A rich perfume wafts through the sixth floor of the Museum of Modern Art, where the installation of Ed Ruscha’s full-dress survey “Now Then” is underway. You sense it before you see it: a room where the white walls are turning velvety brown. A chocolate room.

The McPherson family, dressed in their La Paloma Fine Arts company T-shirts, bustle around their rig. Edan McPherson dips a long squeegee into a pool of melted chocolate, draws the rubber blade across the coarse mesh. His son, Daniel, whisks the prints away, while his sister Robyn feeds fresh paper. His wife, Lynda, and daughter, Kayla, monitor double boilers of chocolate in reserve. The drying racks fill up, two tenths of a pound of dark chocolate coating each sheet. When the chocolate sets, they’ll trim and hang each print, floor to ceiling, like shingles on a Craftsman house.

“Chocolate Room” is an oddity in Ruscha’s influential oeuvre. Of the 85-year-old Nebraska native’s hundreds of projects — paintings, prints, and photo books; dry eulogies of Americana like SPAM cans and Mobil stations and two-lane blacktop — “Chocolate Room” is his only installation. It’s been shown just seven times since its creation in 1970, and never before in New York.

Yet for all that, “Chocolate Room” is a remarkable distillation of Ruscha’s sensibility: chalky humor, sweet gumption, American bleakness, an existentialism that rests on objects of pop culture, like common chocolate.

Christophe Cherix, MoMA’s chief curator of prints and drawings, called it “almost mythical. You read about it, you hear about the insects, the smell.”

“Chocolate Room” might have remained a legend, but in 1995 the curators Ann Goldstein and Anne Rorimer featured it in a survey of conceptual art at the Museum of Contemporary Art in Los Angeles. The museum acquired “Chocolate Room” in 2003.

“I do think we surprised Ed with our proposal to remake the work,” Goldstein, now the deputy director of modern and contemporary art at the Art Institute in Chicago, said over email. They saw the piece as “a union of painting and conceptual art.”

Cherix agrees. In the MoMA show’s floor plan, “Chocolate Room” is crucial, connecting Ruscha’s 1960s pop-art paintings and conceptual books to his prints and drawings in unusual materials like gunpowder and tobacco. Like taste, “Language comes from the mouth,” pointed out Michael Govan, director of Los Angeles County Museum of Art, where the show travels next and where “Chocolate Room” will be remade again. He said “Chocolate Room” was the first piece he and Cherix chose.

For his part, Ruscha seems charmed by the resonance of that wholesome, elegant gesture — wallpaper a room with chocolate. “It’s locked into itself,” he told me. “I’m not sure where it took me, I’m not sure I learned anything from it. It’s just, you know, what it is.”

At the end of March, while La Paloma were still fine-tuning the production, I met Ruscha in MoMA’s Drawings and Prints Study Center. Museum staff had set out examples of his work — including a spare leaf from the original “Chocolate Room,” tan with age. The cornflower stripes on his western-style shirt set off his irises.

In 1970, the curator Henry Hopkins invited 47 artists to the United States Pavilion for the 35th Venice Biennale. More than half withdrew in protest of the Vietnam War. Hopkins set a room aside for rotating printmaking projects, and Ruscha’s was first.

The way he tells it, “Chocolate Room” was “very much ad hoc. I was on the plane to Venice, and I said, good Lord, what am I going do?” Ruscha is the modest kind of genius.

“I was a little bit tired of making conventional pictures,” he said, “and so I thought I would use unconventional materials.” The previous year he’d made the “Stains” portfolio — paper daubed with everything from Los Angeles tap water to the artist’s blood. In London, he’d been working on “News, Mews, Pews, Brews, Stews & Dues,” six words in gothic font printed with organic substances like axel grease and pie filling. (These series bracket “Chocolate Room” at MoMA.) The background of “Pews” was a mix of coffee and Hershey’s syrup.

“Chocolate has a way of laying itself out, almost like an ink,” Ruscha told me. “And I thought, well, I’ll do the same thing, and I’ll avoid making any pictorial statement and make what amount to shingles on a wall.”

