Revealing the secrets of ancient and heritage materials (L'Edition #12)
(Article from L'Edition N°12 - march 2020)
Ingenious imaging and spectroscopy techniques are helping researchers probe the secrets of matter and obtain more detailed characterisations of materials in the cultural heritage, archeological, paleo-environmental and paleontological fields.
In the 19th century, Friedrich Nietzsche once said that it is only possible to truly understand and interpret a work of art in progressive stages. Little did he know what the 21st century would bring! Today’s methods – synchotron, electronic microscopy, multispectral imaging and dating methods – are all pushing back the limits of the possible and lifting the veil on the innermost secrets of artworks, including how they were made.
This is a job for IPANEMA! The European Institute for the Non-Destructive Photon-Based Analysis of Ancient Materials (IPANEMA – Université Paris-Saclay, CNRS, Ministère de la Culture, UVSQ) funds the development of advanced techniques and tools for the study of samples in 2D or 3D as well as statistical analysis of datasets. It also makes these assets available to the ancient and heritage materials community. IPANEMA believes that interdisciplinarity “enriches research projects through cross-fertilisation,” says Loïc Bertrand, the lab’s founder and coordinator of DIM MAP, a large, interdisciplinary network dedicated to the study of ancient and heritage materials.
IPANEMA’s researchers have often figured as trailblazers. Recently, the team worked on textile samples from the Louvre and the Musée du Quai Branly that were in an exceptional state of preservation, having been in contact with objects of corroded copper thousands of years old. By performing statistical analysis on tomographical images, they were able to identify the different phases of the material to better understand the origin of this phenomenon. This was a first!
The SOLEIL synchrotron and IPANEMA are collaborating on a project called MATRIXS4H, or “Massive tabular array for inelastic X-Ray scattering for Heritage”. Now that IPANEMA has completed the proof of concept on specimens from the French museum of natural history and on painter’s pigments, the plan is to install, on the synchrotron’s Galaxies beamline, an innovative system to analyse ancient materials by means of inelastic hard X-ray scattering. The new system, currently in the process of assembly, is to come on stream in mid-2020.
“While it is difficult to study hybrid organic or inorganic materials using imaging techniques, the power of the latter is extremely valuable to the researcher,” emphasises Loïc Bertrand. “Very dynamic, they offer the high spatial resolution needed to distinguish the heterogeneity of the materials under scrutiny. For instance, to get a good understanding of degradation mechanisms, one must study the laws governing them on different scales, i.e. millimetric, micrometric and nanometric.”
Determining the mechanical and chemical heterogeneity of ancient materials
This objective will be well served by the future platform known as MUSIICS (which stands for “Multiscale InfraRed Imaging platform for Complex Systems”), headed by Alexandre Dazzi from the Chemistry and Physics Laboratory (LCP – Université Paris-Saclay, CNRS) and Mathieu Toury from IPANEMA, with support from DIM MAP. The plan is to add a multiscale dimension to the LCP’s existing infrared (IR) nanoscope platform to study the chemical and mechanical heterogeneity of materials at the macro, micro and nano level. All in all, three new IR spectromicroscopy instruments will be added.
The name of this novel technology is atomic force microscope infrared spectroscopy. AFM-IR was developed at the LCP under the supervision of Alexandre Dazzi who notes that “conventional IR microscopy offers a maximum resolution of about a dozen micrometers (μm) whereas AFM-IR achieves a resolution of several nanometers (nm). It is so sensitive that one can study molecular deposits only 1 nm thick.” Needless to say, this level of performance has not gone unnoticed by members of the ancient and heritage materials community!
So far, the team has examined textile fibers 5,000 years old, ancient musical instruments and the pigments used in Mondrian’s paintings. Each sample represents a new challenge. “The instrument used to perform AFM-IR consists of a miniscule lever equipped with a fine tip that registers the object’s surface and delivers infrared light spectra. This lever, highly sensitive to the mechanical interaction between the tip and the object’s surface, registers how hard or soft it is. However, sometimes the mechanical variations are so heterogeneous that they cloud the results. Going forward, the protocols will have to be optimised to ensure imaging success.”
One of the platform’s future acquisitions is a new microscope for imaging in “peak force” mode. The idea is to control the force with which the lever interacts with the surface and do away with mechanical interaction problems. “We will be the first in the world to use this model, which the Bruker company will put on the market in 2020,” declares Alexandre Dazzi. “With a system like this, it will also be possible to scan samples presenting a large surface area.”
