Vincent Van Gogh was known to have painted over 30 self-portraits between the years 1886 and 1889. That is until this week when it was revealed that a hidden self-portrait by the artist has been discovered behind one of his paintings, covered by layers of glue and cardboard for more than a century. The image was found when art conservators took an X-ray of Van Gogh’s 1885 “Head of a Peasant Woman” painting ahead of a forthcoming exhibition. They discovered the concealed image at the back of its canvas hidden by a sheet of cardboard, according to a press release from the National Galleries of Scotland.

His collection of self-portraits places him among the most prolific of 19th century self-portraitists. Van Gogh used portrait painting as a method of introspection and a way of developing his skills as an artist.

In a letter to his brother Theo dated September 16, 1888, Van Gogh writes about a self-portrait he painted and dedicated to his friend and fellow artist Paul Gauguin.

“The third picture this week is a portrait of myself, almost colourless, in ashen tones against a background of pale veronese green. I purposely bought a mirror good enough to enable me to work from my image in default of a model, because if I can manage to paint the colouring of my own head, which is not to be done without some difficulty, I shall likewise be able to paint the heads of other good souls, men and women.”