Column One: The most difficult jigsaw puzzle in the world

I AM almost too scared to lift my eyes as we leave behind the intense Umbrian sunlight and cross the threshold of the Upper Basilica of St Francis.

My last visual memory of the Italian gothic treasure is the compellingly awful image viewed by millions of people around the world - an immense cloud of white dust that filled the television screens as part of the vault crumbled and tumbled down to the high altar below, bringing with it priceless frescos by Cimabue and burying two Franciscan friars and two technicians beneath the rubble.

It is two years since the earth began to move in the central Italian regions of Umbria and Marche and continued to shake for weeks until it had claimed the frescos, cracked convent walls, made statues and bell towers tumble and forced 30,000 people out of their homes.

The Upper Basilica of St Francis, like many of the churches in Assisi, has been closed to the public since then. In the gloom, my eye is drawn along the ceiling of the nave to the Evangelists' Vault to a cream-coloured segment. This was once Cimabue's St Matthew in the Vault of the Evangelists. The jagged hole with loose bricks that risked bringing down the surrounding masonry has been covered and reinforced. The neutral painted surface is a temporary measure but there is no certainty that what remains of St Matthew - 120,000 fragments conserved in boxes - will ever be replaced.

The church is still a building site, inestimable damage has been done, but this great artistic tribute to St Francis is injured, not maimed. Giotto's cycle of the life of the saint on the lower walls is reassuringly intact, visible behind thick covers of plastic and machines with cherry-picker buckets to help the artisans to reach the higher levels.

Above the entrance, the damaged arches, also decorated by Giotto, have been strengthened and much of the frescoed surrounds restored. The eight saints who peeled off are still waiting to be pieced back together in a specially created laboratory; their spaces still hidden behind wooden platforms used by the restorers.

In two months' time, the Upper Basilica is due to reopen to the public, and two of the eight saints must be back in their rightful place - Saint Vittorino and Saint Rufino, whose shattered face was one of the first pieces to be retrieved after the quake.

For the moment they are still in a specially constructed art laboratory, off the lower Basilica. In this airy white-walled space, immense life-size photographs of the shattered saints are laid out on trestle tables. The photographs are covered with transparent plastic; clusters of fragments that match the image below are laid out on top. Segment by segment, matching pieces are glued together. The blank spaces - and there will be many - are to be coloured in to enhance the legibility of the work, a decision similar to that used for the restoration of Leonardo Da Vinci's Last Supper in Milan. The pressure to have the Basilica ready in time for the opening is immense. Assisi is the second most important Italian religious site of the Year 2000 Jubilee declared by the Roman Catholic Church. The Franciscan friars, the local authorities, town businessmen and central government want as much repaired as quickly as possible. Otherwise they fear that many of the 13 million expected visitors may not make the trip. They also want to silence those doom-mongers who argued that all had been lost and Italy would never be capable of restoring one of its most priceless art treasures. At a weekend seminar in Assisi, those involved in the pounds 19m restoration said they were on target to meet the late November deadline. The tympanum, the triangle-shape that sits above the rose window on the dusky pink facade, has been made safe with resins and struts. The bell tower has now been almost liberated of its scaffold covers. The repaving of the piazza in front of the church, in its original pink and grey hues, is near completion. Giuseppe Basile, of the Central Restoration Institute, said: "In October 1997 there were critics who said there was no hope, because all that had collapsed had been reduced to just coloured dust. We have shown that this was unfounded." High technology, in the form of a computer program that digitally photographs, analyses and records the fragments, indicating other similar pieces, has been crucial. Yet even this is unlikely to be able to save Cima-bue's St Matthew, a work that was already damaged before the quake. The cream-coloured space at the end of the basilica may become permanent.