Italian Art Restorers Used Bacteria to Clean Michelangelo Masterpieces

Researchers deployed microbes to remove stains and grime from the marble sculptures in Florence’s Medici Chapels

Tomb of Giuliano di Lorenzo de' Medici with Night and Day
The team used bacteria to clean the tomb of Giuliano di Lorenzo de' Medici, Duke of Nemours (pictured here). Allegorical sculptures of Night and Day flank the marble sarcophagus. Aleksandr Zykov via Flickr under CC BY-SA 2.0

In 1520, Cardinal Giulio de’ Medici—soon to be Pope Clement VII—commissioned Michelangelo to construct an ornate tomb for his family. In addition to creating a marble interior whose “strange proportions and thicknesses” defied classical conventions, per Encyclopedia Britannica, the Renaissance artist sculpted likenesses of two Medici dukes (Giuliano di Lorenzo and Lorenzo di Piero), four allegorical figures representing different times of day, and the Madonna and Child.

Over the centuries, stains and dirt have accumulated on the statue-lined mausoleum in the Florentine San Lorenzo complex’s Medici Chapels, leaving its once-spotless sarcophagi the worse for wear. Luckily, a team of scientists, art conservators and historians has identified an unconventional tool for removing this grime from Michelangelo’s sculptures: bacteria.

As Jason Horowitz reports for the New York Times, researchers dedicated much of the past decade to cleaning the chapel—but a few obstinate spots remained. To finish the job, the team turned to several strains of bacteria, including Serratia ficaria SH7, Pseudomonas stutzeri CONC11 and Rhodococcus sp. ZCONT. According to the Observer’s Helen Holmes, these microbes consumed oil, glue and phosphates present on the marble statues, removing discoloration and other buildup.

Restoration efforts began in November 2019, when the Medici Chapels Museum asked Italy’s National Research Council to conduct an infrared spectroscopy analysis of the stained tombs. The assessment revealed traces of calcite, silicate and organic materials, offering what the Times describes as “a key blueprint” for biologists tasked with determining which types of bacteria would best clean the statues’ surfaces.

Art restorer Daniela Manna tells the Times that the project, which came to a halt during the Covid-19 pandemic but resumed following the chapel’s reopening last October, was “top secret.” She and her colleagues plan to reveal the restoration’s results in detail later this month.

Tomb of Lorenzo di Piero de' Medici, Duke of Urbino, flanked by statues of <em>Dawn</em> and <em>Dusk</em>
Tomb of Lorenzo di Piero de' Medici, Duke of Urbino, flanked by statues of Dawn and Dusk Public domain via Wikimedia Commons

The team tested 8 bacteria strains chosen from a collection of 1,000, ultimately opting to use non-toxic varieties without spores to clean the marble.

Speaking with the Times, Manna says, “It’s better for our health, for the environment and [for] the works of art.”

Though multiple environmental factors have contributed to the tombs’ sordid state, Alessandro de’ Medici’s corpse seemingly caused the most destruction. Likely the son of Lorenzo di Piero, Duke of Urbino, and an enslaved African woman, Alessandro was the first individual to hold the title of Duke of Florence. As historian Catherine Fletcher writes in The Black Prince of Florence, Alessandro’s body was “unceremoniously dumped” in his father’s sarcophagus following his assassination in 1537. When workers interred the duke in the chapel, they failed to properly disembowel him, leading his remains to seep into Michelangelo’s marble creations. Per the Times, the “deep stains [and] button-shaped deformations” left by Alessandro’s decaying corpse provided a veritable feast for SH7.

Using microbes may seem like an unexpected method for cleaning works of art. But the Verge’s Mary Beth Griggs points out that restorers have employed similar techniques at Milan Cathedral, Pisa Cathedral and a cemetery near the Leaning Tower of Pisa. In 2011, researchers in Valencia, Spain, also used microbes to remove debris from Antonio Palomino’s 17th-century frescoes in the Church of Santos Juanes.

“As in nature we find different species of bacteria that feed on almost anything, we are convinced that we can eliminate other substances from different types of materials,” said Pilar Bosch, a biologist who worked on the Valencia restoration, in a 2011 statement.

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