Pictures from The Barnes Foundation, via The History Blog

The brown paper often found on the back of old artwork is highly acidic, and can ruin priceless paintings if that acidity seeps into the paper upon which the work is done. Naturally, conservationists work to address this. But when The Barnes Foundation of Philadelphia sent five watercolors by Paul Cézanne—as part of a group of 22 total works—to the Conservation Center for Art & Historic Artifacts (CCAHA), also in Philadelphia, for treatment last January, they got back more than just the original artwork.

While painstakingly removing the backing from the 1885-1886 watercolor entitled The Chaîne de l’Etoile Mountains, conservator Gwenanne Edwards noticed random watercolor brush strokes. When the entire backing was removed, a very rough and indistinguishable pencil and watercolor sketch was revealed.

Behind Cézanne's Trees, the conservators found another unknown work, this one a much more detailed graphite sketch of a house with rolling hills behind it. After studying the sketch, Denis Coutagne, president of the Société Paul Cezanne in Provence, determined it was a drawing of Pilon du Roi peak in the Massif de l’Etoile mountain range, a frequent subject for Cézanne.

Albert C. Barnes himself purchased the paintings from Leo Stein, author Gertrude Stein's brother, for the bargain price of $100 each (after informing the cash-strapped Stein that there were no other willing buyers, perhaps for his own benefit) in 1921. Since then interest in Cezanne's work—and the corresponding value—has boomed: A watercolor study for his famous work The Card Players, discovered in Dallas in 2012 after a six-decade absence from public view, sold for $19 million at auction that year.

Any new discoveries, particularly those that contribute to an understanding of the artist's process, are remarkable. "These are a perfect example of how much we still don’t know about this collection," Martha Lucy, a consulting curator at the Barnes and an expert on its Renoir and Cézanne holdings, told the New York Times. "You can see how they’re made, and for anyone who cares about Cézanne, that’s an amazing thing to get to see," she added.

The discovery was met with the appropriate enthusiasm. "There were screams of delight," said Barbara Buckley, the senior director of conservation at the Barnes.