ANDREA SEABROOK, host:

There's a newly discovered painting by Vincent Van Gogh. It had been sitting in storage at a museum in the Netherlands. But the portrait wasn't lost in some dusty corridor. It was hidden under the paint of another Van Gogh, and it was revealed by a giant X-ray machine. Now, that's Science Out of the Box.

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SEABROOK: Koen Janssens' head of the international team of researchers that uncovered the hidden painting. He's in Antwerp, Belgium. Mr. Janssens, Van Gogh often painted over his older works. What you found was underneath a painting called Patch of Grass. What does this new painting look like?

Mr. KOEN JANSSENS (Head of International Team of Researchers): Well, what you see in the discovered portrait is a fairly detailed picture of a Dutch rural lady, which has a cap on and is clothed in a kind of dress, very similar to some of the other portraits, which Van Gogh has painted in the same period.

SEABROOK: I understand that art historians knew for many years that there was something underneath this painting. What did you do differently that they didn't do to uncover the actual painting or reveal the actual painting?

Mr. JANSSENS: Rather than just shine X-rays through the painting and get an image of that, like people get with medical equipment, what we did was we used a focus beam. By slowly scanning this beam over the painting, we can more or less interrogate a painting at all its positions for how much mercury is there, how much lead is there and other elements.

SEABROOK: One of the most amazing things is that you have an idea of the colors Van Gogh used in the portrait. How did you figure that out?

Mr. JANSSENS: The mercury - we knew was there as mercury sulfite - which in the art world is known as vermillion, which is a reddish pigment. Similarly, for instance, there is also chromium. We can assign that a green color because there is a pigment, chromium oxide, present.

SEABROOK: Do you know where the portrait might fit in with the evolution of Van Gogh as an artist?

Mr. JANSSENS: So, in his initial period, when he's still in Holland, he lives in a rural village called Nuenen. And so all the villages in Nuenen were painted fairly regularly by Van Gogh. So, he had an almost naturalistic tendency of trying to express the - yeah, their not-so-terribly-well economic situation. So, the sorrow on their faces.

And actually the portrait we have discovered is part of a larger series of Dutch rural ladies and gentlemen. It's quite comparable.

SEABROOK: It's somewhat ironic, isn't it, that the portrait underneath fits into this very important time in Van Gogh's painting career. And the picture over it, Patch of Grass, is kind of thought of as, well, not his best work.

Mr. JANSSENS: Yes, indeed. When we visited the museum, the Kroller-Muller Museum, where they have 86 Van Goghs, it was no problem to get it out of the reserves and borrow it for a week to move it to the synchrotron because they had more interesting pictures to show to the public.

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Mr. JANSSENS: I noticed now with all the attention it's getting now, now they moved it back into the public galleria.

SEABROOK: I see.

Mr. JANSSENS: I think it will stay there for a while.

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SEABROOK: Koen Janssens is a chemistry professor at the University of Antwerp in Belgium. Mr. Janssens, thank you very much.

Mr. JANSSENS: Okay. Thank you.

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SEABROOK: Mr. Janssens is headed to Denver next week. He'll present his findings at a conference on X-ray analysis. But even if you're not going to that conference, you can see a video of how the painting was revealed. It's on our Web site, NPR.org.

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SEABROOK: It's NPR News. Transcript provided by NPR, Copyright NPR.