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.