A clear polysaccharide residue coats the outermost fibers of the cloth. In places, that residue has changed to a caramel-like substance through a Maillard reaction. Cooking is a Maillard reaction. That brown substance forms the images.
One possible modality of formation of the image has to do with the gases that are released from a newly dead body before the liquid putrefaction products start destroying organic substances around the body. These gases 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). These exuded gases react with the polysaccharide layer on the cloth in a Maillard reaction and, in fact, does produce the color we see in the carbohydrate layer on the cloth. However, there is a problem. Whatever created the image on the Shroud acted only in a vertically collimated way—within a very few degrees of stricture—both up and down with no horizontal component. Gases simply do not act like that.
The polysaccharide coating is fragile and can be broken off the fibers... so the more the Shroud is handled, the more of these coatings will be shattered.
Interesting. Awhile back I heard a theory that the image was made by energy moving through the cloth (as Christ was resurrected). I suppose they are assuming that the energy is what caused the reaction, energy that would have been contained to the form of the body and act in a more directed fashion, rather than a diffuse fashion like gases would be expected to behave.
It's all very fascinating, but the science sometimes gets over my head. Thanks!