Be careful not to make a very common mistake: even if it is painted on a wall, Leonardo da Vinci's Last Supper is NOT a fresco, which explains why it's so hard to preserve and restore, and why the room's climactic conditions and the wall's moisture levels must constantly be monitored while at the same time keeping away dust.
The traditional fresco painting technique is advantageous because it allows murals to be preserved for long periods of time, but is disadvantageous in the fact that the artist is forced to quickly paint colors over a thin layer of still-wet plaster before it dries (in fact, fresco means fresh in Italian). Leonardo, who was the most free and independent spirit of the Renaissance, couldn't tolerate constraints and restrictions, and was not one to adapt to the lime's drying times. Days could pass before he even set foot in the Cenacle, or he would suddenly paint non-stop for hours and hours with extraordinary energy and concentration.
For these reasons, Leonardo decided to paint the Last Supper using an innovative technique for fixing colors to walls. He wanted to be able to work on the scaffolding of Holy Mary of Grace as if he were sitting comfortably in front of an easel with tempera colors, able to constantly change his work.
Unfortunately, just a few years after finishing the work, the technique proved to be somewhat of a disaster: the colors began to peel off from the plaster, and the damage immediately appeared to be irreparable. The preparative layer that Leonardo had set hadn't retained the colors correctly, and the entire painting was fragmented and cracked.
It's very likely that the first "restorer" of the Cenacle was Leonardo himself when he returned to Milan ten years later. Many witnesses and critics of the time likened the work to a slowly setting sun that you have to hurry to see before it disappears altogether.
FUN FACT: some of the damage the Cenacle suffered has the friars themselves to blame. Leonardo's work was in their refectory, and in order to pass more quickly from the dining hall to the kitchen, the friars opened a door in the painting, erasing the legs of Christ!
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