It was the find of a lifetime -- a Picasso buried under another Picasso. Now they're both on view at SFMOMA.

Pablo Picasso, Sc�ne de Rue (Street Scene), 1900; oil on canvas; 18 3/4 in. x 26 1/4 in.; Collection SFMOMA, bequest of Harriet Lane Levy Pablo Picasso, Sc�ne de Rue (Street Scene), 1900; oil on canvas; 18 3/4 in. x 26 1/4 in.; Collection SFMOMA, bequest of Harriet Lane Levy Photo: - Photo: - Image 1 of / 4 Caption Close It was the find of a lifetime -- a Picasso buried under another Picasso. Now they're both on view at SFMOMA. 1 / 4 Back to Gallery

Will Shank always suspected something was buried beneath Picasso's "Scène de Rue," a somber street scene painted by the precocious 19-year-old Spanish artist in the fall of 1900 during his first stay in Paris.

Some of the brushwork doesn't line up with the figures and stands out from the comparatively smooth surface of this small painting, which is in the collection of the San Francisco Museum of Modern Art, where Shank was chief conservator for many years. And hairline cracks in the surface reveal bits of bright orange, red and emerald green that seem at odds with the picture's muted grays and browns.

Shank's suspicions were confirmed in 1997, when "Scène de Rue" was loaned to the National Gallery of Art in Washington, D.C., for an exhibition of Picasso's early works. As a matter of course, National Gallery conservator Ann Hoenigswald, a Picasso lover who had often X-rayed his paintings to better understand his methods, exposed all the works in the show to infrared light and X-radiography. She'd seen images beneath other Picassos before, but examining "Scène de Rue" was a revelation: Under the surface was a complete picture. It appeared to be a cabaret scene.

For a conservator, "it was a discovery of a lifetime," says Shank, who later organized an intriguing little exhibition about the find, "Hidden Picasso," on view at SFMOMA's Koret Visitor Education Center. It includes a full-color digital reconstruction of the can-can dance hall scene beneath "Scène de Rue."

Shank's research and consultation with other conservators and curators convinced him that the picture "hidden" under "Scène de Rue" was a prototype for what some consider Picasso's first masterpiece, "Le Moulin de la Galette," a Lautrec-like painting of a Parisian boite. Owned by the Solomon R. Guggenheim Museum in New York, it depicts an unsavory club where men in top hats and white cravats mingle with plume-hatted ladies of the night. Picasso (1881-1973) painted it during the fall of 1900, a prolific period in which he produced several other works focusing on the Parisian nightlife previously portrayed by Toulouse-Lautrec and Renoir.

Working with SFMOMA interactive media specialist Tim Svenonius, Shank took high-definition digital photographs of "Scène de Rue" to capture the colors visible through the cracks, then meticulously transferred them to a black-and-white radiograph of the underlying forms to reconstruct the buried picture.

"This has never been done before," says Shank, a tall, genial man with a trim goatee, standing in front of the digital Picasso. It's displayed in a light box so there is no pretense that it's actually a painting. The reconstruction required a degree of educated guesswork -- Shank used similar Picasso works from the same period as a color guide -- but the conservator thinks it's close to what the artist originally painted before he decided, for whatever reason, to cover it over with a completely different picture.

"If I had to quantify it, I'd say we got 90 percent of Picasso's palette and 95 percent of his shapes," says Shank, who now lives in Picasso's hometown of Barcelona and works as an independent conservator and curator.

SFMOMA supported the effort, which was done here, but the funding came from the Bilbao Guggenheim, where "Hidden Picasso" was first shown in 2004. There, the digital reconstruction was exhibited alongside "Le Moulin de la Galette," loaned from the Guggenheim in New York, and SFMOMA's "Scène de Rue." For security and climate-control reasons, the installation at the Koret Center uses reproductions of the paintings. But there's a cool interactive video about the digital reconstruction process (accessible online at www.sfmoma.org/hiddenpicasso) and film footage of Paris in 1900 shot by Thomas Edison. The real "Scène de Rue" is on display just around the corner in the second-floor galleries.

