True fresco, the standard mural technique of Leonardo's era, requires an artist to paint quickly onto wet plaster, which absorbs the pigment as it dries and binds it permanently into the wall. That speed did not suit Leonardo's working method, which favored slow reworking, layering and correction over rapid, irreversible strokes. So for The Last Supper he abandoned fresco entirely and painted in tempera and oil directly onto dry plaster instead, a technique that let him work at his own pace but that never chemically bonded the paint to the wall the way true fresco does.
The consequences arrived almost immediately. Within about twenty years of its completion around 1498, visitors were already recording that the paint was flaking and discoloring, and by the mid-sixteenth century, roughly fifty years after Leonardo finished it, one observer described the work as already so ruined that the figures could barely be made out. The refectory's location in a low, damp part of Milan, prone to flooding, made the underlying wall itself an unstable surface for anything painted on it, compounding the technique's own weaknesses.
The mural has been restored repeatedly since, most extensively in a twenty-two-year conservation project completed in 1999, which removed centuries of earlier repainting and grime to reveal what could still be recovered of Leonardo's original surface underneath. What museum visitors see today is consequently not simply a five-hundred-year-old painting but the end result of an unusually long and difficult conservation history, layered on top of a technique that began working against itself almost from the moment it was finished.