may have been damaged further by the vibration . From 1951 to 1954, another clean-and-stabilise restoration was undertaken by Mauro Pelliccioli. The painting's appearance by the late 1970s had become badly deteriorated. From 1978 to 1999, Pinin Brambilla Barcilon guided a major restoration project which undertook to stabilize the painting, and reverse the damage caused by dirt and pollution. The 18th- and 19th-century restoration attempts were also reverted. Since it had proved impractical to move the painting to a more controlled environment, the refectory was instead converted to a sealed, climate controlled environment, which meant bricking up the windows. Then, detailed study was undertaken to determine the painting's original form, using scientific tests (especially infrared reflectoscopy and microscopic core-samples), and original cartoons preserved in the Royal Library at Windsor Castle . Some areas were deemed unrestorable. These were re-painted using watercolor in subdued colors intended to indicate they were not original work, while not being too distracting. This restoration took 21 years and on 28 May 1999 the painting was returned to display, although intending visitors are required to book ahead and can only stay for 15 minutes. When it was unveiled, considerable controversy was aroused by the dramatic changes in colours, tones, and even some facial shapes. James Beck , professor of art history at Columbia University and founder of ArtWatch International , had been a particularly strong critic. [ 13 ] Michael Daley, director of ArtWatch UK, has also complained about the restored version of the painting. He