NDT&E International 49 (2012) 40–46

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Stimulated infrared thermography applied to help restoring mural paintings

J.L. Bodnar a,n, J.C. Candore a, J.L. Nicolas a, G. Szatanik b, V. Detalle c, J.M. Vallet d

a GRESPI/ECATHERM, UFR Sciences Exactes et Naturelles, BP 1039, 51687 Reims, Franceb PAINTING RESTORER, 60, avenue Jean Jaur�es, Batiment 9, 92190 Meudon, Francec LRMH, 29 avenue du Paris, 77420 Champs sur Marne, Franced CICRP, 21 rue Guibal, 13003 Marseille, France

a r t i c l e i n f o

Article history:

Received 22 July 2011

Received in revised form

16 March 2012

Accepted 19 March 2012Available online 28 March 2012

Keywords:

Finger tapping analysis

Detachment detection

Fresco

In situ analysis

Louvre

Mural painting

Non destructive testing

Painting on canvas

Salt detection

Stimulated infrared thermography

UNESCO world heritage

Work of art

95/$ - see front matter & 2012 Elsevier Ltd. A

x.doi.org/10.1016/j.ndteint.2012.03.007

esponding author.

ail address: jl.bodnar@univ-reims.fr (J.L. Bodn

a b s t r a c t

In this work, various examples of the use of the stimulated infrared thermography to help restore mural

paintings are presented. First, the principles of this technique are expounded. Then, examples of devices

used for the study are described. Finally, we show the possibility of in situ detection of detachments in

different mural paintings: ‘‘Saint Christophe’’ belonging to the ‘‘Campana’’ collection in the ‘‘Louvre’’,

painted walls in the ‘‘Saint Florentin’’ church in ‘‘Bonnet’’, painted ceilings in the ‘‘Saint-Savin-sur-

Gartempe’’ abbey (classified in the UNESCO world heritage), and the ‘‘Cocteau’’ frescoes in the ‘‘Saint

Pierre’’ vault in ‘‘Villefranche-sur-Mer’’.

& 2012 Elsevier Ltd. All rights reserved.

1. Introduction

Within the framework of art restoration, the research laboratoryof historical monuments (LRMH), the interregional center forheritage conservation and restoration (CICRP) and the restorersuse many methods and instruments. These include visible photo-graphy, ultraviolet photography, infrared photography, static infra-red thermography, optical interferometry, X ray radiography,spectroscopy, liquid or vapor phase chromatography, the particleaccelerator AGLAE, nuclear analysis, finger tapping method etc.[1–7]. They are high performance methods in their applicationsfields. But, they are not universal. On one hand, their implementa-tion can take a long time and be strenuous. For example, thedetection of defects in very large mural paintings (in churches) byfinger tapping analysis can be cited [1]. On the other hand, theirprinciple can be limited to observe the natural radiation of thestudied sample. It does not allow the hidden defect detection

ll rights reserved.

ar).

(static infrared thermography). The GRESPI/ECATHERM of ReimsUniversity has worked for approximately 20 years in the develop-ment of a particular non-destructive testing method: stimulatedinfrared thermography. This method is non-destructive. It is with-out contact and is used for the thermophysical analysis of thinmaterials. It was already implemented to detect and characterizevarious types of localized or extended hidden defects (delamina-tions, cracks, inclusions etc.), in various types of materials [1–16]. Itseems liable to be used in the field of mural painting restoration. Itexplains why, for a few years, several research teams, among witchthe GRESPI/ECATHERM, work in this field [17–30]. In this paper, wepresent various examples of the stimulated infrared thermographyused to help mural paintings restoration. First, the principles of thetechnique are presented, then, examples of devices used for thestudy are shown. Finally, we show the possibility of in situdetachments detection in different murals paintings: ‘‘Saint Chris-tophe’’ belonging to the ‘‘Louvre’’ ‘‘Campana’’ collection, paintedwalls in the ‘‘Saint Florentin’’ church (‘‘Bonnet’’), painted ceilings inthe ‘‘Saint-Savin-sur Gartempe’’ abbey (classified in the UNESCOworld heritage), and the ‘‘Cocteau’’ frescoes in the ‘‘Saint Pierre’’vault in ‘‘Villefranche-sur-Mer’’.

J.L Bodnar et al. / NDT&E International 49 (2012) 40–46 41

2. Principle of the stimulated infrared thermography

The principle of stimulated infrared thermography is simple.First, the analyzed sample with a flux of photons is excited. Theabsorption of theses photons produces an increase of the tem-perature in the vicinity of the lighted area. This variation oftemperature leads to a variation of the infrared radiation of thestudied material. It can be observed by an infrared thermographycamera. The thermo physical phenomena used by this NDTmethod are thermal conduction and thermal radiation. Thephotothermal signal collected by the infrared radiometer thusdepends on parameters governing these physical phenomena:thermal conductivity, thermal emissivity, thermal diffusivity,temperature, specific heat and density. In addition, theses para-meters can be correlated to the following: aspect of surface,presence of delamination, presence of cracks, internal structureof the material, progress of a physical and chemical transforma-tion, drying and sedimentation, etc. This NDT method then allowsthe study of these different parameters. Finally, the principle ofthe method ensures it is non-destructive, without any contact,flexible and easily customized according to needs.

3. The experimental device used for the study

In the long term, we wanted to conceive and produce a deviceusable by restorers. Then, it must be simple to use, sturdy,portable and cheap. We also took into account the (potential)important dimensions of the defect. So, a pulsed excitation waschosen for use. This is the method generally used for stimulatedinfrared thermography. This is a well-known method. It enables a

Fig. 1. Example of experimental device used for the analysis of mural paintings.

Fig. 2. The studied academic fresco (a), the position of intern

fast analysis method. Finally, it is an easy to implement. Sohalogens lamps to light the studied sample were chosen. It is acheap device and easy to monitor. Its light spectrum is approxi-mately flat in the visible domain. Finally, this choice allows asymmetric excitation of the studied sample. In a third stage, a‘‘long wave’’ infrared camera of thermography was chosen. It isthe best wavelength range for the study of ‘‘cold’’ samples (20 1C).This choice makes possible not to detect the excitation light.Finally, the use of two type of camera was possible. The first onesare quantum detector and cooling cameras (‘‘thermovision 782’’and ‘‘JADE 2’’). There are the first available cameras used, but areexpensive. Then, a bolometer camera (‘‘FLIR’’ A20) was put to use.It is a cheap camera. Its NETD equals about 100 mK. It wassufficient for our first study. It is better for an industrial transfer.Finally, electronics and a computer, for the photothermal analysis,were implemented (Fig. 1).

4. Experimental results

The first mural painting analyzed is a partial copy of ‘‘SaintChristophe’’, from the ‘‘Campana’’ collection in the ‘‘Louvre’’(Fig. 2(a)). It is a pre-study sample of the original ‘‘Saint Chris-tophe’’ (Fig. 3(a)). It was painted, using the primitive Italiantechnique, in 2005. It is a modern copy: a block of plaster coveredby a mixture of lime and plaster. The whole is covered with apictorial layer. It was painted ‘‘al fresco’’ and represents the InfantJesus. The length of the academic fresco is 20 cm. its width is20 cm. and its thickness 3 cm. To approach possibilities of thephotothermal method for the defect detection, it contains4 defects (Fig. 2(b)). It is 4 inclusions of plastazote (emalene),an insulating material. The first defect is located in the upper andleft part of the fresco. Its depth is equal to 3 mm. The seconddefect is located in the upper and right part of the fresco. Its depthis equal to 5 mm. The third defect is located in the lower and leftpart of the fresco. Its depth is equal to 3 mm. The last defect islocated in the lower and right part of the fresco. Its depth variesbetween 3 mm and 10 mm. The experimental conditions retainedfor the study are the following: The duration of the excitation isequal to 120 s. The power of excitation is equal to 2n250 W. Theduration of analysis is equal to 300 s. The infrared camera used isa ‘‘CEDIP JADE 2’’. An example of photothermal results obtained ispresented in Fig. 2(c). It is the photothermal response of thestudied fresco, 190 s after the end of the excitation. This figureshows clearly 4 more important infrared signatures. Thesessignatures correspond with a maximum increase of temperatureequal to 12 1C (20 1C-32 1C). They are due to the insulatingproperties of the plastazote. Indeed the principle of our nondestructive testing method consists in exciting the analyzedsample with visible light. This energy, during the radiation/material interaction slightly warms the surface of the work ofart. The heat then propagates by conduction in the fresco. At theplace of the insulating defect, this distribution was slowed down.

al defects (b) and the photothermal results obtained (c).

