Scholars have discovered a previously unknown portrait by James McNeill Whistler beneath a landscape painting from 1862, the Art Newspaper reports.

As they were hanging the exhibition “An American in London: Whistler and the Thames” at the Dulwich Picture Gallery in London, curators noticed bumps on the surface of The Last of Old Westminster, 1862. Curator Margaret MacDonald then asked for an X-ray of the landscape, which revealed an entirely different composition. A portrait, flipped sideways, of a young woman reading, believed to be Joanna Hiffernan, Whistler’s young mistress and model.

The discovery brought up one central question: why would Whistler have painted over a portrait of his lover? Based on her research, MacDonald says it is likely the the cash-strapped artist did not have another large canvas available when he received the commission for The Last of Old Westminster. MacDonald’s research will appear in a new catalogue raisonne of Whistler’s work.