In Mary Cassatt’s 1878 painting Little Girl in a Blue Armchair, a fidgeting young child slouches into the pillowy embrace of a turquoise chair. The girl’s scruffy black and brown dog sleeps on the seat next to her, adding to the tranquility of this domestic scene.

Mary Cassatt , Little Girl in a Blue Armchair, 1878, oil on canvas. COURTESY NATIONAL GALLERY OF ART, WASHINGTON, COLLECTION OF MR. AND MRS. PAUL MELLON.

The canvas is a quintessential Cassatt. However, recent cleaning of the work and infrared images taken by the National Gallery of Art in Washington, D.C., reveal that brushstrokes from someone else’s hand are also present—Cassatt’s friend and colleague Edgar Degas. The French artist subtly changed the shape of the room. He had the floor intersect with the back wall at an angle, rather than perpendicularly, creating negative spaces that are strange and unexpected.

Upon discovering these details of Degas’s intervention on Cassatt’s canvas, a team of experts at the National Gallery decided to explore further. They organized the exhibition Degas/Cassatt to investigate the previously unknown depth of the pair’s artistic relationship. The show, which opens May 11, will feature a selection of 70 paintings, drawings, and works of mixed media by both artists to highlight their artistic dialogue.

In Degas’s egg tempera on canvas painting Rehearsal in the Studio, ca. 1878-1879, the floor of the dance studio intersects with the back wall at an angle. He created a similar angle in Cassatt’s Little Girl in a Blue Armchair. ©SHELBURNE MUSEUM, SHELBURNE, VERMONT. COURTESY COLLECTION OF SHELBURNE MUSEUM. GIFT OF ELECTRA WEBB BOSTWICK.

Degas first met Cassatt (who was born in Pittsburgh but spent much of her life in Paris) on an 1877 visit to her Montmartre studio. “He recognized right off the bat that they had a shared sensibility,” says the show’s curator, Kim Jones, and he invited her to participate in the Impressionist exhibition he was organizing with his fellow “independent” painters. Their introduction marked the beginning of a friendship that lasted nearly 40 years. A lack of existing correspondence between the two makes it difficult to discern the specific details of their interactions, but their artworks—particularly those created between the late-1870s and the mid-1880s, the period of the Impressionist exhibitions in Paris—leave behind compelling clues about their friendship.

Between 1879–89, both Degas and Cassatt took major risks with their art, experimenting with unconventional media such as tempera, distemper, and metallic paints. In Degas’s Portrait after a Costume Ball (Portrait of Mme. Dietz-Monnin), for instance, the artist juxtaposed patches of smooth, matte pigment with wide strokes of metallic paint and delicate applications of pastel to create a textural and frenetic surface. Cassatt tested these materials as well in works like Woman Standing, Holding a Fan and Lydia Seated on a Porch, Crocheting. She also used metallics to add a subtle sheen to oil paint. In The Loge, she incorporated small bits of shimmering, simulated gold paint throughout the canvas to vitalize the scene.

Mary Cassatt, The Loge, ca. 1878-80, oil on canvas. COURTESY NATIONAL GALLERY OF ART, WASHINGTON, CHESTER DALE COLLECTION.

Jones selected many other works that demonstrate Degas and Cassatt’s ongoing artistic exchange. For example, one compositional element commonly employed by both artists was the s-curve, in which the movement of a model’s arms complemented the contraposto or angular pose of their torso to create an elegant, lithe s-shape. The s-curve can be seen in Degas’s pastel and charcoal work Woman Bathing in a Shallow Tub (ca. 1885) and his 1894 drawing Morning Toilet, among others. In conversation with Degas, Cassatt executed s-curves in her ca. 1889 drypoint print Young Girl Fixing her Hair and in the oil painting Child Picking a Fruit (1893).

Mary Cassatt, Child Picking a Fruit, 1893, oil on canvas. © VIRGINIA MUSEUM OF FINE ARTS. COURTESY VIRGINIA MUSEUM OF FINE ARTS, RICHMOND, GIFT OF IVOR AND ANNE MASSEY. PHOTO: TRAVIS FULLERTON.

Degas often enlisted his friends to model for his paintings, but none more frequently than Cassatt. She is the subject of at least eight of Degas’s works, and scholars believe her likeness appears in many more. In most of his depictions of the American artist, she plays a specific role. The unfinished painting Mary Cassatt, however, is the only true portrait that he painted of her. She owned the piece and displayed it in her studio. (Degas also owned numerous works by Cassatt.)

The location depicted in the portrait is vague, but the table in the background and small pictures Cassatt holds suggest that she might be in a photography studio. Artists and dealers commonly had cartes de visite taken to document works in their possession. By depicting Cassatt as the subject of a portrait, likely holding objects associated with her craft, Degas establishes her as his peer and as a successful artist in her own right.

Edgar Degas, Mary Cassatt, ca. 1879-84, oil on canvas. COURTESY NATIONAL PORTRAIT GALLERY, SMITHSONIAN INSTITUTION. GIFT OF THE MORRIS AND GWENDOLYN CAFRITZ FOUNDATION AND THE REGENTS’ MAJOR ACQUISITIONS FUND, SMITHSONIAN INSTITUTION NATIONAL PORTRAIT GALLERY, SMITHSONIAN INSTITUTION / ART RESOURCE, NY.

It is a common misconception that Cassatt was merely a pupil of Degas, when in fact both artists learned from and respected one another, and executed daring experiments using unconventional materials.

Says Jones, “she’s a much edgier artist than people give her credit for.”