X-radiography or a "negative" of a painting that helps show damage.
The National Gallery of Art houses almost 4,000 paintings. Over time, they can be damaged by sunlight, humidity and grime. So every year, a team of eight conservators chooses approximately a dozen paintings to restore to their natural beauty.
In some galleries in Europe they like to display a painting in its natural state, with cracked paint, water damage, and faded colors, says Christensen
"They feel it's part of the history of the picture as an object," says Christensen. "But our philosophy is that the painting is a visual illusion and its function is impaired if you show all the damage."
That's why she and other conservators painstakingly work on tiny sections of a painting -- sometimes the size of a postage stamp -- for weeks at a time, trying to restore it to its original glory.
Removing discolored coating brightens paintings' hues
Today, she's working on a picture of monks.
"We are looking at a painting by an unknown 15th century painter from France and it shows one of the miracles of St. Benedict," says Christensen. As with most old paintings, there's a layer of varnish to remove.
"We use this thing that looks like a giant Q-tip, dip it in solvent and roll it against the picture," says Christensen.
Artists use varnish as a final coat to make colors look rich and as a protective layer. When natural varnish ages, Christensen explains, it creates a yellow film over the whole painting surface, and that's especially distorting on paintings with cool tones.
"Blue looks green and the white looks yellow," says Christensen.
Better care of paintings comes with scientific advances
She uses a gentle chemical that dissolves the varnish but not the paint.
"Before the 20th century, the kinds of things people used to take off these discolored varnishes were very harsh materials like lye, urine," says, Christensen.
Scientific advances, including x-radiography, have helped conservators immensely.
Christensen points to what looks exactly like an X-ray of the painting. You get a negative of the picture; the light blue sky looks white, the monk's black robe looks black.
It also shows what restoration has been done on the painting before it came to the National Gallery.
She points to an area of the sky on the painting that looks like clouds. But on the X-ray the damage can be seen because that cloudy area has a different texture than all of the other paint.
"This is actually a road map of where the damages are going to be," says Christensen.
Conservators also use infrared reflectography, which works on the principle that black absorbs heat and white reflects it. On a computer monitor reflecting the infrared image, the restorers can actually 'see through' the artist's drawing underneath all the layers of paint.
"There were two little monks drawn in here walking away and the artist decided to paint them out later on," says Christensen. "So you can sort of get into the artist's mind a little bit."
A jobs that stays behind the scenes
Unlike the artist who usually takes ownership of a painting by boldly signing his or her name on it, conservators are content to get no credit. Viewers will never know about the reams of research done, the millions of spidery cracks that have been filled or the colors that have been "in-painted" before the painting is displayed again. And that's fine with Christensen.
"Your own skills as a painter are not important unless they are in the service of getting the vision of the artist across," says Christensen. "So you have to sort of be humble in a way. There are some in the field who felt that really good artists don't make good restorers because they may be a little bit too free in rearranging what they don't like."
But her work on this particular monk painting has meant months and months on a painting for which the artist himself isn't well-known -- or known at all.
"It's not a Leonardo, yeah," Christensen says. But she adds that it's beautiful to look at. "And also, it's part of our history. It's evidence of our past, where we came from and I think that's important to all of us to know ... to know how we got from there to here. And that's part of the journey."
It's a journey that includes a stop in the hushed room where Christensen and her colleagues do the behind-the-scenes work that almost no art lovers will ever see.