But paint is just one of the physical clues perused by scientific art authenticators. If a painting is on canvas, the cloth’s fibers can be analyzed under a microscope to determine their probable age and origin. (McCrone once discovered that a purported 18th-century Russian religious painting contained Dacron polyester, a synthetic that didn’t become available until the 1950s.) Paintings done on wood panels can be dated using dendrochronology, the study of how tree ring patterns were affected by historical changes in weather. Ultraviolet light can reveal portions of a painting that have been altered — and possibly uncover a forger’s attempt to make an obscure artist’s work appear to be that of a master.

Recently, a Dartmouth College team headed by computer scientist Hany Farid unveiled a new tool — computer analysis of paintings. According to a 2004 article published by Farid and his colleagues in Proceedings of the National Academy of Sciences, the method uses a process called "wavelet decomposition," in which a painting is converted into a digital image and then subjected to mathematical and statistical analysis. In a manner similar to handwriting analysis, the scientists can map the distinctively personal pen or brush techniques used by different artists — details that are invisible to the unaided eye, but which set apart the work of a master from, say, one of his pupils or a forger. Farid’s team tested the process by analyzing eight drawings by 16th-century artist Pieter Bruegel the Elder, along with five drawings that until recently were attributed to the master. The researchers found that the authentic Bruegel sketches all had a similar digital signature, while the imitations were not only different from the genuine drawings but different from each other as well — suggesting that they were done by multiple imitators. A similar analysis of "Virgin and Child with Saints Jerome and Francis," by 16th-century Italian Renaissance master Pietro Perugino, confirmed art historians’ long-standing suspicions that Perugino himself probably did only part of the painting. Similarly, at the University of Oregon, professor Richard Taylor has used fractual geometry to break down the distinctive drip patterns of Jackson Pollock paintings — and to call into question the authenticity of some newly discovered works.

While digital analysis of artwork has not yet gained wide acceptance in the art world, it could be the wave of the future. But even now, science already plays a major role in differentiating frauds and copies from genuine masterpieces. As McCrone once put it, whether a scientific authenticator judged a painting to be genuine or a fraud, the expert "can be more confident in that belief than anyone who lacks objective scientific evidence."