The conservation laboratory at New York’s Museum of Modern Art (MoMA) is a brightly lit sanctuary where 20th-century masterpieces are brought for cleaning, restoration and, in case of damage, repairs. Although the operations performed here are painstakingly slow, the place looks more like a hospital emergency room than a painter’s studio. There are X-ray machines for diagnosis, and steel carts stocked with cotton swabs, scalpels and needles. For the conservators at work here, watercolors and paintbrushes are instruments of last resort.

Michael Duffy is highly trained in the techniques of his trade, and he’s had experience as a studio artist himself, but he would have to be a Picasso to know exactly what to do with the work at hand. Actually, he is standing in front of Picasso’s 1907 Les Demoiselles d’Avignon, the shocking canvas (called by Picasso biographer Patrick O’Brian the “anarchist bomb tossed into Western European painting”) that launched the stylistic revolution known as Cubism. Now almost 100 years old and one of the icons of MoMA’s collection, the nearly eight-foot-square painting depicts five imposing pink-skinned nudes whose figures owe as much to geometry as anatomy and whose faces recallAfrican masks and ancient Iberian sculpture. When the museum’s conservators and curators decided the painting needed restoration, Duffy got the nod.

Much of the work was done at MoMA’s temporary home, a converted Swingline stapler factory in Queens, while its main space in Midtown Manhattan was undergoing an expansion. The restored Les Demoiselles will be unveiled at the grand reopening of the newly renovated MoMA, designed by Tokyo-based architect Yoshio Taniguchi, on November 20.

Duffy, 43, is an avid bird-watcher—a pursuit that demands the kind of intense observation he employs in the lab. He says the technical aspects of the work, such as varnish removal, are quite straightforward. What’s trickier is bringing a painting back to an artist’s original intentions, which can mean undoing what other restorers have done in the past. In this case, when chief conservator James Coddington joins Duffy one morning this past winter, the conversation turns to what Picasso might have done about a few vertical cracks that run through the standing figure on the right side of Les Demoiselles, most likely caused when Picasso rolled up the canvas not long after he completed it; it remained rolled up in his studio for years.

“The cracks were probably visible when Picasso still owned the work,” Duffy observes. “He even said he liked to see this kind of damage or aging because it imbued a work with a life of its own.” Coddington agrees, but says Picasso might have a different point of view today. “Did these damages look exactly like this when Picasso saw them?” he asks. “They may be more evident now. There’s more dirt in them, and various treatment materials have gotten down in there and darkened them.” They will have to decide, in dialogue with the museum’s curators, if they should fill in the cracks and mask them with watercolors, as previous restorers have done, or leave them as is.

“Our goal is to respect the artist’s intent,” says Duffy, “but at the same time to make it a visually coherent work of art and not make you ask, ‘Ooh, what is that? Is that damage?’ ” He points out where an earlier restorer had gone too far, covering up some of Picasso’s original paint as he masked some cracks with watercolors. “It’s a very tough call,” Coddington explains, “as to what should and shouldn’t be retouched.” French artist Edgar Degas was known to rant about any attempts to restore old-master works in the Louvre and refused to let the museum have his own work. “He was infuriated by the fact that the Giorgiones, the Rembrandts, the Watteaus had fallen into the hands of pedantic functionaries,” his friend historian Daniel Halévy recalled, quoting Degas: “Touch a Rembrandt, does one know what one is touching? Does one know how it’s made? It is a mystery.” Picasso could be even more acerbic. In his 1983 essay “Crimes Against the Cubists,” biographer John Richardson said Picasso had “a healthy horror of varnish and virtually all forms of restoration.” If a painting developed serious damage, his attitude was “Too bad!”

As the conservators analyzed Les Demoiselles with X-rays, infrared light, even microscopic examination of a tiny sample of paint taken from the edge of a crack, they found the painting in remarkably good shape.“Picasso’s materials have really held up,” says Duffy, “and it’s just a matter of getting rid of the old restoration materials, which brings back the original beauty of the paint.” Coddington says their glimpses into the substructure of the painting showed that in terms of technique, it was “executed in the most classical manner.”

It’s what happened to the painting after Picasso finished it that conservators must grapple with now. At the urging of his Surrealist contemporary André Breton, Picasso sold Les Demoiselles to the collector Jacques Doucet in 1924. Doucet had the painting “lined” (a reinforcing canvas was glued and pressed onto its back) before it was restretched and framed. Some of the glue affected the paint, causing small blisters in places. MoMAbought the painting in 1939, and it has been restored on several occasions since. In 1950 it was retouched and varnished with a synthetic resin. In 1963 it was infused with a wax resin adhesive, which was supposed to strengthen the lining and protect the painting from changes in humidity and temperature. But the wax seeped through the canvas, and the excess had to be removed from the painting’s surface, leaving waxy residues. These were all, as John Richardson viewed them, “crimes against the Cubists.”

Picasso, Braque and their followers chose to use flat, or matte, paints to break with the sense of illusion in 19th-century painting traditionally enhanced by varnish. “Instead of using eye-fooling devices to make things recede as far as possible from the onlooker,” Richardson wrote, “the Cubists were out to bring things as far as possible back within reach: they wanted to make the picture surface the equivalent of reality, not a representation of it.” Coddington points to a glossy area of the canvas where varnish has not yet been removed. “Once that varnish comes off you’ll see that some of Picasso’s paint is a little glossier, other parts are more matte,” he says. “The varnish diminishes those differences, and they are not trivial, they are very much part of Picasso’s intention. It’s a painterly quality, but it also differentiates flesh and background. Those differences are often subtle, but they are ultimately where the thrill and life of the picture reside.”

It was the conservators who first suggested, in 2000, that some of that thrill was gone. They had been removing varnish from other paintings of the same era, and their eyes were sensitized to the condition of Les Demoiselles. One of Picasso’s small, preliminary oil sketches, which had never been varnished, offered a guide to what the work should look like, as did some other paintings he’d made around the same time.

It would take months, and infinite patience, as Duffy dampened one swab after another with solvent and rolled it over a bit of varnish, not scrubbing but letting the solvent work, then wicking the varnish off into the swab. This time, the varnish will stay off. “If dirt and grime should fall on the painting, as it undoubtedly will,” says Coddington, “a surface cleaning to remove it will pose no risk whatsoever.” I ask what kind of solvent they use to remove dirt. “A mild enzymatic solution,” Duffy answers. “That’s the term we use.”

Coddington laughs. “Which we take straight from our mouths,” he says. “Spit cleaning.”

Even after working so intimately with Les Demoiselles, the two conservators still seem a bit stunned by the painting. Coddington is especially struck by Picasso’s defiantly modern, unpainterly attack—smudges he didn’t bother to paint over, brushstrokes he literally x-ed out and left that way. For Duffy, who has restored other Picassos, working on this painting is very different. “There’s something about it that gives you a jolt every time you get near it,” he says. “When you get up close you sort of lose yourself in the way the paint is applied, but when you step back you say, ‘Wow! Look at this painting I’m next to!’ It’s always a shock.”