French art expert Pascal Cotte said Leonardo da Vinci built the painting up in layers, the last being a special glaze whose optical properties increased the illusion of a three-dimensional face. Above the glaze Leonardo painted details such as the eyebrows. Cotte said: "That could explain why the eyebrows have disappeared – they have faded because of chemical reactions or they have been cleaned off."
He has uncovered a host of secrets about the Mona Lisa using a 240 megapixel camera. It can measure light so sensitively as to see through the top paint surface and uncover the layers below.
For example, infra-red imaging shows Leonardo moved the position of a finger on the left hand "to give a more relaxed position, consistent with the smile", Cotte said.