Analyzing Artists’ Pigments by Far-Infrared Microspectroscopy

The authentication and preservation of art pieces are primarily achieved by the chemical identity of materials used in them. By comparing the materials used relative to the time period in which the art was supposedly generated, conservators can rapidly differentiate authentic pieces from well-made copies. In addition, identification of the materials used can also direct conservators on the appropriate cleaning and preservation methods to slow the natural degradation processes of those art pieces. In a number of cases, it’s also crucial to identify formerly applied preservation materials.

Fourier Transform Infrared (FTIR) Spectroscopy

Fourier transform infrared (FTIR) spectroscopy is a proven method to test and identify artists’ materials.^1-10 Measurements are usually performed in the mid-IR spectral region (4000-400 cm^-1). The KBr optics typically used in FTIR spectrometers impose a low-frequency cutoff at about 400 cm^-1. Mercury-cadmium-telluride (MCT) detectors used in FTIR microscopes further limit the low-frequency cutoff to the 700-500 cm^-1 range. These low frequency cutoffs restrict the effectiveness of mid-IR in identifying many inorganic pigments found in artwork, which frequently have characteristic absorption bands in the far-IR region (600-10 cm^-1). Raman spectroscopy, another main vibrational spectroscopy method for the molecular identification of cultural heritage materials, has a basic spectral range well below 400 cm^-1. However, fluorescence arising from both pigments and binding media is a frequently-encountered issue in the investigation of artistic pigments using Raman spectroscopy.¹¹

The efficacy of far-IR spectroscopic characterization of pigment materials has been shown for macro samples.^12-15 The far-IR microspectroscopy study of pigments, however, has been an untapped field because of such challenges as the low radiance of thermal sources, comparatively low responsivity of room temperature detectors, and reduction of throughput by aperture masks. Latest advances in optical materials and design have risen above many of these challenges and opened the far-IR region for the microspectroscopic application in pigment characterization. In this article, the microspectroscopic analysis of a number of pigments using the Thermo Scientific™ Nicolet™ iS50 FTIR Spectrometer and the Czitek™ SurveyIR™ Microspectroscopy Accessory (Figure 1) is demonstrated.

Image 1. Nicolet iS50 FTIR spectrometer with the SurveyIR microspectroscopy accessory in the sample compartment.

Experiment

Measurements were performed in attenuated total reflectance (ATR) mode using the SurveyIR diamond ATR attachment, or by reflectance with the samples on a low-E glass microscope slides. Spectra were gathered using 256 scans at 8 cm^-1 spectral resolution. Mid-IR spectra were measured with the aid of a Ge on KBr beamsplitter and DLaTGS detector with a KBr window allowing collection from 4000 to 400 cm^-1. Far-IR spectra in the range of 1800-50 cm^-1 were gathered using a solid substrate beamsplitter and a DLaTGS detector with a polyethylene window. Polyethylene is typically used as a window material in the far-IR, but its robust absorbances around 1460 cm^-1 and 770 cm^-1 do not permit measurements in these particular regions. Changing the beamsplitter between measurements was easily achieved with the automatic beamsplitter exchanger (ABX) on the Nicolet iS50 spectrometer. The special ABX design enables automated switching between measurement modes without any loss of purge. Modifications of the SurveyIR accessory were not necessary to allow it to measure spectra in both mid- and far-IR regions.

Results and Discussion

To exhibit the utility of the far-IR technique and performance, mid- and far-IR spectra of a coated paperboard were gathered. The coating on the paperboard had kaolinite (Al₂O₃*2SiO₂*2H₂O), calcium carbonate (CaCO₃), and a polyester binder. The coating spectra were gathered in two modes using the SurveyIR diamond ATR microspectroscopy attachment. First, a spectrum was gathered using a conventional KBr substrate beamsplitter and KBr window DLaTGS detector. This spectrum is shown in Figure 2B in the 1850-400 cm^-1 region. The optical configuration of the FTIR was then modified to the far-IR mode. The IR spectrum using this design was measured from 1850-50 cm^-1 and is illustrated in Figure 2C. The total spectral quality and signal-to-noise ratios are fairly reasonable. Many absorption bands originating from kaolinite and calcium carbonate are observed below 400 cm^-1 beyond the range of MCT detector and KBr window cut-offs. While these inorganic materials also display features in the mid-IR, broadening the range into the far-IR region offers additional spectral details that can be helpful in identifying materials or components in mixtures.

