What artistic technique did Leonardo use in the Last Supper to achieve tonal variation?

Leonardo employed a technique known as "fresco secco," which involves applying paint to dry plaster, for The Last Supper. This method allowed him to achieve tonal variation and nuanced shading in his depiction of light and shadow. While traditional fresco requires painting on wet plaster, which then becomes integral to the wall as it dries, Leonardo's choice to work on a dry surface created complications but granted him a greater flexibility in color and detail.

Using oils on dry plaster allowed him to manipulate the paint's texture and richness, lending depth to the figures and their expressions. It also facilitated the layering of colors, contributing to the overall realism and emotional impact of the artwork. This innovative approach was a departure from standard practices of the time and helped to showcase Leonardo's mastery in creating lifelike imagery.