Development of non-linear optical microscopy on skin-based materials
Non-linear optical microscopy or multiphoton microscopy is an optical technique widely developed in the biomedical field, but relatively new and promising for the analysis of heritage materials. This technique enables non-invasive three-dimensional imaging with micrometric resolution. It has the advantage of being able to combine several contrast modes, including two-photon excited fluorescence and second harmonic generation (SHG). The latter signal comes from non-centrosymmetric structures (e.g. cellulose, plaster, fibrillar collagen).
Collaborative research carried out by the Conservation Research Centre (CRC) and the Laboratory for Optics and Biosciences (LOB) at the Ecole Polytechnique has highlighted the great potential of non-linear optical microscopy (NLO) for studying heritage materials, particularly those made of skin. This technique provides access to morphological information on a microscopic scale and enables the state of conservation of collagen in these materials to be assessed. Thanks to the correlative approach developed between multiphoton microscopy and nano-infrared spectroscopy, it has been possible to identify the chemical signature associated with the signal changes in NLO microscopy during collagen degradation. More recently, a method for quantifying alteration in parchment based on the measurement of polarisation-resolved SHG signals has been developed to detect the early stages of parchment degradation.
Since 2021, the doctoral research project CaraColl - Characterisation and monitoring of physico-chemical changes in collagen during the deterioration of parchment - focuses on understanding NLO microscopy signals associated with the deterioration of parchment. As part of the Equipex+ ESPADON project, a non-linear optical microscope will be installed at the CRCC in 2024. The instrument, unique in the field, will be optimised for the analysis of heritage materials and open to the community.
Material study of ancient manuscripts from the Mont Saint-Michel
The Mont Saint-Michel abbey in Normandy, France, was a major pilgrimage site in Christianity, and this monastery was also a major spiritual, cultural and intellectual center. Its scriptorium produced numerous manuscripts between the end of the 10th and the middle of the 13th century, which are today conserved for the major part in the patrimonial library of Avranches in Normandy, as well as in other French libraries and archives (Paris, Rouen, Bordeaux, Caen) or abroad (Berlin, London, New York, Vatican…). Since 2017, more than one hundred of them are accessible online on the virtual library of Mont Saint-Michel (https://emmsm.unicaen.fr/).
This exceptional collection constitutes a testimony of the writing and illuminating practices over a large period of time, for that reason a research project was started in 2018 to characterize the different constitutive materials, namely the parchment, the coloring materials and the inks, in order to retrace the copyists and the artists practices. The present research study focuses on manuscripts from the Romanesque period (end of 10th – 12th century), in particular those dated before 1100, and was initiated on the fifty-four manuscripts conserved at the Avranches library. Thanks to the support from the Ministry of Culture and Patrimex (FSP) analytical campaigns could be carried out on site at the library and six manuscripts be brought to Paris to study the complex illuminations by imaging techniques. A documentary was produced of these measurement campaigns at the Avranches library (link here). The research project is currently being extended to Mont Saint-Michel manuscripts from the 11th century conserved in other collections in France and abroad.
The results of this material study will both confirm and complete the attributions made on the basis of the codicological study of the manuscripts, and provide markers for the manuscripts produced in this scriptorium during this period. In addition, the study will aim to gain a better understanding of the deterioration observed in certain pigments, so that these decorations can be better preserved. The data gathered on the various materials will enrich the manuscript records in the Mont Saint-Michel Virtual Library, and will serve as a reference for the scientific community when undertaking comparative approaches with other Norman manuscript collections. Information are also being incorporated into the new exhibiting area of the manuscripts at the Avranches Scriptorial.
Gilt leathers – Study of the provenance and degradation of the decors
The ancestors of our wallpapers, gilt leathers, also known as Cordoba leathers, were very popular in Europe between the 16th and 18th centuries. They were mainly used as tapestries or wall hangings in hotels, castles and large middle-class homes. Despite their name, gold is not used in the manufacturing process of these decors, instead the golden colour is obtained by the application of a yellow varnish over a silver leaf glued to the leather. Today, it is still very difficult to date gilt leather decorations and attribute them to a particular workshop, as they are rarely signed and only their stylistic characteristics allow us to speculate on their origin.
The work carried out by the CRC in collaboration with two gilt leather specialists, conservator Céline Bonnot-Diconne and art historian Jean-Pierre Fournet, as well as other research laboratories, led to analyse the different constitutive materials making up these decorations. Comparison of physico-chemical and stylistic data is carried out on a corpus of gilt leathers, in order to identify physical or chemical markers that can be used to attribute these decorations to a particular workshop or region. The CORDOBA project (FSP 2014-2015) focused on the physico-chemical analysis of the silver leaf within these decorations using ion beams (PIXE and RBS) on the AGLAE accelerator (C2RMF). These analyses made it possible to determine the chemical composition of the leaves as well as their thickness, in order to gain a better understanding of the raw materials, as well as the impact of the manufacturing stages of these decors on the silver leaves. Studies are currently continuing on the other constituent materials, in particular the varnish and the protective layer on these decorations, as well as the leather.
Tarnishing, or even blackening, of the silver leaf on gilt leather hangings is a recurring problem in collections, and mainly affects the so-called "silvered" areas, which have only a very thin organic protective layer on the surface. This deterioration can be limited to superficially small areas, but can also lead to a general darkening of the entire decor, which profoundly affects the legibility of the work and harms the aesthetics of the whole. The CORD'ARGENT project (FSP 2015-2016) was set out to gain a better understanding of the degradation processes of the silver leaf within these decorations and to identify the various factors that play a major role in their appearance, in particular climatic conditions, the constitutive materials or the restoration treatments.
