## 4 Discussion  
Solubility and optical characteristics are often crucial considerations for conservators, and an intriguing correlation between the two emerges through the study of cleaning. The observation of morphological variations induced during cleaning allows for an understanding of the effectiveness of new green solvents in applications similar to real cases. The data collected from various analytical techniques confirm the predictions developed using RED calculations in the Hansen diagram, compared to what is expected from the TEAS diagram.  
All tested FAME can be utilized to solubilize two of the most commonly used low- and non-polar substances in the cultural heritage sector. The best results were obtained from FAME characterized by carbon chain lengths ranging from C16 to C18, both in the removal of microcrystalline wax and Regarlez paint applied on canvas painted with acrylic and oil. The FAME were applied in gel form through a single step with a contact time of 5 min. This method was chosen to avoid potential interactions with the support and to compare the effectiveness of the different tested FAME. A second application, considering the different efficiency of the tested FAME, would have led to total cleaning in some areas, with the possibility of interacting with the surface, thus not allowing a real comparison. Optical microscopy images confirmed the hypotheses developed through macroscopic observations, highlighting greater interaction between the paints and the canvas painted with acrylic. This result is attributed to the aging process, which causes acrylic to retract more compared to the oil-painted layer, as observed in SEM images. Microscopy observations correlated with spectrophotometric analyses show that with an increase in non-polar carbon chain length, solubility typically increases. However, greater efficiency was achieved with FAME characterized by saturated chains compared to FAME characterized by structures of the allylic and bis-allylic methylene groups.  
The overall analysis data allowed us to assess the efficacy of the gel formulation in both removal and interaction with the underlying canvas, evaluating the presence of surface residues through optical microscopy and FTIR spectra, enabling us to achieve the objectives set in the introduction. Different gels were defined for the various canvas compositions, highlighting the extensive customization possibilities of the gel based on specific case needs. Further studies are necessary to evaluate the stability of gel-FAME-emulsifier systems, for example, through aging cycles.  
In synthesis, concerning the canvas painted with oil colour and coated with microcrystalline wax, the optimal results, following various analyses, were achieved using Gel 3 (the best surface cleaning seems to have been obtained with Gels 1, 3, and 4 from imaging observation; with optical microscopy observation, the best cleaning results were obtained with Gels 1, 3, 4, and partially with Gel 5; colorimetry revealed a \(\Delta\)E value of 1.36 for Gel 3). For the canvas painted with oil and coated with Regalrez resin, the best outcomes were obtained with Gel 1, Gel 3, Gel 7, and Gel 4 (the best cleaning appears to be given by Gels 1, 2, 3, 4, 5, and 7 from imaging observation; with optical microscopy observation, the best results were provided by Gels 1 and 7; colorimetry showed the lowest values for Gels 1, 4, and 2).  
For the canvas painted with acrylic and coated with microcrystalline wax, the most favorable results were obtained using Gel 3 (imaging showed cleaning results with Gels 1, 3, 5, and 6; optical microscopy observation indicated Gel 3, and partially Gel 5; colorimetry showed a lower ratio value for Gel 3 and Gel 5). Regarding the canvas painted with acrylic and coated with Regalrez resin, the optimal results were achieved with Gel 3 and Gel 5 (imaging demonstrated better cleaning results with Gels 3, and partially scarce results with Gel 5 and 2; optical microscopy observation confirmed those results).