On Thursday at Sotheby’s in New York, the Cleveland Museum of Art is putting 32 old-master paintings up for auction, and the J. Paul Getty Museum is offering 15. In the meantime the Pennsylvania Museum of Fine Arts and the Carnegie Museum of Art are selling five paintings each, and the Art Institute of Chicago is selling two Picassos, a Matisse and a Braque at Christie’s in London. Last week the New Jersey Historical Society sold 17 items at Christie’s in New York, including a 120-piece dinner service used to entertain President Martin Van Buren that went for $17,000.

A few years ago sales like these were likely to have gone unnoticed. Yet deaccessioning — the art world term for selling pieces from a museum’s collection — has become a dirty word and the focus of increasingly intense attention. Cultural institutions like the National Academy Museum and the Rose Art Museum at Brandeis University have generated controversy by selling or even considering selling items to cover operating costs, a practice forbidden by the professional association for art museum directors.

Allaert van Everdingen’s “Fishing Boats in a Harbor,” from the Cleveland Museum, is being sold. Credit Sotheby's

So even though all of the sales — with the exception of the historical society’s — are to be used to generate funds for future acquisitions, institutions that deaccession these days find themselves on the defensive. “Part of the normal biological clock of museums is to examine their collections,” said David Franklin, the director of the Cleveland Museum, which hopes to gain about $1 million from its sale. “We should be constantly refining and upgrading. I’ve given the message to all the curators that I regard deaccessioning as a normal act, and I encourage them to reassess the collections constantly.”

Some curators and museum directors worry that the problematic practice has been conflated with the routine culling of collections integral to a museum’s mission. Others protest that the entire profession is being tarred with the same brush, when the museums selling work to pay bills tend to be smaller institutions that are struggling to stay alive in an especially difficult economic climate.

In part the criticism surrounding deaccessioning has focused on whether museums have been open about selling work from their collections. The Indianapolis Museum of Art on its Web site, lists every object being sold, why it is being offered and the ultimate sale price or estimated value. The site has also begun to link newly acquired artworks to the sold objects that enabled their purchase. “Without doing all of those steps, museums expose themselves to ridicule and risk unnecessarily,” said Maxwell L. Anderson, the museum’s director. “There is nothing devious going on. They are just not handling it in a way that is candid and explained.”

The Metropolitan Museum of Art is required to list in its annual report the total cash proceeds from art sales each year and to itemize any deaccessioned objects valued at more than $50,000 each. It must also sell those pieces at auction and provide advance public notice of a work’s being sold if it has been on view in the last 10 years. These rules were imposed by the state attorney general in 1972, after an investigation into the museum’s sale of pieces from its modern art collection to help finance the purchase of Velázquez’s “Juan de Pareja.”

In the last three fiscal years the Met has sold close to $3.7 million worth of objects from its collection, including a statue of the Egyptian goddess Sekhmet, a first edition of Vitruvius’s “Ten Books on Architecture,” and Albert Bierstadt’s oil on canvas “Rocky Mountain Goats.” The museum used some of the proceeds from the Sekhmet to purchase other Egyptian pieces: a vizier statuette, a canopic jar and a sculpture of a king’s head; the Vitruvius, sold with 26 other works, enabled the purchase of 19 objects, 8 of them leaves from an unpublished Italian manuscript copy of the “Ten Books on Architecture”; the Bierstadt proceeds have yet to be used.

Adam D. Weinberg, the Whitney’s director, above, said selling pieces refines museum holdings. Credit Fred R. Conrad/The New York Times

“It’s sometimes portrayed in the press as if we’re trading houses, with objects going in and going out,” said the museum’s director, Thomas P. Campbell. “That’s not the case. It’s like a gardener pruning a tree over a long period of time. Deaccessioning is a healthy part of the management of any museum collection. We’re not playing the market.”

A 1910 gift from Mrs. Russell Sage that formed the basis of the Met’s American furniture holdings originally included 430 colonial pieces; now the cache numbers 190. “In the intervening years more than 50 percent” of that collection “has been deaccessioned for reasons of quality or condition,” Mr. Campbell said.

The Museum of Modern Art considers the review and revision of its collection part of its mandate. The museum’s first major gift, in 1931 — a group of works dominated by Cézannes from one of the museum’s founders, Lillie P. Bliss — came with the stipulation that the Modern should sell whatever it needed to buy works deemed more important (which it has). MoMA’s first director, Alfred H. Barr Jr., described the museum as “metabolic” in terms of its programs and the changing nature of its holdings. “We were predicated on an evolving collection,” said Glenn D. Lowry, the museum’s director. “We seek to acquire broadly when it comes to contemporary art and let the next generation or even the generation after that determine what is essential.”

“Many of the greatest works in our collection are the result of deaccessioning,” Mr. Lowry added, citing as examples Gerhard Richter’s Baader-Meinhof series, Andy Warhol’s Campbell’s soup cans, van Gogh’s “Postman Joseph Roulin” and Picasso’s “Demoiselles d’Avignon,” all of which were bought with funds raised by selling other artworks.

The policies of each institution vary on selling works from their collections. If a living donor to the Met objects to his gift’s being deaccessioned, the Met will honor that wish. The Modern takes the opposite tack, refusing any gift that prohibits a possible future sale. The Whitney Museum of American Art will not sell any work by a living artist because it could damage that artist’s career. But it will trade a living artist’s work for another piece by the same artist.

Richard Armstrong, the Guggenheim’s director, said “trading up” should not be the goal. Credit Chester Higgins Jr./The New York Times

The Met did that last year, exchanging Pat Steir’s oil on canvas “The Water Series: The Port Reflected at Night in the Waterfall” for her “Sixteen Waterfalls of Dreams, Memories and Sentiment.” Ms. Steir said it was her idea. “I wanted them to have a better painting than the one they had,” she said.

A few years ago the Whitney sold some “secondary” works by Marsden Hartley, said Adam D. Weinberg, the Whitney’s director, to get Hartley’s portrait of a wrestler, “Madawaska — Acadian Light-Heavy.”

“The Whitney has a great collection of Hartley,” Mr. Weinberg said. “One thing it did not have was a portrait.” He added that the Whitney did the same thing with a John Sloan. “One of the key reasons is to refine the collection, to make it a better, stronger example of a particular artist’s work,” Mr. Weinberg said. “At the end a museum collection is about quality.”

It was in part to protect this type of freedom that museums like the Whitney opposed temporary regulations from the New York State Board of Regents that imposed stricter guidelines in 2008 governing deaccessioning. “They did not allow for that philosophical, aesthetic decision that is fundamental to the curatorial exercise of judgments of quality and improving a collection,” Mr. Weinberg said. Under such regulations, he added, “we could not have deaccessioned Hartleys for a better Hartley.”

While collections “should be dynamic,” Richard Armstrong, director of the Solomon R. Guggenheim Museum, said, institutions should not be acquiring work with an eye toward trading up. “It’s not good to be thinking of the collection as a log-rolling exercise: ‘I’ll take two of that if you let me get one of those,’“ he said. “That’s just not the way to make progress as a collecting institution.”

At most museums items designated for sale go through a rigorous evaluation process, starting with curators and ending with approval by the full board. Such sales are often precipitated by new galleries or building additions. The Cleveland Museum’s coming sale, for example, was prompted in part by the reinstallation of its collection after a renovation. “If you’ve determined that a work is less significant or in poor condition or never been on view, it becomes superfluous,” said Mr. Franklin, the museum’s director, “and to store it becomes a luxury we can’t afford.”

Deaccessioning is “kind of a Humane Society,” he added. “Maybe some of these works can be loved by someone else.”