Thermal behavior of mercury carboxylates as paintings' degradation products

Mercury long-chain carboxylates have been identified recently as degradation products resulting from saponification occurring in painted artworks. Saponification belongs among the degradation processes endangering undesirably the appearance and stability of painted artworks, significant treasures of humanity. The mechanism of saponification has not been still fully understood because of the enormous complexity of both painting materials and factors triggering the deterioration. Moreover, the properties and stability of metal soaps resulting from this degradation are also poorly understood, complicating the choice of suitable conservation treatment. Relining, a heat-based restoration technique for reinforcing deteriorated canvases, can induce irreversible changes in paint layers, being applied inappropriately. Within this study, we report thermal behavior and stability of mercury palmitate (Hg(C16)₂), mercury stearate (Hg(C18)₂), and their respective mixtures with linseed oil in the temperature range of 25-150 degrees C, employing a combination of techniques, including DSC, TG-MS, FTIR, XRPD, and in situ high-temperature FTIR and XRPD. It was observed that while Hg(C16)₂ and Hg(C18)₂ undergo partial decomposition around 150 degrees C, in a mixture with linseed oil, they decompose rapidly at significantly lower temperature (around 100 degrees C). The decomposition of mercury carboxylates results in the formation of metallic mercury, a volatile toxic substance, and free fatty acids, reactants capable of further development of saponification in paint layers. In addition, a structural polymorph of Hg(C16)₂ and Hg(C18)₂ with a different arrangement of carboxylate groups around the mercury atom was formed after the heat treatment during the cooling down at ca 120 degrees C as documented by in situ high-temperature XRPD and FTIR.