Benjamin Hall and Dr. Lawrence Rees, Physics And Astronomy

Using the resources provided by this grant, Dr. Rees and I set out to classify the pigments used in artistic oil painting according to their elemental composition. This we did using proton-based Particle-Induced X-ray Emission (PIXE) spectroscopy. Our goal in this portion of the research was to test the feasibility of creating a baseline set of known compositions against which to compare unknown and or mixed pigments.

Procedures

The actual methods used to produce the targets for PIXE analysis are detailed in the referenced thesis—here I will primarily talk about the issues raised and or dealt with during the preparation phase of the process.

PIXE is a non-destructive method of analysis, meaning in part that no pieces of the subject are removed. For large subjects, such as paintings, this requires allowing the proton beam to exit the evacuated beam-line and bombard the target in atmosphere. Our baseline targets, however were to be run under the internal-beam operating regime , necessitating a correlation check between the two regimes. This turned out to be the major point addressed in the research—is it possible to correlate features seen in the internal regime, where targets are produced under very strict cleanliness protocols accurately to features exhibited by targets produced under more realistic conditions and analyzed externally.

Testing this required the production of two sets of targets, one for each regime. Ten pigments were chosen from five general color-groups . The targets were prepared as described in the referenced thesis. Eight additional targets were created by mixing a colored pigment with one of the two white pigments analyzed. These eight were only analyzed externally, and served as our control.

Another issue that arose while analyzing the targets was the lack of an accurate way to determine the number of protons that hit the target. This count is used as part of the algorithm to convert raw spectra into parts-per-million concentrations. We ameliorated this problem by determining the proton current (charges per second) exiting the beam-line and then numerically integrating this over a fixed run-time; we thus had a good, although not perfect, count of protons on target.

Results

Qualitatively, we were able to distinguish six of the eight control targets using the baseline spectra and compositions created under the internal regime. The two that failed are pigments that depend heavily on organic compounds which PIXE is unable to resolve due to the very low (< 1 keV) energies of their characteristic X-rays .

A sample of the spectra produced by our analysis is found in Fig. 1. This one corresponds to the pigment named “Manganese Blue.” Coincidentally, this target shows why the common names of pigments are not always useful in determining their composition—the pigment is notable for its lack of manganese.

For more details about exact results, please consult the thesis referenced above.

References

• O. Enguita, T. Calderon, M. T. Fernandez-Jimenenz et al. “Damage induced by proton irradiation in carbonate based natural painting pigments,” Nuclear Instruments section B 219-20, 53-56 (2004).
• B. Hall, B.S. Thesis PIXE Analysis of Oil-Paint Pigments: Proof of Principle, Brigham Young University Department of Physics and Astronomy, 15 (2006).
• Ibid. 12-13.
• Ibid. 13-14.
• Ibid. 10.
• Ibid 5.
• Ibid 15.