[d_theorist]

Whatever else you might want to say about the shroud, one thing we know for sure is that it is not a painting. Analysis shows that the image is extremely superficial, only penetrating about 200nm into the fibres of the cloth. This is inconsistent with a painted image.

[ted_dunning]

Could easily have been sprayed on.

This is a common technique done with a mouth tube in older times, a bulb sprayer later and an air brush most recently.

[svieira]

There is nothing on the fibers that is visible under a high powered microscope. Paint (even spray-paint) is quite visibly distinct from the fiber it covers.

[tsimionescu]

When it is relatively fresh, or well protected. Who know what is left after 600 years of handling and sunlight exposure and being saved from a fire.

[svieira]

We have known paintings from the time period - the paint is still visible under a microscope.

[tsimionescu]

Yes, because those paintings were preserved as paintings. And they were not designed in the first place to look like something other than a painting.

[tsimionescu]

I was using "painting" in the broader sense of "man-made work of mostly bi-dimensional visual art".

I'd also say that I'm pretty sure the mysteriousness of the technique is way overblown. Sure, we don't know exactly what it was, but there are trace pigments on the cloth, and ~600 years of handling and being displayed without much protection, plus being doused in water ~500 years during a fire, will wash off pigments in ways we can't fully predict.