Ray Rogers spent a while analysing the vanillin content of various bits of shroud with a view to establishing some kind of vanillochronology, but I cannot find among the vast literature published on shroud.com that anyone has considered whether lignin might be responsible for the image colour. Rogers declared that it was prominent at the nodes of the fibres, which may well be true, but I wonder if it might not also coat the outside of the fibres as well. During the processing of flax, there is an attempt to remove the lignin from the fibres, but even in modern linen, some lignin remains. Could it be (a constituent of) the ‘impurity layer’? It turns yellow quite readily in quite a short time at quite low temperatures, and with exposure to light, much more easily than cellulose, more easily than starch, and for all I know more easily than sugar as well. Has anyone else looked into this, and if so, with what conclusion?
Rogers loved these sorts of good-thinking questions.
Image is from Scientific Method Applied to the Shroud of Turin – A Review by Raymond Rogers.