## 3 Results and Discussion
### FTIR Spectroscopy  
The high complexity of both soiling mixture and historical varnish, in terms of number and variety of constituents, makes the interpretation of the spectra complex. Nonetheless, thanks to the direct observations of the spectral signals and the PCA, relevant results were achieved, and some decisive information was gained, mainly about the Velvesil(tm) Plus test. The two bands of the Portland cement and kaolin inorganic fractions, with the absorbance maxima falling respectively at 533 cm\({}^{-1}\) and 468 cm\({}^{-1}\) (Figure 4), were identified as diagnostics of the cleaning effectiveness. Figure 4: Fourier transform infrared (FTIR) spectra of the Velvesil(tm) Plus test from t\({}_{0}\) (bottom grey line) to t\({}_{4}\) (top)—Velvesil(tm) Plus (red) and soiling mixture (blue) diagnostics bands are highlighted. The intensity reduction of these spectral features were able to provide quality information about the capability of the employed products to remove the soiling from the varnished surfaces. This reduction was clearly observed in the spectra collected in the Velvesil(tm) Plus test areas. They evidently showed a significant drop in the intensity, until disappearing, from t\({}_{0}\) to t\({}_{4}\), proving the positive cleaning effect of this cleaning procedure (Figure 4). However, FTIR spectroscopy analysis showed that the use of Velvesil(tm) Plus has some disadvantages to be taken into consideration in the art conservation field: after cleaning, residues of the product were detected on the surface, proved by the persistence of the silicone bands in the 1500-1000 cm\({}^{-1}\) range and 800 cm\({}^{-1}\). These hypotheses were corroborated by the results of the PCA shown in Figure 5a. The analysis explains a total of 88.73% variance by the first two components: 73.23% and 17.50% of the total variance, respectively for the PC1 and the PC2. Figure 5: Score plots and loadings of the test with Velvesil(tm) Plus (**a,b**), water in agarose (**c,d**), and Tween 20 (**e,f**) are displayed. In the score plots, the application time is expressed in minutes by the labels. In the scores plot, four areas were recognizable, corresponding to the four quadrants. Spectra of the original varnished surface (grey stars) were placed in the first quadrant (I), where both PCs assume positive values. This resulted in being the quadrant in which the spectra of well-cleaned surfaces were better described. In this portion, the clean areas were supposed to be represented. On the opposite quadrant (III), where the PCs were both negative, the spectra corresponding to the soiled condition (the green square) were grouped. The II quadrant (PC1 negative-PC2 positive) assembled the spectra corresponding to a mild effect of the cleaning and a presumed presence of sweat. Finally, in the IV quadrant (PC1 positive-PC2 negative) of the scores plot, the spectra characterized by the presence of the residues of the Velvesil(tm) Plus were positioned, as already shown by the direct observation of the spectra. The distribution in the scores plot was supported by the loadings plot (Figure 5b); the range portions, where the PC1 showed high variability and assumed lower values, included the diagnostic bands of the soiling and the silicone residues. The presence of such a residue that could affect the distribution of the green squares and triangles in the second quadrant, could cover the effect of the cleaning. Under this scenario, the cleaning at 45 and 25 min could also be suitable. As concerns the water in agarose and the Tween 20 cleaning tests, the PCA approach turned out to be powerful and fundamental to visualize the difference among the spectra that, by a preliminary visual approach, were all very similar to each other and little descriptive. In the new space described by PC1 and PC2, the variability of the spectra was well displayed and the discussion of the scores plot by the loadings could guide to a hypothesis very similar to those introduced by the colorimetric data analysis.