What medium did Leonardo use for The Last Supper?

Leonardo used tempera on gesso for The Last Supper.

The mural is often casually called a fresco, but that description is misleading. In a true fresco, pigment mixed with water is applied to freshly laid, wet plaster, allowing the colors to bond as the plaster dries. Leonardo instead worked on a dry wall prepared with layers of gesso, pitch, and mastic, then applied tempera over the surface.

This unconventional method suited Leonardo’s working style. Unlike traditional fresco, it allowed him to work slowly, make revisions, and pursue subtle expressions and effects of light. He also applied an undercoat of white lead to increase the brightness of the colors.

The technique created serious conservation problems. Because the paint did not chemically bond with wet plaster, the mural began deteriorating soon after its completion in 1498. Humidity, pollution, structural changes, and later restorations compounded the damage. Today, the work is best described as a tempera mural on a gesso-prepared wall, not an ordinary oil painting or conventional fresco.