Handle With Care

From the Coliseum in Rome to Notre Dame in Paris and the Brandenburg Gate in Berlin, Europe's cities teem with historic monuments.

Many of these architectural and cultural wonders can't be appreciated to the full, however, because they are covered with thick layers of grime and soot. Moreover, this black crust doesn't just hide masonry and sculptural details, it can also damage the monument itself.

One of the main components of the smog polluting the majority of Europe's cities is sulphur dioxide. It can react with limestone, a material commonly used in heritage buildings, and transform it into a damaging gypsum layer, which causes pieces of the surface to flake off.

"These deposits are particularly harmful," said engineer Benigno Morlin Visconti Castiglione, director of the Veneranda Fabbrica del Duomo, the institute responsible for Milan's famous cathedral.

"Because of this, the black crust absolutely has to be removed if you are serious about doing a proper renovation."

Sulfate munching bacteria

An innovative and environmentally friendly way of removing this unwanted deposit is emerging in the conservation scene -- bacteria. Italian researchers recently used bacteria to do a test clean of a small area of Milan's cathedral. They found the biological process was less damaging and more effective than a chemical treatment often used by conservators.

"The ornamental lunette we cleaned to compare the two methods is up there on the twentieth story," said scientist Francesca Cappitelli, pointing at the cathedral's marbles spires which soar up to 106 meters (350 feet).

For the project, the researchers grew the sulfate-reducing bacteria Desulfovibrio vulgaris, which are naturally found in the environment. This mass of single-celled organisms was then placed on marble surface for 48 hours, explained Dr. Cappitelli, a researcher at the Agricultural Faculty at the University of Milan. She's part of a multidisciplinary team from several Italian institutes who carried out the study.

"The bacteria transform the sulfates [on the marble surface] into gases and these gases are released into the air, which is an environmentally friendly method," she said.

Importantly, the test on the decorated lunette showed that the bacteria ate most of the damaging black crust, without eating the underlying "noble patina" -- a deposit that occurs naturally as buildings age.

"You want to preserve the noble patina because it gives the stone character," said Cappitelli.

Damaging chemicals

Chemical cleaning of stone monuments has come a long way since the 1970s and 1980s, when solutions such as concentrated hydrofluoric acid and sodium hydroxide were applied haphazardly, often causing permanent damage to building facades.

One popular example among conservators, according to an article by architect David Boyer in the periodical "Traditional Masonry," is Rutland Square in the Scottish town of Edinburgh.

During renovation, some of the square's sandstone storefronts were improperly cleaned with chemicals which turned the pale-yellow stone a rusty orange.

"Other shops received a hydrofluoric acid cleaning, which bleached the warm tones and color out of the stone, leaving it a sterile white," Boyer writes.

Since then, advances have been made, and chemical cleaning is now an established method used by conservators world-wide. However, it still has several limitations: it can corrode into the stone's surface.

This was verified in the test carried out by Francesca Cappitelli's group, which found the chemical treatment ate away more than just the crust covering the marble -- it had actually changed the granular structure of the stone's surface.

"It removes whatever it comes in contact with and sometimes you can have over treatment. It removes too much and harms the surface," Cappitelli said.

An additional concern about chemical treatments is that they are toxic for both humans and the environment. During such treatments, chemicals are applied to the monument, and then rinsed off with water, which can lead to contamination of the surroundings.

Questions remain

Although biological cleaning using diverse types of bacteria is being researched across Europe, caution is still necessary to make sure the innovative process doesn't make the situation worse in the long run.

"You have to be careful that you don't introduce the wrong thing that causes a new set of problems," warns Peter Martin from Masonry Conservation Research Group at Aberdeen's Robert Gordon University.

"The underlying principle is that you are trying to replicate nature where one organism disposes of another and that's fine if you are convinced that is going to happen. But if you imagine some of the biological control systems that have gone wrong -- like the cane toad -- you have to make sure you don't introduce a new set of problems," he said.

Meanwhile, Francesca Cappitelli and her co-researchers have already received an Italian patent for their biocleaning treatment and she's waiting for the results of an application for an international patent.