The most distinctive characteristic of a bull was the metal seal (bulla), which was usually made of lead, but on very solemn occasions was made of gold, as those on Byzantine imperial instruments often were (see Golden Bull). On the obverse it depicted, originally somewhat crudely, the early Fathers of the Church of Rome, the Apostles Saint Peter and Saint Paul, identified by the letters Sanctus PAulus and Sanctus PEtrus (thus, SPA •SPE or SPASPE). St. Paul, on the left, was shown with flowing hair and a long pointed beard composed of curved lines, while St. Peter, on the right, was shown with curly hair and a shorter beard made of dome-shaped globetti (beads in relief). Each head was surrounded by a circle of globetti, and the rim of the seal was surrounded by an additional ring of such beads, while the heads themselves were separated by a depiction of a cross. On the reverse was the name of the issuing pope in the nominative Latin form, with the letters "PP", for Pastor Pastorum ("Shepherd of Shepherds"). This disc was then attached to the document either by cords of hemp, in the case of letters of justice and executory letters, or by red and yellow silk, in the case of letters of grace, that was looped through slits in the vellum of the document. The term "bulla" derives from the Latin "bullire" (""to boil""), and alludes to the fact that, whether of wax, lead, or gold, the material making the seal had to be melted to soften it for impression.
In 1535, the Florentine engraver Benvenuto Cellini was paid 50 scudi to recreate the metal matrix which would be used to impress the lead bullae of Pope Paul III. Cellini retained definitive iconographic items like the faces of the two Apostles, but he carved them with a much greater attention to detail and artistic sensibility than had previously been in evidence. On the reverse of the seal he added several fleurs-de-lis, a heraldic device of the Farnese family, from which Pope Paul III descended.
Since the late 18th century, the lead bulla has been replaced with a red ink stamp of Saints Peter and Paul with the reigning pope's name encircling the picture, though very formal letters, e. g. the bull of Pope John XXIII convoking the Second Vatican Council, still receive the leaden seal.
Original papal bulls exist in quantity only after the 11th century onward, when the transition from fragile papyrus to the more durable parchment was made. None survives in entirety from before 819. Some original lead bullae, however, still survive from as early as the 6th century.