Angew.Chem.Int.Ed.http://doi.org/f2dzcp(2013)

Credit: © THE PROTECTED ART ARCHIVE/ALAMY

'Chrome yellow' paint pigments — lead chromate species that sometimes contain sulfate — are often found in late-nineteenth-century artwork, for example Van Gogh's Falling Autumn Leaves (Les Alyscamps; pictured). They are known to gradually darken on exposure to sunlight, and understanding the degradation process is important to the storage and restoration of these paintings. Recent studies have shown that the degradation rate depends on the pigment's crystalline structure and also its sulfate concentration — sulfur-rich, orthorhombic species darken more rapidly.

Using scanning transmission electron microscopy and electron energy-loss spectroscopy with high spatial and energy resolution, He Tian from the University of Antwerp, Belgium, and co-workers have now studied the degradation of a century-old bright-yellow oil-paint sample at the nanoscale. The sample was characterized before and after an artificial ageing treatment — a 40-day UVA–visible light irradiation that converts it from bright yellow to deep brown. They observed that the original and aged samples comprised four types of particle, with a size distribution ranging from 20 to 300 nm, some of which had core–shell morphologies. The particles consist of phases of PbCrO4, PbSO4 (often observed together, suggesting a solid solution PbCr1−xSxO4) and Cr2O3 (either as small particles or forming the shell of larger particles). Electron energy-loss near-edge structure spectra showed that the PbSO4 phases remained unchanged on degradation, whereas a significant amount of PbCrO4 was converted into Cr2O3 — and this process was faster for sulfur-rich phases.

Combining these findings with previous observations on this material, Tian and colleagues propose a model for the ageing process in which PbCrO4 converts into Cr2O3 through the dissolution of chromateions. These ions undergo a reduction reaction at the interface between the paint and surrounding microdroplets of aqueous solution, and precipitate as Cr2O3 at the surface of all of the particles. This model agrees well with the ageing process observed for all four types of particle.