They would not have recognised him. Although the hullabaloo surrounding his Angel of the North has made his name famous, Gormley is not a recognisable celebrity. After all, he does not much resemble the rounded, faceless forms used in the sculpture on show now at the RA - although they are all cast from his own naked body.
A walk through Critical Mass - the title of his intentionally unnerving piece - confirms Gormley's mischievous nature, in spite of its sombre rhetoric. It consists of 60 cast-iron figures, some tightly curled up, some prostrate. All have been strewn, or else suspended, around the neo- classical facade of Burlington House in the semblance of the aftermath of some appalling occurrence. The learned antiquarians and geologists who inhabit the buildings around the courtyard are confronted by rusting replicas of Gormley's buttocks whenever they look out of their windows.
One of Gormley's jokes is the slightly different stories he told to encourage them to accept the imposition of his installation. He told the chemists, for example, that Critical Mass was a work about the earth's raw materials, the geologists that it was about historical archaeology and the Linneans that it was about the biological and botanical forces of nature.
Much of his recent work, including the monumental Angel of the North erected just outside Gateshead, has displayed these twin boyish traits - a genuine zeal for making the world a better place, coupled with a secret love of the "wizard wheeze".
Both impulses are essential parts of the man. Last week, along with England's striker, Alan Shearer, he cheerfully autographed the giant Newcastle football shirt that adventurous fans had draped over his Angel. He regards the act of appropriation as a compliment and the black and white shirt will now be auctioned for charity.
The incident was a symptom of Gormley's leap from the avant garde into popular culture. Despite all the righteous criticism about the expense of constructing the Angel, it has been fondly accepted already by the people of the North-east - like a defining architectural feature, a famous bridge for instance. The Angel has also established its creator as the nation's chief advocate for public art.
Gormley had already won the Turner Prize in 1994, but it was the arrival of the 65ft Angel at the verge of the A1 this February that really drew national attention to his work. That apocryphal moment on the train north when he allegedly sketched a human shape, with outstretched wings instead of arms, on the back of an envelope has turned out to be a pivotal moment in his career.
"Antony has been changed by the Angel," says James Bustard, head of visual arts for Northern Arts. "I worked with him on earlier projects in the 1980s as well as on the Angel, and back then he was someone who worked largely in the controlled environment of the art gallery. Now, through the phenomenon of the Angel, he has become an articulate spokesman for public art."
These days Gormley is a regular on the radio championing the idea that art should be able to survive outside the "specialised conditions" of the gallery. Occasionally he is confronted by enemies such as David Lee, the conservative editor of the magazine Arts Review, who has repeatedly satirised the sculptor's work in his pages under the heading "Gormless". Undeterred, Gormley's mission is to replace antiquated representational ideas of art with a new reflectiveness. "I want to cut through right to the synapses!" he declares.
He feels that commerce and advertising have robbed conventional artistic images of their potency. Therefore, while his work still aims to be visually arresting, Gormley avoids the cynical use of shock tactics. "There is a paradox in my attitude," he admits. "After that initial shock or disorientation, if a work is to be of value, it has to be reflective. I am interested in whether art can re-link itself with the built world," he says.
Gormley believes he can detect those public-art projects that have been commissioned to provide no more than a local amenity. "A lot of public art is just a kind of garnish - an attempt to humanise a bleak real-estate development," he says.
"He is hugely energetic and has very high standards," recalls Tony Durcan, the deputy director of libraries and arts for Gateshead Council, who commissioned the Angel. Mr Durcan strongly denies any implication that the phenomenal cost of the piece - pounds 800,000 - along with the delayed completion date were ever subjects of regret. "In 1994, when we first agreed to the project, we had hoped that it might be finished for 1996, The Year of Visual Arts," he says. "But in the end we, like the artist, thought it would be better to get it right and be late."
Critical Mass at the RA was first shown indoors in Vienna in 1995. Since then the 60 figures have been stored in a relative's barn, but Gormley, who is acknowledged by all who know him as an eminently plausible chap, claims the work sprang to mind again immediately when the academy invited him to show in the courtyard space this summer.
The title of the work is a reference to the nuclear age and to his own childhood fear of the bomb. He felt that in Vienna the figures inevitably took on resonances of the Holocaust. Once in place in Piccadilly, however, Gormley says he was concerned that Critical Mass should "infiltrate" the neighbouring architecture and make the viewer reflect on death tolls in Bosnia, East Timor, Rwanda and Northern Ireland. He talks of the piece "infecting the space", thus revealing his ambivalent attitude to the 230- year-old academy.
"There is something complacent about this establishment. I am not sure it is what it wants to be. They are still running on 18th-century principles and it is all part of this myopic self-contentment, this English net-curtain mindset. And yet this same country is the second biggest arms dealer to the world after the US."
Gormley insists that he would like to be more than "just a pain in the arse". The deep roots of an ancient organisation such as the Royal Academy, he suggests, surely have the potential to sustain new growth. Clearly, he would not mind a part in it.
At 48, Gormley is tall, lean and cycles everywhere, retaining an eager, youthful appearance. He lives in Camden with Vicken, his wife of 18 years, and their three children, Ivo, Guy and Paloma, and he crosses London each day to work in his Peckham studio.
Gormley was brought up in Hampstead Garden Suburb in north London. His father was a pharmaceutical manager, his mother a physiotherapist. As the youngest of seven children in a family so devoutly Catholic they used to kneel in the dark together each evening to pray, Gormley has enough self-knowledge to admit to having an element of the attention-seeker in himself: "It is probably because I am the last in the family and so was never able to do anything that had not been done before.
"I was actually thrown out of my prep school, I think it was for talking too much," Gormley recalls. "I am not sure if that was all of it, though. There were other things. It was a terrible place and a terrible time." Following his brothers on to Ampleforth, the Catholic public school in Yorkshire, was a heartfelt relief. He discovered art at school and formed a lifelong friendship with the painter Humphrey Ocean. "Antony could draw beautifully and naturally," says Ocean, "and we were all rather in awe of that. He had a kind of ease when putting down a line."
Ocean identified early Gormley's competitive instinct. "We were sent out for the night with a compass and a map, and, despite my own lack of enthusiasm, his determination meant that we won the prize for the best team. I think he would admit to a competitive and ambitious nature, but then I think that most artists have that, whether they would admit to it or not." Gormley went on to study at the Slade School of Art and had his first solo exhibition at the Whitechapel Gallery in 1981.
Gormley's ability to talk about his art has "not hindered" his progress either, Ocean gently suggests. These are the verbal skills Gormley has effectively deployed to reshape an old work of art for an entirely new venue and they are also the skills that have persuaded the people of the North-east to take a huge metal humanoid superstructure to their hearts.
One day when all that remains of Gormley's work is the catalogues, the newspaper photographs and the rusting hulk of his Angel, there may still be somebody left who remembers that Gormley was for some time the British artist least likely to shrug his shoulders and simply say: "I don't know what it means. What does it say to you?"Reuse content