Beneath the familiar faces of hundreds of paintings lining the walls of the National Gallery of Art in Washington, D.C., a host of secrets sit, waiting to be discovered.

Whether a Picasso or Da Vinci, every canvas holds brushstrokes that might conceal something else: a secondary painting, perhaps, or a mystery waiting to be untangled by John Delaney, the museum's senior imaging scientist.

On a typical day, he patiently places a painting in front of a specialized camera and completes a scan, which will reveal not just what's under the paint, but also what is happening at each layer of paint that lies under the surface.

Delaney is part of a community of scientists, curators, and art historians who have assembled a toolbox of high-precision x-rays, lasers, microscopes, and software to unveil some of art's greatest mysteries. These technologies have helped unlock groundbreaking information about both art and artist alike.

Here are some of these technologies and the mysteries that they've helped uncover.

#1 Picasso Under the Paint

View Images Picasso's painting, ''Le Gourmet.'' PHOTOGRAPH BY B. CHRISTOPHER, ALAMY

One of Picasso's most distinctive pieces from his Blue Period, "Le Gourmet," depicts a child scraping his bowl for the last morsels of food. But underneath the azures and cobalts on the canvas, there's a hidden portrait—of a veiled woman gazing serenely out into the distance. This hidden portrait, which Picasso worked on before his Blue Period, uses bolder dabs of white paint and may contain other unknown pigments.

The ability to view early sketches and paintings hidden underneath the surface has been around for decades, thanks to infrared reflectography, which was used to uncover the veiled woman at the National Gallery in the mid-1990s.

But Delaney's technology goes far beyond seeing a concealed drawing. His infrared imaging method takes pictures of Picasso's mysterious woman at a range of infrared exposures. Delaney then turns these images into a flipbook of sorts, allowing viewers to see the little boy fading as the woman emerges from the canvas.

The two images are a boon for Picasso enthusiasts, who are interested in how Picasso painted throughout various periods of his life.

"It points closer to [the artist's] methods and techniques and how they evolve," Delaney said.

#2 Bi Discs

Round and flat, jade bi discs resemble 45 rpm records—except they're green, 5,000 years old, and hide several secrets.

The circular discs were crafted by Neolithic Liangzhu artisans in Southeast China. They were then buried with members of the elite.

"It's the first time this kind of circular shape shows up in Chinese history ... so there are real questions about how they were made," said Janet Douglas, a conservation scientist at the Smithsonian Freer Gallery of Art and Arthur M. Sackler Gallery, who reconstructed how the discs may have been crafted.

First, Douglas compared the pieces under a microscope to understand how they were carved and polished. She also examined jade and carving tools from an archeological site in Dingshadi, located in Jiangsu Province, to learn more about how the discs were crafted.

"One of the first things people think when they look at these bi discs is, 'Oh, the Chinese invented the wheel,' but actually the Chinese developed a rotating wheel and rotary tools to work jade," she said.

To sculpt a bi disc, the Liangzhu started by sawing off flat slices of jade with a string saw, a leather strap embedded with rough sand. They then shaped the jade into a circle with a stone saw. To make the center circle, the Liangzhu spun the bi discs against a sharp point that resembles a modern-day drill bit.

"So that's why they were creating objects in circular shapes: because they found a way to mechanize the process," she said.

#3 Recognizing Shakespeare

View Images Shakespeare, shown in a verified engraving (left) and the uncertain "Cobbe portrait." ILLUSTRATIONS FROM HERITAGE IMAGES/CORBIS

Shakespeare, shown in a verified engraving (left) and the uncertain "Cobbe portrait."

There are two portraits of Shakespeare where there is no doubt that he is the man depicted: One is an engraving of the poet in the first compilation of his works; the other is a memorial bust in his hometown of Stratford-upon-Avon.

But there are dozens of other portraits of Shakespeare where the Bard's identity is contested. Enter art historian Conrad Rudolph and electrical engineer Amit Roy-Chowdhury.

They've developed a new form of facial recognition software to analyze the portraits of history's elite (and not-so-well-known) to add some objectivity to the academic kerfuffle over who is actually pictured on a canvas or in a sculpture.

"We provide a quantitative body of information that allows the curator or the art historian to decide in their own mind whether it is, in fact, Mary Queen of Scots, for example," said Rudolph, who works with Roy-Chowdhury at the University of California, Riverside.

