The Louvre had its vast courtyard, ripe for exploitation. You could hardly miss it. The British Museum, too, had its courtyard, a smaller but still impressive affair of two acres or so. But - and this may seem an odd question to ask - where was it, what had happened to it? Perhaps the most extraordinary aspect to the British Museum story is the way its courtyard was completely lost for nearly 150 years, until Foster and his trusted aide Spencer de Grey revealed it in a formidable architectural conjuring trick.
The British Museum has been around for nearly 250 years, but its present building in Bloomsbury, designed by the classicist Sir Robert Smirke, was built between 1823 and 1850, so spanning the period from late Georgian to early Victorian. Then something strange happened. The museum, which also functioned as a national library, immediately ran out of space. In 1857 the glorious Round Reading Room, designed by Smirke's younger brother Sidney, landed in the courtyard like a flying saucer. And a big one: at 140 feet across, its copper-clad dome is 28 feet wider than the dome of St. Paul's cathedral. The rest of the courtyard was filled up with bookstack outbuildings nicknamed "the iron library" because of their metal shelves. Only librarians ever penetrated this twilight zone. The narrow strips of open space left round the edges were left dank and dripping, and the fine Portland stone facades to the lost courtyard went into a long, long, sleep.
Meanwhile, come the 1990s, visitor numbers to the British Museum hit 6.5 million a year, and on busy days in Bloomsbury, you risked being trampled by hordes of tourists picking their way round the Byzantine complexities of the place, baffled and disorientated. The problem was the gigantic cuckoo in the BM's nest - the by now independent British Library. Obviously the Library needed its own home elsewhere, but it took decades to agree where to put it and decades more to build it. But finally the day came. The cuckoo flew away.
Once the final trolleyload of books had trundled off to the new British Library in St. Pancras, the centre of the museum was left empty, eerily quiet but for the scuttle of mice in the deserted bookstacks and the drip of water on mossy, crumbling stones. And then the builders moved in with their hammers and tower cranes and excavators and girders and, in the fullness of time, thousands of triangular sections of glass, each computer-designed to fit in one precise spot in the latticework.
It has been a titanic operation, conducted like keyhole surgery as the museum continued to function all around it. Before a stone could be laid or a girder hoisted aloft, nearly a million cubic feet of rubbish - demolished library stacks and excavated rubble and soil, mainly - had to be lifted out by crane. That's the equivalent of three times the volume of the Museum's enormous Egyptian sculpture gallery, but it is scarcely surprising when you consider that the courtyard is the size of the football pitch at Wembley, and it was full of junk.
Once the space was clear, they had to pour 16,000 tons of concrete for new foundations and the main floor of the courtyard, restore the 65,000 square feet of the courtyard facades - which had got very crumbly in places - and of course rebuild the missing south portico. That used more than 1,000 tons of stone. Slightly the wrong stone, as we now know: more on that later.
