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Van Gogh Versus the Sulfates

March 1, 2011

The title of this article sounds like one if those early "space opera " style science fiction films of the 1950s and later. A recent discovery has shown that some of Vincent van Gogh's paintings have a discoloration caused by a sulfur reaction.[1-4] You notice that the word in the title is "sulfate," not "sulphate," and that the usual spelling of the element is sulfur, and not sulphur.

For the first part of my career, I always wrote sulphur, possibly because that's the way it was spelled in my father's college chemistry book. That spelling is used in England and the UK , but sulfur is the usage that's common in the US . If we let Google be our arbiter, sulfur beats sulphur 18,600,000 hits to 12,500,000, or about fifty percent. To complicate matters, the thio- prefix , from the Greek word for sulfur, is used for some sulfur compounds. In 1990, IUPAC decided the spelling should be sulfur, but old habits die hard.

Not only is sulfur a problem for lexicographers , it's the bane of metallurgists . Sulfur, being an element with low atomic number (16), is reasonably abundant in the Earth's crust , where it exists at about five times the concentration of copper , but a thousand times less than oxygen with which it's chemically similar. Many metal oxides are hard and melt at high high temperatures, and their presence in alloys at small concentrations often enhances mechanical properties . Metal-sulfur compounds generally melt or decompose at lower temperatures, and they're soft. Molybdenum disulfide (MoS₂) is so soft that it's used as a high temperature lubricant . Presence of sulfur in superalloys , even in very small concentrations, can seriously spoil their mechanical properties.

You may not have any superalloys in your house, but sulfur still causes problems for you. The tarnish on your silverware is caused by sulfur in the atmosphere, in the form of hydrogen sulfide (H₂S) that reacts with silver oxide to form the black tarnish product, silver sulfide , Ag₂S,

Ag₂O + H₂S -> Ag₂S + H₂O

The gasoline in your automobile is somewhat more expensive since the petroleum used in its manufacture needs to be scrubbed of sulfur as a pollution preventative; and also because sulfur will poison the catalyst used in your automobile catalytic converter . Sulfur in coal poses a similar pollution problem, sometimes forming sulfuric acid in the atmosphere and the resultant acid rain . You shouldn't taste your oil, but if it has less than 0.5% sulfur, it's considered to be "sweet ".

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

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

Oil paint is simply a pigment , which is generally an inorganic mineral , in a vehicle . The vehicle, which was exclusively linseed oil until recently, allows the transfer of pigment to the canvas. The linseed oil bonds the pigment to the canvas as it "dries." What actually happens is that the oil polymerizes . What results is a relatively permanent representation of what the artist intended. The reality is that chemistry often intervenes to change a painting's colors. Usually it's a photochemical darkening of the protective varnish on the surface, but in some of Van Gogh's works there's a different chemical process taking place.

The most common yellow pigment is yellow ochre , which is the hydrated iron oxide , Fe₂O₃ • H₂O. The iron oxide, Fe₂O₃, itself is red (it's called hematite for that reason), but it takes just one water molecule bound to it to change the color to a golden yellow. For some of his works, Van Gogh used a different yellow pigment. This was lead chromate , PbCrO₄, which at the time seemed like a better yellow, but it was realized years later that the "chrome yellow" would degrade upon exposure to light, turning brown.

Van Gogh's use of the new yellow pigment began after his leaving Holland for France . In France, he learned about the new pigment from his artist friends, and he painted sunflowers using the chrome yellow pigment for his friend, Paul Gauguin , who hung them in his bedroom.[1]

It turned out that even after protecting Van Gogh's painting from ultraviolet , the wavelengths of light responsible for this photochemical reaction, some of his yellows were still turning brown.[1-4] As a first step, a team of chemists decided not to attack any of Van Gogh's paintings directly, but to analyze specimens from three archived paint tubes at the Royal Museum of Fine Arts (Antwerp). Only one paint specimen browned upon exposure to a 500 hours of intense ultraviolet light, that from a tube that belonged to the Flemish painter, Rik Wouters (1882-1913).[1]

As was revealed through X-ray absorption near-edge spectrometry and X-ray fluorescence spectrometry at the European Synchrotron Radiation Facility (Grenoble, France), the non-browning paints were very pure, but the browning paint contained sulfates.[3] The sulfates were responsible for reducing Cr⁶⁺ to Cr³⁺, a change that actually greens the pigment, but results in an enhanced darkening effect. Analysis of a small paint chip from a Van Gogh painting confirmed this effect.[4]

Figure caption

An optical micrograph of a paint chip taken from Van Gogh's "Bank of the Seine."
No, the black upper crust is not a sulfur compound, it's varnish. The sulphates are located in a three micrometers thick region between the upper varnish and the paint.
(Source: University of Antwerp, Department of Chemistry)

A detailed description of this work, by a large team of researchers from the Universit degli Studi di Perugia (Perugia, Italy), the University of Antwerp (Antwerp, Belgium), Delft University of Technology (Delft, The Netherlands), Centre de Recherche et de Restauration des Muses de France (Palais du Louvre, Paris, France), Van Gogh Museum (Amsterdam, The Netherlands), the Netherlands Cultural Heritage Agency (Amsterdam, The Netherlands) and the European Synchrotron Radiation Facility (Grenoble, France), appears as a two part paper in the journal, Analytical Chemistry .[3-4]

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