Fig. 3. The studied ‘‘Saint Christophe’’ fresco (a) and the finger tapping analysis (b).

Fig. 4. Experimental results obtained by finger tapping (a) and photothermal analysis (b) of the ‘‘Saint Christophe’’.

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This thermal effect leads to a lower decrease of temperaturestraight above the inclusions. It leads finally to a more importantinfrared signature for this place. This experimental result clearlyshows the possibilities of the photothermal method for thedetection of defect located in mural painting. It also shows thatthe method is indeed non-destructive (even if the increase ofsurface temperature is not insignificant). The analysis of a realfresco appears to be possible.

The second mural painting analyzed is the genuine ‘‘SaintChristophe’’. This work of art represents Saint Christopher carry-ing the Infant Jesus. It is attributed to Tommaso del Mazza. It waspainted between 1385 and 1390. It is a part of the ‘‘Campana’’collection at the ‘‘Louvre’’ (Fig. 3(a)). Its height is 85.8 cm, itswidth 74 cm. At the time of our study, this fresco was in arestoration phase. We therefore took advantage of this restorationto study the possibilities of the photothermal method to helpfresco restoration. To obtain references results, this work of artwas initially analyzed by Gabriella Szatanick. She is a restorer andused the finger tapping method (Fig. 3(b)). The results obtainedare presented in Fig. 4(a). It shows a lot of grey area at theposition of abnormal finger tapping response. It is the defectspositions. In a second stage, this work of art was analyzed withthe photothermal method. To obtain a sufficient spatial resolu-tion, it was analyzed zone by zone. The experimental conditions

retained for the study are the following: the duration of theexcitation is equal to 120 s. The power of excitation is equal to2n800 W. The duration of analysis is equal to 900 s. The infraredcamera used is a ‘‘Thermovision 782’’. In Fig. 4, the resultsobtained on the whole fresco are compared. Fig. 4(a) relates tothe finger tapping analysis. Fig. 4(b) relates to the photothermalanalysis. The comparison between the two figures shows first thatthe defects detected by the photothermal method are alsodetected by the finger tapping method. It shows then, that inthe conditions of implementation of the photothermal method,the defects detected are less significant than by the finger tappingmethod. Apparently, it is due to the too short duration of theexcitation. It does not allow the detection of the deepest defects(example of the defect situated on the upper and right part of themural painting). Indeed the detected defect by photothermalmethod, at the same time, led to the same acoustic signature ofthe finger tapping analyses. The non detected defects located atthe upper and right part of the fresco, gave a different sound,characteristic of a deep defect. This study shows first, thepossibility to analyze a genuine work of art by stimulated infraredthermography. It then shows that the photothermal method isfaster and more objective than the finger tapping analysis (indeedthis method is highly dependent on the restorer’s experience). It alsoshows that the photothermal method is less destructive than finger

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tapping analysis for the defects located near the surface of muralpaintings Finally, it demonstrates that the photothermal method,throughout the study of the time appearance of defect signature,seems to allow the dimensional characterization of the defects.

The third mural painting studied is a painting on canvas. The aimof this study is to approach the possibility of the photothermalmethod for the detection of detachment of this kind of work of art.The painting on canvas studied is the ‘‘Saint Martin’’ of the ‘‘SaintFlorentin’’ church in ‘‘Bonnet’’ (Fig. 5(a)). It dates from the 19thcentury. Its height is about 2.5 m, its width 1.5 m. To start with andlike in the previous case, we developed a finger tapping analyze of thework of art (Fig. 5(b)). The obtained result shows a multitude of‘‘hollow’’ tones around the circumference of the work of art (Fig. 6(a)).This art work is then analyzed, by using stimulated infrared thermo-graphy. The experimental conditions retained for the study are thefollowing: the duration of the excitation is equal to 300 s. The powerof excitation is equal to 2n500 W. The duration of analysis is equal to600 s. The infrared camera used is a bolometer camera (‘‘FLIR’’ A20).An example of photothermal result obtained is presented in Fig. 8. Itfirst shows clearly more important photothermal signature (max-imum increase of temperature equal to 8.3 1C (11.9 1C-20.2 1C) atthe place located by the finger tapping analysis. It shows then thepossibilities of the photothermal method to detect detachment ofpainting on canvas.

