The Martyrdom of Saints Processus and Martinian

The two men on the rack were, by Roman tradition, Saint Peter's own jailers. Processus and Martinian guarded the apostle in the Mamertine prison below the Capitol, were won over by his preaching and baptised, and were put to death for it under Nero. Their relics were moved into Saint Peter's in the 9th century, and they still lie in a porphyry urn under the central altar of the north transept.

That is the altar this canvas was made for, on a commission that came through Cardinal Francesco Barberini, nephew of Pope Urban VIII, and the museum dates it to 1629. Valentin puts the torture right at the front, the executioners bent close over the bodies with their faces hidden, while angels come down with the martyrs' palms. Damp was the enemy inside the basilica. In the 18th century its altarpieces were replaced, one by one, with mosaic copies that would not decay, and the copy of this one, finished in 1737, still hangs over the altar the painting was made for.