For the last few years, Ms. Villafana and her co-authors have been refining a new microscopy technique for conservation to create “virtual” non-destructive cross-sections. This is a very exciting development for our field, particularly for those of us working with materials–such as works of art on paper–that don’t typically allow for sampling. And for paintings conservators more accustomed to taking traditional cross-sections, this technique has promise for in-situ analysis of paint layers through varnish.
To summarize, the virtual cross-section image is created using pump–probe microscopy, a non-linear optical microscopy technique developed for the biomedical field, which allows non-invasive detection of biological pigments indicative of skin cancer. Because skin tissue is highly scattering, this technique was developed to be inherently confocal, meaning that the signal is generated only at the focal point, creating less scattering, and less spectral noise. The approach is naturally suited to the highly scattering pigments, binders, and supports making up materials of cultural heritage. However, the complexity of art objects render the technique more difficult to apply.
Pump–probe microscopy achieves high resolution in three dimensions with a maximum image area of up to 1mm square. The penetrating depth ultimately depends on the material composition of the object under study. The technique is typically operated at two wavelengths: 810nm and 720nm and modulated to create a series of images at different inter-pulse delays. These images can then be colored according to the molecular composition of the specific material and stacked to create a 3D rendering.
With this presentation, Ms. Villafana shared case studies illustrating ongoing research into cultural heritage materials using pump–probe microscopy. The first project investigates applications of pump–probe on paper substrates bearing coatings of lapis lazuli pigment. With this technique, it is possible to produce an image illustrating the physical structure and condition of paper fibers underlying the paint layer. She observes that the pigment particles cluster around the fibers, as seen in the slide below. She is interested in further investigating the natural heterogeneity of lapis lazuli crystals, noting that samples from different parts of the world exhibit different delay behaviors. She plans to complement her pump–probe analysis of lapis lazuli pigments with SEM-EDS, Raman, and FTIR.
Villafana also presented on preliminary research using pump–probe to investigate historical methods of pottery manufacture. After finding that pump–probe delays of hematite are dependent on firing temperature, Ms. Villafana started using mock-up clay bodies fired under different conditions (Oxidized at 1800F and 2300F/Reduced at 1800F and 2300F) to examine the difference in delay behaviors from the exterior to the interior of fired clay. She has found that higher temperatures and oxidation both result in shorter lifetimes. Further study will focus on phase change and particle size.
I quite curious to see how this technique develops in the near future. Will pump-probe (or something like it) be able to replace traditional cross section techniques within the next 5 to 10 years? What other techniques are being developed out there that might be able to achieve similar results?
See the following two links for more information:
Villafana, et al., full-text PDF of recent research published in The Proceedings of the National Academy of Sciences of the United States of America
Article about Pump-Probe Microscopy in Science News, from Science, AAAS
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AIC Blog Archives: Conservators Converse
Former Blog of the American Institute for Conservation