Figure 2
J. Chillida

Ravages of time. Researchers took a small flake (leaving a white spot) from the dark region (formerly red) just below the lower of two reddish, flower-shaped elements at the left of this mural. The mural is in the 14th century Monastery of Pedralbes in Spain. X-ray diffraction along with first-principles calculations led to a proposed sequence of chemical reactions that explains the darkening of the red color over time.