Philippe Walter

Clément Sanchez

Laurence De Viguerie

Ludovic Bellot-Gurlet

Corinne Chanéac

Christian Bonhomme

Thibaud Coradin

Frédéric Lemaître

Nathalie Capron

Kamal Boubekeur

Aymeric Spiga

Beaucoup À Marie

Agathe Pierre

Valentina Graycy

Silvia Clara

Roger-Charles Sophie

Elsa, Rémi, Thiéry Muriel Agnès

" Du Côté Du

Zeinab, Chirine, Maxime, Estelle Laetitia Laura

Lionel

Anne-Déborah Chellda

Paul

Michael

Yvain Audrey

Kamel Ainsi

Agnès, Joël, Laurent, Clémence Suzanne Camille

Nobu, Matthias Marion

Claire Sophie

Short abstract

Mediums were used by painters in order to modify the texture and drying properties of their paint. During the 19th century, British artists developed a particular medium made of siccative oil, mastic resin and lead acetate trihydrate. The so-called "gumtions" form gel-like materials in a relatively short time, outperforming the existing paint media. This thesis contributes unveiling the chemical processes involved in the formation and ageing of gumtions.

As a first step, we focused on mastic resin since it is a key component for the preparation of gumtion. The triterpenic fraction of the resin was identified and quantified using GC and GC/MS. Moreover, we took advantage of Spectroscopic Ellipsometry so as to study the optical properties of varnish thin films as well as their behavior (swelling) under various atmospheres.

Then, we reproduced historical recipes that helped us afterwards to define simplified formulations to deepen the understanding of the chemical interactions between the gel components, made of oleanolic acid (commercial triterpenoid) and a lead compound (acetate or oxide). They were investigated at di erent scales by spectroscopic (FTIR, MAS-NMR) and supramolecular analyses (Cryo-TEM, SAXS). The use of these complementary techniques gives an overview of the gel's structure and formation: rapidly, a coordination complex is formed between lead and the carboxylic acid moieties of the triterpenoids, that organize into 2D objects leading to the solid-like behavior of the material. After few months ageing, we observed the self-assembly of crystalline nanoparticles into lamellar structures, witnessing the dynamic occurring in the material even after gelation.

Résumé court

Les médiums étaient utilisés par les peintres afin de modifier la texture et le séchage de leur peinture. Au 19ème siècle, des artistes britanniques ont développé un médium composé d'huile siccative, de résine mastic et d'acétate de plomb trihydraté : le « gumtion ». Ce matériau de type gel surpasse les additifs alors existants. Dans cette thèse, nous contribuons à la compréhension des processus chimiques impliqués dans la formation et le vieillissement des gumtions.

Dans un premier temps, nous avons centré l'étude sur la résine mastic, car il s'agit d'un élément clé dans la préparation des gels. La fraction triterpénique de la résine a été identifiée et quantifiée par GC/MS. De plus, nous avons étudié par Ellipsométrie les propriétés optiques de vernis sous forme de films minces, ainsi que leur comportement (gonflement) sous di érentes atmosphères.

Puis, en reproduisant des recettes historiques et afin d'approfondir la compréhension des interactions chimiques entre les composants du gel, nous avons développé des formulations simplifiées à base d'acide oléanolique (triterpène commercial) et d'un composé de plomb (acétate ou oxyde). L'utilisation de techniques d'analyses complémentaires aux échelles moléculaire (IR, MAS-RMN) et supramoléculaire (cryo-TEM, SAXS) indique dans un premier temps la formation d'un complexe de coordination entre le plomb et les fonctions acides des triterpénoïdes, qui s'arrangent en objets 2D expliquant le comportement viscoélastique du matériau. Après plusieurs mois de vieillissement, nous avons observé l'auto-organisation de nanoparticules cristallines en en lamelles, témoignant du caractère dynamique de ce matériau même avec gélification.

Résumé

Dans cette thèse, nous nous intéressons aux "gumtions", des gels hybrides ajoutés à la peinture à l'huile au 19ème siècle (médiums). Ces matériaux de type gel sont composés d'huile siccative, de résine mastic et d'acétate de plomb trihydraté (Pb(CH 3 COO) 2 •3H 2 O).

Très utilisés à cette époque et notamment en Angleterre, ils sont probablement à l'origine de la dégradation majeure de certaines oeuvres picturales (noircissement, craquelures).

Pourtant ils n'ont été que très peu étudiés du point de vue physico-chimique. Ce travail a pour but de caractériser la structure de ces gels ainsi que les processus chimiques mis en jeu lors de leur formation et de leur vieillissement.

Pour ce faire, après avoir décrit les matériaux étudiés ainsi que leur utilisation en peinture (Chapitre I), nous déterminons la composition chimique de la résine mastic, ainsi que les propriétés optiques et de gonflement de films de vernis mastic (Chapitre II). Puis, grâce à une étape de reconstitution de recettes historiques de gumtions, nous préparons des formulations simplifiées appelées "Systèmes Modèles" afin de mieux comprendre les interactions entre les di érents composés (Chapitre III). Enfin, l'analyse des gels à di érentes échelles, moléculaire et supramoléculaire, est présentée (Chapitre IV).

Etude de la résine et du vernis mastic

Le deuxième chapitre présente l'étude de la résine et du vernis mastic, composant essentiel des gumtions. La résine mastic est un composé naturel (exsudat du Pistacia lentiscus) composée principalement d'une fraction polymère (poly---myrcène) et de triterpénoïdes.

Dans une première partie, la composition de la fraction triterpénique de trois résines mastic commerciales (Kremer, Laverdure, Chios Mastiha Growers Association) et trois résines anciennes (plus de 150 ans, provenant de la collection du Muséum National d'Histoire Naturelle), ainsi que la proportion absolue des di érents triterpénoïdes ont été déterminées par des techniques chromatographiques (GC et GC/MS respectivement).

Les résultats obtenus indiquent tout d'abord une similitude de composition chimique entre les trois résines modernes (les variations mineures peuvent s'expliquer par l'origine de la résine, la période de récolte, les conditions de conservation. . . ). L'analyse quantitative quant à elle a permis de confirmer que les triterpénoïdes représentent 60 à 70% de la masse totale de la résine mastic, le reste étant principalement du polymère. La résine mastic est donc un mélange de nombreux triterpénoïdes dont les principaux sont l'(épi)lanostérol, la 28-nor-olean-12-èn-3-one, la --amyrone, la 28-nor-oléan-12,17-dièn-3-one, la --amyrin, la 28-nor-oléan-?,17-dièn-3-one, le 3-oxo-lup-20-ène, l'oléan-dièn-3-one, l'acide moronique, l'acide oléanonique, l'aldéhyde oléanonique, l'acide oléanolique, l'acide ursonique et les acides (iso)masticadiénoïques. Concernant les résines anciennes, seulement une très faible proportions de triterpénoïdes a pu être détectée et identifée (5 à 18% en masse) témoignant de l'auto-oxydation des composés avec le temps.

La seconde partie de ce chapitre présente l'utilisation de l'Ellipsométrie pour l'étude des propriétés optiques du vernis mastic sous forme de films minces (quelques centaines de nanomètres), ainsi que son comportement sous di érentes conditions atmosphériques. En plus du vernis mastic, l'étude a été étendue à di érents matériaux utilisés en peinture de chevalet (vernis dammar, huile de lin et blanc d'oeuf) ainsi qu'un vernis âgé d'environ 70 ans prélevé sur un tableau du 16ème siècle (La Transfiguration, Bellini, ca. 1550) validant ainsi l'utilisation de cette technique pour ce type de matériaux. Les films ont été déposés par dip-coating et analysés en mode statique et dynamique. Le premier mode a permis de déterminer les dispersions d'indice de réfraction en fonction de la longueur d'onde des vernis, qui sont comparés avec ceux donnés dans la littérature obtenus par d'autres techniques. Le second mode a permis d'évaluer le séchage des films, ainsi que leur gonflement sous humidité variable, vapeurs de solvants, exposition aux UV etc. Le vernis mastic a donc un indice de réfraction qui varie de 1.54 à 1.52 entre 400 et 1000 nm. Le gonflement des films de vernis mastic quant à lui dépend du solvant utilisé: l'épaisseur augmente de plus de 30% sous isopropanol, contre 15% sous isooctane, tandis que le gonflement est presque nul quand le film est exposé à 100% d'humidité.

Reconstructions de recettes historiques et élaboration des systèmes modèles

Le troisième chapitre répertorie les recettes historiques de gumtions qui ont été utilisées afin de reconstituer des médiums les plus proches possibles de ceux utilisés au 19ème siècle.

Cette étape de reconstitution nous a permis tout d'abord d'appréhender la formation des gels et également de tester l'influence de di érents paramètres (présence d'huile, présence du polymère de la résine, nature du composé de plomb, proportions) sur la gélification. Il ressort notamment de ces expériences que les composés déterminants pour la formation d'un gel sont les triterpénoïdes contenant une fonction acide carboxylique, et certains composés de plomb (acétate, oxydes). Le broyage des composants est également une étape clé qui conditionne la gélification ; les di érents tests e ectués (temps de broyage, exosition à l'air. . . ) indiquant l'implication de processus chimiques autres que l'augmentation de la

General introduction

Throughout the history of painting, artists have been experimenting and developing recipes to prepare innovative materials they could add to their paint, so as to adjust its consistency and optical aspects. The aim of this thesis is the characterization of "gumtion", a hybrid organic-inorganic gel, described by British oil painters manuals during the 19th century.

This material is composed of siccative oil, mastic resin and lead acetate. The so-called gumtions form gel-like materials in a relatively short time. They would provide, at least according to the manufacturers, all the properties required for the art of painting.

While the unique properties of gumtion gels were known in the 19th century, their formation mechanism and chemical structure has remained unclear. Over and above the fundamental aspects, the characterization of these materials is crucial to develop adapted strategies for the conservation of works of art. Here, a contribution is made through the elucidation of the chemical and structural processes involved in the formation, drying and ageing of gumtion gels.

In the first instance, careful consideration has been given to the mastic resin and varnish.

The molecular composition of natural mastic resins has been investigated by Gas Chromatography coupled with Mass Spectrometry (GC-MS). The main triterpenoids have been identified and quantified using the Internal Standard (I.S.) method. The behavior of mastic varnish in the form of films has been analyzed with Spectroscopic Ellipsometry (SE) under various atmospheric conditions. In particular, the e ect of exposure to water and organic solvents vapors on the optical and swelling properties of the films has been monitored.

This study was then expanded to other materials relevant for the study of oil painting.

Historically acurate reconstructions of gumtions were then made in order 1) to understand the behavior of these materials and the constraints faced by the artists when preparing the gumtions, and 2) to develop simplified formulations -called Model Systems in this study (MS) and based on our observations during reconstructions-to deepen the understanding of the material's chemistry, notably at the molecular level.

The last experimental chapter focuses on the multi-scale analyses of the MS gels. Rheological studies confirm their gel nature. Spectroscopic techniques (FTIR, 13 C and 207 Pb NMR) shed light on the molecular interactions involved in the gelation process. Structural analyses, including cryo-microscopies and Small-Angle X-Ray Scattering (SAXS), complete this study by providing information on the supramolecular organization occuring during gelation.

Chapter 1

Context

This first chapter deals with the existing literature on the topic. It is divided in two parts:

the first one describes the historical context and presents our system of interest from the point of view of technical art history. In the second part, we translate it into chemical terms, looking for analogues in the existing literature covering hybrid gels.

1 Historical context

Use of mediums

Paint is made of at least a pigment dispersed in a binder. Throughout the history of oil painting, artists have experimented and mixed additives (solvent, driers, mediums. . . )

to their paint in order to modify its optical properties, texture or drying time [START_REF] Simonot | Etude expérimentale et modélisation de la di usion de la lumière dans une couche de peinture colorée et translucide[END_REF][START_REF] De Viguerie | Revealing the sfumato technique of Leonardo da Vinci by X-Ray Fluorescence spectroscopy[END_REF]). For example, chalk (calcite CaCO 3 ) was employed by 15th century painters blended with lead white to obtain specific optical e ect, in particular transparency, as the refractive index of calcite is very close to the one of the linseed oil (around 1.48). It was also added to increase the viscosity of the paint and allow the artist to produce desired textural e ects, such as impasto [START_REF] De Viguerie | Re-interpretation of the Old Masters' practices through optical and rheological investigation: The presence of calcite[END_REF].

Depending on the period, di erents e ects were expected by the painters. If the recipes and the ingredients do not change much, their use varies, resulting in di erences in texture.

This allows the artists to tune the consistency of their paint to obtain the e ects desired.

For instance, in the early days of oil painting, the paint is smooth and liquid with good leveling properties [START_REF] De Viguerie | Revealing the sfumato technique of Leonardo da Vinci by X-Ray Fluorescence spectroscopy[END_REF]. As an example of this, one can cite the glazes (very diluted, lightly pigmented thin organic layers) used by Leonardo da Vinci for example to achieve the famous natural impression of depth and shadows [START_REF] De Viguerie | Revealing the sfumato technique of Leonardo da Vinci by X-Ray Fluorescence spectroscopy[END_REF].

From the late Renaissance [START_REF] Salvant | Caractérisation des propriétés physico-chimiques des matériaux de peinture employés par Van Gogh: les peintures blanches[END_REF], paint and mediums became thicker, and some artists used the visible texture of brushstrokes as part as their artistic purpose (Fig. 1.1).

Megilp 1 is one of them. It is made of siccative oil, mastic resin and a lead compound. Then many variations, in particular some mediums, also called gumtion, were developped. The invention of gumtion is attributed to the artist, painting restorer and author Ibbetson at the beginning of the 19th century (Carlyle (2001) p103). In his book published in 1803, he writes that his primary purpose was to prepare a material with which to restore paintings without altering their transparency or giving an "meager or oily" aspect [START_REF] Ibbetson | An accidence, or gamut, of painting in oil and water colours. Part I[END_REF] p.16). Regarding its name, Ibbetson admits himself that he gave gumtion a "ludicrous appellation", without giving any further information. [START_REF] Osborn | Handbook of young artists and amateurs in oil painting, being chiefly a condensed compilation from the celebrated manual of Bouvier[END_REF] p.79 goes even further, a rming that gumtion is an "abominable name, which we are actually ashamed for the honor of the art to put down [. . . ]".

