BRUSSELS — The man in the respirator mask sits in the window, impassive on his chair, breathing as evenly as he can into the aluminum foil balloon he cradles on his knees. After six hours he has filled more than 20 of them, piling them up behind him like a cloud.
“In the end they’ll be about 100,” said Will Lunn, the director of the London-based Copperfield gallery, explaining “Exhaust,” a marathon performance piece by the British conceptual artist David Rickard, whom he represents. Over 24 hours, Mr. Rickard turns the air a human being requires for one day into an enormous shimmering sculpture.
First seen at the Goethe-Institut in London in 2008, this demanding work (the artist isn’t allowed to eat or drink, and the carbon dioxide levels in his blood must be regularly checked) was the storefront display at the second annual edition of A Performance Affair, a fair exclusively devoted to the sale of performance art.
The four-day fair, which finished Sunday, occupied two floors of the Vanderborght Building, an Art Deco former department store, and brought together more than 30 artists hoping to attract the attention of Belgium’s famously discerning and risk-taking contemporary art collectors.
“Eight years ago, we’d just sell limited-edition photographs of the performance,” Mr. Lunn said. “At this fair you can acquire the performance.”
A one-off enactment of “Exhaust,” complete with documentation and the performance’s residue of balloons and breathing equipment, is priced at 10,000 euros, about $11,000, he said.
Performance is the medium of the moment in the art world. In May, Lithuania’s ecologically aware indoor beach opera, “Sun & Sea (Marina),” won the Golden Lion award at the Venice Biennale, making it the second performance in a row to win the top prize for a national pavilion.
The previous edition’s winner, Anne Imhof, who represented Germany, staged a performance at Tate Modern in London in March. Tate has 17 performances in its collection, with “several more currently in the process of being acquired,” said Duncan Holden, the head of the galleries’ press and communications department.
But while museums have been embracing performance art, the investment-minded commercial art world has been slower to get on board. There is one obvious reason.
“It’s ephemeral,” said Will Kerr, a co-founder of the nonprofit A Performance Affair. “You see performance all over the place,” he said, but in the market it is the “weakest link.”
“Dealers just use performance as a hook to sell the work of other artists,” Mr. Kerr said. “It’s seen as entertainment. Take an Instagram pic, then walk away. The model is not mature.”
Sotheby’s, Christie’s and Phillips have yet to sell a single live performance artwork, according to the auction houses’ press offices. They, and potential buyers of such pieces, understandably ask themselves: What, exactly, is being sold?
In an effort to standardize the answer to that question, the A.P.A. fair, in collaboration with Chantal Pontbriand, a Montreal-based curator and writer, has drafted a protocol that addresses issues such as a performance’s duration, how it can be recreated and what materials are left with the owner once it is over.
The organizers hope to devise a protocol that will be widely adopted by artists and galleries. Without a certificate clarifying such practical details and their legal ramifications, performance art will remain a hard sell — and re-sell.
“The acquisition of a real performance is something new,” said Frédéric de Goldschmidt, a Brussels-based collector who is a member of the A.P.A. selection committee. “You buy something immaterial.”
In an interview, Mr. de Goldschmidt recalled that in 2012 he paid about €2,500 for a performance by the French artist Philippe Parreno. Mr. Parreno’s piece, titled “Transubstantiation” and involving the artist preparing some of his deceased mother’s secret recipes, was never activated.
Mr. de Goldschmidt asked Phillips if it might auction the performance, but the piece lacked any documents.
“There was no duration, no practical details. There were a lot of ambiguities,” said Mr. de Goldschmidt, who in the end kept the piece.
In the case of “The Banquet,” a new performance by Ariane Loze, a young Brussels-based artist, a buyer does at least acquire a set of 12 printed scripts. Available in four European languages, each contains the lines from a 2016 video of an uptight middle-class dinner party in which Ms. Loze plays a dozen characters.
Bon mots such as “We all have to find a way of defining ourselves” and “Take time to be lonely and enjoy it” can then be woven into a collector’s dinner party. The limited-edition scripts cost €495 per set.
Evann Siebens, a Vancouver-based former ballet dancer, on the other hand, has created an archive of gestures that references the history of performance art from Allan Kaprow through Gilbert & George to Marina Abramovic.
For €1,000, Ms. Siebens will recreate a gesture photographically and as a performance, which will then be documented on video and preserved on a memory stick in a presentation box. In this case, the buyer gets plenty of material for the money.
Without established performance stars like Ms. Abramovic or Tino Sehgal, or an established market, sales were always going to be a rarity. But the Brussels-based collector Tobias Arndt said he was interested in buying a version of Ms. Loze’s “The Banquet.”
For Mr. Arndt, the market for performance art will continue to be limited. But it will, in his view, appeal to collectors as a relatively inexpensive way to be part of today’s “experience” art economy.
For collectors, performance is “the next step,” Mr. Arndt said.
“It has the potential to be an event,” he added. “Performance is a direct aesthetic experience, and then you can share it on social media. It’s not just about decorating your house with good art. It’s also about doing crazy things on Instagram.”
In the street outside the Vanderborght Building, John Yee, a passing business operations manager from San Francisco, was mesmerized by the sight of Mr. Rickard breathing into his latest foil balloon. It was 9.30 p.m. by that time, and the artist had inflated more than 40 of them.
“I don’t know if I understand it, but it’s cool,” Mr. Yee said, adding, “It feels very European.”
Interest piqued, he then walked into the fair.