Picasso was famous for reworking and transforming his images and was known to repaint canvases. "It's not something he did to save money as a young, struggling artist, he did this his whole life," Shank says. He mentions Henri-Georges Clouzot's 1956 film "The Mystery of Picasso," in which the artist, painting on glass, "turns a still life into a woman, and a woman into a bull, then turns the bull into a flower. This was like a game he played to make the subject matter of his painting continuously evolve until he had what he considered a final product. But he often left clues on the surface. So you could tell there was something under there."

Nobody knows why Picasso painted over the cabaret scene, with its cancan dancers, women in red dresses and top-hatted gentlemen (in "Le Moulin de Galette," couples are dancing, not just watching). Shank conjectures that the painter wanted to recast the scene on a larger scale. "Scène de Rue" is only 2-by-3 feet; "Le Moulin de Galette" is four times that size.

"He loved the image, he was a habitue of the cancan nightclubs," Shanks says. "He drew the patrons, the dancers, repeatedly in his first several visits to Paris. He loved the idea. It seems to me likely that he wanted to make a bigger statement."

Shank spent a lot of time staring at the 8-by-10-inch X-rays Hoenigs- wald sent him before he hit on the idea of trying to reconstruct the full picture, which he and others familiar with Picasso's work of the period assumed was a "moulin something," as he puts it. Then a lightbulb went on: Because you could see virtually all the colors through the cracks, why not apply them to the black and white radiograph and make a color image?

He and Svenonius first looked at "Scène de Rue" through a binocular microscope. Even without the microscope, some of the colors were easily visible with the naked eye, Svenonius says. "There's this red-orange that's kind of sprinkled throughout, and a kind of emerald green, and a salmon color that leap out at you because the surface painting is predominately brown and gray."

The major step was taking high-definition digital photographs of the entire surface of "Scène de Rue," then transferring the colors onto the X-ray, whose skeletal shapes were brought into sharper focus.

"We took small samples, say 1-inch square, and superimposed them on the black-and-white X-ray, finding the exact location of that patch," says Shank, who spent a couple of months on the project. "Hundreds of thousands of tiny specks of color were sampled directly from the digital files of the visible painting."

Some things were easy, like the hats and dresses. But even those required a degree of extrapolation. For example, if there were specks of orange-red in a dress, the team surmised the whole dress was that color. The flesh tones and the dark foreground and background were far more difficult to figure out.

"There were moments when we would come up with green for flesh tones and think, 'there's something wrong with the equipment or with us,' " Shank says. "But in fact, there are some unusual things in the painting." He points to the reproduction of "Le Moulin de la Galette" and says: "We're not talking realistic flesh tones here. This is how Picasso painted in 1900. There are extremely abstracted faces and other elements. This was his palette. So it's not too huge a leap of faith to assume that this other painting of such a similar subject was painted with very similar colors."

Based on the scarcity of underlying color and bold brushstrokes on the right side of the canvas, they concluded that Picasso didn't finish the whole scene. "The only thing we had to finally guess at were the backgrounds, that brown color there," Shank says. Adds Svenonius: "We found little specks of the ochre up top, and specks of that green gray down below, and we had to say, 'Well, maybe it was broad wash over the whole area.' "

Shanks is amazed that Picasso simply didn't white out the underlying painting with gesso and give himself a blank canvas for "Scène de Rue." The artist was so visually acute, he says, that he was able to paint over the "hidden" picture piece by piece and create an entirely new image. Perhaps the most intriguing figure in the underlying picture is a black-cloaked woman off the left side. Most likely she's "La Celestina," the procuress in Spanish art and literature who arranges sexual trysts.

"There's certainly enough there that you can image what the painting would've looked like before he covered it up," Svenonius says. "That was our goal, not to pretend that this is a Picasso."

For Shank, the reconstruction job was a way to further our understanding of the artist and his process, "and how things evolved from one thing to another for probably the most important artist of the 20th century. It was thrilling to have a part in bringing this to light for the first time in 100 years. Picasso did leave clues, but certainly didn't envision the technical possibilities that would be available to us at the end of the 20th century. I think he would be tickled."

Hidden Picasso: 11 a.m.-5:45 p.m. daily except Wednesday, and until 8:45 p.m. Thursday, through May 28 in the Koret Visitor Education Center at the San Francisco Museum of Modern Art, 151 Third St., San Francisco. Admission: $7-$12.50. (415) 357-4000, www.sfmoma.org