In a highly unusual outcome to conservation efforts, the Barnes Foundation has discovered it owns two previously unknown Cézanne sketches - even collector Albert C. Barnes was most likely unaware of their existence.

The two works, unmentioned in any correspondence and not included in the master compendium of Cézanne's works, are on the backs of two watercolors that are permanently hung in the foundation's galleries on the Benjamin Franklin Parkway.

The works had been taken down a year ago for needed conservation.

Curators and conservationists at the Barnes, known for its impressionist and early modern collection, including 69 works by Cézanne, were stunned by the discovery. Plans are set to display both the "old" and the "new" works in double-sided frames beginning April 10 for eight weeks.

"We've had [the watercolors] out of the frame before," said Barbara Buckley, the Barnes' senior director of conservation and chief painting conservator. "But the backs were covered with brown paper. That's one of the reasons they were sent [for conservation]. Brown paper is very acidic, and they needed acid-free paper."

When 22 works were sent to the Conservation Center for Art and Historic Artifacts in Philadelphia for treatment in January 2014, the two Cézanne watercolors were among them.

Within a month, Buckley received an e-mail from Mary Schobert, the center's director of conservation: "You might be interested to know we found something."

Once the glued-on brown paper backing was painstakingly removed from Chaîne de l'Étoile Mountains (1885 or 1886), the conservators discovered that Cézanne had begun a fluent sketch of trees, laying down a bit of pencil work, and then building the drawing with color. The center of the sketch is so unfinished, it is difficult to determine what it represents.

On the back, or verso, of Trees (c. 1900), once the brown paper was removed, conservators discovered a detailed depiction of houses and the same Étoile range that Cézanne returned to repeatedly in his sketches and paintings. This sketch contains no color at all.

"We had no reason to think there was anything there," said Buckley. Another Cézanne watercolor had been conserved in 2007, and nothing was found on the verso, she said.

The conservators also found a quick sketch of a bowler hat on the back of Cézanne's The Smoker, a drawing done in 1890-91, and a few other things. But the two sketches - of trees and of mountains - are by far the most significant finds.

It is extremely unusual for unknown Cézanne drawings to emerge from obscurity. About 15 have come to light in the last 30 years, according to Barnes officials.

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

The newly discovered sketch of the range, and the known watercolor Chaîne de l'Étoile Mountains, show the same landscape from different perspectives.

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

Cézanne also almost always used both sides of the pages in his sketchbooks, filling them completely. There are about 18 or 19 of these books, Lucy said.

But the two newly discovered works are on the backs of rather expensive single sheets, not sketchbook paper - a more uncommon dual use.

Both Lucy and Buckley believe that Albert Barnes, who bought the Cézannes from Leo Stein in Paris in 1921, was unaware he had acquired "four for the price of two," as Lucy put it.

No mention is made of verso drawings in the Barnes-Stein correspondence. It is probable that Stein also did not know of their existence.

There are markings from a Paris dealer on the verso, however, so it is likely the dealer sealed the backs with the brown paper after logging the watercolors into inventory and before selling them to Stein.

The known watercolors - the l'Étoile and Trees - are normally on display in the Barnes galleries. As such, the trust indenture that governs the foundation's operations forbids their removal and display elsewhere - even in the same building.

Barnes officials queried the state Attorney General's Office about a plan to display the new discoveries temporarily in an education room in the gallery before returning them to their original location.

Barnes officials argued that the display would be for only eight weeks and the paintings were already down for conservation. The attorney general made no objection, Barnes officials said.

"As part of our educational mission, we felt it was important for the public to see these," said a Barnes spokeswoman.