Marina Bousvarou

Gilles Tournillon

Jean-Christophe C Dupré

Delphine Jullien

Wood Science and Technology III: methods to examine panel paintings and their preventive and remedial conservation

Keywords: panel painting, cradle, climate variation, stereo correlation monitoring, mechanical behavior

de documents scientifiques de niveau recherche, publiés ou non, émanant des établissements d'enseignement et de recherche français ou étrangers, des laboratoires publics ou privés.

Introduction

The aim of the presented work is to define what happens to a panel painting in case of climate choc change, to calculate the range of tolerance to environmental variations and estimate the time needed to readapt to climatic norms as defined by ICOM. This could lead to a re-evaluation of the climate control in the Fabre Museum in response to the universal need to reduce energy consumption in museums.

We wish to evaluate the time of reaching stability, giving the possibility to fragile panels to better resist to climatic variations without submitting them to damage. Hopefully, this project studying the mechanical behavior of painted panels during climatic changes will initiate further reflection on how to better understand the influence of environmental conditions on the behavior of this type of paintings, improving decision making for the artworks treatment.

Over the years, various methods of remedial conservation have been used to conserve wooden panel paintings. In the two case studies presented here, the wooden supports had been restored in a comparable and rather interventionist manner in the past, with the result that the long-term conservation of the artworks was compromised.

By the beginning of 2010, each of the two panels examined had several cracks. Their paint layer suffered from losses as well as progressive deterioration, partly due to the constant cracking and locked-in strains of their wooden supports. These facts led the Fabre Museum to undertake new remedial conservation treatments. The aim was to better understand their condition and to apply the most appropriate solutions in order to conserve the two works of art in the best possible way.

For one of the two panels an innovative research was carried out before, during and after its remedial conservation (Bousvarou and al. 2020a;Jullien and al. 2023;Nicolas 2019). Conservation decisions were taken on the basis of data collected during six years of continuous stereo-correlation image monitoring, combined with other complementary analyses (Dupré and al. 2020) within the permanent exhibition rooms of the Fabre museum (Bousvarou andStépanoff 2018, 2020b). For the other panel painting, monitoring was carried out only after its treatment in order to verify its mechanical behavior.

Wood Science and Technology III: methods to examine panel paintings and their preventive and remedial conservation Both remedial conservation treatments of the wooden supports were carried out by the same conservators.

Identification of the study cases

Panel N°1: The life of Holy Agnes (1519, Germany) (Figure 1) Oil on lime tree Unframed dimensions: Height 97 cm; Width 78 cm; Thickness 0,8 cm Weight: 7,206 kg panel with a resin cradle with sliding screws -system Remedial conservation between 2011 and 2013 Monitoring in 2017, in 2021-2022 and since July 2023

Panel N° 2: The Holy Trinity Crowning the Virgin (early 16th century, Flemish or Spanish) (Figure 2) Oil on oak Unframed dimensions: Height 113,7 cm; Width 97,8 cm; Thickness 0,48 cm Weight: 15 kg panel with a resin cradle with self-tensioning spring system on sliding screws -system. Remedial conservation between 2017 and 2022 Monitoring between September 2016 and July 2023 2.1 Comparison of condition report before remedial conservation It is important to note that each of the two wooden panels is unique, as is usually the case with works of art. However, some structural comparisons can be made.

The two panel paintings appear to be quite similar in terms of their original technique and the methodology of previous conservation treatments observed. Another point of similarity would be the cause of their deterioration, as well as the similar methodology of recent remedial conservation. The main difference lies in the protocol used for the manufacture of the new cradle, in order to control any mechanical disturbance in the behavior of the wooden support.

Similarities

In both artworks the technique of the paint layer is oil paint with thick stripes on thin layers of oil paint. The preparation is white gesso and protein binder, not very thick, and gold leafs cover about 20% of the surface.

Both wooden supports are made up of several separate planks joined together with glue, missing wooden dowels and a fixed cradle at the back that doesn't appear to be original. The panels had been considerably thinned at some unknown time in the past, certainly during conservation treatment. The panels had many cracks, resulting in deterioration of the paint layer, which in both panels suffered from losses along the cracks and progressive tenting in large areas.

The main similarity between these two paintings is that they are now in very good condition, apparently stable within the current environmental conditions that correspond to the climate recommendations for museums (20°C +/-5%, 50% RH +/-5%).

