## 4 Discussion
### Fluid Absorption and Swelling Dimensional Changes  
Regarding the swelling of historical wood damaged by xylophagous insects, it can be seen that depending on the level of damage [10; 28], the thickness of the wood, and the time in which it is in contact with a solvent, it will dimensionally change differently (Figure 22).  
In the case of less polar solvents, the migration inside the samples was very fast, at less than 2 min for acetone and up to 5 min for white spirit, compared to water, where, due to the increased polarity, the absorption occurred more slowly; the absorption/migration time increased to 10 min (Figure 23).  
Figure 21: (**a**) Panel flatness before treatment; (**b**) changes produced during treatment.  
Figure 22: The swelling ratio according to the thickness of the specimens and the immersion time (15 min for both types).  
The solvents were absorbed in different amounts and volumes (g and cm\({}^{3}\)). Along with this absorption, different degrees of swelling occurred, as exemplified in Tables 5 and 6, which show that the largest volume is represented by the swelling in acetone, at approximately 68.65%. If we refer to the initial mass of the samples and the maximum volume obtained after extraction, it can be concluded that the mass and volume after immersion do not increase equally or in the same proportions with any of the solvents used.  
During the experiments observing the changes in the wood that were produced by the solvents, it was found that the greatest absorption occurred in the samples that were partially immersed in water; however, the greatest level of swelling occurred with the acetone solvent.  
There are different mass increases in all three solvents. In the case of water and white spirit, evaporation is slower and remains longer in the volume of wood, while acetone, being very volatile, evaporates much faster.  
During the research, it was found that in the case of acetone, once it evaporated from the wood, the initial mass of the sample decreased from 63 g to 58 g. This phenomenon can be explained by the fact that the rapid evaporation of acetone also involves water from the initial moisture content of the wood in the evaporation process, thereby reducing the density of the wood.  
In the case of the white spirit solvent, after immersion, the mass of the sample increased to 212 g, resulting in a quantity of 77 g of remaining solvent in the wood. After 24 h of slow evaporation, the mass reached 182 g, including 47 g of solvent, an additional 30 g having evaporated. It was found that after 24 h, approx. 38.96% of the absorbed amount had evaporated.  
Analyzing the samples with polychromy, it was found that white spirit, even if it migrates to the painted surface, does not then destabilize the polychrome layer, does not wet it, and does not produce additional cracking, as seen in the case of acetone or water.  
Figure 23: Migration of the solvent into the wood after the first 2 min: (**a**) water; (**b**) white spirit; (**c**) acetone.  
Regarding the samples immersed in acetone, it was observed that micro-cracks appeared on the wooden surface and on the polychrome layer, where the already existing micro-cracks became more pronounced, making the layer unstable and brittle (Figures 24 and 25).  
On the polychrome samples that were in contact with water, it was found that after migrating to the polychrome surface, the water caused wetting and destabilized the layer. On the reverse side, the initial deep cracks stood out, an accentuated cracking of approximately 0.8 to 1 mm occurred, and this continued throughout the thickness of the specimen (Figure 26).  
Figure 24: Cracks produced in the wood under the polychrome layer, with accentuation of the originally existing cracks.  
Figure 25: Magnified view of the cracks and swelling after immersion in acetone.