The $140 million Broad in Los Angeles is one of several private museums that have opened across the country in the past decade. (Ryan Miller/Courtesy of the Broad)

From its expansive grounds and curious absence of crowds to the unassuming mix of blockbuster and lesser-known artists, the newly expanded Glenstone feels unlike any museum you’ve ever seen.

But in the landscape of the art world, Glenstone illustrates a growing trend: It’s a private museum, one of several that have sprung up across the country in the past decade.

“You have very wealthy people who love art, and as they build their collection, they realize they want to share it with a wider audience,” says Ian Wardropper, director of the Frick Collection in New York, which was opened in 1935 to display the collection of steel and railroad magnate Henry Clay Frick.

In a country full of museums, however, the purpose of private museums — a broadly defined type of institution based on a single collector’s vision — isn’t always clear — besides, that is, giving an uber-wealthy art collector a glitzy vanity project and perhaps a tax write-off.

What such museums offer, says one expert, is an opportunity to see art that might otherwise be hanging in a wealthy home or stuck in storage.

“What I’ve realized is how many collectors there are, and how much of our cultural heritage is in private hands,” says Barbara Chamberlain, who teaches a graduate course on private collections at Johns Hopkins University and has worked for more than a decade with insurance giant AIG to preserve such collections.

Glenstone opens its expanded complex this week. (Calla Kessler/The Washington Post)

The new wave of private museums is evident everywhere: Glenstone, founded by Mitchell and Emily Rales, will unveil a vastly expanded — and more accessible — institution this week with several pieces from the couple’s deep collection of contemporary and modern art. In a culturally quiet corner of Arkansas, Alice Walton, an heir to the Walmart fortune, opened Crystal Bridges in 2011, showcasing her vast trove of important American artists. In Los Angeles, the $140 million Broad, whose walls are largely filled with the art owned by a single, avidly collecting couple, Eli and Edythe Broad, opened in 2015. (Los Angeles’s architecturally significant Getty Center, funded by the trust of Getty Oil baron J. Paul Getty, is a smidgen older than the rest, having opened in 1997.)

And there are more to come: Newspaper magnate Peter M. Brant will open a second museum next year in New York; the Rubell family, which already had a museum, is building 100,000 square feet of gallery space for its collections in Miami; and filmmaker George Lucas is erecting the Lucas Museum of Narrative Art in Los Angeles, promising a strange brew of Star Wars mementos and works by Norman Rockwell.

Many of these institutions are so well funded and so loaded with artistic treasures that visitors don’t necessarily know they’re private museums. The “private,” after all, rarely means the public can’t visit. So, what makes these museums different from, say, the Metropolitan Museum of Art?

For one thing, curatorial freedom. When it came to amassing the collection on the walls of Washington’s Phillips Collection, steel heir Duncan Phillips “didn’t have to answer to a board of trustees. Or even the public. He was driven by his passion,” says museum director Dorothy Kosinski.

Duncan Phillips was “driven by his passion” when establishing his art collection, says the current director, Dorothy Kosinski. (Photo by Photography by Alexander)

Private museums are also much more likely to “reflect the personality of the owner,” says Wardropper. For example, Frick loved portraits. He had several pieces by one artist he appreciated, Flemish painter Anthony van Dyck; he didn’t collect nudes; and his Upper East Side home’s fabric-covered halls display works in no particular chronological order. They’re hung just as one might hang art in an ordinary home.

At the Broad, as with Glenstone, the approach is more contemporary but equally predicated on the vision of its owners, rather than simply museum standards.

“We have no admissions desk,” says Joanne Heyler, the Broad’s founding director. “We don’t have a ton of signage. We make a really big effort to not be prescriptive and didactic.”

In the first year, 800,000 people came through the Broad’s doors, the bulk of them younger than 35. “We lean in the direction of sharing the work with the public,” Heyler says. “Because what’s the point of art if it isn’t seen by people?”

Private museums are also a way to fill gaps in what we know about artists and their works. The Raleses, for example, have dedicated a room in their new Pavilions to Cy Twombly’s sculptures, which are rarely exhibited.

Heyler, for one, doesn’t think private museums will be replacing publicly funded institutions anytime soon. “It’s such a small number of collectors who are going to have the means to do something like this,” she says.

Designing and building a multimillion-dollar museum, and amassing the number of works to make it worth a visit? “It’s a huge undertaking.”