When archaeologists began to uncover Pompeii’s crimson frescoes, releasing the beautiful Roman art from its entombment in volcanic ash, they also exposed it to daylight and a host of airborne chemicals that had been kept at bay for two millennia. As a result, many of the paintings are now darkening to an alarming extent.
Suspicions over the cause centre on cinnabar, the red pigment based on mercury sulphide that was used in the frescoes. The light was thought to be changing the mercury sulphide from the red hexagonal form characteristic of cinnabar to the cubic form known as metacinnabar, which…
