Daveb of Wellington writes by way of a comment:
Colin’s observation that coloration is concentrated on the crowns of the topmost fibres must I think be significant, and has to be the signature of some kind of contact process, if indeed it was a process. I suspect he might well be correct in saying that it couldn’t be radiation, as radiation would give a more pervasive result extending to other fibres, and beyond the crowns.
There may be another possibility that explains imaging at the crowns of the fibers.
Rogers was confident that residues of starch and saccharides were there on the cloths in what he called an impurity layer that he believed was only 200 to 600 nm thick. It is thought, perhaps, that after hand spinning the fibers of the flax plant into yarn, individual hanks of yarn were bleached and dried. When it was time to weave the yarn (thread) into cloth, warp threads were strung vertically on a loom so that weft threads could be passed over and under them. On the loom, the warp threads were lubricated with crude starch to make weaving easier. This reduced friction and lessened the chance of fraying. When a length of linen cloth was finished it was removed from the loom and washed in the suds of the Soapwort plant (Saponaria officinalis). After washing out most of the starch, the linen was laid out across bushes or hung to dry.
Washing, even with repeated rinsing, is not perfect. Soapy residues and small amounts of starch remain in a water soaked cloth. As the cloth dries, moisture wicks its way to the surface to evaporate into the air. As the water makes its way to the surface it carries with it dissolved starch fractions and saccharides: glucose, fucose, galactose, arabinose, xylose, rhamnose, and glucuronic acid. As the water evaporates into the air these chemicals are deposited as a super-thin coating on the crown of the fibers, the very outermost fibers of the thread. Chemists say this superficial residue of reactive saccharides is at the evaporation surface of the cloth.
Thus linen cloth made in this ancient way, with the yarn bleached before weaving, lubricated with crude starch and washed in Saponaria officinalis is ready for image formation. All that is needed are the right reactive chemicals and a mechanism to get the right quantity of the chemicals to the cloth’s surface in the right places at the right time. The amines that come from a dead body before it decomposes, cadaverine and putrescine, are just what is needed, according to Rogers.
Many things would affect how the images would form as the amines met the saccharides: ambient temperatures and humidity in the tomb; the body chemistry of the corpse influenced by diet, disease and possible trauma; the application of different burial spices; and the quantity of residue and evenness of its coating on the cloth. Even the tightness of the weave that affects porosity is a factor. Nonetheless some imaging would take place. The process would continue until the reactants were exhausted or until fluidic bodily decomposition products formed and ravaged the images and the cloth. Soon the cloth would rot away along with the body. However, if at the right moment, the cloth and the body it enshrouded become separated, and if the tomb had been opened so that cloth might be preserved, we might very well have something of a picture of the once enshrouded body on the shroud.
Rogers did some experimenting. He produced the color and some very course images that showed some fuzzy resolution. As he noted: “You can argue all you want about resolution. The Maillard colors are somewhere on that cloth. Where do you think?”
It is important to note that linen cloth, as typically produced after the twelfth century and into our era, will not produce amine/saccharides images. In the first century each hank of yarn or thread for the cloth was bleached before weaving. Such bleaching did not result in uniformly white yarn and because many hanks of yarn were required to make linen cloth, the cloth was not uniformly white. We see this, for instance, in the Turin Shroud. It has a broad variegated appearance where yarn from one hank was joined with yarn from another batch during weaving. The yarn ends were laid side by side pressed together. The overlapping ends are often visible to the unaided eye and correspond to streaks of different off-white color in the weave.
Bleaching after weaving, as was done in the medieval bleaching fields of Europe and as it is done in modern mills, prevents a reactive coating. Bleaching after weaving makes for better quality linen but it does not allow an image to form.
It is also especially important to note that there will be two such chemical coatings on the cloth. The side of the cloth that faced the sun and dried the fastest will have a dominant coating of starch fractions and saccharides from the soap. The other side will have a lesser coating. Both sides will react to the amines since some of the vapors will diffuse through the cloth. Indeed, we should have a more distinct image on one side of the cloth and a less distinct image on the other side. That is the significance of the discovery of a second facial image on the Shroud as recently reported in the peer-reviewed scientific Journal of Optics of the Institute of Physics in London (April 14, 2004).
From spectral analysis, microscopy and image analysis, we see that this is how the cloth of the Shroud of Turin was manufactured. From this, and from a modern knowledge of pathology and chemistry, we can hypothesize that this was how the images were formed on it. Are there many open questions? Of course.