Every major American art museum is sitting on assets that, from the outside, look enviable. They’re called works of art. If they’re by Vincent van Gogh or Frida Kahlo or Jackson Pollock, they may be worth tens, or even hundreds, of millions of dollars.

Even if they’re by less famous artists and consigned to storage — along with perhaps 90 percent of any given museum’s collection — they can still be valued at eye-watering amounts. Set beside, say, a scary budget deficit or the prospect of having to lay off employees, this knowledge can take on an almost voluptuous glow.

To counter the constant temptation to regard art works as a way to get quick cash, the museum world heavily polices the sale of works from permanent collections — otherwise known as deaccessioning. The powerful Association of Art Museum Directors, made up of directors of museums in the United States, Mexico and Canada, has long frowned on any museum that sells off art for purposes other than acquiring new art.

AAMD’s frowns have an effect. Museums that dare to ignore its guidelines — as the Berkshire Museum in Pittsfield, Mass., did in 2018, ultimately selling more than 20 works from its collection to raise money for a renovation — are censured, sanctioned and publicly shamed. For a renegade — or perhaps simply desperate — museum director, a decision to sell works from the collection, even if it’s to raise money deemed necessary for survival, might mean career death.

However, in an unprecedented move, and as a direct result of the coronavirus pandemic, the AAMD has recently relaxed its guidelines. It’s too soon to gauge the effect, but it is already big news in the art world. Once unthinkable, the notion of selling off a Claude Monet or two to plug a budgetary hole — or to fend off a total financial meltdown — is suddenly something to contemplate.

The only problem, of course, is that once you’ve sold a Monet, or a Norman Rockwell, or an Albert Bierstadt, it’s very hard to get it back.

“There are many members of the association who think that this is way too much, and others who think it’s way too little,” Brent Benjamin, the AAMD president and the director of the Saint Louis Art Museum, told the Art Newspaper.

No one doubts that they’re a sign of how bad the picture is looking for museums now.

Since mid-March, when museums began closing because of the coronavirus outbreak, income from admissions and retail has evaporated. Turmoil in financial markets has caused endowments to plummet. Fundraising has been severely constrained. And for many museums, it has quickly become a question of figuring out how to survive.

The revised guidelines touch on two areas. First, they state that any museum that “decides to use restricted endowment funds, trusts, or donations for general operating expenses” will not be censured or sanctioned. The idea here is to allow museums the financial flexibility they need in an uncertain economic environment.

The more striking announcement concerns deaccessioning. According to AAMD, museums may now “use the proceeds from deaccessioned works of art … to support the direct care” of their collection.

The language here is careful. But there’s no doubt: This represents a major departure, and a recognition that many art museums are in financial free fall.

AAMD says it recognizes “the extensive negative effects of the current crisis on the operations and balance sheets of many art museums.” It acknowledges, too, the impossibility of knowing when revenue streams might return to normal.

The new guidelines are temporary, and are “not intended to incentivize … the sale of art.” But their effect may do just that.

Conscious of the significance of its about-face, AAMD has tried to look as though it is still walking calmly in the same direction. The organization’s “long-standing principle” — that money raised from the sale of art from a permanent collection may not be used for general operating expenses — remains in place, according to the announcement in mid-April. Museums that choose to deaccession must maintain records of how money was raised and how it was used. They must have in place “a board-approved policy outlining what expenses it considers direct care, and the policy must be publicly accessible.”

All this caution and qualification make sense. But in reality, almost any museum expense — from the salaries of curators, conservators and other staff members to costs associated with building maintenance and utilities — could conceivably fit the description of “direct care of the museum’s collection.”

And let’s be honest: It’s very theoretical. At a practical level — especially during a crisis — money inside big institutions such as art museums is fairly fungible. If a work of art is sold, the money raised is difficult to silo.

In normal times, and despite the strict guidelines, deaccessioning goes on all the time. The results can be transformative, restorative or alarming — sometimes all three at once. In 2011, for instance, the Museum of Fine Arts in Boston sold eight works from its permanent collection to raise the money it needed to buy a single painting, “Man at His Bath,” by Gustave Caillebotte. The paintings it auctioned off included canvases by Monet, Paul Gauguin, Alfred Sisley, Camille Pissarro and Pierre-Auguste Renoir.

At the time, it seemed an extraordinary exchange. But after taking into account that the MFA has one of the world’s greatest Impressionist collections; that most of the works it chose to sell tended to languish in storage; and that paintings by Caillebotte — an artist regarded as increasingly important — rarely come on the market, the decision began to look rational (if still controversial).

More recently, the Baltimore Museum of Art selected work by white men whose art they own in abundance — such as Andy Warhol and Franz Kline — to raise money for the acquisition of new work by underrepresented groups, especially women and African Americans, including Amy Sherald, Charles Gaines, Faith Ringgold and Lynette Yiadom-Boakye.

Here, too, the thinking was sound, and only mildly controversial. “I don’t think it’s reasonable or appropriate for a museum like the BMA to speak to a city that is 64 percent black unless we reflect our constituents,” Chris Bedford, the museum’s director, told Artnet in 2018.

And yet the practicalities of deaccessioning are rarely straightforward.

To begin with, there is a tension inherent in any decision by a museum to raise money by selling art. To justify selling the works, museums must play down their importance. But to get top dollar at auction, they must talk up their value.

This awkwardness can be managed. But the assumption that there are endless riches in a museum’s storage rooms just waiting to be tapped is unrealistic.

In 2018, Michael O’Hare, a professor of public policy at the University of California at Berkeley, argued in the San Francisco Chronicle that the Art Institute of Chicago could offer free admission in perpetuity by selling 1 percent (by value) of its collection and putting the resulting money in an endowment. The interest on the endowment alone, O’Hare said, would cover the cost.

His argument rested on dubious premises. Attempts to assign credible value to the works in a museum’s permanent collection are notoriously fraught. The vast majority of the financial value of a museum’s collection often resides in a very limited number of works, almost all of them on permanent display. So if a museum wants to avoid selling its most precious works, selling off 1 percent by value probably would mean selling vast amounts of art.

Logistically, that would be challenging, as Martin Gammon, the founder of Pergamon Art Group, pointed out in a 2018 response to O’Hare in the Art Newspaper. The museum probably would face lawsuits from donors who gave works with conditions attached. The market, too, may not cooperate with such a scheme.

Museums, after all, guarantee scarcity, because when they own works, it’s understood that they won’t come back on the market. This is a crucial factor in keeping prices high. Once undone, such a reliable assumption could trigger a market collapse. The bad publicity generated by a prestigious museum’s desperate-looking fire sale also would be likely to affect prices.

Then there are the administrative costs. Deaccessioning involves time-consuming research. Museums that want to sell works have an obligation to look into restrictions and secure necessary approval from donors and their heirs. There also are costs involved in selling at auction.

So the potential gains from any kind of large-scale deaccessioning are almost invariably less than simple market valuations may suggest.

What museums need now is a combination of direct help and the flexibility to act in their own best interests. The new guidelines are an effort to provide the latter. But I believe the old principles remain sound. Even in these unprecedented circumstances, museum directors should not panic or be drawn into overly short-term thinking.

The choices they face aren’t simple. In some cases, they may have to act to ensure their very survival (if they fail at that existential level, their entire collections may hit the market). But they have been entrusted with the care of things that are, collectively as well as individually, of profound and lasting importance. It is their job to safeguard these collections for the future, not to sift them with a view to finding parts of them wanting, expendable and convertible to cash.