31-07-2025
Old oil paintings are suffering from chemical 'acne'
WHEN AN OIL painting is dried and finished, it is supposed to stay that way. Yet when Ida Bronken, an art conservator, began to prepare Jean-Paul Riopelle's 'Composition 1952" for display in 2006, she noticed drops of wet paint were trickling down the canvas from deep within the masterpiece's layers. Equally odd were the tiny, hard, white lumps poking through the painting's surface, as if it had a case of adolescent acne. Other sections seemed soft and moist; some paint layers were coming apart 'like two pieces of buttered bread", Ms Bronken says.
At the time, she was stumped. 'I just stared at the artwork and thought 'Why is this painting acting so strange?'" She soon found out that such behaviour is unexpectedly common in oil-based paintings. There are pockmarks in the red roofs of Vermeer's 'View of Delft" and surprisingly rough surfaces in the black dress of 'Madame X", painted by John Singer Sargent. Damage of this kind has been blamed on everything from air bubbles and glass spheres to insect eggs and sand—all unfairly, as it turns out.
The true culprits instead are positively charged metal ions, such as zinc and lead, present in paint pigments. Over time these react with negatively charged components of oil called fatty acids, which have been severed from the rest of the oil molecules by light, heat and humidity. This process, known as saponification, produces a kind of soap called metal soap, with potentially disastrous consequences. In the past 20 years oil paint's predilection for saponification has been illustrated in masterpieces by Rembrandt, Georgia O'Keeffe and Francisco de Goya, with surveys suggesting it is under way in 70% of oil paintings in museum collections. 'I like to think of paintings as little chemical factories," said Katrien Keune, head of science research at the Rijksmuseum in Amsterdam.
To discuss the problem and what might be done to overcome it, Dr Keune and her colleagues convened a conference in April that brought together some 200 painting researchers and conservators from around the world, including the Metropolitan Museum of Art in New York, the National Gallery in London and the Art Institute of Chicago. They agreed that metal soaps are not all bad and, in fact, help the early drying process. But as the decades stack up, the pimple-like balls, paint drips and wet surfaces that Ms Bronken observed on 'Composition 1952" can begin to emerge. Sometimes a hazy crust known as efflorescence forms, obscuring the artwork below.
Twentieth-century artworks are particularly vulnerable, partly due to changes in paint formulation. In oil painting, double bonds in the long carbon chains of the oil react with oxygen from the air as the artworks dry. This creates new chemical connections that stabilise the final, cured layers. But the linseed oil used in traditional oil paint became harder to source after the first world war, prompting the use of herring, sunflower and safflower oils as substitutes. These oils contained fewer double bonds, leaving the paintings' layers much weaker. The salutary replacement of toxic lead white with zinc-based pigments likewise caused problems, such as delamination—where painting layers lift, and sometimes fall, off.
So what can be done? One priority is to test more thoroughly the cleaning fluids and adhesives used to remove dirt and repair paintings. Some researchers worry that these solutions could penetrate into paint layers and accelerate saponification. Even water is under scrutiny; conservators are increasingly choosing to clean paintings with high-tech tissues and gels that release only a scintilla of solution. Then there is the problem of water in the air. As the costs of energy have risen, many museums have relaxed a strict 48-52% humidity range to a range as wide as 40-60%. As a result, some institutions are putting especially vulnerable oil masterpieces behind glass, where humidity levels can be optimised for the painting in question.
Many questions remain: should soapy acne be cleaned away, or will that lead to unsightly damage? What about oil drips sliding down a canvas? Wipe them off, and a conservator might accidentally remove material deliberately placed there by an artist. Do nothing, and they might cause further harm. Though few answers have emerged so far, it is clear that watching paint dry has become a pursuit of tremendous cultural value. Art conservators across the world hope that it one day brings fewer dramatic consequences.
Curious about the world? To enjoy our mind-expanding science coverage, sign up to Simply Science, our weekly subscriber-only newsletter.
