I just saw this post. Were there ways that the recipient could actually authenticate the sender based simply on a seal that looks like it could be copied?
You’re asking two questions here: 1) whether seals could be forged and 2) whether the recipients could tell if a seal was “the real thing” or not. My specialization is in medieval Britain, and my answers will come from that perspective, so I hope that time frame can get to what you're looking for.
In short, yes, seals could be forged and often were, but whether a recipient could verify them is a little more complicated, and it takes a little background on how seal use developed to answer fully. In northern Europe from the 11th to 13th centuries especially, the nobility started using seals much more than ever, and over this period, we see seal designs develop into a complex system of self-representation, in a very literal sense. This is a mostly non- or partially-literate society, and that’s kind of a hard concept for us to wrap our minds around, since we’re living in a society that’s completely and entirely bound up in reading and writing. If we need to communicate in any way, writing is either the go-to or the next best thing, and we’re taught that from an extremely early age. For medieval Europeans in the early/high middle ages, this isn’t the case. It isn’t that they just didn’t learn literacy, it’s that literacy as we know it did not exist outside mostly religious or high academic contexts. To a medieval noble, writing looked like a clunky, indirect middleman—where’s the authority? Where’s the immediacy? Where’s the personal presence?
This is where seals come in. Seals were how medieval people represented themselves in the most physical, literal way possible without actually being with whatever document the seal was attached to. Some of the earliest seals (I want to say 10th-11th centuries but I’m not completely sure) include hairs or bitemarks belonging to the sealer in the wax as a way of making themselves even more physically present in the seal. Until the later 12th century, only royalty and the pope used seals. The first time we see documented evidence of an attempt to nail down authority and authenticity was from Pope Alexander III in the late 12th century; he came up with the idea of a sigiullum authenticum while trying to regulate the emerging non-royal seals being used by lesser nobility. A seal matrix (the cast that you press into the wax) would include an image and be surrounded by authoritative Latin text that generally said something like “This is the true seal of Such and Such, Countess of…)” The images could be very sophisticated, but that’s another question—all this is to say that a seal was a person’s authority in physical form. The medieval scholar Peter Abelard skirted around the idea that a seal was in fact an incarnation of the person. So, if you are a medieval count drafting a charter, you have a cleric write down everything you dictate, theoretically in a room full of important witnesses, and then you and your witnesses attach your seals by little strips of parchment to the bottom of the charter. That charter would then be as good as if you were standing in front of whomever reads it and saying the words with your own mouth.
So when we talk about medieval seal forgery, we’re talking about identity theft of the highest degree. Seal matrixes and signet rings were extremely important to keep out of the wrong hands. For perspective, one 12th-century source notes that when Ranulf Flambard, King William Rufus’s Keeper of the Seal, was kidnapped while on a ship in the Thames, “he took the ring which he wore on his finger, and his notary took the King’s seal, and they threw them into the middle of the river, so that their enemies should not be able to circulate deceitfully forged writs throughout England where the ring and the seal were known.” Of course, you could make a convincing forgery if you just have a few good wax impressions of the seals, like Westminster Abbey did in the 11th century when they produced a few forged charters with Edward the Confessor’s seal. Forgeries of pre-Norman Conquest seals were especially useful because, well, there were no pre-Norman Conquest rulers left to prove them wrong.
Verifying a seal was another matter, and here, it seems to have come down to physical examination. The papacy kept working to address forgery and authenticity, and in 1199, Pope Innocent III examined a document sealed by Emperor Henry of the Holy Roman Empire, but the seal was damaged, so he couldn’t tell if it the legend said Ludovicus or Henricus. He concluded that the damage invalidated the document. Something similar happened with King Louis IX of France in the late 13th century—he had to invalidate one of his own charters because the seal was so damaged that only the legs remained and he couldn’t be sure it was his own, until another charter with the exact same impression was brought up to compare it to—an exact match, according to the source.
Again, this overview only covers a pretty narrow time frame, even if it was a pretty important one for northern Europe. After about 1250, seals become a lot more standardized and common, to the point that just about anyone could or did have a seal, and I know a lot less about them after that point--I'd love some backup from anyone with expertise further down the road!
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