The yellow problem

Three of the most-reproduced paintings in the world share a defect nobody planned. Their yellows are chemically unstable — and each one is failing in a different way. Which means every colour palette sampled from a famous painting, including the ones on this site, is a reading of a surface that has moved.

The short answer

Paintings do change colour measurably, and conservation science can now say by how much. In van Gogh's Sunflowers chrome yellow is browning; in Seurat's La Grande Jatte zinc yellow has darkened toward ochre; in Munch's The Scream cadmium yellow is turning off-white. Same colour, three different chemical failures.

Chrome yellow: van Gogh, and a reduction you cannot yet see

Van Gogh built Sunflowers out of chrome yellow — lead chromate — including a sulfur-rich lemon variety that turns out to be the unstable one. Synchrotron analysis of the Van Gogh Museum canvas found the pigment undergoing photo-induced reduction, with roughly 35% of the chromium at the paint surface converted from Cr(VI) to Cr(III). That is the chemistry that takes a pale lemon toward olive brown.

The unsettling detail is the timescale. Researchers note the change is not yet obvious to the naked eye. The painting is browning on a schedule slower than human attention, which means the version reproduced in every book is already slightly wrong and nobody can see it happening.

Today's Sunflowers palette →

Zinc yellow: Seurat, and a painting that changed while he was alive

Seurat finished A Sunday on La Grande Jatte in 1886. Darkening of specific brushstrokes was recorded in 1892 — six years later, and a year after his own death at 31. He had used zinc yellow, zinc potassium chromate, in the bright yellow-green passages. It moves toward a dull ochre as Cr(VI) reduces to Cr(III), most aggressively under strong light, high humidity and sulfur dioxide, which describes an industrial city with reasonable precision.

There is a particular irony here. Seurat's entire method was built on optical precision — small separate touches of colour intended to combine in the viewer's eye rather than on the palette. The one variable he could not control was whether the pigments would still be the colours he chose. The Art Institute of Chicago and the Rochester Institute of Technology have since produced digital reconstructions of the original.

Today's La Grande Jatte palette →

Cadmium yellow: Munch, and the one that is not about light

Everyone assumes light is what damages paintings. In the c. 1910 version of The Scream it mostly is not. Yellow strokes in the sky and the figure's neck have turned off-white, and thickly-painted water is flaking. Synchrotron study found the driver is moisture, combined with mobile chlorine compounds, oxidising cadmium sulfide into cadmium sulfate. Photodegradation plays the smaller role. The museum's resulting guidance is about humidity — keep the work below 45% relative humidity — not about lux levels.

Today's The Scream palette →

Why all three are yellow

This is not coincidence. The brilliant yellows that arrived in the nineteenth century were new industrial chemistry — chromates and sulfides — and painters adopted them for their intensity long before anyone understood how they would age. Chrome, zinc and cadmium yellow all reached the studio during precisely the decades when European painting became most preoccupied with colour itself.

The generation that cared most about colour got the least stable pigments in the history of the medium. Van Gogh, Seurat and Munch were not careless; they were early adopters, and the bill arrived a century later.

The counter-example: what stability actually looks like

Set against this, Vermeer's blues are almost unchanged. Natural ultramarine — lazurite ground from lapis lazuli mined in what is now Afghanistan — was more valuable than gold by weight in the seventeenth century, and it is chemically stable. Analysis for the Girl in the Spotlight project found a high-quality grade in the headscarf of Girl with a Pearl Earring, and suggested the stone had been heat-treated before grinding to yield a more intense blue.

So the blue you see is close to the blue Vermeer laid down, and the yellow you see in Sunflowers is not. Cost and permanence turn out to be correlated in a way that is slightly depressing: the pigment that bankrupted painters is the one still telling the truth.

What this means if you are sampling a palette

• A palette taken from a painting is a reading of today's surface. It is a real measurement of a real object, but not a record of the artist's intent.
• Expect yellows and greens in nineteenth-century work to be duller and browner than originally painted. Blues, earths and blacks are generally more trustworthy.
• Varnish and photography add their own bias on top. Different reproductions of the same painting genuinely disagree.
• None of which makes the palettes less useful — it makes them historical. Design with them knowingly.

See all 25 painting palettes → · How they were sampled

Sources

• Monico et al., “Evidence for Degradation of the Chrome Yellows in Van Gogh's Sunflowers”, Angewandte Chemie Int. Ed. (2015)
• ESRF — Van Gogh's Sunflowers: evidence of chrome yellow degradation
• Electron energy loss spectroscopy and the darkening of zinc potassium chromate in La Grande Jatte — Anal. Bioanal. Chem.
• “Probing the chemistry of CdS paints in The Scream” — Science Advances
• Munchmuseet — why The Scream is fading
• “Out of the blue: Vermeer's use of ultramarine in Girl with a Pearl Earring” — npj Heritage Science