APPLICATION OF TERAHERTZ IMAGING METHODOLOGIES FOR THE INVESTIGATION OF PAINTINGS

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1 APPLICATION OF TERAHERTZ IMAGING METHODOLOGIES FOR THE INVESTIGATION OF PAINTINGS Marcello Picollo, Institute for Applied Physics Nello Carrara of the Italian National Research Council (IFAC-CNR) Via Madonna del Piano 10, Sesto Fiorentino, Italy, Yohsei Kohdzuma, Nara National Research Institute for Cultural Properties. Nara , Japan Kaori Fukunaga, Iwao Hosako, National Institute of Information and Communications Technology (NICT), Koganei, Tokyo , Japan. ABSTRACT In recent years, Terahertz (THz) Time Domain Spectroscopy (TDS) imaging techniques have been proposed as a non-invasive means of studying artworks in order to identify the different compounds which constitute multilayered painting systems. In the present paper, reflectance THz-TDS images were acquired by using a portable THz-TDS imaging system, T-Ray 4000 by Picometrix, which was placed in front of the painting at a distance of approximately 20 mm. The intensity of the reflection is reported as gray scale, with the highest magnitude appearing as white/high gray levels and the lowest as black/low gray levels. The reported THz-TDS measurements were taken on a laboratory panel painting, on a tempera panel painting by Giotto from the Uffizi Gallery, and on the mural painting of the Takamatsuzuka tomb (ca. 700 CE), Japan. (C.E. = common era, a politically correct way of referring to western calendar dates without basing on the Christian calendar.) INTRODUCTION Terahertz (THz) Time Domain Spectroscopy (TDS) imaging system have many applications as a new non-invasive methodology for the investigation of various materials. Since the energy of THz waves emitted from existing systems is extremely low, i.e. of a magnitude similar to that of room temperature, this electromagnetic region is considered to be completely safe for operators, living organisms, and objects. Hence, it can be applied to the study of living matters, including human beings [1,2]. In recent years, THz-TDS technology has also been proposed for the study of artworks in order to identify the different compounds which constitute multilayered painting systems [3-12]. THz-TDS imaging has been performed in the reflection and/or transmission modes, depending on the analyzed objects. In particular, THz-TDS reflection imaging uses THz pulses that propagate in specimens. In this technique, pulses reflected from the surface and from the internal boundaries in the reflective index of the specimen are detected. In general, the internal structure of the analysed object is observed in cross-sectional images obtained by using micro-specimens taken from the work that is being analysed. Also, in THz-TDS imaging, a map of the layer of interest can easily be obtained without the need to collect any samples. THz-TDS imaging makes it possible to fill the information gap on panel paintings between two imaging diagnostic techniques, namely IR reflectography and X-ray radiography, as it provides useful data on the internal physical structural information of non-metallic objects without contact with the investigated artworks. This non-invasive cross-section image of the object is acquired by extracting the reflected pulse from a particular interface of two media with different refractive indexes. Figure 1. T-Ray 4000 by Picometrix system in front of the Polittico di Badia panel painting by Giotto.

