Buried treasure: British Museum's secret treasures

Any public museum is like a great iceberg. You are awestruck by the dimensions of what you can see, and you would be doubly astonished if the waters were to recede all of a sudden – as so often happens in these climatically dark days – and you were able to glimpse the extraordinary dimensions, shapeliness and unexpected beauty of what had remained hidden all these years.

It is a little like that with the British Museum. Founded in 1753, it has an almost overpowering architectural presence. Everything that surrounds it seems laughably pygmyish and toyishly vernacular by comparison. The fanfare of its great neo-classical portico seems to proclaim it to be the great national repository of antiquities because it looks so much like a monumental national antiquity itself. You think you know what it is – and therefore you think you know what it contains – because you can tell from the moment that you peer into it, somewhat awestruck, fumbling with the bicycle clips.

Walk through the doors that lead into the Great Court, and these first assumptions seem to be confirmed. Directly ahead of us we see the hanging banners for The First Emperor exhibition, currently being staged in the Reading Room, and another, just off to the left, for a show devoted to the Emperor Hadrian, which is due to open in July. Sir Frank Francis, in the first general guide to the museum published in 1971, told us in no uncertain terms what he thought the museum was all about. In his introduction, its former director and chief librarian wrote: "The British Museum at Bloomsbury is... a national museum of antiquities." So there we have it. This is the place where we go to see brilliant antiquities – Greek, Roman, Egyptian, for example. The very idea seems to go hand in hand with notions of precious, untouchable, awestruck remoteness.

Is all this true of the British Museum in 2008? Not any more, 40 years on. It is now an institution capable of marvellous surprises, near acrobatic flexibilities. It looks stuffy and brow-beatingly over-serious, but it isn't any more, not really. Many of those treasures to which Sir Frank once referred are now accessible to the public in ways of which he could not even have dreamed. What is more, there are secret treasures inside these hallowed walls that could by no stretch of the imagination be regarded as antiquities, and these too are available for the public's delectation as they never were in the past.

The best way to begin your tour of discovery of the museum's secret treasures is to go to the back of the Great Court, and take the lift to the upper floor. Stepping out, you are now in Room 90, an area devoted to prints and drawings. Immediately ahead, you will see a great drawing of enormous dimensions by Michelangelo. In the room where you are now standing, and in the galleries off to the left, changing exhibitions of prints and drawings are shown. Directly behind Michelangelo you will see a pair of glass-panelled doors, with a brass bell to the right of them. Ring that bell, and you can be admitted to the Prints and Drawings Room for students, where the British Museum's great collections can be studied by anyone who chooses to make an appointment – provided that you are carrying a valid form of ID, and that you know what you want to see. (And in order to find out what there is available to be seen, consult the BM's website, where at least 250,000 catalogued references to prints and drawings from the collections are currently online – and the number is increasing by the day.) There is space for 12 people at a time, and it costs nothing to use the room. Appointments can be booked in advance, though it is not essential to do so.

Before entering that room, I spend time in Room 90, examining a newly hung exhibition of American prints from the first half of the 20th century, from the so-called Ash Can School at the turn of the century to the period of Pollock and Abstract Expressionism, American art's greatest invention. The collection itself, and the fact that it is here and not at Tate Modern, for example, is the first of our great surprises. It proves, at a stroke, that the British Museum is not only interested in the gloriously distant past. Its first curator of modern art was appointed in 1975 – just four years after Sir Frank published his guide – and it has been collecting American prints assiduously throughout the Eighties and Nineties.

"Our perception of what is in the museum," Stephen Coppel, curator of the BM's modern collection and author of the show's catalogue, tells me, "is skewed by the fact that the shows devoted to materials from the ancient civilisations generally take place on the ground floor, and they are therefore the first things that the public sees. There are practical reasons for this – the light, or load-bearing issues, for example. Prints, on the other hand, are light and easily transportable."

Prints tell us new stories about the art of a nation, and ones that are often significantly different from what painting or sculpture has led us to believe. Take American art, for example. The screen-print was born during the Depression era. Once used in advertising, the technique was quickly adopted and exploited by artists. Edward Hopper made great paintings in the 1930s and 1940s. Before that, he had learnt his trade as a very proficient etcher – some of the etchings of the kind we see here were the launch pads for his paintings. They caused his paintings to crystallise, he once said.

But prints are more than mere launch pads. They can also be great works of art in their own right – think of Goya, Rembrandt and Dürer, for example, all viewable in quantity here. There is a wonderful flexibility about prints. They can be disseminated in multiples. They don't have that preciously unique feel about them. They can be messed with to a certain extent – tones adjusted, moods modified – once they're done. But the lines of an etching can't be messed with – a few scratches of the needle, and you're out. That explains the mood of pernickety edginess you often see in great etchings.

This story of American art is utterly different from the narrative Alfred H Barr concocted about 20th-century art when he took over as director of New York's recently founded Museum of Modern Art in 1934, although there are interesting links. The prints in this show, in the range of their social documentation, in their recording of political change, seem more akin to folk song than to opera, more on Woody Guthrie's dusty side of the sidewalk. They feel as if they are of the street and for the street, both casual, and casually embracing, in their democratic reach. Nothing is out of bounds, we feel. In one by George Bellows, we see a Thursday evening dance at the local madhouse in Columbus, Ohio, in which each face seems to be quite distinctively of itself, and in another the sky-yearning splendour of the newest of the new skyscrapers. And over here is a brilliant scene of two boxers slugging it out in the ring, also by Bellows. It is called A Stag at Sharkey's, and he produced it in 1917. This is what a non-gilded life is like, Bellows seems to be saying to us: non-stop pugnacity. Many of these prints were commissioned by the American government as part of the New Deal programme to cover the walls of public institutions – schools, libraries, law courts. It was part of a massive programme of reconstruction, a way of getting artists kick-started. There were no rich patrons here to use religion or scenes from high society as a means of self-aggrandisment.

