Optical Function of the Vermilion Stratigraphy in Leonardo da Vinci's Sfumato Technique: Empirical Study on Saint Jerome

This technical note presents empirical findings on the three-dimensional optical stratigraphy of Leonardo da Vinci's sfumato technique, based on physical reconstructions of Saint Jerome in the Wilderness. The study demonstrates how the internal original vermilion (HgS) base layer acts as an active optical reflector under point-source directional lighting, as described in the Codices. By reflecting light through upper glaze layers, this internal red layer creates an illusion of inner radiance and volume. The research highlights the necessity of analyzing Renaissance paint stratigraphy in three dimensions 3D at an angle, bridging the gap between historical source texts, physical material practice, and scientific imaging.