"The Son" by Edvard Munch

This photo provided by the Augsburg State Prosecutor's Office shows "The Son" by Edvard Munch. It is among 79 works now shown on the Lost Art website and among the approximately 1,400 works German authorities confiscated from the Munich residence of Cornelius Gurlitt, son of Hildebrand Gurlitt, an art dealer who worked for the Nazis. (Getty Images / November 22, 2013)

The outrage sparked by the clumsy handling of a Nazi-looted art trove in Munich, which was revealed this month, shows the urgent need for transparency in the art world, from museums to auction houses to private collections.

For years this rarefied world has functioned like a private club. Many institutions, especially in Europe, have kept their World War II-era provenance files discreetly locked away, or have even quietly accepted questionable art from moneyed donors. Dealers too have looked the other way — until a painting stolen during the Holocaust is suddenly, fortuitously spotted, and an auction block turns into a crime scene.

The Munich artworks — many of which were apparently either confiscated from Jewish families, bought for a fraction of their value or pulled off museum walls as "degenerate art" — are only the latest glaring example of the need for openness.

Authorities discovered the hoard in 2012 in the course of a tax investigation of Cornelius Gurlitt, 80, the son of art dealer Hildebrand Gurlitt, who amassed more than 1,400 artworks now estimated to be worth more than $1 billion.

For nearly two years state authorities treated the Gurlitt discovery as a mere tax case, not an urgent moral matter of Nazi-era restitution. The Bavarians, and later the national government, kept the discovery under wraps to assess the artworks and to determine whether Gurlitt committed any crimes. Their secrecy violates the spirit of an international nonbinding accord governing art looted during the Nazi era.

In 1998, Germany and 43 other countries, including the U.S., signed the Washington Principles agreeing to take steps to return art stolen by the Nazis. Among the steps is publicly identifying artworks that may have been confiscated during the Nazi period and creating a central registry so that claimants can seek restitution. The principles draw no distinction between state and individual collections.

When the Gurlitt story broke, German officials at first resisted calls to publish the list of works. After a world outcry, they hastily listed 25, then 79 and, as of this week, 118; at least 970 are being investigated as Nazi loot. They might well learn lessons from other European countries who have suffered public relations disasters over Nazi looted art.

Austrian state museums, after years of stonewalling and secrecy, have returned a dozen masterpieces by Gustav Klimt. Among them was "Portrait of Adele Bloch-Bauer II," which was successfully reclaimed in an eight-year battle by Los Angeles' Maria Altmann. (It later sold for a then-record $135 million to the Neue Galerie in New York.)

The Dutch also learned the hard way. In 2006 they returned 202 valuable works to the heir of Jacques Goudstikker, an Old Masters dealer whose works had been plundered by top Nazi Hermann Goering. and divided among Dutch museums after the war.

As the Gurlitt story made headlines, the Netherlands Museum Assn. coincidentally released the results of a four-year review of all the museums in the country — a list of 139 artworks of "dubious or definitely suspect origins." Works by Matisse, Klee and Kandinsky are on the list, in institutions such as Amsterdam's Stedelijk museum.

Still, very few countries have made the comprehensive effort the Dutch have. In Germany, the Bavarian state museum hasn't publicized its provenance information, though there are works in the collection known to have changed hands during the war. In Austria, state museums are bound by new restitution laws, but not private museums. And in the Netherlands, as the museum association representative told the Associated Press, the investigation wasn't exhaustive: More works of dubious origin, especially those held in private homes, are likely to surface.

Even when works are listed, restitution may be more a matter of honor than law – claims are fought; statutes of limitations have passed. There is also considerable legal leeway for buyers of stolen art in Europe. They can say they bought a painting in "good faith," without knowledge of the theft — a "don't ask, don't tell" approach that would seem to be undermined by the proliferation of experts and registries aimed at vetting Nazi-looted art.

Anonymity also lets collectors play hide-and-seek. The heirs of German Jewish art dealer Max Stern found three paintings that had belonged to Stern in a Cologne auction catalog this year. One seller returned a painting. Another seller withdrew the two remaining paintings, asking the auction house not to reveal his or her identity, Bloomberg reported.

The truth is this: Any work coming up for auction, offered in donation or held in state or private collections that has gaps or shifts in its ownership between 1933 and 1948 might have been stolen or obtained under duress. The records relating to such works must be made easily available, and the job of sorting through the documentation should be left to professionals who specialize in tracing Nazi art theft — and to the claimants themselves.

Of course, this kind of transparency takes real commitment to the principle of restitution. It would pry open some dirty secrets. It would mean museums and individuals losing paintings to their rightful owners, under legal and international pressures that are not as cozy as the insular art world.

But then restitution is not a rarefied, aesthetic matter. It's not genteel. As direct heirs die and statutes of limitations pass, it is an ever more urgent matter of simple justice.

Anne-Marie O'Connor is the author of "The Lady in Gold: The Extraordinary Tale of Gustav Klimt's Masterpiece, 'Portrait of Adele Bloch-Bauer.'"