How do scientists spot a 5,000-year-old blue where no paint is visible?

They make it glow. Egyptian blue, considered the first synthetic pigment, has an extremely powerful, long-lived infrared luminescence when visible light shines on it. Our eyes can't see that infrared glow, but instruments can, so scientists can detect the pigment on objects that look completely unpainted, and even find traces of it on paintings made as late as the 16th century, long after its use was presumed to have died out.

The pigment itself, with the chemical formula CaCuSi4O10, was made by heating quartz sand, a copper compound, calcium carbonate, and a touch of alkali together at temperatures between 800 and 1,000 degrees Celsius for several hours. Egyptians wanted it because natural blue minerals like azurite, or prized stones like turquoise and lapis lazuli, were too rare and too hard to work to supply the color on demand, so instead they synthesized it. The earliest known evidence of this comes from an alabaster bowl dated to roughly 3250 BC, and the color stayed in use for thousands of years, through the Greco-Roman period, before the knowledge of how to make it was lost entirely in the fourth century AD.

No ancient Egyptian text ever explains how the pigment was made. The earliest surviving recipe comes instead from the Roman architect Vitruvius, writing in the first century BC, who described grinding sand, copper, and natron together and heating the mixture, shaped into small balls, in a furnace. Roman sources also credit a man named Vestorius with carrying the manufacturing technology from Alexandria to Italy, after which Egyptian blue became the blue pigment of Roman antiquity. Its traces then fade during the Middle Ages, though it still turns up in surprising places: Raphael used it in his Triumph of Galatea. Interest in how it was made only revived when the chemist Humphry Davy investigated it in 1815.

Today that infrared glow is pointing scientists toward new uses: because the pigment can split into ultra-thin nanosheets in water and glows in a part of the near-infrared where neither fat nor hemoglobin absorbs much light, researchers are exploring it for medical imaging, security inks, and even coatings that could help cool rooftops in sunny climates.