For Mona Lisa, Leonardo used a thin grain of poplar tree and applied an undercoat of lead white, rather than just a mix of chalk and pigment. He wanted a reflective base. Leonardo painted with semi-transparent glazes that had a very small amount of pigment mixed with the oil, so how dark you wanted your glaze to be depends on how much pigment you use. He used more like a “wash”, which he applied thin — layer by layer. Here you can see two colors of contrast — light and dark. When you apply thin glaze over both of them, you can see it starts to unify the contrast but also brings depth and luminosity. The lead white undercoat reflects the light back through the glazes, giving the picture more depth and in essence, lighting Mona Lisa from within.
This was fascinating, not a wasted moment in the whole thing. I’ve read, watched, and listened to a lot of analysis of the Mona Lisa over the years, but Payne’s detailed explanation both added to my knowledge and clarified what I already knew.”