A chemical ghost on linen fibers
The image on the Shroud of Turin is not a painting or a stain. Microscopic examination reveals no pigments, dyes, or brushstrokes. Instead, the image exists as a chemically altered layer on the topmost linen microfibrils. This discolored layer is extraordinarily thin, measuring less than 0.2 micrometers (200 nanometers), which is about 2% of the fiber's radius. The image is the result of a chemical change—specifically, the oxidation and dehydration of the polysaccharide structure of the linen itself. This process formed a series of conjugated carbonyl groups, a chemical structure similar to a scorch, but one that lacks the fluorescence typical of a high-temperature burn.
In 1978, the Shroud of Turin Research Project (STURP) conducted 120 hours of non-destructive testing on the cloth. The team of scientists concluded that no known artistic or scientific method from the medieval period could account for the image's unique characteristics. One of the most unusual properties is the three-dimensional information encoded in the image's shading. When processed with a VP-8 Image Analyzer, a device developed for NASA, the varying light intensity of the image translated directly into a topographically accurate 3D relief of a human body. Photographs and paintings uniformly fail this test, producing distorted results.