## 3 Results and Discussion
### Colorimetric Investigation
As regards the Tween 20 (3%) in agarose, it seemed to be more effective than the water in agarose in removing the dark matter: after cleaning, \(\Delta\)E* in SSw area varied from 2.30 (t\({}_{1}\)) up to 3.82 (t\({}_{4}\)) and L* grew (\(\Delta\)L* max = 3.11 after 90 min), signifying bleaching due to the removal of black particles, facilitated by the surfactant action of the solvent. Observing the \(\Delta\)PC scores trend in Figure 2a, three phases are recognizable: in the first phase, i.e., within 45 min, both \(\Delta\)PC1 and \(\Delta\)PC2 rose constantly; in the second phase, i.e., between 45 and 90 min, a drop of \(\Delta\)PC2 was followed by a third phase described by a plateau. As a matter of fact, the second phase was the most effective one: L* kept rising, showing a removal of the dark matter but a different action was observed for b* suggesting a change in the yellow tone. This was confirmed by the loading investigation (Figure 2c) in which \(\Delta\)PC1 described L* variability, whereas the \(\Delta\)PC2 was influenced by a* and b* coordinates. In the Sw area, \(\Delta\)E* values varied from 4.94 (t\({}_{1}\)) up to 6.55 (t\({}_{4}\)) (Table 1): the variation was supported by the decreasing trend of L* and b*, ascribable to an unexpected darkening and to a yellow component variation of the surface feature after  
Figure 2: Principal component analysis (PCA) results on the colorimetric dataset expressed in differential PC scores vs. cleaning time for the three methods in the SSw (**a**) and Sw (**b**) area; in (**c**) the loadings are displayed. the cleaning. \(\Delta\)PC score supported this variability suggesting also, in this case, that the major action was carried out between 45 and 60 min of application time (Figure 2b). Concerning the Velvesil(tm) Plus cleaning test for the SSw area, \(\Delta\)E* varied from 2.37 to 5.29 with an increasing trend since t\({}_{2}\). As in the Tween 20 test, L* resulted to be the parameter that most affected this variation (\(\Delta\)L*max up to 3.24 after 45 min) due to the dark layer removal and to the consequent surface bleaching. About at* and b*, the variation of the latter was not negligible (Table 2) suggesting a modification of the yellow component. Additionally, in this case, the \(\Delta\)PCs scores endorsed these observations by displaying the presence of three different phenomena occurring in three different moments: in 5, in 15, and after 15 min. In the first 5 min, \(\Delta\)PCs described a rising of L* that seemed to represent the best conditions for the dark matter removal. After 5 min a rise of \(\Delta\)PC1, respectively linked to a drop of \(\Delta\)PC2, could raise some doubts about the advantage of using a long application time of Velvesil(tm) Plus from the chromatic feature point of view. The results obtained for Sw are as unexpected as those obtained for the Tween 20: the value of \(\Delta\)E* varied from 8.41 up to 16.95. If combined with a substantial reduction in the b* value (\(\Delta\)b*max = 16.91 after 45 min) that seemed to drag this trend, it could point to a change in the yellow component. This behavior is well described by the \(\Delta\)PCs scores (Figure 2b). The results of the Velvesil(tm) Plus test highlighted the strongest action. In fact, this highly non-polar and surfactant product seemed to interact also with the hydrophobic components of the sweat, probably inducing a transformation in the aesthetic properties of the surface.