What experimental technique caused rapid deterioration of The Last Supper?

Painting on dry plaster caused The Last Supper to deteriorate rapidly.

Unlike a traditional fresco, which applies pigment to wet plaster so the color bonds with the wall as it dries, Leonardo painted on a dry surface. He used a mixture involving tempera and oil over prepared plaster, allowing him to work slowly, add fine details, and revise passages after they had dried.

That flexibility suited Leonardo’s method but made the mural fragile. The paint did not become chemically integrated with the plaster in the durable way associated with buon fresco. Moisture from the wall, damp conditions, smoke, steam, candle soot, and later damage all accelerated flaking and discoloration.

The deterioration began surprisingly early: by 1517, observers were already recording damage. The work was later harmed by misguided restorations, a doorway cut through its lower section in 1652, and wartime exposure. The phrase “painting on dry plaster” is therefore more accurate here than “fresco,” even though the work is commonly called a mural or fresco in casual descriptions.