As is the case with megilp, gumtion recipes are listed under various names: Gumtion [START_REF] Ibbetson | An accidence, or gamut, of painting in oil and water colours. Part I[END_REF](Ibbetson, & 1828;;Field, 1835;Osborn, 1845) As Carlyle (2001) points out, although megilp and gumtion gels have been mainly used and developed by 19th century British artists, one can assume that such recipes were known earlier, all the ingredients being present in the artists' workshops (at least boiled oil containing lead and mastic varnish). According to [START_REF] Maroger | Main features and applications of organogels in cosmetics[END_REF], Rubens, in the early 17th century would have prepared a Medium composed of "a little more than a spoonful of the black oil with an even spoonful of mastic varnish". This mixture would have allowed such as the liver, kidneys and the nervous system [START_REF] Retief | Lead poisoning in ancient Rome[END_REF][START_REF] Wani | Lead toxicity: A review[END_REF]. It is also highly soluble in water (s = 456 g•L -1 at 20°C) and has long lasting e ects when absorbed or released in the environment. Acute or chronic poisoning is called Saturnism, still referring to the alchemical system.

Historically lead acetate was obtained by dissolving lead oxide in vinegar, then evaporating and filtering (Gay-Lussac, 1828). Lead acetate trihydrate, whose chemical formula is

Pb(CH 3 COO) 2 •3H 2 O (here written as Pb(OAc) 2 •3H 2 O)
, crystallizes in the monoclininc system (space group C2/m), forming whitish translucent crystals. The crystallographic structure of lead acetate trihydrate has been studied by various teams (Bryant et al., 1984;[START_REF] Harrison | Lead(II) carboxylate structures[END_REF][START_REF] Rajaram | Crystal structure of lead acetate trihydrate[END_REF] but remains intricate to elucidate.

Indeed, it is not clear whether the lead atom exhibits eight-or nine-coordination (lead is known to have a wide range of coordination numbers, from 3 to 12, according to [START_REF] Vijayan | The crystal and molecular structure of lead hexa-antipyrine perchlorate[END_REF]).

Figure 1.10 presents the di erent structure of lead acetate trihydrate. The hypothesis that the lead atom exhibits a nine-coordination seems then to be the most likely, since two teams proposed similar crystalline structures.

In the recipes collected by Carlyle (2001), one can note that authors do not mention the use of anhydrous lead acetate (Pb(CH 3 COO) 2 ), which indicates either the artists did not know this compound or that they used only the trihydrate form, which is likely considering the chemical storage in artists workshops.

It may be that painters were using lead subacetate (Pb(CH 3 COO) 2 • 2 Pb(OH) 2 ) since this compound rapidly forms when lead acetate trihydrate is exposed to air at room temperature, as demonstrated by Bryant et al. (1984) thanks to their infrared (IR) and solid state Magic Angle Spinning Nuclear Magnetic Resonance (MAS-NMR) spectroscopies study. According to the authors, Pb(CH 3 COO) 2 •3H 2 O can be stored a few weeks in a tightly closed container, but when exposed to air at room temperature (or even under an infrared beam) the white and opaque lead subacetate forms within half an hour (or even faster when heated).

In his transcription of the Gumtion recipe, Field seems to specifically mention the use of lead subacetate: "[. . . ] In the using of sugar of lead, if the acid abound which it does usually in the purer and more crystalline kinds, its power of drying is weakened, and it may have some injurious action upon colours [. . . ]" [START_REF] Field | Rudiments of the Painters' Art; Or, A Grammar of Colouring: Applicable to Operative Painting, Decorative Architecture, and the Arts[END_REF] pp 140-1).

In this case, the author refers to the loss of acetic acid which was initially present in the compound and was an indication that the lead acetate is more "pure" and thus less appropriate to obtain a good drier which, in addition, can injure the paint color. Bryant et al. (1984) showed that the transformation of lead acetate into subacetate is indeed accompanied by the evaporation of water and acetic acid. Indeed, a freshly purchased lead acetate trihydrate produces strong acidic emanations, while after few days or weeks (depending on the storage), the smell is much lower, witnessing its (at least superficial) degradation into lead subacetate. This suggests that Field (1858) is indirectely advising the use of lead subacetate. To counter this potential problem, Field advises to add lead oxide in the preparation: "in this case a small addition of some of the pure oxides, such as litharge, ground fine, will increase the drying property of the sugar of lead, and correct its injurious tendency" (pp 140-1). In addition, in the historical sources, the authors often recommend the very fine grinding of lead acetate before preparing the gumtion: "sugar of lead, very finely ground" (Templeton, 1846, Medium No. 2 ); "finely ground" (Gilchrist, Etty's Medium, 1855 and Standage, Megilp, 1892); " 1 2 oz. grd Sug Lead" (Roberson, Etty Mixture); "finely-ground sugar of lead has been di used [in linseed oil]" (Edwards, Megilp, 1856). These instructions suggest that as a first step, the lead acetate was ground in air before mixing in the oil or in the varnish, which drastically enhanced its surface-bulk ratio and thus the proportion of lead subacetate formed.

State of the art : studies on gumtion and megilp

In this work the reconstructions were based on the recipes collected by Carlyle (2001).

The Artist's Assistant gives an exhaustive overview of the materials employed by British artists during the 19th century, and it compiles a wide range of historical sources from oil painting instruction manuals, colormen's archives and British historical treatises.

In the literature 5 scientific publications were found that were specifically dedicated to megilp and gumtion materials. The di erent approaches consist of reconstructions of historical recipes, chemical characterization, or their identification in 19th century paintings.

In parallel, reconstructions were carried out to investigate the structure and properties of these gel systems. Townsend et al. (1998) carried out the most extensive study on such systems. Megilps were prepared and analyzed (as made or mixed with paint, before and after natural ageing) using Scanning Electron Microscopy coupled with Energy Dispersive X-ray analysis (SEM-EDX), Fourier-Transform Infrared spectroscopy (FTIR) and Direct Temperature-resolved Mass Spectrometry (DTMS). Then real samples from J. M. W.

Turner were analyzed and results compared to those obtained from modern preparations.

The molecular characterization of such materials remains complex, as the authors conclude that they could not highlight variations between megilp FTIR spectra and the ingredients alone (oil, mastic resin and lead compound).

In some publications, the authors were aiming to identify gelled media in paintings samples using mainly separative techniques, such as mass spectrometry. The main issue of such

Chemical description of gel systems

As in this work we are focusing on the study of gel materials, it is important to specify the main properties and behavior of gel systems.

Gel materials

The term gel includes a wide-range of chemical systems with various fields of application such as cosmetics [START_REF] Maroger | Main features and applications of organogels in cosmetics[END_REF]), biomedical (Ferreira et al., 2018), food [START_REF] Banerjee | Food Gels: Gelling Process and New Applications[END_REF] or restoration of works of art [START_REF] Chelazzi | Gels for the cleaning of works of art[END_REF]. There are many ways to define a gel and, as underlined by Weiss & Terech (2006), all definitions try to connect the micro-and macroscopic properties of gel materials. However, Flory's version seems to have reached a consensus among the scientific community, which defines gel as "a continuous macroscopic structure that is permanent on an analytical time scale and is solid-like in its rheological behavior below a certain stress limit" [START_REF] Flory | Introductory lecture[END_REF]. He then introduces four categories of gels (lamellar gels, covalent polymer networks, weak-bonded polymer networks and particulate disordered gels), but all fulfill his general definition.

The main di erences are attributed to the distinct driving mechanisms for the gelation process: cross-linking vs. self-organization. Generally, this distinction is referenced as "chemical" or "physical" gels respectively. The former are thus qualified with respect to the covalent nature of the bonds (typically formed by polymerization) and their irreversible nature. These gels are relatively stable at high temperature and at di erent pH. It is necessary to provide enough energy to "break" the gel (the energy of a single covalent bond is around 300 kJ•mol ≠1 ). The latter are conversely sensitive to changes in parameters (temperature, pH, ionic strength, etc.) and therefore reversible. This is due to the weak bonds such as electrostatic interactions, Van der Waals forces, fi stacking, hydrogen bonds etc. [START_REF] Terech | Gels from small molecules in organic solvents: Structural features of a family of steroid and anthryl-based organogelators[END_REF], that structure the material into a supramolecular network.

The next section will focus on specific gels which properties and composition seem very close to the behavior and components characterizing gumtion mediums.

Supramolecular Gels

Supramolecular gels result from the self-assembly of Low Molecular Weight Gelators (LMWG) (George & Weiss, 2006). Those molecules of typically < 2000 Da aggregate and trap the solution, thus providing the gel with its solid-like behavior thanks to surface tension [START_REF] Terech | Low Molecular Mass Gelators of Organic Liquids and the Properties of Their Gels[END_REF]. The materials containing an aqueous liquid are referred to as hydrogel, whereas organogels designate gels containing an organic liquid.

Usually two parameters are taken into consideration to describe supramolecular gels:

• the gelation temperature (T gel ), below which the gel forms. Because of their selfassembled nature, organogels are thermally reversible. This means they flow when heated above T gel , and reform when they cool. This cycle can be repeated until thermal decomposition of the material is reached [START_REF] Abdallah | Organogels and Low Molecular Mass Organic Gelators[END_REF].

• the Critical or Minimum Gelation Concentration (CGC or MGC), that is the minimal amount of LMWG required to form the gel at room temperature. For organogel, CGC is typically in the order of 1-5 %w [START_REF] Abdallah | Organogels and Low Molecular Mass Organic Gelators[END_REF][START_REF] Van Bommel | Organic Templates for the Generation of Inorganic Materials[END_REF]Weiss & Terech, 2006). When less than 1 %w is required, the LMWG is called "supergelator" [START_REF] Fujita | Design and Function of Low Molecular-Mass Organic Gelators (LMOGs) Bearing Steroid and Sugar Groups[END_REF].

Organogels

In an organic liquid (less polar than aqueous solutions), LMWG generally aggregate into filament shape (fibers, rods, cylinders. . . ) which self-arrange to form an interlinked 3Dnetwork [START_REF] Abdallah | Organogels and Low Molecular Mass Organic Gelators[END_REF][START_REF] Steed | Anion-tuned supramolecular gels: A natural evolution from urea supramolecular chemistry[END_REF][START_REF] Van Bommel | Organic Templates for the Generation of Inorganic Materials[END_REF]. As summarized by [START_REF] Esch | Recent advances in smart hydrogels for biomedical applications: From self-assembly to functional approaches[END_REF], the formation of the mono-dimensional objects at the beginning of the gelation process needs the LMWG to be subject to specific uni-directional intermolecular interactions. Also certain criteria such as the solubility of the newly formed filaments in the solvent or the parameters promoting the formation of the 3D architecture (via connecting points so-called "junction zones") needs to be controlled. That is why these materials, which are highly tunable, show great potential and are still widely studied.

In organogels, the LMWG can be of diverse types, leading to various structures. Among the wide-range of LMWG identified are included alkane chains (Abdallah & Weiss, 2000a), fatty acids [START_REF] Nikiforidis | Organogel formation via supramolecular assembly of oleic acid and sodium oleate[END_REF], sugars [START_REF] Prathap | Sugar-Based Organogelators for Various Applications[END_REF], crown ethers [START_REF] Park | A crown-ether-based moldable supramolecular gel with unusual mechanical properties and controllable electrical conductivity prepared by cation-mediated cross-linking[END_REF], urea-based molecules [START_REF] Steed | Anion-tuned supramolecular gels: A natural evolution from urea supramolecular chemistry[END_REF], steroidal skeleton [START_REF] Lin | A novel gelator of organic liquids and the properties of its gels[END_REF], etc. Their global structure and intramolecular chemistry a ect the nature of the liquid encompassed and the final gel architecture. Then, as reported by [START_REF] Llusar | Inorganic and Hybrid Nanofibrous Materials Templated with Organogelators[END_REF], this supramolecular assembly can serve as template to form 3D-shaped inorganic or hybrid condensed structures, that can be used for a variety of applications.

Metallogels

In some cases, a metallic compound can be part of the supramolecular gel as a labile ion interacting with the organic molecules, or incorporated in the LMWG structure (forming a coordination complex). The gel obtained is thus called a metallogel [START_REF] Dastidar | Metallogels from coordination complexes, organometallic, and coordination polymers[END_REF][START_REF] Piepenbrock | Metal ion and anion-based "tuning" of a supramolecular metallogel[END_REF].

Because of the presence of a metal in their structure, metallogels are extraordinarily versatile. For instance, one can tune or make use of their optical or magnetic properties useful for electrochemical or catalysis applications [START_REF] Mitsumoto | A Multi-Redox Responsive Cyanometalate-Based Metallogel[END_REF][START_REF] Zhang | A promising heat-induced supramolecular metallogel electrolyte for quasi-solid-state dyesensitized solar cells[END_REF]. Unlike organogels, they show a relatively high mechanical strength and thermal stability [START_REF] Wang | Hydrogels triggered by metal ions as precursors of network CuS for DNA detection[END_REF].

Examples of supramolecular gels

We focused on systems containing similar molecules as the one present in the materials studied in this work.

The use of triterpenoids derivatives as LMWG was first documented by Bag and co-workers in the early 2000s (Bag, Maity, & Pramanik, 2005b, 2005a). Their work on arjunolic acid functionalized with a 3,5-dinitrobenzoate group shows the supergelator (CGC < 1 %w) properties of this LMWG in aliphatic alcohols (methanol, ethanol and propanol). They also demonstrated the importance of the electron donor-acceptor interaction in the gel formation, by adding an electron-deficient compound (for instance, picric acid) to the system. It reveals that T gel increases with the picric acid concentration, interestingly reaching a maximum at a stoechiometric ratio between the acid and the triterpenoid derivative [START_REF] Bag | Donor-Acceptor Interaction Promoted Gelation: Visual Observation of Color Change[END_REF].

The gelator properties of other tritepenoid derivatives based on oleanolic acid (naturally present in many plants, including Pistacia lentiscus) were investigated later by [START_REF] Hu | A simple oleanlic acid derivative as potent organogelator[END_REF] and Lu et al. (2013) (Fig. 1.13). The first team studied 2,3-dihydroxy-imino-oleanic acid, while the second analyzed oleanolic acid conjugated with tripeptides. In both cases, the LMWG form organogels with various solvents (toluene, benzene, xylene and chloroform).

