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.