The aqueous colloidal mediums used in the Middle Ages maintain their airy, light colors even in decay. Therefore, whatever is that yellowed on the Shroud cannot be a medieval paint medium. Only the oil paint mediums and some of the varnishes yellow. Old paint cannot be compared with modern paint. The refrective indices and the structure of old paint radically and erratically change.
Some opaque paints become transparent and the underlying tracing lines show through.
Dr. McCrone stated, the fire of 1532 would not have changed the color of vermilion, because the Shroud was folded. The change of color on both, vermilion and red ochre, would have taken place to different degrees from layer to layer, and we would see a checkerboard effect on the Shroud. Our microscopist takes his knowledge of medieval paint techniques from old books, such as Cennino Cennini and Eastlake. There is great professional doubt about the identity of some of the pigments, materials and methods described in old texts. Some of the pigments are either not recognizable or obsolete and the methods are no longer in use and forgotten.