Observations of the dried gel with Scanning Electron Microscopy (SEM) show filaments (60-100 nm diameter) entangled in a 3D network, forming cavities with an average porosity around 400-500 nm [START_REF] Hu | A simple oleanlic acid derivative as potent organogelator[END_REF]. They also demonstrate the impact of the solvent trapped in the gel morphology (for instance, benzene-based gels display a denser fiber network). Both studies highlight the role of hydrogen bonds in the self-assembly process In the historical recipes studied in our work, a metallic compound, Pb(OAc) 2 •3H 2 O or PbO is used to prepare gumtion and megilp materials. Lead (II) has a flexible coordination sphere, since it is known to exhibit a coordination number ranging from 3 to 12 [START_REF] Vijayan | The crystal and molecular structure of lead hexa-antipyrine perchlorate[END_REF]. This atom is therefore prone to interact with various ligands, that can be used as LMWG to form metallogels. [START_REF] Knichal | A new small molecule gelator and 3D framework ligator of lead (II)[END_REF] prepared a organo-metallogel by mixing lead acetate trihydrate to 5-allenyl-1,3-benzenedicarboxylic acid in N,N-dimethylformamide under solvothermal conditions (100 °C, 72 h). According to the SEM images, the gel obtained consists of an enmeshed filament structure, with an average diameter of around 40 nm. Authors specify that the material does not recover its initial solid-like behavior after mechanical deformation, which attests the involvement of strong bonds in the network formation.

Another example of the formation of a metallogel involving lead (II) ions (among many others including Cu 2+ , Ni 2+ , Fe 2+ . . . ) is given by [START_REF] Wang | Hydrogels triggered by metal ions as precursors of network CuS for DNA detection[END_REF]. In this study, the authors mix a 0.4 to 1 molar ratio of Pb(OAc) 2 •3H 2 O to an aqueous solution of lithocholate (molecules derived from a gonane skeleton). The outcome is a hydrogel composed of an entanglement of fibrils with various sizes (from tens to hundreds of nanometers) (Fig. 1.14).

FTIR spectra indicate the interaction between the metal and the carboxylic acid groups of the lithocolate, leading to 3 coordination modes (monodentate, bidentate bridging and bidentate chelating). Increasing the amount of the metal in the solution tends to a single configuration, where the monodentate mode is preferred. Once again, FTIR (in the 3500 cm ≠1 region) gives information about the intramolecular hydrogen bonds occurring in the gel. The authors also point out that the metallogel obtained has a strong thermostability (up to 90 °C) unlike regular hydrogels, explained by the coordination interaction that is not easily influenced by temperature. Finally, they demonstrate the possibility of synthesizing metallic nanoparticles (CuS in their example) using the metallo-hydrogel (composed of lithocholate coordinated with a metal and trapped water) as a precursor. After adding a precipitator (thioacetamide solution), they obtain a connected network of isotropic, homogeneous in size and shape, and crystalline nanoparticles (Fig. 1.14 bottom). This easy one-step method is a good example of the interest of studying metallogels and to better understand the chemical processes in which they are involved.

To summarize, organogels are the consequence of weak mono-dimensional interactions (often hydrogen bonds) of small organic molecules, the LMWG. The mono-dimensional interactions lead to the formation of filament-like structures which aggregate into entangled 3D-newtorks. The resulting hierarchical porous structure sustains the organic or aqueous solvent, mainly thanks to surface tension, to form an organogel or hydrogel. Because the structure mainly stems from weak interactions, the gel obtained is thermoreversible and recovers its solid-like behavior after shaking. However, when a metal is added to the system, either as part of the LMWG or freely in the solution (metallic salts for instance), the driving force for gelation becomes coordination bonding. The so-called metallogel has a higher strength and thermostability compared to the organogel. Moreover, thanks to the presence of the metal in its structure, the gel can be used for many applications.

Conclusion

Gelled mediums were sold by British 19th century colormen, but there is still a lack in their full characterization. Since the last studies on this type of materials, new analytical tools adapted for the study of gels (and supramolecular organogels) have been developped. This work reopens the research from this angle in order to take benefit from these experimental techniques, as well as all the recipes and ingredients described by Carlyle (2001). We chose to confine the study to one specific recipe (Gumtion 1 ) and to proceed to its chemical simplification.

1 Gumtion recipe by Ibbetson reported by Carlyle, 2001 (p.394)

Mastic resin and varnish study

The aim of this chapter is to study the mastic resin and mastic varnish as they are key components of the gumtion. Because of its natural provenance, mastic resin is a complex material composed of various chemical compounds.

The first part is dedicated to the identification and approximative quantification of the main triterpenoids present in the mastic resin we used to formulate the gels, thanks to Gas Chromatography and Mass Spectrometry (GC-MS) techniques. The second part investigates the feasibility of investigating the optical properties and mechanical behaviour during drying and under certain atmospheric conditions of thin films of mastic varnish and paint materials using in-situ Spectrometric Ellipsometry (SE).

Characterization of mastic resin by Gas Chromatography and Mass Spectrometry 1.Introduction

Gas Chromatography (GC) and GC coupled with a Mass Spectrometer (GC-MS) are very appropriate techniques to study complex organic compounds such as natural resins and have been widely used and described in the literature [START_REF] Aksamija | Etude chimique des matériaux résineux: oliban, dammar et mastic: application à des prélèvements artistiques et archéologiques[END_REF]Assimopoulou & Papageorgiou, 2005b;Azémard, 2014;Daher et al., 2010;Dietemann et al., 2009;Mills & White, 2012;Nevin et al., 2009;Papageorgiou et al., 1997;Prati et al., 2011;Samet, 1998;Scalarone et al., 2005Scalarone et al., , 2003;;Seoane, 1956;Sharifi & Hazell, 2012;Thoury, 2006;van den Berg et al., 1998;van der Doelen, 1999;van der Doelen et al., 1998).

As detailed in Chapter I ( §1.2.), mastic resin is composed of a polymeric fraction (cis-1,4-poly---myrcene) and numerous triterpenic derivatives with four or five cycles forming oleanane, ursane, lupane, euphane and dammarane types of structures. More than thirty triterpenes have been identified in mastic resin samples. However the described triterpene whether they could be used as reconstruction materials.

Materials and method

Mastic resins

Modern mastic resins were purchased from three di erent suppliers: Kremer Pigmente Unfortunately there is no record of their exact provenance, they only mention "mastic (en larmes) 1 ." is indicated on their containers.

Chemical reagents and sample preparation

All resin samples were trimethylsilylated with the following protocol as reported in the literature [START_REF] Azémard | On the tracks of sandarac, review and chemical analysis[END_REF]. First, ca. 1 mg of each mastic resin was weighed, ground in an agate mortar and mixed with 200 µL of a 80:20 volumic ratio of N,O-Bis(trimethylsilyl)trifluoroacetamide (BSTFA, Aldrich, Ø 98 %) and trimetylsilyl chloride (TMCS, Aldrich, Ø 98.0 %). The solution was then heated at 70 °C during 30 min to let the reaction occur. After cooling at room temperature, the reagents were evaporated under a gentle stream of Argon. The dried residuals were eventually dissolved in 50 µL of cyclohexane (Carlo Erba Reagents, RPE, Ø 99.8%) in order to obtain a concentration of 20 mg•mL -1 of derivatized mastic resin. An amount of 0.25 mg of betulinic acid (Aldrich, 90%), used as internal standard as described below, was introduced at the beginning of the process to get a final concentration of 5 mg•mL -1 .

Chromatographic analyses

Equipment

For compound identification, the derivatized extracts were analyzed with a gas chromatograph Shimadzu QP 2010 SE equipped with simple quadrupole. Experimental conditions concerning the mass spectrometer were electron impact 70 eV with a scan of 40-800 uma, ion source temperature 250 °C, interface temperature 300 °C. GC separations were achieved on a ZB5MS (30 m, 0.25 mm I.D.) fused-silica capillary column with a 0.25 µm film dimethylpolysiloxane (5% phenyl groups) coating from Phenomenex. The quantitative analyses were performed on a Trace GC from ThermoFisher Scientific equipped with an apolar column (DB5, with the same characteristics as the GC-MS column described above).

A volume of 1 µL of each sample was injected and heated from 150 to 250 °C at a rate of 10 °C•min -1 and then increased from 250 to 320 °C at 15 °C.min -1 (for a total experiment time of 39 min). The injector and detector temperatures were set to 320°C and the split ratio was 10 mL•min -1 .

Data treatment and interpretation

Since the same protocol and column were used in GC and GC / MS, we were able to use the first technique to quantify (using the internal standard) and the second technique to identify the compounds:

• The GC/MS allowed us to identify mastic compounds, thanks to the previous publications (especially Assimopoulou & Papageorgiou (2005a)) on the basis of mass spectral interpretation, according to molecular ion (M + ), base peak and main fragments.

• Quantization was performed by GC andwas easier to implement thanks to the FID (Flame Ionization Detection) detector whose signal is proportional to the product mass.

For the quantitative analysis, the internal standard method has been used (previously detailed in [START_REF] Kononenko | Qualitative and quantitative studies of chemical composition of sandarac resin by GC-MS[END_REF]). An internal standard is a chemical substance that is added in a known amount to the sample in order to calculate the relative quantity of the compounds present in the sample. The internal standard is a compound that is very similar, but not identical to the chemical species of interest in the samples. Here we used betulinic acid as an internal standard (Fig. 2.1) that enabled us to estimate the proportions of the main components and compare the di erent resins studied. The peak areas have been measured on the chromatograms and the corresponding triterpenoid concentration has been calculated.

Results

Free triterpenoid identification and quantitative analysis on modern mastic resins

First, modern mastic resins from three di erent suppliers were analyzed by GC-MS after trimethylsilylation in order to identify their main triterpenoids: one from Chios Mastiha Growers Association (CMGA) and two others from well known suppliers for paint and restoration materials. As in-depth quantitative analysis of mastic resin from Chios Mastiha Growers Association has already been performed (Assimopoulou & Papageorgiou, 2005a, previous research on sandarac resin (Kononenko, 2016). We did not focus on the minor components identification but aimed at the comparison of the di erent resins based on their main components.

The total amount of free triterpenoids over the total resin samples is comprised between 60 and 73 %w. This means that ca. 35 %w of the resin is not analyzed by the GC experiment and thus corresponds approximately to the polymeric fraction (1,4-cis-poly---myrcene) naturally present in the resin. In fresh commercial resin samples, the total polymer fraction has to be linked first to the amount of monoterpenes initially present in the resin after exudation and thus the exudation conditions, the growing environment of the trees, etc. Dietemann et al. (2009) also showed that the storage conditions (light or dark environment) influences the polymerization process in the mastic resin. Without further information on the history of the resins analyzed, we cannot conclude on these polymer fraction slight di erences. oleanonic acid, (3) olean-dien-3-one, (4) (epi)lanosterol, (5) 28-nor-olean-12-en-3-one and (6) (iso)masticadienoic acids (Fig. 2.1). Some variations can be seenn such as the oleanonic acid amounts (12.5, 11.4 and 7.1 %w for the mastic resin from Laverdure, Kremer and CMGA respectively) or the moronic acid (from 12.5, Kremer, to 14 %w, CMGA). The similarity observed indicates the same provenance, all of them likely coming from the Chios island. These results underline the CMGA's monopole existing now and also suggests the little chemical variation in all the Chios island, despite di erences in the date of harvest (they were bought at di erent time, between 2012 and 2016, we can thus assume they were not harvested the same year). This has to be put in perspective with the great di erences existing in the literature about the exact composition of mastic resin.

As a comparison, Assimopoulou & Papageorgiou (2005a) had also quantified the CMGA resin with the main components being: (1) (iso)masticadienoic acids, (2) 28-nor-olean-17en-3-one, (3) oleanonic aldehyde, (4) oleanonic acid, (5) --amyrone, (6) 28-nor-olean-12en-3-one. It can be assumed that such di erences with our results come from changes in the environmental conditions of harvest and storage between the CMGA resin that they analyzed in 2005 and that analyzed in 2018. However this assumption would imply a significant change in the conditions of production between these two dates: we compared three resin batches from di erent providers and dates and found little di erence between them. A more likely hypothesis is that the di erences observed are mainly due to the analytical procedure such as the nature of the column, the derivatization method and the quantification method, which was not indicated in Assimopoulou 2005. A common procedure should therefore be adopted to ensure comparisons and reproducibility in the field.

Analysis of aged mastic resins

Three historical samples, at least 150-years old, from the collection of the National Museum of Natural History (Paris, France), have been also analyzed. The aim was to check whether those old resin samples, mastic tears preserved in closed containers, could be interesting candidates to perform historically accurate reconstructions of 19th century recipes. They represent the earliest resin samples, preserved as tears (and not prepared as varnish or basalms) that have been analyzed quantitatively.

Previous publications characterized mastic resin or varnishes after artificial ageing (Azémard, 2014;Dietemann et al., 2009;Nevin et al., 2009;Scalarone et al., 2003). Ageing of terpenic resins usually proceeds mainly by autoxidation, followed by, with further ageing, cleavage reactions and reticulation processes. Major components of mastic and dammar triterpenic fraction undergo Norrish I reactions leading to highly oxidized by-products (Azémard, 2014). It has also been shown that strong oxidation already occurs in mastic resin within acid. In contrast to previously published results on mastic resin ageing, fewer compounds could be detected by GC-MS with no common degradation products identified. We can thus hypothesize that the resins are in a high state of degradation with higher molecular weight compounds formed. It is not possible here to conclude whether those di erences between the fresh resin samples and the historical samples are only due to the resin ageing or if they have also occurred as a result of di erences in the provenance, exudation and storage conditions of the resins. Further ageing experiments on raw mastic tears, and with complementary analytical techniques more suitable for the detection of polar compounds, would be necessary to fully explain the compositions of these historical mastic resins. In any case, it appears clear that these 150-year old resin samples cannot be used for the reconstructions of varnishes or paint medium, or to mimic natural ageing of paint varnishes.

Conclusion

To conclude, mastic resin is composed of a complex mixture of more than 60 di erent triterpenoids. The samples purchased from three di erent suppliers were found to be very similar despite minor variations in the relative distributions of their components likely due to di erent shrub environments or genotypes, ages or exudation and storage conditions. It may be that the di erences observed in the literature for the described composition of mastic resin are mainly due to di erent techniques and experimental protocols. Quantitative analysis by the method of internal standard revealed that the identified triterpenoids represent 60-70 %w of the commercial resins analyzed. As a comparison, unique 150-year old samples were investigated and the triterpenoids identified were found to be only ca. 5

to 15 %w of the sample analyzed. Further experiments on mastic tears should be carried out to establish the di erent mechanisms involved in triterpenoids degradation over the centuries.

