History renewed

The obvious way to look in Berlin's Potsdamer Platz is up. Up at the dazzling steel and glass skyscrapers that symbolise the explosion of western economic bravura. Look down, however, and the eye might be drawn by a line of cobbled stones that show where the Berlin Wall carved its way across the square until 20 years ago this November. Everywhere there are reminders of a city brutally divided and dramatically reborn. The Brandenburg Gate, once half-hidden from the West by 10 feet of concrete, is now thronged by tourists and street performers; just across Tiergarten park is the Reichstag, the seat of government, triumphantly rebuilt in 1999.

It is possible to hire a Trabant, the comedy car that symbolised the dysfunctional tragedy of the East, and take a Trabi Safari along the grand boulevard of Unter den Linden to Museum Island. There, by the River Spree, are some of Berlin's great institutions: the Bode Museum; the Pergamon, where the reconstructed Ishtar Gate, taken from Babylon, stands; the National Gallery and, behind protective boards, the Neues Museum.

The building was left a ruin after it was bombed twice during the Second World War. Trees grew out of the rubble of the north-west wing, the grand entrance hall with its monumental stairway was obliterated and nearly every room was ruined by bombing or neglect. Now after five years of labour, the restoration of the building is scheduled to be completed at the cost of some $255 million (Dh937m) in time for its opening on October 16 - almost 66 years after the first bombs fell.

For Professor Matthias Wemhoff, the first director of the museum, this is both a cultural symbol and an affirmation of a city reunited. "In a way it was the end of the time that existed after the war for us," he says. "It was symbolic of the East and West coming together because we were able to bring all the museums together. For many years, there were two Egypt museums and two archaeological museums so this is a very important moment for us.

"It takes the two collections way back to their roots, to the beginning of the museum when there was the collection of European and Egyptian art. Now we can show all our archaeological collections together instead of being divided into a lot of museums and other places. "We have a new quality of museums now in Berlin. I think, for example, that the State Museum, which includes all the museums on the island, is now something really like a national museum, not just because we have our collections back but our written resources are together and it means we can work with objects and present our treasures. We can work with the British Museum and with the Louvre in Paris, arrange exchanges and plan joint exhibitions."

Already, the mighty marble statue to Helios has been lowered into its new home in the north-west wing, while a newly rebuilt domed hall in the south-east of the building awaits the arrival of Nefertiti, the fabled queen of Egypt in about 1300 BC. If Her Majesty, in all her inscrutable elegance, takes the limelight, it will be the English architect David Chipperfield who will pick up the plaudits. Chipperfield, who won the international competition for the rebuilding of the museum in 1997, has eschewed flashy statements. Instead, he has worked as much as possible within the tradition of Friedrich Stüler, who built the museum between 1843 and 1855, and within the framework of the original rooms and galleries.

"It does not look very great from the outside," says Wemhoff, "but it is a great building inside. With two great courts in the middle and a lot of rooms, it is a wonderful building. Because every room has a great atmosphere it will be as if the visitor wakes up every time they move from room to room. It is a wonderful feeling. "I like the architecture of David Chipperfield very much. It is a kind of new architecture which has a great empathy for the old building because the new work is not so dominant as is so often the case.

"Some of the rooms you have a feeling that not a lot has been done, that there is no great replacement of what has survived. But in other parts, you find a room which was destroyed and you see the ruins. Some of the galleries have the atmosphere of ruins. For example, where the stucco was washed off the columns only the stone has been left on the bases, brickwork has been left exposed and traces of fire damage preserved."

What Chipperfield has achieved is the blending of the old and the new by using recycled bricks, some of which are covered in slurry to make them blend. Many of the frescoes from the elaborately decorated galleries have been damaged and bricks have been colour-washed to soften the contrast. Walls have been kept rough, the rendering not replaced, and kept "untidy", with the statuary left damaged. Many in the Berlin cultural elite, particularly those opposed to the perpetuation of "ruin culture", which has seen, for example, the preservation of a few sections of the Wall and the stark shell of Kaiser-Wilhelm-Gedächtniskirche in the old West, chose the iconoclastic architect Frank Gehry to carry out the design. He had just completed the Guggenheim Museum in Bilbao and came up with a concept as spectacular and contrasting to the original as one might expect. Chipperfield, who also built the uncompromising Anchorage Museum in Alaska and has a new design for 6 Burlington Gardens, an extension to the Royal Academy in London, argued that the strengths of the building should be accepted and that as little as possible should be reconstructed.

