The oil painting by N.C. Wyeth depicting the artist with his wife and five children is an affectionate rendering of the famously creative brood from Chadds Ford.

If you could rotate the canvas 180 degrees, however, and peer through the top layer of paint with a kind of X-ray vision, you would behold a snarling villain, his face lit by the red glare of an iron furnace.

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In fact, three scientists have done just that, unveiling their find yesterday at an American Chemical Society conference in Washington.

Using a high-intensity X-ray beam and sophisticated optics, they created a full-color reproduction of the earlier Wyeth work. It had been hidden for decades, ever since the artist painted his family portrait on top of it.

Scientists often have used X-rays to look at covered-up paintings in the past, and did so with this one in 1997. But that method, the same one doctors use to scan broken bones, is fairly primitive and is limited to a black-and-white palette.

The new technique, developed by physicists at Cornell University, relies on a phenomenon called X-ray fluorescence. It unlocks a broad spectrum of color that is sure to whet the appetite of the art-history world.

Christine Podmaniczky, associate curator for the N.C. Wyeth collections at the Brandywine River Museum, was delighted after getting a sneak peek at the reproduction last week.

"I am amazed," she said, noting that the colors in the hidden painting are similar to those in another Wyeth work from the same series.

The covered-up painting was an illustration for a short story, "The Mildest-Mannered Man," in a 1919 issue of a popular review called Everybody's Magazine. It is the melodramatic tale of a love triangle in which villain Slag Harshmeyer, shown charging in the picture, meets an untimely end in the furnace flames.

Wyeth, a prolific illustrator, was known to have painted over some of these works once they had appeared in print.

He painted the family portrait over the Slag Harshmeyer illustration around 1927. The portrait, titled Study for Wyeth Family mural, was done in preparation for a larger work that Wyeth intended to paint in his living room, but never did. Son Andrew Wyeth, who would go on to artistic fame himself, is shown as a young boy at the far right, chin resting on his fist.

The Slag illustration, meanwhile, lay hidden for 60 years, known to scholars only from the black-and-white reproduction in the magazine. (Slag's fist actually can be seen faintly with the naked eye, but nobody noticed it.) Wyeth, who died in 1945, left no record of what happened to the illustration.

The first clue to its rediscovery came in the 1990s, when Podmaniczky found a partial label from Everybody's Magazine stuck to the back of the family portrait. An X-ray was eventually taken, and the resulting black-and-white image, though muddled, was identified as the forgotten magazine illustration.

The full-color re-creation would have to wait for a team of art sleuths led by Jennifer Mass, senior scientist at the Winterthur Museum and an adjunct professor in the art conservation department at the University of Delaware.

Mass, who had earned her Ph.D. in inorganic chemistry at Cornell, was aware of the school's high-intensity X-ray source, called a synchrotron. Housed in a circular underground tunnel, the device generates X-rays with up to a million times the intensity of what is used in a hospital.

Mass learned that physicists could use the device to look beneath the surface of paintings. In 2007, the Wyeth canvas became one of the first test subjects.

The key to the method: When an atom is struck by an X-ray, it loses one of its inner electrons, and a "hole" is left behind.

An outer electron then falls down to occupy the hole, and energy is released in the form of a secondary X-ray - a process called fluorescence.

Each element on the periodic table fluoresces with a unique signature. And it is known which elements were used to make various pigments in paint - cobalt for blue, chromium for green or yellow, for example. So by measuring the fluorescence of covered-up paints, the scientists can deduce their color.

Arthur Woll, senior staff scientist at Cornell's X-ray facility, built a special easel so the X-ray beam could be moved in tiny increments across the surface of the painting. He added specialized "confocal" optical equipment so that the fluorescence could be measured at various depths below the painting's surface.

Woll, Mass, and others spent a week in 2007 babysitting the device round-the-clock as it slowly moved across the painting's surface, a fraction of a millimeter at a time.

In one spot, the scientists could detect chromium in the paint that Wyeth used for the round green bottle in the family portrait. Beneath it, they detected a brownish, iron-based pigment that Wyeth used in the background of the earlier illustration.

Other techniques were used to round out the analysis.

Cornell physicist Sol Gruner, director of the synchrotron facility, praised Mass for being equally fluent in the languages of art and science. She "is one of an emerging, growing group of scientists who have a foot planted in both worlds," Gruner said.

Andrew Wyeth died in January, before the reconstruction of his father's work was complete. But he knew it was under way and was intrigued. He also had seen the faint black-and-white X-ray from the 1990s.

In 2005, the younger Wyeth told The Inquirer that his father might have been surprised at the care taken to detect his covered-up illustrations, believing they were "a bunch of rubbish."

N.C. Wyeth preferred to be thought of as a fine-art painter, not an illustrator, though it is his illustrations, such as the powerful images he created for an edition of Treasure Island, that have earned him the most acclaim. Those, at least, are almost all intact - some of them in the Brandywine museum's own collection.

Wyeth did more than 1,300 illustrations, of which about 400 are known only by their reproductions in magazines or elsewhere. Some of the originals for those 400 are known to have been destroyed, while others were painted over. For some, the exact fate is unclear.

Podmaniczky, the museum curator, is sure there are more hidden works waiting to be found.

Contact staff writer Tom Avril at 215-854-2430 or tavril@phillynews.com.