2 In this paper measurements taken on a laboratory panel painting and on two artworks were collected by use of a portable THz-TDS imaging system, T-Ray 4000 by Picometrix. The imaging system worked approximately in the THz range. It requires a scanning time of about 10 minutes to investigate an area of 150 mm 2. For the analysis, the detection unit was placed in front of the painting at a distance of approximately 20 mm (Fig. 1). The intensity of the reflection is reported as gray scale, the highest magnitude appearing as white/high gray levels and the lowest as black/low gray levels. In particular, the data acquired on the panel painting provide some basic knowledge about the application of THz-TDS imaging technique to a wood panel painting of the type investigated. Subsequently, THz-TDS data obtained on a tempera panel painting by Giotto from the Uffizi Gallery and on the mural painting of the Takamatsuzuka tomb (ca. 700 CE 1 ) in Japan confirmed that one of the principal advantages of using THz in the art-conservation field is that the THz-TDS can provide cross-section images in a non-invasive way that helps conservators to understand the artists techniques and to detect the presence of previous conservation treatments. In particular, mapping (area information) of the layers of interest in a multi-layer structure cannot be obtained, so far, by other diagnostic and analytical methods. TEST PANEL As reported in the introduction, THz-TDS imaging techniques can provide non-invasive cross-section images. The first part of the experimental approach to define the potential for the application of this technique to the study of artworks was focused on the investigation of a test panel, which was prepared following traditional middle age recipes for making panel paintings. Figure 2. Panel test: in the red square is reported the area investigated with the T-Ray 4000 by Picometrix system. Here, the final structure of the mock-up was constituted, starting from the wooden support, with a first layer of gypsum (gesso grosso) and animal glue, a canvas (in this case the canvas was a modern-industrial linen canvas), a final layer of gypsum (gesso fine) and animal glue that was flattened with very thin sandpaper in order to have a perfectly smooth surface to be painted. The paint layers were made with six different pigments spread on the panel with an egg-tempera medium and with three diverse thicknesses, as reported in figure 2. The pigments, namely yellow ochre, cinnabar, green earth, viridian, azurite, and ivory black, were chemically characterised and ground before being mixed with the binding medium. Figure 3. Time domain THz graph (left) and non-invasive cross-section image of a linear section of the test panel (right). In figure 3 the time domain graph of the propagation of the THz radiation into the structure of the test panel is reported together with the reconstruction of the non-invasive cross-section of a linear section of the test panel. As these graphs show, the position of the different refractive index discontinuities versus 1 C.E. = common era, a politically correct way of referring to western calendar dates without basing on the Christian calendar.

3 time of acquisition of the THz signal through the depth of the panel structure due to the presence of the diverse materials is evident. The false colour THz image of the area framed with a red line (Fig. 2) is reported in figure 4. In particular the presence of the linen canvas is revealed by the reconstructed THz image (Fig. 4) at around 175 ps of delay of the THz radiation. Figure 4. False colour THz image of an area of the test panel (left) and reconstructed image at about 175 ps delay showing the presence of the linen canvas (right). CASE STUDIES The first reported case study was the Polittico di Badia (c ) by Giotto di Bondone belonging to the Galleria degli Uffizi. This tempera wooden painting (334 cm x 91 cm), formerly an altarpiece in the Badia Fiorentina church, was investigated with the THz imaging system in December 2008 during its restoration. Usually, THz images are reported in grey scale, with high values of either transmission or reflection shown in white. As an example, a non-invasive cross-section image of an area of the mantle of San Benedetto is shown in figure 5. During the conservation treatment it was not possible to determine the depth of penetration into the paint layers of a superficial fracture in the paint surface. The THz crosssection of a linear segment of the acquired THz image (Fig. 5, solid line) revealed that the fracture was present not only in the entire thickness of the paint layers, but also in the preparation layer, which was completely missing. Moreover, as figure 5 demonstrates, in a past conservation treatment only the inner paint layers were re-added to the painting for the portion of the solid line in the area comprise between the two dashed lines. Figure 5. RGB image (detail) of the mantle of San Benedetto (left) and the non-invasive cross-section image along the solid line A-A of the painting where the depth of the fracture is evident (right). Furthermore, THz-TDS imaging technique provided useful information about the presence of gold leaves even when they were beneath the inner paint layers. Because the reflectance of gold in the THz region is very strong, the shape of the gold foil is visible even when it is under the paint. In addition, this noninvasive imaging technique revealed that there was a linen canvas between two preparation gesso layers because the image obtained at the interface between the two layers showed a regular pattern which, as figure 4, shows, appears in the test panel. This information was useful in confirming the great importance of this panel painting as a milestone artwork that demonstrates the passing of the medieval artists

