FAR INFRARED SPECTROSCOPY OF PIGMENTS IN ART

meddelelser 2 · 2008 - artikel med peer review 33

Figure 1A: Sample AM1 from Abbazia del Monte, painted by artist Giuseppe Milani in 1773-74. Panoramic view of sampling area, Angel on red cloud under the cupola. Photo: xxxxxxxxxxxxxxxxxx

Far InFrared SpectroScopy oF pIgmentS In artelSebeth langholz KendIx, SIlvIa pratI, rocco mazzeo, edIth JoSeph, gIorgIa ScIutto

meddelelser 2 · 2008 - artikel med peer review44

The aim of this paper is to evaluate the per-formance of FIR spectroscopy for the detection of inorganic compound as a complementary technique to Raman and MIR spectroscopy. Recent studies reported the use of FIR spec-troscopy to distinguish between different micas [Mookherjee et al, 2004]. Early studies highlight the analytical potential of FIR for the investiga-tion of minerals in the 200-50 cm-1 region [Karr et al, 1969] other studies discuss the assignment of internal and external vibrations in the FIR re-gion for gaseous, liquid and solid phases [Finch et al, 1970]. However, only few articles about FIR spectroscopy applied in the field of cultural heritage have been published [Karr et al, 1969; Kendix et al, 2008] and for this reason the crea-tion of a FIR library of artistic and corrosion ma-terials is of the outermost importance. In this pa-per we present the preliminary results obtained in FIR transmission mode on a selection of eight standard pigments and on a real case study.

Figure 1 B: Sample AM1 from Abbazia del Monte, painted by artist Giuseppe Milani in 1773-74. Close-up of sampling area AM1. Photo: xxxxxxxxxxxxxx

INTRODUCTIONMIR spectroscopy has been widely used when investigating samples taken from works of art for instance paint samples [Casadio et al, 2001; Mazzeo et al, 2007], parchment [Bruni et al, 1999], glass [Bacci et al, 1996; Orlando et al, 2007], and corrosion products [Mazzeo et Joseph, 2007]. However, MIR spectroscopy becomes less useful when investigating inor-ganic components such as oxides or sulphides, which are inactive in the MIR region. So far Ra-man spectroscopy has been employed to over-come such limitations [Burgio et al, 2001; Bell et al, 1997] and by means of Raman spectroscopy it is possible to observe the heavy oxides and sulphides in paint samples. In the presence of organic compounds Raman spectra can be af-fected by fluorescence and several studies are being carried out to reduce this phenomenon by means of mathematical [Osticioli et al, 2006] or instrumental methods [Whitney et al, 2006].


Figure 2: FIR spectra of Bremer blue (top, blue spectrum) and Milori blue (bottom, red spectrum). In or-der to accommodate several spectra in one figure the intensity be-tween spectra have been altered.

Figure 3: FIR spectra of Viridian green (top, blue spectrum) and Bavarian green earth (bottom, red spectrum). In order to accommodate several spectra in one figure the inten-sity between spectra have been altered

MATERIALS AND METHODSSTANDARDSEight standard pigments are examined by FIR spectroscopy. All standard pigments are pur-chased from Kremer Pigmente [Kremer, 2008]. The pigments are; Bremer blue (an artificial azurite, 2CuCO3Cu(OH)2 K10180), Milori blue also known as Prussian blue (Fe(CN)6FeNH4,

K45200), Viridian green (Cr2O3 2H2O, K44250), Bavarian green earth (K11100) which contains Al-, K-, Mg-, Ca- and Fe-silicates, lead tin yel-low (type 1, Pb2SnO4, K10110), Barium yellow (BaCrO4, K43940), Cinnabar (HgS, K10625), and burnt Terra di Siena (K40450) which contains a mixture of hematite (Fe2O3) and amorphous silica (SiO2). Small amounts of MnO2, alumina and calcium carbonate are also present but in


small quantity (1-10%). The chemical formula of pigments as well as information about impuri-ties is available from the supplier’s homepage [Kremer, 2008]. Lime white (a mixture of CaO and CaCO3 in Italy know as Bianco di San Gio-vanni) purchased from Zecchi in Florence [Zec-chi, 2008] is also shown for sample comparison.

