When the National Gallery of Art underwent renovations a few years ago, curator Kimberly Jones took down a painting by Mary Cassatt. The 1878 masterpiece, “Little Girl in a Blue Armchair,” was looking “a little dingy,” Jones recalls. After some preliminary tests, conservators removed a layer of yellowing varnish, and the painting came alive.

“We uncovered the most glorious, sumptuous paint surface you can imagine. It’s absolutely spectacular, rich and lush and vibrant,” Jones says. “Before, you couldn’t really appreciate the richness of the texture and the variety of the paint strokes.”

Something curious also came to light: A small patch of brush strokes that didn’t look like the American impressionist’s handiwork at all.

“There are these little horizontal strokes of greyish paint, and you don’t see anything like that in the rest of the painting, but you do see it in Degas,” Jones says.

Art historians have long known that Cassatt and French impressionist Edgar Degas worked closely together in Paris, and many thought their relationship was one of mentor and student — due, in part, to the fact that Degas was 10 years older than Cassatt. The new NGA exhibit “Degas/Cassatt” shows that they were actually colleagues, pushing each other to reach greater artistic heights, Jones says.

Degas’ work on “Little Girl in a Blue Armchair” is an excellent case in point, Jones says.

Cassatt said in a letter circa 1903 that Degas helped with the painting, but what he specifically did, no one knew. Infrared analysis shows that Cassatt originally painted the scene with a single wall in the back. Degas (presumably) turned part of that wall into a corner, adding depth. This small change may have helped Cassatt transform a pretty good painting into a great one, Jones says.

“What Degas did was very discrete, very subtle, and then he left it to her to resolve,” Jones says.

Cassatt had to make adjustments to accommodate the wall’s new angle. She ended up with a more-dynamic arrangement that enhanced her bold statement about childhood. At a time when most painters depicted children as living dolls, Cassatt captured a little girl in glorious, disheveled reality.

Inspiration ran both ways. In “Woman Standing Holding a Fan” (1878-1879), Cassatt employed metallic and matte paint at a time when respectable artists exclusively worked in oil. Degas responded by using metallic paint and three different kinds of matte mediums in “Portrait After a Costume Ball” (1879).

“They are doing a little one-upmanship on each other,” Jones says. “I think that is the fun part about the relationship.”

The two sharp-tongued artists had their differences and occasional falling-outs, Jones says, but their mutual respect and admiration always won out in the end.

Peeling Back the Surface

Art historians use infrared imaging to see through the layers of a painting and look for changes the artist made along the way.

— When Edgar Degas changed a flat wall into a corner, he helped Cassatt create a more interesting painting. The final version pulls you in like a whirlpool, making your eyes spiral from the dog to the girl to the far-off couch.

— At some point, Cassatt tried placing a second dog toward the back of the room, perhaps in an attempt to make that wall seem farther away.

— The dog is Mary Cassatt’s Brussels Griffon, Baptiste.

— Cassatt hid her original line by raising the back of the girl’s chair. She left heavy blue paintstrokes to give viewers a peek into her painting process, curator Kimberly Jones says.

— “[Degas] is really involved in this painting even before he even touches the canvas,” Jones says: The girl was Degas’ friend’s daughter.

50 Shades of Nothing

It’s the setup for a great bodice-ripper: Two hot-headed painters working in close quarters during Belle Epoch Paris. Unfortunately, there isn’t a shred of evidence that Mary Cassatt and Edgar Degas were anything more than friends, curator Kimberly Jones says.

“There’s nothing. No sly comments, no wry observations in letters, no asides. If something were going on, somebody would have said something. Artists are terrible gossips,” she says.

In fact, the more Cassatt overturned conventions in her paintings, the more conservative she had to be in her personal life.

“She was already risking her reputation, just by being an artist and by hanging out with these crazy impressionists,” Jones says. “If there had been even one whiff of impropriety, she wouldn’t have been taken seriously as a painter.”

National Gallery of Art, Fourth Street and Constitution Avenue NW; through Oct. 5, free; 202-737-4215. (Archives)