Claude Monet, ‘The Bridge at Argenteuil,’ 1874, oil on canvas, after conservation treatment. (Williams, Greg)

For more than 30 years, Ann Hoenigswald has been studying, treating and cleaning some of the most famous paintings of the 19th century. In an immaculate laboratory at the National Gallery of Art, she restores Renoirs, Manets and other works of the impressionist and post-impressionist schools.

Claude Monet painted “The Bridge at Argenteuil” in 1874 with its blue water and sky and its white clouds and sail. Yet by the 21st century, the important painting was looking dull and needed to be cleaned.

“It struck me it was much too yellow. What disturbed me was that yellow varnish had accumulated in the interstices of the brushwork. With the magnifying loupe and the microscope, you see how thick the varnish layer was and how it altered the intention of the artist,” said Hoenigswald, senior conservator of paintings at the National Gallery, who works on the fading canvases.

Now that the refreshed Monet has been rehung in the newly arranged 19th Century French Galleries, Hoenigswald talks about her satisfaction with revealing the artist’s intentions.

‘There’s a return to the palette which was intended by the artist. The whites were no longer yellow, the blues were no longer green and the purple shadows emerged, as did the crisp texture of the brushwork,” she explained. “However, what is always the most striking is the sense of space which is reestablished when the discolored varnish is removed. It is particularly apparent in landscapes. The relationship between foreground, middle ground and background makes sense again.”

Hoenigswald works from left to right, and that’s how she moves as she describes what was hidden. Treatment time for each painting varies because the investigation and analysis differ from canvas to canvas.

“Monet generally bought his materials from local color merchants. One can see that this painting was not done in one sitting,” she said. “It is apparent that some of the paint was applied wet into wet, but it is also clear that lower layers often had dried thoroughly before additional layers were added.”

This exacting work takes place in a pristine laboratory, far below the footsteps of the millions of visitors to the gallery.

On her table right now is a still-life by Edouard Manet — “Flowers in a Crystal Vase,” painted around 1882.

“The slightest discoloration of varnish or even a layer of grime alters the visual intent of the picture. You lose the definition in the brushwork and the clarity of the tones,” Hoenigswald explained.

“The varnish made this Manet greenish.’’

On this visit, she’s working on the painting. By the table is a small trolley with her basic tools: fine tweezers, small spatulas, hand-rolled swabs, or toothpicks that she adapts to the task.

Half of the cleaning is complete, and an examination through the microscope shows the difference. “I need to make sure that the balance that the artist intended is reestablished,” she said.

The green has been gently wiped with mineral spirits on a cotton swab to show that Manet was using a cool blue. “The colors reemerged, the energy of the brushwork was apparent again and you regain the sense of space and form,” she said.

“There is a magical moment when it all comes together.”