One famous designer chair is oozing goop. Another has exploded into puffs of foam. A bookcase’s shelves bubbled as gases formed within.

The culprits? Plastic. And time.

Preserving and restoring furniture from bygone eras has been a skill treasured for centuries among designers, curators and collectors alike. Every day, armies of experts are fanning out to period rooms and homes, to stabilize delicate ebony and tortoiseshell inlays and flecks of gilding on furniture made before World War II. The profusions of modern plastics, however, have created repair challenges unlike any known before.

Some of the problematic midcentury plastics used in furniture were formulated for military use. The domestic goods created from these polymers were marketed as versatile, affordable and easy to clean. Now, several of the more experimental objects are falling into mysterious decay.

Collectors and scientists have started investigating how to stave off further damage and extend the life spans of endangered pieces designed by important innovators. In some cases, it turns out, the best solution is to maintain serenity and accept the materials’ innate fragility, inevitable decline and weird odors.

“Le plastique, it’s fantastique, but it’s toxique,” said Benoist F. Drut, owner of Maison Gerard, a furniture gallery in New York. He occasionally deals in objects that hardly anyone knows how to fix, including 1960s inflatable PVC armchairs that can lose their luster when exposed to the bodies of people wearing sunscreen and develop holes along their folds when deflated.

Adventurous mid-20th-century designers and manufacturers set out to test the limits of new plastics, and part of the fun was that no one knew how well the materials would age. The makers came up with unprecedented forms, too, just to see what would happen.

Why not, as Italian designer Gaetano Pesce proposed in the 1960s, mold polyurethane foam lounge chairs into the shapes of the colossal marble feet on Michelangelo’s David?

Dr. Al Eiber, a retired physician in Miami, acquired a Pesce foot in the 1970s and mournfully threw away its ruined remains two decades later. He and his wife, Kim Kovel, came home from a short trip to find that its filling had inexplicably burst through its pinkish outer layer.

“It was like a nuclear explosion in our living room — foam had ripped through the skin,” Eiber said. The toes looked cancerous, and “the whole top of it, just boom!” He tried to donate its components to a museum for autopsy, he said, “but no one was interested.” He also owns a resin and Styrofoam bookcase by Pesce, which bulged and warped as gases formed in its depths. He drilled a hole to release the pressure, and since then the shelves have supported books well. “Except for that slight irregularity, it’s been great,” Eiber said.

In the last decade, scientific studies have been conducted on timeworn plastic to determine how to identify ingredients and cope with decay. Marc Mineray, a design historian and dealer who owns Galerie 47 in Paris, said specialists had learned to protect and repair the transparent inflatable seats that have been for sale at Maison Gerard. They were designed in the 1960s by the Paris-based Vietnamese inventor Quasar Khanh.

When the Khanh chairs are deflated, they must be wrapped in sheets to shield the sharp wrinkled edges from breakage. If the surfaces end up perforated, patches can be cannibalized from other works by Khanh that are deemed unsalvageable. “You have to sacrifice one to repair the other,” Mineray said. If the PVC becomes abraded or discolored, mild soap can sometimes undo the damage, he added, but there is “no miracle to hope for.”

Prospects may also be gloomy for biodegradable works that Berlin-based artist Jerszy Seymour made in 2007. He splashed blobs of a thermoplastic polyester, tinted in bright pinks and yellows, across sand blocks to form a suite of furniture.

It belongs to the Vitra Design Museum in Weil am Rhein in Germany.

Susanne Graner, the head of the museum’s collection and archive, said the pieces “were stable until last year, and then they started to drip.” A yellowish clear liquid with “a distinctive smell” puddled at their feet, she said.