The ENBRUNI project is in line with the current challenges of French and European research by being at the interface of several fields structuring the action plan of the APP ANR 2023. The team is composed of three UMRs and one UAR in the humanities and materials sciences, and is associated with professionals in the field of conservation-restoration of built heritage, whose complementary skills are in line with the interdisciplinarity of the project. It also relies on the metal working group (GT Metal) of the CNRS/Ministry of Culture scientific project created after the fire at Notre-Dame Cathedral in Paris. The impact of climate change on heritage buildings is at the heart of the concern of heritage conservation actors. Lead metal, which covers or adorns many buildings, has taken on a dark-red staining colour in recent decades that differs significantly from its usual grey appearance. The formation of plattnerite, a lead dioxide, is responsible for this, without modifying the corrosion resistance properties of this metal. However, it remains poorly understood. The problematic of the ENBRUNI project is therefore built from a concrete field observation. This research aims to contribute to the understanding of the appearance of the phenomenon of the dark-red staining on lead used in historical monuments. It is articulated in three axes. The first axis is devoted to the establishment of a thorough report on the emergence of this phenomenon: from the collection of archival data (photographs and climatic data), the aims is to compare the evolution of the dark-red staining on the national buildings, according to the uses of lead (rooftop, sculpture, geographical orientation), with the environmental changes (evolution of pollutants such as ozone, the dioxide of sulphur, the dioxide of nitrogen, the pluviometry, the temperature...) during the last thirty years. The second axis aims at characterizing the mechanisms of dark-red staining formation by the implementation of ageing in climatic chambers (sulphur dioxide, nitrogen dioxide, ozone) and multi-scale studies of real samples taken from a selection of buildings under restoration works (Saint-Pierre de Beauvais cathedral, Notre-Dame de l'Assomption cathedral of Clermont-Ferrand, the Sainte-Chapelle, Notre-Dame de Paris cathedral...). Finally, the third axis will be dedicated to the communication of the conclusions of this research. Indeed, in a context where the lead issue is essentially based on public health injunctions, the results obtained will contribute to an update of the knowledge acquired so far on the ageing of this material used for the roofing and the decorations of prestigious buildings of our heritage.

Our ambition is to create the foundations of spectral metrology by: - A robust and accurate measure of the distance between two reflectance or radiance spectra, - The characterisation of the spatial variations of reflectance spectra acquired from a coloured surface perceived as homogeneous but which is spectrally not uniform. The necessity to control the stability and the accuracy of such optical measures is always required in civil industry, e.g. in military, Aerospatiale, medical applications, or in the valorisation of the cultural heritage. There is no metrology solution corresponding to this need while the use of spectral sensor increases. This lack is related to the problem complexity: measure processing is related to Digital Sciences, but the validation and interpretation comes from Physics. DigiPi proposes to break this boundary by combining two laboratories, one expert in digital processing of colour and the other one expert in the characterisation of complex heterogeneous materials and modelling their optical properties. As part of this bi-disciplinary approach, we will produce new expressions of similarity/distance measure between spectra embedding constraints of robustness, genericity and uncertainty reduction. Our contribution will integrate the mathematical specificity of the spectrum, which is not a vector or a probability density function, but closer to a series or a function. Another contribution of DigiPi will decompose the distance measure in subparts allowing straightforward interpretations well reflecting the spectral variability. There is no metrology without standards or references. We will create surface ranges ordered by the change in different manufacturing settings. The characterisation by various methods (profilometry, microscopy, spectrophotometry ...), and physical modelling of the optical properties of the paint layers will allow us to better understand the spectral variability of a pigment according to the physical and chemical characteristics. In particular, we will adapt existing models to embed parameters never previously integrated. All this knowledge will allow us to measure the rank correlation between the spectral distance measurements and the data characterisation or parameter models to obtain a complete validation. These surfaces and reference data will allow the development of this metrology beyond DigiPi. The second objective is to produce digital attributes characterising non-uniformity colour from hyperspectral images. This objective is related to the analysis of the micro-textured surface appearance. Our scientific contribution will be to define digital attributes that must be metrologically valid in the physical sense (spectral aspect) and that can be correlated with models of human vision (colour and subjective aspect). The digital attributes will be compared using rank correlations to morphological and chemical parameters extracted from the physical models describing the paint layers of reference surfaces. In parallel, fractal image models coming from the work of the CIE TC8-14 will be used to assess the uncertainty stability whatever is the spatio-chromatic content. Distance/similarity measures between spectra and attributes characterising the non-uniform appearance correspond to industrial, economic, medical, environmental ... requirements. The last part of DigiPi will establish the performance gain in two use cases: one related to industrial quality control in production of colour products and the other in cultural heritage context to characterise colour shading-off in a wide collection of royal vellums.