The coe cients A, B (nm 2 ) (C (nm 4 ), etc.) are positive and characteristic of the material studied. When ⁄ ae OE then n(⁄) ae A. A gives the refractive index range; B and C give the dispersion shape.

SE presents many advantages. The measurement itself is fast (a few seconds) and as an optical technique, it is contactless and non-destructive. Analysis can be performed at room temperature and atmospheric pressure, but controlled atmosphere is also possible. and are precisely measured (± 0.001°) thus a good estimation of the film parameters relies on a good model. The main limitation would be the preparation of uniform films. But thanks to the use of micro-probes (300 µm spot), even heterogeneous films can be analyzed.

Context and previous works

The role of the varnish Until the end of the 20th century, scientists and art historians considered varnish mainly as a protective layer for the art work (De la Rie, 1989). In fact, it was initially used for an aesthetic purpose. Indeed, the application of the varnish makes the paint surface smoother, which limits the light di usion and thus desaturation of the colors. Moreover it is known that the closer the value of the refractive index of the varnish and the paint's binder (e.g. linseed oil), the greater the color saturation (De la Rie, 1989).

Optical constants determination

The literature reports the use of the Becke line test to determine the refractive index of varnishes used for painting [START_REF] Polikreti | Spectroscopic Ellipsometry as a tool for the optical characterization and ageing studies of varnishes used in Post-Byzantine icon reconstructions[END_REF]. This method was developed at the end of the 19th century and the principle is to successively immerge the material studied into di erents liquids with various referenced refractive indices and observe it under the polarized light microscope. Upon moving the microscope stage, a halo forms and moves towards the material with the greatest refractive index, allowing the relative determination of the refractive index of the material.

The main drawbacks of this method are not only that it gives relative information, but also its obvious lack of precision as well as its destructive nature (immersion of the materials studied).

In the early 2000s, a team from the University of Cyprus investigated the use of SE on various materials traditionally used in varnish recipes (such as natural resins, oil or eggwhite) as well as modern resins employed from the middle of the 20th by artists and restorers (such as acrylic resins) [START_REF] Christofides | Fine art painting characterization by spectroscopic ellipsometry: Preliminary measurements on varnish layers[END_REF][START_REF] Polikreti | Spectroscopic Ellipsometry as a tool for the optical characterization and ageing studies of varnishes used in Post-Byzantine icon reconstructions[END_REF][START_REF] Polikreti | Optical Characterization of Varnish Films by Spectroscopic Ellipsometry for Application in Artwork Conservation[END_REF]. After determining the refractive indices of their materials (Fig. 2.7), they mainly show that thanks to SE it is possible to discriminate natural from synthetic resins, which is a key capability for varnish identification and conservation purposes. [START_REF] Polikreti | Spectroscopic Ellipsometry as a tool for the optical characterization and ageing studies of varnishes used in Post-Byzantine icon reconstructions[END_REF] We can see that the refractive index is around 1.53 for most of the studies on resins (mastic and dammar) and eggwhite. It is lower for linseed oil with values around 1.48. We can note that the wavelength is not specified in some cases and that some of the methods are di erent which may explain the variability observed.

Those studies show that SE can give insight to the optical properties of painting materials.

However, no investigation about the thin film dynamic was carried out, such as the study of the drying or the swelling of the films, despite their importance.

Film swelling and the e ect of the solvents treatment

Understanding the e ect of solvents used in restoration processes on the materials present in an oil painting is crucial from a conservation point of view. Varnishes on paintings naturally deteriorate with time under various conditions (such as light and/or humidity).

The main problems detected are both physical (brittleness, flaking) and chemical (yellowing, blanching) (Dietemann et al., 2009;[START_REF] Genty | Les chancis des vernis et des couches picturales des peintures de chevalet à l'huile : Contribution à la[END_REF], damaging the integrity of the artwork.

That is why over the painting's life, restorers are required to partially remove and replace the varnish. To that end, they use specific solvents or solvent mixtures carefully chosen to interact with the nature of the material they want to remove. The most common chemicals are isopropanol (iPr), isooctane, white spirit, acetone or ethanol [START_REF] Nicolaus | Manuel de restauration des tableaux[END_REF]. However, [START_REF] Hedley | A Study of the Mechanical and Surface Properties of Oil Paint Films Treated with Organic Solvents and Water[END_REF] cite the work of Stout (1936) 1 to highlight that it has been known since 1930s that the solvent might reach and a ect the paint layer.

Hence, several studies have been carried out to determine whether solvent treatment can a ect and damage the painting below the varnish layer. [START_REF] Stolow | A modified apparatus for measuring the swelling of polymer films in solvents[END_REF] describes a setup based on a viscometer where the material studied is placed between two plates. When exposed to the solvent, the film swells involving the movement of the plates. The stream then moves forward or backward to keep a constant flow. After proper calibration, one can then establish the value of the film swelling. Two other methods for the determination of the paint film swelling: dimension change under the optical microscope or weight gain/loss (Phenix, 2002a[START_REF] Phenix | E ects of Organic Solvents on Artists Oil Paint Films: Swelling[END_REF]. According to Phenix (2002a), the latter is the most commonly used (as well as for polymeric materials). Although those procedures are e ective, once again they appear to be imprecise and hardly reproducible. [START_REF] Chen | Equilibrium Swelling of Hydrophilic Polyacrylates in Humid Environments[END_REF] first mention the use of SE to study the swelling of polymeric films under humid atmosphere. Since then, this technique has proved its worth for the study of solgel films in our team [START_REF] Boissière | Porosity and Mechanical Properties of Mesoporous Thin Films Assessed by Environmental Ellipsometric Porosimetry[END_REF][START_REF] Hakalahti | Interfacial Mechanisms of Water Vapor Sorption into Cellulose Nanofibril Films as Revealed by Quantitative Models[END_REF] and in various applications, for example biological films, micro-electronics, spectroelectrochemical sensing or ionic separation [START_REF] Ogieglo | In situ ellipsometry studies on swelling of thin polymer films: A review[END_REF]. However, and to our knowledge, to date this kind of study has not been carried out on varnish and paint materials yet.

In this chapter, the aim is to demonstrate that SE is a valuable tool for optical constants measurements and the study of the paint material's swelling during exposure to solvents used for restoration and cleaning.

1 Stout, G.L., 'A preliminary test of varnish solubility', Technical Studies in the Field of the Fine Arts 4

(1936) 146-161. This reference could not be consulted.

Materials and methods

Setup

Static measurements

Analyses were carried out on a Variable Angle Spectroscopic Ellipsometer (V.A.S.E. from J.A. Wollam Co., Inc.) in the visible range (⁄ = 400 ≠ 1000 nm) at an incident angle 0 = 70°and the acquisitions were performed using the V.A.S.E. software.

Dynamic measurements

Monitoring the drying of the film

To observe the evolution of the thickness h and refractive index n of mastic varnish films during drying, we carried out repeated static measurements (every 20 s). The impact of UV exposure was investigated by illuminating the sample with UVB radiation (280-315 nm) during drying.

Environmental Porosimetry experiments

For the Environmental Porosimetry (EP) experiments, an environmental chamber (equipped with two pinholes to allow the passage of light) was installed on the stage of the previously described ellipsometer to control the atmosphere around the sample. We used the setup shown in Figure 2.8. The chamber was connected to a flow controler (Solgelway) monitored with Regul'Hum software. It was linked to dry air (N 2 , flow 5 L•min -1 ) and to two wash-bottles connected in series containing the appropriate solvent, one at 50°C and the other at room temperature.

• For water sensitivity experiments, the ramp was 0% RH (1 min), 100% RH in 20 min (1 min), 0% RH.

• For organic solvents experiments, the ramp was p p 0 = 0 (1 min), p p 0 = 0.95 in 20 min (1 min), p p 0 = 0 (Fig. 2.9). EP measurements were acquired every 20 s. We obtain the evolution of n and h during the vapor adsorption and desorption experiment.

Films preparations

Varnish films were dip-coated on silicon wafer substrates (Fig. 2.10) at very low humidity (0-10% RH using N 2 as dry air, T = 18-23°C) to allow good reproducibility in the films thickness [START_REF] Bindini | Critical Role of the Atmosphere in Dip-Coating Process[END_REF] at di erent withdrawal speeds (from 0.05 to 1.5 mm•s -1 ).

It is interesting to compare these results to the study of [START_REF] Polikreti | Spectroscopic Ellipsometry as a tool for the optical characterization and ageing studies of varnishes used in Post-Byzantine icon reconstructions[END_REF] (Fig. 2.7). As they prepared the varnishes following Byzantine recipes their formulations are not exactly the same as we prepared, and the optical constants obtained are quite di erent from ours. However, the dispersion laws follow the same tendency: n values decrease for dammar, mastic and eggwhite respectively.

In addition, the commercial and experimental dammar varnishes display two di erent n dispersions indicating they do not have exactly the same composition. Because the Lefranc & Bourgeois varnish is a commercial product, we do not have access to its precise composition or proportions. FTIR analyses (Appendix B) suggest a very close composition between the two dammar varnishes.

Historical varnish

As a real case study, residues of an historical varnish (dated from 1947) were recovered from cotton swabs employed for the restoration of Transfiguration of Christ (Fig. 2.13 left), a 15th century oil painting from Giovanni Bellini. As the solution is not completely transparent, the Cauchy model is not valid and we allowed our model to fit the extinction coe cient k. The results are presented in Figure 2.13 (right: n in orange, k in pink). For this particular case, SE could help us to make some assumptions regarding the varnish composition. Indeed, n values over the wavelengths range are close to the resinous varnishes previously analyzed. However, the dispersion shapes are not identical and the historical varnish shows lower n values (from 1.540 to 1.507) compared to the fresh varnishes, closest to the linseed oil n dispersion. Moreover, according to [START_REF] Polikreti | Spectroscopic Ellipsometry as a tool for the optical characterization and ageing studies of varnishes used in Post-Byzantine icon reconstructions[END_REF], this varnish does not appear to contain synthetic resins, such as Paraloid B72 or poly(vinyl acetate) (Fig. 2.7) despite their wild use in restoration processes in this period. The restorers might have wanted to keep an historical varnish recipe, and we can assume that the varnish is made of natural resin in oil. Furthermore, to explain the values measured, then several factors should be considered:

• the ageing of the varnish modifies its optical properties (Dietemann et al., 2009;[START_REF] Genty | Les chancis des vernis et des couches picturales des peintures de chevalet à l'huile : Contribution à la[END_REF][START_REF]The refractive index of a solid film of linseed oil rise in refractive index with age[END_REF].

• the varnish analyzed is certainly a mixture between several components used to make the varnish (resins, oil, turpentine) but also impurities coming from the restoration itself (removal chemicals such as white spirit, inpainting. . . )

To deepen the materials characterization, spectrometric techniques such as GC-MS would be appropriate.

too sti for use, put a little more oil[. . . ]", Ibbetson, 1828 p.16).

Despite the varying proportions Carlyle identified three main formulations for gumtion (Carlyle (2001) p.107), which are identified here as:

• Original recipe: oil, mastic resin, solid lead acetate 1

• Variation A: oil, mastic varnish, solid lead acetate 2

• Variation B: oil, mastic varnish, aqueous lead acetate 3 Among all the recipes available, we selected the ones containing the most details of the quantities required and the synthesis process.

Because they are gelled Mediums all the preparations are expected to gel or at least become thick enough to modify the paint properties. The artists empirically adjusted the protocols in their workshops to achieve the desired results.

After the reconstructions of the recipes, the approach consisted of testing di erent parameters to establish whether they have an impact on the gelation process. For instance, we wondered whether all the components (oil, triterpenic or polymeric fraction of the resin, lead compound) were necessary to form the gel, and what is the impact of their relative proportions. It was also questioned whether the nature of the di erent components (the metallic compound, the oil and the resin) can also have an influence on the gelation.

Finally, an exploration of the synthesis process and the external parameters were carried out to determine if these factors can be a key aspect of the gel formation and hence should be monitored.

The primary aim of this chapter is to define the key parameters involved in the gelation process. The second aim was the elaboration of simplified gel materials, called in this work "Model Systems" (MS), in order to deepen the understanding of the chemical interactions occurring in the gels. 1 As cited in Carlyle, 2001 (pp.394-395)

The historical recipes

This section lists the most precise gumtion recipes reported by Carlyle (2001) pp.395-398.

2.1 Original recipe: oil, mastic resin, solid lead acetate and then two parts of mastic varnish are to be added, and well mixed with the preceding.

By this means a white creamy emulsion is obtained, which, though opaque in use, becomes quite transparent as it dries."

Although containing much information, these texts show how delicate the formulation of reproducible protocols from the ancient recipes can be. However, it is crucial to translate these recipes into a chemical process, to further characterise these gumtion recipes.

Reconstructions

Preliminary experiments indicated that the historical gumtion recipes indeed form opaque gel-like materials with a yellowish color (likely due to the presence of the oil); however di erent textures and gelation time were obtained with slight variations in the protocol used. That is why it was necessary to set a very precise protocol to ensure complete reproducibility.

The choice of ingredients

Three di erent "cold drawn" linseed oils1 were used, purchased from di erent suppliers (Laverdure, Kremer Pigmente and Schminke). We also employed two di erent mastic resins (Laverdure 2015, and Kremer Pigmente 2016) both originally harvested from the island of Chios. To make the varnish, we used rectified spirit of turpentine from Sennelier.

The sugar of lead referred to in the historical recipes was interpreted as lead acetate trihydrate. Even if the literature reports the quick transformation of this compound into lead subacetate (Bryant et al., 1984), meaning that the artists certainly used a partially subacetate converted compound, we chose to use lead acetate trihydrate Pb(

CH 3 COO) 2 •3H 2 O (s)
(Sigma Aldrich, Ø 99%) to be in the same conditions as the artists and follow their recommendations. Lead oxide, also purchased from the same supplier, is a mixture of --and --PbO (99+%).

Scientific protocols

This section describes the synthesis protocols used to prepare the gumtions, based on the recipes listed above. They are preceeded by instructions for the preparation of a mastic varnish and the saturated solution of lead acetate, necessary for some gumtions recipes.

