16 years - the Warhol Foundation gave the time capsules to the museum when it was founded, in the early 1990s. Yet, of the 610 capsules, only 19 have been fully catalogued; 91 have been inventoried; and 40 or so have been peeked into, with notes made of their more interesting contents. It would be easy to label the stuff "junk", but they're really archives - and the value of archives increases as years pass and we become further removed from their creation. Besides, many of the items are far from mundane: they're a chronicle of mid-20th-century high life. For instance, Warhol's invitations to the White House and to Truman Capote's Black and White Ball, the "party of the century"; his ticket to Maria Callas's debut at the Metropolitan Opera; or his pass to Xenon, the scandalous New York disco of the 1970s.

It's always been difficult keeping pace with Warhol. Collecting became a daily preoccupation for him (even when he went on trips, he brought home not just typical souvenirs but also, say, the porcelain, cutlery and menus he used on an Air France Concorde). He would collect far quicker than I can catalogue. You may wonder where he kept all this stuff. His home, a four-storey townhouse on the Upper East Side, was chock-full - his kitchen and bedroom were the only rooms you could walk through. And whatever couldn't be fitted in the house would be moved to his studio-warehouse, near the Empire State Building. Warhol not only had a hoarding tendency but also the luxury of enough space to keep the stuff. It seems that he came to think of his capsules as art. Several times he considered exhibiting them. At one point, he even planned to sell his collection. His idea was that each box should be sold, unseen, for the same price: purchasing them would be a game of chance, in which there would be winners and losers, depending on which items you ended up with.

Many of the items are very valuable: they aren't all airline cutlery and unpaid bills. He had world-class collections of art deco, Native American objects and American folk art. Indeed, when he became interested in collecting the so-called "higher end" objects, such as gemstones, he sought experts' advice to guide his acquisitions. His collection was a very serious matter to him. Much of it, unsurprisingly for someone so fascinated by the cult of celebrity, relates to the famous people he knew. There are photographs of Warhol with Truman Capote and Pope John Paul II; there are cards from Elizabeth Taylor and Mick Jagger; as well as autographed photos of Hollywood stars from the Golden Age which he began collecting at the age of 12. And for all those enthralled by his relationship with Edie Sedgwick, there's a get-well card to him on the day after he was shot (June 3, 1968), sent by her from the New York State Psychiatric Institute. It's extremely affectionate - with a charming Beatrix Potter watercolour on the front - but written in an unsteady hand. It also sheds light on Sedgwick's tragic fate: she had left Warhol's circle in early 1966, and by 1968 was frequently hospitalised with drug addiction and mental illness. Within three years she'd be dead.

Also interesting are the books and records autographed by Ireene Wicker Hammer, aka the Singing Lady, who told children's stories on American radio when Warhol was young. His diaries reflect the boyish enthusiasm and surprising excitement he felt when, as an A-list celebrity himself, he got to meet her. This sort of item offers a fascinating insight into Warhol the person. It's clear from the capsules that, although he was a superstar who lived his life so publicly, there was a lot about him that people really didn't know. Which brings us back to his foot fetishism. Warhol's archives reveal how fond he was of feet and shoes. He made his name with whimsical ink drawings of shoes, in New York's Sunday newspapers, and this collection includes many examples of his own footwear (from penny-loafers and worn-through cowboy boots, to paint-spattered Ferragamos and Cuban-heeled "Beatle boots"), plus the shoes of other celebrities, notably Clark Gable's. It's just me and a colleague doing the cataloguing. It would take two people, working full-time, nine years to get through every item in the unopened time capsules. So who knows what weird, wonderful gems still await discovery?

Telegraph, London