The software will key in on two main facial aspects: what researchers call "local feature similarity" (the corners of the mouths or eyes) and the "anthropometric distance similarity" (the width of a mouth or the distance between the eyes).

From there, an algorithm will calculate the probability of the portrait subject's identity. A computer's objectivity, however, has its limitations. For instance, a portrait of a young Shakespeare would look radically different from a portrait of middle-aged Shakespeare—because facial characteristics change shape as people age. Training a computer to recognize this will be tricky, Row-Chowdhury said.

#4 Did Picasso Use House Paint?

View Images "The Red Armchair," left, and an ad for the Ripolin housepaint that Picasso used. PAINTING BY PABLO PICASSO, DANITA DELIMONT/ALAMY; SCAN FROM CHRONICLE/ALAMY

"The Red Armchair," left, and an ad for the Ripolin housepaint that Picasso used.

As the avant-garde movement emerged in the early 20th century, artists began challenging the existing establishment with new styles and materials.

For years art historians knew Picasso was among the first artists to eschew traditional paint for a quick-drying, glossy French house paint called Ripolin. But no one had any idea which of his paintings included the industrial paint, or in what concentration he may have mixed it with other paints.

"If you looked at a painting and saw bright colors, smooth finish, and enamel-like surface, [then we assumed] that must be Ripolin," saidFrancesca Casadio, a senior conservation scientist at the Art Institute of Chicago who has spent eight years chasing down Picasso's paint. "Until now, art historians only had a chance to make these determinations based on observation."

But now, conservation scientists like Casadio can use high-resolution imaging—instead of relying on the naked eye—to determine the type of paint an artist used.

Casadio collected microsamples from two Picasso paintings, including the famous Cubist portrayal of his mistress, "The Red Armchair." She then brought the samples to Argonne National Laboratory near Chicago, where she worked with physicist Volker Rose to analyze what kind of paint was used.

To do this, Casadio used what's called a nanoprobe to look inside the individual pigment particles. Various chemicals in these particles (and the absence of other chemicals) indicated the paint's true origin.

Her study not only revealed more about Picasso's use of house paint—it also added to discussion about Ripolin's cultural influence in early 19th-century France. It had even fewer impurities than she expected.

"The brand was used not only because it had good advertising but because it had really good quality," Casadio said.

And thanks to her effort, conservationists now know that Ripolin is a stable paint, which will help them keep Picasso's canvases vibrant and well-preserved for centuries to come.

#5 How Blue Was the Sky?

View Images The pigment smalt. PHOTOGRAPH FROM SSPL/GETTY IMAGES

Sometimes in art, gray skies might not mean cloudy skies.

A variety of pigments in paint can fade or discolor over time due to chemical reactions with the environment.

In Bartolomé Esteban Murillo's "The Heavenly and Earthly Trinities," the sky and an array of clouds surround the heavenly figure of God, who watches over the earthly trinity of Mary, Joseph, and young Jesus. When Murillo first painted this biblical image in the late 1600s, he used a kind of powdered cobalt glass—called smalt—to color the sky blue.

But over time smalt degrades and its blues become browns and grays. For many years, Murillo's sky was blotched in differing shades of gray and blue, but no one could explain exactly how or why the colors degraded.

"For many, many years, scientific groups working in the conservation of paintings were wondering how this pigment was fading," said Loïc Bertrand, the director of IPANEMA, a French organization—based at a particle accelerator—that researches ancient European artifacts using advanced scientific techniques.

Knowing how the paint degrades—and how quickly it degrades—can help art historians imagine how the painting may have looked originally.

So Bertrand's organization, working with scientists at the National Gallery in London and the Louvre in Paris, gathered microsamples from five paintings that used smalt between the 16th and 18th centuries.

The tiny sample collected from Murillo's "The Heavenly and Earthly Trinities" looked like a layered cake—if you looked at it under a microscope. The scientists were interested in what kinds of paint were contained within those layers. To determine that, they used a technique called x-ray spectroscopy, which can easily differentiate various chemicals in the paint. (Related: "X-rays and Operas: A Collaborative Restoration.")

The advanced imaging showed why the smalt lost its color. It has to do with potassium, an ion that was present in the smalt pigment to help preserve it. When potassium reacts with the environment, it destabilizes and slowly leaches out of the paint. And this leaching causes the smalt to fade from bright blues to more drab grays.

