation of   a -,  b - and  e -CuPc in artists ’  paints  J. Raman Spectrosc.  2012 ,  43 , 1772 – 1780 Copyright © 2012 John Wiley & Sons, Ltd. 1 7 7  5   samples. We found the same discriminating criteria for the IRspectra recorded with a Nicolet 380 FTIR spectrometer and SmartOrbit ATR accessory with a diamond crystal.  Artists ’    paints The number of absorption bands induced by the presence of CuPcbluevariesfrom3to14,dependingontheartists ’ paintsample.Forthe oil-based paints, many CuPc bands are hidden by the bindingmedium. Theseoverlapscon fi rm that inthe case ofpaints samples,FTIR analysis should be combined with another analytical tech-niquetobereliable. [21] TheabsorptionbandsattributedtotheCuPcpigment (Table 6) have been isolated to focus on the polymorphicmarkers as de fi ned by the study of the dry pigments.Among the bands assigned to the phthalo blue used in theWinton 516, Grif  fi n 514, Blockx 451 and Graham 141 formulations,the band position at 721 – 722cm  1 is much closer to the  a -CuPcband position than to the  b - or the  e -CuPc. The bands observedat 864 and 769cm  1 for the Graham 141 sample and the bandobserved at 770cm  1 for the Grif  fi n 514 (Fig. 2(b)) sample con fi rmthepresence ofan a -CuPc pigment.Four bandmarkers arepresentin the FTIR spectrum obtained for the Liquitex 316 sample (Fig. 2(b)). The corresponding band positions totally  fi t the respectiveband positions observed for the dry pigments of   b -CuPc. Indeed,the band positions at 879, 781 and 731cm  1 are due to a CuPcpigment showing a  b -crystalline structure. Moreover, the band at1102cm  1 , which is absent in the  a - and  e -CuPc FTIR spectra, butcommon for the  b -form, has been measured for the Liquitex 316sample.Concerning the Flashe 036 and the Rembrandt 576, the bandsobservedat781and729 – 731cm  1 suggestalsoa b -CuPcpigment.The FTIR results obtained for the Flashe 036 sample are in accor-dance with XRD results, which indicate a  b -CuPc instead of PB15:0(corresponding to the unstabilised a -CuPc). The positions recordedfor the Flashe 036 paint from 2009 differ only by 1 wave numberfrom the FTIR measurements performed by Learner on a Flashe036paintfrom1994. [27] IntheFTIRspectraobtainedfortheLiquitex314 (Fig. 2(b)) and Rembrandt 583 samples, two bands that areconsideredaspolymorphicmarkerscouldbeobserved.Thesebandpositions (771 – 774 and at 727cm  1 ) match with the presence of  e -CuPc. Raman spectroscopy Dry pigments Previously, Scherrer  et al  . [19] proposed different discriminationcriteria to distinguish the stabilised and unstabilised  a -,  b - and e -CuPc blue pigments. For the CuPc polymorphs distinction, theauthors used the shift of the strongest band corresponding to themain macrocycle stretching vibration (1518 – 1530cm  1 ). This shiftisnowexplainedbyaphenomenonofpeakshiftingwithincreasinglaser power. The peak shifting which seems to be a speci fi c featureof phthalocyanine pigments is reversible when a decreasing laserpower is used. [28] For the Raman study, a special care has beentaken to control the laser spot after measurements. In addition,thelaserpowerappliedformeasurementsdidnotcauseanyvisiblealteration on the analysed samples. Using a constant setting, theRaman spectra obtained for the pure pigment samples are verysimilar despite the different crystalline structure (Fig. 3(a)). In addi-tion, the shifts reported by Shaibat  et al  . [29] and Tackley  et al  . [30] for the  a -CuPc and  b -CuPc distinction turned out to be unreliablefor the CuPc samples investigated in this work.If we examine the averaged band positions calculated for drypigments(Table7),wecanseethattwobandpositionsdiffersignif-icantly according the type of CuPc pigment. Indeed, the bandsobserved at 124 – 125cm  1 for the PB15:3, PB15:4 and PB15:0 Table 5.  Average of the position of IR bands recorded between 1600and700cm  1 forthePB15:0,PB15:1,PB15:3,PB15:4andPB15:6samplesIR band position Wave number /cm  1 PB15:0 PB15:1 PB15:3 PB15:4 PB15:6 a -CuPc  a -CuPc  b -CuPc  b -CuPc  e -CuPc1613 1612 1610 1610 16091508 1508 1509 1507 15071465 1465 1465 1464 14651422 1421 1421 1420 14201334 1334 1334 1334 13331287 1289 1288 1288 1287 — —  1175 1174  — 1167 1167 1165 1165 11631121 1121 1120 1120 1119 — —  1101 1100  — 1091 1092 1090 1089 10941069 1070 1068 1068 1068900 900 901 900 901 864 866 878 877 880770 771 781 781 774 754 754 754 754 754 719 721 731 729 726 The values presented in bold indicate the polymorphic markershighlighted for dry pigments of CuPc. Figure 2.  FTIR spectra in the1600-to700-cm  1 regionobtainedfor(a)drypigmentsamplesofPB15:1,PB15:3andPB15:6;(b)Liquitex316,Liquitex314and Grif  fi n 514 artists ’  paint samples. C. Defeyt  et al  .  Copyright © 2012 John Wiley & Sons, Ltd.  J. Raman Spectrosc.  2012 ,  43 , 1772 – 1780 1 7 7  6  
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