Image 2. Measurement of coated paperboard by ATR. (A) Image of sample area through diamond ATR crystal before and after contact, allowing the user to verify the sample area

being measured. (B) Mid-IR spectra with characteristic low end cut-offs of MCT and DLaTGS detectors indicated by dashed lines. (C) Spectral features seen in the far-IR spectrum indicative of calcium carbonate and kaolinite seen below the KBr and MCT low

wavenumber cut-offs. Spectral regions around 1460 and 770 cm^-1 are blanked due to the strong absorbance of polyethylene detector window at these locations.

The mid-IR and far-IR spectra of a contemporary paint are illustrated in Figure 3, as another comparison. These spectra were recorded with the aid of the SurveyIR diamond ATR microspectroscopy attachment. The observed mid-IR spectrum is consistent with polymers of ethyl acrylate and methyl methacrylate,^16-18 typical for a contemporary polymer-based paint binder. However, the far-IR spectrum permits the identification of the inorganic pigment, zinc oxide, or “zinc white” since it has a characteristic absorbance peak at 380 cm^-1. Evidently, measurement of the mid-IR spectrum alone would not enable a positive identification of zinc white.

Image 3. White paint containing zinc oxide. Top: Mid-IR spectrum showing only shoulder of zinc oxide absorption at 380 cm^-1 (dashed line). Bottom: Far-IR spectrum clearly showing zinc oxide’s absorption peak at 380 cm^-1.

Far-IR microspectroscopy was also used in the analysis of pigments from a furniture piece in the collections of the Philadelphia Museum of Art (PMA). The piece, a wardrobe illustrated in Figure 4, was supposedly fabricated in China in the 18th century during the Qing dynasty. To assist in planning the conservation treatments, the decorative surfaces were analyzed to establish whether they are consistent with the 18th century in China or are later additions. A cross-section from a sample taken from an inner portion of the wardrobe is also illustrated in Figure 4.

Image 4. Qing dynasty lacquer clothes wardrobe and inset shows a cross-section from the interior used for analysis. (A) Exterior, (B) interior, and (C) cross-section analyzed from the compound wardrobe. The asterisk represents the original decorative surface. The layers above this point are later additions. Accession number: 1940-7-2, Philadelphia

Museum of Art; Artist/maker unknown Qing Dynasty (1644–1911); Lacquered wood with painted and gilt decoration; brass fittings. Purchased with the Bloomfield Moore Fund, 1940.

Far-IR microanalysis was performed on the yellow and red layers of the cross-section. The images acquired from the red pigment particles are illustrated in Figure 5, Section A. Spectra of the red layer acquired by diamond ATR microscopy and a vermilion reference are illustrated in Figure 5, sections B and C, respectively. The vermilion reference was from the Forbes pigment collection at Harvard University.¹⁹ The two spectra display a high degree of likeness and substantiate a positive identification. Vermilion is made from the mineral cinnabar, HgS, mercury (II) sulfide. The observed bands are compatible with those from trigonal α-mercuric sulfide (cinnabar mineral) and are attributed to the lattice vibrations of the crystal.²⁰ Vermilion was a basic red pigment used in the making of Chinese lacquerware²¹ and its presence offers vital evidence of the wardrobe’s authenticity. The captured images of the yellow particles (A), the far-IR spectra of the yellow lacquer layer pigment (B), and orpiment reference (As₂S₃) (C) are illustrated in Figure 6. Both spectra were recorded in the external reflectance mode from samples compressed onto low-E glass substrates. An aperture mask corresponding to 250 µm at the sample was used to target the pigment extracted from the yellow layer. The reference orpiment pigment was also from the Forbes collection. Orpiment is a mineral formed geothermally and was used as a yellow pigment from antiquity into the early 20th century. Major absorption bands are specified in the figure. The absorption bands between the yellow pigment and the orpiment reference spectrum match and correspond with former reports.^22-23 The results clearly specify that the yellow pigment is orpiment. In the cases of both pigments, the lack of features in the mid-IR regions highlights the significance of far-IR measurements.

Image 5. Far-IR ATR spectra: (A) images of red pigment extracted from wardrobe sample, (B) far-IR ATR spectrum of wardrobe sample and (C) vermillion reference.

Image 6. Far-IR ATR spectra: (A) images of yellow pigment extracted from the wardrobe sample, (B) far-IR reflectance spectrum of yellow wardrobe sample and (C) orpiment reference.

