Mona Lisa is in Baltimore at the Walters Art Gallery.

Well, it's not the Mona Lisa. This one was painted by an admirer of Leonardo, for a wealthy patron.

"She's one of about a half-dozen Mona Lisas of the era that still exist," says Gary Vikan, assistant director for curatorial affairs and curator of medieval art at the Walters.

Her portrait hangs at the entrance to a small but delightful show, "Artful Deception: The Craft of the Forger."

Maybe that's why she's smiling.

The show came about in answer to a challenge issued by Thomas Hoving, former director of the Metropolitan Museum of Art. In an article called "The Forgery Boom" in Connoisseur magazine a year ago, Hoving said that nearly 60 percent of the artworks he'd seen offered for sale were forgeries. He suggested museums come forward with shows of their fakes. But, he asked, "What art museum in America these days would have the courage to put it on?"

The Walters, for one, which gathered from its collection two dozen objects -- forgeries, works of questionable authenticity and, for the sake of comparison, genuine works. Ranging from the 4th century B.C. to the 20th A.D., they include a "signed" Courbet that a forger copied from a lithograph; "Renaissance" jewelry made in the 19th century; and an illuminated manuscript by the notorious "Spanish Forger," who, working in Paris in the early decades of the 20th century, used genuine medieval vellum for his bogus manuscripts.

"A lot of museums are embarrassed about talking about fakes," says Vikan. "For us, forgeries are no problem, the proportion is so small." He says the occasional piece of chicanery just points up what a great collector Henry Walters was. Walters' philosophy was that everyone makes mistakes; in the 1920s, he kept a cabinet where he exhibited $100,000 worth of his.

There's a long tradition of fraud in the art world. Probably the most famous modern-day art forger was Han Van Meegeren, who, in the 1940s, made more than $2 million unloading the "Vermeers" he had painted. Some faked baked potatoes made in 1966, rip-offs of sculptor Claes Oldenburg, shrivel by comparison. But the best fake of all time is not known, of course. It's too good.

"Henry Walters would have wanted us to do this, to clean house," says Vikan. But while a museum like the Walters -- or a private collector -- may suffer the disappointment of bad investments and quietly remove bogus works from the wall, the classification of a work as fake is not carved in stone -- or fired in terra cotta.

For example, when the Walters forgery show opened, one object that was in limbo was a terra-cotta St. Joseph, two-thirds life size. The statue entered the United States just before World War I as part of a nativity group that included a Virgin and Child. The National Gallery now owns those two, and a recent lab test -- thermoluminescence dating -- placed them at around 1910. The thermoluminescence technique is used in dating materials like terra cotta that are fired during manufacture; since the firing removes any radiation, the more radiation in the object, the older it is.

The fakes show gave the Walters the chance either to redeem St. Joseph or condemn him to the basement. In August, a tiny sample of terra cotta was removed from the statue for testing. Meanwhile, Joseph went on display in the show in September under the damning label "Guilt by Association."

But on Sept. 23, the report came in from the Research Laboratory for Archaeology in Oxford, England: "It is estimated that the material of the sample was last fired between 380 and 580 years ago." That worked out just right, because the man to whom the statue had once been attributed, Matteo Civitali, died in 1501.

"It was a wonderful surprise," says Vikan.

But even if something proves to be fake, that doesn't mean it's valueless.

A forgery, says Vikan, "can still be a fine piece, and something that can reveal a lot about a period or culture."

This Mona Lisa, for instance. "Historically, it's a significant work," he says. "It wasn't a fake when it was made. It could become a fake, had the Mona Lisa disappeared 150 years ago, and this turned up 20 years ago."

It's a twist on the question of an artist's intentions. Reinhold Vasters, a 19th-century German goldsmith whose work is in the Walters show, knew exactly what he was doing when producing "Renaissance" jewelry and decorative arts. So successful was his deception that his work eventually fooled not just Henry Walters, who acquired an engraved bowl and a pendant, but the Metropolitan Museum of Art as well. The Met's "Rospigliosi Cup" -- until recently attributed to the great Renaissance sculptor Benvenuto Cellini -- was one of Vasters' finest.

Purchased at the turn of the century, the bejeweled pendant with biblical scenes on either side and the elegant bowl with the glass bird dipping into it were revered objets d'art at the Walters. But just eight years ago, a curator at London's Victoria and Albert Museum found Vasters' papers, including drawings, in the files.

Included were drawings of familiar Renaissance treasures. It was naturally assumed that Vasters made the drawings out of admiration for the artists, to train his eye. But a closer look revealed that they were actually studies for work Vasters was planning -- the creation of those very same treasures. The Met confirmed this when it dismantled the "Rospigliosi Cup" and found 19th-century construction.

Why didn't Vasters shred the evidence? Probably because he wanted posterity to know how good he was. Vikan calls him the "nearly perfect forger" -- a master who wasn't around for the thrill of his discovery.

This is the rare case where the forger is just as good an artist as the original one. Vasters' work isn't schlock. The bowl at the Walters is gorgeous. And do the lovers of the former "Cellini cup" care that it is Vasters' 19th-century work of art rather than a Renaissance piece?

"The problem is, he created so much he begins to distort perceptions of what Renaissance art should be," says Vikan.

Vasters is the scholar's nemesis.

"Curators are custodians of things that are beautiful as well as history. No more than you would break something beautiful, you wouldn't perpetuate a lie." Take away the financial part -- the deception, the not getting your money's worth -- and forgeries become a question of damage to art history.

Examples of too much of a good thing were the Byzantine enamels that showed up around the turn of the century in the collection of a wealthy Russian painter, M.P. Botkine. Until then there had been only a half-dozen such 11th-century creations. Suddenly, there were about 180. The large medals depicted different saints -- but they were too perfect, even for the holy. None of the gold showed signs of wear. Comparison analysis this year showed that most of the enamels were fakes. All had the same amount of lead, but some contained uranium and chromium, which weren't used in coloring enamels or glass until the late 18th century.

A Byzantine enamel, if genuine, could go for "upwards of six figures," says Vikan. Condemned as a fake, it may suddenly plummet in worth to virtually nothing but its gold value. But the Botkines are such handsome pieces, says Vikan, that "there will always be a trade in them."

Art fakery, he says, implies "big money, important people being fooled and duped; it implies controversy and criminality, people getting away with things on people who have a lot of money and not so much sense."

But, at least for visitors to the Walters show, that attitude disappears quickly. As long as you aren't paying for it, it becomes a delightful puzzle. There are four "brain teasers" here, where visitors may test their curatorial mettle with works of art and their imitations: Can you tell the real Se`vres Louis XVI clock from the 19th-century copy? It's a little like comparative shopping at Bloomingdale's.

"In looking at fakes you learn a lot about yourself, about the period you're going after," says Vikan. "When you take the fake away from the original, it's like hearing a fine tenor and then hearing Pavarotti. But if you hear nothing but Pavarotti, you can't fully appreciate how superhuman he is."

"Artful Deception: The Craft of the Forger" will be at the Walters Art Gallery, 600 N. Charles St., Baltimore, through Jan. 10.