The blockbuster show at the Tate isn't just about paintings. It's about sandwiches, T-shirts, jigsaws...
The staff of Sotheby's and Christies must be kicking themselves. At the upmarket high-concept chain of London snack bars Pret a Manger, the latest part of Paul Cezanne's legacy goes on sale this week: a sandwich, price pounds 2.20.

The painter would have recognised its Provencal ingredients, mozzarella, avocado, pine nuts, basil and Mediterranean bread with sun-dried tomatoes. His "bathers" might have picnicked on something similar. What they couldn't have known is that they were eating the prototype for a product called the "Cezannewich". The ghost of Cezanne - not a man renowned for his sense of humour - must be looking down from his beloved Mont Sainte-Victoire with his head in his hands.

The Cezannewich is the brainchild of the Tate Gallery in London whose major retrospective of the Impressionist's paintings opened on Thursday. Tickets are selling at 2,000 a day; over 300,000 visitors are expected. For the Tate, a sandwich with a memorably silly name makes a good marketing gimmick. For Pret a Manger it's a godsend: their last marketing tie-in was with Pret a Porter, Robert Altman's terminally bad film about the fashion industry. But the Cezannewich is also a symbol of how important the sale of merchandise has become to the nation's great galleries and museums, and never more so than during a blockbuster exhibition.

The day when a museum shop consisted of postcards, a wall of academic tomes and two dozen flaky leather bookmarks has gone. For this show, the Tate's sales space has been enlarged to house Cezanne T-shirts, tea-towels, jigsaws, pencils and apple-dappled silk head-scarves. There's a cookbook ("When Cezanne was once asked to name his favourite dish his simple reply was `potatoes in olive oil'," the chef Alain Ducasse reveals) plus a selection of elegant jugs and jars, as advertised in the artist's still-lifes. Chateau Calissane, "Cuvee Cezanne at the Tate" wine can be bought at the restaurant and from selected off-licences.

The Tate's last blockbuster, Picasso, in 1994, shows just how much money there is to be made from our desire to possess as little bit of an exhibition, to take a slice home with us. 272,000 people saw the Picasso show and, among other objects, the gallery sold 20,000 catalogues, 32,500 shorter guidebooks, 10,500 posters, 4,000 T-shirts, 1,400 commemorative phonecards and a staggering 200,000 postcards.

Which is bad news for the postman who has to deliver them, but good news for the Tate's accountants. Picasso-related goods generated over pounds 200,000 in profits. Weighed against the Government's annual grant to the Tate - almost pounds 19m this year - that sort of figure is a drop in the ocean. But because it's not earmarked for salaries or building works, because it is "discretionary income", cash made by the gallery from retailing is particularly valuable.

For other institutions it may be vital. With pounds 4m overheads and no government funding, the Royal Academy would be hard-pressed to survive without the pounds 500,000-plus it makes each year from its restaurant, retail and picture framing services. "This is not a bonus," the RA's Secretary, Piers Rogers, says: "It is an essential part of our income."

"After all," says David Breuer, managing director of the RA's commercial offshoot, Royal Academy Enterprises, "it makes sense for everyone. All manufacturers are looking for designs. And the museums and galleries are the repositories of design, of what you might call the pictorial wealth of the nation."

Merchandising Cezanne, a popular dead artist, whose works are out of copyright, sounds like a licence to print money. But for Celia Clear, head of Tate publishing and retail, finding the right Cezanne merchandise involves an element of tightrope-walking. On the one hand, the gallery needs to make money. On the other, its has an image - as national treasure house, public service, serious arts provider - to uphold. Getting it right is, you might say, the difference between a Cezannewich and a McDonald's Chicken McCezannewich.

Clear maintains that British gallery-goers still "don't want the cuddly bunny, they want the book", pointing to the pounds 25,000-worth of books sold by the Tate shop in the first week of January. The Cezanne doll, on sale when the exhibition moves on to Philadelphia, won't be making an appearance in London. And as for the idea of, say, a Cezanne teddy bear: "It simply wouldn't be right for us. You know the kind of bear I mean. The type that says `Kiss me Kate' if you buy it on Southend Pier and `Brighton is best' if you buy it at Brighton. We wouldn't make enough extra money to make it worth having the bear taking up space and diluting the feeling of what the exhibition's about.

"I'd like people to leave our shop feeling it's a cheerful reflection of what they've just seen, not something completely unconnected. We've nothing that either isn't specially made for us or doesn't directly make use of Cezanne's art. When you come to see one of the greatest shows of the century, you don't want to come out to trivia."

There is, potentially, a lot of trivia about. The moment it becomes public knowledge that a gallery is going to do a major show, the phone starts ringing with ideas for spin-offs. Some companies even swing into production, gambling that they will be able to sell to the galleries. If they do, that's maybe 20 per cent of their sales taken care of in an instant. If they don't, their Cezanne (or Monet or pre-Raphaelite or Renoir) greetings cards and address books still look pretty in the high street, and the sales may anyway ride the wave of interest a big show creates.

It's not uncommon for salespeople to turn up on spec in the Tate shop, having lugged suitcases of samples from Pimlico Tube. "I get sent lots of things," says Rosemary Bennett, the Tate shop's buyer. "Funny chocolates with strange designs. Lots of cheap souvenirs. Tacky key rings. Some very nasty and cheap T-shirts. And then there are some images that just don't translate very well into products - for example, I didn't go for a Cezanne tie, and I've turned down tablemats recently. We've always got to be very careful that we never diminish the integrity of the image."

In this respect, it's fair to say, the Tate may be erring on the side of caution. "You do get Americans coming into the shop," Bennett says, "and they ask, `Where's the gift shop?' You have to explain to them that this is the gift shop."

Cezanne finishes in April, but the agents of merchandise never sleep. There's always the Tate's all-year-round collection to be considered - a Roy Lichtenstein ruler here ("Wham!"), a Stanley Spencer fridge magnet there - the day-to-day "branding" of the gallery's products. And that's not to mention the next big show, still a whole two years away.

"As soon as Cezanne is over," Clear says, "we shall sit down and see what's gone right and wrong with it, and apply that to Bonnard. I think the appeal will be very similar." More pottery, then? "Well, we'll be asking ourselves things like `Does Bonnard speak jugs or does he speak ...bathroom tiles?" She laughs mischievously. And, for just a moment, I can imagine Celia Clear biting into a Chicken McCezannewich.