A bronze, alien-like figure in Mimmo Paladino’s mysterious sculpture “Canto Notturno” was foaming at the mouth. No one at the Minnesota Landscape Arboretum knew why.

“It was probably possessed,” said volunteer John Dean with a shrug.

Dean spotted the eerie phenomenon while giving the 6-foot-tall sculpture its biannual bath. Paladino was notoriously secretive about his artwork — this 1984 piece shows a tall woman in a billowing cape, reaching behind her to touch the “alien’s” antenna — so it’s possible that the sculpture had been inhabited by a guest from another universe.

But on this drizzly gray day in late April, Dean, two other volunteers and arboretum gardener Erik Lemke weren’t there to think about the mysteries of art and the cosmos. They had begun cleaning the 26 sculptures in the arboretum’s Harrison Sculpture Garden, and in the coming weeks would start on 35 more sculptures scattered throughout the woods and in the water. Spring was coming.

The care and maintenance required to upkeep massive metal sculptures is far beyond the white vinegar and water used to clean the aftereffects of winter from household objects. Volunteers trained in sculpture cleaning come out once or twice a year, armed with giant sponges, Orvus detergent soap and a water sprayer to combat surface level gunk. In the process, they may discover scratches or tooth marks from animals, insect nests, weather-related damage and man-made scars.

These issues would never crop up if the sculptures were housed inside, in climate-controlled museum galleries. But the natural world is part of the aesthetic experience of viewing these sculptures, right down to the collection of plants selected to surround them.

“There’s a lot of mud in this eye,” said Wendy DePaolis, motioning toward the alien’s left eye. The arboretum’s curator, DePaolis initiated the sculpture garden cleaning program when she started the job three years ago.

Volunteer Diane Shelgren pointed her water sprayer and pulled the trigger. DePaolis also spotted a penny inside a small bowl that the alien holds in front of its bare chest. Someone probably thought it was cute to put it there. But its zinc-copper alloy is a potential source of corrosion.

“You have to take it out,” DePaolis said. “It actually damages the patina.”

Cleaning the sculptures is vital to ensure they have a long life.

Last year, Barbara Hepworth’s “Summer Dance” — two rectangular, white-painted bronze slabs with circles carved into them — got marked up by a kid’s blue crayon.

When things like this happen, the Midwest Art Conservation Center steps in. It has a regular working relationship with the arboretum, visiting once or twice a year for ongoing maintenance.

“Weather really takes a toll,” said Megan Emery, the center’s chief conservator. “Metals are susceptible to corrosion, developing in damp, humid, wet environments. It’s also important to maintain protective wax coating on some metals, such as bronze.”

Bird guano is another hazard. Because of its acidity, it can corrode the surface of metal pieces, especially bronze works.

Sometimes, conservators need to repaint, as they did with Louise Nevelson’s “Night Gesture,” a welded-aluminum structure that was once black but now is gray. “It is common for painted outdoor sculpture to deteriorate at a faster rate,” Emery said.

That particular work is also popular with a turkey or a squirrel that has been chewing enthusiastically at its base. With the help of the Nevelson Foundation and the Getty Conservation Institute, which is working on long-term paint solutions for Nevelson’s works, such naturally inflicted damage is being addressed. During the repainting process, areas are filled in but no metal is added.

The volunteers happily sprayed away at the hulking, geometric sculpture. Between scrubs and spritzes of water, another surprise popped up — a dry and graying empty wasp nest that had been hidden in a nook of the piece.

“We knocked out old ones, and now they are starting to form again,” Lemke said. “If we get rid of the wasp nests now, we don’t have to contend with them when guests start coming.”