Disclosing the Binding Medium Effects and the Pigment Solubility in the (Photo)reduction Process of Chrome Yellows (PbCrO4/PbCr1– x S x O4)

Abstract

Abstract The darkening due to chemical alteration of chrome yellows (PbCrO4/PbCr1–xSxO4) is a phenomenon threatening a large number of 19th–20th century paintings, including the Amsterdam Sunflowers by Vincent van Gogh. Our earlier studies have proven that the alteration is due to a Cr(VI) → Cr(III) reduction with Cr(V)-species that are formed as long-lived intermediates and that PbCr1–xSxO4 (0 < x ≤ 0.8) types undergo reduction more readily than monoclinic, S-free, PbCrO4. In this context, there is still lack of knowledge about the effects of the chemical properties of the binding medium (i.e., chemical composition and drying process) and the solubility of chrome yellows on the overall reduction pathways. Here, we study a series of naturally and photochemically aged mock-up paints prepared by mixing chrome yellow powders (PbCrO4/PbCr0.2S0.8O4) with either linseed oil or a water-based acrylic emulsion as the binding medium. Equivalent paints made up of the highly soluble K2CrO4 were also investigated and used as benchmarks to provide a more in-depth understanding of the influence of the solubility on the chromate reduction pathways in the two different binders. A combination of synchrotron radiation-based Cr K-edge X-ray absorption near edge structure (XANES), electron paramagnetic resonance (EPR), and UV–Visible spectroscopy measurements shows that: (1) the Cr(VI) reduction results from the interaction between the pigment and the binder; (2) the process is more significant in oil, giving rise to Cr(V)- and Cr(III)-species as well as oxidized organic compounds; (3) the lightfastness of the chrome yellow pigment is enhanced in the acrylic binder; and (4) the tendency toward chromium reduction increases with increasing solubility of the pigment. Based on our findings, we propose a scheme for the mechanism of the (photo)reduction process of chrome yellows in the oil and acrylic binder. Overall, our results provide new insights into the factors driving the degradation of lead chromate-based paints in artworks and contribute to the development of strategies for preserving them over time.

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