The Barnes Foundation accidentally discovered unfinished sketches of famous French painter Paul Cézanne on the back of two of his paintings.

Cézanne is one of the most influential artists in the history of the 20th century painting who has inspired some of the modern artists.

The unfinished sketches were found while art conservators in Philadelphia were doing a conservation treatment on the paintings featuring French landscapes-- "The Chaine de l'Etoile Mountains," and "Trees."

"It was a very fortunate day in Philadelphia when they found these," said Denis Coutagne, president of the Paul Cézanne Society in Provence, in a telephone interview with the New York Times.

The paintings were purchased in 1921 from Leo Stein by Albert Barnes for $100 each. Experts believe that Barnes was unaware of the hidden sketches, Reuters reported.

"These sketches offer a window into Cezanne's artistic process, which is truly invaluable," Barbara Buckley, senior director of conservation at the Barnes Foundation, said in a statement.

The unfinished sketches- a graphite drawing and a watercolor with graphite-- will be available for public viewing from April 10 to May 18. The foundation is actually prohibited from moving or rearranging the works on the walls, but the Pennsylvania attorney general approved that they do so temporarily. The display aims to show the public and provide them a new insight on how Cézanne worked.

"I don't want to say that these are spontaneous, but there's more spontaneity," said Martha Lucy, a consulting curator at the Barnes and an expert on its Renoir and Cézanne holdings, to 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."