Differences

The Life of Holy Agnes (panel N° 1), is about 10% smaller than the Holy Trinity Crowning the Virgin (panel N° 2). It is made up of three planks of lime, partly tangentially cut, with the presence of nodules and several xylophagous insect holes. The condition report notes six linear meters of cracks in the wood, some of which are very complex.

The thickness of the panel is 8 mm, due to thinning in the past, making it thicker than panel N°2. The surface is slightly convex and, as the conservators noted at the time of treatment, "very sensitive to changes in humidity due to its partial tangential cut". The back was reinforced at some unknown time Wood Science and Technology III: methods to examine panel paintings and their preventive and remedial conservation with an 18 mm thick cradle of unspecified softwood. It consisted of three uprights and five crosspieces glued together. As described by the conservators in their 2011 report, "This extremely cramped frame has led to the partial splitting of the two joints and to the numerous cracks present on each board".

The Holy Trinity Crowning the Virgin (panel N° 2) is made of four vertical oak planks. Three of them are quarter sawn (radial grain) and the fourth is flat sawn (tangential grain). The painting was cut on three of its four sides at an unknown time, which means that it came from a larger composition. Historical research into its provenance is still underway. The wooden support, slightly thicker than panel N°1, has been thinned to 10 mm. Fifteen line cracks were counted in the upper and lower areas. At the back of the panel, probably at the same time as the thinning, a complex auxiliary cradle of coniferous wood (Abies spp. (fir)) was added. It consisted of seven horizontal crossbars of 35 mm (width) by 15 mm (thickness) section made of fir (Abies sp.) glued onto the original oak planks. The entire surface of the back was covered with a layer of brown paint. The surface showed a 15 mm concave deformation between the upper and lower edges, but no insect galleries were observed.

Comparison of recent remedial conservation steps

The painted layer of the two artworks was restored by two different teams but the proposal was identical and involved consolidation, then deep cleaning of the original paint layer and finally restoration of the areas of loss. Finally, natural resin varnish was applied to the painted surface.

For the wooden supports of the two paintings, the proposal for remedial conservation, as fundamental as that for the paint layer, was similar and seemed appropriate in terms of art conservation standards. The two remedial treatments where operated by the same restoration workshop and are objectively efficient ensuring good conservation of both artworks. It consisted of removing the previous cradle on the back by mechanical and chemical means. All the planks were separated to remove the old animal glue that was no longer effective. The planks were reassembled by applying new animal glue and inserting missing dowels.

For each panel painting a new auxiliary cradle had to be adjusted at the back in order to hold the planks together and permeate to control unwanted stresses causing damage. For panel N°1 the cradle was designed on the basis of empirical experience and technical literature, permit lateral displacement of the planks (Figure 3). For panel N°2 the cradle was designed with a self-tensioning spring system also permit out of plane displacements, following the analysis of several scientific data collected during the first four of the six years of the study, which made it possible to define the hygro-mechanical behavior of the artwork and that of the new cradle (Figure 4) (Mazzanti and al. 2020).

Methods of study

The panels' studies were carried out in a hermetic showcase, glazed on both sides and equipped with a relative humidity control system, in order to characterize the response of the different structures (panel and cradle, old or new) to hygro-thermal variations. The shape, the curvature variations and the strains on the surfaces of the panels were measured on both sides using a non-contact optical method (Dupré et al. 2020). It's important to say that before being subjected to any experimental procedure the condition of the paint layers was inspected and verified by a paint conservator.

A team of three universities of science and technology (Poitiers (F), Florence (I), Montpellier (F)) and also independent researchers (Nth, F) ensured the elaboration of the methodology adopted. The tests were carried out after remedial conservation of panel N°1 instead of before, during and after remedial conservation of panel N° 2.

To measure 3D displacements, a stereo image acquisition was used, coupled with a mark tracking technique and digital image correlation method for the image analysis. The choice of the image analysis depends on the contrast of the surface. On one hand, the mark tracking technique consists in detecting and following some dots (black or white) on the surface during the shape variation. On the other hand, the correlation methods require a texture on the surface (natural in the case of panel N°1, i.e. the paint layer) in order to follow several small areas by a local matching between two images of different mechanical states. In both cases, the natural contrast is not sufficient on the back side but also on the Wood Science and Technology III: methods to examine panel paintings and their preventive and remedial conservation front side of panel N°2, where small round marks (Japanese paper reversibly glued with a weak adhesive) have been placed on the two sides and the mark tracking technique has been used. Images of the panels were taken every 30 minutes and recorded on a computer.

