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Figure 5. (A) General overview of the mural painting from Thubchen Lakhang temple in Lo Manthang, Nepal. (B) Close-up of th green sampling area (East wall) before and after cleaning. olenna 1s a natural variety of ochre whose colour has been stabilised and homogenised by burning the pigment at a relatively high temperature. The main ingredients in burnt sienna are iron oxide in the form of haematite (Fe2O3, 30-50%) and amorphous silica (10-30%) mixed with minor components of MnO,, Alumina, alkali, quartz and calcium carbonate, which are present in the range 1-10%.!° The Raman spectrum of burnt sienna, which is shown in Fig. 4(B), shows clearly the presence of haematite by bands at 225(Aj,), 299(E,), 410(E,) and 613 (E,) cm™’. It is a matter of debate as to whether another Raman band observed at 659 cm7! belongs to aniron impurity in the form of magnetite (Fe3O,) whose bands are known to be observed at 665 cm7! (Aig), 550 cm™! (To¢) and 310 cm™! (E,).7°?” The Raman band observed at 659 cm™! could then be attributed to magnetite; however, the expected magnetite bands at 550 and 310 cm7! (of medium and weak intensity, respectively) are not visible in our recorded Raman spectrum of burnt Sienna. Therefore, the opinion that the band at 659 cm™! can be attributed to a disorder band in the crystalline structure of haematite caused by exposure to high temperature during burning is more likely to be correct.”*”? Cao et al.8 speculated that the band at 659 cm7! can be attributed to the nanostructure of the a-Fe,O3, as the band changes drastically in intensity according to the source. This hypothesis is backed by Bersani et al.” who also mention the possibility of the band coming from a disorder-induced activation of a forbidden IR mode. The FIR spectrum has bands that clearly indicate the presence of haematite by bands at 262, 319, 393, 460 and 529 cm™!.°° In the MIR spectrum, the bands at 700, 779, 798 and 1084 cm7! can be attributed to quartz.'? Quartz is not observed in the Raman spectrum of Sienna. This can be due to the particle size being of approximately the same size as the spatial resolution of the Raman microscope, which means that in micro-Raman Both the olive green pigment and the preparation layer are examined by FIR spectroscopy. Approximately 0.40 mg of the T2 olive green sample was ground and mixed with PE as described above in the experimental methods. Figure 6(A) shows the recorded FIR spectrum of the olive green pigment compared with an FIR spectrum of kaolinite, which has strong bands at 537, 470 and 432 cm7!. The kaolinite shown here has a high iron oxide content, as suggested by the FIR band at 393 cm™!, which is also present in our T2 sample. n the FIR spectrum of synthesised malachite, we observe strong bands at 525, 430 and 303 cm7!, which are all present in the T2 sample. The band at 430 cm7!, observed as a weak band in the T2 sample, is overlapping with the band from kaolin. Water or hydroxide bands are also present in the sample, as suggested by the multiple bands present in the 300-50 cm7! region of the FIR spectrum. These vibrational bands can be attributed to the hydroxide groups present in the malachite or also to air. In the same region, strong background absorption is visible, possibly due to instrument or environmental interference or other components not yet recorded in our FIR spectroscopic database. Small trace amounts of azurite, brochantite, calcite and vermilion are also expected in this sample as reported in the literature." Brochantite is not present in our FIR database, and although azurite, calcite and vermilion find place in it, their spectra do not correspond with the bands observed in our sample. We can conclude by FIR spectroscopic examination of our olive green T2 sample that it contains malachite and kaolinite with high iron oxide content.

