Poison in the Frescoes: A Painter's Apprentice Who Knew Too Much About the Medici

Florence, October 1494. A seventeen-year-old painter's apprentice notices something wrong with the blue.

The fresco in the private chapel of the Via dei Bardi had been drying for three days when Luca Bertolini first saw it: a slight dulling at the boundary between two sections of plaster, a surface anomaly invisible to anyone who had not spent four years learning to read painted walls the way a physician reads a pulse. He touched it. A faint white residue came away on his fingertip.

He recogn...