Letterlocking
en.wikipedia.org
en.wikipedia.org
Basically, opening centuries-old letters that are sealed with this technique is usually a destructive process: you might end up rendering some portions of the letter unreadable.
As such, many of these letters have never been opened! They might contain interesting things, but we have no idea.
Some researchers figured out a way to "unfold" X-rays of these intricately-locked letters, to render the letter legible without having to actually open it! It's a pretty cool technique.
The underlying paper is here: https://www.nature.com/articles/s41467-021-21326-w
It seems almost impossible to find these videos or channels intentionally, you can only stumble upon them.
I remember when they figured out how to use spectral analysis to 'see' the solvents that soak into paper from the ink, allowing them to read words that had flaked off due to the ink or the paper delaminating, especially at the edges of paper.
If sealing wax is going to be used anyway, why not just fold the letter and seal with wax like normal?
Not that I've ever done that, as a courier, in a live-roleplay game, ever. Repeatedly ;D
Depending on the security level of the letter, of course, a non-letterlocked letter might be pretty readable even if sealed. A simple letter where the seal authenticates the sender but doesn't protect the contents might simply be folded in three and sealed closed - you can bend and flex such a letter without breaking the seal to read most of it. A more important letter being _protected_ by a seal might be folded into an ersatz envelope and then sealed on the join ... but that's most of the way to a basic letterlock.
So, like all communications there's a tradeoff between complexity and security, and whether you're using the seal merely to authenticate the sender (which was pretty common) or also to protect the contents.
So it's not so much that the seal holds the lock closed, as that the seal obscures the lock to the point that opening non-destructively is much harder.
Well now I'd love to hear more about this!
It was a ~1000-person national level game in a fantasyish setting, and one of the game elements was that you were explorers and colonists in a strange new land, and you could send messages to your supporting factions "back home" to request support or supplies or whatever. Those messages had to be carried by a ship belonging to a trade house, and any supplies that your supporters sent you had to come back by the same method, and all the trade houses were represented by player groups on the field...
So yeah, a group of players would write to their "sponsors", and my group (as one of the trade houses) would open their letter, see what they were requesting and then forward the letter to the game organisers. The organisers would play the part of the remote sponsor, and send a letter back, which we'd open and read to see what orders the players were being given and what support they could expect (then we'd reseal the letter and deliver it). This positioned us super well to sell them exactly what they needed to achieve those goals, at an only slightly elevated "rush" price.
We didn't open every letter, and we didn't scam everyone - we made most of our money legitimately. But it was a fun side-game.
If there are other pieces of paper held in the wax like a 3D matrix of sorts, it gets much more difficult to undo then redo the seal.
Rumor has it that Cardinal Richelieu's men had ways to read wax sealed documents.
How many of these documents have multiple layers of meaning embedded in them? Steganography, euphemisms, inside jokes, shibboleths, what have you. I wonder how many things these letters say that we simply cannot read.