(http://img2.tfd.com/wiki/0/08/Leonardo_da_Vinci_%281452-1519%29_-_The_Last_Supper_%281495-1498%29.jpg)
By the 1970s, Leonardo da Vinci's 15th-century mural masterpiece,
The Last Supper, was badly deteriorated. Italian officials then undertook a major restoration project to permanently stabilize the painting and reverse the damage. The painting's original form was determined using original sketches and scientific tests, including infrared reflectoscopy and microscopic core-samples. The restoration took 21 years, and the painting was put back on display in 1999.
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Source: Free Dictionary Encylopedia