THERMOPHYSICAL CHARACTERIZATION OF
SURFICIAL LAYERS: INPUT TO THE
CONSERVATION OF HERITAGE OBJECTS

Jean-Marc Vallet, Kamel Mouhoubi, Jean-Luc Bodnar

► To cite this version:

Jean-Marc Vallet, Kamel Mouhoubi, Jean-Luc Bodnar. THERMOPHYSICAL CHARACTERIZATION OF SURFICIAL LAYERS: INPUT TO THE CONSERVATION OF HERITAGE OBJECTS. Stone 2025 15th International Congress on the Deterioration and Conservation of Stone, Sep 2025, Paris, France. pp.98-101. hal-05370389

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Stone 2025

15th International Congress on the
Deterioration and Conservation of Stone

Paris 8-12 September 2025
# THERMOPHYSICAL CHARACTERIZATION OF SURFICIAL LAYERS: INPUT TO THE CONSERVATION OF HERITAGE OBJECTS

M. Kamel<fnref n="1" />, V. Jean-Marc<fnref n="1" />* and B. Jean-Luc<fnref n="2" />

## Abstract

We carried out stimulated IR thermography using a high-frequency camera to investigate its potential for studying the surficial pictorial layers of wall paintings. We studied the response to flash illumination of experimental samples painted with lead white and malachite using glue or lime as a binder. The results show that a higher image acquisition frequency than standard IR cameras can deliver enables differentiation between painting techniques. These results show that we can study variations in the thermal response of the surficial painted layers.

**Keywords:** Stimulated IR thermography, high speed camera, wall painting, malachite and lead white, high frequency, pictorial technique characterization.

## 1. Introduction

Stimulated infrared thermography is a non-destructive and contactless technique for analyzing cultural heritage (Gavrilov et al., 2014; Maldague, 2001). Classical infrared cameras allow the study of surface and subsurface thermal properties of murals and provide valuable information for conservation purposes (Bodnar et al., 2012). Researchers have recently started investigating the potential of high-frequency IR cameras in cultural heritage applications (Boubanga Tombet et al., 2022; Kouser et al., 2023). The present study, conducted at CICRP, aims to demonstrate the specific interest of using this type of IR camera to enhance the detection and detailed characterization of pictorial materials and their alterations in mural paintings. We then performed investigations on experimental samples.

## 2. Materials and experimental method

We studied three test samples simulating a wall painting (A, B, C; see Figure 1). Their supports are cubes (10 cm) made of limestone from the South of France called "Estaillades" stone. We first applied a mortar made of natural hydraulic lime and river sand on the support, then a final fine render made of hydrated lime and sand. We then painted two bands using different pigments, one using malachite and the other lead white, using different techniques (samples B and C: glue-based; sample A: lime-based). Some areas exhibited the blackening of malachite due to its transformation into tenorite (CuO) and the presence of laitance (whitening of the malachite in some areas). We performed stimulation using two studio flash lamps (Elinchrom Style 600) at 300 W each. The flashes were equidistant and positioned at the same height as both the camera and the test samples. The flashes were triggered simultaneously. We captured the transient thermal response of the samples using a high-performance infrared camera (Flir X6981 SLS), capable of full-frame acquisition at 1000 Hz. To study the influence of acquisition speed, we first carried out several experiments at 125 Hz frame rate using a standard lens to record the response of the entire samples to the same stimulation.

We then normalized the temperature responses to allow reliable comparisons between the different zones and experiments, independently of any influence from initial heating variations, which may result from local absorptivity or slight fluctuations in the flash. We characterized the cooling behaviour of the materials subjected to a brief but significant thermal disturbance, following the approach described in the literature (Parker's model), by analysing the cooling curves (Parker et al., 1961). We focused the study on the temporal evolution of this signal within Regions of Interest (ROI) that were defined on the different materials and surface states. We compared these cooling curves, particularly in a Log-Log representation (log(normalized ΔT) vs. log(time)), with the aim of identifying specific thermal signatures linked to surface features and material interfaces.

## 3. Results and discussion

As a first test, we studied malachite-painted areas at 125 Hz in full frame. The thermal behaviour of malachite painting, avoiding blackened zones caused by the degradation of malachite into tenorite, and areas affected by laitance, showed

<footnote n="1">¹ Centre Interdisciplinaire de Conservation et Restauration du Patrimoine (CICRP), Marseille, France</footnote>

<footnote n="2">² ITheMM (Institut de Thermique, Mécanique, Matériaux), Université Reims Champagne Ardennes (URCA), Reims</footnote>

<footnote>*Corresponding author</footnote>
significant differences (Figure 2). These distinctions are particularly evident during the initial moments of cooling (typically < 100 ms). When we look at the variation of the thermal signal by making a diagonal comparison between samples A and B (Figure 3), this divergence becomes clearly apparent. The same phenomenon exists on the white parts of the samples painted with lead white (Figure 4), even though it is not easy to see with the naked eye. In this way, we can study all the samples with the same stimulation and compare the areas of the different samples in full frame. Figures 3 and 4 show 9 significant points at 125 Hz, whereas we could only obtain around 3 points with a standard bolometer camera, which would have a maximum full-frame frequency of 50 Hz. The divergence may result from the difference in painting technique and/or the presence of an interface that we can detect. When we compare samples B and C, which have been made using the same painting technique (Figure 5), no divergence of the thermal signal by diagonal comparison exists. It therefore seems possible to differentiate between the glue technique and the lime technique.