At the same time, the weight of the structure was measured and recorded during the variations of relative humidity inside the showcase, in order to assess the absorption and the loss of moisture by the artwork. (Dupré and al. 2020) The long-term observation of panel N°2 has allowed us to better understand how the damage to the wooden support occurred, its impact on the wooden structure and how previous restorations have altered its mechanical behavior (Dupré and al. 2017;Jullien and al. 2020;Jullien and al. 2021). The study of panel N°2 helped to refine the conservators' initial remedial proposal for conservation and design a new cradle with self-tensioning spring system based on technological science, which will ensure preventive conservation on this panel. The study of panel N°1 is currently underway. It is expected to characterize its' existing new cradle, manufactured in 2011 and possibly complete the study carried out on the panel N°2 between 2016 and 2023.

Phases of analysis

Measurements were carried out on the panels in different configurations in order to characterize the response of the different structures to hygro-thermal variations.

During each experimental phase inside the hermetic showcase, the relative humidity (RH) was programmed as far as possible to identical cycles of increase and decrease of about 10 +/-5% RH between 50 and 65% RH. The paintings were monitored during several stages and these RH cycles at a constant temperature which was around 24°C due to the equipment installed in showcase.

For The Holy Trinity Crowning the Virgin (panel N° 2) most of its constituent materials gave been identified and characterized by chemists and other research laboratories. The measurements were recorded in various configurations:

-The panel with the old cradle and frame: stabilization at 55% RH then 65% -The panel with the old cradle, without the frame: 65% RH then 55% RH and stabilization -The panel alone, once the old cradle was removed by the conservators: same as above, during different time lapses and complementary tests. -The panel with the newly designed cradle, before and after the restoration of the paint layer: same as above during different time lapses and adjustment of the cradle's tension.

The Life of Holy Agnes (Panel N° 1) was also examined in the same showcase but only during the last phase.

The measurements were taken in the following configurations:

-The panel with the new cradle, after remedial conservation and restoration of the paint layer: from 60% down to 55% RH until stabilization -Same configuration 55% RH then 65% and stabilization

Results and discussion

The mechanical behavior of the two panels, after remedial conservation, shows interesting differences during variations of relative humidity.

The panel N°1 (Figure 5) reaches a maximum slight 0.4 mm curvature 2 days after the 10% change of humidity; 13 days later, this curvature disappears, due to the equilibrium of the moisture in the panel. The horizontal strain reaches about 0.12% in 27 days. The panel N°2 (Figure 6) curvature of 1.5 mm 4 days after the 10% change in humidity; it takes 30 days to the panel to reach its moisture equilibrium and thus the curvature to disappear. The maximum horizontal strain, 0.15%, is obtained in 53 days. The maximum curvature is higher for panel N°2. Several factors may influence this difference: the dimensions of the panels (panel N°2 is 1.2 times larger), their thickness (panel N°2 is 0.6 times thinner) and the type of connection between the cradle and the panel (panel N°2 has self-tensioning springs). The durations to reach the maximum curvature and the moisture equilibrium are higher for panel N°2. Panel N°1 is thinner, it should reach its moisture equilibrium faster. That is not the case. The difference may be due to the permeability of the back surfaces: panel N°2 has a coating on the back side whereas panel n°1 doesn't, and moreover presents xylophagous holes counted on 15% of the surface. The characteristics of the paint layers may also have an influence. The maximum horizontal strains are slightly higher for panel N°2, that may be due to the specie of wood, this tendency agrees with the coefficients of oak and lime given in the literature (total tangential shrinkage coefficient: lime 9%; oak 10%).

Panel N°1 shows a smaller maximum curvature. We assume that stresses are then greater because of restrictive movements. The cradle with springs allows more movements. The next step in the study will be to add springs to the cradle of panel N°1 to compare these behaviors, and deduce the effect of the springs.

Conclusion

After analyzing the monitoring data from the two panels, it seems necessary to multiply the cases. For example, choose different panels with different types of auxiliary cradles or secondary supports, or different essences of original wooden supports. Combine studies of the panels with those existing on canvas and study in a more general way study the climate in the exhibition rooms in relation to that outside and, overall, the effect on works of art made of organic materials. Then relate this to works made of metal and also to biological or chemical degradation.

