Restorers save frescoes by injecting walls with a quake-proof resin that will last to the next millennium
On September 26 1997, a violent earthquake rocked central Italy. In the hilltop town of Assisi, the 13th-century Basilica of St Francis, a historic landmark known the world over, was seriously damaged. Frescoes by Cimabue crashed to the grounds and others, some by Giotto, were damaged, writes Paul Bompard.
Italy's restorers immediately rushed to the scene. Among them was Ulderico Santamaria, a chief chemist at the Central Restoration Institute in Rome, whose job was to develop the materials with which to repair the damage.
Professor Santamaria said: "Most of the damage was in the frescoed ceiling vaults, some had fallen, others were badly cracked."
There is a space, enough to move around, between the roof-beams of the basilica and the vaults underneath. The vaults support the surface plaster on which the frescoes are painted. The problem was to consolidate the cracks and fissures between the vault and the frescoed plaster with some kind of adhesive filler that would satisfy four basic needs: to stand up to future tremors, to stand the test of time, to be removable and not damage the frescoes.
"We knew we could insert small plastic tubes from above and drip the adhesive down between the vault and the plaster. And we could also inject from below," said Professor Santamaria.
"The real problem was finding the right material to use. It also had to set quickly or the added weight of the liquid could have caused more damage."
Professor Santamaria and his team first looked at epoxy resins. "These, however, proved to be too different from the vault structure. Anything containing cement was also out of the question. So we took some commercial formulae based on hydraulic lime. We took out some ingredients and added a few."
Lab tests showed that this adhesive would spread out effectively between vaults and plaster. But uncertainty remained regarding its other properties.
"We made mock-ups of the restoration, trying many different mixes of the ingredients, and subjected them to tests. We put them in a special chamber to simulate ageing, weathering tests with extremes of temperature and very high and very low humidity, and then saw if they maintained their strength and could be removed without damaging the plaster.
"As a result of these tests, we used an adhesive, that we know will last at least 50 years, but probably hundreds or thousands, that will support the plaster even in the face of another earthquake, will not damage the frescoes chemically and can, anytime in the future, be removed without too much difficulty."
The vaults have now been restored and the basilica will be formally inaugurated by the pope on November .