Public art has been one of the great cultural success stories of the past 15 years. From Antony Gormley's imposing Angel of the North near Gateshead in north-east England to Anish Kapoor's reflective orb of stainless steel in Chicago, people flock to see these giant sculptures in communal settings. More than simple tourist attractions, the very best examples become iconic shorthand for the places in which they reside.
But unshackling art from traditional gallery spaces does come at a price. It was revealed last week that a towering new monument by the Turner Prize-winning artist Mark Wallinger had been delayed - and it wasn't just an escalation in the construction budget that was to blame. How much it might cost to keep it graffiti-free was also a major concern.
Commissioned by - and known as - the Ebbsfleet Landmark Project, Wallinger's huge white horse was designed as a welcome to the UK for Eurostar rail passengers zooming through the Kent countryside. But it was evidence of how much Gormley's piece has entered the public consciousness that most people refer to it as the "Angel of the South".
Perhaps such reverence might have prevented people from scrawling all over it in marker pen and spray paint - but the authorities were not prepared to take that chance, and included a budget for removing graffiti over 80 years as part of a new set of costings that saw the proposed bill rocket from £2 million (Dh11.8m) to £12m.
It might sound ridiculous, but the authorities were right to be cautious. Not least because police had to be called when enterprising Newcastle United fans memorably took ownership of the Angel of the North in 1998... by wrapping the sculpture in an enormous shirt of their favourite football club. Gormley, though, was said to be touched by the gesture - and there is a school of thought that artists working in the public realm should be prepared for that same public to interact with and perhaps change the work.
Although one imagines they might draw the line at painting over it completely, which someone unwittingly did to a Banksy work last week. The Bristol building on which the Oscar-nominated graffiti artist had painted The Gorilla in a Pink Mask has recently been turned into a Muslim cultural centre. Its owner, Saeed Ahmed, whitewashed the wall as part of the renovations, blissfully ignorant of its artistic significance. "I thought it was worthless. I didn't know it was valuable. That's why I painted over it. I really am sorry if people are upset," he told the Bristol Evening Post.
Some people were upset, but it's not as if this is the first time Banksy's work has been obliterated. After any initial shock and anger have subsided, most people tend to see his graffiti as a more transient form in any case - to many, it's simple vandalism. Banksy himself refuses to be riled when eager council workmen not conversant with contemporary art paint over his murals.
Such are the dangers faced by public art, and one suspects Banksy sees the funny side. The venerable sculptor and painter Maggi Hambling is less amused, however, every time somebody scrawls on her beautiful, burnished-steel, scallop-shaped sculpture on Aldeburgh beach, on the English east coast. It was meant as a tribute to the composer Benjamin Britten, but its detractors regularly inscribe "reviews" such as "Move this tin can" and "rubbish" on its shells.
"If I didn't like a work of art, I would not look at it. The last thing I would do would be to vandalise it," said Hambling in the local paper after yet another less-than-generous public review. And the saga took a rather ironic turn when someone stuck a note to the piece with the inscription: "People who desecrate works of art are ignorant bigots." Imagine doing that to the Mona Lisa.
So, short of employing a whole host of outdoor gallery assistants to stand guard over famous works, or asking artists to use only anti-vandal paint, there's actually very little that can be done to protect the pieces of art seen by millions of people every day. Except, it seems, to make them really, really good. Another Gormley sculpture series is currently in situ at the top of an Austrian mountain range. The iron figures stand, naked, looking across the peaks and valleys, and their effect has been such that moves are afoot to make them a permanent addition to the scenery.
The only "graffiti"? Some have had coats put on them by concerned mountain trekkers, eager to look after these beautifully thoughtful representations of humanity. Now that's a seal of approval.