Dec. 5, 2001 -- The clues could be as subtle as a tiny scratch made with a needle. Sometimes they were more obvious like the smooth, unhesitating lines that outlined a figure.
Combined, Philadelphia Museum of Art curator Mark Tucker believes it's clear that the great 19th-century American painter Thomas Eakins had a secret technique for achieving the remarkable realism often praised in his work: he traced from photographs.
And recent science suggests he was not alone.
According to the collaborative work of an artist and surface scientist, artists dating as far back as 1430 traced their images from optical projections of photographs or real life.
"We want to unambiguously prove that artists used lenses way back in 1430, so then every artist since then is suspect," explained Charles Falco, a surface scientist at the University of Arizona. "People who have not seen the visual evidence are skeptical. But once I show them, they are converted."
Bach With a Tape Recorder?
Some art historians have felt affronted by the claims. They argue that a year or two of scientific analysis can't possibly topple the decades of analysis by art scholars that barely mentions such tracing techniques.
"It's like saying Bach had a tape recorder and recorded noises from the forest for his music," said Walter Liedtke, curator of European painting at the Metropolitan Museum of Art in New York City. "It's ridiculous."
To detect the use of tracing, Tucker and his colleague Nica Gutman used infrared reflectography to examine the pencil drawings underneath the paint of Eakins' works. An infrared reflectography camera detects infrared light that's shined on a painting and reflected by paint layers.
Outlines of etchings used under a painting appear black on the camera's screen since these materials absorb the light.
When examining some works like Eakins' "Shad Fishing at Gloucester on the Delaware River," (1881) Tucker and Gutman noticed that the underdrawings were made up of "unhesitating" lines that had "unmistakable traced qualities" to them.
Furthermore, Eakins' smooth lines perfectly matched photographs the team recovered from Eakins' estate. Tucker believes the artist traced the photographs by projecting the images onto his canvas using a primitive, slide projector-like instrument called a magic lantern.
In other works, such as "Sailboats Racing on the Delaware" (1874), the Philadelphia team used a stereo microscope (a microscope with two eyepieces) to detect very fine markings made in several paint layers of the work.
Tucker believes Eakins made the small marks from projected images to mark coordinates for his drawings. He then used the coordinates to "connect the dots" and draw the scene.
"These marks didn't just occur in the lower layers of the paint," said Tucker. "We found them in low layers and even in very late stages of the painting."
Sometimes Eakins traced from one photograph, but more commonly, Tucker believes, the artist acted as an editor and traced images from multiple photographs that he projected one by one onto his paintings.
Although artists openly use all kinds of projection methods today to create their art (Andy Warhol once championed such techniques in his "Factory" studio), it appears Eakins may have wanted to keep it under wraps.
In one work, for example, the curators noticed Eakins used a tiny spot of touch-up paint to hide a needle-thin scratch mark that had likely served as a coordinate. And when asked if Eakins had used photographs as models, the artist's widow asserted her late husband preferred using real life as his model, according to Tucker.
"Critics were ambivalent about whether this was a legitimate technique or not," said Tucker. "So I think there was a lot of pressure on Eakins not to divulge."
In fact, Eakins might have been doing what artists have done for centuries.
Tracing in the 15th Century
For the past few years, surface scientist Falco and artist David Hockney, have acted as art detectives looking for signs that artists from as far back as the 1400s used projections to trace their work. Hockney recently authored a book on the subject, Secret Knowledge: Rediscovering the Lost Techniques of the Old Masters, in which he proposes the introduction of optical tools led to an increase in realism in art starting with the 15th century.
Falco explains that images projected by a camera or more primitive device (like a slide projector-like instrument called a camera obscura or a concave mirror) have qualities that could never be observed without the aid of an optical tool.
Vanishing points, for example, are where parallel lines appear to converge in an image (the way railroad tracks appear to join in the distance, for example). Paintings with more than one vanishing point, says Falco, suggest they were modeled on more than one projected image.
Much like Eakins may have used projections of multiple photographs in his work, earlier artists may have traced multiple images on their canvases.
Another clue is the depth of field in a painting. When projected by a lens, objects in the background appear fuzzy, while those in the foreground are clear. Falco says many early works have this quality, which could never be seen by the naked eye of the artist.
Still, Falco and others emphasize that even if artists used tools to trace their work, their reputations as masters are hardly diminished.
"Optics might have helped certain artists achieve certain results with greater facility, but it's still not easy," said Gary Tinterow, a curator at the Metropolitan Museum of Art. "You still have to be an artist to conceive a composition and execute it. These devices don't do that for you, they're just tools."