Where sampling is possible, analytical techniques employing large instruments allow high-resolution chemical imaging of paint constituents. This lecture explained the principles of these analytical methods and their added value compared with portable instruments. They provide data that enable us to better understand the history of painting techniques, as well as pigment alteration phenomena that modify the color of works of art over the long term.
Analytical techniques based on very large-scale research infrastructures (TGIR) such as synchrotron radiation and nuclear reactors enable us to analyze the constituent elements of a painting and produce high-resolution chemical images. The brightness of the beams, their flux and the possibility of selecting their energy enable very precise, highly spatially resolved measurements to be taken, thanks to the use of dedicated optics. Over the last 15 years, this has enabled us to move on from point-based analysis methods to incomparably sensitive 2 or 3-dimensional analytical imaging methods. X-ray fluorescence spectrometry and neutron activation reveal the distribution map of certain chemical elements across the entire surface of a work of art. The documents obtained show certain modifications in the composition of the painting during the creative process (change of composition, repentance, superimposition of several paintings, etc.).
X-ray diffraction revealed the complexity of a pigment generally considered to be well known: lead white, also known as ceruse. Most often, it turned out to be made up of two lead carbonates, cerussite and hydrocerussite, whose proportions varied according to the synthesis methods used to prepare it. On the same work by Leonardo da Vinci(La Vierge à l'Enfant, avec Sainte Anne, Musée du Louvre) or Matthias Grünewald(Le altable Issenheim, Musée de Colmar), it has been demonstrated that artists chose one or other of these qualities to realize their projects. The crystalline structure of hydrocerusite, which leads to the formation of sub-micrometric, platelet-shaped crystals, plays an important role in the interaction properties between pigment and light.
