Modern pigments in street art: stability of monoazo and isoindoline yellows mixed with 2PbCO3‧Pb(OH)2, ZnO, and TiO2 white pigments

The physical and chemical stability of commercial monoazo (PY1) and isoindoline (PY139) yellow pigments was investigated when dispersed in linseed oil and combined with three inorganic white pigments: 2PbCO 3 ‧Pb(OH) 2 , ZnO, and TiO 2 , forming binary and ternary mixtures. These systems were subjected to accelerated aging in a climatic chamber at 30 °C and 60 % RH. Surface and subsurface changes induced by artificial aging were analyzed using a multi-technique approach, including colorimetry, Fourier Transform Infrared Spectroscopy in Attenuated Total Reflectance mode (ATR-FTIR), micro-Raman Spectroscopy (μ-RS), and Time-of-Flight Secondary Ion Mass Spectrometry (ToF-SIMS). Evident color changes were observed, highlighting the impact of metal oxides on the degradation of yellow pigments. Titanium white promoted marked degradation at the surface level, leading to the near-complete disappearance of the yellow components. This effect was attributed to the photocatalytic activity of nanosized TiO 2 . Conversely, despite its known photocatalytic properties, ZnO did not trigger significant degradation, a result linked to its larger particle size, which limited its reactivity. 2PbCO 3 ‧Pb(OH) 2 , on the other hand, mainly affected the oil binder's drying process and promoted chemical oxidation of the finely dispersed fraction of the yellow pigments, while leaving larger, undegraded aggregates unevenly distributed across the surface. These findings shed light on pigment-metal oxide interactions in modern paint systems and offer valuable insight for understanding the long-term behavior of such materials in contemporary artworks and street art exposed to outdoor environments. • TiO₂ caused the strongest fading of PY1 and PY139 via photocatalysis. • Lead white sped oil drying and oxidation without major pigment loss. • ZnO induced mild pigment loss and formed zinc–carboxylate soaps. • All mixtures showed strong yellow loss, with TiO₂ giving ΔE up to 73. • Multi-technique study revealed surface and subsurface aging effects.