Figure 5 (A) General overview of the mural painting from Thubchen Lakhang temple in Lo Manthang, Nepal. (B) Close-up of th green sampling area (East wall) before and after cleaning. olenna 1s a natural variety of ochre whose colour has been stabilised and homogenised by burning the pigment at a relatively high temperature. The main ingredients in burnt sienna are iron oxide in the form of haematite (Fe2O3, 30-50%) and amorphous silica (10-30%) mixed with minor components of MnO,, Alumina, alkali, quartz and calcium carbonate, which are present in the range 1-10%.!° The Raman spectrum of burnt sienna, which is shown in Fig. 4(B), shows clearly the presence of haematite by bands at 225(Aj,), 299(E,), 410(E,) and 613 (E,) cm™’. It is a matter of debate as to whether another Raman band observed at 659 cm7! belongs to aniron impurity in the form of magnetite (Fe3O,) whose bands are known to be observed at 665 cm7! (Aig), 550 cm™! (To¢) and 310 cm™! (E,).7°?” The Raman band observed at 659 cm™! could then be attributed to magnetite; however, the expected magnetite bands at 550 and 310 cm7! (of medium and weak intensity, respectively) are not visible in our recorded Raman spectrum of burnt Sienna. Therefore, the opinion that the band at 659 cm™! can be attributed to a disorder band in the crystalline structure of haematite caused by exposure to high temperature during burning is more likely to be correct.”*”? Cao et al.8 speculated that the band at 659 cm7! can be attributed to the nanostructure of the a-Fe,O3, as the band changes drastically in intensity according to the source. This hypothesis is backed by Bersani et al.” who also mention the possibility of the band coming from a disorder-induced activation of a forbidden IR mode. The FIR spectrum has bands that clearly indicate the presence of haematite by bands at 262, 319, 393, 460 and 529 cm™!.°° In the MIR spectrum, the bands at 700, 779, 798 and 1084 cm7! can be attributed to quartz.'? Quartz is not observed in the Raman spectrum of Sienna. This can be due to the particle size being of approximately the same size as the spatial resolution of the Raman microscope, which means that in micro-Raman Both the olive green pigment and the preparation layer are examined by FIR spectroscopy. Approximately 0.40 mg of the T2 olive green sample was ground and mixed with PE as described above in the experimental methods. Figure 6(A) shows the recorded FIR spectrum of the olive green pigment compared with an FIR spectrum of kaolinite, which has strong bands at 537, 470 and 432 cm7!. The kaolinite shown here has a high iron oxide content, as suggested by the FIR band at 393 cm™!, which is also present in our T2 sample. n the FIR spectrum of synthesised malachite, we observe strong bands at 525, 430 and 303 cm7!, which are all present in the T2 sample. The band at 430 cm7!, observed as a weak band in the T2 sample, is overlapping with the band from kaolin. Water or hydroxide bands are also present in the sample, as suggested by the multiple bands present in the 300-50 cm7! region of the FIR spectrum. These vibrational bands can be attributed to the hydroxide groups present in the malachite or also to air. In the same region, strong background absorption is visible, possibly due to instrument or environmental interference or other components not yet recorded in our FIR spectroscopic database. Small trace amounts of azurite, brochantite, calcite and vermilion are also expected in this sample as reported in the literature." Brochantite is not present in our FIR database, and although azurite, calcite and vermilion find place in it, their spectra do not correspond with the bands observed in our sample. We can conclude by FIR spectroscopic examination of our olive green T2 sample that it contains malachite and kaolinite with high iron oxide content.

Related Figures (5)

Figure 1. (A) FIR spectra of polyethylene (PE) coated with a thin film of water (W) recorded in the FIR region of 650-50 cm—". PE has one broad band at 540 cm~" and one relatively sharp band at 73 cm~". All other bands can be attributed to water. (B) Cross-sectior of a PE pellet containing 0.6 mg of the pigment terra di Sienna (burnt). The cross-section shows homogeneous distribution of the pigment in the polyethylene. No air bubbles are visible and the PE pellet has a homogeneous thickness of approximately 1.0 mm (Optical Microscopy, original magnification 50x). The FIR spectra of PE coated with a thin film of water is shown in Fig. 1(A). PE has one noticeable band at approximately 73 cm~! and another weak and broad one at 540 cm~!, which is usually covered by other bands coming from the pigment samples. Another band, whose origin is unknown (coming from the instrument, water or air), is at around 50 cm~!, however, this band is not always present in the FIR spectra. The remaining bands seen in Fig. 1(A) can be attributed to water, which is present on this PE pellet as a thin film. Both water and PE bands have an intensity of less than 20% transmission and are therefore usually covered by actual pigment bands when PE pellets are mixed with a pigment. Figure 1(B) shows the cross-section of a PE pellet containing 0.6 mg of the standard Kremer pigment terra di Sienna (burnt). The PE pellet has a diameter of 13 mm and a homogeneous thickness over the entire cross- section of approximately 1.05-1.08 mm. The cross-section shows a homogeneous distribution of the pigment in the PE. No air bubbles, which could disturb the recording of the FIR spectra and create artificial noise bands in the spectra, are visible in the PE pellet. The scratches on the surface are due to the cutting method (razor blade) of the PE pellet. The melting of PE to produce PE pellets involves exposing the sample to elevated temperatures for a brief period and this could possibly cause thermal degradation of the sample. Just how much or how little of the sample
