The artist's signature, preceded by the letter "F", meaning "fait par" (fecit), and the date the painting was made, are inscribed on the skull in brown paint. The lettering is arranged to match the curved shape of the skull. In this way, Ribera seeks to integrate the inscription into the painting to signify his presence, as the person who "made" the painting. It should be noted, however, that the signature does not rule out the possibility that the work was done in the studio.