Surveys suggest that saponification—a slow chemical reaction that damages oil paintings from within—is under way in roughly 70% of oil paintings in museum collections. Positively charged metal ions (chiefly zinc and lead) present in paint pigments react over time with fatty acids that have been severed from the oil molecules by light, heat and humidity. The result is a substance called metal soap. Symptoms include pimple-like lumps poking through the surface, wet paint trickling down the canvas decades after completion, softened paint layers, and a hazy crust known as efflorescence that obscures the image. Damage has been documented in masterpieces by Vermeer, Rembrandt, Georgia O'Keeffe, Francisco de Goya and John Singer Sargent.
Paintings from the 20th century are especially susceptible. After the first world war, the linseed oil traditionally used in oil paint became harder to source and was replaced by herring, sunflower and safflower oils. These substitutes contain fewer carbon double bonds, which weakens the chemical cross-links that stabilise cured paint layers. The replacement of toxic lead-white pigments with zinc-based alternatives also caused problems, notably delamination—where paint layers lift off the canvas.
Museums have debated how aggressively to treat affected works. Conservators increasingly clean paintings with high-tech tissues and gels that release only a tiny amount of solution, rather than conventional fluids that might penetrate paint layers and accelerate saponification. Many institutions have also placed vulnerable oil paintings behind glass to control humidity. A standard range of 48-52% relative humidity has been relaxed in some museums to 40-60% as energy costs have risen, prompting concern that wider swings may worsen the problem. In April 2025 some 200 researchers and conservators convened at the Rijksmuseum in Amsterdam to discuss the issue.
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