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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 (MoS
2) 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 (H
2S) that reacts with
silver oxide to form the black tarnish product,
silver sulfide, Ag
2S,
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).
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, Fe
2O
3 • H
2O. The iron oxide, Fe
2O
3, 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, PbCrO
4, 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 Cr
6+ to Cr
3+, 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]
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]
References:
- Claus Habfast, "X-rays show why van Gogh paintings lose their shine," European Synchrotron Radiation Facility Press Release, February 11, 2011.
- By Amina Khan, "The mystery of the discolored Van Goghs," Los Angeles Times, February 14, 2011.
- Letizia Monico, Geert Van der Snick, Koen Janssens, Wout De Nolf, Costanza Miliani, Johan Verbeeck, He Tian, Haiyan Tan, Joris Dik, Marie Radepont and Marine Cotte, "Degradation Process of Lead Chromate in Paintings by Vincent van Gogh Studied by Means of Synchrotron X-ray Spectromicroscopy and Related Methods. 1. Artificially Aged Model Samples," Anal. Chem., vol. 83, no. 4 (February 14, 2011), pp, 1214-1223.
- Letizia Monico, Geert Van der Snick, Koen Janssens, Wout De Nolf, Costanza Miliani, Joris Dik, Marie Radepont, Ella Hendriks, Muriel Geldof and Marine Cotte, "Degradation Process of Lead Chromate in Paintings by Vincent van Gogh Studied by Means of Synchrotron X-ray Spectromicroscopy and Related Methods. 2. Original Paint Layer Samples," Anal. Chem., vol. 83, no. 4 (February 14, 2011), pp, 1224-1231.
Permanent Link to this article
Linked Keywords: Space opera; science fiction; Vincent van Gogh; painting; sulfur; college; chemistry; England; United Kingdom; UK; US; Google; thio- prefix; Greek; International Union of Pure and Applied Chemistry; IUPAC; lexicography; lexicographer; metallurgist; atomic number; abundance of elements in Earth's crust; copper; oxygen; transition metal oxides; metal oxides; alloy; mechanical properties; molybdenum disulfide; lubricant; superalloy; tarnish; silverware; hydrogen sulfide; silver oxide; silver sulfide; gasoline; automobile; petroleum; pollution; catalyst; automobile catalytic converter; coal; sulfuric acid; acid rain; sweet crude oil; The Starry Night (1889, Oil on canvas); oil paint; pigment; inorganic compound; mineral; binder; vehicle; resin; linseed oil; polymerization; photochemical; varnish; yellow ochre; hydrate; iron oxide; hematite; water molecule; lead chromate; Holland; France; sunflower; Paul Gauguin; ultraviolet; Royal Museum of Fine Arts (Antwerp); Flemish; Rik Wouters; X-ray; absorption near-edge spectrometry; X-ray fluorescence spectrometry; European Synchrotron Radiation Facility (Grenoble, France); Bank of the Seine; University of Perugia; Universit degli Studi di Perugia (Perugia, Italy); University of Antwerp (Antwerp, Belgium); Delft University of Technology (Delft, The Netherlands); Center for Research and Restoration of Museums of France; Centre de Recherche et de Restauration des Muses de France (Palais du Louvre, Paris, France); Van Gogh Museum (Amsterdam, The Netherlands); Netherlands Cultural Heritage Agency (Amsterdam, The Netherlands); Analytical Chemistry.