Innovative dating with iron and carbon
Lucile Beck and Philippe Dillmann are experts at revealing the innermost secrets of ancient materials. The former works at the Laboratory for the Measurement of Carbon 14 (LMC14), the latter at the Archeomaterials and Alteration Prediction Laboratory (LAPA), both part of the Nanosciences and Innovation for Materials, Biomedicine and Energy Unit, a.k.a. NIMBE (Université Paris-Saclay, CNRS, CEA). At the LMC14 lab, the team develops techniques to determine the age of materials that, in theory, cannot be dated using carbon 14. This method, introduced in the 1950s, measures the residual radiological activity of the radioactive isotope of carbon (14C) present in samples of organic origin. “Since the 1980s, we’ve been measuring the 14C content using accelerator mass spectrometry, like in the laboratory,” says Lucile Beck. Once extracted, the carbon is reduced to graphite and put through the particle accelerator, which separates the different isotopes of carbon and counts the carbon 14 atoms. “This is the conventional method used to date wood, charcoal, paper, wool, ivory or the shells of Foraminifera,” but not metal.
Working hand in hand with the LAPA lab, the team devised an unprecedented absolute dating process for iron and steel. “First, we had to identify the method used to produce steel from iron at the time. It was done in a charcoal-fired kiln at high temperature (1,200°C),” recalls Philippe Dillmann. “Due to the high temperature, the carbon in the charcoal was diffused into the metal in the form of carbon dioxide (CO2) and carbon monoxide (CO), and then remained trapped there in the form of iron carbide.” Researchers were able to extract the carbon from the metal and date the tree that yielded the charcoal. In this way, indirectly, they could establish the age of the metal. “We proved that the tie-rods and reinforcements found in the choir of Beauvais Cathedral, Sainte Chapelle in Paris and Amiens Cathedral had always been part of their respective edifice. These elements date back to the time of construction and were not added subsequently,” comments Philippe Dillmann.
Transcending the limits of carbon 14 dating
Recently, Lucile Beck has been working on dating the pigments based on lead carbonate (e.g. white lead), an ingredient in 16th century cosmetics used to whiten the face of ladies at court as well as in pigments in 19th century history paintings. “It was long thought that white lead could not be dated using carbon 14, considered to be like blanc de Meudon, a mineral pigment,” she indicates. Then she got an idea when reading the earliest recipes for the synthesis of white lead. “In the 4th century, they positioned metallic lead above a recipient filled with vinegar and surrounded it with organic matter, such as horse manure or grapevine stalks, then left the whole business for a few weeks in a confined space. This yielded a white powder that was washed, then ground to make pigments or cosmetics.” Eureka! As it fermented, the organic manure released CO2, which reacted with the metallic lead to form lead carbonate. “Since the carbon 14 signature is in the carbon of the lead carbonate, it’s possible to date white lead!”
The team is currently focusing on the white pigments present in Renaissance paintings. “Often, these pigments are a mix of white lead, blanc de Meudon and a binder. Our task is to isolate and extract each ingredient separately” by means of a particular thermal method. “When one heats the mix to 400°C, the white lead and only the white lead decomposes into CO2. The decomposition of the blanc de Meudon requires a temperature of 600-700°C.” Once recovered, the CO2 is subjected to the standard protocol for analysis by accelerator mass spectrometry.
“Today, the minimal mass of graphite must be between 0.2 and 1 mg in order to obtain a coherent measurement. In other words, one must start out with a quantity of between 5 and 20 mg of matter, which is not possible with precious or limited materials.” The PATRIC14 project funded by the DIM MAP network aims to lower this threshold by using a new injection source by gas.
“The CO2 produced by heating the sample is injected directly into the accelerator.” This source, expected to be operational by year-end 2020, will make it possible to skip the graphitisation stage and subsequent manipulations.
Publications
∙ Beck L. et al., Thermal Decomposition of Lead White for Radiocarbon Dating of Paintings, Radiocarbon 61, 2019, pp 1345 – 1356.
∙ Bertrand L. et al., Synchrotron-Based Phase Mapping in Corroded Metals: Insights from Early Copper-Base Artifacts. Anal. Chem. 2019, 91, 1815−1825.
∙ Dazzi A. et al., AFM-IR: Technology and Applications in Nanoscale Infrared Spectroscopy and Chemical Imaging. Chem. Rev. 117:5146–5173 (2017).