Many have long wondered just how the artist achieved such delicate shading on this woman's face, and especially around the mouth. Prolonged staring reveals nary a brushstroke; only a somewhat sly smile "that seems to disappear when looked at directly". Finally, after the use of a technique called X-ray fluorescence (instead of the more invasive and controversial paint sample for chemical analysis), scientists have proven that da Vinci created his trademark shadowing "by applying up to 40 layers of extremely thin glaze thought to have been smeared on with his fingers."
The Telegraph then notes that with "the drying times for the glaze taking months, such effects would have taken years to achieve."
Something about this detail suddenly throbbed with metaphor, I'm afraid. That is, of course it takes many, many years for a woman to cultivate a smile that one suspects is there, but one can't be sure of, and which certainly disappears when scrutinised head on.