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A canvas doesn't always contain only the picture visible on its surface.
Artists sometimes reused supports or painted over earlier compositions.
Modern technologies such as X-rays and X-ray fluorescence imaging can reveal elements of those hidden images without simply removing the visible painting.
For example, researchers investigating Vermeer's Girl with the Red Hat obtained clearer evidence of an earlier portrait of a man underneath it.
The hidden figure has features including long hair, a broad-brimmed hat, a white collar, and a cloak.
Scientists were even able to map some of the pigments associated with the concealed composition.
That means one artwork can sometimes physically contain another artwork that disappeared from human sight centuries ago.
© National Gallery of Art