More than 1,500 people attended the black-tie reception celebrating the 75th anniversary of the National Gallery of Art. (Kate Patterson for The Washington Post)

Darting into a room filled with priceless paintings by van Gogh, Degas and Gauguin, Catherine Conover spotted van Gogh’s “Still Life of Oranges and Lemons With Blue Gloves” — in an ornate gilt frame.

“This was in our living room, without a frame,” said the daughter of the late philanthropist Paul Mellon. She turned to look at van Gogh’s “Green Wheat Fields, Auvers.” “This one, too. I think they look better without a frame.”

A few steps away hung another painting from the family collection: Degas’ “The Riders,” a beautiful composition of men and horses. “This was my father’s favorite kind of art,” she said.

And now, instead of hanging in a private home, the paintings adorn the walls of Gallery 83 of the National Gallery of Art.

To kick off its 75th anniversary, the NGA hosted a black-tie reception Wednesday night for 1,400 art collectors, donors large and small, and other patrons. On the surface, it was a celebration of past, present and future. The subtext was the importance of traditional, old-fashioned philanthropy. The gallery is such a fixture in Washington that it’s hard to remember that it didn’t open until 1941, thanks to the vision of one man.

“Andrew Mellon’s idea was an American idea,” said Earl “Rusty” Powell III, the NGA’s director for the past 24 years. “It was the idea that America should have a great national gallery.”

Traditionally, wealthy art lovers of the 19th century built and donated to local museums, creating cities with great art collections such as New York and Boston. Mellon, a wealthy businessman from Pittsburgh, was inspired by the National Gallery in London and Madrid’s Prado. A former Treasury secretary and ambassador to Britain, Mellon donated his art collection to the nation and paid for the imposing neoclassical West Building on the Mall to house it. He insisted that it be called the National Gallery of Art — it was his gift to the nation, not a monument to himself, and he wanted it to inspire others.

“He built it hoping they would come, and they came,” said Powell, ticking off the names of other great collections — Widener, Rosenwald, Kress and Dale — donated to the gallery. Mellon’s son, Paul, continued the family tradition by not only giving his vast private collection but also paying for the construction of I.M. Pei’s modern East Building.

Champagne flowed at the gala. The gallery donors large and small and other art-lovers were there for the celebration. (Kate Patterson for The Washington Post)

Now, billionaire David Rubenstein, who became one of the gallery’s five trustees last year, is following in their footsteps. He donated $10 million for the expansion of the East Building (along with $10 million from former NGA president Victoria Sant and her husband, Roger, and $10 million from current trustee Mitch Rales and his wife, Emily) and is trying to encourage others to emulate Andrew Mellon’s patriotism.

“I’m trying to get people to do more of giving things to their country, like the Mellons did,” Rubenstein said, “and I’m trying in my own modest way to do it. But I hope other people will begin to do it as well.”

Andrew Robison, a senior curator at the gallery for 42 years, said that the National Gallery has enormous meaning nationwide, which gives him an edge with donors.

“I never ask for everything,” he said. “I always say, when I go see a collector: ‘Support your local museum, but give us a slice of pie. For the nation.’ ”

Many of those donors were invited to Wednesday’s celebration. It was the biggest party in the gallery’s history (as far as anyone can remember), with a crush of people who drank champagne, swarmed the buffets and wandered through the permanent collection, where they were allowed to take selfies with the priceless art. A plate crashed onto the marble floor and broke into pieces. It was an accident, not performance art. Now it was officially a party.

The mood was expansive — partly because of all that champagne, partly because there were so many people running around, but mostly because this is a big year for the gallery: a year-long anniversary with a $75 million endowment drive and the reopening of the East Building this fall, both designed to ensure the NGA’s position as one of the top art institutions in the world.

National Gallery President Frederick Beinecke with his niece, Kit Krugman, at the reception. (Kate Patterson for The Washington Post)

“The biggest thing this year is not this party. It’s the opening of the East Building,” NGA President Frederick Beinecke said. “It’s by far the most important thing we’re doing.” The expanded space will house the 20th-century collection, which Powell calls the future of the gallery. “All art was modern art at some point,” he noted.

For the gallery’s 50th anniversary, then-director Carter Brown spent five years persuading collectors to give a portion of their prize artworks to the gallery. Now Powell is trying to raise money to add to the existing $750 million endowment fund — a critical but harder task, because no one gets their name on a wall next to a beautiful painting.

Again, the Mellon legacy is crucial. To mark the 75th anniversary, the Andrew W. Mellon Foundation has pledged $30 million if the gallery raises $45 million within five years.

“We’ll do it,” said Powell, with the confidence of a man who has two billionaire philanthropists on his board.