Since the 12th century, papal bulls have carried a leaden seal with the heads of the Apostles Saint Peter and Saint Paul on one side and the pope's name on the other. Papal bulls were originally issued by the pope for many kinds of communication of a public nature, but by the 13th century, papal bulls were only used for the most formal or solemn of occasions. Papyrus seems to have been used almost uniformly as the material for these documents until the early years of the eleventh century, after which it was rapidly superseded by a rough kind of parchment.