The fourth work of art studied is a mural painting of the naveof the abbey of ‘‘Saint- Savin-sur-Gartempe’’. This Abbey is classifiedsince 1984, because of the exceptional quality of its mural paintings,

Fig. 5. The studied painting on canvas entitled ‘‘Saint Martin’’ of the churc

Fig. 6. Finger tapping (a) and photothermal (b) analysis of the painting on

as a UNESCO world heritage site. These mural paintings werecompleted at the end of the 11th century. They are characteristicof the Romanesque period. They represent religious themes inspiredby the Bible, the major source for the art in the middle Ages. Theyillustrate the Genesis and the Exodus. The vault of the nave, which ismore than 40 m long, is completely covered with mural paintingsover a surface area of about 500 m2 (Fig. 7(a)). Because of theantiquity of this work of art, it has to be restored. Prosper Merimeeinitiated the first restoration campaign in 1835. The second wasdeveloped in 1967. The last is in progress (since 2005). Thiscampaign is focused on the restoration of the painting of the nave.For our research work, it was the chance to test the possibilities ofstimulated infrared thermography in real conditions of restoration:on a scaffold, in the dust, in a limited time, simultaneously withother art restoration work and with a temperature of only a fewdegrees. The results presented here concern the study of the frescoentitled ‘‘ La traversee du desert’’. The zone surrounding the backwheel of the wagon was more particularly studied (Fig. 8(b)). Toobtain a reference result, Brice Moulinier and Emmanuelle Cante,restorers, first developed a finger tapping analysis of this work of art.The result obtained shows two abnormal finger tapping responses.The first is located in the upper part of the wheel of the wagon. Thesecond is located in the lower part of the wheel (Fig. 8(a)). In asecond phase, we developed a stimulated infrared thermographyanalysis. The experimental conditions retained for the study arethe following: The duration of the excitation is equal to 300 s. Thepower of excitation is equal to 2n500 W. The duration of analysis

h ‘‘Saint Florentin’’ of ‘‘Bonnet’’ (a) and its finger tapping analysis (b).

canvas entitled ‘‘Saint Martin’’ of church ‘‘Saint Florentin’’ of ‘‘Bonnet’’.

Fig. 7. The murals paintings of the abbey of ‘‘Saint Savin sur Gartempe’’ (a) and the studied mural painting entitled ‘‘la traversee du desert’’ (b).

Fig. 8. Finger tapping (a) and photothermal (b) analysis of the mural painting entitled ‘‘la traversee du desert’’ of abbey of ‘‘Saint Savin sur Gartempe’’.

J.L Bodnar et al. / NDT&E International 49 (2012) 40–4644

is equal to 720 s. The infrared camera used is a bolometer camera(‘‘FLIR A20’’). An example of photothermal result obtained is shownin Fig. 8(b). First, it shows more important photothermal signature atthe place located by the finger tapping analysis. Theses signaturescorrespond to a maximum increase of temperature equal to 3.4 1C(10.3 1C-13.7 1C). There is then a reasonable correlation betweenthe results supplied by both physical methods. Second, Fig. 8(b)shows some difference between the two statements, especially in thelower part of the wheel. In this part of the work of art, thephotothermal method indicates two defects, while the finger tappingmethod detects only one defect. We then asked the restorer a newfinger tapping analysis. The new finger tapping statement made byrestorers then confirmed the presence of two separate defects. This isa very important result. Indeed in the restoration phase, the fingertapping result would have led to an incomplete consolidation.Because, only one detachment could have be restored. The photo-thermal statement, indicating two defects, avoids this problem. Tofinish this study, we can say that first the photothermal methodcan be used in real conditions of restoration. We can then underlinethe possibilities of the photothermal method to separate two closedefects.