SAMPLING

Real samples have been collected from a mu-ral painting made by Giuseppe Milani in 1773-74, which covers the inside of the cupola in Abbazia del Monte (Cesena, Italy). All samples where investigated with micro-ATR-FTIR spec-troscopy in the MIR region for cross-section identification (non published data). Supple-mentary FIR spectroscopic examination was conducted on a single sample and is present-

Figure 5: FIR spectra of Cin-nabar (top, blue spectrum) and Terra di Siena (bottom, red spectrum. In order to accommodate several spectra in one figure the intensity between spectra have been alteredaltered

Figure 4: FIR spectra of Lead tin yellow, type 1(top, blue spectrum) and Barium yel-low (bottom, red spectrum). In order to accommodate several spectra in one fig-ure the intensity between spectra have been altered


ed here. Figure 1A show the general area of an Angel sitting on a red cloud from where the sample is taken. Figure 1B is a close-up of the sampling area. The sample AM1 is a pale red sample taken from a dotted area of a cloud.

FIR SPECTROSCOPYThe spectrometer applied for these examina-tions is a Thermo Nicolet Nexus 5700 using a solid-substrate™ beamsplitter operating in the FIR region of 600-90 cm-1. A Parker/Balston Self-Contained Gas Purge Generator is attached, purging the spectrometer with dry air for mini-mizing interference bands generated from ambient moisture. The detector is a DTGS Poly-ethylene detector used with a spectral resolu-tion of 4 cm-1. One spectrum is the sum of 64 scans, collecting time is approximately 1 min. All spectra have been baseline corrected and the background has been subtracted with one PE pellet inserted in the pathway. The spectrum is collected in transmission mode. Polyethylene (PE) pellet were prepared as reported else-where [Karr et al, 1969; Finch et al, 1970; Ken-dix et al, 2008] with an anvil die heated on a magnetic stirrer to a temperature above the melting point of PE (155°C); in our case the die is heated to 180°C in a fume cupboard. Ap-proximately 0.5-1.5 mg of sample is mixed with 70 mg of PE and pressed for 2 min at 3 tons to a transparent pellet in the heated die. Transpar-ent PE is an inactive material in the FIR region and gives rise to only two small bands in the FIR spectrum, which is subtracted when collecting the pure PE background spectrum. The pure PE

bands can be observed as a sharp band at 73 cm-1 (outside the region shown in the spectra) and as a broad but weak band at 550 cm-1.

RESULTS AND DISCUSSIONThe eight inorganic standard pigments shown here are preliminary results collected for our FIR library database. They have been chosen for their different structure and absorption in the IR spectroscopic region. A pigment like Milori blue has its strongest band in the mid IR region (at 2090 cm-1), whereas silicate pigments like Bavarian green earth and Terra di Siena both have their strongest band around 1100-1000 cm-1. Lead tin yellow and Cinnabar have their strongest bands below 600 and 400 cm-1 re-spectively, which makes Cinnabar difficult to observe in normal IR spectroscopy. Common for these eight standard pigments is that they all have specific and characteristic bands in the far infrared region, which makes FIR spectros-copy a viable method for their identification.

FIR spectra of two blue pigments, Bremer and Milori blue, are shown in Figure 2. Cu-O stretching vibrations from Bremer blue are observed at 493 and 404 cm-1, Cu-OH stretching at 457 and 345 cm-1, O-Cu-OH bending 305 and 239 cm-1, out-of-plane bending modes at 197 and 173 cm-1

and external vibrations at 251, 187, 140 and 129 cm-1 [Frost et al, 2002; White, 1974]. Milori blue have bands at 511, 496 and 243 cm-1. The band at 496 cm-1 is assigned to Fe-C and C-Fe-C in-plane bending or Fe-C≡N bending [Berrie, 1997].

FIR spectra of Viridian green and Bavar-ian green earth are shown in Figure 3. The FIR

Figure 6: Spectra of sam-ple AM1 (red spectrum, middle), Terra di Siena (blue spectrum, top) and Lime white (orange spec-trum, bottom). In order to accommodate several spectra in one figure the intensity between spec-tra have been altered


spectrum of Viridian green (Cr2O3 2H2O) have vibrations at 565, 552, 475 and 410 cm-1. Bands for Viridian green are expected at 555, 481, 420 and 402 cm-1 according to literature [Newman, 1997]. These vibration bands are not quite in agreement with the ones observed in our FIR spectrum. This could be due to pollution from anhydrous Chromium oxide, which have bands at 566(s), 443(w) and 416(w) cm-1 [Newman, 1997]. The FIR spectrum of another green stand-ard pigment Bavarian green earth has bands a 509, 458, 395, 371, 264 and 228 cm-1. In particu-lar the band at 395 cm-1 is thought to be due to a SiO4-SiO4 coupling in silicate [Farmer, 1974].

Figure 4 shows the FIR spectra of two yel-low pigments. Lead tin yellow vibration bands are observed at 567, 490, 449, 419, 359, 317, 289, 263, 246, 179 and 166 cm-1. Barium yel-low has bands at 417, 390, 373, 336, 187, 139 and 114 cm-1. The three bands at 417, 390 and 373 cm-1 have been assigned to the ν4- vibrations of the chromate [Ross, 1974].

