Image copyright Evening Standard

Thirty years ago, Pop-Art's bad boy, Andy Warhol, consigned 300,000 of his everyday possessions to sealed cardboard boxes. Now, as the final boxes are opened for the first time, what do they tell us about the man who turned Campbell's soup tins into art?

It's a stuffy summer evening on Pittsburgh's unfashionable North Shore. Cars jam the freeway, thanks to endless repairs to the crumbling flyovers. This famous old US industrial city is showing its age.

But in the shadow of the highway spaghetti lies another world - the ultra-cool converted warehouse that houses The Warhol. Seven floors devoted to Pittsburgh's most illustrious artist-son, Andy Warhol, master of gaudily-coloured multiple Marilyn Monroes, Elvis Presleys and cans of soup.

Here, at The Warhol, splendour reigns - museum-perfection. Air-conditioned to a nicety, slate-grey floors, gleaming gift-shop, walls papered with multiple images of pink Warholian cows.

And in the luxuriously appointed studio theatre, a buzz of expectation.

The place is full to bursting with fans, acolytes and casual drop-ins. Everyone has paid their $10 (£6) for the privilege of seeing inside the humdrum cardboard box that sits, very alone, on a table on stage.

Because this, as a hand-scrawled label announces, is TC 528. It's the last-but-one of 610 mystery parcels or Time Capsules that Andy Warhol sealed up over the course of the last 13 years of his life. They contain hundreds of thousands of objects, artefacts as the curators know them - a Work of Art, according to Warhol.

Image copyright The Andy Warhol Museum Image caption Andy Warhol, Time Capsule 262, courtesy of The Andy Warhol Museum

Or rubbish, as you and I would doubtless consider them. Because the 608 Time Capsules that have already divulged their contents and are in the process of being catalogued and preserved by The Warhol's staff include everything from flyers from galleries, junk-mail, fan-letters, gallery-invitation cards, unopened letters, solicitations for work, freebie LPs, a lump of concrete, eccentric pornographic assemblages by Warhol's friends and associates, thousands of used postage stamps that the artist tore from envelopes, packets of sweets and - inevitably - unopened Campbell's soup tins, by now rotting and hard to preserve.

Image copyright The Andy Warhol Museum Image caption Andy Warhol, Time Capsule 44, components, courtesy of The Andy Warhol Museum

And then, in the really unsavoury corner, toenail clippings, dead ants, a mummified foot and used condoms.

But there are also strips of photobooth photographs that Warhol took to create his celebrated portraits and some original completed pieces by him that, for whatever reason, he chose not to exhibit.

"It's a selection," says Benjamin Liu, who was personal assistant to Warhol and who's here tonight for the unveiling of TC 528.

"All the times I was helping him packing and sealing them, it's not like he'd just throw stuff in. It was, like, this goes here, this goes there. The Time Capsule is also a source-material for a lot of his work. Even the personal stuff that people sent to him, he might look at it and go: 'Maybe that could be so-and-so'."

On stage tonight at The Warhol, the audience - and Benjamin Liu - are watching carefully as the first objects are lifted, like some priceless golden treasure from an Egyptian tomb, by the blue-latex-gloved hands of chief cataloguer Erin Byrne.

Image caption Erin Byrne (right) and Benjamin Liu (next to her) opening capsule 528

There's something more than faintly absurd in the fact that what she's holding is no gilded helmet or obsidian amulet, but a pile of Christmas wrapping paper, a greetings card from Mick Jagger and Jerry Hall and - to his surprise and the audience's hooted amusement - a metallic belt buckle ("very camp!" whispered one spectator to me) that had been sent to Andy by a certain Benjamin Liu, standing onstage tonight. Sent, that was, of course, as a Warhol fan, long before he had had his 15 minutes of fame as part of the Warhol menage.

"There is an element of surrealist about Warhol. These odd, disparate objects put together have a kind of curious poetry," says British art historian Tim Marlow.

"But he is an archivist and a collector and someone who understands 'product' and is fascinated by it, and these are assemblages of that. And they're still-lives as well."

Image copyright The Andy Warhol Museum Image caption Andy Warhol, greeting card from Time Capsule 21, courtesy of The Andy Warhol Museum

In the 1950s Warhol earned a living as a commercial graphic artist, which may be one reason why he is fascinated with mass-produced images and goods - and why advertising, packaging, and things drawn from the surface of everyday life became the warp and weft of his creative work. As history knows, those cans of soup, the multiple Elvises and Marilyns are his Pop-Art trademark.

"I think with Warhol, broadly, the joke's on us," says Marlow. "The joke's on our culture, but we're complicit in that. We are not being laughed at - we join in the laughter."

Another perspective comes from the late Malcolm McLaren, an ardent fan. According to him, "Andy's adage was, 'I can simply be an artist without doing any art: I am the art.'"

Image copyright The Andy Warhol Museum Image caption Andy Warhol, telephone message from Time Capsule 21, courtesy of The Andy Warhol Museum

But above all, says Marlow, the Time Capsules - started long after Warhol nearly perished when, in 1968, one of the peripheral figures from The Factory took a potshot at him - are essentially about death. "He said, 'Everything I do is concerned with death,' and I think the 'celebratory' portraits (of Marilyn, Elvis etc) deal with death, and so do the Time Capsules."

You can hear an echo of this too when, in 1981, Warhol was asked whether he minded getting older. "Oh no," he said, "I'm old. They call me 'granny'. They say I'm old. They're just mean!" He was 53.

In The Warhol's theatre, the blue-gloved archivists have done their stuff. The contents of TC 528 lie strewn across the examination tables on stage.

Image caption The contents of Time Capsule 528

What does it all amount to? So much rubbish - greeting cards, business cards, an ashtray lifted from a fashionable restaurant, a photograph of Elvis Presley, Christmas wrapping paper and ribbon, a "do-not-disturb" sign from the Beverly Wilshire Hotel. Yet Warhol - ahead of his time in so many ways - selected these objects with care, chose to give them their own 15 minutes of fame. Not for nothing was one of his most celebrated films entitled Trash.

And clearly someone thinks they have inherent value. He or she has just paid the astonishing sum of $30,000 (£19,000) to be the one to open the very last Time Capsule of all.

Image copyright The Andy Warhol Museum Image caption Andy Warhol, Time Capsule 21, exterior, courtesy of The Andy Warhol Museum

Listen to Andy Warhol: Time Regained on Thursday 11 September at 09:00 BST and 21:30 BST on BBC Radio 4 or catch up on iPlayer

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