Murals painted by the Maya and other pre-Columbian cultures often contain a distinctive blue pigment that somehow manages to last for centuries in humid rain forests, as has this mural from Cacaxtla in Mexico. Miguel José-Yacaman, a materials scientist at the National Autonomous University in Mexico City, may have discovered the secret of the paint’s unusual durability. It’s been known for decades that the main ingredient is a white, powdery clay called palygorskite that, when boiled with indigo dye, turns a dark blue. The way the paint deflected X-rays told earlier researchers that it was made up of needle-shaped crystals. Using an electron microscope, José-Yacaman found not only indigo but also particles of iron, manganese, chromium, and titanium, in grapelike bunches of about 1,000 atoms each, embedded within the clay (inset). The particles reflect a slightly different shade of blue than the indigo, giving the murals a distinct color; the clay around the particles protects them from corrosion by water and air. José-Yacaman and his colleagues intend to commercialize the blue pigment, which would be ideal for art conservation and restoration projects. Says José-Yacaman: If you want to paint something so that it lasts 1,000 years, you might use this.