PHILADELPHIA -- Two unfinished sketches have been discovered on the reverse side of two watercolours by Cezanne -- and officials at Philadelphia's Barnes Foundation museum says the collector who bought them more than 90 years ago probably never knew they were there.

The Philadelphia-based foundation said the sketches -- one graphite and the other watercolour -- were found during conservation work on the reverse sides of two Cezanne watercolours depicting the landscape of southern France.

Officials said in a news release that the sketches haven't been seen since at least the early 20th century, "most likely prior to Dr. Albert Barnes's purchase of the works from Leo Stein in 1921."

"We've had (the watercolours) out of the frame before. But the backs were covered with brown paper," said Barbara Buckley, the foundation's senior director of conservation and chief painting conservator, told The Philadelphia Inquirer. "That's one of the reasons they were sent (for conservation). Brown paper is very acidic, and they needed acid-free paper."

Officials said Cezanne often worked on both sides of the paper in his sketchbooks and on larger sheets, producing thousands of such drawings over the course of his career, but they were usually done "to experiment with line and colour."

Buckley said the sketches, which were on the back of watercolours normally on view in one of the galleries, offer a window into Cezanne's artistic process.

On the back of "The Chaine de l'Etoile Mountains," conservators found that Cezanne had begun a sketch of trees with pencil and then colour, but the centre of the sketch is so unfinished it's hard to determine what it represents. On the back of "Trees," conservators found a detailed depiction without colour of houses and the same Etoile range that was often the subject of the artist's sketches and paintings.

"We had no reason to think there was anything there," said Buckley, who added that nothing was found on the back of another Cezanne watercolour after conservation work in 2007. Barnes officials say 15 unknown Cezanne drawings have been found in the last three decades.

Martha Lucy, a Drexel University assistant professor of art and art history and a former Barnes curator, said Cezanne frequently walked along a route that looked out over the Etoile range near his home in Aix-en-Provence in the south of France.

"Cezanne walked frequently there and did many depictions of it," Lucy said.

Both Lucy and Buckley said they believe that Albert Barnes was unaware that he had acquired, in Lucy's words, "four for the price of two." The correspondence between Barnes and Stein contains no mention of the drawings, so Stein likely did not know about them either.

The foundation plans to display the works in double-sided frames that will allow viewing of both sides from April 10 through May 18, after which the watercolours will be returned to their original locations.