Chemical Analysis of Art: Macro X-ray Fluorescence Scanning

Visual art is among some of the most longstanding and expressive forms of artistic representation. Categorized by a variety of movements and developing alongside individual and social philosophies, art offers an immersive glimpse into human history. However, this contribution of art is predicated on its preservation. As the world of visual art evolves, a prominent area of interest deals with the materials employed to produce masterpieces, and how well they endure the test of time. In efforts to optimize conservation attempts, authenticate the origin of paintings, as well as elucidate the composition and properties of an art piece, minimally invasive and robust analytical methodologies are imperative. One such prevalent method is scanning macro X-ray fluorescence, or MA-XRF, which effectively provides quantitative chemical information for paintings (Targowski et al., 2015).

On its surface, paintings are nothing more than blended palettes of colour, arranged in interesting configurations. However, a more intricate view of the painting arises when considering the specific formulations of the paint, and the interactions they have with parchment and canvases. In essence, paint consists of three components: the pigment, usually sourced from salts containing various metals that impart colour; the binder, a polymer that facilitates adhesion and allow for pigments to thicken; and the solvent, which reduces the paint viscosity for easier application (Kwaambwa, 2013). Variations in the type of metallic pigment or ratio of ingredients influence the binding and appearance of the paint with different media. One example is iron-gall inks, which functioned as a precursor to paint and supplementary writing material for 1500 years (Kolar et al., 2012). The constituents of iron-gall inks – ferrous sulphate, plant tannins, and a binder – slowly led to the colouration changes and structural degradation of parchments. Often times, inks would contain additional mineral pigment contaminants, such as compounds of copper, zinc, manganese, and titanium, which were also main elements in paints; the presence of these transition metal ions would contribute to the deterioration of the animal membrane that historically comprised parchments (Targowski et al., 2015). As such, the ability to quantify and evaluate the distribution of metal pigments in a painting is invaluable, and informs preservation procedures.

As a commercial and non-invasive research tool, MA-XRF is used to examine the elemental composition of objects (Targowski et al., 2015). In brief, when an object is exposed to mono-energetic X-rays, the constituent atoms are ionized after absorbing photons. After ionization, electrons from higher orbitals are demoted and emit secondary photons; the energy released in this process corresponds to the excited state of the atom, with each transition of electrons from one shell to another producing a particular spectral line (Targowski et al., 2015). This information is collected in a point-wise fashion for an entire painting, allowing for the distribution of elements to be mapped over large areas (Targowski et al., 2015; Saverwyns, Currie, and Lamas-Delgado, 2018). The scanning apparatus is fitted above the painting and slowly scans the canvas millimetre by millimetre (Figure 1). A commonly used scanner for paintings is the M6 Jetstream, manufactured by Bruker, which employs a rhodium-target microfocus X-ray tube and can scan areas up to 80 x 60 centimetres (Saverwyns, Currie, and Lamas-Delgado, 2018). These scanners also utilize poly-capillary optics, which essentially function as a lens, in order to adjust the size and intensity of the X-ray beam and ultimately increase image resolution (Arkadiev and Gruev, 1994; Saverwyns, Currie, and Lamas-Delgado, 2018).

As a whole, macro X-ray fluorescence scanning is a powerful tool that enables large-scale, non-invasive analysis of paintings. By looking past the superficial appearance of an artwork, further insight can be gained relating to the pigments used by the artist, historical conservation treatments, and past painting layers, shaping the future of preservation efforts. By operating on the principles of analytical chemistry and materials science, this technique truly merges the essence of science and art.

References

Arkadiev, V.A. and Gruev, D.I., 1994. Principal Possibilities of Kumakhov Lenses. X-ray and UV Detectors. [online] Available at: <https://doi.org/10.1117/12.180015> [Accessed 31 Jan. 2020].

Kolar, J., Malešič, J., Kočar, D., Strlič, M., De Bruin, G. and Koleša, D., 2012. Characterisation of Paper Containing Iron Gall Ink Using Size Exclusion Chromatography. Polymer Degradation and Stability, [online] 97(11), pp.2212–2216. Available at: <https://doi.org/10.1016/j.polymdegradstab.2012.08.005> [Accessed 31 Jan. 2020].

Kwaambwa, H., 2013. A Review of Current and Future Challenges in Paints and Coatings Chemistry. Progress Multidisciplinary Research Journal , [online] 3(1), pp.75–101. Available at: <http://citeseerx.ist.psu.edu/viewdoc/download?doi=10.1.1.1015.3398&rep=rep1&type=pdf> [Accessed 31 Jan. 2020].

Saverwyns, S., Currie, C. and Lamas-Delgado, E., 2018. Macro X-ray Fluorescence Scanning (MA-XRF) as Tool in the Authentication of Paintings. Microchemical Journal, [online] 137, pp.139–147. Available at: <https://doi.org/10.1016/j.microc.2017.10.008> [Accessed 31 Jan. 2020].

Targowski, P., Pronobis-Gajdzis, M., Surmak, A., Iwanicka, M., Kaszewska, E.A. and Sylwestrzak, M., 2015. The Application of Macro-X-ray Fluorescence and Optical Coherence Tomography for Examination of Parchment Manuscripts. Studies in Conservation, [online] 60(sup1), pp.167–177. Available at: < https://doi.org/10.1179/0039363015Z.000000000221> [Accessed 31 Jan. 2020].