# A Multi-Analytical Approach to Investigate Fresco Paintings in a Hypogeum Environment
## 3 Results
### Spectroscopic Investigations  
IR-R was used on the fresco \"Madonna in trono con Bambino\" using different filters (715 nm-850 nm-1000 nm) to reveal possible hidden details, such as retouches, underdrawings and pentimenti, beneath the surface of the fresco. The main results of these measurement are reported in Figure 5, with images focused on the area where the Child [PERSON] is present (details of its face, the hand of the [PERSON], their robes, and part of the throne where they are seated). Because superficial salt efflorescences are largely transparent in the infrared, whereas the pigments absorb in that region, IR imaging yields a far clearer reading of the fresco--an essential step before any cleaning intervention. In the 715 nm band (Figure 5c), subtle surface modelling becomes evident, such as the nuanced shading of the [PERSON]'s hand resting on [PERSON] shoulder (contoured in Figure 5f with yellow arrows), revealing a more elaborate execution than can be discerned with the naked eye. The presence of thin shading details and _chiarscuro_ effects (Figure 5f) are visible only on this specific wavelength (715 nm), suggesting that the artist was involved in a more sophisticated and accurate  
Figure 4: Hygrometric tomography of the fresco “Madonna in trono con Bambino”. compositional process. The detection of these nuances through imaging techniques reveals that the artist intentionally used light and shadow to enhance the realism and depth of the figures. IR-R technique exposes areas of past reintegration and retouching (e.g., the _rigatino_ restoration technique for the halo of [PERSON], contoured in blue in Figure 5c,e). Conversely, no pentimenti or preparatory under-drawings were detected at the longer wavelengths typically used to reveal them (850 nm, 1000 nm). Elemental analysis by portable X-ray fluorescence (XRF) was carried out on the two most representative and best-preserved frescoes in the hypogeum--\"Madonna in tronto con bambino\" and \"Teoria di Santi\". The resulting spectra, presented in Figures 6 and 7, include measurements on both the lime-based substrate and the principal color areas (white, yellow, red, brown, green, and black). Figure 5: IR-R images obtained with different filters: (**a**) Visible; (**b**) 850 nm; (**c**) 715 nm; (**d**) 1000 nm. The magnifications of areas marked in blue and yellow in the image (**c**) are also shown to highlight the details observed with IR-R: area marked in blue for _rigatino_ technique (**e**) and yellow arrows for shading and _chiaroscuro_ effect (**f**). By comparing the data obtained with the XRF database for pigments [35], the various pigments were assigned as shown in Table 1. From the analysis of the lime substrate (Figure 6a), in addition to calcium, iron, and sulfur were also found, the former probably due to the use of small quantities of iron-based earths mixed with the lime matrix, and the latter due to the presence of both sulfate-based salt efflorescence and gypsum. For this reason, a contribution of these elements was always observed in the subsequent pigment analyses. Figure 6: XRF spectra of the fresco “Madonna in trrono con bamboo” measured at the following points: (**a**) limestone; (**b**) white pigment; (**c**) yellow pigment; (**d**) brown pigment; (**e**) green pigment with a magnification of the area between 1 and 3.5 keV; (**f**) black pigment. The bottom right of each spectrum shows the image of the measurement point: red circles indicate the exact point of measurement. The overlayed spectra (blue, orange, and magenta) are referred to the different voltages (8, 15, 45 kV) used for the detection of light and heavy elements. The XRF survey on the white pigments of both frescoes shows calcium as the dominant element, with only minor iron and sulfur. Because no lead, tin, barium, zinc, or titanium were detected, the use of lead white, tin white, barium-based \"fixed\" white, zinc white, or titanium white can be ruled out. The white layer is therefore consistent with chalk, i.e., calcium carbonate (CaCO\({}_{3}\)).