Chemical analysis of byssal threads
http://www.acheiropoietos.info/proceedings/JaworskiWeb.pdf
As noted , not all researchers agree about the findings of pigments on the veil. It references Fanti who thinks it is linen. I have seen in my research that the term byssus was once known to be used for linen according to Websters, Oxford, etc.
and I still have no answer as to why the image on the veil is not a true mirror - for example - the lock of hair.
http://www.shroud.com/pdfs/jaworski.pdf
Does that make it more or less likely that it is a painting? The paper theorizes that it makes it less likely to be a painting.