It is fairly easy for an art conservator to see the damage done to a work when using a scalpel to clean away centuries of gunk from the surface: Mistakes end up as paint flakes on the floor or as a gash in the painting. When a laser cleans paintings, however, the damage may be subtler.

Using profilometry; colorimetry; and optical, vibrational and analytical mass spectroscopy, researchers from Spain, Greece, the Netherlands and Portugal analyzed damage done by a 248-nm KrF excimer laser to egg/tempera-based paints -- the most popular types used before the 15th century. They found that the worst-case scenario, direct laser contact, could induce paint discoloration. The radiation inflicts little if any damage when a layer of varnish is present.

The study appeared in the Sept. 15 issue of Analytical Chemistry.