The scene is about as nondescript as you can get. The side of a photocopy shop in Shoreditch, east London. Beneath a billboard bearing an advert for Flora Buttery, a peeling wall is obscured by scaffolding and some black wooden boards.
Passers-by walk on oblivious to the fact that behind the hoarding lies an estimated £500,000-worth of graffiti, which is being painstakingly chainsawed and chiselled off the brickwork by a crack team of seven specialist builders.
Of course, this isn't just any bit of spray-paint; Girl with Red Balloon is by the street artist Banksy. That means it is gold dust for collectors around the world and is why it is at the heart of yet another contentious operation on the streets of Britain that starts with extracting a Banksy from a building and ends with flogging it off at auction (to howls of protests from betrayed locals). As a case in point, Kissing Coppers, a Banksy mural that was removed from the wall of a Brighton pub 10 years ago, sold in the US on Wednesday for £350,000.
But have you ever wondered how anyone actually gets street art off the street?
The Sincura Group, the secretive events-and-concierge company which is responsible for the removal of Girl with Red Balloon, has already "salvaged" one other and is organising the sale of at least eight former walls.
Keen to show the care taken to preserve the artwork and stung by the wave of criticism – and death threats – its staff have received, Sincura gave The Independent exclusive behind-the-scenes access to see the removal.
The entire process lasted a month, cost tens of thousands of pounds and involved masked builders, 24-hour security, a diamond chainsaw and a Picasso expert…
Once the scaffolding and hoarding are erected, the first person to get to work on the masterpiece is a professional art restorer who has previously helped to resurrect chapel ceilings, Constables and Picassos. He spends a week "stabilising" the graffiti, starting by placing a microfilm over the top to protect the surface of the paint (when it is taken to be fully restored, the whole wall will be dipped into a bath and the film will be dissolved). A sheet of perspex is then attached for added protection.
The operation is delayed by a whole week as the saws that the builders had hoped to use prove to be too short. They are forced to import a new chainsaw with a special diamond blade from the United States – at about one metre long, it is the largest on the market.
Acrows are placed on the outside of the building to prop up the wall before any cutting starts. Machines to extract all the dust are also essential, especially as the men are working in a space only about 1m wide.
And as soon as the builders leave, a security guard is posted inside the scaffolding until they return in the morning.
A fortnight of cutting can now begin, using the chainsaw and a circular saw. The brickwork is frustratingly thick – more than half a metre – which makes taking the whole wall away impossible. Embedded electrics and cables further complicate the procedure.
A team of three builders drills pilot holes in each corner of the structure, bricks are removed to allow space for the chainsaw to be manoeuvred in, and then they start cutting, about 22cm into the wall. They slice through the back, removing the artwork in two pieces, one containing the girl and one the balloon.
The chunks of building are perilously heavy. The team has learnt its lesson after removing a previous Banksy, showing two children playing with a "no ball games" sign, from a wall in Tottenham in July last year.
Tony Baxter, the director of the Sincura Group, says: "The problem we had with No Ball Games was we didn't reduce it – it was a 6.5-tonner, we put it on the back of the lorry and we broke the axle."
This time, the men use hand saws to slim the pieces down until they are just one brick thick.
The heavy lifting
The hoarding remains up until the very last second. Only once the lorry has driven up is one board swiftly drilled out to reveal the precious cargo, which has been mounted on wooden pallets.
It takes seven men (one of whom has opted for a Banksy-style disguise of hood and rubber mask) to lift it on to the flatbed truck. About half a ton of bricks have been removed. Banksy has left the building.
The wall is taken to a top-secret, temperature-controlled location in the countryside to be worked on by a team of six restorers.
It would usually take eight months, but they have only just over two before Girl with Red Balloon goes on display in London to be auctioned.
It cost Sincura £28,500 to extract the piece from the wall and restoration will swallow up another £40,000.
The next time the work is permanently displayed, it is likely to be in an oligarch's living room or used as a glass-covered floor in a luxury hotel. What's left in Shoreditch is a gaping hole of exposed brickwork.
Bricks are fitted into the vacated place, the wall is replastered and a clean surface is left behind. It is a desolate sight for any E2 street-art fan. But the blank canvas proves too devilishly inviting for the area's teeming mass of street artists.
In no time, the east wall of 36 Great Eastern Street has a new piece of graffiti: "BANKSY FOR SALE. CALL 020 7 774 1000". Dialling the number gets you the head office of Goldman Sachs.
The Stealing Banksy? exhibition, featuring eight Banksy wall murals, runs from 24-27 April at the ME London hotel; stealingbanksy.com
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