So Ruscha and the dealer Brooke Alexander gathered up every tube of Nestlé chocolate paste they could find. The master printer William Weege was stationed in the Pavilion, and he and Ruscha ran the syrup through the silk-screen press, onto deluxe, handmade Fabriano paper. They trimmed the edges and tacked up the sheets four high.

“Chocolate Room” was playful and abstract, evocative in the way of scents — and slightly brutal, a fragrant blank. Ants marched in. Visitors drew peace signs and antiwar slogans with their fingers.

“I remember not being too insulted by it,” said Ruscha. “I’d prefer that nobody graffiti that thing here,” at MoMA, he added, “but, you know, it’s possible.” Not to mistake his ease for apathy, but Ruscha seems to embrace contingencies. Certain parameters are fixed, others wild. “It’s like a quiet fun house,” he said.

Of the MoMA version, Cherix said: “We want to preserve the presentation as much as we can.” But when I talked to Ed about that, he couldn’t care less.”

Cherix laughed. “He said, well, the work changes, chocolate changes color, that’s what it is.”

Indeed.

“I don’t try to replicate what’s been done before,” Ruscha said. “I don’t stand up there with a color sample and demand accuracy.”

For accuracy, there’s La Paloma. “They’re fabricators extraordinaire,” said Ruscha.

While Ruscha was in Venice cornering the market on Nestlé syrup, Ron McPherson, Edan’s father, was working the press at the storied Los Angeles print shop Gemini G.E.L. That’s where he first met Ruscha. When Ron started La Paloma, shifting to fiberglass and metal fabrication, they continued working together — in 1985, for instance, his company built a set of giant curved stretchers for Ruscha’s mural in the Miami-Dade Public Library rotunda. When the call came in 1995 to revive “Chocolate Room,” Ron was his man.

In late March, I visited La Paloma’s vast workshop on the fringe of the San Fernando Valley. Ron and his team, now led by his son, Edan, have installed every “Chocolate Room” but the first: “Los Angeles twice, Palm Springs, Anchorage, Reno, Oklahoma City.” The New York install is different. Ruscha’s survey is an occasion to dial in the “Chocolate Room” recipe for the ages.

To get the shingles to hang straight, they chose a coated paper. They experimented with different chocolates, too. “We wanted to stick with a commercial mix,” Edan said. “Something you could get consistently.”

They’d always used regular Hershey’s bars before. So long as the brand uses pure ingredients, it’s a matter of color. They’d winnowed it to three — Hershey’s regular, a dark Ghirardelli’s and a darker Callebaut — and sent samples to Ruscha’s studio. He chose the Callebaut. MoMA’s team matched it with a touch-up paint, to fill in around the walls’ edges: Benjamin Moore Arroyo Red.

Most of all, La Paloma wanted to prevent the powdery, fatty bloom that sometimes, not always, covers the chocolate’s surface. The trick, as chocolatiers know, is to temper the chocolate, and keep it tempered as it’s shaped.

The double boilers they’d been using to melt the Hershey’s bars were inexact. Now they travel with the ChocoVision Revolation Delta, an electronically regulated, air-heated, self-stirring metal bowl that puts chocolate through its paces, up to 115 degrees or so, then back down to 86. Then, explained Edan, “you add a little bit of unmelted chocolate, and that tempers it.” It’s called “seeding”: “The seeding tells the cocoa butter molecules how to realign correctly inside the chocolate.”

“We printed and then everything looked fine,” Edan added. They hung up some test sheets at LACMA. “But then several days later it started to bloom.”

They realized the light bulbs they’d rigged over the press to keep the chocolate flowing were a little too hot, and it was separating again. Now they use controllable heat lamps and a voltage regulator. The crew monitors the chocolate ink with an infrared thermometer. During printing, the temperature is kept around 95 degrees.

All this precision — but the result, almost by design, remains capricious. Every installation is different. MoMA is the first version using dark chocolate, which Ruscha said in an email, “was a visual choice and was more appealing.” A few of the corners went on thicker than the rest, and look slightly toasted. The exhibition team sent photos to Ruscha. The artist approved.