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.