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San Cristoforo", Giulio Benso (1592-1668), 74x90 cm oil on canvas (162 cm), from the collection of Professor Antonio Ceci (1920). Right: Flax fibers from a painting covered with layers of different materials (Glue, plaster, paint, etc.) Observed by scanning electron microscope. © Anthony MAGUERESSE and Alessia MELELLI

The use of renewable plant resources (such as flax fibers) to design durable materials is an important milestone in the transition to a bioeconomy based on the rational use of biomass. However, compared with materials derived from petroleum resources, these new ecological materials often face distrust of their strength and durability. Research on extreme aging has brought time-scale issues to scientists, who are often forced to adopt artificially accelerated processes in the laboratory, at the risk of deviating from reality.

Flax is a plant that has been cultivated for thousands of years and used to make various objects, especially canvas. This is why the research team studied the canvases of four Baroque paintings exhibited at the Civic Art Museum of Ascoli Piceno from the 17th to the 18th centuries in the Baroque period, and traced their preservation methods. This work is in addition to the work on analyzing 4000-year-old Egyptian mortuary linens published in Nature Plants in September. This work was conducted by the same French team with the Louvre Museum, FEMTO-ST (Besancon), and LMGC. (Montpellier) Cooperating with the University of Cambridge.

One of the challenges of this research is the limitations associated with the collection and analysis of samples that must preserve historical paintings, as any samples obtained from heritage objects must be returned. Take 4 square centimeters from the area on the back of each canvas that is not visible and does not reduce the quality of the work.

Subsequently, the researchers used several very high-resolution optical analysis methods to study the fiber structure of the samples without degrading them. They used an atomic force microscope [1] to map the surface of the sample and the stiffness of the nanofibers. They also used two-photon microscope [2], nuclear magnetic resonance spectroscopy [3] and infrared spectroscopy [4] to analyze the organization, biochemical evolution and structure of the flax fibers that make up the fabric.

Contrary to intuition, the results show that the fibers of the fabric hundreds of years ago degraded more significantly than the fibers of the ancient Egyptian mortuary linen. Compared with modern flax yarns, almost all ancient fibers have universal stiffness. However, the yarns in the painting also exhibit other structural defects, such as fiber fragments due to oxidation, and fungal attack that severely destroys the fiber structure by forming "tunnels" and breaks in the fibers.

In terms of aging, the fibers of these paintings have been in a state of tension, but before they were acquired by the Civic Art Museum, they were also exposed to changes in humidity and temperature and air pollution during exhibitions in churches or private collections. In addition to these factors The role of other ageing agents such as glue, plaster and paint can make them more similar to new-generation objects such as composite materials, in which several materials coexist-usually with very different properties-to form new material combinations and improve the performance of the original material. Scientists have proved that certain protection techniques applied in the 19th century, such as a glue lining made of flour and animal glue, caused the long-term degradation of these paintings. In the final analysis, paintings that have undergone the greatest environmental changes or that have been processed during preservation are the most degraded.

This work provides valuable information about the evolution of natural fiber behavior and properties. It can be used to design durable and robust ecological materials, and it can also provide valuable information about the protection status of historical relics. Although this work focuses on flax, the method is also applicable to other fibers used historically, including hemp used for ropes and sails. The team is now studying flax textiles from other periods and origins. In 2022, the French National Research Agency (ANR) ANUBIS project will be launched, led by IRDL, in collaboration with SOLEIL Synchrotron, University of Cambridge, Egyptian Museum in Cairo, and INRAE.

[1] The atomic force microscope is based on scanning the surface of the sample to be analyzed with a very fine tip of a few microns in diameter. After extensive computer processing of the signal, the morphology of the sample can be determined.

[2] Two-photon microscopy is an optical imaging technique that combines fluorescence to perform non-destructive imaging of samples with a depth of about one millimeter. It allows visualization of the structure and internal orientation of the cellulose chains that provide the tensile strength of the flax fibers.

[3] NMR spectroscopy is based on nuclear magnetic resonance, which is the characteristic of certain atomic nuclei that release energy (ie, relax) after being exposed to electromagnetic radiation. The restored relaxation characteristics correspond to a very precise frequency, which can be used to describe the composition and tissue state of the molecules in the sample under certain conditions.

[4] Infrared spectroscopy is a non-destructive method that can work in the infrared region of the electromagnetic spectrum. This technique is used to indirectly identify the chemical composition of samples.

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