Shedding light on the molecular interactions during the aging process of yellow ochre pigment in oil employed in art paints

Abstract

Yellow ochre commercial oil paint used in art and restoration was submitted to accelerated aging at 378 K between 5–720 h. SEM-EDS analysis shows particles with size in the range of 1.4–2.5 μm with high iron concentration in the paint aged from 360 h. EPR spectra showed a variation of g values between 2.2256 and 2.1719 (Fe³⁺) for samples aged 5–720 h, respectively, attributed to changes in the goethite chemical environment. Infrared analysis showed the presence of carboxylate groups in the aged material as the result of the molecular interaction between the product of linseed oil hydrolysis (palmitate) and the goethite surface through hydrogen bonding (R–C–OO⋯H–O–FeO). Such data is also supported by the Zeta potential of the pigment surface, which decreased as a function of aging time. The results show the change in the chemical environment of goethite is proportional to the increase in oil oxidation/hydrolysis products. The EPR and Zeta potential proved to be promising for the study of the molecular interactions in oil painting aging and can be used for new pigment/media formulations and for proposing new materials aimed at inhibiting the hydrolytic/oxidative processes of oil paints.