score:9
From Wikipedia:
In their oldest attested form, as used in the ancient Near and Middle East of the 8th century BCE onwards, bullae were hollow ball-like clay envelopes that contained other smaller tokens that identified the quantity and types of goods being recorded. In this form, bullae represent one of the earliest forms of specialization in the ancient world, and likely required skill to create.
Based on this alone, it seems to me that the makers of bullae served a function basically comparable to that of a notary public today. They were a third party attesting to the validity of the token inside. If I don't know you but I do recognize the bulla you have as one made by an authority I trust, and if I can also see that the bulla is unbroken (so that the token inside has not been replaced or tampered with) this greatly increases my trust in the validity of the token inside.
Just as with a notary seal and signature today, forgery would have been a possibility. I'm not sure how common this was historically, but if the system was widely used, it must have had at least some reliability. If the bullae were too easily faked or tampered with, you are right to think they would not be very useful. Or if the makers of the bullae were corrupt and willing to verify false tokens, this would also have been just as problematic and potentially discovered in time.
Upvote:3
It is important to realise that bullae pre-date any sort of writing. Originally, they could not contain enough information to form a full contract.
The tokens inside represented quantities of goods, such as sheep or jars of oil. They were used by Mesopotamian accountants, and one use might well have been just to keep track of temple property. Tokens were probably just held in leather bags originally. The bullae were a later development, with the advantage that to see the number of tokens inside, the bulla had to be broken open, and this would have been obvious. One possible use would have been as a bill of lading. The delivery man would be given a bulla at the start of the journey, and a priest could check that the number of jars of oil arriving at the temple was the same as had been sent. A broken bulla would imply the bill of lading had been tampered with on the journey. A seal would prevent a new bulla being created with different tokens inside and passed off as the original – the new bulla would lack the correct seal. A well-made seal would have carvings that would be too difficult to reproduce.
It then became common to write on the bulla a depiction of the tokens inside, so that the number could be checked at any point on the journey, without invalidating the bulla. It is these depictions of tokens on the outside of the bullae (pictograms) that gradually developed into writing. Eventually, the bullae were flattened out into just a tablet with the tokens shown on its surface, sufficient for most purposes.
Contracts are quite complex things, and it would have taken a long time for writing to get to the point where a contract could be fully specified in writing. However, contracts can be used even in pre-literate societies. You just need one of more witnesses to the contract, who could be called on if necessary to recount the contract. A seal would be sufficient to identify the witnesses, and just a few marks on a tablet might be enough to remind the witness of what the contract was about. For most early societies, a trusted witness was far more reliable than a few marks on a piece of clay.
A good source on bullae and writing is "Ancient Mesopotamia" by Susan Pollack.