Figure 1. (A) FIR spectra of polyethylene (PE) coated with a thin film of water (W) recorded in the FIR region of 650-50 cm—". PE has one broad band at 540 cm~" and one relatively sharp band at 73 cm~". All other bands can be attributed to water. (B) Cross-sectior of a PE pellet containing 0.6 mg of the pigment terra di Sienna (burnt). The cross-section shows homogeneous distribution of the pigment in the polyethylene. No air bubbles are visible and the PE pellet has a homogeneous thickness of approximately 1.0 mm (Optical Microscopy, original magnification 50x). The FIR spectra of PE coated with a thin film of water is shown in Fig. 1(A). PE has one noticeable band at approximately 73 cm~! and another weak and broad one at 540 cm~!, which is usually covered by other bands coming from the pigment samples. Another band, whose origin is unknown (coming from the instrument, water or air), is at around 50 cm~!, however, this band is not always present in the FIR spectra. The remaining bands seen in Fig. 1(A) can be attributed to water, which is present on this PE pellet as a thin film. Both water and PE bands have an intensity of less than 20% transmission and are therefore usually covered by actual pigment bands when PE pellets are mixed with a pigment. Figure 1(B) shows the cross-section of a PE pellet containing 0.6 mg of the standard Kremer pigment terra di Sienna (burnt). The PE pellet has a diameter of 13 mm and a homogeneous thickness over the entire cross- section of approximately 1.05-1.08 mm. The cross-section shows a homogeneous distribution of the pigment in the PE. No air bubbles, which could disturb the recording of the FIR spectra and create artificial noise bands in the spectra, are visible in the PE pellet. The scratches on the surface are due to the cutting method (razor blade) of the PE pellet. The melting of PE to produce PE pellets involves exposing the sample to elevated temperatures for a brief period and this could possibly cause thermal degradation of the sample. Just how much or how little of the sample
Figure 2. (A) From top to bottom: FIR, MIR and Raman spectra of barium chromate (Lemon yellow) BaCrOx,. (B) From top to bottorr FIR, MIR and Raman spectra of Aureolin (Cobalt yellow) CoK3(NObz)g-3H20.
Figure 2. (A) From top to bottom: FIR, MIR and Raman spectra of barium chromate (Lemon yellow) BaCrOx,. (B) From top to bottorr FIR, MIR and Raman spectra of Aureolin (Cobalt yellow) CoK3(NObz)g-3H20.
Figure 3. (A) From top to bottom: FIR, MIR and Raman spectra of Naples yellow, Pb2Sb207. (B) From top to bottom: FIR, MIR and Raman spectra of Lead-tin yellow, Pb2SnO,.
Figure 3. (A) From top to bottom: FIR, MIR and Raman spectra of Naples yellow, Pb2Sb207. (B) From top to bottom: FIR, MIR and Raman spectra of Lead-tin yellow, Pb2SnO,.
Figure 4. (A) From top to bottom: FIR, MIR and Raman spectra of Malachite, Cu2(CO3)(OH)s. (B) From top to bottom: FIR, MIR and Raman spectra of Sienna (burnt), containing mainly haematite Fe2O3 and quartz.
Figure 4. (A) From top to bottom: FIR, MIR and Raman spectra of Malachite, Cu2(CO3)(OH)s. (B) From top to bottom: FIR, MIR and Raman spectra of Sienna (burnt), containing mainly haematite Fe2O3 and quartz.
Figure 6. (A) TOP: FIR spectrum of T2 olive green pigment from Thubchen mural paintings marked with M = malachite, K = Kaolin, W = water and PE = Polyethylene. MIDDLE: FIR spectrum of Kaolin with high iron oxide content. BOTTOM: FIR spectrum of the synthetic pigment malachite. (B) TOP: FIR spectrum of T2 substrate from Thubchen mural paintings marked with C = calcite, K = Kaolin with high content of iron oxide and PE = Polyethylene. MIDDLE: FIR spectrum of calcite (Bianco San Giovanni from Zecchi). BOTTOM: FIR spectrum of Kaolin with high content of iron oxide (visible by band at 398 cm—).
Figure 6. (A) TOP: FIR spectrum of T2 olive green pigment from Thubchen mural paintings marked with M = malachite, K = Kaolin, W = water and PE = Polyethylene. MIDDLE: FIR spectrum of Kaolin with high iron oxide content. BOTTOM: FIR spectrum of the synthetic pigment malachite. (B) TOP: FIR spectrum of T2 substrate from Thubchen mural paintings marked with C = calcite, K = Kaolin with high content of iron oxide and PE = Polyethylene. MIDDLE: FIR spectrum of calcite (Bianco San Giovanni from Zecchi). BOTTOM: FIR spectrum of Kaolin with high content of iron oxide (visible by band at 398 cm—).
Related topics:
Condensed Matter PhysicsChemistrySpectroscopyFar InfraredArtist Pigments
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