A characteristic head study by Ubaldo, possibly executed in connection with his celebrated altarpiece of 1759 in the church of Santa Maria Maggiore at Castel San Pietro Terme (D. Biagi Maino, Ubaldo Gandolfi, 1990, p.249, no.7, and fig.40), although in the latter the saint wears different robes, his left arm is extended and the key lies at his feet. The altarpiece, which was the artist's first public success, displays throughout the influence of Guido Reni, also strikingly evident in the present picture