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J. Bianca Jackson, Ph.D., who reported on the project, explained that it involved a fresco, which is a mural or painting done on a wall after application of fresh plaster. In a fresco, the artist’s paint seeps into the wet plaster and sets as the plaster dries. The painting becomes part of the wall. The earliest known frescoes date to about 1500 B.C. and were found on the island of Crete in Greece.

“No previous imaging technique, including almost half a dozen commonly used to detect hidden images below paintings, forged signatures of artists and other information not visible on the surface has revealed a lost image in this fresco,� Jackson said. “This opens to door to wider use of the technology in the world of art, and we also used the method to study a Russian religious icon and the walls of a mud hut in one of humanity’s first settlements in what was ancient Turkey.�

The technology is a new addition to the palette that art conservators and scientists use to see below the surface and detect changes, including fake signatures and other alterations in a painting. Termed terahertz spectroscopy, it uses beams of electromagnetic radiation that lie between microwaves, like those used in kitchen ovens, and the infrared rays used in TV remote controls. This radiation is relatively weak, does not damage paintings and does not involve exposure to harmful radiation.

“Terahertz technology has been in use for some time, especially in quality control in the pharmaceutical industry to assure the integrity of pills and capsules, in biomedical imaging and even in homeland security with those whole-body scanners that see beneath clothing at airport security check points,� said Jackson, who is now with the University of Rochester. “But its use in examining artifacts and artworks is relatively new.�

Artists, including some of the great masters, sometimes re-used canvases, wiping out the initial image or covered old paintings with new works. They often did this in order to avoid the expense of buying a new canvas or to enhance colors and shapes in a prior composition. Frescoes likewise got a refresh, especially when the originals faded, owners tired of the image on the wall or property changed hands

The scientists turned to terahertz technology when suspicions surfaced that a hidden image might lie beneath the brushstrokes of a precious 19^th century fresco, Trois homes armÉs de lances, in the Louvre’s Campana collection. Giampietro Campana was an Italian art collector in the 1800s whose treasures are now on display in museums around the world. When Campana acquired a work of art, he sometimes restored damaged parts or reworked the original. Art historians believe that Campana painted Trois homes armÉs de lances after the fresco was removed from its original wall in Italy and entered his collection.

Jackson said that Campana’s painting in itself is valuable, and the terahertz revelations may have added value by showing that an authentic Roman fresco lies under it.