At the time of the competition, he said: "The analogy that I always use is that of a broken Greek vase. You restore it by bedding the fragments in white plaster so that you can discover the figure and the form and see what supports it and gives it substance - not to reinterpret it or, worse, attempt to replicate it." In an interview with the art historian Wolfgang Wolters last year, which is soon to be published in book form, he said: "The building had such power and was so impressive, and we were aware that whatever we did should celebrate this... It seemed to me that the ruin had established its own authenticity. That authenticity was one created by accident and therefore it contained many other stories inside of it - the main story being the intentions of the original architect. So I think that in terms of authenticity and integrity, there were two original motivations. The first was that we should try not to lose, through the repair of the building, the 'undressed' condition, which imbued it with a power that 'completing' the building would have lost. Secondly, the remaining fragments were in danger of becoming scenographic and somehow totemic if they were not put back into some meaningful context, because the alternative would be a strange collage of broken and unbroken pieces. Therefore I think that this desire to establish the meaning of the original building was very important."

Though Chipperfield used as many scraps and remnants of the original as he could - thousands of pieces, including bullet holes - and incorporated them into the building, he was not afraid of the new. The Egyptian gallery once elaborately decorated with 19th-century frescoes and columns with scenes of Egyptian life from the era of Nefertiti's father, the pharaoh Akhenaten, who died in about 1336 BC, now has a glass roof held aloft by stark columns that contrast with the exposed brick of the ruined walls. Yet even here murals that were intact have been preserved. The destroyed north-west wing consists of three long galleries in uncompromising concrete.

"There are some moments with all of these junctions coming together which are fabulous," he says. "You sometimes get a new concrete wall, you get a piece of new brickwork filling in for the old brickwork, and then you get a bit of restored cornice and all of a sudden you think: 'Wow, that is something that no one else would have ever done.'" Nowhere is there more of a contrast in architectural styles between Gehry and Chipperfield than the main stairway. While Gehry offered a design that contrasted the solidity around it with a floaty conceit of curves and surprising shapes, Chipperfield opted for a massive concrete stairway, something close to the original, which rises between the bare walls in monumental style.

Chipperfield argues: "The problem is that if you design a building and the shape is really exciting and the photo montage shows an incredible fantasy, everyone says: 'Wow, this is going to be fantastic.' But if you try and design something quieter and say: 'When you touch this it will feel beautiful', how do they know? It's much easier to sell impact than reality, and it's much more difficult to explain reality before it exists."

Wemhoff needs no convincing: "It is wonderful. It is extraordinary what has been achieved. The stairway has a wooden roof held up by strong oak trusses. Now it is like an ancient Roman basilica." As delighted as he is about the building, the reunification of the collection, which has been housed in museums opposite the 17th-century Palace of Charlottenburg, a few miles from the city centre, is perhaps more important for the professor.

"We spent years bringing together all our artefacts. It was my daily work," he says. "In the last months of the war they were taken to so-called safe places outside Berlin. The American army comes and takes things away, then the British Army comes and then the Russians, who took the rest. They gave some back to the GDR in 1958 and the western states gave things back to West Germany, so all parts of the collections needed to be tracked down and brought together. Many were lost and of the 3,500 objects that are in the highest category, more than 3,000 are still in Russia."

With the museum opening, the next stage is to build an entrance building - known as The Cube - that will create an underground promenade, connecting all the museums and giving the island a coherence that might be envied by the thinkers behind the UAE's Saadiyat Island. To celebrate its opening in 2013, plans are being made to show an exhibition on the Vikings with the British Museum and the National Museum of Denmark.

The opening of the museum might just reinvigorate the debate over the validity of foreign powers owning great works that have been taken from other countries. Iraqis have often demanded the return of the Ishtar Gates, and many Egyptians would like to see Nefertiti, discovered by a German team in 1912, returned. Wemhoff has the classic western museum director's answer: "It is the heritage of all mankind. I think it is very important that some objects are not just in the country but are available to be shared in the states of all the world."

And Nefertiti as the symbol for a German national museum? He laughs: "It's like the Mona Lisa in The Louvre. Most of our visitors have no idea what they have come to see when they land at the airport. They know Nefertiti and the Wall and no more, but now that will change because now we do not have the same kind of cultural visitors that we had before." Tourism to Berlin has risen from just over 700,000 in 1993 to more than two million last year, and many will make their way to the new museum.

"My only concern is whether you can see the thought that went into everything as you go through a room," says Chipperfield. "I took someone round and I realised that in the finished rooms there's less to say and I'm worried that some of the qualities that were there during the process will get lost. The demonstration of effort is always a futile one, and a vain process, but I think actually the richness comes through.

"The amount of work undergone to make the process look easy will never be understood."