Watching Ancient Paintings through Synchrotron-Based X-Ray Microscopes
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Watching Ancient Paintings through Synchrotron-Based X-Ray Microscopes M. Cotte and J. Susini Even if attitudes are changing, our study of works of art using synchrotron instruments is usually perceived to be exotic. To the community of curators and conservators, synchrotron instruments can appear as enormous, and sometimes frightening, exotic machines. Likewise, for the community of chemists and physicists working with synchrotron sources, works of art can be seen as complex and exotic samples. However, these two communities are finding an increasing interest in collaborating. On the one hand, scientific analyses aim to complement historical knowledge and assist curators in the future preservation of artworks. On the other hand, museum objects will certainly become more and more appealing in materials research as they can provide researchers with examples of very longterm evolutions, hardly reproducible at the laboratory scale. Paintings, and more generally all cultural heritage objects, have inestimable value, not only from artistic, aesthetic, and historic points of view, but also because they give us a unique testimony to the technical, physical, and chemical practices through the ages. Some materials, such as natural pigments, may intrinsically retain information about their geographical origins, and can be used to identify the trade routes of the ancient world. In parallel, artificial materials used in arts and crafts provide evidence of the evolution of ancient technologies. Accessing the chemical formulation of paintings can be particularly rewarding, providing us with new insight into techniques and knowledge developed by the artists and artisans years, decades, or even centuries ago. Long-term changes in the chemical composition of materials encompass another important area where detailed analysis is required. The understanding of degradation processes is essential to enable curators and conservators to identify the best techniques to use to preserve a given object. From the chemistry perspective, paintings can be seen as the archetype of puzzling materials. They often present a complex multilayered structure, with layer sizes ranging from a half millimeter to some micrometers. These layers can be non-uniform, containing pigment grains of a few micrometers. As already mentioned, these objects are particularly precious. Hence, non-invasive techniques are usually favored for analysis. When we are allowed to acquire a sample, in particular for a deeper analysis
of the entire stratigraphy, the fragment size is generally only a few hundred micrometers. After embedding the fragment in resin, we polish it perpendicularly to the painting surface, to reveal the whole multilayered structure (see Figure 1c). Various analyses can then be carried out, with different nondestructive microanalytical techniques.
From the chemistry perspective, paintings can be seen as the archetype of puzzling materials. Paintings are a complex mixture of tens of ingredients: inorganic and crystallized products, such as pigments and pla
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