Effect of volatile organic compounds on the stability of inorganic pigments in oil-based paintings

The present study seeks to understand the interaction between paintings and Volatile Organic Compounds (VOC) present in museum environments, evaluating the stability of inorganic pigments and predicting possible risks to cultural heritage. The VOC chosen in this study to determine these effects on pigments were those studied most frequently, such as acetic acid, formic acid, and formaldehyde, and in addition hexanal and 2-butanone oxime, also found in museum environments. Accelerated laboratory ageing tests were carried out with models of oil paintings or powdered pigment, prepared from malachite, lead white and gypsum and exposed to the VOC-saturated atmosphere. After 42 days of exposure, FTIR-ATR analyses confirmed the formation of metal carboxylates such as lead acetate, formate and hexanoate, as well as copper acetate and hexanoate. These degradation products are caused by VOC attack on the paint surface. For some models, colorimetric analysis also identified significant total color variation (∆E), with differences visible to the naked eye, in addition to decreased lightness. The lead white and gypsum changed from white to a yellowish color. Malachite tended to turn blue when exposed to acetic acid. All samples exposed to 2-butanone oxime showed ∆E > 5 an acceptable tolerance limit, thus reiterating the importance of the effect of this particular VOC. In EPR and color analysis, carried out with powdered malachite, exposure to acetic acid, hexanal and formic acid resulted in similar EPR spectra, decreased lighting (L^⁎) and color shift to blue. Exposure to 2-butanone oxime caused an elongated axial Cu (II) site (g// > g_⊥), reduced lighting, and a slight color difference. However, exposure conditions occurred at high concentrations of atmospheric contaminants, which is uncommon in natural museum environments, this study demonstrates what can occur over years of exposure, reiterating the importance of studies involving indoor air quality and its effect on historical heritage.