Since walnuts are enveloped in a thin rind, which partakes of the nature of ..., if you do not remove it when you make the oil from them, this skin tinges the oil, and when you work with it this skin separates from the oil and rises to the surface of the painting, and this is what makes it change.
Taken from The Notebooks of Leonardo da Vinci edited by Jean Paul Richter, 1880.