## 4 Final Discussion and Conclusions  
This study investigated the stability of modern paints containing manganese violet PV16 (NH\({}_{4}\)MnP\({}_{2}\)O\({}_{7}\)) when exposed to weathering agents, specifically high relative humidity and sulfur dioxide. In the field of cultural heritage, PV16 is used mainly in paintings, but little research exists on its conservation. This study further investigates the susceptibility of this pigment to degradation in indoor conditions and mixed with different modern binders (oil, alkyl resin, acrylic emulsion, and styrene-acrylic emulsion). After the accelerated aging, color changes from violet to grey occurred in all the samples, and in the grey areas, degradation of both organic and inorganic fractions was observed. At the inorganic level, three different types of crystals were formed in all samples, except in the oil one, where only one type of crystal was visible. These crystals have different shapes and elemental compositions. In fact, they were identified as a mixture of phosphates and sulfates and, in particular, (NH\({}_{4}\))\({}_{2}\)Mn\({}_{2}\)(SO\({}_{4}\))\({}_{3}\) and (NH\({}_{4}\))\({}_{2}\)(HSO\({}_{4}\))(H\({}_{2}\)PO\({}_{4}\)). Both the formation of the crystals and color changes seem to be more related to the pigment itself and, in particular, to the hygroscopicity of its phosphate group and the electron configuration of Mn ions. The main chemical reactions that occurred in the organic matrix of all samples are hydrolysis, oxidation, and sulfonation. The effect of hydrolysis is more evident in the acrylics; oxidation is more evident in the commercial and styrene-acrylic samples; and sulfonation is evident in all samples, but less in the commercial oil one. Furthermore, comparing the self-made and the commercial oil-based samples, some differences were noticed. The commercial sample showed the greatest optical change on the surface, and the prevalent degradation reaction seems to be the oxidation of the organic binder, whereas in the self-made oil sample, the color change was only at the edges, and the principal reaction appears to be sulfonation. These differences could be related to the pre-heated treatment of the oil in the self-made sample and the presence of additives in the commercial sample. Considering all the results obtained, the most degraded paints after exposure to SO\({}_{2}\) in a humid environment were those based on alkyl resin, styrene-acrylic emulsion, and commercial oil paint, while the more stable paints were the acrylic-based and the self-made oil-based samples. The results obtained proved promising for future investigations and considerations of the materials under investigation. One example is the understanding of the role and the chemical stability that the two types of oil, i.e., stand-oil and non-pre-polymerized types, have regarding the resistance of the manganese pigment to acid attacks. Another consideration for future studies is the use of models that reproduce paintings on canvas to study the possible influence of the substrate and the presence of other pigments. Finally, it is important to investigate whether this type of degradation effect can also be found in real works of art, for example, in modern and contemporary paintings, to possibly proceed with studies on its correct conservation. It is important to underline that geographic areas that are more susceptible to this problem are those where SO2 levels are still high, such as Mediterranean and Asian countries, including China, India, and Southern Italy. Therefore, the survey should be carried out starting from these areas. This study has contributed to the expansion of knowledge relating to modern and contemporary paints containing manganese-based purple pigment, highlighting their susceptibility to exposure to pollutants, especially depending on the different binders with which it is mixed. These results therefore highlight the importance of museum environmental-monitoring measures, underlining the need for constant refinement.