Figures Figure 1 .Fig 4

 14 Figures Figure 1. The Life of Holy Agnes, (1) front and back (2) side after restoration, ©CICRP-Emilie Hubert-Joly Figure 2. The Holy Trinity Crowning the Virgin, (1) front and back (2) side after restoration, © Danièle Amoroso Figure 3. The new cradle for The Life of Holy Agnes designed by conservators: (1) design drawing, (2) wood structure, (3) sliding screw system, © Gilles Tournillon Figure 4. The new cradle for The Holy Trinity Crowning the Virgin designed by conservators: (1) design drawing, (2) wood structure, (3) sliding screw and self-tension spring system, © Gilles Tournillon Figure 5. The Life of Holy Agnes, RH variation, shape deformation and horizontal strain viewed from front side at different steps, ©Pprime Figure 6. The Holy Trinity Crowning the Virgin, RH variation, shape deformation and horizontal strain viewed from front side at different steps, ©Pprime

Acknowledgments

We would like to thank the contributors and institutions that participated in this study or made it possible: From Fabre museum: director, Michel Hilaire and Curator, Florence Hudowicz Conservators : Daniel Jaunard, Patrick Mandron, Philippe Hazaël Massieux, Amoroso Danièle, Anne Baxter LMGC (Laboratory of Mechanical and Civil Engineering) Univ. Montpellier, CNRS, Montpellier, France: Chrysochoos Cécile, Kurtz Samuel, Valorge Patrice Univ. Clermont Auvergne, CNRS, Sigma Clermont, Institut Pascal, Clermont-Ferrand, France : Gril Joseph Institute PPRIME, Univ. Poitiers, CNRS, Poitiers, France: Hesser Franck Research laboratory for historical monuments (LRMH), Champs-sur-Marne, France: Maurin Emmanuel Department of Agriculture, Food, Environment and Forestry (DAGRI), Univ. Florence, Italie: Mazzanti Paola, Linda Cocchi, Paolo Dionisi-Vici, Lorenzo Riparbelli, Giacomo Goli, Marco Fioravanti, Marco Togni and Uzielli Luca From OPD (Florence): Marco Ciatti, Andrea Santacesaria, Ciro Castelli Regional cultural affairs office (DRAC) Occitanie, France Materials list -Springs Smalley (https://www.smalley.com/wave-springs/crest-crest-shim-ends) -IDS UI-3480CP-M-GL high resolution monochrome camera, 1/2", 2560 x 1920 pixels² -Industrial scale SARTORIUS, Combics Series, type CAW1P-1-60ED-I, maximun load: 60 kg, accuracy: 2 g -CEAM Preservatech, MiniOne, http://www.preservatech.com/ -deformometric Kit, GESAAF, University of Florence Wood Science and Technology III: methods to examine panel paintings and their preventive and remedial conservation

Contribution by each author M. Bousvarou initiated and participated in the scientific study and discussions, obtained funding, coordinated the different teams, participated in writing the article D. Jullien initiated and participated in the scientific study and discussions, coordinated the scientific teams, participated in the analysis of the results and in the writing of the article J.C. Dupré initiated and participated in the scientific study and discussions, conducted the image correlation measurements, participated in the analysis of the results and in the writing of the article G. Tournillon did the remedial conservation of wooden supports, initiated and participated in the scientific study and discussions


Delphine Jullien
email: delphine.jullien@umontpellier.fr

Marina Bousvarou
email: m.bousvarou@montpellier3m.fr

Pierre Stepanoff
email: p.stepanoff@montpellier3m.fr

J C Dupré

Cécilia Gauvin

Riparbelli Lorenzo
email: lorenzo.riparbelli@unifi.it

Monitoring

Jullien

Monitoring the deformation of an historic painting on a wooden panel (16th century panel painting): effect of the frame, the cradle and microclimate variations

Keywords: wood, panel painting, cradle, deformation, environment, cultural heritage, monitoring, remedial conservation

de niveau recherche, publiés ou non, émanant des établissements d'enseignement et de recherche français ou étrangers, des laboratoires publics ou privés.

Introduction

After six years of continued and constant scientific observation, we were finally able to switchoff the machines and are now pleased to present to the public some results of this experiment. Throughout this time, the panel painting The Holy Trinity crowning the Virgin (figure 2, 3), (Flemish, beginning of the 16 th century) was analyzed and treated by a unique collaborative team which consisted of three engineering laboratories from scientific universities, two studios of painting conservators and the staff of the Musée Fabre. In order to explain this scientific research project, the whole experiment took place in one of the exhibition galleries of the Fabre Museum in full view of the visitors [START_REF] Bousvarou | Un panneau mouvant, La Sainte Trinité couronnant la Vierge In Dans le secret des oeuvres d[END_REF]Bousvarou, 2020b;[START_REF] Pelletier | Les oeuvres à l'épreuve des musées[END_REF]. The visitors could appreciate the complex nature of the artworks conservation lead by a unique collaboration of experts (Bousvarou, 2020a).

