The National Gallery of Scotland has discovered a previously unknown Van Gogh self-portrait. He was hiding behind his painting "Portrait of a Peasant Woman in a White Cap", which is kept in the same gallery.

Gallerists saw the portrait when, as part of the cataloging process, they shone through the "Portrait of a Peasant Woman" with X-rays. Apparently, it was sealed in preparation for the exhibition, held at the beginning of the 20th century. The portrait is painted on the back of the painting and has been hidden for over 100 years under layers of glue and cardboard. To create it, Van Gogh used lead white - a much heavier paint that appears on x-rays.

Experts believe the painting is part of a series of experimental Van Gogh self-portraits, five of which are on display at his museum in the Netherlands. They are written on the reverse side of earlier canvases created from December 1883 to November 1885, when the post-impressionist lived in Nuenen.

Now the restorers will try to unstick the self-portrait from the Portrait of a Peasant Woman. “The challenge will be to remove glue from layers of oil paint using the difference in solubility between animal glue and oil paint,” said National Gallery of Scotland Senior Senior Conservator Leslie Stevenson.