Why an Italian museum unleashed bacteria on Michelangelo's marble masterpieces

When it came time to clean sculptures by Michelangelo at the Medici Chapels in Florence, Italy, conservators knew they needed a new approach.

For nearly a decade, teams have been working to clean the marble works in the New Sacristy — searching for solutions that wouldn't damage the fragile stone Michelangelo worked with to sculpt the room's elaborate tombs. 

So rather than applying chemicals or using abrasives, the experts made a "bold decision" to use bacteria-infused gel to clean away centuries worth of dirt and grime.

Now, the stains that marked the sculptures have been washed away.

"Of course, the signs of times and centuries will stay — we cannot think of setting the clock back 600 years," said Paola D'Agostino, director of the Bargello Museums which oversees the Medici Chapels.

"But what is so fulfilling is that now finally you have a sense of harmony when you enter the New Sacristy."

Scientists had tested the effectiveness of 11 different bacteria strains on marble by applying a grid of the available options in small squares to the back of the sacristy's altar. 

A bacteria known as Serratia ficaria SH7 proved the most effective, feasting on Michelangelo's works, and was chosen as the preferred cleaning microbe.

"Once the bacteria had finished eating…. Then the bacteria dies and when they remove the gel, they were able to verify that they only ate the traces that they had been created for," D'Agostino said.

It's technology that has been used in the chemical sector, she added, to clean up waste sites in parts of Italy.

Spoiled by a de Medici

D'Agostino attributes the discolourations, in part, to the remains of a Alessandro de Medici, a ruler of Florence, who was entombed at the sacristy — though the body was not properly prepared for burial.

"Over time ... all the organic liquids from his body started to stain the marble sarcophagus," D'Agostino explained. "It has been like this since the late 16th century."

The remains aren't the only thing discolouring Michaelagelo's masterpieces, however. 

Michelangelo's works at the New Sacristy are among the last pieces he started in Florence and were long the favourites of other artists. 

Those artists made copies by applying plaster to the sculptures, leaving behind residue in the process. Over time, that residue stained and turned the marble grey-yellow in places.

Bacteria 'not a typical tool' for conservators

The unconventional approach of using bacteria for cleaning artwork is an example of conservators using "the best tool possible" when it comes to restoration, said Emy Kim.

"For conservators, cleaning is an irreversible step because you can imagine when you take dirt off, you can't exactly put it back on if you feel like you've gone too far. At least not in the same way," said Kim, an assistant professor of artifacts conservation at Queen's University in Kingston, Ontario.

Significant care is taken when determining what solution to use on a given work. In the case of Michelangelo's marble sculpture, Kim said she would avoid anything acidic or abrasive.

But how a material will react can be unpredictable, meaning that often there's no single correct method.

Using bacteria, however, presents a novel, and possibly more environmentally-friendly opportunity for conservators.

"I would say it's not a typical tool in most conservators' arsenals, and I think since we are always looking for a new tool. I think it's a very interesting avenue," she said.

Depending on the experiment's outcome, Kim says that conservators will be looking to compare the effectiveness of bacteria-based cleaning to existing methods.

"It could be a game changer for some very specific situations, but with any tool, the tool is only good if the user knows how to use it," she said.

'A new light'

D'Agostino says that the bacterial treatment of Michelangelo's marble masterpieces will not need to be reapplied.

"The statues will need to be maintained and preserved, but only by traditional means of dusting," she said.

But what makes D'Agostino and her colleagues happiest about the restoration is that visitors will now be able to enjoy the works in their natural colour.

"After centuries and centuries of artistry, conservation practice and science, the combination of all these different expertise created a protocol that worked to bring a masterpiece to new light."

Written by Jason Vermes. Produced by Sameer Chhabra.

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