Tikalon Blog by Dev Gualtieri

Van Gogh's Fading Reds

April 9, 2015

Children are subject to many small disappointments throughout their childhood. In my youth, we lived in an older house, since demolished, that had interior door locks opened by huge keys. I was gifted with one of those keys at about age six, and I was heartbroken when it was lost.

One smaller disappointment was finding my small, red, plastic airplane bleached to pink after exposure to the Sun's rays after wintering on a porch. Later, I learned about the mechanism of photochemical bleaching of organic dyes.

Red plastic airplane, Vikingplast article no. 113

Red plastic airplane.

Photochemical bleaching is the reason why museums don't allow flash photography.

(Photo by Gwafton, via Wikimedia Commons.)

Some inorganic materials change color in response to light, also. An F-center, named after the combination of the German words, Farbe (color) and Zentrum (center), is an anionic vacancy in a crystal. Such vacancies include missing oxygen atoms in oxides, and missing chlorine or fluorine atoms in halides. Light absorbed by electrons at these vacancies modify the absorption spectrum of visible light, such that transparent materials, which appear to be white in polycrystalline form, become colored.

F-centers will change white potassium chloride (KCl) powder to a pleasant violet color, while table salt (sodium chloride, NaCl) will turn a brownish-yellow. Since F-centers are a type of doping, it's possible to make F-center lasers, usually from single crystals of the halides. Excitation with intense electromagnetic radiation, such as X-rays and gamma rays, will produce F-centers.

Although F-centers are one way for inorganic materials to change color, a color change is usually effected by chemical modification. One simple example of how environmental exposure can cause a color change is the response of cobalt (II) chloride (CoCl₂) and copper (II) chloride (CuCl₂) to humidity. While anhydrous CoCl₂ is sky blue, its hexahydrate form, CoCl₂·6H₂), is a deep purple. Anhydrous CuCl₂ is brown, its dihydrate form, CuCl₂·2H₂), is azure. These color changes have been used for decades as humidity indicators.

The primary photochemical process of concern in the preservation of oil paintings is the darkening of the protective varnish on the surface. In some cases, however, more damaging photochemical processes take place in the pigment, itself. In a previous article (Van Gogh Versus the Sulfates, March 1, 2011), I wrote about the photochemical color change of the yellow pigment of later works by the Dutch artist, Vincent van Gogh.

While in France , van Gogh learned about a new yellow pigment from his artist friends. This new yellow pigment was lead chromate , PbCrO₄, which at the time seemed like a better yellow than that in common use; namely, yellow ochre , which is the hydrated form of iron oxide , Fe₂O₃·H₂O.

Vincent van Gogh (1853-1890), "The Starry Night" (1889, Oil on canvas).

(Museum of Modern Art, Accession number 472.1941, via Wikimedia Commons.)

Eventually, this "chrome yellow" was found to turn brown when exposed to light. The culprit in this color change was found to be trace quantities of sulfates that reduced Cr⁶⁺ to Cr³⁺.[1-2] The valence change shifts the pigment color to green, causing the darkening effect.

Another pigment used by van Gogh and many artists of his period was red lead, Pb₃O₄, also called minium. This pigment, known from antiquity, will whiten upon extended exposure to light through the formation of anglesite (PbSO₄), or lead carbonate, PbCO₃, also known by its mineral name, cerussite. This pigment will darken, also, under other circumstances through conversion to the beta-phase of lead oxide, β-PbO₂, plattnerite, or lead (II) sulfide, PbS, galena.[3]

While Picasso had his "blue period," van Gogh had his "wheat series" that ran from 1885 until his death in 1890. His first such painting, Wheat Sheaves, was done with rather drab colors, while the later ones, painted in France, were more colorful (see figure).

Left, Wheat sheaves in a field, August, 1885; right, Landscape with wheat sheaves and rising moon, July, 1889; both from the Kröller-Müller Museum, via Wikimedia Commons.

A team of scientists from the University of Antwerp (Antwerp, Belgium) has recently published an analysis of the bleaching of the red pigment in van Gogh's, “Wheat Stack under a Cloudy Sky,” an oil on canvas painted in 1889 and presently at the Kröller-Müller Museum in the Netherlands.[3-5] Says Koen Janssens, research team leader and a professor in its Department of Chemistry,

"Normally, the idea is these paintings are there for a hundred years, or five hundred years, and they're static – nothing really changes... but the opposite is actually true when you look in detail."[5]

The Antwerp team examined a minute white particle from the pond area in the painting, and they used X-ray powder diffraction mapping and tomography to determine the chemical phases present. The equipment, at the DESY synchrotron light source, PETRA III, allowed a depth profiling of the specimen.[4-5] What they discovered was an extremely rare lead mineral, plumbonacrite, 3PbCO₃·Pb(OH)₂·PbO), which is the first report of this chemical compound in a painting created prior to the mid-20th century.[3]

Vincent van Gogh (1853 - 1890), Korenschelf onder wolkenlucht (Wheat stack under a cloudy sky)

Vincent van Gogh (1853-1890), Wheat stack under a cloudy sky (Korenschelf onder wolkenlucht), oil on canvas, October 1889.

(© Collection of the Kröller-Müller Museum, Otterlo, The Netherlands and used with permission.)

This observation enabled a conjectured reaction pathway for such pigment bleaching in which both light and carbon dioxide are required. Red lead is converted into plumbonacrite when exposed to light, and the plumbonacrite reacts with carbon dioxide to form lead carbonate, cerussite, and its hydrated form, hydrocerussite.[5] The light irradiation moves electrons from the valence band to the conduction band in the semiconducting red lead.[4] This reduces the Pb₃O₄ red lead to PbO, which reacts with the CO₂ from the environment to form plumbonacrite. The plumbonacrite is converted into the cerussite compounds by further absorption of CO₂, thereby whitening the red pigment.[4]

Of course, none of this work would have been possible without the sample from the Kröller-Müller Museum, and the authors acknowledge this in their paper. Also acknowledged are the Helmholtz Association for the PETRA III light source at DESY, and the University of Antwerp.[3]

Van Gogh's Fading Reds

References: