Using UV-Vis Spectroscopy to Analyse the Surface of Oil Canvases

Diffuse reflectance spectroscopy is extremely useful for analyzing rough surfaced samples and powders. However, it is difficult to analyze most of the rough surfaced samples with in-compartment diffuse reflection accessories due to their large size.

Nevertheless, they can be easily examined using a diffuse reflection fiber optic probe, specifically in the visible region due to the availability of high throughput fiber optics. This article illustrates the analysis of color differences in paint on canvas using a diffuse reflection fiber optic probe.

Instrumentation and Experimental Procedure

Using a UV-Vis spectrometer in double-beam mode with a 1.1nm data interval, a 2nm SBW, and a reduced slit, the diffuse reflectance measurements were performed. The UV-Vis spectrometer was linked to the Omni-Diff diffuse reflectance fiber optic probe (see Figure 1) via the two fused silica fibers with a 1000µm diameter core and the FiberMate2 fiber optic coupler (see Figure 2). The Omni-Diff consists of a digital camera to perform real-time imaging of the sampled area.

The Omni-Diff diffuse reflection fiber optic probe with its digital camera

Figure 1. The Omni-Diff diffuse reflection fiber optic probe with its digital camera

The FiberMate2 fiber optic coupler

Figure 2. The FiberMate2 fiber optic coupler

As shown in Figure 3, a canvas was applied with 14 different oil paints purchased from two manufacturers, and then allowed to dry before analysis. The Grumbacher oil paints like Naples Yellow, Yellow Ochre, Sap Green, Burnt Umber, Mars Red, Mars Yellow, Cadmium Red, Alizarin Crimson, Indian Red and Paynes Gray were used. The Winsor and Newton oil paints such as Cadmium Yellow, Cadmium Lemon, Cobalt Blue and Ultramarine were also investigated.

Oil paints on canvas: Paynes Gray (1), Indian Red (2), Alizarin Crimson (3), Cadmium Red (4), Mars Yellow (5), Mars Red (6), Burnt Umber (7), Ultramarine (8), Cobalt Blue (9), Cadmium Lemon (10), Cadmium Yellow (11), Sap Green (12), Yellow Ochre (13), Naples Yellow (14).

Figure 3. Oil paints on canvas: Paynes Gray (1), Indian Red (2), Alizarin Crimson (3), Cadmium Red (4), Mars Yellow (5), Mars Red (6), Burnt Umber (7), Ultramarine (8), Cobalt Blue (9), Cadmium Lemon (10), Cadmium Yellow (11), Sap Green (12), Yellow Ochre (13), Naples Yellow (14).

Based on the center of the painted area, all spectra were recorded, and the sample was photographed at each point.

Results and Discussion

Figures 4 to 7 show the recorded spectra, which are approximately grouped based on the color. Besides the unpainted canvas, two grayish samples were shown in Figure 4. As anticipated, only few unique spectral bands could be observed in either the spectra of the Burnt Umber or Paynes Gray. The reflectance of the Burnt Umber is slightly higher over 550nm, resulting in a slightly browner color. The canvas is adequately covered by these two paints to mask any change in reflectivity of the canvas occurring at 425nm.

Diffuse reflectance of the grayish samples. Canvas spectrum shown for reference.

Figure 4. Diffuse reflectance of the grayish samples. Canvas spectrum shown for reference.

The spectra of the colors categorized as reds are shown in Figure 5. All the four paints were shown to have a stronger reflectance over 600nm than the gray samples, as expected from their reddish color. Unlike the other two paints, the Indian Red and Mars Red are not strongly reflecting in this spectra range, as is consistent with their more muted colors. The strongest reflectance of the Cadmium Red was at 625nm in the orange region of the spectrum.

Diffuse reflectance spectra of the reddish samples

Figure 5. Diffuse reflectance spectra of the reddish samples

The spectra of the yellowish paints were shown in Figure 6. The Yellow Ochre and Mars Yellow were found to be spectroscopically similar. Some blue-green components were found to be present in the Naples Yellow, with the band at 500nm, and some violet components at around 400nm peak.

Diffuse reflectance spectra of the yellowish samples

Figure 6. Diffuse reflectance spectra of the yellowish samples

The spectra of the three remaining samples were shown in Figure 7. As expected, the strong bands of Cobalt Blue and Ultramarine were observed in the blue at around 470nm. Further, a band indicating a reddish hue for the Cobalt Blue was also observed at around 700nm. The third spectrum indicated a yellowish green colored paint instead of a blue color, with a distinct band of around 550nm.

Diffuse reflectance spectra of the bluish samples

Figure 7. Diffuse reflectance spectra of the bluish samples

Conclusion

From the results, it is evident that the differences in the colors of paint on canvas in the visible region can be detected using the Omni- Diff fiber optic probe.

This information has been sourced, reviewed and adapted from materials provided by Harrick Scientific Products, Inc.

For more information on this source, please visit Harrick Scientific Products, Inc.

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