Through this project we wished to understand and resolve the serious deterioration problems of The Holy Trinity crowning the Virgin artwork due to the deformation of the wooden support leading to the formation of cracks, paint flaking and loss. To this end, the panel was continuously monitored through a controlled microclimate change and also through changes of its physical condition. The monitoring before, during and after conservation treatments aimed to demonstrate the advantages of this procedure for future conservation decisionmaking [START_REF] Dupré | Poster : Suivi du comportement mécanique d'une peinture sur bois, soumise à des variations d'hygrométrie. 6 ièmes Journées du GDR 3544[END_REF]Jullien, 2020;[START_REF] Jullien | Poster : Suivi des déformations d'un panneau de bois peint du patrimoine : effet du cadre, du parquetage et de variations hygrothermiques[END_REF]. Thus, the painting was monitored during several stages: in its initial poor state of conservation, after the removal of its old frame, after the removal of the cradle and the remedial conservation of the wooden support and finally after the restoration of the paint layer. During each of these stages, the relative humidity (RH) was programmed as far as possible identical cycles rising and diminishing in increments of about 10% RH between 51 and 65% RH. The mechanical behavior of the panel was observed during these RH cycles at a constant temperature. The showcase was hermetic with controlled relative humidity and glazed on both sides.

After acquiring a set of data in order to understand the hygro-mechanical behavior of the panel, conservators and researchers created a new cradle, serving as an auxiliary conservation support that improved conservation conditions, avoiding further cracking and loss of paint. The new cradle was also designed to enable the panel to tolerate a wider range of ambient relative humidity conditions (figure 4).

Identification of the artwork

Title:

The Holy Trinity crowning the Virgin, Date: beginning of the 16th century, Flemish Inventory number: D61.1.379 Technique: oil on wood Unframed dimensions: 113,7 x 97,8 x 0,48 cm Weight: 32 kg panel with the cradle and frame, 15 kg panel with the cradle, 11 kg panel without the cradle.

Historical background

The attribution of this painting is complex but it seems to have been created at the beginning of the 16 th century by a Flemish painter, close to artists from Bruges such as Gérard David, Adriaen Isenbrant or Ambrosius Benson. The theme was identified as the Coronation of the Virgin by the Holy Trinity although the holy spirits' figure on the left is missing and two of the angels' heads are cut off, proving the existence of a larger initial composition. This panel is related to three other versions of the similar composition and style: of which one was found in the Church of Santa Maria, Portugalete near Bilbao, Spain, An Immaculate conception in a similar style was found at the national museum in Warsaw, Poland and a Virgin Mary was located at the museum of Troyes, Troyes, France.

Description of the condition before treatment

The original panel support consists of four vertical oak planks, three cut radially and one tangentially. The boards were glued together with three butt-joints, but had originally been attached by dowels. The panel was reinforced by a complex auxiliary fixed fir cradle which consisted of seven crossbars and six vertical bars, divided in six pieces for the four central ones. The cradle was glued and nailed onto the back of the planks and the whole back of the painting was coated with a thick brown paint layer which, after testing, appears to be very rigid and not very permeable. The panel was presented in a softwood frame, 135 mm wide and 40 mm thick on average. Height rigid steel plates held the panel in its frame (figure 3).

This painted panel has exhibited mechanical problems for many years. In 1963, the last conservation report suggested a drastic treatment consisting of an additional thinning of the panel and the transfer of the paint layer onto canvas [START_REF] Claparède | Archives, manuscrit historic documents[END_REF]. The deterioration of the paint and panel could be observed though the fifteen large vertical cracks on the painted side which mostly affected the entire thickness of the panel. In 1998 it was necessary to apply facing papers to prevent the cracks in the paint layer from extending further. The concave shape of the panel had a deflection of 4 and 16 mm (at 55 %RH) respectively (figure 6.a) at the bottom and top edges, with tenting and flaking of the paint layer were observed, caused by the instability of the wooden support.

Methods of study and sampling

Before being subjected to the experimental procedure the paint layer was examined and consolidated. At each step or change it was re-examined and treated if necessary to prevent losses.

