Characterization of metal carboxylates relevant for degradation of oil paintings by complementary XRPD and ssNMR

Abstract

Saponification occurring in paint layers represents a serious degradation process affecting the appearance and stability of paintings, leading for example to protrusions, efflorescence, darkening, delamination, exudates etc. A substantial part of saponification is formation of metal carboxylates, resulting from the interaction between metal cations (e.g., Pb 2+ , Zn 2+ ) released from pigment particles (e.g., lead white, red lead, zinc white) with fatty acids (usually palmitic and/or stearic) released from triglycerides making-up oil-based binders. Metal carboxylates can adopt variable structures from ionomers to amorphous complexes to crystalline phases, and up to now the mechanism of their crystallization is not elucidated. Moreover, crystal structures of most metal carboxylates are not determined. This paucity complicates the study of the degradation process and clarifying of factors promoting or inhibiting the saponification. However, without knowledge of degradation mechanism it is impossible to find a suitable strategy to prevent it.Within the study of miniature paintings by combination of non-destructive spectroscopic and diffraction techniques (X-ray fluorescence, infrared spectroscopy and X-ray powder diffraction), unusual patterns of crystalline metal carboxylates together with the red pigment cinnabar (HgS) were detected Fig. 1 [1], indicating the possible effect of the cinnabar on the formation of these carboxylates. PS-44-1 Poster SessionActa Cryst. (2021), A77, C1138-C1139 reference [2]. We revealed that both hexadecanoate (C16) and octadecanoate (C18) chains are present in one crystal structure, creating the statistical disorder at the ethyl end of the chains.