Leonardo da Vinci painted The Last Supper during the years 1495-1498. He did not follow the true format for painting a fresco, which requires that pigments are applied to a newly plastered and still wet wall. This procedure allows the pigment to bond with the plaster as it dries but does not permit changes in the work as it progresses. Leonardo chose to paint on dry plaster, which allowed him to make changes but also left the work much more vulnerable to changes in y humidity.
The paint started flaking in the early 1500s and was described as "unrecognizable" by 1556. In 1726, the first unsuccessful attempt at restoration involved using oil paint over missing sections and varnishing the entire work. In 1821, an attempt was made to remove the painting for repairs, based on the impression that it was a true fresco. Part of the mural was badly damaged before the effort was halted. The mural was again studied and cleaned 1901-1908. Using modern photographic techniques, The Last Supper was again extensively researched beginning in 1978. Based on information gained, the painting was cleaned, restored, and repainted where necessary. It was reopened for limited public viewing in a climate-controlled environment in 1999.