On a recent afternoon behind a scruffy door in South London, remarkable alchemical transformations were taking place under the watchful eye of the painter Frank Bowling. Wearing industrial masks, a team of assistants brushed and dolloped ammonia, gold powder, acrylic gel and water onto a dripping canvas hung onto Bowling’s studio wall. Looking dapper in a fedora and a green velvet jacket, the 87-year-old artist directed proceedings from a wheelchair in the center of the room. “Put gel on the edges of the square. No, you’re putting it on the flat,” Bowling said, guiding the action on the canvas with a laser pointer. “Dust that with the gold. Brush the water all over.”