Mastic varnish

• In a glassware, put 15 g of mastic resin

• Add 30 g of turpentine

• Magnetic stirring 15 min

• Let it rest 30 min (the insoluble part sinks to the bottom)

• With a Pasteur pipette, recover the liquid and store in a sealed glass bottle

As described in the literature (see Chapter I), and as we could demonstrate (see Chapter II), mastic resin is approximately made of 66% of triterpenoids and 33% of polymeric fraction.

We could assume then that in 2 g of mastic varnish prepared (~0,667 g of resin) we have 1.47•10 -3 mol of triterpenoids (taking an average triterpenoid molar mass ~455 g•mol -1 ).

Saturated solution of lead acetate

• In a glass beaker, put 5 g of Pb(OAc) 2 •3H 2 O (s) (s = 44 g/100 mL)

• Add 10 mL of distilled water

• Magnetic stirring 5 min

Original recipe of Gumtion

• Grind 1 g of mastic resin (less than 1 min) in an agate mortar

• Add 1 g of Pb(OAc) 2 •3H 2 O (s)
and grind with mastic resin (1 min)

• Add 2 g of linseed oil gradually (0.5 mL) and grind

• Store in a sealed glass jar

Variation B

• Put 2 g of linseed oil in a glass jar With this protocol, the systems gelled within a few hours (ranging from about 2 to 6h depending on the proportions). The gelation time was identified as the moment when the material does not flow anymore as we turn the vial upside down. This would establish that our experimental protocols are relevant to reproduce the recipes used by the artists.

We then carried out several tests, described in the next section, to identify the key parameters of the gelation process.

Variations and definition of model systems 4.1 Evaluation of the gelation parameters

Influence of the presence of oil

As described in the literature, siccative oils are known to interact with metallic compounds such as lead or zinc, forming metal soaps, which can dramatically a ect the visual aspect of paintings [START_REF] Hermans | Metal soaps in oil paint: Structure, mechanisms and dynamics[END_REF]. Artists sometimes actively seek the reaction between the lead compound and the oil. For instance, lead treated oil was used from the 12th century and is composed of a drying oil, such as linseed oil, heated with litharge (--PbO) (de [START_REF] De Viguerie | The drying of linseed oil investigated by Fourier transform infrared spectroscopy: Historical recipes and influence of lead compounds[END_REF]. In their experiment, they found that a dark viscous liquid is formed and the more lead is added to the preparation, the less it flows (de Viguerie et al., 2009).

One could assume this reaction is involved in the gelation process occurring in gumtions.

In order to explore the role played by the oil, we ground Pb(OAc) 2 •3H 2 O in mastic varnish with the following protocol:

• In an agate mortar, put 1 g of Pb(OAc

) 2 •3H 2 O (s)
• Add 2 g of mastic varnish

• Grind 1 min

• Store in a sealed glass jar

We could see that even without adding the oil, the mixture forms a gel. Thus, oil is not a pre-requisite for the gelation process (even if it reacts with lead) and was probably added by painters to adjust the flow of the gumtion and subsequently allow its proper mixing with the paint.

Influence of the amount of lead acetate trihydrate

We also varied the amount of Pb(OAc) 2 •3H 2 O in two formulations (Original recipe, and the mixture between mastic varnish and PbOAc (s) ) to see whether there is a minimum quantity needed to form the gel, as shown in Tables 3.1 and3.2. In the original recipe which includes oil, at least approximately 0.40 as a molar ratio between the lead compound and the triterpenic fraction1 was required to form a gel. In contrast, in the system without oil, a lower ratio between Pb(OAc)

Influence of the presence of the polymeric part of the mastic resin

To investigate which part of the mastic resin is involved in the gelation process, we removed the polymeric fraction of the resin and prepared a varnish, inspired by the protocol described in Dietemann ( 2003) :

• Grind 5 g of mastic resin

• Add 15 mL of dichloromethane and stir to dissolve the resin

• Add 40 mL of methanol

• Filter to remove the white polymer formed during the previous step

• Let the solvents evaporate and re-dissolve in the chosen solvent (turpentine in our case)

After removing the poly---myrcene from the mastic resin, and thus having prepared a "triterpenoids varnish", we ground-in Pb(OAc) 2 •3H 2 O and very quickly obtained a thick white gel -in less than 1 min. This shows that the gelation depends on the interaction between the triterpenoids and the Pb(OAc) 2 •3H 2 O. In the same way as the oil, the polymeric part of the resin does not seem necessary to form the gel, but might have an impact on the gel properties.

We then proceeded to buy from Sigma Aldrich two commercial triterpenoids known to be present (or with a close structure) in mastic resins (see Chapter II): oleanolic acid (Ø 97%)

and lupeol (Fig. 3.1) so as to investigate whether or not they could form a gel with the following protocol:

• Mix 1 g of turpentine and 50 mg of the triterpenoid

• Add 45 mg of Pb(OAc) 2 •3H 2 O s
ground at first in air and mixed with the varnish afterwards. This points out a sort of "passivation" of the lead acetate and a decrease of its reactivity presumably due to air exposure. If it is probable that the increase of the surface to bulk ratio interferes in the gel formation, there are as well chemical mechanisms taking place during the grinding step.

Once again, an e ect of the grinding time could be observed (5 min grinding in air leads to gel formation in about 25 min, vs. 5 min when the lead acetate is ground directly in the varnish). Moreover, waiting a few hours after the grinding of the lead acetate before mixing it with the varnish extends the gelation time even further (reaching more than 1 h after a 16 h exposure to the air).

As stated by Bryant et al. (1984) (cf Chapter I), lead acetate trihydrate is subject to rapid transformation into lead subacetate Pb(CH 3 COO) 2 •Pb(OH) 2 when exposed to air. This could explain the decrease of reactivity observed after air-grinding the lead acetate: when ground in the mastic varnish, Pb(OAc) 2 •3H 2 O is somehow protected from the oxygen in the air and its surface increases without forming lead subacetate which is less reactive than the pure acetate.

We also investigated the possibility of a mechanochemical process involved in the gel formation, and in particular the hypothetical formation of radical species. Indeed, unstable radicals could potentially promote a chemical chain reaction. Therefore, we carried out preliminary Electron Paramagnetic Resonance spectroscopy (EPR) experiments that are detailed in Appendix D. However, we could not highlight the formation of radical species during the grinding. Further experiments should be carried out in order to confirm these results.

It was evident from these experiments that grinding is a key step for the gel formation and its rate. The greater the grinding time, the lower the time it takes for the gelation to occur.

That could be explained by two factors: the increase of the lead acetate crystals surface and, in the case of grinding in varnish, the potential evaporation of the solvent (turpentine) leading to a slight increase in the lead concentration in the sample. The exposure to air of the lead acetate also plays an important role, likely favoring the formation of lead subacetate, and increasing the gelation kinetics.

Summary of the findings and the proposed simplified formulations (Model Systems)

Thanks to the tests on the influence of the di erent ingredients pertaining to the gelation parameter, we were able to establish some preliminary evidence with reference to the gelation process. The oil and the polymeric fraction of the resin are not necessary to obtain a solid-like material that does not flow. Moreover, the reaction involves only the triterpenoids containing carboxylic acid moieties, such as oleanolic acid. Regarding the

Characterization techniques

Attenuated Total Reflectance Fourier-Transformed Infrared spectroscopy Principle

Fourier-Transformed Infrared spectroscopy (FTIR) is a vibrational spectroscopic technique allowing access to information about molecular structures via an absorption spectrum.

When the energy of the IR beam sent to the system studied is close to the vibrational energy of a chemical bond, it is absorbed, which results in a decrease of the recorded signal.

Consequently, the spectrum displays characteristic bands of the chemical function and its vibrational mode (elongation of the bonds, deformation of the angles).

The apparatus is composed of a source, an IR-transparent crystal (refractive index n 1 )

where the sample is deposited and a detector. The IR beam hits the sample (n 2 ) at an incidence angle and is theoretically completely reflected towards the detector (Descartes law). The Total Attenuated Reflectance (ATR) mode is based on the formation of an evanescent wave entering the first few micrometers of the sample (if n 1 > n 2 ), attenuating the total reflection and decreasing the final signal.

Setup

The analyses were carried out on a Cary 630 device equipped with a Cary 5 bounce Zinc Selenide ATR accessory (from Agilent). Liquid (varnishes) and soft samples (gels) were evenly spread on the large ZnSe crystal, which grants a longer pathlength for the infrared beam, making multi-bounce possible, thus enhancing the detection limit. The spectra were acquired with Microlab software with the following parameters: 650-4000 cm ≠1 , 32 scans, spectral resolution 4 cm ≠1 . A new background was registered between each measurement.

Nuclear Magnetic Resonance

Principle

A nucleus is made of protons and neutrons, which have a mass, an electric charge and an intrinsic kinetic momentum. The amplitude of this latter is called spin. The nuclei then possess a nuclear spin I resulting from all their protons and neutrons spins.

The Nuclear Magnetic Resonance (NMR) is a physical phenomenon occurring when an atom with nonzero nuclear spin (I " = 0) is placed into a uniform external magnetic field and excited by a high frequency radiation.

nucleus: μ = " • Î. Because " is specific to the nucleus (Table 4.1), it is possible to tune the excitation radiation frequency in order to observe the magnetic behaviour of the nucleus of interest.

Table 4.1: Some characteristic of the nuclei studied.

Nucleus Natural abundance (%) Spin I Gyromagnetic ratio " (MHz•T -1 )

1 H 99.9 1/2 42.576

13 C 1.07 1/2 10.708

207 Pb 22.1 1/2 9.033
In the absence of an external magnetic field, the spin moments of a nucleus in a sample have random direction (the total spin moment of the sample, also called the macroscopic magnetization, equals zero). When a magnetic field B 0 is applied, it interacts with the spins that are divided among di erent energetic states. This phenomenon is called the "Zeeman e ect". Because of its quantum nature, the spin can only occupy discrete energetic states defined by (2I + 1) levels proportional to B 0 . In the case of 13 C and 207 P b (I = | 1 2 |), two energetic positions are possible (and -). As described by the Boltzmann distribution

N - N - = e E kT
the population is slightly predominant in the lowest level -. Even if the spin population di erence is low, explaining why NMR spectroscopy is not a very sensitive technique, there is still a residual magnetization 1 .

When placed into a magnetic field B 0 , the spin precesses along B 0 axis at an angular speed Ê 0 = "B 0 . The Larmor frequency is then defined as follow

‹ 0 = Ê 0 2fi
During the NMR experiment, a short and intense radiofrequence pulse (B 1 , perpendicular to B 0 ) is applied (its frequency needs to respect the resonance condition:

‹ 1 = ‹ 0 ), leading
to the appearance of a total magnetic moment (transverse component). After the pulse, the magnetization recovers its equilibrium state (the system relaxes) through a precession move along B 0 (the transverse component disappears as the longitudinal component is restored). The return to the equilibrium is defined by two characteristic independant relaxation times:

The higher ‹ rot increases, the fewer the rotation bands. By varying ‹ rot , it is possible to define the iostropic chemical shift (" iso ). Coupled to the determination of the tensors (static shape), we are able to characterize the chemical environment of the nucleus studied.

Many pulse sequences have been developped then to increase the signal-to-noise ratio (S/N). Among them, one can cite two methods that have been used in this work:

• Cross-Polarization (CP) The e ciency of this method is based on the di erence of the gyromagnetic ratio (") between two nuclei present in the sample. In our case, we carried out 13 C{ 1 H} and 207 Pb{ 1 H} CP. As presented on Table 1, "1 H > "13 C and "207 P b , thus when the excitation is directed to 1 H, the magnetization can be transfered to the nucleus of interest ( 13 C and 207 Pb in our case) if the the frequency of each nucleus is tuned to match the precession frequency (Hartmann-Hahn's condition).

• Carr-Purcell-Meiboom-Gill (CPMG) Echo Train Acquisition A fi pulse applied after the first fi 2 pulse results in refocusing the spin magnetization. This phenomenon is called an echo and is used to enhance the signal. During the CPMG experiment, a series of fi pulses is added to the sequence (• ms after the first fi 2 pulse). After Fourier transformation, the NMR signal is decomposed into numerous spikelets (spaced by • ). When the signal is broader than the bandwidth of the pulses, as for 1 H ae 207 Pb CP experiments were conducted on a Bruker Advance III 800 SB spectrometer (magnetic field: 18.6 T) at 100 K (-173 °C) with a 3.2 mm rotor.

For MAS experiments, the gels were put inside the rotor, then let few minutes straight to allow the material to fill the rotor uniformly. Then, the rotor was immersed in liquid N 2 to freeze and have solid state behavior in the spectrometer.

NMR spectra were processed using Topspin software and the line shapes and CSA parameters were simulated using Dmfit program [START_REF] Massiot | Modelling one-and two-dimensional solid-state NMR spectra[END_REF]. In this study we follow the Haeberlen convention: CSA = " 33 ≠ " iso and ÷ CSA = |" 22 ≠" 11 | CSA [START_REF] Haeberlen | Advances in magnetic resonance[END_REF]. the form factor P (q). In the case of polydisperse objects, the di erent P (q) are summed up, giving an average form factor. In concentrated samples, the scattering pattern contains also information about the interaction between the contiguous objects, the structure factor S(q). These are the key parameters to describe and understand the system studied.

Setup

SAXS measurements were acquired on a laboratory device (LIONS-NIMBE CEA, Saclay, France) at 1.542 Å (Cu source) under vacuum with a Xeuss 2.0 apparatus (Xenocs).

The intensity is plotted against the scattering vector q = 4fi ⁄ • sin(◊) , with ⁄ being the wavelength and 2 ◊ the scattering angle. The displayed q-range from 2 • 10 ≠2 to 1 Å ≠1 was attained with a single sample-to-detector distance of 54 cm. The latter was calibrated with tetradecanol and the detector count was normalized by direct beam measurements.

The samples were inserted in quartz capillaries (diameter 1 mm, wall thickness 0.1 mm, WJM-Glas) before gelling. Acquisition times were of 45 min, except for the first two hours of the kinetics where it was set to 5 min to monitor the sample evolution with an adequate time-resolution. The treatment of the 2D images and of the resulting scattering profiles was performed using pySAXS. Standard procedures were applied to subtract background scattering and to normalize the intensities [START_REF] Linder | Neutrons, X-rays and light: Scattering methods applied to soft condensed matter[END_REF].

