How did thin layers of oil glaze help Leonardo da Vinci paint the Mona Lisa?

Leonardo da Vinci utilized thin, translucent layers of oil glaze to achieve the highly illusionistic and soft effects characteristic of the Mona Lisa. Unlike fast-drying tempera, oil paint allowed Leonardo to apply these delicate layers to build gradual, subtle shifts from light to dark.1

This technique was essential to his mastery of sfumato, a method of shading that produces imperceptible transitions between colors and tones without hard lines or borders. By layering the oil glazes, Leonardo was able to blur outlines and gently model the sitter's face, creating a sense of sculptural depth and harmony that reflects his deep understanding of human anatomy. He likely worked on the painting intermittently over many years, adding these multiple thin glazes to refine the work throughout his late period.213