Short answer
Some historic pigments change chemically long after the painter finished. Chrome yellow, used widely by Van Gogh, can darken at the surface as its chromate is reduced; smalt, a cobalt blue glass, can lose its colour as potassium leaches out; many red and yellow lakes fade in light; and some arsenic and lead pigments degrade into other compounds. Synchrotron and imaging studies have traced these changes molecule by molecule, and the results guide how such works are lit.
Chrome yellow is a family of lead chromate pigments, some containing lead sulfate, that nineteenth-century painters valued for bright, cheap yellows. Darkening of chrome yellow areas is observed in paintings of the period, most famously the versions of Van Gogh's Sunflowers. A series of studies led by Letizia Monico used synchrotron X-ray methods to find why: in artificially aged samples, and then in microsamples from two Van Gogh paintings, part of the chromium at the surface had been reduced from its hexavalent state to trivalent compounds, forming a thin altered layer, one to three micrometres deep, that darkens the yellow. Later non-invasive mapping of the Van Gogh Museum's Sunflowers found two kinds of chrome yellow — a light-fast lead chromate and a light-sensitive, sulfate-rich form — with the sensitive form present in more than half the chrome yellow areas.
Smalt is powdered blue potash glass coloured by cobalt, used widely from the sixteenth to the eighteenth century. It often degrades dramatically, turning skies, draperies and backgrounds grey, brown or colourless. Analysis of smalt particles from paintings in the National Gallery, London, and the Louvre showed why. In intensely blue particles the cobalt sits mainly in tetrahedral coordination in the glass; in colourless altered smalt the coordination has shifted towards octahedral, and the shift correlates with loss of potassium. The potassium ions that leached out of the glass had been stabilising the arrangement that makes cobalt blue. Once they leave, the blue goes, and the change cannot be reversed.
Organic lake pigments — reds from madder, cochineal and brazilwood, yellows from plant dyes — fade in light because their colour depends on dye molecules. In a study of a seventeenth-century still life by Abraham Mignon, researchers showed that the light-induced fading of an organic yellow lake, together with the breakdown of the arsenic sulfide yellow orpiment into other compounds, had flattened the light-to-shadow modelling of a yellow rose, making it look quite unlike what the painter intended. Similar losses are thought to explain purplish or grey skies where a blue was mixed with a fugitive red, and green foliage turned blue where a yellow lake glaze has faded from over blue underpaint.
These changes are irreversible, so what museums can do is slow them and explain them. Knowing which areas contain light-sensitive chrome yellow or lakes lets conservators set light levels and exposure more precisely, and it is part of why many paintings are shown at modest illuminance with ultraviolet removed. Technical studies also change how works are read: a dull grey passage may once have been blue, and a painter's apparently muted palette may be the result of fading rather than choice. The history pages on this site cover the pigments themselves and their toxicity; this page is about how they change on the wall. Knowing the pigment does not let an owner diagnose a painting — that needs analysis.
| Pigment | Change seen | Mechanism found | Evidence |
|---|---|---|---|
| Chrome yellow (lead chromate, sulfochromate) | Surface darkening | Reduction of Cr(VI) to Cr(III) in a thin layer; sulfate-rich forms more sensitive | Synchrotron studies of Van Gogh paint samples; mapping of Sunflowers |
| Smalt (cobalt potash glass) | Blue to grey or colourless | Potassium leaching shifts cobalt coordination | National Gallery and Louvre samples |
| Organic yellow lakes | Fading | Photodegradation of the dye | 17th-century still life by Abraham Mignon |
| Orpiment (arsenic sulfide) | Loss of yellow; new compounds | Degradation to lead arsenates and arsenolite in the paint | Same still-life study |
| Red lakes | Fading, especially in mixtures with white | Photodegradation of the dye | Widely observed; see lake pigments |
Why: The reproduction may be colour-corrected, while the original has pigments that have darkened or faded.
Fix: Treat both as partial records; technical studies explain which areas have changed.
Why: Possibly an unstable chrome yellow or a degraded varnish; the two look similar.
Fix: Keep the work out of strong light and ask a conservator; only analysis can tell the causes apart.
Each statement is labelled by kind — established fact, a standard’s requirement, observed market data, a convention, or Colourwise’s own interpretation or analysis — with the strength of the evidence behind it.
FactStrong evidence
Darkening of chrome yellow involves reduction of chromate to Cr(III) compounds in a thin (1–3 µm) surface layer; in one aged historic tube-paint sample about two-thirds of the surface chromium had been reduced.
FactStrong evidence
Reduced chromium was found in original paint microsamples from two paintings by Van Gogh, confirming that chrome yellow reduction takes place in real paintings.
FactStrong evidence
Mapping of Van Gogh's Sunflowers (Van Gogh Museum) identified a light-fast monoclinic lead chromate and a light-sensitive sulfate-rich chrome yellow, the latter present in more than half of the chrome yellow regions.
Caveat: One painting; other versions and works differ.
FactStrong evidence
In discoloured smalt from National Gallery and Louvre paintings, cobalt coordination shifts from tetrahedral towards octahedral, correlating with loss of potassium, which stabilises the tetrahedral cobalt responsible for the blue.
FactStrong evidence
In a 17th-century still life, light-induced fading of an organic yellow lake and degradation of orpiment irreversibly changed the intended modelling of a yellow rose.
Reviewed 29 September 2026. Colourwise summarises its sources in its own words and does not reproduce standards text or proprietary colour data. Spotted an error? Tell us.