The last work of art studied is a mural painting of the ‘‘SaintPierre’’ church in ‘‘Villefranche-sur-Mer’’. This church is entirelycovered by mural paintings made by Jean Cocteau in 1956 and1957 (Fig. 9(a)). Theses mural paintings, though relatively recent,already suffer from the effects of their surroundings and from thebehavior of our contemporaries. They are already slightly degraded.Particularly, restorers’ statements show delamination and also rashesof salts (Fig. 10(a)). The presence of salt in mural painting can bedangerous for the work of art. Indeed, in the crystallization phase,salts can lead to a detachment of the pictorial layer and then to adeterioration of the work of art. The possibilities of the stimulatedinfrared thermography to the salt detection were then approached.The part of the ‘‘Saint Pierre’’ church analyzed is the mural paintingentitled ‘‘Saint Pierre livre aux soldats de Pilate’’ (Fig. 9(b)). Restorersfirst analyzed this work of art. The result obtained is presented inFig. 10(a). It shows 3 types of defect connected to the capillary ascentsand to the crystallization of salts. In red, it is the rashes of salts. Inblue, it is the bubbling. In green, it is the lacuna. In a second phase,this work of art was studied by stimulated infrared thermography.The experimental conditions retained for the study are the following:The duration of the excitation is equal to 60 s. The power of excitation

Fig. 9. The murals paintings of the church ‘‘Saint Pierre’’ of ‘‘Villefranche sur mer’’ (a) and the studied mural painting entitled ‘‘Saint Pierre livre aux soldats de Pilate’’ (b).

Fig. 10. The visual and finger tapping statement of the restorers (a): Rashes of salts (red), bubbings (blue) and lacunas (green) and the photothermal results (b) obtained

during the analysis of the mural painting entitled ‘‘Saint Pierre livre aux soldats de Pilate’’ of the church ‘‘Saint Pierre’’ of ‘‘villefranche sur mer’’. (For interpretation of the

references to colour in this figure legend, the reader is reffered to the web version of this article.)

J.L Bodnar et al. / NDT&E International 49 (2012) 40–46 45

is equal to 2n400 W. The duration of analysis is equal to 180 s. Theinfrared camera used is a bolometer camera (‘‘FLIR A20’’). An exampleof obtained result is presented in Fig. 10(b). It shows first moreimportant photothermal signature (maximum increase of tempera-ture equal to 8.5 1C (24 1C-32.5 1C). at the place located by therestorers. It shows then the possibilities of the photothermal methodfor the salts detection.

5. Conclusion

In this work, the possibilities of the stimulated infraredthermography to help restoring mural paintings are approached.First, the study of an academic fresco shows the possibilities todetect ‘‘plastazote’’ inclusion by the photothermal method. Then,the study of the mural painting entitled ‘‘Saint Christophe’’, fromthe ‘‘Campana’’ collection in the ‘‘Louvre’’, first confirms thepossibilities of defect detection by the photothermal method.Then, it shows that the photothermal response is often correlatedto the traditional finger tapping response. In a third stage, thestudy of the mural painting entitled ‘‘Saint Martin’’ in the ‘‘SaintFlorentin’’ church (‘‘Bonnet’’) shows the possibilities of detectingdetachment of painting on canvas. Fourth, the study of the muralpainting entitles ‘‘La traverse du Desert’’ in the ‘‘Saint-Savin-sur-Gartempe’’ abbey (classified in the UNESCO world heritage)shows first that the photothermal method is usable in realconditions of restoration. It also shows that the photothermalmethod seems to allow a detection and separation of two closedefects. Finally, the study of the mural painting entitled ‘‘ SaintPierre livre aux soldats de Pilate’’ in the ‘‘Saint Pierre’’ church

(‘‘Villefranche-sur-Mer’’) shows the possibilities to detect saltdamage by stimulated infrared thermography.

These results show that the photothermal method seems to bea new tool in helping mural painting restoration. This study alsoshows that the photothermal method seems, with regard to thefinger tapping method, to allow a more precise, faster and moreobjective detection of defects. It is also less destructive than fingertapping analysis for defects located near the surface of muralpaintings.

These very encouraging results have now to be confirmed andcompleted. We have to define and implement the most completephotothermal method. The need then is to define and implement theless disturbing (smallest variations in temperature, smallest variationin humidity) photothermal method (random excitation?).

Moreover, the possibilities of the photothermal method for thedefect characterization need to be studied. (This characterizationwould be able to lead a 3D representation of the defect whereasthe finger tapping analysis can give only a 2D representation).Finally, we have to study the post treatments procedures tohighlight the useful photothermal information in a better wayand to remove the artifacts of detection. Studies pointing in thatdirection are under way.

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