Still, the art conservationists working on the Murillo are left asking themselves one question: Did the Baroque painter use a pigment that faded on purpose, perhaps as an artistic statement?

"The question of intentionality is always a difficult statement ... For that we need to rely on past comments about the painting's color but also on material evidence," Bertrand said.

#6 Finding a Lost Leonardo

View Images Scientists use an endoscope to look behind a mural. PHOTOGRAPH COURTESY DAVE YODER

Engineer Maurizio Seracini's quest to find Leonardo da Vinci's lost masterpiece "The Battle of Anghiari" has spanned 36 years with all the mystique, triumph, and despair of a Dan Brown novel—but with way more science.

Seracini is certain that Leonardo's famous experimental mural is behind the "Battle of Marciano" in the Hall of 500 in the Palazzo Vecchio, Florence's City Hall. (Related: "Finding the Lost da Vinci.")

But another painter, Giorgio Vasari, may have obscured Leonardo's mural when he was assigned to paint the "Battle of Marciano" in the mid-16th century.

Vasari was a known admirer of Leonardo and in the past had hidden another famous work of art behind an altar.

When Seracini first started to poke around the Palazzo Vecchio in the mid-1970s, he climbed a scaffold in front of Vasari's fresco and discovered the words "Cerca Trova"—"Seek and You Shall Find"—painted on a soldier's small green flag.

The phrase led Seracini to wonder: What is there to seek?

Over the years, he's applied many diagnostic tools to answer that very question.

Funded by the National Geographic Society, Seracini planned to search for "The Battle of Anghiari" by using two nuclear-based technologies—a bomb-detection device and a neutron back-scatterer—that had the potential to detect Leonardo's trademark linseed oil paint without disturbing Vasari's battle scene.

But there was one problem. Both methods use radiation, which left government officials uncomfortable with the idea. Instead, officials in Florence asked Seracini to look for "The Battle of Angiari" with another technique called endoscopy.

Similar to the medical procedure, art endoscopy uses a tiny camera to explore inaccessible spaces. In this case, Seracini's team drilled six small, centimeter-wide holes in already-fractured portions of the Vasari fresco. They then threaded a tiny probe with a tiny camera attached through the holes.

In November 2012 the endoscopy analysis ended with mixed results.

X-ray data from grit samples removed from behind the wall described black pigments similar to those in the glazes of other paintings like the "Mona Lisa." But it didn't find Leonardo's actual mural.

#7 Who Did What, Exactly?

View Images Tree rings were a key clue in untangling the lineage of "The Armorer's Shop." IMAGE FROM NORTH CAROLINA MUSEUM OF ART/CORBIS

It looks so natural. Industrious Dutch craftsmen surround a pile of resplendent silver armor. But contrary to the simplicity of this quiet working environment, the armorers and the armor were painted by two artists, 20 years apart.

For about 50 years, the North Carolina Museum of Art attributed "The Armorer's Shop" to David Teniers the Younger. Teniers, who worked in the 17th century, was thought to have painted the scene on a wooden panel.

But when Noelle Ocon, the museum's conservator of paintings, took an x-ray of the painting in November 2000, she was surprised to see thick lines that indicated separate wooden panels, and probably separate artists.

Intrigued by this discovery, she and her fellow conservators contacted Jennifer Mass, a senior scientist of art conservation at the University of Delaware, to find out whether the painting was a pastiche—a work of art that was changed or added to by another artist.

"They wanted more in-depth information about what they were seeing and what it actually meant about the construction of the work and the artists that had been involved," Mass said.

By measuring the tree rings in the oak-wood panels, Mass pinpointed the age range of each panel, allowing her to estimate when each portion was painted and which part was painted first.

Then she scanned the painting to analyze what chemicals were contained within each paint layer.

Her scan revealed thicker layers of paint where the later artist had conjoined the paintings.

With the help of art historians, Mass and Ocon concluded it was likely that Jan Brueghel the Younger had painted the pile of armor in the 1620s, when Teniers the Younger was in his teens. Then, about 20 years later, Teniers added the slice-of-life details, replacing a suit of armor with a table and depicting some blacksmiths working in the background.

This was the only painting confirmed to be the work of both artists. There is no record of them working together.

"Whether you can call it a collaboration or an appropriation is difficult to say," Mass said. "We'll never know if Teniers had Brueghel's input or approval when he made it into a larger painting."

Either way, at least we now know that Teniers armed himself with all the credit, since he was the only artist to sign the work.