The different constituent materials of the artwork were identified and characterized. Specialists from the laboratory of research for historical monuments identified the wood of the panel as oak, and that of the auxiliary cradle as Abies spp (fir tree). The chemical identification of some pigments (green, red, yellow, those of the ground layer) and the stratigraphy of the paint layers, and also the chemical identification of the brown paint layer on the back of the panel were conducted by researchers from the Mediterranean institute of biodiversity and ecology.

Several series of studies were carried out on the panel inside and outside the showcase. The shape, its' variations (relative curvature) and the strains on the surfaces of the panel were measured on both sides of the panel by a non-contact optical method (Dupré, 2020). To measure 3D displacements, a stereo-images acquisition has been used coupled with a mark tracking technique or digital image correlation method for the images analysis. The choice of the image analysis depends to the contrast of the surface. In one hand, the mark tracking technique consist in detecting and following some dots (black or white) on the surface during the loading. In another hand, the correlation methods need a texture on the surface (natural in our case, i.e. the painting) to follow several small areas by a local matching between two images of different mechanical states. In this case, the natural contrast is not sufficient on the back face but also on the front face, then small round marks (Japanese paper reversibly glued with a weak adhesive) have been placed on the two faces and the mark tracking technique has been used. Images of the panel are taken every 30 minutes and recorded on computer. Simultaneously, the variations in the relative humidity in the showcase and the weight of the structure were also measured and recorded to evaluate the absorption and the loss of moisture by the artwork. These measurements were carried out on the panel in different configurations in order to characterize the response of the different structures to hygro-thermal variations: with the original cradle and the frame, on the panel with the original cradle, and on the panel alone, after the cradle had been removed by the conservators in their studio.

Diagnosis:

The scientific study confirmed that the old auxiliary wooden cradle restrained the natural movements of the panel because it was fixed. Variations of humidity applied to a constrained wood induce locked-in strains [START_REF] Mazzanti | A review on compression set shrinkage related to the mechanics of wooden painted panels[END_REF]. It was necessary to remove this cradle in order to limit the damage to the panel, and therefore to the artwork as a whole. It was replaced with a self-tensioning spring system, designed to be less restrictive to the natural movement of the panel [START_REF] Ciatti | Structural conservation of Panel paintings at the Opificio delle Pietre Dure in Florence: method, theory, and practice[END_REF][START_REF] Marcon | Mechanical study of a support system for cupping control of panel paintings combining crossbars and springs[END_REF][START_REF] Marcon | Methodology for the design of crossbeams and springs system for cupping control of wooden panel paintings[END_REF]. This new system would enable the panel to tolerate a wider range of ambient humidity conditions in the museum.

Remedial conservation

The treatment consisted of removing all previous additions. The vertical and horizontal bars of the cradle, glued and/or nailed on the back side were removed. Old glue residues were eliminated. The four planks were correctly reassembled and the lost dowels were replaced. All these treatments were done by the conservators from "Atelier Tournillon". The data gathered through the different measurements taken during the experiment was analyzed by the team of Italian engineers and the French wood conservator who collaborated to design and manufacture a new cradle and a novel way of fixing it to the original boards. Observations made since the addition of this newly designed auxiliary cradle indicate that it has reduced the rate of damage to the paint layer, these observations are ongoing. In addition to the work on the panel support, the paint layer was cleaned and the old repaints were completely removed before the restoration of the losses was carried out.

Experimental results

Mechanical effect of the frame and of the cradle the panel

The panel stabilized at 65% RH showed a strong concave shape with a 15 millimeters maximum deflection (Figure 6a). While the removal of the frame had almost no effect on the shape (slight twist of +/-1mm) (Figure 5), the removal of the cradle reduced the curvature to 3 mm (Figure 6g).

Effect of moisture variation on the panel with the original cradle

An increase and decrease of humidity have been carried out at each state of the panel. When the panel with the cradle underwent an increase in humidity from 50% to 65% RH, a relative convex curvature was observed (Figure 6f), with a maximum deflection of about 2 mm instead of 1.5 mm for the panel with the frame (Figure 6b). Moreover, the horizontal swelling of the panel reached a maximum of 0.1% in strain in some areas. We attribute the convex curvature variation to the effect of the horizontal crosspieces which were oriented in the longitudinal direction of the wood. Because they were much more rigid than the wooden panel itself, oriented in the direction of the wood grain, they limited the lateral swelling, especially of the reverse side. A decrease of humidity (Figure 6d) had the inverse effect increasing the concave curvature by 2 mm and a shrinkage of about 0.1% in the horizontal direction.

