Far-Infrared Microspectroscopy on Artist's Pigments

Table of Contents

Introduction
Experimental
Results and Discussion
Conclusions

Introduction

Identifying the chemical components of materials used in art is useful for their authentication and restoration. Authentic pieces of art can be distinguished from fakes by comparing the chemical identities of the materials used to those prevalent in the period when they were made. Also, this can also help in restoring and preserving the art and identify previous chemicals used in its preservation.

Fourier transform infrared spectroscopy (FT-IR) is commonly used to investigate art materials,1–10 mainly in the mid-IR frequency range (4000–400 cm-1). However, the type of detector used imposes a limitation on the spectral range available. For example, KBr is limited to 400 cm-1 and mercury-cadmium-telluride (MCT) to above about 700–500 cm-1. This is particularly important for investigating art as most inorganic pigments have strong absorptions in the far IR region (600–10 cm-1). Although another technique used for this purpose, Raman spectroscopy,11 has a spectral range well below 400 cm-1, but its disadvantage is the fluorescence from the components.

Far-IR spectral investigation can be done on macro sample.12–15 However, this technique has not been studied for smaller samples because of issues such as low radiance of thermal sources, relatively low responsivity of room temperature detectors, and reduction of throughput by aperture masks. Recent advances in spectroscopy have overcome these challenges. In this study, we characterize several micro-pigments using a Thermo Scientific™ Nicolet™ iS50 FTIR Spectrometer and the Czitek™ SurveyIR™ Microspectroscopy Accessory (Figure 1).

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

Experimental

Spectra were obtained using the SurveyIR diamond ATR attachment or by reflectance on low-E microscopic glass slides at 8 cm-1 resolution and 256 scans. Mid-IR spectra were measured using 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 collected using a solid substrate

beamsplitter and a DLaTGS detector with a polyethylene window. The beamsplitter was easily changed using the automatic beamsplitter exchange (ABX) on the Nicolet iS50 spectrometer, which is automatic and there is no loss of purge. The SurveyIR accessory was used, without modifications, to measure in the mid- and far-IR range.

Figure 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- and far-IR spectra of modern paint (Figure 3) is consistent with that of ethyl acrylate and methyl acrylate,16–18 typically found in modern polymer-based paint binders. The far-IR measurement detects Zn white and inorganic pigment with a peak at 380 cm-1, which would not have been detectable otherwise.

Figure 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.

Results and Discussion

Spectra of a coated paperboard were obtained to demonstrate the use of the far-IR method. The coating contained calcium carbonate, kaolinite, and a polyester binder. The spectra were collected in two modes using the SurveyIR diamond ATR accessory. The first spectrum was collected using the traditional KBr substrate beamsplitter and KBr window DLaTGS detector in the 1850–400 cm-1 region (Figure 2B). After changing the FTIR configuration to the far-IR mode, the spectrum was measured over the range 1850–50 cm-1 (Figure 2C). Several peaks for calcium carbonate and kaolinite were observed below 400 cm-1, beyond the limits of the KBr and MCT detector. This provides more details about the inorganic materials, which is useful for identifying them in mixtures.

Far-IR spectroscopy was used to analyze pigments of a wardrobe (Figure 4) from the collections of the Philadelphia Museum of Art. The wardrobe was supposed to be fabricated in 18th century China by the Qing dynasty. The decorative surfaces were analyzed to help conservation efforts, to check if they were truly from the 18th century or were later additions.

Figure 4. Qing dynasty lacquer clothes wardrobe and inset shows a cross-section from the interior used for analysis.

Diamond ATR spectra of the red layer were obtained and compared to a vermilion reference from the Forbes pigment collection at Harvard University (Figure 5).19 A good correlation was seen between the two spectra. Vermilion is produced from cinnabar, mercury sulfide. The observed bands are consistent with those from trigonal a-mercuric sulfide (cinnabar mineral) and are attributed to the lattice vibrations of the crystal.20 Vermilion was a common red pigment used in Chinese lacquerware21.

Its presence in the furniture sample proves that furniture piece is authentic. The images of the yellow particles, their spectra in the external reflectance mode from flattened samples on low-E glass, and the reference pigment orpiment (As2S3) are shown in Figure 6. A 250 µm aperture mask was used to target the yellow pigment. Orpiment is a geothermal pigment used since historic times into the 20th century. Distinct absorption bands of the pigment were seen and agree well with that of the reference and previous reports,22,23 indicating the pigment is orpiment. The lack of bands in the mid-IR region emphasizes the importance of far-IR measurements.

Figure 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.

Figure 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

Although mid-IR spectroscopy combined with microscopy is a useful analytical technique for art materials, many materials, especially those containing inorganic pigments cannot be completely characterized only by mid-IR operation. The analysis of two layers of an antique artifact shows that more information can be obtained using microspectroscopy accessories and far-IR spectroscopy. This information can be used to date and authenticate artworks and to plan conservation efforts.

References

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  16. M.R. Nelson, Authentic or Not, ChemMatters, April 2011, pp. 15-17.

This information has been sourced, reviewed and adapted from materials provided by Czitek.

For more information on this source, please visit Czitek.

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