One of the favorite assertions of the believers is that only a miracle could have created the ghostly, lifelike image; there’s no known technique, they say, by which human hands"”especially medieval hands"”could have created it. But at least two reputable researchers, professional skeptic Joe Nickell and forensic anthropologist Emily Craig, have shown that a medieval artist could easily have created the image, using red ochre pigment applied with a dust-transfer technique, like the one employed to make brass rubbings from tombstones.
Source: Shelfari: The Inquisitor’s Key: The Shroud of Turin and the Ultimate Game of "What if?"