Laser technology has revealed that Donatello's David, one of the most important statues of the Italian Renaissance, was once decorated with gold leaf.
The priceless piece, created by the artist in the 1440s, is being given its first facelift in more than 100 years at Florence's Bargello Museum.
The work has been opened to the public, who can watch the restorers using a medical-grade laser to clean off the residue that has built up on the statue, which was the first free-standing nude bronze to have been created since classical times.
Ludovica Nicolai, the chief restorer, said that the laser - normally used to treat the eye condition glaucoma - has confirmed suspicions that Donatello put gold leaf on the statue to highlight its wavy hair and clothing.
"We have almost finished the front of the statue, and we have found traces of gold leaf on his right sandal, on the helmet of Goliath, and on David's hair," she said.
The restorers have been using lasers that are customised to shoot a "longer pulse" than the ones used by surgeons. She said the beams were able to reach through to the original gold leaf without damaging it.
The gold had been buried under a thick layer of wax that was applied to the statue in the 18th century. Ms Nicolai said that when David was kept at the Uffizi Gallery he was covered in a "mixture of wax, oils and pigment".
The Uffizi wanted to darken the statue's colour so it would match the other sculptures in its collection. But those visiting the Bargello today can see that the statue's true colour is light bronze.
The restorers suspected that Donatello had "glued" gold leaf to the statue after they X-rayed it in 2006.
Donatello used the same technique in another work, Attis, which was restored in 2005 and is also on show at the Bargello.
The museum said the gold leaf is "particularly fragile", meaning the operation was necessarily "so long and delicate".
The decision to allow visitors to view the work was taken to educate the public about the work of restorers.
Beatrice Paolozzi Strozzi, the director of the museum, said: "We did not want to hide the statue for 18 months, so we put the work on view. People are fascinated to see what normally happens in a lab."
The front of the statue has been completed, and the restorers expect to complete their work by the end of the year.
A full restoration of Florence's other statue of David, by Michelangelo, was carried out in 2004.
Michelangelo's David, which stands 17ft high and weighs five tons, is a colossus by comparison and was scrubbed of its black marks and stains with a special abrasive paste.
