Michelangelo's David risks crumbling down under its own weight because of the statue's weak ankles.
Alarm bells sounded after researchers carried out a series of centrifuge tests on small-scale plaster replicas of the marble masterpiece. Apparently, damage caused by the statue's inclination is placing the great art work at risk, according to researchers at Italy's National Research Council (CNR) and the University of Florence.
The experiments revealed that under high-stress conditions, the statue would break along small cracks currently visible in the left ankle and in the lower part of the carved tree stump supporting the right leg.
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Such cracks are not unknown to experts.
"They were first detected between 1852 and 1872 and nowadays they are more extensive than in 1872," Giacomo Corti, a researcher at CNR's Institute of Geosciences and Earth Resources, and colleagues wrote in the Journal of Cultural Heritage.
Although they have been covered up with plaster over the years, the hairline cracks tend to reappear. They do not indicate an imminent collapse, but could prove devastating in the event of a major earthquake.
The experiments consisted of several centrifuge runs. In the centrifuge, the 4-inch plaster replicas were affected by a force stronger than gravity, but otherwise playing the same role.
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Positioned both vertically and inclined forward at angles of 5, 15 and 25 degrees, the models were subjected to increasing rotational speeds up to rupture.
"In the vertical position, the gypsum model and, by analogy, the original masterpiece are in the most stable conditions, as indicated by rupture occurring at the highest acceleration well above the conditions of equilibrium," the researchers wrote.
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They noted that fractures typically occur in the lower portions of the legs. But as the statue's angle of inclination increases, the models break at lower rotational speeds, while cracks tend to develop progressively higher along the legs -- close to the right knee for inclinations between 15 and 25 degrees.
"This means that the higher the inclination, the more unstable the statue becomes," the researchers said.
The tests suggest the 5,572-kg marble statue would break under its own weight if standing at an inclination higher of 15 degrees.
In such a scenario, the statue's pose and poor quality marble would contribute to the collapse.
Michelangelo carved David from a single block of marble that two other sculptors, Agostino di Duccio and Antonio Rossellino, discarded as it had an imperfection.
On Sept. 8, 1504, the towering sculpture, acclaimed for its depiction of male physical perfection, was displayed beside the main doorway of the Piazza della Signoria in Florence.
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Representing the biblical hero who killed Goliath, the sculpture marked a watershed in Renaissance art and established Michelangelo as the foremost sculptor of his time at the age of 29.
The statue remained in its original location, at the mercy of the elements, until 1873, when it was moved to its present location in the Galleria dell'Accademia.
According to the researchers, the micro fractures currently visible on David are the result of a long-lasting, small forward inclination of about 5 degrees during the statue's time in Piazza della Signoria between 1504 and 1873.
"The research confirms previous hypothesis and could help in the preservation of this masterpiece," Cristina Acidini, superintendent of Florence's museums, said.
She reassured that the cracks do not pose an immediate threat.
"Since 2001, David's micro fractures are constantly monitored and no variation has been recorded so far," Acidini told Florence's daily La Nazione.
Image: from left to right: Michelangelo's David; details of the fracture system affecting the lower sections of the statue's legs; approximate location of fractures in the replica statue with angle of inclination between 0 and 5 degrees. Boxes with red contours shows the position of micro fractures in the original statue. Credit: CNR.