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Finding The Culprit For Van Gogh’s Darkening Yellows

Van Gogh's Vase with Sunflowers is getting a pigment check-up this week. Credit: Wikimedia Commons.

The Sunflower still-life series is possibly Vincent van Gogh’s most famous work. Unfortunately the warm yellow hues that make the paintings memorable come from pigments that don’t have a long life-expectancy.

During the 19th century, chrome yellow pigments came in to fashion among painters and then quickly went out again, as artists realized that the vibrant yellow color was unstable and would lose its vibrancy when exposed to light.

For example, Claude Monet and Pierre-Auguste Renoir both steered clear of the chrome yellow pigments. But van Gogh threw caution to the wind and continued to use chrome yellow until his suicide in 1890—a tragic hint, perhaps, of his own instability and imminent breakdown.

This week, conservation scientists at the Van Gogh Museum in Amsterdam have been taking a closer look at chrome yellow pigments in Vase with Sunflower, and a few other paintings, to learn more about the degradation problem.

The museum’s staff has recruited a group of traveling conservation scientists from Perugia, Italy, called MOLAB to come help out. (MOLAB has a sophisticated collection of analytical equipment that can study artwork without harming it.)

The yellow pigments in Sunflowers gone to seed are also suffering their own aging problems. Credit: Wikimedia Commons

It’s not the first time MOLAB has worked on the chrome yellow issue. Last year, they worked with almost the same team of van Gogh experts to discover why chrome yellow turns from bright yellow to a dingy brown in the presence of sunlight.

Here’s why: Chrome yellow pigment is primarily lead chromate (which, incidentally, is the pigment for yellow school buses and was also briefly used to color candy in the 1800s—yikes!). Different yellow hues can be made by mixing in lead chromate oxide and lead chromate sulfate.

The darkening of the yellow paint occurs because the chromium in the pigment is reduced from a hexavalent (Cr6+) to a tetravalent (Cr3+) state. The team also reported that sulfur compounds in the paint seem to exacerbate the process. The sulfur is probably present from lead chromate sulfate added to modulate the yellow hue.

MOLAB has been back in Amsterdam this week to study more van Gogh paintings, says Costanza Miliani, a lead researcher with MOLAB. The plan is to see if sulfur is indeed a culprit in chrome yellow degradation.

Hopefully the team will eventually find a way to thwart and reverse this breakdown so that van Gogh’s vibrant yellows aren’t lost forever.

2 Comments

  • Apr 28th 201200:04
    by john bartlett

    Reply

    Is todays Chrome Yellow subject to the same instability?

  • May 10th 201212:05
    by Sarah Everts

    Reply

    Great question. I’m told by the MOLAB folks that modern day chrome yellow paint is stabilized, typically by encapsulating the problematic pigment with an inert material such us sodium silicate. You can read more about it here: http://pubs.acs.org/doi/abs/10.1021/ac102424h

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