"It was the most exciting discovery of my career," says Hoenigswald. Thereafter, she sought permission to X-ray every Picasso painting that crossed her path, lecturing and publishing scholarly articles on her findings.

Hoenigswald quickly learned that Picasso deliberately painted on top of his work. Unlike other artists, he did not scrape off a discarded painting or cover it with a white ground layer. "His creativity was spurred by the concept of metamorphosis," she said. "He used the discarded shapes and colors to influence his next composition." And he continued this practice throughout his life, long after poverty ceased being a motivation to reuse old canvases.

The National Galley has only 15 or so Picassos in its collection, so the 1997 exhibit was an opportunity to see numerous works by the artist that arrived on special loan. Hoenigswald was alert to certain clues: a thickness or impasto that implies heavy brushwork beneath the paint, or a certain crackle pattern from two layers drying at different rates.

"I knew what to look for," she said. "Sometimes I don't even need a loop or a microscope. This top layer of paint had cracks -- I could see underlying bits of orange and yellow colors with my naked eye. I asked myself, What is that doing in a muted blue palette?"

Hoenigswald added that time was a crucial factor. "There are always constraints when a show is about to be mounted," she remembered. "The museum registrar wants the art handlers to take the pictures to the gallery space. Plus I had to ask permission. Not every museum wants their paintings X-rayed. You've got to take it out of its frame and there's always a small risk involved when handling the work."

As her assistants removed the Picassos from the shipping crates, Hoenigswald surveyed the possibilities. The "Rue de Montemartre" was small. "That's an advantage because you can X-ray a small painting in a couple of hours," she said. "A large painting can take up to three days. After all, we use regular X-ray-size film and then must piece it together like a mosaic."

Hoenigswald's long-standing friendship with her counterpart at San Francisco's Museum of Modern Art, Will Shank, also helped her focus on "Rue de Montmartre." They had met in the late '70s when Shank interned as a graduate student at the American Museum of Art at the Smithsonian.

"Obviously, when she called I said 'sure' in the interests of science," Shank remembers. "But more vivid was her call a few days later when she said, 'You'll never believe what we found underneath "Rue de Montemartre."'"

Hoenigswald's black-and-white X-radiograph hinted at a fully realized nightclub scene. This, she recalls, was a rarity. More often, her images uncover only fragments of earlier drawings. Additionally, the ability to trace a connection from sketch work to later painting is not always possible. This time, the image's connection to "Le Moulin de la Galette" did not require extensive detective work.

"She had that painting, on loan from the Solomon Guggenheim, right there, too," Shank remembers. "We quickly made the connection that both images featured men and women in formal dress, visiting a nightclub."

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I met Shank in San Francisco in May 2000, three years after Hoenigswald had made the "Rue de Montmartre" X-radiograph. A few months earlier, Shank, who had served as SFMOMA's chief conservator from 1991 to 2000, had resigned to consult. His first project as a freelance curator would be "A Hidden Picasso," which was scheduled to open at SFMOMA during the 2001-02 season.

Shank had pulled a Xerox copy of the X-radiograph from a manila folder. On the left was a group of well-dressed people out for a night on the town: men in top hats, women in bonnets. They were looking at something to the right of the painting, which is harder to decipher. It was, Shank assured me, the swirling dress and petticoats of cancan dancers, no doubt from one of the Montmartre's seedy nightclubs.

"My guess is that Picasso found the composition too crowded," he said. "There was too much going on so he abandoned it. But the idea remained and when he took it up again, he worked on a canvas that was four times the size of this one."

The image changed in other ways, Shank remembered. The dancers were deleted, and in the official painting it is the spectators who are dancing in a scene of gleeful revelry.

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