The BBC cult classic series Antiques Roadshow usually features heartwarming and occasionally maudlin tales of people discovering the value of their family heirlooms. It’s a medal from the war, or a toy belonging to a great-grandmother. They present their objects to professional appraisers who tell them a little about the thing and what it might be worth. 

Last year an episode of Antiques Roadshow aired that was unexpectedly prescient: It contains the answer to the question of why a piece of tokenised cryptoart, in the form of an NFT, is worth anything at all. 

The episode aired in October, and the guest was a Brighton man who brought in a piece of metal cladding. He had prised it off the side of a local swimming pool (or lido, to be precise and British about it). The reason he had done so was because the cladding bore a stencil of a rat using a jackhammer. Above the rodent was the legend: BANKSY. 

The Brighton man explained that he knew it was an original Banksy. The street artist had operated in the seaside town in his early years, and examples of his work litter the streets there. He simply wanted to know what it was worth. After all, it was even signed by the artist.  

The show’s specialist, Rupert Maas, then proceeds to gently upbraid the Brighton lido raider. As it turns out, the artwork in hand is probably worth nothing at all. That’s because it was rejected for a certificate of authenticity from Pest Control, the body that Banksy created to authenticate his works. With the certificate, it would have been worth £20,000 or so. Pest Control doesn’t certify works that were by Banksy’s hand, but removed from the public domain, Maas explains.  “I think the message here is that, if you do see a piece of graffiti art out there, leave it, leave it for the public,” Maas says. 

The Antiques Roadshow episode neatly demonstrates the value of authentication. It matters if an artist links themselves to a thing. The episode is now even linked to from Pest Control’s official questions and answers page. 

But you might say, individual art works are unique and non-fungible. Since each one is slightly different, it might be easier for an artist or appraiser to assign different valuations for each one. What if the artworks were perfect copies of each other? How then to assign a different value to individual pieces?

This is the argument made by the “but NFTs are just JPGs” brigade. The claim is that perfect replicas of images are indistinguishable by the viewer and therefore must be equally valuable (or not valuable at all, as the claim goes). 

Another Banksy piece helps illustrate this scenario. Helpfully, it even involves money. In 2004, merrimakers at the Notting Hill carnival found ten pound notes raining from the sky. Instead of Queen Elizabeth’s visage, however, the wan smile of Princess Diana gazed out at them. The Di-faced Tenners were issued by the Banksy of England, and bore the legend “Trust No One.” The same notes were also showered on people at the Reading Festival and on the platform of the Liverpool Street tube station, a major station close to Banksy’s old stomping grounds of Shoreditch. 

In Banksy’s own telling, he made over 100 million pounds worth of Di-faced Tenners. He claims he stopped circulating them when the notes started to be passed of as genuine pounds sterling at shops. The British Museum even has a Tenner in its collection of notes and coins. 

So what’s a Di-faced tenner worth these days? Lots of them are now floating around, and there’s even an internet sub-culture devoted to identifying counterfeits of the counterfeit art-money. If you picked up a wad of notes at one of those festivals, or at the tube station, you wouldn’t be able to get it authenticated by Pest Control, since it would have the same status as a stencil prised off a building. 

The closest thing are Tenners that come with a certificate of authenticity from Banksy’s former agent, Steve Lazarides. He’s selling Tenners for £2,250 a pop, and he’ll vouch for their provenance. 

To complicate matters, Banksy wasn’t being entirely truthful when he said the Tenners aren’t in circulation any more. In fact, he still issues Tenners, except they form part of the official certificate of authentication issued by Pest Control. Each certificate comes with a serial number written on a Tenner. The Tenner is then torn in half, with one half kept by Pest Control and the other half glued to certificate. Any one wishing to authenticate a piece would need the right fragment of the note to match up with Pest Control’s half. 

It turns out Banksy’s clever scheme is a form of medieval cryptography known as a chirograph. It cryptocurrency terms, it’s a little like public-key encryption, which is how all cryptocurrencies are secured. The public key represents the half kept by the collector, and the private key is the piece stored by Pest Control, as Clinton Freeman, who analyzed the scheme on his blog RepRage noted.

So why is authentication worth anything at all? As Banksy’s art and meta-art shows us, it’s the difference between some graffiti on a piece of metal and tens of thousands of dollars. Except that with a blockchain, you don’t need to be a multi-millionaire pseudonymous street artist maintaining his own authentication service for as long as the market for your works exists. Instead, you can offload all of that record-keeping to a blockchain. Just inscribe your signature on the immutable ledger and it’ll be there for anyone to verify, at any time. 

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