There really isn’t a way to measure this, that I can see. How would you decide? The original painting used as artwork for the cover of
America holds the highest recorded price for the sale of a piece of cartoon artwork anywhere in the world, but is that important to you if you are seeking to complete your collection of Tintin-themed biscuit tins or chocolate wrappers?
Any piece of artwork by Hergé would be unique, so desirable - but do you count a picture of Tintin to be more sought after - and thus “rarer†than, say, a picture of Mrs. Finch? By all estimates there must be more images of Tintin than Mrs. Finch by Hergé by some considerable marging, but I’d guess that the more “common†Tintin would be more wanted. But what if it was a never-before seen by the public image of Mrs. Finch, on a page which wasn’t published, which showed a piece of story not known about - would that be a rarity of greater standing, even if it didn’t show Tintin? It would be up to the individual.
I wouldn’t knock back a set of signed French first editions if someone wanted to give them to me, but I wouldn’t go out of my way to buy them, even if I had the money. I’d be happy to own even one of Hergé’s pieces of commercial art that he did for advertisements over a signed first edition.
Hergé seems to have been an inveterate letter writer (the largest part of the archives is the correspondence - more than 50,000 items), so again while many, if not all, will be unique (I imagine he may have had a standard or form letter for replies to fans), they may not be scarce. Whether people who have them tend to sell them, who could say, so they might not change hands - so there might be a lot of them, but comparatively few on the market.
You would also have to look at what constituted the rarity - some first editions had vanishingly small runs, so to find one at all, unsigned, would probably make it more collectible than a signed
714 or
Picaros. But there was some discussion a while back about an edition of
Tintin in Tibet in Tibetan*, which was suppressed and destroyed, of which only one copy seemed to be still in existence. It has no real connection to Hergé (he wrote it of course, but never saw this copy), but it would still be a rare book, possibly the rarest, but would many people actually want it?
*
Update: I beg your pardon - it wasn’t in Tibetan, it was
in Greek, and a second copy may have emerged… Rarity possibly halved at a stroke!
