Initial stages of metal soaps` formation in model paints: The role of humidity

A Critical Review on the Analysis of Metal Soaps in Oil Paintings

“…Other environmental factors, such as light and temperature, may influence these phenomena. [ 16 , 59 , 60 ]…”

“…[ 39 , 69 ] Data from FTIR can aid in monitoring the formation of metal soaps in oil systems and in distinguishing the amorphous and crystalline soaps. [16] An example is provided by the analysis of the painting “ The woodcutter (after Millet)” (1889) by Vincent van Gogh, where different kinds of zinc soaps were identified by Hermans et al. The results highlighted that their IR absorptions differed from standard materials, in particular the asymmetric stretching vibration of COO − is broader and shifted towards higher wavenumbers respect to the absorptions related to metal soaps synthesised and used as references.…”

“…[ 9 , 10 , 11 , 12 , 13 , 14 , 15 ] Metal soaps may cause several undesirable and often irreversible changes in the painting's appearance: the abovementioned protrusions on the surface, the presence of efflorescence, darkening or contrarily translucency, losses, delamination and cracking. [ 16 , 17 , 18 , 19 , 20 , 21 ] While metal soaps are often related to degradation phenomena in oil paintings, thus influencing the stability of the painted systems. Nevertheless, it was observed that, sometimes, their presence does not produce any significant effect in multilayered paint films.…”

Formation of Metal Soaps In Oil Paintings Under Variable Environmental Conditions

“…However, several studies have shown how increasing values of relative humidity speed up the formation of carboxylates as a consequence of hydrolysis phenomena of drying oil [9,13,29]. As hydrolysis proceeds, an FTIR band at about 1710 cm − 1 , attributed to the carbonyl stretching ν(C = O) of the free fatty acids, increases, while the carbonyl stretching band of the ester groups decreases [11,19,22]. In the studied lead white paints, the band of the free fatty acids cannot be distinguished because it is hidden by the stretching vibration band of the ester groups at 1735 cm − 1 .…”

“…Although the formation mechanism of metal soaps has been extensively studied, the correlation with the environmental conditions of conservation still needs a deeper investigation. Some studies have demonstrated that certain conditions of temperature and relative humidity favour the formation of metal soaps and their subsequent crystallization [9][10][11][12][13][14][15]. However, in most of the published works, extreme conditions of ageing were used, which are far away from the real conservation environments.…”