4 technique to those of the Renaissance. The obtained results proved that THz analysis can provide information of value to both historians and conservators. The second case study relates to one of the most important Japanese artworks. Indeed, unlike western countries, only a few mural paintings survive in Japan, and the mural painting of the Takamatuzuka tomb (ca. 700 CE) in Asuka near Nara, discovered in 1972, is one of these. Because of their severe deterioration, the 16 rocks that formed the tomb, and consequently the mural painting, were disassembled in 2006 and placed in a dedicated conservation laboratory. One of the west wall rocks, which has female figures, was investigated with the T-Ray 4000 THz-TDS imaging system. To comply with the strict safety regulations, the THz imaging system was placed above a polyethylene sheet, and the measurement was performed at a distance of more than 100 mm, as shown in figure 6. Hence, the spatial resolution and the sensitivity of the system were greatly reduced in comparison with the example reported above. Figure 6. Instrumental set-up for the acquisition of the THz-TDS images on the Takamatuzuka tomb mural paintings. The red solid line in Figure 7 shows the detail of the female figures with the reconstructed cross-section image obtained in a non-invasive way by the THz system. In the same figure the reconstructed images of the surface of the painting, the preparation layer, and the surface of the support stone are shown along with the cross-section image. Some parts on the surface of the painting exhibit strong reflection, which could be related to the presence of the re-crystallised calcite. The uncertainty of the reflection signals from the calcite preparation layer may be due to scattering effects in the porous structure of the most deteriorated layers. From these preliminary data it is evident that higher resolution could have been achieved if the THz imaging measurements could have been performed without the polyethylene sheet. These results suggest that THz imaging is a very promising technique for the analysis of the conservation condition of the preparation layer of mural paintings. Figure 7. RGB image (detail) of three female figures from Takamatuzuka tomb mural paintings (left) and the non-invasive cross-section image of the painting and the reconstruction of three images at different time delays (right).

5 ACKNOWLEDGEMENTS The authors would like to thank Cristina Acidini (Soprintendente per il Polo Museale Fiorentino), Antonio Natali and Angelo Tartuferi, Director and Curator of the Uffizi Gallery, respectively, who kindly gave permission for the analysis of the Giotto painting, and Stefano Scarpelli, Susanna Bracci, and Irl N. Duling III for their collaboration in the examination and analysis of the Giotto painting. In addition, the authors are most grateful to Marina Ginanni, conservator at the Conservation Laboratories of Polo Museale Fiorentino, for the preparation of the test panel painting. REFERENCES [1] Institute of Physics, T-rays, VISIONS, 22, available from (2008). [2] H. Hoshina, Y. Sasaki, A. Hayashi, C. Otani, K. Kawase, SPIE Newsroom, DOI: / , (2009). [3] W. Köhler, M. Panzer, U. Klotzach, S. Winner, M. Helm, F. Rutz, C. Jördens, M. Koch, H. Leitner, in Proceedings of the European Conference of Non-Destructive Testing, P181, Berlin, [4] K. Fukunaga, Y. Ogawa, S. Hayashi, I. Hosako, IEICE Electronics Express, 4(8), 258, (2007). [5] I. Hosako, N. Sekine, M. Patrashin, S. Saito, K. Fukunaga, Y. Kasai, P. Baron, T. Seta, J. Mendrok, S. Ochiai, H. Yasuda, in Proceedings of IEEE 95, 1611, (2007). [6] J.B. Jackson, M. Mourou, J.F. Whitaker, I.N. Duling III, S.L. Williamson, M. Menu, G.A. Mourou, Optics Communications 281, 527, (2008). [7] A.J.L. Adam, P.C.M. Planken, S. Meloni, J. Dik, Optics Express 17, 3407 (2009). [8] J.B. Jackson, J. Labaune, G. Mourou, I. Duling, C. Lavier, M. Menu, in Proceedings of SPIE 7391, O3A Optics for Arts, Architecture, and Archaeology II, Munich, ed. by L. Pezzati, R. Salimbeni, (2009). [8] J.B. Jackson, M. Mourou, J. Labaune, J.F. Whitaker, I.N. Duling III, S.L. Williamson, C. Lavier, M. Menu, G.A. Mourou, Meas. Sci. Technol. 20, , (2009). [10] E. Abraham, A. Younus, J.C. Delagnes, P. Mounaix, Appl. Phys. A 100, 585, (2010). [11] J. Labaune, J.B. Jackson, S. Pagès-Camagna, I.N. Duling, M. Menu, G.A. Mourou, Appl. Phys. A 100, 607, (2010). [12] K. Fukunaga, M. Picollo, Appl. Phys. A 100, 591, (2010).