When Ian McMahon’s “Sojourn” at T+H Gallery closes on April 22, the artist will destroy his work. He has to. It’s the only way he can get his giant sculpture out the door.

He spent 12 days building “Sojourn,” his latest large-scale, materially fragile, time-limited work. He sealed together seven industrial-size plastic bags and inflated them, then coated their insides with a thin layer of sprayed plaster. Once the plaster (1,500 pounds of it!) dried, he peeled the plastic away with a hot knife. The result – a massive, pillowy structure – nearly fills the gallery space. It skims the ceiling, and balloons over the iron bars that hold it up.

McMahon has said that he thinks of his works as moments in time rather than sculptures. Indeed, he turns traditional notions of sculpture, and particularly monumental sculpture, on their head. At more than 10 feet high and nearly 20 feet long, this work has the size of a public art piece that withstands weather and time passing, but “Sojourn” is no immobile object on a pedestal.

It breathes, shifts, and settles. Since it was built, one corner has lifted off the floor. The plaster has cracked, and an errant visitor – an unmonitored toddler, say – could cause irreparable damage. McMahon built a sealed door into one end; when you peer through a hole in it, you can see light filtering through the plaster. That’s how thin it is. Yet “Sojourn” is a behemoth straining against its scaffolding, a Pillsbury doughboy on steroids.

The gallery screens videos of the artist’s earlier works in another room: He takes down giant curtains of plaster with the quick swoop of a metal bar tethered to the ceiling. It’s thrilling to see something so imposing crumble so quickly. The idea that a big sculpture in a commercial gallery is too delicate to last prompts a similar thrill; it can’t be sold (T+H instead offers prints depicting “Sojourn”). The sculpture hums with the transient, unpredictable, hard-to-commodify energy of performance art.

Sadly, unlike McMahon’s destruction of his curtains, the end of “Sojourn” won’t be a public performance – space and safety considerations won’t allow it. All the more reason to see it before it goes.