## 3 Results
### Imaging Documentation  
Examination of the wall painting fragments under raking light (Figure 8a) highlighted specific details that indicate that the wall paintings may have been created using the _a secco_ technique. No characteristic technical hallmarks of _a fresco_ painting are present: no evidence of the typical shiny surface given by the carbonatation crust, no technological incisions made on the fresh plaster (to help in the process of drawing), and, most importantly, no traces of smoothing or polishing of the surface made by trovel blades [24; 47]. This hypothesis is sustained by the registered FTIR data that indicate, for some of the investigated samples, the presence of an organic binder. The use of the _a secco_ technique would also explain the conservation state of the pictorial layers--mainly very fragile, with poor adhesion to the support, a consequence of the natural deterioration of the organic binder. Figure 8: Details in raking light (**a**) and under UV fluorescence examination (**b**). Hyperspectral imaging details that highlight the behavior of the pigments at specific wavelength values: 954 nm (**c**); 1600 nm (**d**). Images registered at different wavelengths of the SWIR region (Figure 8c,d) suggest that a carbonaceous material may have been used for the black pigmented areas, most probably carbon black. This hypothesis is based on the fact that some of the pigmented areas are visible in infrared light (these areas correspond to a pigmented layer based on a mixture of red and black pigments). Carbon-based blacks were frequently used by the Romans, alone or admixed with other pigments in order to obtain different shades, such as brown (obtained from hematite darkened red with amorphous carbon) [48; 49]. Given the large areas of lost polychromy as well as the poor visible trace of some of the pigments (mainly the yellow ochre pigments), the SWIR hyperspectral data cubes were processed using linear spectral unmixing (LSU) with the aim to enhance the presence of the pigments used for the surface decoration of the wall painting fragments. The LSU algorithm was applied since it is able to generate abundance maps of pixels with a specific spectral profile. These are constructed by identifying the endmembers and the fractional abundances that correspond to each pixel. Moreover, LSU calculation allows classifying areas with mixed pixels based on their statistic values [50; 51]. The main endmembers (corresponding to the various pigmented areas) were selected using the regions of interest (ROI). The results of the applied algorithm are displayed as abundance maps (Figure 9), with bright pixels representing higher abundance. The classification algorithm returned unsatisfactory results--the actual distribution of the red pigment, for example, did not perfectly match with the resulting fraction (Figure 9b). Further analysis indicated that the results of the LSU classification were affected by the presence of a synthetic adhesive used by the archaeologists to fix the wall painting  
Figure 9: Results of the LSU classification obtained on a selection of samples. Visible color image (**a**); abundance map of the red ochre pigment (**b**); abundance map of the identified adhesive (**c**). Bright pixels represent higher abundance. fragments on a Plexiglas support immediately after excavation. As it can be seen in Figure 9c, this adhesive partially contaminated the surface of the wall painting fragments, including some of the pigmented areas. In the hyperspectral data cube, the spectral response of the adhesive could be slightly traced in the interval between 2454 and 2204 nm (characteristic infrared band for synthetic resins were also observed in some of the investigated samples under FTIR analysis; these samples were considered contaminated and excluded from the study). The presence of this organic consolidant also explains the high absorption in the UV region registered for some of the areas (Figure 8b).