GROSSE POINTE SHORES, Mich. — The Edsel & Eleanor Ford House kept secret its 2013 sale of an oil painting by French post-impressionist Paul Cezanne to a private buyer for $100 million to help protect Detroit-owned artworks under threat due to the city's bankruptcy.

The sale appeared on the nonprofit institution's 2013 tax form and removes from the 1929 Grosse Pointe Shores mansion a painting that had been in the Ford family since the mid-20th century, the Detroit Free Press reported (http://on.freep.com/1CaRTTa ) Friday.

Ford House president Kathleen Mullins confirmed to the newspaper the sale of "La Montagne Sainte-Victoire vue du bosquet du Château Noir," which was painted around 1904. It depicts a mountain in southern France. The buyer's name was not released.

Mullins said Ford House officials didn't release news of the sale when it occurred for fear of causing problems for the Detroit Institute of Arts, which was the focus of debate over whether city-owned pieces of its collection should be sold as part of Detroit's bankruptcy.

An $800 million promise from foundations, major corporations and the state to helped protect the DIA's art from possible sale.

Detroit Institute of Arts Director Graham Beal, who was unaware of the Cezanne sale until recently, said the publicity and price would have emboldened creditors in their arguments against the museum's absolutist stance against selling art.

"I am very glad the Ford House proceeded with such caution" in announcing the sale, Beal said.

The Ford House, which is on solid financial footing and carries no debt, is using proceeds from the sale to create a special endowment for preservation, conservation and restoration. It includes the 87-acre estate of auto industry pioneer Henry Ford's only son on the shores of Lake St. Clair as well as furnishings and objects inside the mansion.

Mullins said Ford House trustees received an unsolicited offer for the painting in mid-2013 but at first turned it down. The buyer came back with other offers and the Ford House's board, which includes Ford family members and the family's lawyer, gave the idea more consideration.

"This was really a once-in-a-lifetime offer," Mullins said. "The family thought it was a way to guarantee the estate would be taken care of the way Eleanor would have wanted."

Mullins said the Cézanne sale was done within the legal terms of Eleanor Ford's will. She said the board is not selling more art. The buyer of the Cézanne painting also had offered to purchase a watercolor by Cezanne in the collection, she said, but the board said no.

The Ford House, which draws about 60,000 visitors a year for tours and events, has a separate operating endowment of $86 million.