Effect of moisture variation on the panel, without the original cradle, after conservation

When the panel without the cradle experienced a desorption from 64% to 56% RH, the curvature of the panel varied by about 4 mm (Figure 6h), which is twice the effect observed with the cradle (Figure 6d). As the initial curvature is small, this humidity variation allows to pass from a slight negative curvature to a positive one. The horizontal strains were in the same order as in the case of the panel with the original cradle, about -0.1%, but were less homogeneous. Indeed, each beams are not reinforced by the cradle and local strain can now occur due to local properties or defects of the beams.

Consequences of the supports' remedial treatment

The results obtained from this scientific project showed that the presence of the frame had very little influence on the shape of the panel with its original cradle, whereas the removal of the cradle modified its curvature by 3 mm. As the relative humidity increased, a slight convex curvature was observed when the panel is with its original cradle; and a higher concave curvature was observed, when the panel is without the cradle. Horizontal swelling was observed in both cases. As expected, it is larger in the case of the panel without cradle. The stability of the panel in terms of curvature or strain was reached in four weeks, and thus independently of its state (with or without cradle). The behavior of the panel with its new cradle is still in analysis.

Conclusion

This innovative experiment made it possible to measure the deformations and strains of this 16th-century painting on a wooden panel support, with and without its frame, and with and without the original cradle during relative humidity variations. The effect of moisture variation on the panel with the new cradle is under progress and will be the object of further communication.

The microclimate inside a display case designed specifically for this study was monitored and the response of the artwork to variations in humidity was measured. These measurements were carried out continuously, in the museum exhibition space, in full view of the public. Advances in imaging equipment and data processing suggest that continuous monitoring of artworks within museums is indeed possible. The further development of this type of within museums would allow a wider range of measurements to be obtained, on different types of panels, different structures, different woods, etc. The information thus gained would make it possible to regulate the climate in museums in an informed way, limiting the risk of the degradation of artwork and it would contribute to current reflections concerning the climatic regulation of the museums in relation to their energy consumption, in order to meet the current requirements of energy reduction.

Figures

Figure 1 :Figure 2 :

 12 Figure 1: Exhibition room N° 3-Rubens room, hermetic show case, 3D cameras, The Holy Trinity crowning the Virgin, 16 th century, oil on wood, Anonymous, Montpellier, Musée Fabre, front side September 2019. © CNRS Figure 2: The Holy Trinity crowning the Virgin, 16 th century, oil on wood, Anonymous, Montpellier, Musée Fabre, front side after restauration, September 2021. ©Danièle Amoroso.

Figure 3 :

 3 Figure 3: The Holy Trinity crowning the Virgin, 16 th century, oil on wood, Anonymous, Montpellier, Musée Fabre, back side, before remedial conservation, September 2016, room N° 10-salle du jeu de paume. ©Musée Fabre.

Figure 4 :

 4 Figure 4: Mounting in the hermetic showcase in room N° 3, Musée Fabre, Montpellier, September 2019.

Figure 5 :

 5 Figure 5: Colored relief deformation of the panels surface (in mm) showing shape deformations of the surface, October 2016. © PPrime Institute Figure 6: Effect of a 10% variation of relative humidity on the shape of the panel. © PPrime Institute

  Effect of a desorption from 64 to 56% RH

Acknowledgements

We would like to thank the contributors and institutions that participated in this study or made it possible: Amoroso Danièle, Atelier Amoroso Waldeis, Villeneuve-les-Avignon, France Chrysochoos Cécile, Laboratory of Mechanical and Civil Engineering (LMGC) Univ. Montpellier, CNRS, Montpellier, France Gril Joseph, Univ. Clermont Auvergne, CNRS, Sigma Clermont, Institut Pascal, Clermont-Ferrand, France Hesser Franck, Institut PPRIME, Univ. Poitiers, CNRS, Poitiers, France Joliot Céline, Mediterranean Institut of Biodiversity and Ecology (IMBE), CNRS, IRD, Marseille, France Kurtz Samuel, Laboratory of Mechanical and Civil Engineering (LMGC) Univ. Montpellier, CNRS, Montpellier, France Maurin Emmanuel, Research laboratory for historical monuments (LRMH), Champs-sur-Marne, France Mazzanti Paola, Department of Agriculture, Food, Environment and Forestry (DAGRI), Univ. Florence, Italie Tournillon Gilles, Atelier Tournillon, Sainte-Cécile-les-Vignes, France Uzielli Luca, Department of Agriculture, Food, Environment and Forestry (DAGRI), Univ. Florence, Italie Regional cultural affairs office (DRAC) Occitanie, France BNP Paribas, France


Norman Verschueren

Delphine Jullien

Meriam El Ouahabi

Jean-Christophe Dupré

FURNITURE Wood

Introduction

Materials and Methods

Using a mock-up, we aim to recreate the mechanical stresses applied to the panel during the thinning process and throughout the installation of the cradle.

