The barbe
A “barbe” is visible on all four edges of the poplar panel on which the Mona Lisa was painted. This is the ridge of paint that was revealed on the panel’s unpainted edges when it was removed from the frame to which it must have been fixed before the artist began his work. The existence of the barbe is proof that the complete composition has been preserved.
The paint colors in the barbe are closer to the painting’s original hues, having been partly spared the successive coats of varnish applied after Leonardo’s death.
The split
The thin poplar panel on which the Mona Lisa was painted has changed considerably in the course of its five hundred years of existence. It has become slightly warped, with a convex deformation created by the changes in humidity and temperature to which the painting was subjected. A split some eleven centimeters long appeared as a result, and was stabilized in the past by two dovetail joints fixed to the back of the painting.
The vertical craquelure
Vertical craquelure is visible in the paint layer of the Mona Lisa, especially in the area extending from the split in the wood. This network of fine dark lines undermines the subtlety of the transitions from shadow to light on the figure’s face. The craquelure is due to the convex warping of the very fine poplar panel on which the Mona Lisa was painted─an inevitable result of the humidity and temperature changes to which the painting was subjected over the years.
The diagonal craquelure
A network of fine diagonal cracks is particularly visible in the four corners of the paint layer of the Mona Lisa. This was caused by the pressure exerted on the thin poplar panel by the frame in which the painting was placed.
The lacuna
A lacuna is visible near Mona Lisa’s left elbow. This is the only damaged part of the painting itself, which is otherwise in an excellent state of preservation. In 1956, a mentally unstable visitor threw a stone that broke the glass frame. The impact and the splintering of the glass affected the paint layer and preparatory undercoats of Leonardo’s masterpiece.
As a result, the famous portrait has been displayed behind bulletproof glass ever since that time.