Varnish profiles could be linearly fitted by a power law I = 0.0302 ◊ q 0.813 in the [0.015 ≠ 0.34Å ≠1 ] q-range with a 0.9989 correlation coe cient. When it was not possible to directly subtract the experimental varnish profiles, we used that fit.

Results

Preliminary experiments: rheology

Measurements carried out on gumtion gels have been previously published [START_REF] De Viguerie | A 19th Century "Ideal" Oil Paint Medium: A Complex Hybrid OrganicInorganic Gel[END_REF]. The study demonstrated that gumtion is a gel according to rheological definition, as the elastic modulus G' is at least one order of magnitude higher than the viscous modulus G''over a large stress range, before reaching the flow threshold, also called the yield stress (where G' and G" intersect).

The same experiments were performed on MS1 and MS2 . We can see the emergence of a vibrational band at 1520 cm ≠1 in the gel. This zone has been reported to correspond to "resinate" or "resin-metal soap", that is ester species COO-M formed by the interaction of a metal such as lead or zinc with natural resins [START_REF] Poli | Aging of Natural Resins in Presence of Pigments: Metal Soap and Oxalate Formation[END_REF]. We can see also in the gel's spectrum the band at 670 cm ≠1 that is generally attributed to Pb-O bond vibration [START_REF] Culea | Novel bismuthLeadSilver glasses and glass ceramics doped with neodymium ions[END_REF]. Since we performed 5-bounce FTIR-ATR by just applying the soft gel on the crystal (and not with a press as we did for lead acetate powder), we assume that this band can not be due to remaining Pb(OAc) 2 •3H 2 O crystals in the gel, but results from an intrinsic interaction ocurring in the gel. The figure also displays a mastic resin carbonyl stretching band (1712 cm ≠1 ), since the resin contains a lot of carboxylic acid and ketone groups (see Chapter I). This band does not undergo noticeable change when the lead compound is added, suggesting that only few carbonyl moieties interact with the lead. and broad band around 3400 cm ≠1 , corresponding to weak hydrogen bondings taking place in the gels [START_REF] Nibbering | Vibrational dynamics of hydrogen bonds[END_REF]. could not yet explain this e ect. However, changing the spectrometer and setting up new parameters led to improvements on the spectra:

• according to the Boltzmann law ( N- N -= e E kT ), a lower temperature (100 K vs. 243 K) leads to a largest population variation, and thus a higher experimental sensitivity.

• unlike the previous configuration, a CP experiment was possible, the lower temperature appears to enhance the e ciency of cross polarization from the proton to lead atoms, possibly due to a lower mobility average of the 1 H-207 Pb dipolar interaction.

• the higher the magnetic field of the spectrometer (i.e. 800 vs. 700 MHz) leads to a larger population di erence, and thus a higher polarization.

These conditions allowed the analysis of a gel with stoechiometric proportions between the triterpenoid and the lead, so as to avoid detecting only excess lead acetate trihydrate.

Figure 4.20 compares the spectra obtained for a 18:1 and a 1:1 MS2 gels. Despite the similar line shape, slight di erences are evident, in particular for the " 11 values (-1379 and -1497 ppm respectively, see Table 4.5). Although complementary experiments are needed, it can be said that the local environment of the lead atom is very similar to the acetate trihydrate.

Finally, Figure 4.21 (c) presents the results obtained for a MS3 gel (PbO and oleanolic acid, molar ratio 18:1). The aim was to see whether the gel signal has also an acetate shape despite the absence of lead acetate. Because of experimental issues and due to time constraints, the spectrum obtained has a very weak signal. These are thus only preliminary results and would need to be conducted again in better conditions (lower temperature and an equimolar ratio between the lead and the triterpenoid). However, we can see that the gel's signal covers a very wide CS range (from ≥ 3000 to -3000 ppm), indicating that we detect the remaining PbO that did not react (3000 < CSA PbO < -1000 ppm) but also an acetate signal (down to -3000 ppm). This suggests that the lead compound interacts with the triterpenoid to form a new carboxylate species, as assumed earlier with 13 C NMR and FTIR.

Discussion

This study allowed the analysis of various gel systems in solid state. The low temperature protocol first needed a few adjustements, but then proved useful for the study of soft materials such asas the gels with MAS-NMR.

13 C-NMR revealed the formation of a new chemical environment in the acidic region (around 180 ppm) when the lead compound (Pb(OAc) 2 •3H 2 O or PbO) is added to the mastic varnish or the triterpenoid. This confirms the interaction of the lead with carboxylic acid groups of the triterpenoids. Because of the variety of organic compounds present in the mastic resin, the Model Systems 2 and 3 proved to be very suitable to simplify the spectra as well as their interpretation.

207 Pb-NMR was then carried out to give more information about the lead atom's chemical environment. The comparison of the spectra showed almost identical line shapes between Pb(OAc) 2 •3H 2 O and MS2 gels with di erent amounts of lead acetate. This can be explained by the fact that a lot of Pb(OAc) 2 •3H 2 O is introduced in the systems, and the NMR acquisition detects only the excess lead acetate. Indeed, the spectrum of the gel prepared with equimolar proportions has a slightly di erent " 11 value. However, the line shape is again very similar to the lead precursor, this validates the fact that the lead chemical environment in the gel is very close to the one in lead acetate trihydrate. In addition, despite the poor quality of the MS3 gel spectrum (Fig. 4.21), it seems that signal is detected over the lead acetate region. A similar experiments in better conditions is planned to verify this result. All of these analyses confirm the 13 C-NMR interpretations stating that the lead interacts with the carboxylic acids, forming new RCOO•••Pb (acetate) species.

Structural analysis

Cryo-microscopies

Cryo-microscopy techniques (FF-EM and cryo-STEM) were carried out on gumtion and MS gels in order to know more about their structure. Figure 4.22 presents FF-EM micrographs of (a) a gumtion (original recipe, see Chapter II) and (b) MS1 gel. In some areas we can see a "river pattern" that can reach few microns length. This type of feature is characteristic of lamellar structures [START_REF] Yang | Microstructural characterisation of fatigue crack growth fracture surfaces of lamellar Ti45Al2Mn2Nb1[END_REF].

Cryo-STEM analyses were performed on 80 nm cuts of MS1 (Fig. 4.23) and MS2 (Fig. 4.24).

In both cases, the micrographs shows the heterogeneity of the samples, containing both dispersed and aggregated nanoparticles of di erent sizes, as well as more organized structures.

Due to experimental constraints, the experiments were carried out at di erent times after 2014).

In the case of MS2 and MS3 (composed of oleanolic acid), a strong scattering pattern was meaqured due to the fibrillar organisation of the triterpenoid in the solvent (possibly already forming an organogel) that hinders a good interpretation of the curves obtained when lead components are added. Further analyses are needed for MS2 and MS3 (lower q range), but the fact that oleanolic acid in turpentine seems to form an organogel suggests that it may not be a good system to study the structure of MS2 and MS3.

Conclusion

This study focused on the characterization of lead-triterpenoids gels, in order to better grasp the chemistry and structure of the gumtions. These materials were prepared after thorough historical reconstructions, and Model Systems (MS) were formulated to isolate the reactive components and facilitate their determination.

First, as demonstrated with viscoelastic measurements, the gumtions and MS behave as gels. The gel prepared with the commercial triterpenoid (oleanolic acid) instead of mastic varnish is more sti . For example, at equivalent molar ratio, G' and G" values are 2 to 3 times higher in MS2 than in MS1. In other words, the more lead in the system, the more solid-like the behavior. The stronger network could be explained by more junction zones.

To adjudicate on this question, it would be interesting to increase the lead amount in the systems to see whether a plateau is reached in terms of mechanical properties. If so, we could determine an amount of "excess lead" that does not take part into the architecture.

Then, spectroscopic analyses using FTIR-ATR and solid-state NMR gave information about the molecular interactions. Gumtion gel was found not appropriate for FTIR characterization, because its signal corresponds to the combination of its organic components (oil, varnish) that overtake the spectrum. Thus, MS proved to be useful for molecular analyses. FTIR-ATR and 13 C-NMR demonstrated the interaction of the lead compound and the carboxylic acid moieties of the triterpenoids, by the formation of resinate bands in FTIR (1500-1600 cm ≠1 ) and of a new chemical environment in the acidic carbon region in NMR (≥ 180 ppm). In addition, 207 Pb-NMR was carried out to investigate more precisely the local environment of lead in the gels. The results show the wide line shape of Pb(OAc) 2 •3H 2 O detected in the gels, regardless of the amount of lead acetate, meaning either that the lead in the gel has the same configuration as in the precursor or that we detect only excess lead acetate. Nevertheless, the spectrum of the gel with equimolar proportions of lead and triterpenoids has a slightly di erent " 11 than the other gels, indicating a weak di erence in the lead environment, remaining however very close to the initial configuration in the precursor. We could not go further into the determination of the coordination mode of the acetate species formed during gelation, but FTIR-ATR spectra suggest the coexistence of several coordination modes, while NMR indicates only one type of chemical environment for the lead. This could be explained either by the detection of only one predominant geometry (NMR is not a very sensitive technique), or by the presence of several carboxylate species that give more or less the same line shape. More investigations should be performed on those systems to elucidate the lead coordination in the gels, such as high-resolution solid-state NMR on gels containing low amounts of lead compound, or FTIR spectra decomposition.

Regarding the structural characterization of the gels, two complementary techniques were used to probe locally and globally their supramolecular organization. SAXS experiments attest to the rapid (few hours) formation of organized objects in the gels, described as platelet-like objects with a characteristic distance of 2.41 nm. As this value corresponds to the distance between two oleanolic acid molecules acting as Pb 2+ bidentate ligands, one can assume the self-organization of this basic unit. Furthermore, the appearance of two di raction peaks during the kinetics study were linked to the formation of small objects, likely composed of lead and long chain organic molecules. On the other hand, no specific organization could be observed by cryo-microscopies in fresh samples. Indeed, if coordination complexes scatter enough X-rays to be detected with SAXS, they do not give contrast in electron microscopies. Nevertheless, these techniques revealed the presence of lamellar structures forming in the gels after few months curing. Those lamellas are notably composed of lead-containing crystalline nanoparticles, that self-assemble first into filaments and then sheet-like shapes. Further di raction analyses are necessary to determine the crystalline phase of the nanoparticles.

Our multi-scale analytical approach unveiled the molecular interactions occurring in the gel, leading to supramolecular organization and explaining the solid-like behavior of the gel at a macroscopic scale. Moreover, the structural study provided valuable insights on the organization process occuring in two times: gelation (short term) and ageing (medium term). Therefore, we propose the following mechanism, summarized on Figure 4.34.

Pb(OAc) 2 •3H 2 O is first intimately mixed with mastic varnish (mainly made of triterpenoids in turpentine). Thanks to coordination between lead and carboxylic acid groups of the triterpenoids (considered as the driving force for gelation), lead acetate can partially dissolve in the organic mixture. Rapidly, the triterpenoids interact with lead and self-assemble via hydrogen bondings and electrostatic forces into 2D molecular objects dispersed in the material and trapping the solvent (turpentine) through surface tension. This organometallogel (where the complex lead-triterpenoids act as LMWG) macroscopically presents a solid-like behavior. After few months, the system observed with electron microscopy is a mixture of di erent objects and contain assembled as well as isolated nanoparticles (a few tens nanometers in diameter). As we did not yet determine the crystallographic phase of these nanoparticles, two hypotheses are envisaged: these NP could result either from the law of their refractive indices. It was also possible with the same technique to perform dynamic measurements and to monitor the films swelling in di erent atmospheres (water, and iso-propanol and isooctane -two organic solvents used by restorers). This approach had been developed to study the behavior of polymeric materials and it was used for the first time on paint materials in this work. It will be interesting in the future to investigate how gumtion modifies the optical properties of paint: while it was not possible in the time frame of the PhD and with visible SE, it will be with a new system recently acquired in the infrared range (IR-SE).

After the characterization of mastic, and in order to grasp the constraints and the behavior of the materials during gumtion preparation, a reconstruction -based on historical recipeswas carried out. The translation of the old recipes into chemical terms can sometimes be tricky, and the first approach as an investigator, is to set-up synthesis protocols. Once the gumtion preparations had been understood, it was possible to change several variables to target the key parameters involved in the gelation process.

In particular, we evaluated the influence of the lead compound (and its proportions) on the gelation. As expected, the more lead in the system, the faster the gel formation. In addition, we varied the metallic compound, and it appears that the gel forms only when Pb(OAc) 2 •3(H 2 O), PbO, Pb 3 O 4 and 2PbCO 3 •Pb(OH) 2 are employed as the lead source.

These experiments showed the importance of the lead atom, and a strong influence of the low ionic character of the metal-ligand bonds.

In addition, during the reconstructions, the grinding of lead acetate turned out to be a key step for the formation of the gel. Several experiments showed that not only the morphology (and the surface to bulk ratio) of the lead acetate particles a ects the system, but also its exposure to air and thus the formation of lead subacetate that seems to be less reactive.

Furthermore, mechanochemical-induced reactions have been envisioned, but no radical species has so far been detected in Pb(OAc) 2 •3(H 2 O). Further EPR experiments would be required here to reach a conclusion. One can also hypothesize di erent mechanisms (for instance ligands exchange) taking place thanks to the energy provided during grinding.

An exploration of the historical reconstructions led to the smplification of the gel formulations, in order to study the processes taking place during gelation at di erent scales with appropriate techniques. The di erent gels have been studied at molecular and supramolecular scales, thanks to a multi-technique approach. First, spectroscopic analyses (FTIR and NMR) demonstrated the formation of a "resinate", that is a carboxylate complex involving lead and the acidic triterpenoids. From the supramolecular point of view, SAXS experiments highlighted an early structural organization in the gels, composed of 2D objects that organize in layered patterns dispersed in the material. Moreover, cryo-electron microscopy showed that the materials present self-assembled nanoparticles in platelet-like structures. Further experiments would be needed to confirm this but the systems seemed to be highly time-dependent, which reminds us that paint material is a dynamic system, that keeps on evolving even after application onto the canvas.

