The sketches were preparatory to the realization of the frescoes of the Seat of Porto in Naples, one of the five civil buildings in which the representatives of the city seats gathered, which were destroyed in the early 19th century. The canvases depict the people who watch frightened by the divine intervention that saves San Gennaro from the first martyrdom. A third sketch, in compliance with the will of the donee, was sold to carry out works of mercy, and today is kept in the State Museum of Stuttgart: it represents the main scene of the saint coming out of the furnace unharmed.