The Papal bull and the Papal seal.

Papal Bull ofPope BENEDICT XV(1914-22)
Papal seal of Pope GREGORIUS XIII(1572-85)
APapal bull is a special kind of patent or charter issued by a Pope and named for the seal (bulla) that was appended to the end to authenticate it.Papal bulls were originally issued by the Pope for many kinds of gemunication of a public nature, but after the fifteenth century, only for the most formal or solemn of occasions. Modern scholars have retroactively used the term "bull" to describe any elaborate papal document issued in the form of a decree or privilege (solemn or simple), and to some less elaborate ones issued in the form of a letter. Popularly, the name is used for any papal document that contains a metal seal.During Middle Age the lead bullae were used to seal and to authenticate important documents of historic significance issued from the Pope's Office in Rome, under the Pope's authority. For the most solemn bulls, the pope would sign the document himself, in which case he used the formula Ego N. Catholicae Ecclesiae Episcopus (I, N., Bishop of the Catholic Church). Following the signature in this case would be an elaborate monogram, the signatures of any witnesses, and then the seal. Nowadays, a member of the Roma Curia signs the document on behalf of the Pope, usually the Cardinal Secretary of State, and thus the monogram is omitted.The Papal seal.The most distinctive characteristic of a bull was the metal seal, which was usually made of lead, but on very solemn occasions was made of gold (as Byzantine imperial deeds often were). It depicted the founders of the Church of Rome, the Apostoles Peter and Paul, identified by the letters Sanctus PAulus and Sanctus PEtrus. The name of the issuing pope is on the reverse side. example : EUGENIUS PP IIII or SIXTUS PP IIII or LEO PAPA X.This was then attached to the document either by cords of hemp (in the case of letters of justice, and executory) or by red and yellow silk (in the case of letters of grace) that was looped through slits in the vellum of the document. Bulla is the name of this seal, which to ancient observers looked like a bubble floating on water: Latin bullire, "to boil".Since the late eighteenth century, the lead seal 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.
Actually it's very hard to find an authentic Papal lead seal, their rarity and uniquity depends on the age of the seal and also on how long was the period of Papacy. It means that, of course, a lead seal of a Pope that reigned for a few years an who issued a few sealed bullae, is more precious than a lead seal of Pope with a long Papal life.Usually it is difficult to find a single lead seal, better if it is part of a collection. It means that it belongs to someone that collected and papal seals and values their authenticity.An original Papal lead seal had athread inside. In fact originally it was attached on two points of the vellum manuscript called Papal bull or Papal bulla. The Papal bull is an official Papal document hand written on vellum with its Papal seal still attached.Today it is very rare to find a Papal bull still intact, anyway a Papal bull (vellum) also without seal is considered a rare and old manuscript of religious importance.
Usually these threads were made up of silk or hemp; different threads often represent a different legal content. If the litterae cum filo serico are usually documents which grant a favour (litterae de gratia), the litterae cum filo canapis could be used for giving orders or to take legal decisions (litterae de iustitia).
However, it is useful to remember that this kind of distinction may not be always found and it tends to diminish as the centuries go by. The silk thread is made up of a bundle of single threads, usually yellow and red. Starting from the XV Century, we can also find yellow-green or neutral threads.

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