Why did Leonardo da Vinci’s Last Supper begin to deteriorate soon after it was completed?

Leonardo da Vinci’s Last Supper began to deteriorate shortly after its completion primarily due to the artist's choice of an experimental technique.12

Rather than using traditional fresco painting—which requires applying pigment to wet plaster for a permanent bond—Leonardo opted to use tempera or oil paint on two layers of dry preparatory ground. This method allowed him more time to work, but it prevented the pigments from permanently attaching to the stone wall.123

As a result, the paint began to flake within a few years. The condition of the mural was further exacerbated by environmental factors in the refectory of Santa Maria delle Grazie, including dampness, soot from candles, and steam and smoke from the monastery’s kitchen. Over the following centuries, the work suffered additional damage from structural modifications, well-intentioned but harmful restorations, and exposure to the elements during World War II.231