While allowing greater precision of detail, this technique left the mural particularly vulnerable to moisture. Within a few years, paint began flaking off.
In 1556, art historian Giorgio Vasari called the work a "muddle of blots." By 1796, Napoleonic troops occupying Milan were so unimpressed that they converted the refectory into a stable and amused themselves by throwing bricks at the Apostles' heads.
The Last Supper narrowly averted disaster in World War II when a bomb fell on the roof of the church. Protected by sandbags, it went largely unscathed.
As the mural deteriorated, sections were touched up using various chemicals and techniques -- some of which may have been more detrimental to the painting than either the French cavalry or the aerial bombardment.
The latest restoration efforts, which began in 1979 and lasted 20 years, made great gains in revealing color and detail, but a more thorough cleaning could destroy the work beyond recognition.
"The researchers at Leonardo3 have great modular views, as shown by their models of Leonardo's machines," Alessandro Vezzosi, director of the Museo Ideale in the Tuscan town of Vinci, where the artist was born in 1452, told Discovery News.