The FIR spectrum of Cinnabar is observed in Figure 5. Strong bands are observed at 345, 282, 126 and 112 cm-1. Three of the four strong bands (345, 282, 126 cm-1) are in accordance with lit-erature [Mioč et al, 2004; Moenke, 1974]. Many weak bands are observed between 600-400 and 250-150 cm-1. These weak bands probably originate from an impurity in the standard or they could be a mixture of combination bands and lattice vibrations. However, these bands have not previously been reported in literature. One could think the weak bands originate from ambient water content but the weak bands do not overlap with those collected of water vapour or with the normal background noise.

The main vibrations for burnt Terra di Siena are observed at 534, 454, 395, 368 and 312 cm-1

as seen in Figure 5. Like in the case of Bavarian green earth we observe the SiO4-SiO4 coupling at 395 cm-1 [Farmer, 1974]. Weak bands are ob-served in the spectrum, which do not originate from ambient moisture. Again it is quite possi-ble they originate from an impurity in the stand-ard pigment. As stated above in Materials and Methods section we know from the supplier that Terra di Siena contains impurities (MnO2, alumina and calcium carbonate) in small percentages.

In the case study from Abbazia del Monte we have used approximately 0,5 mg of sample for preparation of the PE pellet. The FIR spectrum of sample AM1 is in Figure 6 compared with an ex-ample of a red silicate standard pigment, Terra di Siena and with lime white. The AM1 sample has bands at 538, 520, 467, 395, 368, 313, 225 and 107 cm-1. The band at 395 cm-1, which we have seen before in the two Bavarian green

earth and Terra di Siena standards, is indicative of a silicate pigment with a SiO4-SiO4 coupling [Farmer, 1974]. Sample AM1 and Terra di Siena have very similar band frequencies but the in-tensity of the bands at 538/520 and 466 cm-1 are not the same. We must conclude that the red pigment is a silicate similar to Terra di Siena but not identical. Bands at 313, 225 and 105 cm-1

show clearly the presence of a small amount of calcite in AM1, used as binder for the prep-aration of the mural paintings [White, 1974].

CONCLUSIONThis paper shows that FIR spectroscopy may be a useful technique for the identification of pig-ments in art objects. In an attempt to build a digital FIR library of pigments many pigments have been recorded in transmission mode. Eight of these standard pigments with differ-ent IR absorptions have been presented here. For all the pigments characteristic spectra in the Far IR region have been obtained both for MID-IR active as well as inactive compounds. Band assignation is still a difficult task and it has been possible only for compounds which have been studied in the past. It is the authors’ aim in the future to perform crystal modelling with chemical-physical studies to support the spec-tral interpretation to aid in better differentiating between the pure compound and impurities.

The red sample AM1 from the Abbazia del Monte was identified as a red silicate similar but not identical to Terra di Siena as observed by the SiO4-SiO4 coupling band at 395 cm-1. Calcite was also observed in the sample confirming its use as binder for the execution of these mural paintings.

ACKNOWLEDGEMENTSThe authors wish to acknowledge the support of the European Union, within the VI Frame-work Program, Contract: Eu-ARTECH, RII3-CT-2004-506171 and within Marie Curie EST ac-tion, Contract: EPISCON, MEST-CT-2005-020559.

ABSTRACTIn the field of FTIR spectroscopy, the Far Infrared (FIR) region is less investigated than the Mid In-frared (MIR) but it presents some advantages, when characterising inorganic compounds such as painting materials and corrosion products, which are not active in the MIR. Furthermore FIR spectroscopy is a complementary technique to Raman spectroscopy in particular in the pres-ence of fluorescence. In these experiments we have investigated the FIR region (600-90 cm-1) of eight inorganic standard pigments employed in art. The eight standard compounds have been collected in order to build a large FIR-data-


base library. One case study is also presented.

KEYWORDS Far Infrared spectroscopy (FIR), Transmission spectroscopy, inorganic compounds, Polyethylene (PE) pellets.

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ELSEBETH LANGHOLz KENDIx&

SILvIA PRATI&

ROCCO MAzzEO&

EDITH JOSEPH&

GIORGIA SCIUTTO

UNIvERSITY OF BOLOGNA, M2ADL - MICROCHEMISTRY AND MICROSCOPY ART

DIAGNOSTIC LABORATORY, vIA GUACCIMANNI 42, 48100 RAvENNA,

ITALY

E-MAIL: [email protected]

FAR INFRARED SPECTROSCOPY OF PIGMENTS IN ART

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