Using FT-IR to Analyze Artists Paints

Visitors flock from around the world to view the priceless works of art that are on display in museums, and to ensure the pieces are kept in the best possible condition, conservators face many challenges. One of the most difficult is how to best care for these assets. Before the conservator’s work can begin the materials used in the arts production must be identified.

It is only after these have been determined that recommendations can be made for display, care, restoration, or storage. Along with this, the museum also must have a way of finding out if the artwork is genuine.

Recently there have been several high-profile cases concerning organized forgery enterprises that highlight this concern [1,2], and prove why determining the materials used is so crucial. This problem can be overcome by identifying the materials, allowing the piece to be dated and its provenance known.

Identification of historical materials on works of art presents challenges, due to the coatings on the piece usually comprising of a number of components. The coating materials used by artists have also evolved over time. Older paints consisted of ingredients that could be found in nature, unlike modern, synthetic acrylic-based paints.

Not being as stable as synthetic paints, many natural materials experience degradation over time, causing the artwork to change in color. This can be seen in Van Gogh’s famous painting “Roses,” with the once pink roses having faded to white. This is attributed to his preferred organic red pigment, red lake, that fades extremely quickly when exposed to light.

Fourier Transform Infrared Spectroscopy

Fourier Transform Infrared Spectroscopy (FT-IR) analyses the vibrations of the molecules in a material to provide detailed information regarding the materials molecular structure, and displays the results in a spectrum. A huge benefit of using it for paint sample identification is that FT-IR is a non-destructive technique and, when using microspectroscopy, uses a minimal amount of sample to make an accurate identification.

FT-IR microspectroscopy is therefore particularly useful in the microanalysis of artists’ materials. The quantity of sample required is very small which allows the painting to remain unaltered and unscathed. By its nature, FT-IR microspectroscopy greatly aids the analysis of complex, multi-component samples. In first selecting the target area with microscopical observation, these certain areas that can then be measured spectroscopically.

The SurveyIR is Czitek’s latest system: a compact FT-IR microspectroscopy accessory that can be equipped to any commercially available FTIR instrument. There is no requirement for cryogenic cooling as it uses the standard detector found in most instruments. SurveyIR also has a small footprint and is easy to install.

These two factors allow modern, small FT-IR instruments, to be deployed in situations where it isn’t possible to remove samples from the vicinity of the artwork. SurveyIR also makes use of digital imaging software to observe, document, analyze, and store images.  

Results and Discussion

A painting may be authenticated by identifying of the pigments used in its composition. Fig. 1 is a picture of two samples of paint chips taken from different works of art which, on first glance, appear to be almost the same in color and nearly indistinguishable from one another.

(Left) Acrylic Prussian blue hue; (Right) Oil based Prussian blue.

Figure 1. (Left) Acrylic Prussian blue hue; (Right) Oil based Prussian blue.

Both samples are of the color known as Prussian blue, but they are distinct due to their material composition. The left hand side image in Fig. 1 is of an acrylic Prussian blue hue, first used in the late 1940’s [3]. The right-hand image however, is a paint sample containing the material used to create the original inorganic pigment Prussian blue, first synthesized in 1704 and commercially available by 1724.

This pigment used oil as a binder [4], as opposed to the polymers used in modern paints. These paints are almost the same when viewed with the naked eye; however, the IR spectra in Fig 2 shows that they are vastly different in their chemical composition.

IR spectra of oil based Prussian blue (Top, Red) and acrylic Prussian blue hue (Bottom, Green).

Figure 2. IR spectra of oil based Prussian blue (Top, Red) and acrylic Prussian blue hue (Bottom, Green).

Fig. 2 shows the difference that really sets the two paint chip spectra apart, the band at 2083 cm-1 in the spectrum of Prussian blue (Red). The band at 2083 cm-1 is due to the C N stretch of the cyano groups in iron hexacyanoferrate (Prussian blue). The green spectral line is from Prussian blue hue, where the main acrylic component dominates the spectrum from 1000-1300 cm-1.

When compared to the oil based paint, the actual pigment within the acrylic paint does not take up such a significant portion of the spectrum due to the lesser amount present within the paint. In understanding the composition of the paint samples, collectors have the evidence to investigate claims of legitimacy by confirming the materials coincide with the artists’ time period.

As well as being used to investigate claims of authenticity in older works of art, identifying paint components can be helpful in preserving an artwork. As mentioned earlier it is typical for older works of art to contain materials that may deteriorate under certain environmental conditions [5]. The following example concerns a yellow paint sample removed from a historical painting.

Yellow paint chip imaged with oblique illumination.

Figure 3. Yellow paint chip imaged with oblique illumination.

Before being used with the spectrometer the specimen in Fig. 3 was flattened out, it was then placed onto an IR transparent Potassium Bromide (KBr) window for analysis.

(Right) IR spectra of the art work sample (top, black) and its complimentary components.

Figure 4. (Right) IR spectra of the art work sample (top, black) and its complimentary components.

There were three components within the sample identified using FT-IR. Fig.4 (black) shows that the primary carrier for the paint pigment is linseed oil (red), an oil that’s been used throughout history as a vehicle for pigment in oil based paints. After the linseed oil was spectrally compensated, the pigment could then be identified through a spectral library search, recognizing it as Dalamar yellow (blue).

The third component was confirmed as alumina trihydrate (green), a filler and extender that aids in bringing out the brilliant yellow color. In this case, the Dalamar yellow pigment is a robust azo dye complex, making it more stable and thus not requiring special environmental conditions, as opposed to other natural pigments such as red lake. This is an excellent demonstration of how useful FT-IR microspectroscopy can be in analyzing complex compositions. All three molecular constituents could be identified from a microscopic paint chip, using less than one (1) microgram of material.

Conclusion

There is a rising trend in the conservation and authenticity of works of art in determining their detailed chemical composition. FT-IR microspectroscopy provides a powerful tool for conservators and forensic scientists to analyze an identify complex compositions of materials in artworks.

As well as minimizing damage risk from transport or theft, it also provides a workaround to the several international conventions that prevent artwork and antiquities from being relocated. The unique SurveyIR microscope accessory can facilitate identification at the location of the work.

References

  1. Gates, Anita, “Where Art Forgeries Meet Their Match,” New York Times, 2 May, 2018.
  2. Moynihan, Colin, “Knoedler Gallery Director’s Lawyer Says Other Experts Were Duped by Fake Rothko,” New York Times, 26 Jan., 2016.
  3. Phaidon Press (2001). The 20th-Century art book (Reprinted. ed.). London: Phaidon Press. ISBN 0714835420
  4. Douma, M., curator. (2008). History. In Pigments through the Ages. Retrieved 6 22, 2017, http://www.webexhibits.org/pigments/indiv/history/prussblue.html
  5. Hoeve, Claire L. “A Study of the Discoloration Products Found in Lead White Paint Films.” The Book and Paper Group Anual, vol. 4, 1986, http://cool.conservation-us.org/coolaic/sg/bpg/annual/v04/pubinfo.html

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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