There are 147 prints in this show. In all, the British Museum owns about two million prints and drawings. Of the quarter of a million that are now online, 120,000 have images attached. A hundred new images are added every week. A little later I see the process in action – a Giulio Romano drawing is face down on the scanner, getting the treatment, accepting the inevitable. And what major 20th century works can we see in the prints room, but not online, I ask Coppel?

"Well, there is Picasso. We have about 100 prints by Picasso, marvellous linocuts, but without permission from the artist's estate, we can't reproduce them. The Picasso estate won't let us show them without payment, and we can't afford to pay what they are asking," he says ruefully.

Having rung the bell to the students' room, I sign the register, an ancient-looking ledger that sits on top of what looks like an old-style desk from a schoolroom. Directly in front of me there is a marvellous 16th-century map of Venice.

Now, it is one thing to see a work of art in an exhibition, and quite another to have someone open a leather-bound folder from a cabinet you might have glimpsed in an etching of 1828 by George Cruikshank – yes, you really can see some of the wall cabinets in this room in that etching by Cruikshank – and have someone hand it over to you. What should it be then? A Constable cloudscape, perhaps? Something by Botticelli? A Henry Moore sketchbook from wartime? One of many great drawings by Dürer?

Or perhaps it should after all be a watercolour by JMW Turner because, once again, it comes as something of a surprise to me that great works by Turner should be here at all when I had somehow imagined that they would be in Tate Britain. It is Giulia Bartrum, curator of German prints and drawings, who hands it to me. I stare down at a watercolour of the Colosseum in Rome. Goats are browsing amid the ruins, which look significantly less ruinous than they did a few weeks ago when I last saw them. What astonishes – and it is something that you could never see from an image that has been scanned on to the web – is the quality, the extraordinary vividness, brightness and walking-on-eggshells delicacy, of the colours. I have never before seen a Turner that actually looks as if it might have been painted yesterday. And why does it look so fresh? The man who bequeathed it to the museum stipulated that it should never be lent out, so you will never have seen it in an exhibition outside these walls. The fact is that it has lived in the dark, in this folder, within a carefully recessed mount so that nothing slides across its face and scratches it, for most of its life.

And this fact relates to something else that I have already noticed in this wonderful, long room with its skylights. All the artworks on the walls are behind blue drapes to protect them from the light. They get opened when you wish to see, and then they get closed again.

And what is the most surprising hidden treasure here? Bartrum smiles, impishly. "Well, one of the most valuable works of all, and you may well not believe this, is a cigarette card of an American baseball player." And how long has the British Museum collected cigarette cards? There was a bequest, she tells me, about a decade ago. They were not quite sure whether or not to accept it, but given that they have an extensive collection of 18th and 19th century ephemera ranging from trade cards to greetings cards, which are of enormous help to historians of any given period, they decided to accept the donation of one million cigarette cards, provided that it arrived properly organised and catalogued.

That happened – and the British Museum subsequently discovered that one of these cards was an image of a famous Pittsburg baseball player called Honus Wagner, who was a ferocious campaigner against the evils of smoking. This meant that very few copies were ever produced – he requested that production be halted because his image was being used to market tobacco – and that the ones which remain in existence – perhaps 50 to 60 – are extraordinarily valuable. A fine example of this tiny, insignificant slip of a card sold for $2.8m in September 2007. The British Museum has found itself owning what is popularly described as the "Mona Lisa of baseball cards".

Something is still puzzling me as I question Bartrum. Of the two million prints and drawings that this institution is said to own, only 250,000 are currently described on the web. How then do they know what is in the collection? And how accurate is that figure? She tells me that all acquisitions were inventoried from 1837 onwards – in fact, she pulls out one of the leather-bound ledgers to prove it, and there you see it, in a book first used in 1837: a list, written in the most exquisite handwriting, chronologically organised, of what was bought, and when, and from where, and at what price – I spot an acquisition of prints, numbers 1-200, "after J Reynolds", acquired on 14 July 1838, with the lot number, bought from Messrs Smith for £159 and 12 shillings. That's a lot of money, I say. Prints were highly valued then, Bartrum tells me.

Now, none of these early ledgers is organised alphabetically, only chronologically, so how do they know exactly what they own? Does the British Museum keep a Methuselah chained up in the basement whose memory reaches back two and a half centuries? The fact is that they don't. And when I question Bartrum further, she thinks that the actual number of prints and drawings may well not be two million, but 1.5 million. Perhaps. A certain amount of slippage there. So how do you know what to ask to see if it is not among the 250,000 on the web? Because the fact is that you can't get in unless you know what you want to see.

Well, in addition, there are various books that have been published. And all the most important things have been catalogued... The inescapable conclusion is that there are many more secret treasures in the Prints and Drawings Room of the British Museum that are yet to be discovered.