“Often, the issues can be traced back to the artists themselves. While painting’“The Last Supper’ in Milan in the 1490s, Leonardo da Vinci tried a number of oil-paint experiments, hoping to achieve better effects by painting on dry wall rather than on wet plaster. Within 20 years, the masterpiece was flaking off the wall, creating an ongoing headache as experts argued over which parts were authentic and which had been added by overzealous restorers.”