The global approach adopted for this research allowed the investigation of the molecular interactions occurring in the gel, leading to the identificationn of mesoscopic self-assembly, which explains the macroscopic properties of these solid-like materials. As a result of this work, it can be concluded that lead acetate and carboxylic acid groups coordinate and combine in a supramolecular network trapping the solvent, thus forming a metallogel.

Advanced techniques, resulting from numerous collaborations, were used and adapted to the study of this complex material. One should now consider broader questions to fulfil the global understanding of gumtion gels chemistry and the influence these gels have on the appearance and behavior over time of the works of art of which they are a part.

A strong time-dependent behavior of the gels structure was observed, and as gel Mediums are suspected of being responsible for the deterioration of paintings, conducting an ageing study is necessary to understand the changes taking place in the gel. Short-term and longterm impact would be interesting to consider. Indeed, even a relatively rapid modifications of the structure (a few months) could result in optical changes in the painting, while longer durations could lead to important degradation processes (solvent evaporation, cracks. . . ).

These changes could be monitored in situ thanks to IR-SE after long-term drying, as well as exposure to solvents or humidity .

Another important question lies in the reactivity of the di erent resins, which can also form metal "resin soaps" (as these are called in the field). We highlighted the formation of lead-mastic resin complexes opening numerous questions about the reactivity of metal ions in resins, which has to be put in relation with their reactivity in oil systems (kinetics, stability). This formation of resin soaps could also be involved in degradation phenomena that should be further investigated.

To our knowledge, gumtions are the first examples of metallogels to be synthetized. This demonstrates the power of the empirical approach of painters, turning their art studios into process laboratories: they developed and optimized complex chemical processes that are still di cult to explain today. Paint and mediums, composed of organic binders, pigments, metallic salts etc. are intrinsically hybrid materials, that belong to the History of Techniques as much as to the field of Materials Sciences. A synergistic approach to the study of these materials is now more common and promises to provide many valuable results.

  surface des particules d'acétate de plomb. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1.2 Materials and method . . . . . . . . . . . . . . . . . . . . . . . . . .

  using two spectroscopy techniques: Fourier-Transfrom Infrared (FTIR) and proton Nuclear Magnetic Resonance ( 1 H-NMR) spectroscopies. By comparing the gel and the solid LMWG FTIR spectra, Hu et al. (2009) point out the shift of the O-H stretching band. Variable temperature 1 H-NMR experiments carried out by Lu et al. (2013) indicate the breaking of intermolecular hydrogen bonds when the temperature increases and T gel is reached (gel to sol transition).

  (origin: Chios Island, Greece, 2016), Laverdure (origin: Chios Island, Greece and North Africa, 2015) and the Chios Mastiha Growers Association (CMGA, origin: Chios Island, Greece, 2012). Ancient mastic resins samples were provided by the National Museum of Natural History collection (Paris, France); they have been part of the collection since at least 1850. All samples have been stored in glass jars, in a room without curtains.

Figure 2

 2 Figure 2.7: Refractive indices dispersion laws of several varnishes, obtained by Polikreti & Christofides (2006).

207

  Pb, one can use wideband, uniform rate, smooth truncation (WURST) pulses during the acquisition. The so-called WURST-CPMG sequence provides a wideline spectrum with a good S/N (O'Dell & Schurko, 2008). Setup 13 C NMR experiments were carried out on a Bruker Advance III 500 SB spectrometer (magnetic field: 11.6 T). The experiments were conducted at 243 K (-30 °C) at a ‹ rot = 10 kHz with a 4 mm rotor. For 207 Pb NMR analyses, two setups were used. The first series of experiments were operated on a Bruker Advance III 700 SB spectrometer (magnetic field: 16.3 T), tuned at 146.471 MHz for 207 Pb resonance. The MAS and WURST-CPMG experiments were carried out at 243 K (-30 °C) with 2.5 and 4 mm rotors respectively.

Figure 4 .

 4 Figure 4.7 compares the FTIR spectra of MS1 gels with di erent amounts of Pb(OAc) 2 •3H 2 O. It is interesting to note that, in addition to the band at 1520 cm ≠1 , a second band at 1579 cm ≠1 appears. This could indicate the formation of two di erent resinate species in the gel. In the greater wavenumbers (Fig. 4.7 left), one can note a weak

Figure 4 .

 4 Figure 4.8 represents the FTIR-ATR spectra of MS2 and MS3 gels and their components: oleanolic acid in turpentine and the lead compound (Pb(OAc) 2 •3H 2 O for MS2 and PbO for MS3). In both gels, two carbonyl stretching bands are detected at ca. 1715 and 1689 cm ≠1 . These two bands can in fact be assigned to the oleanolic acid as we can see on Figure4.9: the oleanolic acid signal shows also two carbonyl stretching bands at 1720

historical recipes, reconstructions, and Model System formu- lations 1

  Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2 The historical recipes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

	2	Spectroscopic Ellipsometry on mastic varnish and paint materials . . . . . . 2.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.2 Materials and methods . . . . . . . . . . . . . . . . . . . . . . . . . . 2.3 Original recipe: oil, mastic resin, solid lead acetate . . . . . . . . . . 2.2 Variations A: oil, mastic varnish, solid lead acetate . . . . . . . . . . Results . . 2.1 2.3 Variations B: oil, mastic varnish, aqueous lead acetate . . . . . . . .

1.3 Results . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1.4 Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.4 Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3 Gumtion:

Table 1 .

 1

	Components	Megilp	Gumtion
	Siccative oil	Cooked with litharge	Raw
	Mastic resin	In powder or in turpentine	In powder or in turpentine
		(varnish)	(varnish)
	Lead compound	Litharge (--PbO)	Lead acetate

1: Comparison between megilp and gumtion recipes, according to the texts reported in

Carlyle (2001)

.

  made of raw linseed oil, mastic resin and

	Chapter 2
	lead acetate trihydrate

Table 2 . 1 :

 21 Estimate of the absolute proportions (%w) of the trimethylsilylated main triterpenoids identified in the mastic resin from Laverdure, Kremer and Chios Mastiha Growers Association (%w > 0.5).

		Main triterpenoids identified Mw Laverdure Kremer CMGA
	(1)	(epi)lanosterol	498 0.5	0.6	0.5
	(1')	(epi)lanosterol	498 4.9	4.8	4.7
	(5)	28-nor-olean-12-en-3-one	410 2.0	2.0	2.3
	(7)	--amyrone	424 1.8	2.0	1.6
	(8)	28-nor-olean-12,17-dien-3-one 408 1.8	1.5	1.5
	(9)	--amyrin	498 1.9	2.0	2.0
	(21)	olean-dien-3-one	422 3.7	4.5	4.2
	(26)	Moronic acid	526 12.9	14.0	12.5
	(27)	Oleanonic acid	526 12.5	11.4	7.1
	(29)	Oleanonic aldehyde	438 1.8	2.2	1.4
	(30)	Oleanolic acid	526 0.6	0.9	0.8
	(36)	Ursonic acid	526 0.8	0.9	0.4
	(42)	(iso)masticadienoic acid	526 3.6	3.1	2.9
	(42')	(iso)masticadienoic acid	526 2.2	2.4	1.9
			Laverdure Kremer CMGA
	Total (%w) detected (identified and unidentified)	66.7	73.1	60.2

Table 2

 2

.1 presents the main band assignements. It shows the estimation of the absolute proportions of the main triterpenoids identified. For the three mastic resin sam-ples, the proportions are quite similar. The main components are: (1) moronic acid (2)

Table 2

 2

		.2: Estimate of the absolute proportions (%w) of
		the main trimethylsilylated triterpenoids identified in the
		historical mastic resins.
		Identified triterpenoids	Historical 1 Historical 2 Historical 3
	(43)	dammarenolic acid	0.3	1.2	0.2
	(44)	dammaradienone	0.2	0.2	0.1
	(45)	lupenone	0.1	0.2	0.1
	(*)	nor-oleanene	1	0.0	0.4
	(*)	nor-oleanene	2	0.1	0.0
	(26)	moronic acid	0.2	2.1	0.5
	(27)	oleanonic acid	0.8	3.9	0.7
	(42)	(iso)masticadienoic acid	0.1	0.1	0.1
	(42')	(iso)masticadienoic acid	N/A	0.1	N/A
	Total (%w) detected identified and unidentified 4.9	17.5	5.4

Table 2

 2

	Table 2.3: Refractive indices n of di erent materials according
	to the literature.
	Material	Refractive index n
	Mastic resin	1.536, ⁄ unspecified (de la Rie, 1987); 1.537 at 589 nm (Polikreti
		& Christofides, 2006)
	Dammar resin	1.539, ⁄ unspecified (de la Rie, 1987); 1.539 at 589 nm (Polikreti
		& Christofides, 2006)
	Linseed oil	1.478, ⁄ unspecified (Hess & O'Hare, 1950; Yadav et al., 2009);
		1.4769, ⁄ unspecified (Caldwell & Mattiello, 1932); 1.48, ⁄
		unspecified (Laurie, 1937)
	Eggwhite	1.530 at 589 nm

.3 presents the values of the refractive index n reported in the literature (with various determination methods) for several materials relevant for the study of old masters' paintings.

:

  Dryer and Medium, Bulkley, 1821; Gumtion, Ibbetson, 1828;

	Vehicle, Fielding, 1839.
	Etty's Medium, Gilchrist, 1855 & Standage, 1892; Megilp, Edwards, 1856; Sir William Beechey's Drying
	Oil, Sully, 1873. 3 As cited in Carlyle, 2001 (pp.396, 398): Gumtion, Field, 1835; J. M. Leigh's Medium, Roberson, 1835;
	Another Megelp [Recipe 2], Cawse, 1840; Medium No.3, Templeton, 1846; Medium No.4, Reeves and Sons,
	1852; Vehicle, Edwards, 1856; Another Macguilp [Recipe 3], Sully, 1873; Etty's Medium, Standage, 1892.

2

As cited in

Carlyle, 2001 (pp.395-397)

: Gumtion,

Field, 1835; Etty Mixture, Roberson, 1835; White Macguelph, Arnald, 1839; Gumtion, Osbourn, 1845; Medium No.2, Templeton, 1846; Gumtion, Field, 1850;

3 Variations B: oil, mastic varnish, aqueous lead acetate

  Medium No. 4, Reeves and Sons, 1852 " [. . . ] mixing one part of saturated solution of sugar of lead in water with two parts of linseed oil, stirred or shaken together until they are combined, when two parts of mastich

	varnish are to be added, and all well mixed. This forms a white, creamy, opaque compound,
	but it dries transparently."
	• Vehicle, Fielding, 1839 • Vehicle, Edwards, 1856
	"[. . . ] mixing one part of a saturated solution of sugar of lead in water, with two parts of "A Vehicle is made of the purest gum mastic and sugar of lead, about equal part (by weight), ground very fine with much oil (linseed, poppy or nut oils, unboiled)." linseed or poppy oil. These are to be well stirred or shaken together, till they are combined;
	2.2 Variations A: oil, mastic varnish, solid lead acetate
	• Medium No. 3, Templeton, 1846
	"Mix one part of a saturated solution of Sugar of Lead in Water, with two parts of Linseed
	oil, stir them well together, till they are intimately combined, then add two parts of Mastich

• Gumtion,

Field, 1835 "If [. . . ]

the simple pure linseed oil be used with about an eighth of acetate or sugar of lead [. . . ] ground fine, we obtain variously the opaque mixture called gumtion." • Etty Mixture, Roberson, 1835 " 1 2 oz. grd Sug Lead to 1 oz Mastic Var & 2 oz Lins' Oil [. . . ]" • White Macguelph, Arnald, 1839 "The light [Macguelph] is composed of sugar of lead, made thin with linseed oil, two thirds, and then mixed with one third mastic varnish, which will see to the consistency of butter." • Gumtion, Field, 1850 "Composed of no more than an eighth of the acetate or sugare of lead, with simple oil and strong varnish [. . . ]." • Megilp, Edwards, 1856; "[. . . ] linseed oil, in which a small quantity of finely-ground sugar of lead has been di used.

With equal parts of this compound, and of mastic varnish, a very light megilp is obtained."

2.

Varnish; this forms a white creamy opaque compound

[. . . ]

It is called Gumption, is an excellent drier, and much used."

•

  2 •3H 2 O and triterpenoids is required (around 0.09). It is interesting also to see that the amount of Pb(OAc) 2 •3H 2 O impacts on the gelation time: the more Pb(OAc) 2 •3H 2 O was added, the shorter the gelation time (although not indicated in Table3.1, this e ect was also observed in the original gumtion recipe). For information, the amount of water was determined by the

Karl-Fisher method. A gel without oil with a 1:1 (Pb:triterpenes) molar ratio is 2.4%m (0.2%m in the varnish), leading to 2:1 water/Pb molar ratio.

Table 3 .

 3

	PbOAc (s) (g) Approx. molar ratio Pb/triterpenes gelation?
	0.05	0.076	x
	0.10	0.15	x
	0.20	0.30	x
	0.25	0.38	?
	0.30	0.46	v
	0.50	0.76	v
	1.00	1.5	v
	1.20	1.8	v
	2.00	3.0	v
	4.00	6.0	v

1: Impact of the amount of Pb(OAc) 2 •3H 2 O on the gelation of di erent systems based on the original recipe (oil 4g, mastic resin 1.2g).

  1 (with di erent molar ratios between Pb(OAc) 2 •3H 2 O and the triterpenoids) and the results are presented on Figure4.4

	and Table 4.2. Viscoelastic measurements have been performed on a controlled stress
	rheometer (HAAKE RS 600 ), equipped with a cone/plate geometry (diameter 35 mm,
	angle 2°, gap 103 µm).

Table 4 .

 4 3: Attribution of the main bands detected in the gumtion OAc) 2 •3H 2 O and mastic varnish in di erent proportions. It has been analyzed with FTIR-ATR and Figure 4.6 shows the spectrum compared with mastic varnish and Pb(OAc) 2 •3H 2 O s in the region 1900-600 cm ≠1

	FTIR spectrum.
	venumber (cm ≠1 ) Gl obal assignment Co mponent
	2800-3000	‹(sp 3 C-H)	oil, mastic varnish, turpentine
	1742	‹(C=O)	oil (fatty acids)
	1712	‹(C=O)	mastic resin (acid triterpenoids)
	1458	"(CH, CH 2 , CH 3 )	oil
	1237, 1160	‹(C-O)	oil (triglycerides)
	889, 787	"(sp 2 C-H)	turpentine
	722	fl(CH 2 )	oil (aliphatic chain)
	Model Systems
	MS1 is composed of Pb(

Table 4 .