Conclusions

The Mid-IR FTIR spectroscopy is a useful analytical tool for artworks, and the combination with microscopy is mainly beneficial as minimal sample is needed for investigation. However, many materials, mainly pigments comprising of inorganic components, cannot be completely characterized with FTIR microscopes configured only for mid-IR operation. The analyses of zinc white and two layers of an antique artifact show the advantages of integrating far-IR spectroscopy with microspectroscopy accessories, which allow for more perceptive characterization of pigment materials. The chemical information gathered from the interpretation of these spectra can be used to determine authenticity through dating of artworks, characterize earlier conservation attempts, and plan new conservation efforts.

References and Further Reading

1. Infrared Spectroscopy in Conservation Science, Michele R. Derrick, Dusan Stulik, and James M. Landry, J. Paul Getty Trust, Getty Conservation Institute, Los Angeles, 1999.
2. Meiluna, Raymond J., James G. Bentson, and Arthur Steinberg. Analysis of Aged Paint Binders by FTIR Spectroscopy. Studies in Conservation 35 (1990).
3. Sloggett, Robyn, Caroline Kyi, Nicole Tse, Mark J. Tobin, Ljiljana Puskar, and Stephen P. Best. Microanalysis of Artworks: IR Microspectroscopy of Paint Cross-sections. Vibrational Spectroscopy 53.1 (2010): 77-82.
4. Newman, Richard. Some Applications of Infrared Spectroscopy in the Examination of Paintings Materials. Journal of the American Institute for Conservation 19.1 (1979): 42-62.
5. Manzano, E., N. Navas, R. Checamoreno, L. Rodriguez-Simon, and L.F. Capitan-Vallvey. Preliminary Study of UV Ageing Process of Proteinaceous Paint Binder by FTIR and Principal Component Analysis. Talanta 77.5 (2009): 724-731.
6. Jonsson, Julia, and Tom Learner. Separation of Acrylic Paint Components and Their Identification with FTIR Spectroscopy. Proceedings of the Sixth Infrared and Raman Users Group Conference (IRUG6). Florence, Italy March 29th-April 1st 2004. Ed. Marcello Picollo. Florence: Il Prato, 2005. 58-68.
7. Hodson, J., and J.A. Lander. The Analysis of Cured Paint Media and a Study of the Weathering of Alkyd Paints byFourier Transform Infra-red/photoacoustic Spectroscopy. Polymer 28.2 (1987): 251-56.
8. Lang, Patricia L., Chad D. Keefer, Jessica C. Juenemann, Khoa V. Tran, Scott M. Peters, Nancy M. Huth, and Alain G. Joyaux. The Infrared Microspectroscopic and Energy Dispersive X-ray Analysis of Paints Removed from a Painted, Medieval Sculpture of Saint Wolfgang. Microchemical Journal 74.1 (2003): 33-46.
9. Olin, J.S. The Use of Infrared Spectrophotometry in the Examination of Paintings and Ancient Artifacts. Instrument News 17 (1966): 1.
10. McClure, A., J. Thomson, and J. Tannahill. Infrared Spectra of Ninety-six Organic Pigments. Journal of Oil and Colour Chemists’ Association 51 (1968)
11. López-Gil, Ruiz-Moreno, Miralles. Optimum acquisition of Raman spectra in pigment analysis with IR laser diode and plulsed UV irradiation Journal of Raman Spectroscopy. 37 (2006); 966-973
12. Kendix, Elsebeth L., Silvia Prati, Edith Joseph, Giorgia Sciutto, and Rocco Mazzeo. ATR and Transmission Analysis of Pigments by Means of Far Infrared Spectroscopy. Analytical and Bioanalytical Chemistry 394 (2009): 1023-032.
13. https://spectra.chem.ut.ee/
14. Afremow, Leonard C., and John T. Vanderberg. High Resolution Spectra of Inorganic Pigments and Extenders in the Mid-Infrared Region from 1500 cm-1 to 200 cm-1. Journal of Paint Technology 38.495 (1966): 169-202
15. C. Karr and J. Kovach. Far-Infrared Spectroscopy of Minerals and Inorganics. Appl. Spectrosc. 23, 219 (1969).
16. M.R. Nelson, Authentic or Not, ChemMatters, April 2011, pp. 15-17.
17. 

This information has been sourced, reviewed and adapted from materials provided by Thermo Fisher Scientific – Materials & Structural Analysis.

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