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Clay bullae 
The original bulla was a lump of clay molded around a cord and stamped with a seal. Once the clay has dried, the container (such as a vase or money bag) cannot be violated without visible damage to the bulla, thereby ensuring the contents remain tamper-proof until they reach their destination.
Bullae from antiquity appear in two distinct forms:
- A lump surrounding a dangling cord (as with much later wax bullae and papal bulls made of lead rather than clay)
- A flat, disc-shaped lump pressed against a cord surrounding a folded document (such as papyrus or vellum)
In many cases, fingerprints of the person who made the impression remain visible near the border of the seal in the clay.
Metal bullae 
The term bulla was eventually applied to seals made out of metal. Although the most typical form of bulla was made of lead, it was sometimes made of gold, as the ones affixed to the several Golden Bulls issued by the Byzantine Emperors, Holy Roman Emperors, and various other monarchs in the Middle Ages. A particularly famous type of lead bulla is the one affixed to important documents issued by the Pope, called Papal bulls for the type of seal, where the bulla has an image of Saints Peter and Paul on one side and the name of the issuing Pope on the other.
Gallery of bullae
A bull of Pope Urban VIII. Note the lead seal attached to the cord.