possibly the biggest radiocarbon dating mistake ever

Now that we know that the radiocarbon dating is invalid, meaning we don't know the age of the Shroud of Turin, much of the emphasis in shroud research has turned to trying to understand how the images were formed.

Skeptics have long maintained several positions: the shroud is a painting, a medieval photograph or a rubbing of a bas-relief. More recently, it was proposed that the image was sun bleached using a shadow mask painted on glass. None of these methods survive scientific scrutiny.

Proponents have suggested a number of theories as well. They include chemical processes produced by vapors emitted by a body shortly after death such as cadaverine and putrescine, radiation and an electrical corona discharge. A chemical reaction might be a natural phenomenon but an explanation for radiation or any other form of energy so far lacks a natural source explanation and seems to require a miracle, which clearly is beyond the purview of science. So far, no theory survives scientific scrutiny.

A Filmy Substance of Sugar

When the cloth of the Shroud of Turin was made, flax fibers, about one-fifth the thickness of human hair, were hand-spun together into linen thread. The threads were then woven into a fine herringbone linen sheet that is about three feet wide and fourteen feet long. These fibers, scientists now know, hold the key to how the image was recorded on the cloth whether by a miracle, a faker of relics or an accident of nature. It is on the surface of the fibers that the image resides.

Scientist who examined the shroud in 1978 used to think that some of the white fibers had oxidized and dehydrated and turned brown. Inexplicably, they thought that this was how the image was formed, even if they didn’t know the mechanism for this color change.

Now, scientists know that isn't the case. Instead, a thin filmy substance that coats some of the fibers has undergone a chemical change. It is the coating that has turned brown forming the image. Chemists know what this filmy substance is. It is a polysaccharide substance, a mixture of different sugars and trace amounts of starch. And they also know what sort of chemical reaction is needed to cause it to change color. But they still don't know how this might have happened in a way that would form an image of a man on the cloth.

The substance of the pictures, the starch and saccharide mixture, is extremely thin. From microscopic observations, chemists have estimated that it varies in thickness from about 200 to 800 nanometers. It is as thin as the wall of a soap bubble; thinner than the invisible glare proof coating on modern eyeglasses and thinner than most bacteria. To get an idea of how thin a few hundred nanometers is, it helps to realize that a sheet of copier paper is about 100,000 nanometers thick. 

The coating is only found on the outermost fibers of the thread. In fact, it is only found where the fibers are close to the surface of the shroud's cloth. In other words, the fibers inside the thread, deep in the middle of the cloth, do not have this filmy substance.

The coating can be removed by scraping or by pulling it away with adhesive tape. Over the years, as the Shroud of Turin was folded and unfolded, and spread out across rough surfaces, microscopic bits of the filmy substance flaked away. In fact, when scientists examined the shroud in 1978, they collected samples with adhesive tape. Today, countless tiny bits of the coating, some including parts of the image, are stuck to microscope slides and sampling tapes stored in laboratories around the world.

Scientists have a pretty good idea about how the coating got there. It wasn't brushed on or wiped on as an artist might apply sizing to a canvas before painting. Had that been the case, the starch and sugar mixture would have soaked at least part of the way through the fabric. Capillary action would have pulled the mixture into the middle of the threads. That didn’t happen. Scientists have not found any of the polysaccharide mixture except on the surface.

It turns out that the distribution of the coating is consistent with evaporation concentration. Interestingly, this is consistent with the way linen was made during Jesus' era as described by Pliny the Elder (23 to 77 AD).

If linen cloth is rinsed in a solution of water and dissolved saccharides, and if the cloth was is then dried in the air, a coating forms that is identical to the coating found on the Shroud of Turin. We know that in the first century, threads on the loom were lubricated with crude starch to make weaving easier and to prevent fraying. The starch was then washed out of the cloth by rinsing it in suds from the Soapwort plant. But the starch wouldn't have been washed out completely. Trace amounts of both starch and the numerous saccharides found in the natural soap would have remained in the wet cloth. As the cloth dried, moisture wicked its way to the surface carrying with it starch and saccharide molecules. The dissolved material concentrated at the surface and remained on the fibers as the moisture evaporated into the air.

Such a coating would have remained clear and colorless unless it was exposed to reactive liquids or vapors such as ammonia, or if it was heated sufficiently to caramelize it. We can probably rule out heat because enough heat to cause the coating to turn brown would also have caused the fibers to turn brown. That didn’t happen. When the image is pulled away with adhesive, the remaining fibers are completely clear. This leaves us to consider reactive vapors or liquid. But liquid will not work; for it would have dissolved the saccharides and carried them into the interior of the thread.

One theory is that vapors of cadaverine and putrescine, natural vapors that emerge from a corpse, would have produced an image. An interesting article, the Shroud of Caiaphas, explains this theory in greater detail. But, as the article explains, vapors present problems, as well, because they diffuse at various angles as they emerge from a body. It is unlikely that vapors could produce a high resolution image.

Philip Ball, who for many years was the physical science editor of Nature, a prestigious international weekly journal of science, wrote a commentary in Nature’s online edition in January of 2005. He summarized the problem well when he wrote, "[The] shroud is a remarkable artifact, one of the few religious relics to have a justifiably mythical status. It is simply not known how the ghostly image of a serene, bearded man was made."

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