Lead Carboxylates and Chromatic Parameter Changes in Oil-Based Paintings Exposed to Volatile Organic Compounds

Some of the main concerns when attempting to preserve artwork regard environmental factors, such as luminosity, temperature and, recently, even atmospheric pollution. From this perspective, volatile organic compounds (VOC) stand out due to the wide availability of contaminating sources in internal museum environments. Amongst artistic objects susceptible to VOC interference, paintings are more likely to undergo it because their stability is closely related to each component. Thus, it is crucial to comprehend the interaction between VOC and paintings, specifically those with inorganic and adhesive pigments, and evaluate changes in chromatic parameters and the formation of metallic carboxylates. Mock-ups of paintings were prepared with cadmium yellow, chromium oxide, ultramarine blue, linseed oil, and lead white or gypsum primer. Exposure was arranged within an airtight container containing a VOC-saturated atmosphere: formaldehyde, acetic acid, hexanal and 2-butanone oxime. Color alteration occurred for ultramarine blue (greenish coloration) and cadmium yellow (lost luminosity). Infrared analysis showed the formation of lead carboxylates (acetate, formate and hexanoate) in all pigments. Micrographs of ultramarine models, verified via scanning electron microscopy, showed that an otherwise flat and homogenous appearance replaced the granular aspect of the paint; the flat regions being indicative of carboxylate formation.

Keywords:
carboxylates; pigment; degradation; color change; painting