Colin, in a comment yesterday (I threw in an inline link to Colin’s image file) wrote:

I wasn’t going to start the microscopy for a few days, Thibault, but you asked, so here’s the first using my revised technique (linen on top of heated template, damp cloth on top of linen, gentle manual pressure).

Let me say first of all that the procedure produces very faint scorches, dare I say Shroud-like, so faint that one can scarcely see them at all under a hand lens. Here’s a picture I have just taken at x40, the lowest magnification on my USB microscope.

I’d say the threads and fibres were a pale yellow, with no obvious “patchiness” or restriction to crowns of threads only, but these are early days.