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Deterioration of Zinc Potassium Chromate Pigments: elucidating the effects of pigment mixture and environmental conditions on changes in color and chemical speciation Zinc yellow is a zinc potassium chromate (K2O*4ZnCrO4*3H2O) that is a strong lemon yellow color when manufactured, but has been reported to fade rapidly to a dull, ochre yellow. Most famously, this occurred in the late 19th century masterpiece “Sunday at La Grande Jatte” (LGJ) by French post-impressionist Georges Seurat. This project aims at gaining a fundamental understanding of the darkening of this pigment by systematically controlling and probing critical parameters individually and in combination. Progress has been made in devising new strategies to artificially age model systems simulating the composition of the paints and probe the chemical phenomena involved in the deterioration of the Zn yellow pigment under various environmental conditions. To this end, we have succeeded in designing and constructing a system of environmental chambers that allows us to accelerate the photo-degradation of 6 different model paint films containing different combinations of the chemical components present in the zinc yellow dots of LGJ in various set conditions such as light, humidity and acidity. With this experimental capability we compiled preliminary data indicating key interactions and effects previously overlooked. In addition, we now have sufficient sample size with which to adapt and apply a wider array of analytical tools to probe the cause of the rapid color deterioration experienced by LGJ. Samples will be characterized using various spectroscopic tools, such as X-ray photoelectron spectroscopy (XPS), X-ray absorption spectroscopy (XAS), and infrared (IR) and Raman spectroscopies in order to identify interactions among the components and changes in chemical speciation. The significance of this work goes beyond the specific study of a single painting and contributes to the general study of pigment and color deterioration, with particular focus on chromates, not only widely used by modern artists and found on many paintings in the most important collections in the world, but also of relevance for their industrial applications as paints, inks and anti-corrosion primers. Additionally, learning more about the behavior of the Zn yellow model system may be useful for the development of paint dosimeters to be used as monitors of environmental conditions that may be detrimental to paintings in museums as proposed in the literature.
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