Even the biggest museums are basically icebergs — eye-catching peaks of history, art and culture bobbing atop vast, hidden resources.

Expanding the bergs doesn’t necessarily bring everything to light. Walker Art Center nearly doubled its size with a 2005 expansion and now shows about 5 percent of its collection, up from 2 percent. A year later, the Minneapolis Institute of Arts (MIA) added a wing and opened 34 new and renovated galleries, but it, too, has most of its 80,000-piece collection in storage.

Like homeowners everywhere, most museums have too much stuff. Rather than crowd the walls and clutter the halls, museums store the excess and change their exhibitions periodically to introduce new ideas and freshen things for visitors. Many artworks — especially watercolors, drawings and fabric — are too fragile to be shown for more than a few months at a time because they fade when long exposed to light.

Even cultural trends affect what gets shown, and when, and why.

“There’s a real evolution in museums today,” said Dan Spock, director of the Minnesota History Center in St. Paul. “In the old days you might go to an antiquarian historical society and you’d see every type of maple syrup bucket. That might be valuable for someone deeply interested in maple syrup harvesting, but most people want a broader story now.”

Not enough room

Shaping those stories is the job of curators, experts who pick objects to answer questions, develop themes or illustrate ideas. The History Center’s current Civil War show, for example, uses letters, clothing, weapons, photos and other objects to answer the question: How did the Civil War affect Minnesota families? Most items are from the center’s own collection, but some are borrowed from other museums because they’re a key to Minnesota’s story.

Displaying everything the History Center owns would be not only ridiculous but pretty near impossible given that the collection comprises 225,000 artifacts, 6,000 works of art (landscapes and portraits), 205,000 photos, 50,000 manuscripts, 500,000 books and 25,000 maps. Highlights range from a “duster” worn by a member of the Jesse James bank-robbing gang to Prince’s “Purple Rain” costume, a 1919 guide to St. Paul prostitution and the 2 billionth roll of Scotch tape.

“To curate means to choose,” Spock said. “By selecting, a museum can choose to emphasize some things over others.”

With about 4,000 ceramic bowls, vases, teapots, figurines and sculpture in its 17,000-piece collection, the Weisman Art Museum at the University of Minnesota doesn’t have room to exhibit them all, either, even after essentially doubling its gallery space two years ago. Besides, putting everything out would have been “a kind of hodgepodge with no unifying theme,” said director Lyndel King. So she invited internationally known ceramist Warren MacKenzie to pick what he believes to be the most important pieces.

“Just as a library would never expect that all its books would be checked out at any one time, museums rarely show all their collection,” King said.

The Tweed Museum at the University of Minnesota Duluth makes a point of rotating its 8,000-piece collection, which ranges from 19th-century French and American paintings to mid-20th-century ceramics and extensive holdings of Ojibwe art, including traditional beadwork and contemporary paintings. One of its most popular collections is a series of illustrations of Canadian Mounties that attracts “pilgrims from Canada,” director Ken Bloom said.

“We don’t have a formula” dictating how much of the collection is displayed, Bloom said. Instead the curators mount shows in response to public requests or the interests of faculty members and students. They also invite young American Indian “emerging curators” to develop shows including artists of Indian descent.

“The rationale is to increase the intellectual range of our collection,” Bloom said. “There are many stories that attach to objects or interpretive strategies, so to have a fresh pair of eyes is important.”

By appointment only

About 40,000 fragile, light-sensitive works on paper — drawings, prints, watercolors, illustrated books, collages — make up roughly half of the MIA’s collection. Given the items’ fragility, curator Tom Raisseur organizes thematic shows (flowers, death, Rembrandt etchings) that change every few months. But the museum also has a Print Study Room where, by appointment, anyone can ask to see other works in the collection. Students especially come by to examine techniques (lithography, silk-screen), styles (baroque, neoclassical) or subjects (Belle Epoque fashion, dogs, World War I).

“We don’t have space to put out everything all the time, but if people want to look at our Whistler prints, or explore German Expressionism, they are welcome to ask for them and have an instant exhibition,” Raisseur said.

Walker Art Center likewise samples and rotates its 13,000-piece collection. It makes a point of filling at least five of its nine galleries with works that it owns. In 2015, to celebrate its 75th anniversary as a public museum, it plans to turn the whole building over to the collection. While that still leaves much in storage, the Walker — like the other institutions — takes pride in sending art on the road in shows it has organized or in loans to other institutions. About 60 pieces are traveling now.

So the Walker’s art is “not just upstairs or downstairs [in storage]; it’s out there, too,” visual arts curator Siri Engberg said.