A single corner of a single quad-mosaic image has often been presented as evidence of chemical anomaly in that area. I myself do not think it shows that at all. A useful starting point is Barrie Schwortz’s 'Some Details about the STuRP Quad Mosaic Images" at
https://www.shroud.com/pdfs/quad.pdf, and the images themselves can be found at
STERA, Inc. Image Library.
There are four images, each showing one quarter of the Shroud. Although they are in colour, each image derives from three black-and-white photographs, each taken through a different coloured filter (red/green/blue). Each black-and-white negative could then be coloured, and the trio recombined to give a coloured image.
So far so good. However, the strength of the coloured filters required quite intense light, with the result that “there was a strong illumination brightness falloff from the center.” Although there was an attempt to eliminate this, it failed fairly comprehensively. Three of the images show a very bright sub-rectangular patch of brightness across the lower half, and much darker areas across the top third and bottom corners. This is significant in view of what happened next, which was a series of manipulations designed to enhance the colours, in an attempt to detect chemical differences from place to place.
At this point poor Jean Lorre loses all connection with rationality. “These color images should be interpreted as a chemical composition map”. Different colours, he hoped, showed different chemicals. But look, there is a bright blue field across the top of three of these images - are we to suppose that these blue bands represent a different chemical from the yellower areas below? And look, place the two images showing the dorsal image in their proper positions. From head to feet we see bright clear horizontal bands of yellow, then blue, then pink, then orange - are we to suppose that the entire Shroud was made of different chemicals in wide bands? Oh, and look, every one of the images has a dark green corner - are we to suppose that these green corners are all a different chemical from the yellower areas in the middle of the cloth? No, obviously not. The colours of the images are quite obviously artefacts of the lighting set up, and almost irrelevant as to the chemical composition of what they are photographing.