“I’m working on a very large rock,” said Nicole Grabow, squinting intently at a sparkling boulder of honey calcite in a laboratory at the Midwest Art Conservation Center (MACC).

With a tweezer she carefully lifted a long, thin crystal from a tray and gently nudged it into a narrow channel in the rock, listening intently for it to click into place. With a microscope at hand and the rock’s composition confirmed by microchemical test, Grabow deftly fastened the fragment in place with an acrylic resin.

The broken “rock” was part of a privately owned sculpture damaged in shipping.

“There’s a lot of chemistry in what we do so it’s important that we all have training in organic chemistry, studio arts and art history — the three-legged stool of conservation,” Grabow said of her colleagues at MACC, located in a remote corner of the Minneapolis Institute of Arts.

One of only four nonprofit conservation labs in the country, MACC cleans and restores art of all kinds along with frames, documents, textiles and other objects. Its nine staff members also advise museums about the care and preservation of their collections, and work with first responders in disasters.

“We have always told organizations to have disaster-response plans, because at any time they might have to deal with a water leak, flood or fire, but the whole world became aware of these issues after 9/11 and the recent hurricanes,” said executive director Colin Turner.

While the organization helped museums after Hurricane Sandy and recent Midwestern floods, most of its work is more routine. Projects this spring included repairing cracks and tears in a painting of a founding father of Brainerd, Minn.; removing faint stains from an abstract screenprint by Ellsworth Kelly; re­attaching loose paint in a picture of a Boston oyster house; cleaning 200 years of yellow grime from an English image of a German river town, and patching the broken finger of a white marble maiden.

Surgical caution

Like doctors with fragile patients, art conservators must first of all do no harm.

All of their treatments must be reversible and apparent to other keen-eyed professionals, though not necessarily noticed by casual viewers. Before proceeding, they test ink and paint for permanence, take X-rays, analyze fiber composition and then devise restoration plans specific to each object.

More laboratory than art studio, their offices are equipped with huge tables, elaborate air-filtration systems, sinks with ionized water, freezers, microscopes, and trays of solvents and brushes. They even make their own wooden-stemmed cotton swabs for removing grime from paintings.

Modern art often poses the most challenges because it frequently incorporates unconventional materials. The late German conceptual artist Joseph Beuys, for example, included a can of honey in a sculpture that Walker Art Center owns. The honey rusted the can from the inside out. What to do?

“We worked with MACC to make a very discreet hole so we could drain the honey out,” said Joe King, the Walker’s registrar. “We kept the honey in case we need it sometime in the future if someone decides removing it was a big mistake.”

The conservation honchos also advised the Walker on slowing the inevitable deterioration of PVC, a type of plastic once favored by artists. As it ages and loses its plasticity, the material becomes stiff, brittle and strangely damp.

“It’s called Weeping Barbie Syndrome,” because it affects Barbie dolls made in the 1950s, said King. “As the material migrates to the surface, the dolls look like they’ve been weeping. We’re working with MACC to slow that process. Fortunately we don’t have too many works made with PVC in the collection, maybe 10.”

The Minnesota Marine Art Museum in Winona recently hired MACC to clean four 19th- and early 20th-century paintings — including a J.F. Kensett image of Lake Pepin — and two linen military standards that were raggedy from being hidden in a tavern barrel in 1918.

Sometimes conservators turn up unexpected details. Last year the Marine Museum had a Van Gogh painting examined by a former conservator from the Van Gogh Museum in Amsterdam. She found evidence that Van Gogh had used a perspective device that he’d sketched in a letter to his brother, Theo, and that “there is sand in the paint from the beach from which he painted,” said Jon Swanson, curator at the Marine Museum, where the picture is now on loan.

Spring-cleaning the pictures

Cleaning art is MACC’s bread-and-butter work. Founded in 1977, it earns about two-thirds of its $1 million annual budget restoring art for public institutions and private collectors, and gets the remainder from grants and donations.

Over the past year, it freshened up about 24 pictures for the Minneapolis Institute of Arts, which has launched an Adopt-a-Painting program for fans to pay for cleanings (typical cost: $1,400 to $7,000). Assistant curator Erika Holmquist-Wall prepared a “look book” that allowed enthusiasts to shop for projects. Supporters get to visit the conservation lab and observe the work, plus they have their names attached to the picture for five years.

“I got hooked,” said Al Naylor, a retired banker who, with his wife, Dena, committed to restoring 16 paintings. They include a 19th-century French picture of cows, an Alpine vista, an English landscape and a couple of Dutch scenes.

“Not everyone can afford to buy $3 million paintings for the museum,” Naylor said, “but some of these things are $250 or $1,800 to restore, so theoretically this activity is within the capability of more people.”

One of the most dramatic transformations was “Portrait of Lady Lucy Percy,” a wealthy English countess painted about 1639 by Dutch artist Adriaen Hanneman. In storage since the 1940s, the work “was so filthy it was embarrassing,” said painting curator Patrick Noon. Beneath the guck and discolored varnish, however, was a rose-bedecked court beauty with a peachy complexion and cunning eyes. She now sparkles in Gallery 313.

“It’s been fun to go into storage and find these treasures that really just require a bit of fixing,” said Holmquist-Wall.