<figure><img src="image_2.png" /><figcaption>Figure 1: Studied samples, each with two painted bands. On the left, the band painted with malachite and on the right, the band painted with lead white. Sample A was painted using lime as a binder; samples B and C were made using glue.</figcaption></figure>

<figure><img src="image_3.png" /><figcaption>Figure 2: Temperature responses of malachite applied with lime as the in a dimensionless Log-Log representation.</figcaption></figure>
<figure><img src="image_4.png" /></figure>

Figure 3: Diagonal correlation analysis of temperature responses for the malachite band showing the divergence of the thermosignal variation in the first milliseconds of cooling.

<figure><img src="image_5.png" /></figure>

Figure 4: Diagonal correlation analysis of temperature responses for the lead white band showing the divergence of the thermosignal variation in the first milliseconds of cooling.

<figure><img src="image_6.png" /></figure>

Figure 5: Diagonal correlation analysis of temperature responses for the upper part of malachite bands of samples B and C showing the lack of divergence of the thermosignal variation (with its raw thermogram).
## 4. Conclusion-perspectives

In conclusion, pulsed active infrared thermography, when implemented with a high-speed camera, proves to be a highly effective non-destructive analysis tool for mural paintings. Based on their specific thermal signatures, which we reveal through normalized cooling kinetics, this technique enables the differentiation of pictorial techniques regardless of the pigments used. Indeed, acquisitions at lower frame rates (e.g., 50 Hz) tend to average out or obscure these rapid thermal transients, inevitably resulting in information loss and reducing the precision in characterizing surface conditions and fine structural details. We find the most relevant information pertains to the nature, thickness, and degradation state of the surface pictorial layer, as well as to the characteristics of the first interfaces—precisely within this very short time interval. These features manifest as subtle variations in the shape of the initial dimensionless cooling curve. The key factor is the use of a high acquisition frame rate, which appears essential for capturing rapid transient thermal phenomena that carry crucial information about the most superficial layers. This approach allows for a much finer and more detailed characterization of surface conditions compared to conventional lower-frequency systems (e.g., bolometric IR cameras) or analyses based solely on raw thermal contrast. These detailed insights offer strong potential for refining diagnostics and informing conservation and restoration decisions regarding mural heritage. Future experiments will explore the potential of high-speed cameras operating at even higher frequencies (up to 1000 Hz) to study mural painting techniques in greater detail, such as the thickness of paint layers, and to detect interfaces and defects in the surface layers of a mural—such as early-stage detachments or subsurface cracks.

## Acknowledgements

This work was supported by funding from the Agence Nationale de la Recherche (French National Research Agency) under the future investment programme "France 2030", bearing the reference number "ANR-21-ESRE-0050 - ESPADON - PATRIMEX+".

## References

Bodnar, J.-L., Candoré, J.-C., Nicolas, J.-L., Szatanik, G., Detalle, V., & Vallet, J.-M. (2012). Stimulated infrared thermography applied to help restoring mural paintings. NDT&E International, 49, 40–46. http://dx.doi.org/10.1016/j.ndteint.2012.03.007

Boubanga Tombet, S., Guyot, E., Huillery, R., Calligaro, T., Detalle, V., Bai, X., & Semerok, A. (2022). Active thermography for panel paintings inspection: A comparative study of mid-wave and long-wave Infrared spectral analysis, pp. 457-462, in Proceedings of 16th Quantitative InfraRed Thermography Conference, 4 – 8 July 2022, Paris, France.

Gavrilov, D., Maev, R. Gr., & Almond, D. P. (2014). A review of imaging methods in analysis of works of art: Thermographic imaging method in art analysis. Can. J. Phys., 92, 341–364. http://dx.doi.org/10.1139/cjp-2013-0128

Kouser, H., Boubanga Tombet, S., Carrock, J., Detalle, V., Calligaro, T., Bai, X. & Semerok, A. (2023). Pulse Phase Thermography for Panel Paintings Inspection in Mid-wave and Long-wave Infrared Bands, in Proceedings of 4th Asian Quantitative InfraRed Thermography Conference, October 30 – November 3, 2023, Abu Dhabi, U.A.E.

Maldague, X. P. V. (2001). Theory and Practice of Infrared Technology for Nondestructive Testing. Wiley.

Mouhoubi, K., Bodnar, J. L., Ibos, L., Detalle, V., Vallet, J. M., & Duvaut, T. Identification of frescoes among murals paintings of the inheritance by stimulated infra-red thermography. In 12th Quantitative InfraRed Thermography (QIRT) conference (p. 160).

Parker, W. J., Jenkins, R. J., Butler, C. P., & Abbott, G. L. (1961). Flash method of determining thermal diffusivity, heat capacity, and thermal conductivity. Journal of Applied Physics, 32(9),1679–1684. https://doi.org/10.1063/1.1728417