 4 4: Experimental CSA parameters (in ppm) for the di erent lead compounds studied compared to the literature.In this configuration, for MS3 gels (with PbO) and MS2 gels with a lower amount of Pb(CH 3 COO) 2 •3H 2 O, we could not obtain a good S/N. Hence, we decided to carry out Dynamic Nuclear Polarization (DNP) experiments. The principle is comparable to the CP (see §2.2.2.), but we use the polarization of unpaired electrons (" e ≥ 28025 MHz•T ≠1 ) coming from radicalar species added to the sample before the acquisition. But no DNP e ect could be observed in our materials, on the contrary the signals decrease, and we

	Lead compound	Reference	Experimental conditions	" iso	CSA	÷ CSA	" 11	" 22	" 33
	Pb(NO 3 ) 2	Fayon et al. (1997)	MAS (‹ rot = 1-15 kHz), RT	-3494	55	0	-3476	-3476	-3531
		This work	MAS (‹ rot = 20 kHz), T = 298 K	-3494	-	-	-	-	-
		This work	MAS (‹ rot = 20 kHz), T = 243 K	-3530
	Pb(CH 3 COO) 4	Buston et al. (2001)	CP-MAS (‹ rot = 2-3 kHz), 303 K	-1873	296	0.66	-1988	-1938	-1873
		This work	MAS (‹ rot = 20 kHz), T = 298 K	-1891	181	0.25	-2004	-1959	-1710
	Pb(CH 3 COO) 2 •3H 2 O	Kye et al. (1999) This work	Hahn echo, 299 ± 1 K MAS (‹ rot = 20 kHz), T = 243 K	-1942 -1989	-1041 -970	0.32 0.25	-1256 -1383	-1588 -1626	-2984 -2959
		This work	CPMG, T = 243 K	-1992	-967	0.34	-1343	-1674	-2959
	--PbO	Kye et al. (1999) Fayon et al. (1997)	Hahn echo, 299 ± 1 K MAS (‹ rot = 1-15 kHz), RT	1910 1939	-2215 -2076	0 0	3017 2977	3017 2977	-305 -137
		This work	CPMG, T = 243 K	1963	-2100	0.035	3050	2977	-137
	--PbO	Kye et al. (1999) Fayon et al. (1997)	Hahn echo, 299 ± 1 K MAS (‹ rot = 1-15 kHz), RT	1503 1515	-2673 -2487	0.034 0.15	2825 2945	2733 2573	-1170 -972
		This work	CPMG, T = 243 K	1523	-2573	0.027	2845	2775	-1050

Table 4 .

 4 5: Experimental CSA parameters (in ppm) obtained for the gels (composed of di erent proportions of Pb(CH 3 COO) 2 •3H 2 O and oleanolic acid).

	System studied	Experimental conditions	" iso	CSA	÷ CSA	" 11	" 22	" 33
	Pb(CH 3 COO) 2 •3H 2 O	MAS (‹ rot = 20 kHz), T = 243 K	-1989	-970	0.25	-1383	-1626	-2959
		CPMG, T = 243 K	-1992	-967	0.34	-1343	-1674	-2959
	MS2 gel (molar ratio 31:1)	MAS (‹ rot = 20 kHz), T = 243 K	-1986	-1009	0.35	-1305	-1658	-2995
	MS2 gel (molar ratio 18:1)	CPMG, T = 243 K	-2005	-967	0.34	-1357	-1686	-2972
		CP, T = 100 K	-2010	-932	0.35	-1379	-1708	-2942
	MS2 gel (molar ratio 6:1)	CPMG, T = 243 K	-1993	-987	0.34	-1330	-1670	-2980
	MS2 gel (molar ratio 1:1)	CP, T = 100 K	-2048	-894	0.23	-1497	-1706	-2942

[START_REF] Beck | Multistimuli, multiresponsive metallo-supramolecular polymers[END_REF][START_REF] Roubeau | Thermoreversible gels as magneto-optical switches[END_REF]. Furthermore, the redox character can be

"Translation from French: "mastic (in tears)"

Nowadays, commercial linseed oil is already pre-processed, implying that the oil is not necessary the same as an "historical" raw linseed oil(Carlyle,

2017).

The approximative amount of triterpenic fraction has been evaluated as explained in the mastic varnish protocol, p75.

This is a simplified way to explain the phenomenon. In reality, the spins can be described as a superposition of states, themselves composed of di erent wave functions of the --and --states. It results that the spin can have various directions even at the equilibrium. However, the Boltzmann equation still applies and leads to an overall magnetization of the sample.

Due to experimental constraints, we were not able to measure the viscoleastic properties of MS3.

Acknowledgments

funded by the Doctoral School ED397 and the MiChem labex

The first part of this study consisted of the reconstruction of historical Medium recipes, in particular Ibbetson's "gumtion". This step is important to understand the experimental constraints of the preparation of the gumtion and to experience the painter's experience with the physical consistency of the material during its preparation, essential to their final choice.

Indeed, gumtion is reported to be a "gelled medium" as cited in Carlyle (2001) (p.101), the historical recipes give qualitative description of the properties of gumtion, such as: "consistency of butter" (Bulkley, 1821 p.54;Arnald, 1839 p.11), "white, creamy, opaque compound" (Reeves and Sons, 1852 p.61), "white creamy emulsion" (Edwards, 1856 pp.30-1), "opaque mixture" (Field, 1835 p.208), "sti " (Ibbetson, 1828 p.16). . . As Ibbetson says "A good deals depends on the practice, in preparing Gumtion[. . . ]". Despite all the details provided by the authors, which are cited in Carlyle (2001), it was sometimes tricky to set a precise protocol based on the historical texts. For instance, the quantities for Sugar of Lead are sometimes indicated as following: "about the size of a bean" (Gilchrist, 1855 p.256;Standage, 1892 pp.51-2) or "as much sugar of lead as will cover a sixpence[. . . ]" (Cawse, 1840 p.18). Luckily, some authors specified values in several recipes (in ounce or (tea)spoonfuls) or gave equivalence (e.g. "equal parts (by weight)", Fielding, Chapter 4

Study of the gels 1 Introduction

In this chapter, the focus is on the characterization of gumtions to understand their gelation mechanism. The gumtion gels and simplified formulations (Model Systems, MS), formulated after the historical recipes reconstructions (cf Chapter III) are analyzed through a multi-analytical approach. Preliminary rheological experiments confirm the gel nature of our model systems. Then, spectroscopic analyses, Fourier-Transform Infrared (FTIR) and Nuclear Magnetic Resonance (NMR), are carried out to investigate the molecular interactions occurring in the gels. Finally, the gels are examined at the supramolecular scale by means of cryo-microscopies and Small-Angle X-ray Scattering (SAXS) experiments.

Materials and methods

Gels studied

The reconstructions and tests on the gel recipes (see Chapter III) allowed us to develop three simplified formulations. The gels obtained are then called "Model Systems" (MS) and the exact protocols are described in Chapter III §4.2.

proportions from 1:0.1 to 1:31)

• Model System 3 (MS3): oleanolic acid + PbO (s) (various molar proportions from 1:1 to 1:18)

The solvent used is always turpentine.

Cryo-electron microscopies

The morphology of the gel was studied by using cryo-electron microscopy techniques: freeze-fracture trasmission electron microscopy (FF-TEM) and Scanning-TEM in High Angular Annular Dark Field mode (STEM-HAADF).

Freeze-fracture TEM

This technique allows the analysis of a print of a fracture, thus we observe the topology of the fracture line, giving informations about the structures in the sample. It consists of first freezing, second fracturing the gel, and third creating a replica of the gel by vacuum deposition of platinum and carbon. This last step generates a "cast" providing a planar view of the internal organization of the gel. Then the cast is examined by transmission electron microscopy.

Freeze-Fracture Transmission Electron Microscopy (FF-TEM) experiments were carried out at the Département Innovation Matériaux of LVMH Recherche Parfums et Cosmétiques (St-Jean de Bray, France). The gel samples were forthwith frozen with liquid propane and then kept in liquid nitrogen (-160 °C). Then, fracturing and shadowing were executed into a cool BAL TEC BAF 060 freeze-fracture system. The samples were coated with Pt (4 nm) for shadowing at 45°and C (30nm) for better electronic conduction. The replicas obtained were finally washed in THF and distilled water, and placed on copper grids for observation under a TEM FEI CM120.

Cryo-STEM HAADF

The HAADF system allows the detection of the electrons scattered at large angles. The signal intensifies for heavy atoms (such as lead, Z = 82), and their image is brighter.

In contrast to FF-TEM, this techniques allows the direct observation of the sample. The gels were analyzed at ICPMS (Strasbourg, France) using the Tokuyasu method [START_REF] Tokuyasu | A technique for ultracryotomy of cell suspensions and tissues[END_REF]: the samples were frozen in the presence of sucrose (allowing better cutting) and sectionned into 80 nm cuts. The cuts were stored in liquid nitrogen (-160 °C) and then transfered inside the microscope (Jeol 2100F ) for analyses in Cryo-Scanning TEM in High Angle Annular Dark Field (STEM-HAADF) mode.

Small Angle X-ray Scattering

Principle

Small Angle X-ray Scattering (SAXS) is based on the scattering of X-ray radiation by particles or molecules. The core electrons of the sample interact with the incident X-ray

General conclusions

The study of paint materials and more generally cultural heritage objects is challenging because it involves complex materials both in terms of their composition and their history, sometimes poorly known. The rise of this field of research at the crossroads of disciplines nevertheless makes it possible to obtain crucial information for historians and art conservators. This case study on gumtion chemistry is meant to demonstrate the possibility of characterizing and analyzing a material that was used in the past: gel Mediums as additives for oil paint. Despite its complexity both in composition and dynamics, it was possible to establish rigorous protocols and obtain a variety of information regarding its structure.

Thanks to complementary analytical techniques, it was possible to study di erent aspects of these materials and the simplification of their formulation in Model Systems allowed a better understanding of the physico-chemical interactions involved in their formation.

Gumtions are made from natural compounds which are therefore complex in composition.

Particular attention was drawn to mastic resin. Indeed, while the linseed oil and the polymer part of the resin can be easily removed from the formulation to form gels, the use of mastic triterpenoids is mandatory. The oil and the polymer probably only have a role in the overall behavior of the material, which is a crucial issue for painters, but these materials are not directly involved in the gelation mechanism.

The thorough study of mastic resin with GC/MS carried out in this work allowed its composition to be established and the proportions of the main triterpenoids to be evaluated. Among these, several contain carboxylic acid moieties which are involved in the interactions leading to gelation. It is interesting to note that, although dammar resin also contains acidic triterpenoids including some that are also detected in mastic, such as oleanolic acid for instance, we could not obtain a gel when replacing mastic with dammar.

To complete the study of mastic varnish, we probed its optical properties using Spectroscopic Ellipsometry (SE). Then the study was extended to other paint materials: dammar varnishes, eggwhite and linseed oil. In all cases, it was possible to prepare thin films (a few hundreds nanometers thick) with optical quality, using dip coating. The materials were analyzed as Cauchy layers (transparent) between 400 and 1000 nm, and we determined the dispersion

Appendix A -Gelation tests

We used di erent siccative oils to prepare the gumtions: linseed, poppy seed and walnut oils.

We did not remark any di erence in the behavior of the material and all the preparations formed gels at the same speed. The only di erence stands in the color of the gel, the poppy seed-based gumtion is less yellow than the two other ones, because of the initial colors of the oils.

Regarding the resins, we substituted the mastic with dammar (triterpenic resin), colophony, sandarac and copal (diterpenic resins). The preparations did not form a gel except in the case of colophony: a non-flowing pale yellow material was formed after one day in the jar.

The following protocols were used: We used N-tert-Butyl-alpha-phenylnitrone (PBN) as a spin-trap, and experiments were performed at room temperature and ambient atmosphere. We used an agate mortar for the grinding. Among all the di erent experiments, no radical species was detected in Pb(OAc) 2 •3H 2 O before or after grinding. The di erent configurations of lead acetate trihydrate alone were tested:

Variation on the Original recipe of Gumtion

Then we investigated the impact of grinding Pb(OAc) 2 •3H 2 O in the mastic varnish. We could verify that the varnish itself contains some radical species (known and described in the literature).

• d) Mastic varnish (567 mg) + PBN (11.0 mg)

• e) Mastic varnish (2.011 g) + Pb(OAc) 2 •3H 2 O (0.511 g) ground 1 min. Addition of PBN (6.6 mg) : the intensity of radical species detected is lower than in the varnish alone (d). This could be explained by the amount of PBN that is lower as well.

• f) Mastic varnish (2.011 g) + Pb(OAc) 2 •3H 2 O (0.505 g) + PBN (11.1 mg) ground together 1 min : same result as (d). The amount of PBN does not explain the lower intensity.

• g) Sample (e) allowed to rest one night in darkness: the signal is more intense and apparition of a second radical specie. This suggests a slow radical process.

• h) Mastic varnish (2.003 g) + Pb(OAc) 2 •3H 2 O (0.505 g) + PBN (10.8 mg) ground 10 min : radicals detected, the signal is more intense. This could be explained by the longer grinding time or longer exposure to light.

•

i) The sample (h) is illuminated at 385 nm during 40 s: the signal is doubled, demonstrating a strong e ect of the light exposure on the amount of radicals.

• j) Mastic varnish (2.008 g) + Pb(OAc) 2 •3H 2 O (2.003 g) + PBN (10.2 mg) ground 1 min : the signal is less intense than (e) and (f).

Preliminary conclusions:

• Pb(OAc) 2 •3H 2 O alone does not contain radical species, even after grinding (a, b, c).

• Mastic varnish alone contain radical species, as described in the literature (d).

• When Pb(OAc) 2 •3H 2 O is added and ground in mastic varnish, the intensity of the signal decreases. The more Pb(OAc) 2 •3H 2 O, the less radical species.

• When comparing systems with the same amount of Pb(OAc) 2 •3H 2 O, the longer the grinding, the more intense the signal. This could suggest an impact of the grinding on the radicals formation, but experiment (i) indicates a strong e ect of the light exposure, that can explain the increase of the signal.

Due to time constraints, we could not deepen these experiments. In particular, these are preliminary analyses which results need to be confirmed.