## 3 Results
### Measurements on Ceramic Test-Objects  
A second set of measurements was carried out on another class of test objects, simulating artistic glazed ceramic tiles in terms of materials and their elastic properties. Glaze delamination and spalling is a severe conservation problem, often correlated to water-related deterioration: the damage is mainly associated to the co-existence of the porous ceramic substrate, which may absorb water, and the glazed painted layer, which constitutes a barrier to water and to evaporation. The crystallization of soluble salts and the presence of manufacturing defects enhance the damage. The early detection of adhesion failure, before the glaze falls off and the lacunas impair the visual integrity of the artwork, is highly desirable and should identify hidden cavities a few hundreds of microns deep, when they are still invisible to the naked eye. Figure 10: In (**a**) Roman floor mosaic (ca. 20 \(\times\) 20 cm). In (**b**,**c**) there is a clear detachment from the adhesion surface. In (**d**) the analysis carried out with the device described in this paper, obtained with about 200 measurement points and superimposed on the photo of the handwork for a better understanding of the position of the defects. Three tile models were analyzed in the present work, shown in Figure 11; they belong to a little and heterogeneous collection of antique azulejos and laboratory models [22; 23] of the Laboratory of Acoustics Research applications for Cultural Heritage (LARCH) of INM-CNR (Institute of Marine Engineering, National Research Council, Roma, Italy). The tile models T1-T11-T13 with artificial cavities, different in shape and depth, were manufactured assembling thin technical glasses of the type SCHOTT D 263(r) LA eco [24] (a colorless borosilicate glass, having thickness 0.7 mm, density 2.51 g/cm\({}^{3}\), and [PERSON]'s modulus 72.9 GPa) on small clay substrates using epoxy glue. The physical properties of these technical glasses and of the epoxy glue allowed simulating the physical properties (air cavity thickness, glaze thickness, density and elastic properties) of the real tiles in vibro-acoustic phenomena. The manufacturing process was carried out at room temperature using epoxy glue with medium drying time (30 min) to prevent cracks during the drying process. For the model T1 few pieces of ordinary microscope glass, 0.8 mm thick, were cut and fixed on the clay substrate in order to build the side walls of a square cavity. Successively, the thin D 263 glass was fixed as capping layer upon the previous one, thus forming a hidden cavity having a resulting thickness of 1.3 mm. In order to minimize the cavity thickness, for the other models a different manufacturing process was adopted, building up the cavity's walls only with the epoxy glue. Specifically, for T11 a first uniform layer of glue was spread over the back surface of the thin technical glass used as cap layer, masking a little area to shape the cavity; after drying, a second layer of epoxy glue was uniformly spread over the first layer, in order to fix the whole to the clay substrate. It resulted a hidden square cavity having a final thickness of 0.8 mm. Similarly, T13 was built following the same procedure, masking a triangular area to shape the cavity. It resulted a hidden triangular cavity having a final thickness of 0.6 mm. The manufacturing process was essentially a handmade procedure, however particular attention was paid during any single phase in order to make uniform each model's thickness, and obtain flat surfaces and regular cavities as much as possible. The main features of the tile models are summarized in Table 2.  
These tile models were the subject of research addressing the validation of contactless and frequency-resolved vibro-acoustic imaging methods for the early detection of hidden glazed delamination [22; 23].