## 7 Measurements on Water  
The experience gained from the THz-ARTE project gave rise to a new research line for THz imaging, one that is related to the detection of water in mural paintings as well as mosaic walls and floors. Water should exhibit both a high reflectivity and a high absorption coefficient in the THz spectral range. This feature can be exploited in order to detect water damages in mural paintings and mosaics as well as the presence of moss, lichen, and weeds, containing water, under the artwork surface. In order to test the ability of the proposed system to detect the presence of water, it was decided to carry out a set of laboratory tests. The first tests were made to verify whether it was possible to detect under a ceramic tile, the presence of drops of water that were absorbed by its rear side. In Figure 19,  
Figure 17: Visible (**a**) and THz images of the same area, taken at d = 0.5 mm (**b**) and d = 0.7 mm (**c**). In (**c**), the area on the left side of the red curve shows large variations, while the area on the right side exhibits smaller ones. Size of the images: \(100\times 100\) mm\({}^{2}\). Figure 18: Variation of the signal as a function of the distance of the target. A large variation occurs for small distances. the results of this preliminary measurement are shown. Measurements were carried out by placing drops of water on the reverse side of a ceramic tile and by performing imaging from the opposite side (the \"trampling side\"). Both drops were clearly visible in the THz, and their tracks were superimposed on the morphological features of the tile. More measurements were performed on mock-ups of fresco paintings and stone tesserae mosaic, which were prepared by following the traditional recipes (Figure 20). In addition, a sample simulating the absorption property of mosaic, without the use of mortar, was prepared (Figure 21). It was made using a sponge material as the substrate, i.e., one that absorbs water, and various stone tesserae. The assembled mock-up was placed in a plastic box. Mosaics are of a particular interest because one of the more frequent causes of degradation is biological infestation from organisms, such as cyanobacteria, thiobacillus and green algae, that are able to colonize the layer under the tiles, thus causing the tesserae to become detached from the support. If the ability of the THz radiation to penetrate under the tiles and to detect the water contained in moss, lichen and weeds has been demonstrated, this technique could be an important tool for the conservation of mosaics. Figure 19: Reverse side of a ceramic tile moistened with two droplets of water (**a**) visible image of the ceramic reverse side; (**b**) visible image of the ceramic reverse side with THz image (taken from the other side) superimposed. Size of the THz images: \(50\times 50\) mm\({}^{2}\). Figure 20: A mosaic floor (**a**) and fresco wall painting (**b**) mock-ups used for testing the THz imaging system. In order to perform measurements on the sample without mortar, the 3D scanning prototype was modified so as to allow the horizontal surfaces to be measured (Figure 22). Unfortunately, for mosaic samples, in which sharp angles were present on the surface, it was almost impossible to correct the topology of the sample. Thus, measurements were carried out by moving the probe to a greater distance from the sample, to where the oscillations of the signal due to phase beating are greatly reduced. A THz image was taken of the dry sample described in Figure 20. Subsequently, 5 mL of water was injected with a syringe into the top left corner of the plastic box, under the stone tiles, and scans were then performed at 5, 10, 15 and 60 min after the inoculation. In Figure 23, the THz images of the dry sample and of the sample 60 min after an injection of water was reported. Figure 21: Mosaic mock-up: (**a**) top view, (**b**) detail of the sponge background. Figure 22: Setup for horizontal measurements with the mosaic mockup described in Figure 21a.