The original panel

Pietà, Flemish painted oak panel, Church of Saint-Denis, Liege, Belgium, 1070 x 683 x 3 mm. 4 boards: quarter and half quarter cutting. 19 th century cradle: 10 vertical uprights and 12 horizontal crosspieces, common spruce. Ø Selection of oak boards; Ø Gluing: 10% fish glue / 100 ml of water; Ø Preparation CaCO 3, and oil glazes.

Ø 7 fixed uprights and 9 sliding crosspieces; Ø Gluing: 50% bone glue / 100ml water and press for 24 hours.

Ø Before: 2 months 43% RH / 19°C; Deflection of 3mm. Ø Thinning atmosphere: 80% RH / 24 hours; Tickness 8 mm to 3 mm.

Difficulties

Homogeneity, precision, controlled glue application.

Methodology: 15% humidity variations, monitoring: 10 days / 9 months.

Results and discussion

Conclusion

Author for Correspondence: Norman Verschueren Verschuerennorman@gmail.com

The unthinned panel -7 mm thick After 24 hours:

Over 9 months:

13 mm deflection.
Deflection of 8mm 36% RH.

Blocked, no cracks.

Understanding alterations

Thinning process Release of compressive, tensile and shear stresses.

Installing the cradles New supply of moisture, high compressive stress and crushing of the fibres.

The conservation conditions of cradled panels

Fig. 1 .

 1 Fig.1. (a) Front and (b) back faces of the panel studied. (a) (b)

Fig. 3a .

 3a Fig.3a. Thinning process.

Fig. 4a .

 4a Fig.4a. Front and back, unthinned panel, 818 x 525 x 7 mm.

Fig. 4b .

 4b Fig.4b. Front and back, thinned panel, 1030 x 507 x 3 mm.

Fig

  Fig.5a. Monitoring, unthinned panel Fig.4a.

Fig. 5a .

 5a Fig.5a. Monitoring, thinned panel Fig.4b.

Fig. 6a .

 6a Fig.6a. Detail of two cracks -thinned panel mock-up. Fig.6b. Detail -crack on the original panel.

Fig. 7 .

 7 Fig.7. Fractures at the edges with the uprights. ©BOBAK, S. 1995


Estelle Hervé

Anne-Sophie Dubarry

Thierry Legou

Béatrice Desnous

Laurent Prévot

Clément François

perception development in young infants. (Thierry, 2005; Hervé et al, 2021)  Evaluate the feasibility of simultaneous cortical and subcortical EEG recordings in infants.  Understand the mutual contribution of subcortical and cortical structures during speech processing .  Decipher the neuroplasticity mechanisms supporting the maturation of the auditory system during early development. Consistent results with literature for both cortical ERPs and FFR:  Better non-native speech sounds discrimination in younger infants (Kuhl et al, 2007)  Better encoding of speech sounds features (temporal and frequential domains) in older infants (Anderson et al, 2015) From Skoe & Kraus, 2010. F0 = 100.4 Hz, duration: 170 ms

Materials and Methods

Results

Discussion

FIG.1b: Procedure: passive listening oddball paradigm FIG.1a: Stimuli : synthetic CV syllables /da/ standard /ba/ deviant /ga/ deviant 30 alternating blocks of fast and slow stimulation frequency (Fs).

( Bidelman et al., 2015) 90 infants, aged 6 to 24 months; healthy, no developmental or hearing disorder. Recorded semi-longitudinally and separated into two age groups: the younger (6-to 10-month-olds) and the older (18-to 24-month-olds).

Introduction

A new tool for neurodevelopment of speech perception abilities from lower to higher levels of processing?

  FIG.3b:Neural lag as a function of age.

  FIG.3d: Fast-Fourrier transform of the FFR, group comparison FIG.3c: SNR at vowel time-window as a function of age.

  FIG.2a:Mean evoked activity by electrode for each condition, all subjects (n = 80).

  FIG.2c:Subtracted activity (deviantstandard) for each deviant, group comparison. Grey areas: group-level time window for MMR (see Fig.2b).