The effects of humidity on ester hydrolysis and oxidation in aging oil paint

Water plays a critical role in the chemical processes that occur during aging of oil paint, but the detailed effects of environmental humidity on the interlinked reaction pathways of hydrolysis and oxidation are not well understood. We report the results of long-term artificial aging experiments on oil paints prepared with iron oxide, zinc white, vine black and unpigmented films. The films were frequently monitored with attenuated total reflection Fourier-transform infrared (ATR-FTIR) spectroscopy, after which concentration estimates for various carbonyl species could be extracted using a combination of curve fitting and calibration. Using this thorough analysis approach, it was demonstrated that ester hydrolysis reaches an equilibrium state in iron oxide oil paint. In contrast, in zinc white oil paint the rapid formation of zinc carboxylates drives the ester hydrolysis process towards completion. The large dataset of ATR-FTIR spectra also revealed that the sensitivity of oil paint to humidity and the relative importance of hydrolysis and oxidation degradation pathways both depend strongly on pigmentation. These results shed light on the complex chemistry of oil paint aging, and are important to support decisions on indoor climate conditions for museums. • A large dataset of ATR-FTIR spectra of aging oil paints was collected. • Ester hydrolysis can reach equilibrium in oil paint, which reduces its sensitivity to humidity fluctuations. • The relative importance of hydrolysis and oxidation pathways during oil paint aging depends strongly on pigmentation.