It's the world's most ephemeral art, scrubbed from hoardings and the sides of trains as soon as the paint has dried. Good examples are rare - and valuable. No wonder art gurus are starting to take graffiti seriously.

It may be highly speculative, but why not buy a train? That was the advice of Jeffrey Deitch, New York contemporary art guru and erstwhile head of the art advisory service at Citibank, around 1980 when the graffiti scene was exploding in New York. The underground trains, or "subway cars," were covered in colourful writing sprayed from aerosol cans; the artists worked illegally in train yards by night. "There was this big seminar on art collecting," says Deitch, "and someone from the audience asked me what he should collect. I suggested buying a complete subway car by one of the best graffiti artists, putting it in a warehouse and waiting."

Since then the graf scene has gone international - insiders prefer "graf" to graffiti, which they consider derogatory. You can see the "tags" (painted signatures) of "writers" (graffiti artists) on railway sidings as you trundle out of Paddington, Berlin Central, Rome or Melbourne. London Underground estimates removing graffiti costs them pounds 10m a year. The British Transport Police have a special graffiti squad. But it's not yet too late to invest in a train if you have determination and money.

It all began in America in the late 60s with city youths - especially from the Bronx - competing to get their names up on trains and walls as often as possible. Norman Mailer wrote a whole book about it, Watching Your Name Go By. The writers used tags like Craze, Drax, Siege, Kilo. They developed a calligraphic style of bubble letters, then added outlines, then went 3D, giving the letters perspective. In the 1970s the wild style developed, a tangled maze of lettering which only experienced writers can read. Then landscapes, figures, cartoons and abstract patterning were added, in blazing colours, to create "burns" or "big productions", huge walls, densely covered with graffiti.

Today every country has its leading artists, each with highly recognisable and individual styles which are much admired by those in the know. A wide range of patterning has been invented, but "writing" remains fundamental - what it's all about. Curiously, until now calligraphy has never been regarded as a full-blown art form in the west, though it ranks as such in Islamic art, and in China and Japan. Graf is possibly the first major calligraphic art form the western world has produced, though since graffiti artists are commonly regarded as vandals, very little has been written about graffiti as an art form, as opposed to graffiti as crime.

Graffiti in Britain started with the hip-hop culture which was so hugely promoted in the early to mid 1980s. It arrived in Britain from America in a more or less packaged form, with all of its four aspects equally emphasised - rap music, break-dancing, graffiti writing and deejaying. Graffiti was the only illegal aspect - though graf artists were legally employed to design record sleeves, flyers, backdrops for musicians and other associated visual projects.

"Like the jazz movement of the 40s and 50s, it resonated through popular culture," Deitch points out. "When they look back, people will be amazed. The art world didn't open up to these people - didn't show them. I would have done a show at the Whitney. The US city transport police wiped out the scene by buffing [cleaning] cars and arresting people."

The New Yorkers developed a new paint for subway cars which is impervious to aerosol spray - it can just be wiped off. London Transport have copied them. That means that very little genuine graffiti art survives and, at some future date, that very little will become very precious, according to Deitch. He is planning to curate a big show covering the historical perspective of graffiti art - which may well mark the beginning of an investment boom.

The illegal aspect of graf is where its vibrant energy comes from. This is an art form indulged in by writers aged from 10 to 40: a gifted 10- year-old has been heard of in London. They get their kicks by climbing into train yards at night; it takes a lot of planning and watching. Once they've identified a way in, they may go for two or three nights without painting, just watching who is there. Then they spray in the dark, often without using any light. The best artists can produce extraordinarily controlled lettering and design in these circumstances. The public gets an unfair view of it, since the best paintings are quickly cleaned off by the transport police.

"There are writers who only paint legal sites and others who only paint illegally - and a lot who do both," Julio Abajo, publisher of Graphotism, a new London-based magazine, explains. Graphotism is produced on a shoestring by writers for writers and illustrates, in full colour, recent wall and train paintings. "They often only last 24 hours before they're buffed," says Abajo. "Photographs are the only record. People will come from all over the country to see a good wall and take photos. Writers collect photographs as inspiration for their own work - some people have several thousand."

Even with legal sites, where permission to paint walls has been obtained in advance, paintings are regularly gone over (painted over by another artist). It is against the unwritten rules, Abajo tells me, to add a scruffy tag to a "big production" but it is perfectly acceptable to cover it with an equally ambitious painting. So legal paintings disappear as well as illegals, their existence only marked by photographs.

Graffiti writers live for their art, Abajo emphasises, though it may be necessary to take ordinary jobs during the day - many go into graphic design. Graphotism recently published an interview with MODE 2, a British artist who lives in Paris, which explained the problems of keeping solvent: "It's more survival than living, from just graffiti and drawing and like the only thing I think is gonna work for me is the teeshirt thing where I design my own stuff. I just make them myself, borrow money and then pay back the guy I borrowed from once they're sold. Depending on how quick I sell, it either fills up the fridge over a long time or it means I go back to borrowing more money."

Graphotism has also published an interview with PHASE 2, one of the original pioneers of the art form - he had his first gallery show in 1973 - under the title "The legend speaks out". The Bronx boy explains that graffiti writers do not want to sell their work. "It's not just one man's culture and when you diss [disrespect] it, when you front on the history, front on any aspect of the art, you're dissing the masses out there busting their ass that ain't ever going to be in any gallery, and don't aspire to be, because they can do other things beside shrink their stuff down and out in a canvas and sell it."

Simon Sunderland, tag name FISTO, a 23-year-old writer from Sheffield, was given a five-year prison sentence last March after pleading guilty to 14 specimen charges of criminal damage - otherwise known as writing. The sentence shocked the world of graphic artists. The Visual Artists Branch of BECTU (the Broadcasting, Entertainment, Cinematograph and Theatre trade union) has recruited Sunderland as a new member, and is helping to mount a "Free Simon Sunderland" Campaign.

! 'Graphotism' magazine can be contacted at PO Box 352, Wallington, Surrey SM5 2WJ.

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