A group of European researchers belonging to the Institute of Science molecular technologies and the National Research Council (CNR-ISTM), Department of Chemistry, University of Perugia, the Departments of Chemistry and Physics, University of Antwerp, Delft University of Technology (TUDelft) and ESRF (European Synchrotron Radiation Facility) in Grenoble has begun a careful study to assess and understand the phenomenon of degradation of chrome yellow, which threatens the most famous masterpieces by Vincent van Gogh The important results were published today in the journal Analytical Chemistry.
"The issue has long been known. The various shades of yellow, typical of the vibrant painting Vincent van Gogh and many others as Seraut Impressionists, Pissarro, Manet and Reina, are at risk, "says Constance Miliani, Isthmus-CNR researcher and co-author of the study. "The yellow-based lead chromate (chrome yellow), in spite of the beauty and intensity of colors, are characterized by a lack of chemical and photochemical stability that occurs over time with a marked browning. The old masters used over the centuries to create natural colors of their works until the nineteenth century, painters were immediately attracted to the expressive possibilities of the new formulation of pigments that were introduced in the market through the development of synthetic chemistry. The problem has been noticed is that these modern colors and intense over time show a higher reactivity compared to the elements like light, temperature and humidity, it would have suffered a significant and irreversible trasfornmazione, much more than their natural colors.
Researchers carried out spectroscopic investigations on samples of original paintings and other reference artificially aged. "The precious micro-stratigraphic section in the original samples were taken from paintings Banks of the Seine (1887) and View of Arles with Irises (1888), preserved in the van Gogh Museum in Amsterdam, which show clearly the change in the yellow chromium, Miliani continues. "They were the techniques used μ-XANES (micro X-Ray Absorption Near Edge Spectroscopy), μ-XRF (micro X-Ray Fluorescence), EELS (Electron Energy Loss Spectroscopy), micro-FTIR and micro-Raman. The analysis of these data allowed us to demonstrate that the blackening is related to the reduction of chromium VI to chromium III. In particular, the lead chromate (PbCrO4) degrades to form chromium oxide (Cr2O3 .2 H2O) as the main product of degradation, along with other compounds of Cr (III) as sulphate or acetate hydrates. Furthermore, it was shown that these kind of degradation of Cr (III) are confined to a thin layer of a thickness of 1.2 microns and that their formation is catalyzed by presence in the paint layer of sulphate ions.
The goal of researchers is to understand what conditions favor the degradation of color and the ability to slow it down. In fact, not all the paintings of van Gogh suffered the same alteration, for example in the series of sunflowers version kept at the National Gallery in London maintains the vibrant shades of yellow, while the unaltered version of the van Gogh Museum in Amsterdam is a marked color change brown. It seems that the UV rays are more clues from staff, so the first indication is given as to protect the paintings from sunlight.
"This type of research is essential to advance our knowledge of the paintings on aging and how best to preserve them for future generations, "says Ella Hendriks of the Van Gogh Museum in Amsterdam.
Source cs CNR 10/2011
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