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Chemist Alan Adler, however, doubts that the oxidation was humanly induced. For one thing, the image is only one fiber deep. "If you lift a crossing fiber, you won't find any discoloration below," he says. The application of acids would not achieve such delicacy. Similarly, the fiber-by-microscopic-fiber gradations, even within a single thread, that make up the figure's exquisite "shading" would defy a human hand, were it engaged in either the application of acid or a rubbing process. Finally, Adler, a recognized expert on certain molecules found in blood, notes emphatically of the crimson stains and rivulets that ornament the shroud, "The blood is blood, and it came from a man who died a traumatic death." In fact, he says, both chemical analyses and a telltale yellow-green fluorescence under ultraviolet light indicate the presence of remains of a slightly different substance: the fluid exuded from blood clots. That substance and its invisible-to-the-naked-eye manifestation, he says, were unknown until the 20th century, so if a medieval artist did create the image, "he must have been a genius."
Like many other experts, Adler discounts a once popular theory that the bloodstains are composed of microscopic particles of reddish pigment, bound in a tempera medium. While it is possible that there are traces of pigment on the shroud, says historian Wilson, they are most likely flakes from copies of the image that were pressed onto the shroud in an attempt to rub off some of its sanctity. Adler believes the image must have been triggered by some sort of radiation process. But he stays away from speculation as to whether such radiation could have been divine in origin. "You can't go to the literature and find an explanation," says Adler. "Science can never authenticate this cloth, because there's no lab test for Christ-ness."
Which is not to say that some people haven't played with the possibility. In November, Doubleday plans to publish Garza-Valdes' provocatively titled The DNA of God? Scientifically, Garza-Valdes carefully hedges his statements about the shroud, saying only that "as of now, I have no reason to believe the Shroud of Turin is not the burial cloth of Jesus Christ" and that he thinks the blood on the shroud is human, male and ancient. In the early 1990s, Garza-Valdes asked Victor Tryon, director of the Center for Advanced DNA Technologies at the University of Texas Health Science Center at San Antonio, to help him identify the organisms he believed were present in the shroud samples. To do so, he used a technique that enabled him to make millions of copies of the infinitesimally small segments of DNA contained in sticky-tape samples of the shroud.
Of the tests, Tryon says, "All I can tell you is that DNA contamination is present and that the DNA belonged either to a human or another higher primate. I have no idea who or where the DNA signal came from, nor how long it's been there." It is, he says, not necessarily the remains of blood. "Everyone who has ever touched the shroud or cried over the shroud has left a potential DNA signal there." Tryon quit the project soon after his tests. "I saw it as a multidisciplinary project involving archaeology, physiology and other fields. But I came to believe there was another agenda present too. It was my first encounter with zealotry in science."