The POSTFIRE project addresses the vulnerability and resilience of urban systems, with the main objective of facilitating the recovery of cultural heritage buildings after fires. The project will investigate the behaviour of historic stone masonry after high temperature exposure at the material and structural scale, accounting for the effect of water quenching. The project will investigate the relations between microscopic phenomena and property changes; it will create a property database for the selected materials in post-fire conditions, and material models of immediate applicability in analytical and computer-based methods for structural performance assessment and calculation. The reliability of the database and models will be tested at the structural scale. Finally, the project will propose post-fire assessment guidelines for heritage buildings of France, to extend the benefits of the project among academy, professionals, and national and international normalisation committees.

The interdisciplinary AORUM project aims to study gold as a painting material in artistic practices in Western Europe between the 16th and 17th centuries, a period when it is usually thought gold is no more used in artistic pratices of painting. Its ambition is to gather an original corpus of artworks and to analyse it from a threefold perspective (historical, technical and optical). Indeed, contrary to what the prescriptions of 15th and 16th century painting theoreticians (e.g. Alberti and Vasari) would suggest, gold continued to be used in the 16th and later. This is evidenced by some paintings of such famous artists as Raphael, Dosso Dossi, Frans Floris, Rubens, Rembrandt or Vermeer. There are many other examples, yet gold is the one material that is largely absent from the work on early modern European painting. The objectives of the AORUM project are precisely to bring this largely ignored corpus out of the shadows; to analyse it according to the usual questions of art history (iconography, social history, history of taste, etc) ; to study, in an interdisciplinary approach (Technical art history – physical-chemistry of Heritage artifacts), the history of the gilding techniques; to analyse the optical properties of the different gilding techniques, the optical effects generated with regard to historical and current lighting, again in an interdisciplinary approach (art history - physics); finally, to manage all the data in such a way as to contribute to the EquipEx+ ESPADON, and to valorize these data and the results for art historians, curators, restorers, and the general public.

Although apparently simple, transparency is a highly complex coloration strategy. While abundant in water, transparency is nearly absent on land, with the exception of insect wings. Research effort has been devoted to water transparency, leaving transparency on land – a specific question since optical requirements largely differ in air and water – virtually unstudied. Rare studies have shown diverse structural solutions, poorly known physical properties, unstudied functions and evolution. Understanding transparency cannot be approached without an interdisciplinary perspective and a coherent grasp of both transparency – as a physical phenomenon – and organisms – in their eco-evolutionary context. At the frontiers between biology and physics, this project will (i) characterize the diversity of structural solutions for transparency and their optical, thermal and hydrophobic properties, (ii) test the adaptive functions of transparency for camouflage and communication, and (iii) decipher the processes driving its evolution at large scale. We will work on Lepidoptera, insects with wings covered with scales, a key multifunctional (coloration, thermoregulation, flight, water repellency) innovation abandoned by many species which are transparent. This raises questions about how vital functions are ensured, the functions and evolution of transparency. Objective 1. We will characterize (i) clearwing nanostructural diversity with SEM, TEM, and photonic imaging, (ii) transmission and anti-reflective properties with scatterometry and UV-visible spectrometry, (iii) thermal and hydrophobic properties with IR spectrometry and water contact angles. Physical investigation will help assess the trade-offs between properties, the robustness of global effects to variations in structures, and identify the key elements responsible for the different functions. Objective 2. We will evaluate the roles of transparency in communication and camouflage. Contrasting with the classical view of transparency as exclusively serving camouflage, we change paradigm and hypothesize that transparency serves both camouflage and communication, through iridescence or polarization patterns which can function as private signals. (i) We will determine detectability of transparent prey by birds in experiments in controlled conditions, compare survival values of artificial prey differing in transparent/opaque patterns in field experiments, and examine cognitive aspects with computer games. (ii) With imaging, scatterometry and spectrometry, we will test in comparative analyses whether polarization and iridescence patterns serve communication in sympatric similar species. Objective 3. Transparency has impacts at macroevolutionary level. With comparative analyses, we will (i) reconstruct the evolution of transparency at large scale and examine structure-function relationships, (ii) test whether transparency – which can produce diverse signals for communication beyond a common protective appearance – can promote species diversification and co-evolution of butterfly vision. Bridging the gap between disciplines, this project will bring significant advances to a crucial lack of knowledge: physics will provide thorough quantitative measurements indispensable to answer biological questions; biology will offer an evolutionary framework needed to understand multifunctional photonics, to characterize solutions for invisibility and understand robustness to disorder, promising lines of research. Project feasibility is ensured by the complementary expertise of the biologists D. Gomez and M. Elias, specialists of communication, community ecology and phylogeny, and the physicists S. Berthier and C. Andraud, specialists of optics and physics. Project originality and novelty guarantees highly-valuable results we will disseminate to scientists, trainees, and the general public through a large range of actions.