Cellulose fibers that make up the threads of the Shroud's cloth are coated with a thin carbohydrate layer of starch fractions, various sugars and other impurities. This chemical layer, which is about as thick as the transparent scratch-resistant coatings used for eye glasses, is essentially colorless. However, in some places, the layer has undergone a chemical change that appears straw-yellow. This chemical change is similar to the change that takes place when sugar is heated to make caramel or when proteins react with sugar giving beer its color. And it is the straw-yellow, selectively present in some parts of the carbohydrate layer, that makes up the image we see on the Shroud. When scientists speak of image fibers they are referring to the coating on lengths of fiber that have undergone this chemical change. How and when did this layer come to be on the Shroud?
Most probably when the cloth was manufactured. If the cloth is truly from Jesus' era, then our best source of information is from the Roman encyclopedist, Pliny the Elder (Caius Plinius Secundus) (23-79 CE). In the ancient production of linen on vertical hand looms, warp threads, the up and down threads on the loom, were lubricated with starch to make it easier to weave the weft threads over and under the warp threads. The starch also protected the threads from fraying or unraveling. After weaving, the cloth was washed with a natural soap made from the soapwort plant (Saponaria officinalis). It was then dried flat over bushes in the open air.
Washing, even with rinsing in clear water, never removes everything and small residual amounts of material remain on the cloth. This would include starch fractions, various types of sugar in Saponaria, and other impurities dissolved or suspended in the wash water. As the cloth dries, all these residual chemicals concentrate at the evaporation surface, forming the colorless carbohydrate layer on the outermost edges of the top fibers in the thread. Concentration at the evaporation edge is a very natural phenomenon when cloth is air dried.
Ray Rogers followed Pliny the Elder’s directions for making linen, but added blue dye to the washing solution as a marker. After the cloth was washed with Saponaria officinalis and dried lying flat, the blue dye had concentrated on the topmost fibers of the cloth. The carbohydrate impurities in the starch and washing solution are also concentrated on the surface.
One should not get the mistaken impression that the image was formed by dyes. The blue dye in this demonstration was simply used as a marker to demonstrate how a concentration layer forms. Without the marker, the carbohydrate layer is colorless.
It was later that a chemical reaction to occurred that selectively changed the the carbon double bonds of some of the layers of impurities. This may have been a perfectly natural phenomenon. A Maillard reactions of amines from a human body with the carbohydrate layer will occur within a reasonable time, before liquid decompositions products stain or damage the cloth. The gases produced by a dead body are extremely reactive chemically. Within a few hours, in an environment such as a tomb, a body starts to produce heavier amines in its tissues such as putrescine (1,4-diaminobutane), and cadaverine (1,5-diaminopentane). This does produce the color we see in the carbohydrate layer. But it raises tough questions about why the image are so photo-realistic and why the images were not destroyed by later decomposition products.
These layers, visible with phase-contrast microscopy, are extremely thin. On the Shroud they have been observed to be approximately 180 to 600 nanometers thick. This is in the range of the wavelengths of visible lights. Where they are imaged, that is where they are chemically changed, they are thinner. This is chemically expected. The cellular fibers, which host the carbohydrate layers, are not colored.
The thickness of the fibers from flax plants varies significantly as they do in the yarn of the Shroud. The average thickness of Shroud fibers is about 13 micrometers or 13,000 nanometers (a typical human hair is about 100,000 nanometers thick). Thus the coating, in very approximate terms, is about 1 percent to 4 percent of the thickness of the fibers.
It is important to note that the carbohydrate layer can be dissolved with diimide or stripped away with an adhesive leaving clear, colorless fiber. This is true whether it has undergone a color-producing color change or not.
The yarn (thread) consists of approximately 70 to 120 fibers twisted together in a Z-twist (clockwise). The weave of the cloth is a rare 3-over-1 herringbone twill. It is approximately 350 micrometers thick though in some places it was found to be as thick as 390 micrometers and as thin as 315 micrometers. For comparison, a sheet of typical 20lb paper used in copiers and inkjet printers is 100 micrometers thick, about the same thickness as human hair.
The color of the images is in these carbohydrate layers, composed of a complex mixture of conjugated systems. Ray Rogers summarizes nicely:
"There is absolutely no doubt that the image color exists in a thin layer on the surface of image fibers. The layer is amorphous, and it seems to have an index of refraction relatively close to that of the linen fiber. The layer is quite brittle, and many flakes of the color have flaked off of the fibers. Colorless cellulose can be seen where image color has flaked off. The flakes can be seen and identified on the adhesive of sampling tapes. The flakes have the chemical properties of the intact image color on the fibers.
Non-image areas show an impurity coating on the surfaces of the linen fibers. It is slightly thicker than the colored image layer, as would be expected. When a material is dehydrated it shrinks. When the impurity layer reacted to produce the color, it got thinner.
Can this explain how the images were formed on the cloth?
No! The images are visually discernable representations of a human body as though painted by an artist or photographed by a camera. The image is focused, reasonably proportioned, has highlights and lowlights as though created by reflected light and is sufficiently exposed to be discernable yet not overexposed or washed out.
How representative of the body are the images? We can only infer that they are reasonably so. We cannot know how the cloth was draped across the body. At best it is a reasonable guess that it was loosely draped resting on high places such as the tip of the nose and falling off near the edges. We cannot know how smooth the shelf in the tomb was. We cannot know the ambient temperatures or humidity in the tomb which would be a factor in a chemical process. From forensic pathology certain assumptions can be made with regards to how fast the body might have cooled and, at various times, the concentrations of amine vapors that might have been produced.
If the image was caused simply by contact it would be seriously distorted and it could not account for details in the recesses of the eyes, the side of the nose, etc. Nor would it allow for the back images. The most probable scenario is that gaseous amines released by the body arrived at the surface of the cloth and reacted with the carbohydrate layer to produce the chemical products of the image.
The argument, often heard, that vapors coming off of the body would have simply diffused in all directions and could not have produced a focused image is incorrect. Graham's Law of Diffusion including the effects of Brownian motion, thermophoresis, and funneling effects where the cloth angles away from the body, does permit for a reasonably focused image as though painted on a flat surface.
This is still only speculation. But it is reasonable. The backside images just don't make sense for any form of forged image.
I hope this is of some help.
Shroudie